metallography manual - dcu

J~ META ~06RAPH"'i EUROPE

TABLE OF CONTENTS CHAPTER

3 day programme layout 1

Microstructure & Sectioning 2

Encapsulation 3

Single Point Tools 4

Surface Preparation to Integrity 5

6Thin Film Measurement

Traceability to ISO 9000 7

Microscopy & Photomicrography 8

Group Questionnaire 9

Material Classification & PreparationMethods

10

Own Preparation Method 11

Company Standard to ISO 9000 12

Much of this mataial bas bea1 taken fItBD the boot 'Surface ~~-aDOO &; Mia~ of MaItJiaIs'

B. BOUSFIELD, Buehler Europe Ltd. Coventry, UK

The first of its kind, this book is d~icat~ to the systematic preparation of avast range of material surfaces, looking in detail at the problem ofmicrostruCtUral traceability. Designed to be of practical use, the book has beenwritten in two parts. In the first half, tile book systematically defines theessential procedures involv~ in surface preparation. Having establish~ howto prepare a sample of integrity, the second half of the book illustrates the bestuse of microscopy by discussing, in depth. the different features whichcontribute to informative analysis.

Complete with over 100 stunning colour photo-micrographs and fully illustratedthroughout, this book provides an essential reference for researchers andtechnicians who require a comprehensive overview of microstructural analysis.

CONTENTS:

PART 1: SURFACE PREPARATION: Introduction; Sectioning; Mounting; Single Point Tools; TheNew Concept; Grinding; Polishing; Grinding and Polishing LubricantS; Towards a MetallographicStandard; Characterization: Auditing aOO Traceable Standards; Traditional Methods Only; Preparationof Spray Coatings; Preparation of Composites; Preparation of Minerals; Preparation of PCB's aOOElectronic ComponentS; Thin Film Measurement; Preparation of Soft Materials; Preparation ofCeramics; Hardness; Training in Metallography; Supplementary Materials, Techniques aOO Methods;PART 2: APPLIED MICROSCOPY: The Microscope - A Resume; Microscope Types andNomenclature; Creating the Microscope Image; Objective Aberrations; Improving the Image;MeasurementS; Illumination System; Eyepieces aOO Condensers; Introduction to Interference; SurfaceFinish Interference; Contrast Interference; Video Imaging and Archiving; Polarizing Light Microsopy;Fluorescence, Reflectance and Con-focal Microscopy; Photomicrography; Inverted Techniques;Photomicrography in Practice

£100.00/$164.000471931810 356pp 1992

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J"i. MET ALL06RAPH:'/ EUROPE

CHAPTER 1 DA"'l ONE - TUTOIUALS

roTORIAL MET ALLOG RAPHY AND THEW CR OSTR UCfURFJSECn 0 NIN G

DISCUSSION SEcrIONlNG OPTIONS

nJTORIAL ENCAPS ULA TI 0 N ISING LE PO)NT TOOLS

DISCUSSION ENCAPSULA nON OPnONS

TUTORIAL SURFACE PREPARAnON TO INTEGRITY

DISCUSSION GRINDING I POLISHING OPriONS

TUTORIAL THIN Fn.M MEASUREMENT I HA RD NESS IIMA G IN G AND A R CIDVIN G

SLmEP~ENTATION

PREPARAnON ARTEFAcrs

roTORIAL MICROSCOPY AND PHOTOMICROGRAPHY

SLIDEP~ENTATION

EXAMPL~ OF WELL PREP A RED SAMPLES,EXPLOITING THE OPTICAL MICROSCOPE

1.1

DAY TWO / DA YTBR ££ - PRACTlCALS

INSTRUCTOR DEMONSTRA T10NS

AM PM8.30 DELEGATES SPLIT INTO 3 GROUPS A.B.C. 1.00 STAY IN SAME 3 GROUPS

DAY TWO

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4.30 to 5.00 pm REVEW IN NA liVE LANGUAGE

Otherperloda D8cuSSlON ~ 11NG TO PAEPARA TIOH UUG A FLOW CHART WITH

OPTIONS

DELEGATES OWN PREPARATION

DAY 3

Delegates split into 2 groups X I: Y

lAM IPM II 1.31 I 1.18 I

4.50 COURSE DIRECTORS OVERVIEW OF COMPLETED COURSE WORK

MET ALLOGRAPH\' EUROPE

CHAPTER! MlCROSTRUC11JRE &. SEtnONlN6

Metallography is both an understanding of 'materialsU"Uctures' and the 'science of revealing those suucwres'.The sttuctures of materials can be 'macro' (lowmagnifICation - large field of view) or 'micro' (highermagnifications - small field of view). The materialmicrostrucblre is the 'rmgerprint' of metallurgy Le. themicrostrucblre is directly related to the performance ofthat material. The material structure has a relationship tothe physical and mechanical properties of the material.

MICROSTRUCTURE

H the microstruCnlre is to be correctly interpreted itfollows that the microstrucblre must be a true and faithfulrepresentation. Since most methods for the surfacepreparation of materials involve mechanical working(stress dislocation) which induce damage into the materialcare must be exercised in conn-oIling this damage to aminimum.

MACROSTRUCTUREMacrostructUres are often visible to the naked eye, but ingeneral are aided by the use of magnifying lens,stereoscopic micr~ope (double image) or the single axismacroscope. Surface preparation for the analysis ofmacroStIUCtures is generally confmed to fine silicon carbide

papers.

A macrostructure would be used to defme:. FracttJre morphology and grains. Dendrites in castings. Welding integrity. Porosity. Cracks. Exterior swface condition

MICROSTRUCTURES

Microstructures would be observed using a compoundoptical microscope with fields of view as low as O.18mmand resolution of O.25Jjm. When resolution less than

. O.25Jjm is required it is necessary to use the electron- microscope. The two basic types of electron microscope

being the scanning (for morphology) and the transmission(internal structure). Surface preparation can bemechanical, chemical (electrolytic) or chemomechanical(chemicals used in con.unction with the rindin action.

Copyright 1994 BUEHLER Ltd 2.1

MET AJ..L06RAPH"'i EUROPE

Fig 2.1 Microstructural details

Figure 2.1 gives an indication of the type of detail visibledown the microscope. In order to reveal information suchas grain boundaries and alloying elemen~ it will benecessary to etch the sample. Strucmres not visible in the'as polished' condition can abo be revealed by opticalinterference techniques by depositing an interference coaton the sample prior to observation (anodizing) or usingdifferential interference contrast to highlight interphaseabrasion differentials.1 2

3

12

4

5

10

/I

i 8 7 6Figure 2. 2 Possibl~ defects of through-hole platinr, (1, lack uf plating adhesion; 2, final plating fault.~;3. prima~. plating void; 4. fi~ protrusion; 5, average platinr, thickness; O. laminate void; 7. plating void;x. n(tdule; q, nail h~ading; 10, wicking; 11, knee crack; 12, J'Ot'r connection with rrsin smear)

Not every microsttuCb!re ~ concerned with grains andmaterial identifICation. Very often with an electroniccomponent or engineering ~bly the cross-section ~carried out to observe component dimensionalcompatibility. Compatibility of plated layer from laminatedboards and the cleanliness of the drilled hole in printedcircuit boards (~B) are adeqUately illustrated in thesketch in Figure 2.2.


MET ~06RAPH"/ EUROPE

FA 1THFm. REPROOucnON

Brief mention bas been made to the n~ for samples thatare a faithful reproduction of the original material prior to~bani~-ti woIting. Samples that are a true and faithfulreproduction are often referred to as 'sample of integrity'.So what can 'go wrong' as we mechanically preparematerial specimens for microstructural analysis?Elements can be:-

. Fractwed

. Pulled out

. Washed out

. Etched

. Smeared

. Distorted

. Enlarged

. Transformed

Elements can also be affected by:-- Heat-Pressure

- Surface friction- Force direction

Artefacts occur by incorrect:-. Abrasive type/size. Abrasive rake angle. Abrasive backing. Abrasive function. Lubricant. Surface speed. Grinding surface. Polishing surface

Results are affected by:-. Blunt abrasives. Prolonged I insufficient preparation times. Chemical attack. Cl~JiQess (lack of). Incorrect encapsulation. Resin contraction. Incorrect sectioning

From the four ~ above it can be seen how an incorrectapproach to surface preparation can inttoduce residualdamage leading to an erroneous analysis.

2.3

MET ALL06RAPIrl EUROPE

SEC"nON1N6 Before any sectioning takes place the need to carefullyselect a representative area cannot be over stressed. Themicrostrucmre over the entire component may not beunifoml, therefore, decide beforehand what the object ofany section is for and where best this can be located.

Figure 2.3 Sampling Identity

Figure 2.3 is used to illustrate a simple fonnat fordesignating the sampling position and the swface ofinterest within the sample.

Figure 2.4 Sample I Specimen Defmition

The word sample and specimen is often used to describethe same thing, with the convention as shown in figme 2.4this should now be overcome.

Sectioning is considered to be one of the most imponantsteps in the preparation of surfaces for microstructuralanalysis and before one can proceed from this point it iswise to have some idea of the depth of damage residual inthe sample resulting from the sectioning stage.


MET ALL06RAPH'/ EUROPE

FIgure 2.5 Depth of sectioning damage

This Z axis infonnation is derived by resectioning andpreparing to integrity the surface adjacent to the originalcut (figure 2.5)

WHEEL TRA VERSERATESlowFat

DEPfH OFDAMAGE (fUD)

10

45Soft Bond

Alumina Grit

Bard BondAlumina Grit

SlowFast

20900

Hacksaw Normal 70 + 200

Figure 2.6 Sectioning deformation 0.37% C Steel

Figure 2.6 gives the results from a series of differentsections on a piece of 0.37% Carbon Steel, notice how theresultant deformation or structural damage varies from10JUD to 9O0Jl1U. Without this information it is impossibleto 'tailor' the most appropriate subsequent preparationsteps. To put the 9O0JUD damage into perspective -

(A) 3 samples of 0.37% C Steel, diameter 25mm, whenused on 8" diameter 180 grit silicon carbide paper, for thelife of the paper, would remove 100JJIn i.e. 9 sheets of 180grit paper would be required to remove the 900J1mdamage.

(B) 3 samples of ~ C Steel under the same conditionswould require 18 sheets of 180 grit silicon carbide paper,Le. 50~ per sheet (These figmes are based on optimumcutting conditions; silicon carbide papers will remove twicethe amount quoted but this extra material removal willinduce intolerable levels of residual damage).

MET ALLOORAPm' EUROPE

WHEEL SPEEDIpD

100

D EPnf OFDAMAGE (JDD)

TIMEMINUTES

METALBONDEDDIAMOND

10 ~

METALBONDEDCBN

100 . IS

11m so ~

RaIN BONDEDsn.ICONCARDa

lUX) 9 2

2(XX) 7

Figure 2.7 Sectioning damage Aluminium Alloy.

The resultant structural damage shown in figure 2.7 is usedto illusttate the relationship that exis~ between abrasivetype, bond, operating speed. cutting time and residualdamage. Take the diamond wheel in comparison with thecubic boron nitride (CBN) wheel operating at the samespeed. The CBN wheel manifests a reduced depth ofdamage and cuts in less time. When the CBN wheel speedis increased so does the damage depth and cutting time.Ftnally the abradable resin bonded silicon carbide wheelreduces cutting times dramatically without an increase indamage when the speed is high. This chart indicates thatthere is (I) an optimum cutting speed and (2) an optimumabrasive - resulting in (I) a minimum residual damage and(2) a minimum cutting time.

Figure 2.8 - Sectioning Characteristics.

Sectioning characteristics have been brought together in asingle chart (figure 2.8), illustrating the effects of lubricant.speed, abrasive size and type, abrasive concentration,wheel bond, wheel thickness and mechanical factors.

MET ALL06RAPH'{ EUROPE

Figure 2.9 Characteristics - Metal Bonded diamond/CBNwheels

This final chart is intended to give guide lines when dealingspecifically with metal bonded diamond or CBN sectioningwheels. Speeds, lubricity, operating speeds, wheeldressing, abrasive size and concentration are all addressed.

2.7Copyright 1994 BUEHLER Ltd

SECTIONING OPTIONS GUIDE CHART

Reference the enclosed (reduced) Wall Chart. From this chart we are able toselect the most appropriate cut-off wheel to suit specific user needs. Fromthe 'legend' at the bottom of this chart, notice how abrasive concentration isdepicted by the number of abrasives (low and high). Abrasive bond strengthis related to the number of cross-lines i.e. single cross-line weak bond - multi-cross line strong bond. When dealing with expensive abrasives such asdiamond or cubic boron nitride then the bond/abrasive will be attached to the'rim' of a circular metal disc, this is depicted by an extra semi-circle. This 'rim'can be a resin or metal matrix. When metal matrix, the bond is shown as asquare grid. The type of abrasive is also designated by shape viz square =alumina, triangle = silicon carbide etc. When different size abrasives areused then a numerical system is employed i.e. 5 = small, progressing to 20 =large. The first five wheel types are all intended for ferrous materials ofdifferent degrees of hardness, the wheel abrasive being alumina. Notice the8th wheel on the list, this is also alumina but is a much thinner wheel than theprevious alumina wheels. Being a rubber bonded wheel (as opposed toresin) it is less likely to fracture when slightly flexed. This wheel is intendedfor delicate cutting, it also finds many applications which induced cuttingdamage must be kept to a minimum, viz sectioning of plasma coatedmaterials.

Silicon carbide wheels (6th and 7th) are offered for sectioning non-ferrousmaterials. they can however prove equally successful when sectioningferrous materials though wheel life would be very much reduced.

The rest of the wheels in the range are of the non abradable type(diamond/CBN). Much development has gone into these wheels to makethem very specific to the hard and brittle fracture materials as shown under'Materials Applications'.


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SECTION1N6

Wheels break for a variety ~ ~DSand can be frightening whell they «CUr.Abrasive wheels when manufacturm arecar8:ully tested to emure safe operatingspeem, these operating conditions mlmbe adherm to in use. Wheels can breakbe(:aD-~ they become jammed into theworkpiece, thk Is usually one ~ twofactors (a) the workpi~ (Spedmen) ~moved, or (b) the whee) ~ wandered(moved off uis.)

FAULT: WgF:F:I.BR£AKA6E

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1-_"

SAWPL!

To overcome (a) It win be ~ry toattribute the ca~ of any movement, I'it is s~ caused by dampiug thensiugle pcXnt damps will be necessary. Ifthe s~ Is within the workpi~ and ~released as the cut tak~ place then theworkpi~ shcx1ld be 5treG relieved orincremental cutting adopted.

REf.ONLY

~a.NAPAL TERNA TIVE

STRESS REUEVEREDUCE COOLANT FLOWREDUCE CUTTING FORCETo oven:c:Mne (b) a softer wheel is often

all that is required

FAULT: BURNIN6

Although burning can readily beo~rved on the cut surface, what is I~obvious is the depth to whim theburning hM affected the micr~cture.This depth for example an vary fnMn2Oprn to 25OJIID without a dramaticchange in the top surface burnedappearance. Some materials areadversely affected by thama1 shockingwhel'e It is not ~arlly thetemperature mange but the Iocalisedshock that occurs through im11ftJdent orpoorly directed coolanL Burning canvery ~ be overcome, ~ng thecoolant k appropriate, by reducing thetraverR rate. Softer wheels, althoughthe obvious molce, mU give a reducedwheel lire.

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: SOFTE R: WHEEL

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Al TERNA TIVE

REDUCE RATE Of' TRAVERSECHANGE POSITION OF'LUBRICATION

2.10Copyright 1994 BUEHLER Lm

SECTIONING

FAULT: PICK-UP II. WH££L 6LAZlN6

./J.,~-' -.., . sanER. . WHEEL: c . :

. -.I..1 .

Swarf pick-up is evideut wta merving~ ~phery of cut off ~ after use.This oolKlition is m<Xe in ~ as ~workpiece material ~PS softer IDdwould ~ver be manifest wid! bittlefracture materials. Alttnlgh ~ dnce ofa softer ~ will usually Dve thistroblem. ~ use of oillubricaots must ~beovedooked

'"

"i4

~"WHEEL GLAZING

,~ glazing occurs wIa d:Ie blunt

abrasive is DOt allowed to break free fttXnd:Ie surrounding matrix (resin), d:Ie remedyis to use a harder amsive, softer ~ (X'

increase the pressure. When using baOO

operating equipment d:Ie aInsi ve can re1:roken free by sudden contact of ~ to

workpiece. This however is DOt atedlDically soond recommeOOation

-"T, ... -.

~AlTERNATIVE

/- ., SOfTER

: WHEEl.-.1

\INCREASE PRESSUREHARDER ABRASIVE

FAULT: WIu. NOT CUT

IWhen wheel penetration into dJeworkpece is difficult, yet tM ~ and~ ~tions a~ to be ~--.miz~ie. ~ speed, pressure, lubrication.wheel rood. abrasive type; it will be~ssary to investigate ~ abrasive sizeand concenttati on. With very bardmaterials a coOOition COJId be foooowIae too many abrasives are in contactwith tM workpece at <e time restricting~ iOOividual f<rce ~ssary to ~and *ar ~ material. R~.l1I~-!!g dJeIBlIDber of abrasives in contact 0V0'00IDesthis problem. Reducing ~ ~de size m-an abrasive usually inaeases ~ ability tocut We to ~ lower fcxces required. ThiswiD also give a much ~~ residualstress (structural damage), cuaing timeshowever, coold iocrease.

IREDUCE

:A8RASIVECONCENTRATION

~y

. REDUCE ABRASIVE SIZEI ,..

