erasteel, surface, eng.2010-04:erasteel, surface, eng...surfaces influence of surface quality on...

S U R FA C E S

Influence of surface quality on high performance ASP toolsInfluence of surface quality onhigh performance ASP tools®

EDUCATION

®

S U R FA C E S

Erasteel, surface, eng.2010-04:Erasteel, surface, eng.qxd 10-04-19 13.25 Sida 1

New high-performance tool materials, such as the

latest generation of ASP steels, make operations such

as cutting and forming increasingly more effective.

However, high-performance tool materials of high

hardness provide inevitably a lower toughness. This

lower toughness calls for a material with smaller and

fewer imperfections so that the strength of the tool

can be kept high. Smaller and fewer imperfections

mean, in practice for high speed steels, that there is a

need to reduce the size and amount of oxides, non-

metallic inclusions, carbide clusters, etc. At Erasteel

we have worked continuously with this during the last

30 years. First, with the introduction of the powder

metallurgy process (ASP) and then through various

developments to reduce the amount of non-metallic

inclusions, such as the ESH process and Dvalin™. In

this way we have managed to continuously achieve a

considerably finer microstructure of the steel and,

hence, a steel of increasingly higher strength. As the

bulk material has now reached a high level of per -

formance, owing to the refinement in the production

of ASP grades, we see an increasing trend of failures

initiated at the surface of the tools. These failures are

often linked to the processing and manufacturing of

tools – presently about 1/3 of the failed tools we

receive. This demonstrates that a level has been

reached where the surface finish is critically important

in taking full advantage of the latest material develop -

ment. The intention of this brochure is to raise the

awareness and the understanding of the importance of

surface and surface quality for tools made of the latest

generation of high performance ASP grades.

Introduction

®

ASP is a registered trade mark of Erasteel.®

2


HSS basics page 4

Defects and their influenceon tool properties page 5

Surface quality basics page 7Surface roughness 7Structural alterations 8Chemical alterations 8Residual stresses 9Burr 10

Influence of toolmanufacturing techniques on the surface condition page 11

EDM 11Heat treatment 13Grinding 13Techniques for deburringand surface finishing 16Coating 22Nitriding 25Peening 26Marking 26

Trouble shooting page 27

3

AVOID TOOL FAILURES BYTAKING A CLOSER LOOKAT THE SURFACES


The ASP-process is a powder metallurgy technique, comprising hot–isostatic pressing(HIP) of rapidly solidified gas atomised powder.

During gas atomisation the melt is disintegrated by powerful jets of nitrogen gas intosmall droplets, which solidify at a very high rate. The powder is collected in a steelcapsule which subsequently is evacuated and welded. Finally, after hot-isostaticpressing of the ASP-powder, bars, wire rods, strips and sheets are obtained fromforging, hot and cold rolling and wire drawing.

High speed steel (HSS) combines, in a uniquemanner, high strength with high hardness, and isthus an ideal material in such diverse applicationsas cutting tools, cold forming tools, wear parts,etc. The unique properties of HSS are derived fromthe iron-carbon matrix alloyed with Mo, W, V andCr, which, after tempering, contains evenlydistributed nano-meter-sized carbides whichstrengthen the steel. The HSS also containsbetween 2-20 volume percent of larger carbides,about 1-10 µm large, which are either of MC type(mainly vanadium in addition to carbon) or ofM6C type (mainly molybdenum, tungsten and ironin addition to carbon). The hardness of thesecarbides is about 2800 HV for the MC carbides

and about 1600 for the M6C carbides. These so-called primary carbides provide the high wearresistance of the HSS and to a lesser degree thehardness. Additionally, some HSS grades containup to 15 wt. % of cobalt in order to give someadditional hardness and hot-hardness.

Traditionally, manufacturing of HSS has beencarried out using casting and forging techniques tomanufacture conventional HSS. A substantialimprovement in the properties of HSS wasachieved from the introduction of the Erasteelpowder metallurgy process – ASP steels. With thepowder metallurgy process, the microstructure ofthe ASP steel is much refined with a very evendistribution of finer primary carbides. This resultsin an improved combination of strength andhardness in comparison to ‘conventional’ HSS.

HSS basics

Graphite electrodes

Molten steel

Nitrogen gas

Powder

Hot isostaticpressing, HIP

ForgingRolling

4


Defects and their influence on tool properties

One of the most distinct advantages of ASP steelscompared to other tool materials, such ascemented carbides, is their high strength. Thisallows tool makers to make not only sharp toolsbut also their customers to use these tools underhigh loads, i.e. high feeds and work materialremoval rates. The reason for the high strength ofASP steels is the extremely low level of internalmaterial defects, owing to the unique combinationof fine microstructure with high cleanliness.

Because of the low level and size of internalmaterial defects in ASP steels, there is a highprobability that surface defects owing to theprocessing of the tools, and not the ‘intrinsic’quality of the steel, provide the larger defects. Thisimplies that the high strength expected from thehigh quality of the bulk material is not alwaysachieved owing to defects at the surface in -troduced during manu facturing of the tool. As thetool surface is often the part of the tool that isunder the highest stress, defects at the surface arevery often a limiting factor for the tool life.

At a given hardness, the strength of high speedsteel is inversely proportional to the largestimperfection in that part of the material underhigh tensile stress. In other words, as the size of

the largest imperfection increases the strengthdecreases as:

KIcσF ∝ √d

Here σF is the strength, KIc the fracture toughnessand d is the size of the imperfection initiating thefracture. The im perfections may, for instance, belarge carbides, carbide clusters and strings ofcarbides, as found in conventional HSS, and also,non-metallic inclusions and defects related to thesurface such as grooves (grinding marks,scratches) and thermal damage. Of these im -perfections, it is the largest one which willdetermine the strength of the tool edge. Thismeans that the strength of tool materials iscontrolled by imperfections or defects.

Fracture toughness (KIc), or, more specifically thesensitivity to stress raising imperfections, alsoinfluences the strength of the tool material. Thefracture toughness has been found to decreasealmost linearly with the hardness of the steel,whereas the properties of the microstructure, likecarbide size and distribution, usually have only aminor influence. Thus, the fracture toughness maybe regarded as constant for a given hardness levelfor both HSS and ASP steels.

The largest imperfection, in the part of the material under high stress, sets the maximum strength of the tool. Often, for the latest generation of ASP steels, thebulk material is free from larger imperfections, and surface defects such as scratches or structural changes owing to the processing of the tool are the limitingfactors.

Material Law of nature Properties

F ∝ KIc

√dσ

Fσ FσSurface cracks {

d

Largecarbides

Non-metallicinclusions

Carbideclusters

Level of strength allowedby bulk material

Actual level of strength due to surface quality

5


In the present example, a substantial increase in bending strength is ob served for ASP 2023 from the use of a high quality surface preparation whereas for M2 (a standardHSS made by conventional casting) there is no or very little influence from high quality surface preparation.

This example shows that the surface quality