'9ALTERNATIVE

CHANGE ABRASIVE SIZE

Copyright 1994 BUEln..ER Lm 2.11

SECTIONIN6

FAULT: CONTACT AREAThe contact area is a vitally imJX>nantfactor in the ability to successfully sectionany workpiece or comJX>~nl Thenumber of abrasives in contact relating tothe generated fttCes ~cessary. With thisin miOO it is always wise with abrasivesectioning to take ~ least contact areapossible. This is oot always JX>SSible witha workpiece that changes the area incontact as the cut ~ (circle), onthese occasions either iocremenW cuttinga: the oscillating head system will improvecutting performance. S~en rotation isanother m~ of ~~-!1g a constantcontact area, d1is facility also helps toensure a parallel sedi~ romJK>nCn1 i.e.it reduces any ~ for the ~ todrift.

.t'REDUCE CONTACT

AREA ~J,.~

~At TERNA TIVE

WHEEL osc~nONINCRE~AL CU1TINGSPECIMEN ROTA nON

FAULT: R£SWUAL DAMA6£/WHEEL WEAR

s~ or workpiece residual damage isa factor often ovedooked yet in our fieldof miaostroCb1ral analysis we must 00more dIaD ju.\1 section ~ COInIX>nent, wemust adlieve a sample widt ~ leastresidual damage. Wtae d1is is a JX'ob1ema thiDOC' blade (X' ~ wiD often suffice.These d1in blades are often robber ~and are also re~ficia1 f(X' sectioning themore delicate comJX)~nt. Reducing d)eabrasive particulate size wiD also reducedefonnation (residua] damage).

FAULT-SPECIMEN RESIDUAL DAMAGE

HIGH DEFORMATION LOW DEFORMATION

f{~1\\

0()USE THIN .WHEEL

ALTERNATIVEREDUCE ABRASIVE ",SIZE

f"AULT-RAPIf') WHF"F"I WfAR

Having selected ~ 'best' abr3sive/ 0000combination axupatible with efficientsectioning of a particular material ~noperating fcxces must be ~red to. Touse the ~ urm higrer than theoptimized pressure will certainly reducethe sectioning time resulting in rapid wheelwear, inaeased defcrmation and increasedgenerated heal To inaease the matrixbond strength of the wheel would alsoreduce wheel wear.

. '" . '.,"'"""'~"'" po

"v, - - -: HARDER. wHEEL.1'--

~~.--., ALTERNATIVE

RCOUCE PRESSURE

2.12Copyright 1994 BUEHLER Lm

SECTlONIN6

FAULT: SAMPLE BURN1N6This ackIitional section 00 bIrDing is todraw attention to ~ lubricant or c(Xjantwhich infiuelx:es ~ eflideu;y of anysectioning ~tion. Cutting wheels wiD~te differeDtly as ~ tem~affects ~ resin 1xmd Some wheels fcrexample are designOO to ~te dry aIJd~ ~te efficiently when ~ ~temperamre rises. ~ wheels we arema:e familiar with are dX)5e which are'jetted' with coolant to keep ~ sample(XX)} or ~ totaD Y ~ in axiantavoiding thermal shocking. It is importantwith these ~ to target, wIaepossible, ~ jet of coolant into ~ an aDdarowK1 ~ sample and WOIkpi~Targeting ~ coolant onto ~ ~thereby keeping it (XX)} wiD iIKhlce sampleOOming by effectively hardening ~wheel. Heat must be g~ted wbe:n'working' any mIface. On this occasion itis wise to cxmfiDe beat, where possible, tothe swarf and blade.

_R~~I

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-~T---SAMPLE

; (;jAT£R~WHEEL BONO TOO STRONGSAMPLE BURMNC

I

~EJtE~~

AlTERNATIVE

USE SOntR BONO WHEEL

TRY LESS LU8R!CANT

TRY DRY CUT .SLOWER TRAVERSE

FA UL T: WHE£L P R OFIL£ - IN CORR£ CT

USE

When coolant is used. even when takinginto consideration ~ {X)ints rdised atx>ve.it will be !l~~ to wet ~~. ~Bow onto ~ ~ must re equal 00d1sides if unifcml ~ wear is to takeplace. ~ ~ pofile can also re aclue to correct matrix tKmd as sIK>wn inthesketdl.

'" WHEEL

~

, ,

~

"

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~~POINTED

~ "/""""""""'~~SUR£ COOlANT

BOTH SIDES or WHEEL~-

~/

CHISEl.ALTERNATIVE

KEEP COOLANT FLOW

AWA"" FROM SlOES or

WHEEL

Copyright 1994 BUEHLER Lm 2.13

USESOFTER

SECTIONIN6

FAULT: POOR SCRATCH P A n£RNIt bas been kIXJwn for ~ reflectivity c:t:~ sediooed surface to re used as a guideto ~famation free cutting aDd fcx theweD ~tdl~ surface to re rejected. Thisin fact is an ~ assumption since itis ~ OOrnisbing CX fXX'C cutting bluntabrasive that causes ~ shiny roOOition.Efficient cutting takes place ~ usingsharp abrasives and sharp efficientabrasives leave wen defined saatches. Itis impcxtant thc'efore to recognise a gocx1saatdl pattern sinre this wiD often JXOveto re t1M:: time emcient mettxx1, but aboveall t1M:: metIxx1 that least alters ~ materialstructure i.e. d1e most faithful.

~, ~- ,

~@ RUL(CTMTY HIGI

SOF"TERWHEr-L

~..,:- ~1fA~ rYLl

~~~~~ RE~ LOW

FAULT: STRUC11JRAL DAMA6£

SECTIONING

~

~~

This sketd1 is used to reinforce ~importance of m~1cing the sectioning stage~ most imP<X18nt ~e in ~preparation p'ocedure. Not only caninevocable damage re ~ at dlis stagebut In(X'e imlXJ1'taDtl Y a gcxx1 section canp:1imina.tf": ~ ~ for some of ~sumequent JXeparation stages. ~damage aeated from a given size abrasiveis less when sectioning d1an ~ anequivalent sized abrasive is used in ~

grindingmOOe.

t) [~t:t~~~20 M'CROMETR[ LABRASIVE LOW ~E

~

GRINDING

W////?& I=~~~::::]::::::.~

AlTERNATIVE

INCREASE SHOI:K loBSI)ReING Ol~ CRINDlNiSURfACE USE ':-":';;';[l- ~RA$!V(

Copyright 1994 BUEHLER Lm 2.14

-

SECTIONING

When sectioning coated layers such as plasmacoatings or materials with a friable surface it is wiseto direct the cutting force, first through the coating,followed by the substrate, failure to do this can resultin delamination as shown. When dealing with afriable material or a porous coating it is wise tovacuum impregnate prior to sectioning. The vacuumnecessary will be dictated to a large extent by thesize of interconnecting paths between the pores.When the specimen has been totally immersed it ispossible to use isostatic pressure to increase resinpenetration. A lower viscosity resin will also improveresin ingress. Irrespective of any protectionprovided it is still wise to direct all forces in thedirection of resin - coating - matrix. Notice from the

sketch how not all pores are 'open pores'.

SEcnONlN6FAULT: SAMPLE ORIENTATION


MET .AlJ.06RAPH\' EUROPE

ENCAPSULA 110NCHAPTERS

£NCAPSULA TlON Specimens are mounted or encapsulated to (a) protect thesample material. (b) produce a uniform dimensionallystable size for subsequent automatic machine operation or(c) assist handling for subsequent band operatedprocedures. The 2-types of mounting techniques availableare compression hot mounting and castable cold mounting.Additionally there are two basic types of resins. those thatonce set will remain rigid even when subjected to heat(Thermosetting) and others which once set can berendered plastic when subjected to heat (Thermoplastic).Castable cold mounting resins set after reaching theexotherm temperature and can be thermoplastic orthermosetting. These resins do not require pressure to setand are simply poured into appropriately shaped cups.Compression hot mountings are also available asthermoplastic or thermosetting. This powder mixture isencased within a pressure chamber and requires a specifictemperature relative to pressure to set the resin.Thermosetting resins 'set' at the hot curing temperature andas such can be ejected from the mould chamber when hot ifnecessary. Thermoplastic 'set' below the peak temperamreand must therefore he ejected when cool.

Furin~HeatIng Cooling

~"~

f~

t'~

Solid

THERMOSETTING

Liquidkei-f.!' Heating CoolIng Eject

THERMOPLASTIC

Fig 3.1 - Compression Moulding Conditions

Fig 3.1 illustrates the heating I curing conditions for boththe Thennoplastic and Thennosetting hot mounting resins.


MET ALL06RAPHi EUROPE

RESIN REQUIREMENTS

Ideally we would like a resin to have nil contraction, toadhere to the sample, to have an identical abrasioncharacteristic to the sample and be easy to PomboUnfortunately this is rarely, if ever, possible to achievetherefore the object is to define the 'prime' requirements andrelate these to the resin. From the point of view of a4hesion,only Epoxy resins fully satisfy this requirement with thePhenolics being the least accommodating. With castable coldmounting resins contraction is an important factor. With hotcompression mounting the different coefficient of tbennalexpansion between resin and mount also becomes importantAs an example the coefficient of lbennal Expansion (crE)for some resins are as follows:-

Phenolic 3/4.5 x 10"Acrylic 5/9 x 10"Epoxy 4n X 10"Metals 1/3 x 1 0-'

.

.

.

.

From these figures it is easy to see how the resins would beshnmk onto a round sample when cooled.

Figure 3.2 - Reducing Contraction

Grinding relief between mounting resin and sample is to beavoided when edge analysis is to be carried out To avoidthis sitUation the abrasion rates between sample and resinmust be considered.


MET Au.06RAPH"l EUROPE

ABRASION CONSTANT/If'AT£CLASS MATERIAL

Hd.M~Prek8d 1m-~M-.iaII

~~"-baIaIe~yRaaAaytie

Ccid Mmmtiq f.paIyAayIic~yeIt«AayIio+AV

17010401100m

M~ Cq.-ADoySIerJ(O.37)

A l Ii.iIaII ADoy

3512.5142

Figure 3.3 - Abrasion rates grinding

Figure 3.3 gives comparison rates betWeen various materials.Two important points relating to this chart are (1) thereduced abrasion rates on compression mounting whencompared to castable resins and (2) the reduced abrasionrates of metals relative to the resins.

, S I Carbon/carbon" tee I /Phenolic

Epoxy

(b)(a)

(c)

Fig 3.4 - Relief Grinding

Figure 3.4 gives examples of positive and negative grindingrelief. Additionally at (c) is the example of differentialinterphase relief. This is vital when ~ing differentialinterference contrast, the height difference however must notexceed the resolution of the microscope objective.The following is a list of attractions for both compression andcastable moulds.


(JDD/miD).160440130SXJ

£NCAPSULA TION OPTIONS 6 UIDE CHART- -- -

Reference the enclosed (reduced) Wall Char1. From this char1 we are able tosee at a glance the variety of moulding materials available and the variouscharacteristics relating to each resin.

In the field of materialography, encapsulation is via hot compression mouldingor alternatively the so called 'cold' castable method. The word 'cold'mounting is somewhat a misnomer since exotherm temperatures of 1202 C ishardly 'cold'.

The least abrasive resins are the compression hot mounted variety whichalso tend to be quicker setting and more dimensionally correct. From thechart it can be seen how the various categories have been defined. The firstthree are phenolics and are also available as pre-moulds. The abrasionfactor is important in trying to match, where possible, the abrasion of thesample with the moulding material. This factor is related to micrometres perminute under a given test parameter, this shows the difference between hotand cold mounting.

In addition to the abrasive rate is the 'polishing rate'. From the legend at thebottom of the chart it will be noticed how this has been designated by cross-lines, just one line for 'high' progressing to many for 'Iow'.

The hardness value (Hv) has also been quoted but at these low values theymust be for comparison purposes only. Note how the hardness value has norelationship with the abrasion factor. The abrasion factor is the importantvalue. the hardness is of little importance and again is only quoted forcomparison.

With all these different resins it is important to address the question ofsuitable specific applications, the columns starting from 'use' are intended tohelp. The cleavage (gap between resin and sample on the inside of awasher) and the relative grinding relief when comparing resin to steel (0.4%C) are drawn schematically. .Take for example the black Epomet, this showsno cleavage and nil relief; the black Edgemount on the other hand has veryslight cleavage still with nil relief. The major difference between the two resinsbeing cost. These two resins would be totally unsuitable when the samplematerial is highly abrasive because on this occasion negative relief would becreated.

Copyright 1994 BUEHLER Ltd

ENCAPSULATION OPTIONS GUIDE CHART - COImHUED

Viscosity, the ability of the resin to flow into small areas, is very important inparticular when the sample is porous, when it may be necessary to vacuumimpregnate. Notice with the hot mounts how the Epomet and the Transoptichave been given a 'start' towards being viscous. With the castable resinsrated from high with one drop to low with a continuous flow it is very easy tomatch a specific resin to suit particular needs.

Setting times have been generalised but as can be seen, vary from 5 minutesto 12 hours. Thermosetting resins, those that set with heat and cannot laterbe softened, are shown by shading the upper triangle in the box.Thermoplastic resins, those that can be softened later by introducing heat,are shown by shading the lower triangle in the box. Thermoplastics areusually difficult to polish scratch free.

Finally we have the peak temperature column for compression mountingresins, these figures are only valid with a given pressure. The thermoplasticfor example could require 20~ C if the pressure was to be halved. Thequoted peak temperature for the castable resins is the minimum temperatureduring the exothermic reaction, it will if not controlled rise above this figure.


CIIA MET AlJ.06RAP~ EUROPE

FAULT: CONTRACTION

COMPRESSION MOUNTING

CASTABLE MOULDS.

SAMPl!

COtfTRACTKIN I INSERT PARTCULAT[

AlTERNATIVECOMPRESSION YOI.MINC - REDUCED CO£FFUHT or

THERMAL EXPANSOf RESWCASTA&[ YOUlDS - LOWER RESIN COH1RACTOI

The gap that occurs between sample andmould will always be a problem withresins that exhibit a high degree ofcontraction. This contraction willinevitably, be higher on the inside of awasher than the outside. With hotcompression mounting it is the differencein coefficient of thenna! expansionbetween moulding material and samplethat creates the greater problem.With hot compression moulding it is theinside of the washer only where a fissureis likely to occur. If, as often is the case,a gap is noticed on the outside of acompression moWlt it signifies an uncuredmould.To reduce the area of resin in closeproximity to the sample will reduce theeffect of contraction and coefficientdifferentials. To do this an insen can beintroduced as shown or alternatively useparticulates of ceramic. Anotheralternative would be to section the washergiving a 'C' shaped subjectThe use of epoxy castable resins willovercome most problems as can be seenfrom the 'encaosulation ootions e:uide'.

FAULT: MOULD CRACKlN6

COMPRESSION MOUNrJNGCAST ABLE MOln..DS

~:., Q<~SAlm.E~~I-~

I"" (0'-"1"-

REMOVE SHARPOORNERS

--01..,- i...,~

r INCREASE MOULD$1m

When two different materials 'set' underpressure such as occurs in hotcompression mounting there exists atambient temperawre, localised stresspoints within the mould. This situation ~aggravated when dramatic volwne ratiochanges take place such as occurs at sharpcomers of samples as illustrated. Thisresidual stress can be so great itpropagates cracks within the resin.Removing sharp comers or increasing themould size will usually overcome suchproblems.

;

,...ALTERNAnVE

LOWER RESIN CONTRACI10N

3.7Copyright 1994 B UEln..ER Ltd

@A MET ALLOORAPHY EUROPE

FAULT: MOUNT UNCUREDThennosetting moulding material canoften look quite good from the outside yetbe partiBlly uncured on the inside. Thissiblation can be confumed by crosssectioning the moulded unit Specimensexhibiting what looks like contractionbetween resin and sample outside face aremore often than not examples of partiallyuncured moulds. When d1is internallyuncured condition increases, the specimenwhen ejected from the mould unit, willhave either a burst on the opposite face tothe sample or alternatively the sampleitself will stand proud of the mount Thiscondition is more prevalent whenthermosetting resins are ejected hot ~can be seen from the sketch it will benecessary to vary pressure, temperatt1re orcure period as appropriate

CO~ION MOUN11NO . THERM~ RE.sIN

.,., ~ ,~ :-CRAHU~T£O I

I ~1\M( .~""...:.:::-:...,~ . f~

~~I~~

t'_--'. .

~ ~,,--:0-

~, IaMEPERIOO

ALTERNAT1VE

USE THERMOPLASTIC RESIN

FAULT: MOURTURCUREDCOMPRESSION MOUNTINGTHERMOPLASTIC RESIN

Clear thermoplastic compression moul~often exhibit what is called a cotton balleffect, this can be in evidence on ejectionfrom the mould or can occur as the mouldcools to ambient temperature.There are two types of cotton ball effectsand they must be addressed differently ifthe problem is to be overcome. Firstlythere is the misty effect just below thesample, this can usually be overcome byreducing the rate of cooling, prolongingthe period. When the misty or cloud effectis replaced with a distinct combination ofsmaIl bubbles then reducing the heatingperiod should suffice. There is a word ofwarning however and that is, both ofthese conditions are temperature/pressurerelated. i.e.,. lower the temperature-increase the pressure. Having establisheda good working combination it will benecessary to retain a similar resin tosample ratio to avoid cloud fe-emergence.

0REDUCE

~~l~"~5~

~

l- - Of

...\

-~lOUO [MISTY]

~

r SAMPLE

/--~~--

Go

--~ CLOUD [SMALL BUBBLES)

AlT£I~NA11V(

USE LESS RESIN

3.8Copyright 1994 BUEHLER Ltd

REDUC( IHEATING PERIOD

MET Au.06RAPH-{ EUROPE

FAULT: SAllPLEDISTOR110N

COMPRESSION MOUNTINGTHERMOSETTING RESINDelicate samp~ i~y should not be

subjected to compression mounting, thishowever is sometimes unavoidable. Someusers surround the delicate componentwith a metal ring thus confining pressureto the longimdinal axis. If isostaticpressure is to be used then thermoplasticresins are best employed; the mountingpress pre-load being used until the resinhas softened. Since thermoplastic resinscan be moulded at much lower pressurethan the thennosetting resins it is possibleto achieve satisfactory moulds at muchlower pressure than recommended by theequipment supplier. It is important toremember that these moulding resins arepressure I temperature related and as suchany lowering of pressure must beaccompanied with a rise in mouldingtemperature.

DRY OUTMOULDINGPOWDER

SAMPLE

\... "

~4 CIRCUMFERENTIAl

SPLIT

FAULT: MOUNT ADHESION

C~SSION MOUNTING

THERMOSffilNG RESINOne of the most desirable features ofspecimen moWlting is to have the moWltmaterial stick to the sample. Thiscondition is prevalent with epoxy typeresins and all other resins to a lesserdegree. Having established this verydesirable feature it is not surprising to findthe mount sticks to the metal walls of themoulding cylinder. Lubrican~ areavailable to avoid cylinder wall stickingand should be used at intervaJs compatiblewith the type of moulding resin in use.Another type of sticking occurs mcompression moWlting and that is whenthe moulding resin is squeezed betweenthe ram and cylinder walls. to rectify thiscondition it will be necessary to replacethe worn out ram

FAULT SAMPLE DISTORTION

- SAMPLEr DISTORTION,

:-i CHANCE TO

,...:~~~'" THERMOPlASTIC"-=-:'""~---" REDUCE PRESSURE

INCREASE TEMPERATlIRf

"'--~~

ALTERNATIVE !!$E CAS1A8LE RESIN

3.9Copyright 1994 BUEIn..ERLtd

J"" MET ALL06RAPH"i EUROPE

FAULT: MOULD ADHESION

SPECMN AOttCSOCLubricants are available to avoid cylinderwall sticking and should be used atintervw compatible with the type ofmoulding resin in use.

I.~

-~ SPEa..:Nl

--~j~a.

~

FAULT: POOR DIM£NSIONALST AS nnY

CASTABLE RESINS

~~-~

':'-~t REs..l_J~ ,-

PHENOLICRINGFORM

SAMPLE

Most castable ~ exhibit a degree ofcontraction. d1is contraction leads to anirregular shaped mould dipping in thecentre and not too parallel on the outerdiameter. In addition to this poor shapethere is the Jack of dimensional stabilityattributed to varying contraction betweendifferent resins. This problem can beovercome by using an epoxy type ~(low contraction, low viscosity) or.alternatively mould the sample within a'ring form' as shown in the sketch.m 6H EX OTH£RIIAll castable ~ have a 'peattemperature', this is the minimmnexotherm temperature needed to 'set' theresin. (See encapsulating resin guide).This exotherm temperature is ~lfgenerating and will if not controlled. farexceed the peak temperamre. To controlthe generated heat (1) reduce the volumeof resin (2) increase the sample size (3)blow the exotherm peat temperatureacross the setting resin surface.

FAULT HIGH (XOTHERM

/ 1» C JO'c

7<

8 REDUCE -

At. TERNA flV[

BLOW JOoC AIR ACROSSSPECIMEN SURrACE

3.10Copyright 1994 BUEIn.ER Ltd

11>4 MET ALL06RAPm' EUROPE

FAULT: EXPENSIVE RESINAssociated with improved resincharacteristics is usually a price penaltyLe. better resins are usually . moreexpensive. To compensate for this priceincrease it is possible to support the frontface of expensive resin with aninexpensive resin such as standardphenolic as backing material.

COMPRESSION MOUNTING

COST SAVING

INEXPENSIVERESIN

SAMPLEEXPENSIVE RESIN

FAULT: POOR £L£CTRICAL CONTACT

Good electrical contact between sampleand anode, for example in electrolyticpolishing/etching, is often required. Theapproach can be one of two ways (a) drilla hole through the resin until contact ~made with the specimen after which anelectrical contact can be made or (b)mount the sample in a conducting resin.Conducting resim can be metal filled asused in electropolishing or graphite filledfor scanning electron microscopy.

CONDUCTIVE MOUNTSr METAL CONTACT

OR

METAL OR GRAPHITEINCLUSION

SAMPLE

NON CONDUCTING RESIN

3.11Copyright 1994 BUEm..ER Ltd

E-1Jc.A~ULATIO).J OPT'O~ gUIDE-(!)

BUEHLE~COMPee-SSlO).1 Mo~L. 'D ItJG -

HyEEt'TIVf I

C...Tu.

D£.5t:2rPT10,I AMASlO'.'I POL ISHiUG 'I'

~ACTO2 RATE- CLEAVAGe-26"" /STEEL.

\JSe. ""$(OS,"y ~~~ ~MD

-~A;~:

PUt:T~.,

0. !L4C~ P~'Q.IC\l

72D- 3100 5f.O 4'-

,~~:6f&.'f'r.4LP\I£~

"7 /50""J

4(.

--'~~~GE~ml.PC~~~

7 150

~

-W-3~ ~c-o

--"~~a!~~L~E;CQ.UU~

'7 150

"ZO-mo 4-40DIAu,yL.PJfTHA~

~o ~ ~ .., 150

"::li130ZO-33CO "71 \. 'fe£ FAce

Ao~esIOU ~ , "0

~

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to-noo -&0 PllEUQ.IC 5(.0 0 ~ Gt~'ffAL , ..'PI«~ ~:c.-.& ~,

~UQI 'I8IOL~ , (,0 0 ~ ~ --~ ~CQ.'-M~ ~. ~.

~ !

DlALL'fL. ',...0 ~o rMfMICAL --:-y~

.PtfTHALATJC ~15T ' ~-10-3380 -aL~~ "0 ~"71 \"'Sf'ACf ~

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@4 MET ALLOORAPH'i EUROPE

CHAPTER 4- SIN6L£ POINT TOOlS (STRESS APPUCA TORS)Removal of material by multiple single point tools seems astrange subject to be included in a course ofmetallography. In fact a knowledge of how the materialremoval occurs and the subsequent damage resulting fromthis 'woIting' is vital to our knowledge if progressing thesurface to integrity is to be achieved Materials fall intoone of two groups i.e., (1) Ductile materials wherematerial is removed by 'slip plane dislocation', suchmaterials would be steel, brass, aluminimn etc. (2) Brittlematerials where material is removed by brittle fracmrem~hani.an; such materials would be ceramics, bricks.minerals etc.

RES\A. T:S1t[l \ ~~ RES\A. T :CERAYIC\-rRAl

OEF~M4TION ANaR[5C)UAI. STRESS

RtsIOUAl C~AC..-S

Figure 4.1 - Material Removal Phenomina

These two groups of material removal are shown in figure4.1. Dislocation by slip plane mechanism is shown to nmalong the shear stress direction, this results in a defo1'JDedstructure and a potential residual stress. When material ~removed by crack propogation the result is a residualcracked StI'tlcture with little if any residual stress. The ideain materialography is to progress this residual damage to atheoretical zero by use of smaller sized abrasives anddifferent support surfaces to absorb the cutting shock.From figure 4.1 it should be noticed how the cutting toolfront face has a different angle (rake angle) for the twomaterials. Ductile materials require a positive rake angle(see figure 4.2), brittle fracture materials require a zero tonegative rake angle.


.A MET ALLOORAPH"l EUROPE

~ t:~] a 0Positive lowdeformation Negative high

deformation

Figure 4.2 Optimum Rake Angles

This rake angle varies for different mate~ softer ductilematerials need a higher positive angle, harder materials alower positive angle. The resultant defomation can berelated to the rake cutting angle. Le. a low positive rakeangle will induce greater specimen residual damage thanwould the optimum larger rake angle. This cutting actionis an example of single stress applicators, in the field ofmaterialography we are more involved with 'multiple stressapplicators'. One such example is silicon carbide paper,this is shown in figure 4.3. The rake angle of the cuttingabrasive bas what is called a 'critical angle'. Less than thecritical angle will cut the workpiece (sample), greater thanthis 'ploughs' and eventually rubs (to be explained later).Silicon carbide particles, as they are electrostaticallyexcited on the adhesive backing paper are randomlyorientated The result being a combination of differentrake angles presented to the approaching workpiece.


J~ MET ALL06RAPH'i EUROPE

Steel

[:::j ~

l~ ~=:JCritical angle 900

< Cuts> ploughs/rubs

Steel

C:1

Figure 4.3 - Silicon Carbide Paper when new

!~ ~~~/Positives become negatives

Figure 4.4 - Silicon Carbide Paper In Use

Figure 4.4 shows the effect of these SiC particles as theyare used. The ideal positive angles soon become negativeby breaking or shearing at an angle nonnal to the front faceshear direction.

Cut Ploughed

Fig;ure 4.5 - Scratch Patterns


~

~4 MET ALL06RAPH"l EUROPE

Figure 4.5 shows the effect of using the various angledabrasives. Sharp positive angle abrasives give a clean cut.sharp negative rake angle stan to plough, causing a break-up from the clean cut: This u often accompanied by amaterial build-up each side of the groove as shown.Negative rake angles when used with ductile mataeriabwill soon ~ome blunt, from which point there is a greatertendency to rub than there u to cul The visual effect ofthis last condition is to shine or b~ the surface giving abrighter appearance which to the untrained eye could beinterpreted as a better finish. Looking at the type ofscratch pattern is essential in u-~-~~g the residualdamage. Just as with the surface finim after sectioning thecondition should be a unifoI1D series of clean scratches. Asthe scratches darken so the cutting is less efficient. beyondd1is stage as the surface shines then residual damage(deformation) can be gross. When assessing a scratchpattern following a grinding process where the workpiecehas been orientated then the scratches should have a depthLe. not all on the same plane, some going underneathothers and at a different orientation.

Figure 4.6 - Degrading Abrasive and Deformation Chart

Figure 4.6 bas combined the relationship between thecutting abrasive angle in use and the resulting defonnation.The major point being, when using degrading abrasivessuch as silicon carbide paper they should only be usedwithin the time o~ (T1) period.


@4 MET ALLOORAPH"{ EUROPE

Figure 4.7 - Factors Affecting Material Removal Residual

Damage

There is an important material removal! resultant damagereJationship that occurs relative to a given factor. A list offactors affecting final results have been shown in figure 4.7

COMPRESSED CRYSTAL. J~~

SLIP P! .\.t'Jf DISLOCATIONS

J,.\ MET ALLOGRAPH'l EUROPE

DEPTH OF DAMAGE

If we are to progress the preparation sequence towardssample integrity it is important to have an appreciation ofthe total defonned layer (or heat affected zone) resultingfrom a particular sectioning I grinding or lapping process.

) RtSGuAl STRESS HARD METAL 2~~

son METAL 50.

-- - BRmL£ ~TU~£ lOOK

~ ~To.

I£rAl SPECMH

~RY--

.,:------~-~~~ -, TOTAl OEF'ORMATIONSl.-tPI,AWS ..:::/

(PLASTIC DlSLO~~,:E ... F"RAGWENT£O L.AY[~

~:I;;;;~~~::::.1 SIZ£ CO4T

~~~i~~~~~~~~~ ~ Al~

PAP[R 8.c.c~ ... ~ COAT

Figure 4.8 - Plastic Grinding Using Abrasive Particles

Figure 4.8 depicts the Stn1cnJral artifacts resulting from theuse of SiC or alumina grinding papers. These papers areproduced with variable 'weights', heavy papers (thick)offering more shock absorbing Oess defonnation).Abrasive grains adhere to the paper by the 'bond coat' asshown. The working severity of the grinding paper ~inf1~nced by the 'size coat'. The thin size coat beingintially more aggressive, thicker size coats low in materialremoval offering a longer active working life.

The metal specimen shown m figure 4.8 shows two areasof slip plane dislocation caused by the abrasive grindingprocess, note the strain boundary and the visible~fonnation depth. The visible ~fonnation is that whichwould be optically revealed after cross sectioning theground surface and etching. This etched image would not~.re-.ssa.-rily reveal these slip planes but will reveal adistorted structure m comparison with the parent or truestructure. Additionally to the visible defonnation is the topsurface fragmented layer. When using sharp, optimum tocritical angle abrasives this layer will be extremely small(2JJ1nto nil). As d1ese grains chip and become truncated sothe fragmented layer will mcrease as will the visible~fonnation.

Copyright 1994 BUEHLER LId 4.6

PREFERENTIAL ET.CH ATSLIP PLANE/STRAINBOUNDARY~""

PLASTIC DISLOCATIONSFROM GRINDING

,'-

METAl SPECIMEN)

PLASTIC LAYERASSOCIATED WITHPOLISHING CLOTH

ABRASIVE PARTICLE_S/~-

/\ I DIRECTION4

-""

--"--.c--

\.:.-'""'\

'.:z:~~.- .

CLOTH BACKING

ABRASIVE RESTRICTING SURFACE

CLOTH NAP

.' . -. .. ,: -. '" ",.. '.. " . ; '~"";;.": .A"-':'";-".'..~;;;,I;o""~_I~""",'::.'~'. ...';.:.;~ j..-':.'~':"-:"-~';',,:,:~~"l'~-::';","~'.'.

,I.. c', .. '" ':',...'.~~:.r-,:,:-':

. " ,:.

.' ':' .~:;

:ij:~ .:.'.

AFTERSAMPLE INTEGRITY STAGE ~m

POLISHING STAGEIMPRESSED GRIT /SWARF n

~FINGER, IWATER JET

...~~~~~~

~:::;;;~. W ATE R J ET

SOAP,

- ~~ T~~~N~C -{)-CLEAN ~@

t~ EJC1.H~ ~ (if req u ired)

.

~~::~~~:::J::~=:::::~:~.HOT AIR

"""'",~ ALCOHOL

',:

, -~

~

I'

18OPTICAL

-~ ANAL YSIS @)

SAMPLE

ENCAPSULATION RESIN

FIG CLEANING

iliA MET Au.06RAPH'l EUROPE

During the material removal process it is Hkely that slipplane action has resulted in residual Sttess being presentbeyond the visible deformation layer. In considering apotential total ~formation layer the depth of residualStreSS has to be taken into account. Figure 4.8 indicatesthe additional increments necessary to take this factor intoaccount.

Although this discussion bas been confiDed to plasticgrinding where material removal is by slip planedislocation. a dmilar adjustment has to be made whenmaterial is removed by brittle fracture m~hani~(ceramics. bricks. rocks. minerals etc). From the chart thisis shown to be 100% of the visible depth. not because ofresidual stress but ~ause fine cracks are areas of furthercrack propagation.

Copyright 1994 Bum n..ER Ltd

@A MET AlL06RAPm' EUROPE

CHAM"£R 5CONV£lmONS

SURFACE PR£PARA TlOR 1'0 1RT£6Rm'In the surface preparation of materials for microS1rUcUalanalysis words such as grinding. lapping and polishing areused. To avoid any confusion the following conventionsand definitions are used throughout the book. (theseconventions have been well published and established)

Sample

':!:~::::~-t:: Deformation

Fixed abrasive

Figure 5.1 - Grinding

This is the most aggressive material removal techniq~ weshall use. Although it is efficient at removing material itdoes, as shown, manifest a relatively high level ofdeformation with a ductile material. This is due to theresultant shear forces (RJ acting on the sample. To reducethis deformation with a fixed grinding abrasive it will be~-~~ to progressively reduce the abrasive size and/orincrease the shock absorbing characteristics of the grindingsurface. Grinding is a quick operation.

~SampleImpre.ssed L("""",.~ ~abrasve I "" I II / / /, Deformation

~ Compressed grains

00 8~~~~o 000Rolling abrasive

Figure 5.2 . Lapping

..Do ,."p sorT

&.,.~

,..,~TE~'~ J

A.Wt1,"~J1 M.M,C)

J~ MET AUaOORAPHi EUROPE

'Ibis techniq~ ,-mli5es a flee rolling abrasive and producesa much reduced deformation layer in comparison withgrinding. 'Ibis ~ in part due to the changed resultant force(R,) and the shock: absorbing action of the non-fixedabrasive. 'Ibis technique ~ highly favoured for its extremeplanarity and ~ relevant as a possible techniq~ for anbrittJe fracnJre materiah (not d~). If ~ techniquewas to be used for soft metah then impressed grindingabrasives would occur as shown. Lapping ~ a slowoperation.

F

Polishing

(note the deformationlplucked grains, smear and cloth nap)

FIgure 5.3 - Polishing

Po~g is the step used when scratches are to beremoved from the sample often resulting in a '~'.Defonnatlon is relatively low as shown (should be nil) butthere is always the chance of smearing the sample surfaceif prolonged times or increased pressure is applied

Figure 5.4 - Composite Surfaces

Grinding is quick, not so flat and leaves a relatively high

Sample,...~"'4@\'""'.::.:"

i

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It bas been shown. using the charts, bow operatingparameters can be used to optimise a given preparationprocedure. It is aha shown in the 'options' cban forgrinding and polishing the grinding surfaces available andthe severity rating in relation to each other. What ~~g from all this infonnation is a guide to theappropriate grinding/polishing surfaces relative to specificmaterials.

SAMPLEPR£PARATIONSPECIMENMATERIALSPECIFIC 5.86 TO5.42

The following chIns are intended to guide the user intousing the correct grindin1/polishing surfaces that arespecimen material specific. All too often a grindingswface is selected 'because it is available' and nowhere isit stated in literature where these surfaces must not beused. Take for example the nickel coated diamond discs(ULTRA-PREP), they are not intended as replacementsfor silicon carbide paper, nor are they intended for use withvery hard materials. These discs will 'clog' with swart' ifused with soft tough materials rendering them uselesswithin minutes. These charts therefore will indicate wherebest to be used and by definition should NOT re used withother specimen materials.

The material groups have been generalised and willinevitably be incomplete, they will however cover themajority of materiah likely to be encountered

The Material groups are:-. Very bard materiaJs. Brittle fracture materials. Soft d~e materials. Ductile materials - general. Metal matrix composites. GlassK::eramic matrix composites. Polymer matrix com~tes

Figure 5.36: This chart relates to the use of fixed diamondgrinding surfaces. ~ is the area where incorrect use ofgrinding surfaces is most prevalent Very hard materia1sobviously require a hard abrasive such as diamond in orderto abrade the specimen. This diamond must re wellprotected against impact damage otherwise the diamondwill become dislodged from its housing. It is for thisreason the resin matrix grinding wheel is used. To use thenickel plated wheel for these hard materials would render amuch reduced life due to impact damage (see figure5.36A).

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As the brittle fracwre materials become softer so the use ofnickel bonded diamond grinding becomes ~ (A)The diamond is deposited onto circular Mesa shaped formsadhering via the nickel to a resin fibre support. Theprotruding diamonds are also nickel plated to initiateefficient cutting. There are a series of these surfacesallowing progression to the second integrity stage. Whenobserving the scratch pattern from nickel bonded diamondgrinding a brittle fracture pattern should be observed. Itwill be noticed from figure 5.36 how the Ultra-Prep disc ~iJx;luOOd for planar grinding of MMC ma1erials; ~ ~only possible when the ceramic particulate has a highconcentration. The swart' from cutting the soft metalmatrix will want to weld to the diamond cutting face, it ~the ceramic particulate that will act like a dressing stick asit is being cut and therefore stop the diamond frombecoming clogged.

This grinding surface can also be used for the planargrinding of soft materials w~ alternative methods such assilicon carbide or allDDiDium oxide papers manifestimpressed abrasives. When it is used for soft materials thegrinding surface should be waxed or oiled prior to use andcould additionally require regular dressing to keep thediamond faces clean.

Metal mesh discs (UL'I'RA-PLAN) (C) are very muchconfiDed to the brittle fracture materia]s, in comparisonwith the two previous discs, they induce ~ damage butare slower at removing material. ~~~u..~ the diamond is'charged' onto the steel mesh, and requires the action ofthe ~imen passing over to lodge the diamond into themesh surface, ductile materials would not be a candidatefor this grinding surface. This is confinned by referenceonce again to figure 5.36.

Metiap Platens (D) which are composites of metaJ/resin orceramic/resin require a charged abrasive and as such haverestricted use when material ductility ~ high. They inducea restricted level of defonnation or structural damage andleaves the specimen in an extremely flat condition. Thereare a series of platens offering increased shock absorbingcharacteristics for systematic progression towards sampleintegrity. Note how the use of these have been restrictedto the harder ductile and brittle fracture materials thusavoiding any impressed diamond abrasives from softme~

J~ MET ALLOORAPH"l EUROPE

ULTRA-PREP discs (E) have a very important role to playin the brittle fracture series of materials. note bow they areused in the final integrity stages ie. 2 and 3. The diamondabrasive is em~M~-rl into dome shaped resin spots on aresin fibre support. When observing the scratch pattern itwill be noticed bow the previous stage fracture mec~has changed to what looks like a polished appearance.1bis is due to the material removal med1aDism changing toa type of slip plane dislocation from the previous brittlefracture. This condition is in part shock absorbingdependent as well as particle siu - ie.

20pm nickel plated mIra-Prep disc would inducebrittle fracnue - material removal by crack

propagation.

.

30J1Jn resin bond Ultra-Prep disc would induce apolished appearance - material removal by slip planedis1 <x:ati on.

.

Figure 5.37 - This group of grinding surfaces (H & 1) are

those which due to degradation have to be replaced, as dogrinding papers, or have to be dressed, as do grindingwhee~, in order to present fresb/sharp grains to thematerial to be ground. Grinding whee~ tend to be thevitrified type as opposed to remn bonded. Alumina for theferrous materials and micon carbide for the non ferrous.Grinding stones or ~~ are only to be used where largeamounts of stock are to be removal or alternatively a quickplanar grind is required on a large area. These wheels arenot intended for soft materials. Zirconia alumina papers(K) would be used to planar grind these materials if thegrinding wheel should not be available. The majorityapplication in this group is for silicon carbide papers (L)they have been the backbo~ of all grinding surfaces in thefield of metallography and will continue to have animportant role to play, in particular with routine materials.Wid! polymer materiah and fibre circuit boards forexample it is possible to reach integrity group 2 usingP4000 silicon carbide papers. As a general mle the softerthe material the coarser the final silicon carbide gradeshould be. This is reinforced in figure 5.37 where generalductile materials ~ SiC papers to integrity group 1 andsoft ductile materials to the planar grinding stage only.

«>4 MET ALL06RAPH"i EUROPE

Figure 5.38 - Grinding cloths are those fabric surfacescharged with a diamond ~ve and suitable lubricant.used in ~ grinding mode in the sample integrity stage. Inthe past these cloths have reen called .polishing' clothssince no distinction was ~ between grinding andpo~g. In order to carry out efficient grinding mesesurfaces n~d to have a reduced surface tension and this ~achieved by the weave of the cloth or the porosity within aresin ingressed textile material. The diameter of the weaveand subsequent mesh (denier by weight) dictates the idealgrinding abrasive~. This is reflected in me selection andsubsequent positioning within the chart Finally at the lastintegrity stage the choice is influenced by the specimenmaterial abrasion resistance, hard materials using Texmet2000 (N), soft materia]s using the acetate silk (Q).

Figure 5.39 - Polishing cloths are used as the word implies'to polish or shine the surface' in order to achieve a scratchfree reflective surface. Those ~en materials that aremost prone to scratches. that ~ soft metals, should bepolished with a non-saarcbing abrasive such as ColloidalSilica. The cloth equally should be soft and free fromabrasion such as Mastertex. Notice from figure 5.39 howtbi§ cloth ~ used for slWTy polishing (SP) and notdiamond

Specimens that smear or are 'gummy' will require theaddition of either an acid or an alkaline solution to causespecimen ~lution and in consequence allow theabrasive particles to polish in a defonnation free manner.To ~ end the COOmomet cloth for soft materials ~designated' AK' for attack polish. For general ductilematerials the Clemomet, when used, would be for slurrypolishing (SP).

Ceramic material, silicon wafer etc require a totallydifferent cloth hence the Polimet fcr sluny polishing.

When dealing with general ductile materiab the use ofdiamond polishing can abo be a choice, the Microcloth(high nap) being used for many ~~~c1I!:-~. This ~ of clothcould also be used on difficult MMC materiab where thematrix requires an additiooal polish. Some ceramiccomposites such as PODFA used in almninium castingcould require an attack or ~lution polish hence theCbemomet for AK.

J~ META LLO6RAPH'i EUROPErD lAM 0 lm &RIND IN 6

CLOTHS Through the advent of the B~bler DeW concept inspecimen preparation a greater emphasis has beenplaced in what is the specific fmK:tion ofgrinding/lapping and polishing surfaces, and whenbest to ~ them. Additionally to this ~ the need to~ the type of abrasive most suitable for suchsurfaces. Considering cloths for grinding tOOyshould be without any nap and have some means ofretaining the grinding abrasive. Le. considerationssuch as

.

.

.

.p

have a varying size mesh structure to suit largeand small sized abrasives .

Surface to be porousabrasives to lodge between fibresabrasives to be forced into polymer binder

Coths that are chemotexti1e usually have a gradedresin binder system based on hardness (usuallybetween 70° - 90° shore A). When the resin volumeis high it is re~.ssary to 'buff' the cloth surface inorder to retain the diamond abrasive and lubricatingmedium.

Examples of diamond IliDdin& cloths are:-Utra-padTexmetPeIforated TexmetNyloomtra-polAcetate Silk (qA~)Polimet

Although an explanation of which cloths to use forspecific ~ is given later in the text, it ~thought prudent at this stage to introduce thesubject

Consider the grinding cloth selection at the finalintegrity stage for the following materia1s:-

. Very hard (Tungsten Carbide)

. Medium Hard (0.4% Carbon Steel)

. Very Soft (Aluminium)

. Brittle Fracture (Mineral)

J~ MET AIJ.O6RAPH"l EUROPE

All these four materials could be prepared using aTexmet cloth at the final integrity stage, the resultshowever would be compromised. i.e.

. Soft materials would be severelysaatcbed.

. Deformation would be higher as thematerial becomes softer.

. Surface robbing could cause phase pullout

. Integrity would be compromised.

Taking the soft materlaJs first we ~d a cloth that ~the leut abrasive (soft to the touch), in this case theacetate silk cloth would be suitable. This clothadditionally has a cross-weave pattern which helps inremoving surface tension during grinding. Thereduced smface tension by definition means lesssurface rubbing which is an added advantage whenpreparing friabJe ~ (graphite in iron).

The medium bard materialthe previous example only~ to be more resilientrequirement

Hard materials n~d a hard wearing Oat surfacecloth. Cloth abrasion is not important at this stage.High ~nsity laminAte cloths that are fused togetherunder heat and pressure could be ~d. The mostpopular cloth throughout the world is thecbemotextile 'Texmet', this cloth is not quite sodense. Texmet made from absorbent fibres in a ~binOOr exhibits porosity which is useful for trappinggrinding abrasive and lubricant

Finally, what is required for brittJe fracture mateJialssuch as mineraJs? Soft cloths are unsuitable becauseof interphase relief grinding and ~ffici~t materialremoval rates. The Texmet cloth, althoughmarginally suitable, does suffer from robbing andrelief. The answer therefore is to have a fine weavelow weight denim cloth such as natural silt i.e.mira-polThese four examples serve to illustrate theshortcomings in the traditional blinkered approachwith its 6JIID Texmet and 1 JIID Microcloth for anmaterials.I

;'

requires a mm,lar cloth tothis time the cross-weaveNylon cloths satisfy this

@A MET Au..O6RAPH'i EUROPE~

Since polishing is the step d1at takes place afterachieving sample integrity. its function must be toremove top surf~ scrau:bes. improve reflectivityand where ne~-~ry introduce die submicrometreinterphase relief n~-~ for differentialinterference contrast. The cloth will be soft (nonabrasive) and will exhibit a short nap. One exampleof diamond ~llihin& cloths is Microcloth

D lAM 0 NO PO 1JSHIN 6

CLOTHS

SLURR"'l POlJSHIN6 CLOTHS

J:

The requirement of a slurry polishing cloth isdifferent to d1at of a diamond polishing cloth in thatit d~ not have a high density vertical fibre nap.Slurry polishing cloths are very soft and ~ fibresare non veI1ical. A good example would be theSelvyt cloth ~d for polishing brass and copper.One important function is the retention of liquid andto this end some cloths encompass a foamsubsurface. AnotJr;r example of liquid absorbencywould be the felt type cloths. Absorbent fibres suchas cotton will be better for example than man madefibres.Examples of slurry ~~& cloths are:-

. Mastertex

. OM:momet

. PolimetSuitable slurries would be allImina I magnesia Icerimn oxide I colloidal silica.

SLURR~ &RINDIN& CLOTHSThe thought of using an abrasive grinding particle assmall as O.OSJJIn in a grinding mode at first seems astrange combination. however, if grinding is~~-~~ry to achieve integrity then such small sizeabrasives will be n~-~~ry. If for example thesma]1p-~t element or phase in the specimen to beprepared is say O.5JIID then a grinding abrasive much~a11P:T than O.5JJ1n will be n~-~~ry.The danger when selecting the sluny grinding clothwith such small abrasiv~ is to ensure the abrasivefollows the convention for grinding and notpolishing. i.e. the abrasive must not ~ and fall atthe point of cutting.Exampl~ of slUII'Y eliDdin& cloths are:-

. Texmet

. Utra-pol

VERY HARD MATERIALS

(tungsten carbide)

MBRnTLE FRACTURE MATER~S I(ceramics/ minerals/ refractaries)

00

SOFT DUCTILE MATERIALS(aluminium/tin/lead/ copper)

DUCTILE MATERIAlS GENERAL(ferrous/non-ferrous)

MMETAL MATRIX COMPOSITE (MMC)

(high particulate concentration)0 ;0

0

GLASS MATRIX COMPOSITE N

pCERAMIC MATRIX COMPOSITE

LEGEND(in order of severity)HIGH M=ULTRA-PAD hard woven cloth

: N=TEXMET 2000 chemotextile clothI O=NYLON soft woven ~Ioth. P=ULTRA-POL low denier silk cloth

lOW 0= RAM ocetate silk cloth

FIG 5.38 GRINDINGMATERIAL

CH,A.RGEDCLOTHS-SPECIFIC

DIAMOND ALUMINA

. ~:"J. ~..'i.",,-;';~~

VERY HARD MATERIAlS

(tungsten carbide)

POLIMET/SPBRITTLE FRACTURE MATERIALS

(ceramics/ minerals/ refractaries)ULTRA-POl/DP

CHEMOMET/SP

MASTERTEX/SPSOFT DUcnLE MATERIALS(aluminium/tin/lead/ copper)

CHEMOMET/AK

MICROCLOTH/DP/SPRAM/DPCHEMOMET/SP

DuCTILE MATERIALS GENERAl(ferrous/ non-ferrous)

MATRIX COMPOSITE (MMC)POLIMET/SP

(high particulate concentration)CHEUOMET /SP / At<

GLASS MATRIX COMPOSITE CHEMOMET/SP

CE?;A~IC MATRIX COMPOSITE MASTERTEX/SP

MASTERTEX/SPPOLYMER MATRIX COMPOSITE

CHEUOMET/SP

LEGENDDP-diamond polishingSP-slurry polishing ",

AK-attack polishing

CHEMOUET-hard materials-soft mate-";;:ls when using high ar low pH attack polishingMASTERTEX-saft materials-hard mat~~ials when interphase relief is required

CLOT~S, /MATERIAL5.39-POLISHING SPECIFIC

Cl\4 MET ALLOORA.PH"i EUROPE

Figure 5.40 - This cban gives an indication of the choice ofgrinding or polishing abrasive best suited to a particuJarmaterial group. The question of which type of abrasive,should it be diamond and if so what type? Should aluminaor colloidal silica be used? - how can the pH be changedand what effect will it have?

Firstly consider diamon~ usually offered in two distinctfonDS: monocrystalline (R) and Polycrystalline (S).Diamond can Wo be nabJral or manufactured.MonocrystaIIine as shown in figure 5.40 A have the blockysharp edged form where as polyaystalline are less blockyand are multi-faceted offering more cutting points.

""" 8dw- of monocrystanlne diamonds in META-D! DiamoM

SuSP8ftsi0n8 -- clean and effic~ cutting ection.

Particle size 45 micron. SEM - 450x., . .

Spherical 8h8ped polycry8tatline diamoftd8 ., ME:TAOI SUPREMEprovide n_roU8 cutti". laceta on the particle surface.P8rticle alze 45 micron. SEM - 450..

Figure 5 - 4OA - Diamond Abrasives

m on cx:;ry stallineJ pol ycrystaIlin e

f1)A MET Au.06RAPH\' EUROPE

deformation. lapping is slow. very fiat and bas a reducedsttucmral damage. Composite swfaces (resin matrix withmetal or ceramic panicles) ~n used in the grinding modeoffer extreme flatt!ess and reduced deformation whencompared to grinding and are quicker than lapping.

'+ Grindin&: The abrasive will be fixed (or stationary) at thepoint of cutting.Appearance: shiny and scratched

~ LaUl!in&: The abrasive will be moving (or rolling) at thepoint of cutting.Appearance: dull matt finJsh.

-+ Pol~hin2: The abrasive will be allowed to 'rise and fall'within, for example, the nap of the cloth at the point ofcutting.Appearance: shiny without scratches.

DEFINITIONS

faDA88ASI'¥'E

- . ~~~~~;~~6" ~r.. ~ ~ALMAnIX GaJlfRfGW88L ~~~ .. VUlEDA8aA1IVEMA T£RlAL REM 0" ALSURFACES

. ~ £ ~£-.

GRINDING..-c

~8DaP4CE(MEn.4P)

8Di!CMAnIs:t6S'rAJJCDAMIC PAaT1C1LA'RCBAaGED Asusm

. MEr AL OR GLA8L4PP1NG8[JU' ACE I:~-:d I .

~a.aIB- ~a~ ~ ':0 D AAA8ftGalNlHNG

~a.OrB 8(JRJ' A a

Figure S.S Types of material removal

Figure 5.5 gives 5 examples of different generic materialremoval surfaces.

Grindin&Wh~l

Grinding wheels cover the abrasive binder/matrix group ofsurfaces where it is necessary to wear down thebinder/matrix during use in order to expose fresh abrasives.Examples of this would be the resin matrix or vitrifiedgrinding stone which has a silicon carbide or alilminaabrasive. generally used for planar grinding of ferrous typemate~

S.3

MCSELftA'ra8NGi.BI.A.YDIOMm'IDA8DIrt

AMET Au-oeMPRY' EUROPE

What happens in practise therefore is the polycrystaJlineabrasive does more gentle cutting (Jess defonnation Istructural damage), the monocrystaIline being aggIessive.An additional commercial factor infltJelK:ing the choi~ ofdiamond is that polycrystalline is more expensive. From thechart, figure 5.40, it can be seen how mon<x:rystalline ~ hasbeen used at the planar grind for all materials, continuing toconclude d1e "sample integrity stage' for vel)' hard materialsand earlier as the specimen material becomes softer.

Alumina has been used in the preparation of fenousmaterials and others for many years, it is also an excellentabrasive for polishing many materiah. It can be made byadding water, oil or alcohol to graded alumina powder oralternatively to use the ready made slurry. Micropo1isb n adeagglomerated version is available, duplicating the ~al1P;r

particle sizes; the advantage being a more unifonn scratchpanern. Very simply die choice is simi1ar to that of diediamond in that there are two versions, the more aggressivehexagonal alpha alumina m and the more gentle cubicgamma alumina (U). It follows therefore that polishingwould be caIried out with the gamma and integrity stagewith the alpha, this is confirmed on chart figure 5.40.Alpha alumina's are shown as 5J1ID I IJ1ID I O.3~, theseare all high purity white alumina and can be levigatedpowder fonn or as a slurry. Coarser grades from 25~down to 3JJJn are also used for lapping applications, d1esealpha alumina's do not ~ to be die high purity whiteversion.

Additionally to this range of alumina's, which are ma(k; bycrushing from large particles, is the seeded gel almnina(Masterprep) which is grown. A conseq~ce of d1is is aaggregate particulate that is one tenth the size of anequivalent crushed alum;na Further to this the O.OSJ1lDMasterprep is the harder alpha phase and not the gamma asthe traditional O.OSJ1lD alum;na The net result is a non-agglomerated harder abrasive with a much reduced sizeaggregate that gives efficient material removal com~dwith a superior surface finish. Masterprep is also beneficialwhen automatically dispensing due to the lower sizeaggregate.

Many materials when pomhed have a tendency to smear theupper worked surface. Although almnina is a very efficientmaterial removal abrasive. it will when used with apolishing cloth exhibit this abemtion.

A MET ALLOEiRAPH}' £ OR OPE(.

Fine powder alumina is restricted to gamma O.OS~. When~g with materials that are in any way water soluble it ~pos,gble to make a sluny from polishing oil or alcohoLAltOOUgh all1mina does not benefit from dissolutionpolishing (high or low pH). It does have the addedadvantage of being cbemica1ly inert and when mixed inslurry fonn can be neutral. Le. will not attack inclusions etc.

t1\:

~

J~ MET Au.06RAPH"i EUROPE

COOVeI!ely to die situation above where a high pH couldcause 1D1welcome material surface dissolution there is an~ for a c~r.ai1y inert almnina suspension. ~ ~ theMicropolish C which is a I JIID IK:xagonal gamma alumina(U). Additional to this range of alwnina' s is Masterprep,d1is is an alumina made by a seeded gel process where thepanicles are grown as opposed to die nonnally 'crushed'and sized particle method. This process allows a non-agglomerated suspension to be used with automaticdispensing systems (Metlap 2000 dispenser), the particlesize confined to O.OSJ1ln. Note Masterprep is not shownon die chart, but would be in place of 'U' when

automatically dispensing.

Colloidal Silica is the most important polishing abrasive tobe added to the metallographers range of polishingabrasives. The particle size can be as low as O.OlfJln butfor general use O.OSJ1ID is recommended. The interestingfeamre of this abrasive is its shape, unlike diamond oralwnina which are multi-faceted colloidal silica is sphericalResulting from this shaped abrasive is a surface finish thatis devoid of scratches. ~ with the alum;",.. the pH of the81mI)' is extremely important, Mastennet (W) has a highpH of about 9 which helps when surface dissolution ~required. When compared with alumina as a polishing~nsion it is not so aggressive but gives an improvedscratch free finish. Colloidal silica can usually be dilutedwith water to a ratio of one to one without any loss ofperformance.

~~

When it ~ required to lower the pH an ammonia stab~dversion can be used which allows die addition of a diluteacid again causing dissolution of specifIC materials.

Supplementing this range of polishing abrasives ~Masterpolisb (Y), ~ combiIa the efficient materialremoval qualities of alumina with the ex~llent surface~ resulting from using colloidal silica. Figure 5.40depicts the material specific applications for thissuspension. Finally there is Masterpollib 2 (Z), again amixture of alumina and silica with an additional ingredientwhich creates a ~mical reaction with ceramic materialenabling more efficient cutting.

Figure 5.41 - Lapping is a gentle fonn of material removalthat is less aggressive dIaD grinding and usually gives anextremely planar surface.

VERY HARD MATERIAlS

(tungsten carbide) # # # #

.- - - -.BRITTLE FRACTURE MATERIALS'(ceramics/ minerals/ refractories) * * **

# # N #SOFT DUCTILE MATERIALS(aluminium/tin/lead/ copper)

DUCTILE MATERIAlS GENERAL(ferrous/non-ferrous)

METAL MATRIX COMPOSITE (MMC)

(high particulate concentration)

GLASS MATRIX COMPOSITE -. - - _.

CERAMIC MATRIX COMPOSITE # ## #

POLYMER MATRIX COMPOSITE

LEGEND- -SiC SLURRY* -ALUUINIUU OXIDE, -DIAMOND

FtG ABRASIVES 'MATERIAL SPECIFiC5.41-LAPPING

B DR

8 DR

£ NP $

s uw z

GLASS MATRIX COMPOSITE

CERAMIC MATRIX COMPOSITE ~ .# I - /Ii .. L

D ES (,.,Ar,

- '* ~ #

1. L N N,OPOLYMER MATRIX COMPOSITE

R R R T 0 S

LEGENDA-ULTRA-PREP DISC m~tol bonde-OIAMOND GRINDING W"'iEELC-UL TRA-Pl.AN metal meshD=BUEHL[R METL.AP PLATENSE-UL TRA-PREP DISC resin bondF'GH-VlTRIFlED ALUMINA WHEELIJ-VlTRIF"IED SiC WHEELI<-ZIRCONIA AlUMINA PAPERL-SiC PAPER- =SIUCON CARBIDE SLURRY

* =ALUUINIUU OXIDE SLURRY, -DIAMOND

M-ULTRA-PAD hard wov.n clothN-TEXMET 2000 chemotextile clothO-NYLON soft woven clothP-ULTRA-POl low denier silk clothO-RAM acetote silk clothR-D4AMOND monocrystolnS-DIAMOND polycrystallineT=MICROPOUSH A/hard hexagonal alpha aluminaU-MICROPOlISH B/less hard cubic gammo aluminaV-MICROPOUSH C/hard hexaQonal chemIcally inert aliminaW-MASTERMET colloidal silica high pHX-MASTERMET 2 colloidol silica low pHY=MASTERPOLISH colloidal silica/alumino suspensionZ-UASTERPOLISH 2 colloidal silica/alumino/iron a.,de solution

~fG 5.42-THE COMB~NED SPECIFtCCHART - MATERIAL

.A MET AIJ.06RAPH"! EUROPE

Its application is specific to brittle fracture materials as canbe observed on the chart. Also notice how lappingtechniques are not used under the polishing colwnn. Whenobserving a lapped finim it will have a matt appearancewith brittle fracture materials. From figure 5.41 observeno lapping is to be carried out on materials exhibiting anyductility.

Figure 5.42 - This chart is a compilation of all theinfonnation given in the previous chans. B~hler issuespecific DIALOG methods, these are methods thatconfonn to the concept of 'sample integrity' prior topolishing. The represent sample integrity in the shonesttime. The idea of the combined chart is to compile thisinfonnation as shown enabling method coDSttuction overthe whole range of materials.

GRINDINGPOUSHIN6OPTIONS GUIDE

CHART

This guide is intended to assist in ( I) compiling apreparation procedure from first principles or (2) to modifyan existing procedure given the severity rating wid1in eachgroup of grinding I polishing swfaces. It will berecognised from the previous infonnation. the importanceof selecting the correct size abrasive relative to theinduced residual damage. 'Ibis choice must be made inrelation to the severity of the chosen surface. From thechart it will be noticed bow ~ three important stages havebeen defined Le.

1) Planar grinding (where necessary)2) Sample Integrity3) Polishing (not always required with hard

materials)

A suggested abrasive size has been indicated for the 3stages i.e.

.

.Planar grinding - > 300m >P320Sample integrity - group 1 < 300m < P320

group 2 < 9um < P1200group 3 < 30m

Polishing - < lum.

When selecting grinding/polishing surfaces be aware oftheir severity rating from 5 to 1. '5' being the most severe,'I' being the least severe. Also notice how a grindingsurface in one group has a different severity rating whenpositioned in another group. viz:-

5.33

J~ MET ALLOGRAPH"i EUROPE

\.,

Texmet "9' in sample integrity stage. Group 1 - severity rating 1. Group 2 - severity rating 3. Group 3 - severity rating 4

So bow does the system work? From any given procedureit is possible to select upwards or downwards in severityfrom a given position. Take for example the followingcases :-

Fault 1 Sample shows grinding inteIphase relief

Remed~ 1 Use higher numbered surfaces in die sampleintegrity groups, if severity 2 tty 3 etc.

Fault 2 Sample giving high porosity levels. thoughtto be preparation induced

Remedx 2 Use lower numbered surfaces

Fault 3 Grinding scratches visible after the polishingstage

Remed~ 3 Use higrer nwnbered severity polishing cloth

~rationa1 Check mt for success

Optimise platen/specimen head rotational

speed.Select grinding surfaces relative to residualdamage after sectioningFrom the options in each group select the leastsevere (1) for perfection, the most severe (5)for speed and high material removal rates.NOTE it ~ not ~ssary to use a grindingsurface from all three groups at the sample

integrity stage.Use only one polishing step, select the m~tsevere (5) for planarity, the least severe (1) for

reflectivity.Use vibratory polisher for scratch free,deformation free or controlled differenbaIabrasion polishing (explained in the microscopysection).Use COn'eCt extender to reduce cost.

.

.

.

.

.

.

J~ MET ALL06RAPH'i EUROPE

. Use not only the correct sized abrasive but alsoits correct shape and type.Do not prolong polishing times wmeressarily( S minutes maxim om ) .

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Additionally, this stOne would be used for 'more d1annormal' material removal. When grinding hard materialssuch as tungsten carbide the grinding wb~l would be adiamond, me~ matrix. ~ wb~l would not besuitable for softer materials which would not wear downthe matrix, it would however be most suitable for anbrittle fracture materia1s

Grindine DiscsSingle laid abrasives are bonded to a rigid suppon, oftenmetal. When diamood or albic boron nitride they tend tobe nickel plated onto the surface, the backing when notmetal can be fibrous which improves the shock absorbingcharacteristics. To aid swart removal the abrasives tend tobe in uniform clusters. These discs could be used for anbrittle fracture materials but it must be remembered theabrasive standing proud of its support renders the disc lessrobust than the ~uiva1ent grinding wheel If for examplevery hard ceramia or tungsten carbide was to be groundon d1is surface its life expectancy would be grosslyreduced.

Com~site SurfaceThese engineered composites are made to satisfy thedeman~ of ~n flatness and sample integrity. Thecharged abrasive is intended to imp~ into the compositesurface matrix (epoxy resin) as the sample under pressurepasses over, thereby causing a aiDdin& effect The rate towhich the composite surface wears, is governed by theparticulate material and this must be matched to the samplematerial Le. steel samples would require iron particulates inthe composite surface. Brass samples would requirecopper particulates in the composite surface etc. Theseplaten surfaces have appJications with all brittle fracturematerial and ductile materials excludine soft metals Le.non-alloy, copper, tin. lead, aluminium etc.

La!J!'in& Surface

Lapping is slow and gentle, an abrasive slurry is 'dripped'onto the, often cast iron, platen. The specimen under a~ pressure is lapped as the platen slowly rotates. Thisoperation can be done by band as is the case with thegeologist who thins his sections down to 30JjJD using 600grit powder (silicon carbide or alumina) on a glass plate.Lapping is applicable to all brittJe fracture materials butmust not be used on ductile materials.

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Grindin&l.fg1ishin& Cloth Surfacr.

It may seem strange to call a material removal action on acloth a grinding stage, but from our 'defInitions' this iswhat happens when the cloth or fabric is ingressed with aresin. We therefore have cloth surfaces that can be usedfor grinding or polishing. With the high matrix ~surfaces this can readily be appreciated (PerforatedTexmet). With the resin partially ingressed cloth thesample pressure resists the tendency for the abrasive to'rise and fall' during cutting, ensuring a grinding action.With the softer nap type cloth and reduced pressure thenthe abrasive will 'rise and fall' during cutting giving apolishing action. These surfaces are employed with nearlyevery material and there is a great variety available specificto particular ~imen requirements. The' great variety'can by itself prove very confusing hence the reason forfurther explanations to follow.

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Figure 5.6 - Hand Preparation

Figure 5.6 gives the information necessary to cany out thepreparation of materials by band. On this occasion fourstrips of silicon carbide paper are used, the sample beingtraversed backward and forward. The specimen is rotatedthrough 900 as shown.

5.5

@A MET Au.06R.APH"i EUROPE

After P1200 grit the ~imen requires two polishingstages, one using 6J11n abrasive the other 1 JIm. Bothgrinding and polishing are .wet' operations, grinding withwater, polishing with an extender of lubricant. From figure5.6 notice how the polishing action takes the fonn of afigure eight. with practise this is madily ~hievabJe.

Without much skill. specimens are rarely flat from thistechnique

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Figure S.7 Rotating Platens

Three examples are shown in figme S.7, the first being asimple band tecbniq~ where the sample is ~1d against arotating platen having the appropriate grinding/polisbingsurfaces attached. In order to use all the platen surfaceand to avoid directional grinding/polishing, the specimenwill require moving, if possible in some son of rotation asshown. Specimens prepared this way can be much flatterthan by the stationary mode above but lack the control ofmachine automation as follows.In the next example, three specimens are clamped into aspecimen holder, the pressure being directed via a centralspigot as shown. For a more uniform fon=e the specimen ~driven in a complementary direction to the rotating platen.For unidirectional preparation the relative motions wouldbe contra. This system has many advantages. The onedraw back however is ~ ~ty for planar grinding.

/1)4 MET A.IJ.06RAPm' EUROPE

Planar grinding is ~~~-~~'Y beca~ the action of clampingthe specimens in the specimen bolder dictates a positionwhich rarely is planar betw~n all specimens in ~ bolder.

The last example is where the three samples are ~ tomove within given constraints and although driven asbefore have individual loads enabling each specimen to beplane with the rotating platen. Planar grinding not being~ allows the samples to be prepared on the exactsurface needed relative to specimen residual damage Le.after sectioning a component (sample) the residual damagewas so small (lOJ!ID) that one grinding cloth sUlface ~g3J!ID was all that was required.

SURF ACE TENSION

SURF AC[ TlNSION HIGH

CUTTING LOW

I .~D .8

""~;//':?//ISURrACE TENSION LOW

CUTTING HIGH

Figure 5.8 Surface Tension I Water

Surface tension caused by the specimens, platen surface orlubricating medium will adversely effect material removalfrom the workpiece (specimen). The sketch in figure S.8 hrepresentative of aquaplaning when flushing a grindingsurface with water during im operating cycle. Theoperator of any grinding equipment where water hemployed is asked to listen to the grinding 'sound'. ~ thewater flush increases so the grinding decreases. Undercertain ciICtlm.-~~ vibration will be created through dJissurface tension. this often is totally eliminated by reducingthe water volume. Soap solutions can break down surfacetension.

S.7

@4 MET ALL06RAPHi EUROPE

Composite platen surfaces can ~ be affected by surfacetension, this is why porosity is so vital in theresin/particulate combination. Surface tension willinaease as the specimen surface centre line average isreduced., i.e. as the specimen surface smoothes. To havean emul~~er in the lubricant can ~ help in reducingtension.

Grinding/Polisbing cloths are also affected by surfacetension. Development wort bas shown bow cross-weavecloths remove more material than a smooth cloth ~g thesame size abrasive.

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FIgure 5.9 - Grinding I Polishing Surface

Figure 5.9 shows a series of aggressive cloths. Thosecloths intended for grinding with 9 to 15J1In abrasive, havea course ~nier cross-weave pattern. These aggressivegrinding cloths are cla.c.~fied as integrity group I (to beexplained later).

As we move to a Jess aggressive surface (integrity group2) the denier becomes ~ to be compatible with thereduced abrasive size of 3 to 9}JIn.

The last grinding cloth in any preparation procedure will bean extremely fine denier weave or the resin ingressed typeas shown in figure 5.9.

I1JA MET ALLOORAPH'i EUROPE

Finally, the polishing clodl with i~ dense nap surface fordiamond polishing or the soft 'selvyt' type surface foroxideisilica polishing. The surface tension in this case ~more related to the specimen smoothness. If the specimenis polished beyond a particular smoothness, dlen rubbingrather d1an polishing occurs, resulting in a loss of sampleintegrity and with soft materials the 'orange ~r effect.To overcome the surface tensio~ emn1.~fiers could beused; the prefened route is to ~ a high or low pH oxideor silica polishing suspension.

THE CLASSICALAPPROACH European grit designated numbers are different to those

numbers quoted in the U.S.A. To avoid confusion ~using European sizes the letter 'P' ~ placed before gritnumbers. Comparison of grit numbersw and J11D size ~shown in figure S.lO

USA EuropeanGrit Averase me Averase si%.e

(.-In) (,.D\)Grit

4080

120.180240-320-400.

600-800

lCXX>1200

4281921161853362316129.26.5

412197127785846352S221815

P40P8O

P120-P180P2408P320P400-P6(X)-P8OO

P1(xx)P12(X)-P23OOP4(XX)6.5Figure 5.10 Grit Comparisons

Figure 5.10. the grit comparison chart. gives a comparisonbetween grit nmnbers along with the equivalent ~ size

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Figure 5.11 Traditional Preparation

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Figure 5.11 is a graphic display of the traditional approachto specimen preparation where the manifest residualdamage is removed by subsequent steps, progressing intothe sample. Notice the terms used at each stage Le. Roughgrinding, fine grinding, rough polishing and final polishing.The method of pro~ng the sample through the varioussteps (7) is to o~ the surface finish at each stage andensure scratches from the previous stage are not inevidence. At the completion of all the steps the samplewill be shiny and well polished.

THE NEW CONCEPTWith the knowledge that each ~tioning/grinding/lappingor polishing stage manifests residual damage to theworkpiece (sample) and that a particular combination ofabrasives and abrasive support would 11~, independentof time, leave at best a specific damage depth. This Z ~damage being the criteria to specimen preparation and notsurface ~ It wu also realised that the abrasivebacking or the ability to absorb shock during grindingwould also reduce Z axis damage. These thoughts lead tothe questions (a) What damage exism from the ~tioningstage? and (b) How can we progress this surface tointegrity without ~y creating deeper damage?

Figure 5.12 Traditional preparation within the NewConcept

Figure 5.12 graphically displays the philosophy where thesame number of steps (7) is still employed. but theundamaged portion from the sectioning stage remainsthroughout From this display the question then foremostin our min~ is 'Could we omit some of these steps?'.

«>4 MET Au.06RAPH'/ EUROPE~

Figure 5.13 - Selection

Figure 5.13 having addressed the question of abrasiveselection, bas concluded that three of the steps could beremoved without re~~rily oompromLgng the totalprocedme (sample integrity in the shonest time).

As bas been shown in the sectioning part of this manual,the residual damage can vary from IOJllD to 9O0JllD.Consider the material used in figure S.II/ S.12/ S.13 nowto be sectioned under optimised controlled conditionsgiving a Z axis damage depth. as shown in figure S.14

Figure 5.14 - Preparation Without Logic

To progress the sample through any of the previous stageswould be totally illogical. As shown from the sketch the240grit abrasive induces rather than reduces the Z ~dimension.

From this simplified explanation it can be seen how vitalthe sectioning stage is to any preparation procedure. Theknowledge of residual damage is the factor that dictateSwhat the proceeding steps will be.

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Figure S.lS - Preparation with Logic

figure 5.15 completes the procedure by baving a singlestep only. The assumption mack: in this discussion ~ theabsence of any planar grinding requirements.

Z AXIS CURVESWhen 'working' a material there will be a given depth ofresidual damage relative to a given grindingsurfaceJabrasive size and shock absorbing cutting action.'Ibis depth (z axis) will vary with different materials; anexampJe for 0.37 Steel is shown in figure 5.16

Figure 5.16 - Database ChaIt for 0.37comparison only)

Steel (for

Damage in micrometers is given for a whole series ofsilicon carbick: papers and diamond polishing clothcombinations. To select a procedure using silicon carbidepaper the choice would depend not on surface finish of thesample to be prepared, but the estimated or documentedresidual damage. On dJe polishing side it will be noticedhow the abrasive particle size is followed by 'PS. then anumber (9JJ1n PS2). PS is short for 'polishing surface. thenumber indicative of the polishing severity. 2 being low inpolishing (abrasive) - 8 being high in polishing (relief).

5.12

J..i MET ALLO&RAPH"/ EUROPE

WbtJ1 progJeSSing a preparauon procedure from a knowndamage depth. what are called Z axis curves can be createdas the residual damage decreases with 1ime. This ~displayed in figure 5.17

Figure 5.17 - Z Am Curve Sequence

Notice how some of the steps improve then degrade(1/213) this is ~~~~ silicon carbi~ in use develo~tnmcated particles which eventually induce damage backinto the sample as they remove material. Step No.5 is alsodegrading, this is the final polishing step which utilises anapped cloth in order to achieve a shine, resulting in a slowdegradation of integrity. Step No.4 is a charged diamondabrasive onto a ~abric surface which will continue to.aiDd without degrading from its 'best ~tion ' . This

graphic display bas been related to O.37%C steel and ~shown in figure 5.18

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asca

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Figure 5.18 - Preparation Method for 0.37 Steel

OIM: of the major advantages of depicting preparationmethods by graphic display is the technical be~fiu derivedfrom alternative methods, figure 5.19 is one such example

J~ MET ALL06R.APm' EUROPE

3 SiI~ C8tIide p.,efS1 PoIistW'D .

",--3 Platen ~surtaces \

~-, --- ~--:-'I1 . --ow ', .~ -- ~ r.uh'~--- ' O' Time - -.~ .

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Figure 5.19 - Integrity ChaIt for Material MMC

A four step (3 SiC. 1 Polish) traditional method ~compared with an alternative four step mediod utilisingcomposite platen surfaces and a grinding cloth. Thegrinding cloth ~ a 'polishing type' cloth used in a grindingmode. These two methods are based on the preparation ofa metal matrix composite. The important aspect of thisgraphic display is the information that can be derived fromit. Viz.

Dotted line method (according to the ~play). Exhibits greater integrity. Takes less time to prepare, and. Does not degrade with time

Continuous line method

Does not achieve the same degree of integrityTakes longer. andDe~ with time

.

.

.

When comparing the surface ~ at the 'best position'with both methods. they were considered to be identical.Figure 5.20 sums up the two approaches ie. surface finimV s residual damage.

A MET ALL06R.APB\' EUROPE: TRAomONAL NEW

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HIGHLY POUSHEDFigure 5.20 - Surface Finish or Integrity

In comparing the two approaches it will be noticed that achange of terminology bas taken place. The ~ of wordssuch as 'rough grinding' and 'rough polishing' aremimomers m the new terminology. Since all sampleswould be ground or Japped to integrity (i.e. we wouldne.Br polish a sample prior to achieving a faithfulstructure) this group of steps have been clA~fied as the'sample integrity stage'. The only polishing that takesplace therefore is after achieving sample integrity, this is a~ step and must be kept as shott as possible.

FIgure 5.21 The Concept in use.

@4 MET ALL06RA.Pn EUROPE

Figure 5.21 brings together all the be~fits of the ~wConcept in surface preparation including the terminologyof each stage and the appropriate abrasive for each stage.Notice how the sample integrity stage bas been split intothree groups as the sample progressed to integrity. Withinthese three groups the ab~ve size bas also been included.A severity rating, from 5 the greatest to 1 the least, basbeen introduced for all grinding/iapping/polisbing surfacesused. This rating will apply to planar grinding, the 3groups in the sample integrity and the polishing stage.(The sketch only shows the rating for the 3 groups in thesample integrity stage). The choi~ of surfaces and itseffect on progressing the sample to integrity will be~ussed later under 'preparation options'.

MA T£R1AL REMaV AL

MECHANISM The two modes of material removal are by slip planedislocation for ductile materlab as a result of the shearforce from the abrasive. The other mode of materialremoval is by brittle fracture which is the result ofcracking. These cracks emanating from the shear stresspoint having a major crack nmning along the shear axis,odlers of lesser severity at angles from the major axis.

Figure 5.22 - Deformation I Structural DamageRelationship for Brittle Fracture and Ductile Materials

If we observe the reduction in defonnation with the steelsamples as shown in figure 5.22, material removal is totallyby plastic dislocation. The upper part of this sketch showswhat happens as a ceramic material is progressed tointegrity. Firstly, material is removed by brittle fracture.Towards completion of the preparation however, notice

A MET ALL06RA.PH'i EUROPE

how ductile slip plane mechAni.clnc occur until slip planedisl<x:ation is the only mode of material removal. Thischange..over point is related to the fracttJre toughness andabrasive particle size and can be influenced by the shockabsorbing effect of the cutting abrasive support.

.-r

II'"

Figure 5.23 - Preparation Sequence for Ceramic Materials

Figure 5.23 is an example of a preparation procedure for aceramic material (reaction bonded silicon carbide) showingthe two types of material removal.

EFFECT OFLUBRI CART

We have mentioned the material removal differences thatoccur with changes in surface tension, different lubricantshave an effect on this. In materialography, three differentlubricant types are available. These are, in order ofefficiency: -

1. Alcohol2. Water,

and 3.0ilIt does not always follow however that this order will sti11apply when combining the variables of grinding/polishjngsurfaces with different materials. Oil for example would berequired for soft materim in order to lubricate the cuttingabrasive. The combination of material/grinding-polishingsurface must be carefully considered. The following fourexamples are used to illustrate this

Copyright 1994 BUEJn..ER Lid S.17

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Figure 5.24 - Effect of Lubricant I Identical Oath (Steel)

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Figure S.2S - Effect of Lubricant I Identical Ooth

(Ceramic)

A cbemotextile grinding cloth. dosed with a controlledamount of 6J1Jn diamond at set intervab when grindingsteel and ceramic specimem, was checked for materialremoval at intervaJs of 5 minutes. Figure 5.24 shows theresults from working steel - figure 5.25 the results fromworking reaction bonded silicon carbide. The first point ofinterest from these figures is the compatibility of the Waterbased diamond suspension with both the steel and ceramic.

Copyright 1994 BU~R Ltd

I

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the low figure and faIl off when using alcohol wassurprising. These results would abo show how thereramic is more prone to surface tension as the specimensurface 'SIDOOthes'. On the right hand side of both thesefigmes is the restan (RS) dimension. this is the ma1erialremoval figure after 5 minutes when the specimen has beenreUJrned to its original ground condition. This restanfigure is what would be expected in real life use. Fromthese ~ water is by far the most efficient with thiscombination. It does not follow however that these resultswould be identical if for example a nylon weave grindingcloth was to be used.

The 'smoother' and planar the grinding surface the greaterwill be surface tension, figure 5.26 and 5.27 are examplesof material removal still using the steel and ceramicsamples, only this time using composite platen surfaces(metal/resin).

10t 1

~ PLATmf ItJRPAa (~6)~ WA1DJAL o.4~ ITEm.ABRASIVB 9 pM aAA«*D

Figure 5.26 - Effect of Lubricant I Composite Surface(Steel)

Copyright 1994 Bt~R Ltd

J~ MET ALL06RA.PH"l EUROPE...I(~R(~...

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Figure 5.27 - Effect of Lubricant I Composite Surface

(Ceramic)

With both these figures there ~ a tendency to peak, afterwhich time material removal decreases. this is thought tobe caused by surface tension. Alcohol ~ better for bothmaterials; attributed to surface tension break-down, thereare problems however using alcohol (Surface pick-up andvapour). Taking the first 5 min~ which ~ a real-lifefigure then water ~ nearly as good for the ceramic. oilbeing slightly better for the steel

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Figure 5.28 - Use of Extender

Copyright 1994 BUEm..ER Ltd

eA MET Au.06RAPH'l EUROPE

When using automatic abl3Sive dosing devi~ it ~possible to monitor the dosing period and also theeconomic dosing amount. It does not follow that the moreabrasive dispensed onto the grinding/po1ishing swface thatan ~uiva1ent improvement in material removal will takeplace. Increased abrasive dosing can impede mate.ria1removal. In general there is a point where funher dosingof abrasive will give little if any benefit, this is shown infigure 5.28. From d1is economic dose the interval thenmust be de~.d. It was found empirically that a loss ofeffici~ was not encountered if the abrasive suspensionwas alternated with a compatible solution of lubricant only.thereby reducing consumable costs by half. Theexplanation offered for d1is siUJation is the extender will'clean' the abrasive surf~ extending its useful life.

DIFF£RENT CLOTHSThe experiments used to define lubricant efficiency wasalso used to illustrate how different cloths give totallydifferent results. Seven different grinding cloths. anrecommended for steel were tested for material removal~~. From figure 5.29 it can be shown how 3 at thebottom of the graph are so ;nPffi~P:Dt u to be consideredW1Suitable for this ~~lication. The efficiency of the otherfour varies from 20JJlnS to 28~s (figures taken fromrestart position).

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Figure 5.29 - Grinding using Different Cloths

Polishing cloth selection is covered in the grindingpolishing options guide.

Copyright 1994 B~ER Ltd

li>A MET AIJ.~RAPH'l EUROPE

HIGH um:6RIn'THIN S£C110NS6R1NDIN6ANDPOUSHIN6

Thin sections are now being made for much widerapplications than the traditional requirement of themineralogists Thin sections of bone, ceramics, teeth.JK}lymers, refactories, implant materia]s etc., have madeextta demands on the thin section - that of high integrity.Implicit in this ~rement is the suggestion that themineralogism' thin section was 1ackiI1~ in integrity i.e., itwas not a faithful representation of the true structure. Tosome extent this is true, the mineralogism' thin section washowever faithful in revealing the required information. It isour expectations of the d1in section that has changed andas such the mediad of preparing the thin section mustreflect this change in expectations.

The c~~cal approach to dUn sections would be to lap;ming silicon carbide or alurni~ powder, the materialsmface prior to sticking down on a glass microscope slide.The abrasive particle size would be to 600 grit prior tobonding the chip onto a glass slide. H the chip was to beobserved in reflected light, experience would tell us it hasnot been prepared to integrity.

I-j c:=J

)-

-~. il',.;--.,

SUPPLDefTARY

~-- J I DeKnm DIPI1I ~ naucn8AL ~ ~

Figure 5.30 - Thin Section Protocol

C~yrigbt 1994 B UEIn..ER LId

aD 1;'1 []I ~~~I~ PStJLA:1D

@A MET ALLOORAPH}' EUROPE

Figure 30 shows the two different approaches. where theprimary working with the c~cic;:-cIl approach means theresultant thin section will ~est d1is StructW"al damage.The target therefore is to achieve sample integrity with theprimary working in advance of any bonding operation.

The method of preparation of ~ first surface is identicalto that required for any other material. From ourgrinding/polishing options guide we must make a selection,notice how lapping has reen accommodated in ~ chart(PJanar grind severity 1 - sample integrity group 2 severity2/ group 2 severity rate 2). Should lapping be the chosenmethod it must be followed by a grinding surface in group3 of the 'sample integrity' stage, polishing would n,Q1 be~.&-~ry. The use of diamond grinding discs (Ultra-Prep)have proved most soc~-eS8-ful in dramatically reducingpreparation times.

0Ix:e the high integrity bas been bondf"..d to the glass slide(Epoxy resin 2SO Cps) care should be taken when thinning,not to Ie-introduce a damaged S1J11ctme.

--- "'R(SSuR£. ~ - , --.

I ,,

II,

,,..--

'" '.

,.;~::;,.-f~~ ~",,' '

!.O¥I!R BLOC" UhTtlI -"""'" ~L SP[CIMEr~ Ct"!PS

ARE LMl THE" CL"MP

~

I",," 'r;;~;;; ~ - ~~, ~TATION~~~ =:f ~ - ::::!---:--::~ ~ ~

1:\ , Q) .-~I ~-

-- -ClAWP SCR£W$

SPEC'MLH aLOC~ ,,~~

~- SPtC"'~N ~OCI(

I SPECIM£N CMPBONOtD WItH lAKES'O£ "D

Figure 5.31 Automating Chip Grinding

The medlod of preparation when lapping varies from thatof grinding, in that for grinding, the specimen requires tobe fixed This is done as shown in figure 5.30, where thespecimen chip of varying shape the thickness is temporarilystuck to a specimen block. The s~ens can then be1e\Ielled prior to clamping the blocks in position.

c ~ 1994 B UEIn.ER Lad

1114 MET ALLOORAPIf"{ EUROPE

1 Preparation and optical observation is canied out with thespecimens remaining in the specimen holder. WhensatisfactOrily prepared to integrity, the specimens can bereleased from the specimen block (by beating) prior tofixing ontO a glass slide.

CAPITULATION6RINDIN6&.POUSH IN 6

Figure 5.32 graphically shows the various stages ofpreparation starting with the sectioning and its induceddamage. This strucUJral damage being gradually reducedby a £riDdin& or Jaal!in& process called the 'sampleintegrity' stage, until a faithful Structure ~ achieved. Thisstructure is revealed after carrying out the polishing stageand ~ accomplished by one of the listed methods.

( The machine optimised operating conventions must beob~ed in achieving good results and these have beenlisted JJnder operating conventions figures 3 - 5.33 I 5.34/5.35 (at the end of chapter)

In consttUctlng a method the choice of abrasive function.be it fixed. loose, charged or OOgI'ading must first beaddressed, this is shown in figure 5.36.

OPERA.T~ TECHNIQUE:MATt:R1Al

LAPPING GR1NDING \ POUS"NG

METAL

"~ERAI-

,,

I-~ I

OPERAl1NG METHOO

~MATERIAl I LOOSE A8ASIVt ( ~ ) I CHARC£D. ASRAS~ (GR»!~) I. DE~ING ~ -(~ PAPER~

,-

,

,,

,""ERAI,.

\ HARD M£:r AL

I ~,:,M HARDMETAL

, I

.4 LARGE SIZE (3um) tMEDIUM sonMETAl..Y8Y -.r .Mm'ALa

, lARCE SIZE (w..)I NTIAU Y

Figure 5.36 - Method Construction

With all ~ infonnation and the "grinding/polishingoptions gui~' it should be possible to construct atechnically sound procedure for the preparation of all

@4 MET AU06RAPH"l EUROPE

materials. From ~ derived procedure it should aha bepossible to fine nme the procedure given the severityratings for the operating procedure. in order to ~hieveSAMPLE INIEGRcrY

SAMPLEPREP AM TIONUSING THECHARTS 5.32 TO

5.35

Figure 5.32 shows graphically the progression from thesectioned condition to completion when the microstIUcmreneeds to be observed. In many cases the microStIUCblre ~mble without recourse to etching techniq1ES as indicated.When this is not possible men some fonn of chemicaletching or staining is required.

Figure 5.33 'Operating conventions for planar grinding',this and subsequent cha1U can best be illustrated by using agiven method for example, in chapter 10 of this referencemanld1 Take 0.4% Carbon Steel MFJIAIM using a2OOmm (8") 0 macltJDe.

Planar grind 180g SiC: from figure 5.33 ~ giv~ ahead speed of l2Orpm and a wheel or platen speed of200 rpm operating in a complementary direction with aforce of 5 Ibs (22.N).

.

Sample integrity taken from figure 5.34 - Metlap 6with head speed 60 rpm and wheel speed of 150.Texmet 2000 with head speed 120 rpm and a ~lspeed of 300. Texmet 1000 also with head speed of120 rpm but wheel speed of 250.

Polishing taken from figure 5.35 - On this occasion nopolishing step is quoted. If ~~-~~. sluny po&bingwould have a head speed of 60 rpm with a wheel speedof 80. Diamond polishing would be at 120 rpm headspeed and a wheel s~d of 200.

.

Attention is drawn to the note where contra motion mrecommended in retaining slurry, this of course would notbe possible with softer materials. due to directionalpolishing effecu. Also note how the force is reduced forpolishing, the ~ty for this is greater with softmaterials and is often ignored as the sample hardneMincreases.

C~yrigbt 1994 BUEIn..PR LId

TURE BY: IHINGING

1 DIFFERENTIAL INTERFERENCE CONTRAST

INTERF'ERENCE COATING

ELECTOL YTIC ETCHINGCHEMICAl. STAINING

- - - -,.SECTIONING

STRUCTURALDAMAGE '.N

/I

GROUP , I GROUP 21GROUP 3

I " I I

I I I II PLANAR , SAMPL£ INTEGRITY STAGE I POUSHINGIT- GRIND 1- - ~ T ~;A~E -i

FIG 5.32- THE TRUE STRUCTURE-BY STAGES

",'

>-e~'

~BLE STRVCWRE- BY 1

'CHEMICAL ETCHING- ..

DWTERENT~ INTERFERENCE CONTRASTINTERFERENCE COATINGELECTOlY'TIC ETCHINGCHEMICAl STAINING

~SECTIONING

STRUCTURAlDAMAGE

I ' ,,GROUP , 'GROUP 2 'GROUP 3'- -- -~ --

I I

SAMPLE INTEGRITY STAGE'PLANAR, II,~OUS~INGI

HEAD

PLATEN

OPERATING PLATEN SPEED rpm

so ~rol PUfP9~tH HEAD rpm 120GRINDING OR LAPPING

SURF'ACE

rORCE

PER

25mm2200 1250 ,300 2 1250 1300; 200 1250 ~ I DIRECTIONI mm Imm m Imm m II 1 300 1 ?75 2 I C M . ,..Q ~T. - -1- - + - -t - -1- - t-11~ - -1'* - T17~l:0"~~ :"\.1.

- - - - - T - -t - - - - t- --:J r"K ..~~ 1'.~-- - - - - - -1 I ~1,,;1U .JVU I: . i- - - - - T - -t - - - - t- - "t - 1 -T 5 ~ - -. - - .. -1 I ~vvl ,~ 1 u I . I.. - -,- - T - -t - -,- - t- - "t-M r27S '"250-'- -= - - -: -

+--t t--"t t) ; . - _1- - T - -t - -'- - t- - "t~Q.IZ2} ~Q9-'- ..: - - ..; _II 1 150113Q1?01:* - -1- - + - -t - -,- - t- - "t40-1'0- T~-I== =-==--===:'

+--t t--"t-- . I .',

:~::

:J::

ULTRA-PREP DISCVlTRIFlED WHEELDIAMOND GRINDING WHEEL

V SlUCON CARBIDE PAPERZIRCONIA/ ALUMINA PAPERULTRA-PLANMETLAP No 10CAST IRON LAP

-.

NOTE: CONTRA-DIRECTION OR LOWER HEAD SPEEDS FOR INCRE}.SED MATERIAl REMOV~W INCREASES MATERIAl REMOVAL DIF"FERENCE (MRD)

GRINDING VIBRATION-THIS CAN OCCUR WHEN GRINDING SOfT MATERIAlS OR ALTERN4TIVEL'WHEN SPECIMEN SURFACE BECOMES SMOOTH

REMEDY - USE LOW PLATEN SPEED WITH 120 rpm HEAD SPEED OR 30 rpmHEAD SPEED WITH HIGH PLATEN SPEEDSOAP OR EMULSlnER WILL ALSO ALLEVIATE THE PROBLEM

FlJG5.33-0PERATING CONVENTIONS FOR PLANAR GRlf-J(;ING

NOTE US{ CONTRA F'OR WORt AGGMSSr.'E WPROACHGRINDING ~BRAT~-MS CAN OCCUR WHEN CRN)I'«: son WATERW..$ OR AlTERNATNELY

WHEN SPEC...o. SURFACE BECOMES SWOOTHREMEDY- US[ La. PlATEN SP£ED wnH 1~ HEAD sPEED OR »Pm

HEAD SPEED WITH HIGH PLATEN SPEED~ OR EMVLSIFlER WIlL AlSO Al1£VIAT[ THE PRO8l£W

G 5.34 OPERATI~G J:,O~VENTIONS FOR THE SAMPLE INTEGRITY STAGEC'

~--l~

B:f:J- -

OPTiCAl STAININGDIFFERENTIAL INTERF"ERENCE CONTRASTINTERF"ERENCE COATINGELECTOl YTIC ETCHINGCHEMiCAl STAINING

- - - -.,SECTIONING

=- ~

,mSTRUCTURALDAMAGE ~

' ;

I II II I~I II IJ POUS.tt!NGI..

I I I,GROUP 1 I GROUP 2 'GROUP 3

I. PLANAR

. .SAMPLE INTEGRITY STAGE

OPERATING PLATEN SPEED rpm HEADJ ff~ - ~~o ~ r ~~ ~:jo I~~.~ ;1 P!.ATEN

FORCE IPEP; I

25mm21\ 2001250~300~ 200-' 250; 300;200 Ti56~300mm' Imml mmlmmllmml.mml- m~I,"!!,1 mml

DIRECTIONSURFACE, " ~ I 7" a\ ~ I 17- "1~ I"'" I" ".--

- - - - T - .,. .-- - & to »".,. ~ - .. T -- - ~":'"" - - - - .. ~ --I " I. . "I. "I" .- - - - T - ~ - - -.- to - .,. - - - - T - - - - - - - - - - - -I : I: : "I I"- - - - T - ~ - - - - to -.,. - - -:- T - - - - - - - - - - - -I . I. . . I . I "- - - - ~ - ~ - - -.- to -.,. - - - - ~ ~ - - -"- - - - - - --, I;.: . I- - - - T - ~ - - - - to - .,. - - -" - ~ - - - - - - - - - . - -I . ,.. I " I. .- - - - 1" - - ..;. - -. - to '- .,. - - - - T - - - .". - - - - ." --, ., I I- -,- - ~ - ... - -,- - to - .,. - -1- - T - -.- - - - . _f. - - -

---1"-~ to-"' T" I : : I : . I ; I'~

POUMETTEXMET 1000t.4ICROCLOTHCHEMOMETMASTERTExULTRA-POL 1000

NOTE-

TO RETAIN SLURR'r ON P~1[N USE CONTRACONTPA CAN BE "ORE ErF'ECTM WITH SOME: MATERIAlS

STAGE!POLISHING5.35- OPERATING CONVENTION S FOR THEFIG

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CHAPT£R6 THlRmMlf£ASUREIIEBT//"""" ' ,

. . lARCE'R,---II..

". '\son MATERIALOVER POLISHED

--,III.

-DUCTILE MA!;RIALS SWEAR (OVERSJ~)

Figure 6.1 - Polishing soft materials, in particular whensome fonn of pressure is used, will often result in a topsurface smearing or burnishing. Although this ~exaggerated in the sketch it will take place and be morepronounced if uni-diroctional polishing is used. Thissmeared surface will 'lid' any pore or hole that is presentthus disguising or reducing the visible size. Apparent sizeof a smeared layer will also suffer from size interpretationas illustrated in example No.4.

A solution would be to avoid any rubbing and use non-naptype cloths, even if smaIl scratches are visible - avoid thebigbly polished finisb

2-BRmLE _MA1ERIAlS CRACK tUNDER:S.;£

Figure 6.2 - Brittle fracture materials, as the word implies,are worked by fracturing small particles from the topsurface in order to remove material. This fractureddamage must be reduced, eventually to zero, if realistic

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J..\ MET ALL06RAPH'i EUROPE

measuremen~ ~ to be expected. When dealing with aductile substrate material, as shown in the sketch, thisdamage factor can inadvenently be overlooked since thesubstrate and mounting resin would look perfectlysatisfactory. A solution would be to use shock absorbinggrinding surfaces and small grinding abrasive particles.

/"~--~-~~~'"/- - - ~ ~ LH~~E~ "'

\I III r son METAl. / F1LL£D SPACE

//v

~-SOME RESINS CONTRAC~ (OVERSIZE)

Figure 6.3 - Resin contraction must be avoided or reducedto an absolute m;nimum when the desired coating bas anencapsu)ating resin interface. The danger when opticallyobserving the prepared sample and not seeing any fissureat the coating/resin interface is to assume there must not beany resin contraction. When grinding a soft material it willdeform to fiB any vacant space caused by contraction.Additionally, the swart removed when grinding the softelement can be deposited into any other vacant space, suchas a pore, giving an erron~ous microstructure.

rocusLEVEL f ~GER'- - -..I-

I. "\ I\- - - - -~- -

-or--II.

R(U(F

4-.~_~~~§~E_N!ERRORS DUE TO OPTICALINTERPRETATION

Figure 6.4 - The apparent size of an object when observed

Copyright 1994 BUEm.ER LId

J~ MET ALL06RA.PH'i EUROPE

down the mi~pe is not always a true size. Variousfactors affect the image size such as aperture diaphragmposition (see No 5). A factor affecting the size from aswface preparation point of view is grinding relief.Polishing relief at the resin/coating interface causes anangled face rather d1an a planar face to be presented to themicroscope objective. Due to the diffraction of light theimage size will increase as the lower surface increases fromthe focus level as shown in the sketch.

5-MEASUREMENT ERRORS DUE TO MICROMETEREYEPIECE POSITION

Figure 6.S - Every subject bas a specific line size, if weconsider the simation as shown, then the filar micrometerline (or graticule) can be placed at either edge or in themiddle of this finite boundary between the measuredsubject and its two interfaces. The end result of this beinga variable dimension. To overcome this error the operatormust make his testing position U'aceable to some standard.i.e. cb~k a known dimension from a stage micrometerbefore sinng any component This e1iminAtP_~ operatoren-or by checking to a known standard

Copyright 1994 BUEHLER Ltd 6..3

J~ MET Au.06RAPH'l EUROPE../ "

/ LARGER~ - - ~ -~40RE- ACCURATE

I II I

~,\,)#

.4

. NORMAL THICKNESS

6 - ~tJREMP:NT nRnR.~ n~ m nR)p£T S~

Figure 6.6 - Angle grinding is an established method forintroducing accuracy into ~-sec1ion measurements.angle inserts are supplied enabling components to lie at agiven angle. This inclined angle dictates the increase insize that can be expected. Figure 6.7 shows the expectedincrease in size from 9QO when it is 1: I to 00 where itsincrease is infinite. The ~ful working area from thisgraph is between 40 degrees from the horizontal and ISdegrees maximum. Lower than IS degrees to thehorizontal is too sensitive to a given error ie. 91/2 - 100 is agreater error than 291/2 - 300

REDUCING ERRORS

Figure 6.7 - Taper Sectioning

Another method of achieving an angled ttacea.ble subjectwhich overcomes some of the shortcomings of the above ~given in figure 6.8.


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Figure 6.8 - Angle Ratio

Figure 6.8 depends upon having two identical components.one as the reference. the other. angled. It can be seen howthe actual thickness is derived from the three otherdimensions (A.B &; C).

The methods listed previously have all been destructive inthat a cross-section of the component is required. This ~not always possible or desirable and therefore analternative method must be found.

1---~ "-""

~l -'-.~'Li;" - -.(Ri':ii;

Figure 6.9 Spherical Ball Method (Plan 2:1)

Figure 6.9 illustrates the principles behind the spherical ballmethod. The rotating sphere when dosed with 3J.1Indiamond will grind into and through the plated layer. Thisdome shaped hole can then be optically observed and theplated thickness calculated from knowing L. & La. Thefonnulae is shown in the sketch.


@4 MET AU06RAPH\' EUROPE

CHAPT£R 7 MICROSTRUCTURAL TRACEABwn'TO ISO 9000The idea behind microstroctural traceability is toimplement a system of operations based on referencedstandards that can be defmed and audited. These referencestandards will where possible be statistical. The objectivestherefore will be to. Produce non-empirically derived surface preparation

methods.. Designate statistical and visual parameters for each

stage. Define a standard procedure with auditing andtraceable references

. Carry out a quality control audit

It is imponant to stress the point that it is not the 'method'that is traceable to ISO 9000 but the manner in which themethod is caITied out. monitored and recorded. Anysurface preparation method that gives the desired result ~acceptable. providing visual and statistical data can be usedand audited.

THE METHOD

To develop a non-empirical method along with the desiredvisual and statistical data will be pursued. Consider figure7.1

,

ISTR~~

~J-

-mIE - - , ,Figure 7.1 Procedure Construction I The Best of Three

Each grinding I sectioning step has been displayed as atriangle. the vertical axis being the reduction in structuraldamage. the horizontal being time. The ideal triangle or

Copyright 1994 B~R Lid

~A MET ALL06RAPB"i EUROPE

step therefore would be to have a long vertical directionwith a short horizontal. The preparation stages as can beseen are sectioning and sample integrity. the latter havingthr= groups. The principle is to select from the'sectioning options guide' thr= suitable sectioning wheels.1bis will be followed by three suitable grinding surfacesfrom each group selected from the 'grinding/polishingoptions guide',

The four selected steps chosen for the standard methodhave cross-hatch uiangles. Notice how the choice ingroup three takes the preparation to integrity, othersurfaces in thh group would be quicker, but without thehigh degree of integrity. In the real world it could benecessary to go for speed, the choice will vary with theneeds.

STATIS11CAL IIlm NOTE: The polishing Stage is not included in the\'IsUAL PARAM£T£RS procedure construction since this is a supplementary stage

as and when necessary.

[Reference: Towards a Metallographic Standard Suitablefor ISO 9000 Approval. Microsnuctural Science - Vol 21]

The procedure construction requiring visual and Statisticaldata ~ shown in 7 A/B~ and ~ based on three surfacesin each of the four groups. (one sectioning - three

grinding). The information required at the sectioning Stageultimately requires the time taken to cut (Inins) be divick:dby the cut length to give tan Theta. This then allows theconstruction of the trigons. From these three tests it willbe ~ssary to select the least damaged This surfacechoice will be noted on the sheet 7 A and caIried over to7B. The three tests in 78, which ~ the first in the sampleintegrity stage, will be based on the preferred sectioningsurface and all surfaces must commence from this position.With the SO kg Hv indent it will be necessary to note the Zaxis material removal necessary to progress each grindingsurface to its 'best position'. The best position ~ when thesample will no longer improve. Material removal (J1ID) Bthen related to time to derive the theta trigon angle. Onceagain the surface choice ~ noted and carried over to 7C asthe 'reference' surface. The three swfaces are then Olx:eagain related to the reference surface, the trigon createdand the surface choice made.

At the final sample integrity stage (7D) the new referenceswface is used in again creating die three 1rigons.

Copyright 1994 BUEln..ER Ltd

«>4 MET ALL06RAPH'i EUROPE

Z AXlS 6RAPHlCDISPLA ""

From the twelve trigons it will now be relaJively easy toconstruct the graphic display as shown in figure 7.1.Starting from the best position (that which most closelymanifests sample integrity) and working upwards usingmaterial removal to position each trigon. From the graphicdisplay a method must be constructed that satisfIes aparticular industrial requirement of cost, time and integritylevel required

STANDARD AUDIT

PROCEDURE

The 'standard audit procedure sheets' are included in7E/F/G/H, the information necessary to complete suchsheets bas already been compiled in the relevant'procedure construction sheets' . Once the sheets havebeen completed an audit can take place.

TRAC£ABL£STANDARDS

What has been achieved so far is:-

An acceptable system for defining a preparation methodA means of relating the chosen method to statistical andvisual standards

Providing the documentation is in the form of a closedloop i.e. every process or check is carried out inaccordance with the audit requirements and is suitablydocumented, then ISO 9000 requirements have been metIf however d1is work is to mean anything, it bas to betraceable to some standard and d1is can be done asfollows:-

1. A Sample Material Standard must be identified

2. The 'Sample Material Standard' should be traceable toa traceable institution or failing this an agreed indmttialbody. ~ body could be your own company if DOother industrial standard exis~.

3. There should be traceable reference ma1eria1 with adocumented location.

Copyright 1994 B~R Ltd

J~ MET ALLOOR.APH'i EUROPE

~ 4. Additionally. there should be audit material, taken fromthe traceable material that is to be used for each audit -this will also require a documented location.

s. The audit inlCIYll must be ~ignated and this appliesto firstly the machine °!'ter8tion parameters. that is a-c~t is made at given intervaJs to confmn theequipment/consumable combination has not changedie. If the material removal required at a given stage ~4OJJmS and that 10 minutes was allocated at this stagethen this rate of removal must be confinned Secondly.the surface prepamtion conformance must beapproved To do this the audit material will be usedalong with the sample material and checked against theaudit procedure documents.

6. The audit Jevel must be de~d for both "machineoperation parameters' and "surface preparationconformance' Le. Select from the audit proceduredocuments the minimum number of checks thoughtnecessary to confIrm conformance.

7. Non-confonnance whenever it should occur mayrequire that a fuJ1 audit takes place. this must berecorded in the documentation.

I LocationI

~R&D

I Buehler UK ~ - -- - - -

MachiDe operatioa p8P~ - the start of eYery --lb8tcb

I QC

Audit Internal - ~ -- -- - - - - -Surface preparation coaformance - First Monday or each

:

Audit Level

. - -- - - -I MadaiDe operation parameters . Check Z aD in group 1

and 2 of the sample Inte2rltv sta2eSurfKe preparatioD confm'maDce . Check Z uIs aud

I Ytlual lafm'matiOD In group 1 aad 2.. Check Yisua1

conformance or 2r'OUP 3

I Surface Pl"epantioD conformance. total check against aUJ standard audit procedure laformatiou

NOD.Conf~

Figure 7.2 Traceable Audit Procedure

Figure 7.2 is an example of how such a ttaceable auditprocedure could be consU'Ucted.

Copyright 1994 SUE In.ER Ltd

7A

PROCEDt JRE CnNSTR.UCT1nNi:.~nNrNG

Oplimised machine parameters

SAMPLE mENTITY--

Z-AXIS

mETA (deg)TEST No WHEEL

mENTITYSEcnO~G~(minr

-'

SAMPLE

LENGm

lum)rt

MicrographITest 1 I

Porosity %

Depth ofPorosity J1IDInterface Rclief J1IDResidual Damage J1IDRemarks .'.." '.'.""" ""."...' '.".""".'

MAG/NA

MicrographI Test 2 I

Porosity Depth of Porosity Interface Relief Residual Damage Remarks

%

J.ID1

J1n\

J1n\

~

MAG/NA

McrographI Test 3-- -I

..%

.~

...~

...~

Porosity Depth of Porosity

Interface Rdief Residual Damage.

Renwks . ""

~

MAG/NA

~I..ISURFACE CHOICE TEST No

7Ba

\. PROCEDtJRF:. CONSTRUCTTnN

PLANAR GRINDING

Optimised machine parameters

SAMPLE -mENTITY :

RESmUAL DAMAGE ~

III so Kg Hv n-mENT mEN L~ REMOVALREFERENCE SURFACE No

~ Test No 4

S6

Surface I Measuring IInitia1roD 0 I sequence I ind~

FinalIndent

Material~ova1

Z-AxisTheta

CombinedTime( min )

I Test 4 ,IMicrograph

Porosity %Depth ofPorosity""."' J1D1Interface Relief."""" J1D1Residual Damage' J1D1Remarks """""'.'"

MAG/NA

I Test 5,.1Micrograph

%

...J1n1

J1n1

J1n1

Porosity Depth of Porosity.

interface Relief Residual Damage..

Remarks

MAG/NA

Test 6Micrograph

...%

.~

~

~


Interface Relief Residual Damage.

Remarks . .'.."'..'.".'...

7B2

PROCF-DI JR.:F- CONSTRUCTION

1 st SAMPLE mTEGRITY STAGE

Optimiscd machine parameters

SAMPLE -mENTITY :

RESmUAL DAMAGE J!m

REFERENCE SURFACE No

Micrograph'Test 7 I

Porosity OJ.

Depth ofPorosity J1!DInterface Relief J1!D

Residual Damage J1!DRemarks ,...

MAG/NA

Micrograph(Test 8 I

... 8f.

..JJmJJmJJm

Porosity Depth of Porosity .

Interface Relief Residual Damage.

Remarks

MAG/NA

Micrograph'Tell 9-')

%JUnJUnJUn

Porosity Depth of Porosity Interface Relief Residual Damage Renw"ks ,...

7C

PROCEDtm.E CONSTRUCTION

2nd SAMPLE DIlTEGRITY STAGE


SAMPLEm ENTITY .

RESmUAL DAMAGE J!Jn

REFERENCE SURFACE No I:iA so Kg Hv n-mENT mEN LIP REMO VAL

FinalInd=

MaterialRemoval

Z-Axis~

TestNo

SurfaceLD.

MeasuriDI~u~

Initialindent

CombinedTune( min )

~

m

'Test 10 - I

MIcrograph


%J1D1

J1D1

J1D1

MAG/NA

MicrographI Test 11 1

...0/,

..J1nt

J1nt

J1n\

Porosity Depth of Porosity .

Interface Relief P~dual Damage.

Remarks

MAG/NA

I Test12 .-:- --:3 Micrograph

..0/.

.JJ.In

...JJ.In

...1Jm


Interface Rdief Residual Damage.

Remarks

7D

PROCEDUR~ ~nNSTRucrlnN

FINAL SAMPLE nolTEGRITY STAGE


SAMPLE mENm"Y :

RESmUAL DAMAGE J1ID

~ so Kg Hv INDENT mEN m REMOVALREFERENCE SURF ACE No

I Measunng

!~ence 1: I~terialI Removal

- -

Combined

T Jme( min )

Z-AxisTheta

I~ T est

No

13

- -

IS~II.D.

(JL-

II!!! 13 IMicrograph

Porosity %Depth ofPorosity J1D1Interface Relie( ~ J1mResidual DImIIC J1D1Remarks

MAGmA

'Test 14 IMicrograph

%

J1InJ1InJ1In

Porosity Depth of Porosity Interface Rdief Residual Damage Remarks .'.""' '...' ".

MAG/NA

I Test IS

%., J1In

J1InJ1In


7E

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01\4 MET AlJ.06R.APH"l EUROPE

MlCROSCoP\' AND PBOTOMlCRO6RAPnCHAPTER 6

The metallographer when analysing specimens usuallyrequires a 'Macro' (low magnification -large field of view)or a 'Micro' observation (higher magnification small fieldof view). Figures 8.1 and 8.2 are examples of 2 'micro'scopes. Figure 8.1 is an 'upright' compound microscope,8.2 an 'invened' compound microscope.

MICROSCOPE n'PES

Figure 8.1Upright Compound

Microscope

Figure 8.2Inverted Compound

Microscope

The word 'compound' is used to signify a double opticalimaging system. That is an objective which creates theprimary image and the eyepiece which further ma~jfj~~the primary creating a secondary image. If just onemagnification occurs then the microscope would bedescribed as 'simple' (i.e. magnifying glass). Invenedcompound microscopes are often called Metallographs orProjection microscopes. The advantage offered by theinvened system is that no specimen levelling is necessary,the object just sits on top of the microscope stage.

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Figure 8.3 - (Macro)

Figure 8.3 is an example of a Macroscope (sometimcscalled a macro-microscope, although this is a misnomer).This type of scope can be considered as a camera lenssystem with various sized projected images. It i~ a'simple' scope in that it depends upon a primary imageonly. The most obvious example would be a 35mmcamera. The illustration in figure 8.3 shows a bellowsexten.~ion system whereby the field of view can he varied.The image being reflected via the upper mirror onto aprojection screen or alternatively allowed to pa.~ upwards,with the mirror tripped out. Once projected upwards it(;an ~ imaged into th~ large format cam~rd plane. Bothcamera plane and projection screen arc par-focal (at IhL'same image distance).

St\."r\."osrupic micr()sco~S (figure 8.4) are unlikl..' any 'If tht.'flrevious example.1\ m that they have a d()uhl\.' ()flti..: OL,j.'.Thl..' angle of thc.~ two separate image.' i.'i d\.'~ignl..'d t"I..'mula~ thc avcragc J'Crsons intcrocular di.'\tant.'1..' wh\.'nviewing an object at the least distance of distint.'t vi~i()n (II)inchl.'~). It is because of these two separate rdY.'i that wcfl\."rcciv\.' the imagc in three dimensions (st\."re().'iCoflic).

MICROSCOPE IMAGE 1111.' micm~co~ is u~d basically becau~ thc ~uh.ic~t WL'wi",h to ()hservc i~ .waller than the human I.'yc c.:an rl.'~"v~.In ordL'r to ~c thi~ .wall ohject we havt.' to magnify animagl.' of tht.' ohjl.'ct that is within the vi"ihlL' limit, ()f thl:~y~.

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Figure 8.5 Man's limit of Resolution

Figure 8.5 is used to illustrate the limit of mans visionwhen observing an object at the least distance of distinctvision (10"). This limit as shown is IOOJ.1In. We thereforeneed to magnify any object less than IOOJ.1In in order to seeand resolve. For example a sOJ.1In object would have to beX2 magnified in order for the eye to see.

Figure 8.6 illustrates some of the important elements andpositions of the compound microscope. Four conjugateplanes are shown; the lens principle focus, objective backfocal plane, primary image plane and secondary imageplane. Notice how the mechanical tube length is fIXed. theoptical tube length will vary with different lensconfigurations (objectives). Figure 8.6 depicts a finite

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optical system of l6Omm mbe lengdL ~ dimensionchanges with metallurgical microscopes to a higher valuein order to accommodate the reflected light illuminator.Objectives must be corrected for the operating mbe lengthand should not be used at other mbe lengths (objectives ofnumerical aperture greater than 0.25 will manifest cloudy.images).

The alternative to finite tube length systems is to have aninfinite system incorporating parallel rays along part of theoptic axis. This system is n.o.1 superior to a finite system.It is however, more convenient when a combination ofdifferent ~~~~ries are to be used. Once again themicroscope objectives have to be specially corrected forthis infinite tube length and must not be used with otherobjectives.

BRI6HT FIELDMICROSCOPETE. C HNl Q U£S The most common microscope technique is that of bright

field. The micr~pe being set for what is called KOhlerillumination, this is shown in Figure 8.7

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Light illuminated from the lamp filament travels along theillwnination axis incident to a semi-reflecting mirror. Lightreflected from the mirror passes through the objective andis focused onto the object An image of the object ~

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image plane. The eyepiece further m~ this image togive a secondary image for observation or photographing.

To satisfy KOhler illl1m;ftArion the lamp filament must beimaged in the objective back focal plane. the fielddiaphragm in the object plane and the apermre diaphragm~ in the objective back f<x:a1 plane.

Microscope accessories shown in the sketch are filter andpolariser I analyser.

The specimen when observed in bright field very oftenrequires some form of chemical etching in order to revealmicros1ructUIaldetail

Dart Ground (Dark Field}

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Figure 8.8. Light Intesity Comparison(a) Scratched (b) Etched

Figure 8.8 give an indication of the light intesity differencebetween a scratch and an etched grain boundary.Brightfield shows very little difference between the scratch- the etched boundary however is dramatic. Dark groundon the other band shows an excellent contrast differencebetween background and scratched surface. Tounderstand the principles of dark ground your attention ~drawn to figure 8.9.

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Figure 8.9 Dark Ground illumination

Ught illuminated from the lamp filament is projectedtowards the incident semi-reflecting mirror. The centralportion of this light is blanked off by the means of thedarkground illumination stop, thus allowing an annular ringof light to pass and be reflected into the optic axis. Thisannular ring of light has an inside diameter greater than theobjective field of view. The light then passes through whatis called a 'catoptric' condenser system which focuses theillwnination onto the object plane as shown.

It follows therefore that only light that is 'deflected' by thespecimen will re-enter the optical path, light that has ~nreflected will be reflected out of the optical path. Apolished surface with for example a scratch would manifesta black background, the scratch being the only illuminateditem.

Dark ground fin~ many applications, the following just afew oftbe examples:-. Improved contrast. Feature morphology highlighted. Material translucency identifIed. Sub-surface damage identified. Sub-surface porosity

(Examples of d1is and all other optical techniqueapplications is covered by the series 'Exploiting the OpticalMicroscope' by the author.)

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The bright field microscope shown in figure 8.7 can beused for simple polarising techniques. The polariser couldbe in position for most applications just to reduce glare inthe optical system. Once the analyser is introduced acondition called cross-polars can be achieved. This is whenthe vibration ~tion of the polariser is at 90° to theanalyser. The result being all light is extinguished i.e. lightreflecting from the specimen is totally extinguished. If theanalyser or polariser be slightly rotated (1 - 3°) then thelow intensity subject image will re-appear. This conditionis refened to as un-crossed polars.

POLARlSIN6

To put this technique to use we need to understand whathappens when light is ilK:ident to a subject material.Materials nmmally reflect light, incident upon the preparedsurface, in a vibration direction identical to the incidententry vibration direction. Materials that do not re-orientatethe incident ray vibration direction are said to be'isotropic'. Metals for example are isotropic. Consider thebright field microscope as shown in figure 8.7 with thepolariser in position and a steel specimen placed in theoptic axis. Light when passing through the polariser willextinguish all but one vibration ~tion (North/South).This specific vibration (NS) will be reflected off the steelsample without any noticeable difference. If the analyserbe now placed in the optic axis and set to East/W est(crossed-polars) then all light will be extinguished

Some materials such as minerals react differently to metalsin that when light is incident to the prepared surface thereflected image emerges in tWo specific vibrationdirections, these materials are said to be 'anistropic'.

Consider now an anistropic material being used as thespecimen. Polarised light vibrating in one direction willupon reflection be now vibrating in two directions at 90.to each other. When these two directions fail to coincidewith the analyser vibration direction. an image is created.This can best be illustrated by using a cast nodular ironsample as the specimen. Between cross-polars the ironwill be extinguished but the graphite (anistropic) will beexcited and imaged against this black extinguishedbackground. To convert d1is grey level image into themost beautiful coloured structure it will be necessary toplace after the polariser an accessory called a 'sensitive tintplate' (sometimes called a first order red plate). Thisac~-~cory when placed as suggested changes the black

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extinguished background to that of the 1st order red. (540DIn). die anistropic graphite now a multitude of colours.

The preparation of thin sections bas been covered in thismanual, the microscope required to observe these thinsections requires to be illuminated with traDSmitted lightA schematic layout of a 1st order polarising transmittedlight microscope is shown in figure 8.10

Figure 8.10 - Po1arising Microscope - Transmitted Light

Consider the light emanating from the illuminator at thebottom (as shown). This light incident to the optic axis ~vibrating in all directions until it passes through thepolarlser. A single vibration direction when passingthrough the anistropic material emerges in two vibrationdirections, ultimately to pass through the analyser. Thesetwo rays will, when recombined in the primary image plane,interfere to create the image. (Constructively ordestructively).

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(Differential interferen~ contrast can with a correctlyprepared specimen prove to be the most revealing of any ofthe optical techniques. Providing the abrasive differentialbetween different material phases bas been restricted to theequivalent depth (or less) as the objective resolution then athree dimensional image can be ~ved. This subject hasreceived extensive coverage in the 'Exploiting the OpticalMi~ope' series. also in the book 'Surface Preparationand Microscopy of Materials'. The schematic layout infigure 8.11 illustrates the optical components necessary.

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Light

Basically what we have is the standard bright field reflectedlight microscope with the addition of a polariser/analyserand a beam splitter often called a Normaski prism (namedafter the inventor). From figure 8.11 the polars arecrossed, the prism slowly introduced Wltil the 1st ordergrey extinction position is achieved

DIC depends upon the interference of the ordinary andextra-ordinary ray when re-combined. To have a perfectlyplanar or stepped component the optical path difference

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( (OPD) between the two rays would be identical, resultingin a background colour remaining unchanged inespectiveof the planar or stepped surface. 1bankfu1ly allmec}\~~-11y worked surfaces result in slopes, rather thansteps, as one phase is abraded differently to another. It ~this slope that creates an OPD between the two raysresulting in a colour change. ie. planar surfaces would all.inespective of height be one colour - all sloped surfaceswould be another colour related to the angle of slope. Thisis illustrated in figure 8.12

0 .~RayEX=~Ray

Figure 8.12 - Identification by Slope

CAMERA PLANE

Cameras can be placed anywhere after the secondary imageplane. they do tend however to be placed at what appearsto be an wmecessarily long distance away from the cameraeyepiece (C.F. PL lens) as shown in figure 8.13

PHOTOMlCR06RAPHY

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This long distance is in fact a repeat of the original objectto image conjugate distance. This is done to give a unityposition where the quality of the secondary image ~reproduced. Any other position could re-image opticalabeIrations designed out of the conjugate plane. Iftherefore a single lens reflex camera was to be used itwould have to be fitted with a 'long robe' adaptor.

QuALm' OF MICR06RAPHS

The quality of picttJreS depends. in the main. on the caretaken in the surface preparation, the chosen opticaltechnique and observance of some well defined rules.Very rarely if ever should the microscope be blamed forindifferent results.

FILM

The choice of fiJm in general tenus is to target for thelowest ASA (speed) fiJm ming the largest format. Thissiwation obviously depends upon different circumstancesthe author who always used Kodak 5 x 4 cut film now usesFuji Velvia 50 ASA (ISO) transparencies. because of thepriorities of lectures.

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(UE - underexposure; OE - overexposure)

Figure 8.14 FJlm OJaracteristics

Figure 8.14 shows how the low ASA (ISO) film with imexcellent range of density maximum/minimum has inflicteda high angle exposure curve with its narrow latitude toexposure errors. The high ASA being much less sensitive.very fast but could be lacking in contrast

Fn..TERS

Filters play an imponant role in both balancing reciprocityand contrasting both coloured and black and whitesubjects.

Table 36.2 Guide to CC mien

~ure time(s) F1Itas New film speed Setting

1/815

1030

CC2OC +CC3OB

CC30BCC20M+CCIOBCC20MCCIOM+CCIOR.

ISO 25/150ISO 2S/lSOISO 20/140ISO 16/1~ISO 16/130

Figure 8.1S Guide to CC FDters

Figure 8.15 give an indication of the necessity tosupplement the illumination with different colourcorrection filters as the exposure time necessary becomeslonger. Reciprocity failme is when the intensity/exposmecurve proves no longer arithmetical. i.e. under or overexposed. The materialograpber will in the main beunderexposing and as can be seen from figure 8.15 theaddition of CC2OC + CC30B will be necessary when

8.12

@A MET AJ.J..06RAPH\' £UR OPE

exposing at 1/8 second in order to achieve a faithful colourreproduction. (Ibis assumes the colour temperatures ofthe lamp and film are compatible).

If the light intensity emanating from the specimen was solow as to incur a 30 second exposure, then colourcorrection filters of CCIOM + CCIOR would have to beemployed.

Fn.. TERS CONTRAST

Yellow 8bsorbs blue CC20Y

Red absotbs bkIe and green CC20R

Magenta absorbs green CC20M

Blue absorbs red and green CC20B

Cyan absOfbs red CC20C

Green absorbs bkle and red CC2OG

Figure 8.16 Complementary Colours

Filters can be introduced to complement a particularcoloured feature. These complementary filters are shownin figure 8.16. It is important to reaIise that filters are notentirely confined to colour photography, they also have arole to play in black and white.One of the most enjoyable aspects of being amaterialographer is the ~hnicaIly revealing picblres thatcan be captured when carefully using the microscope.

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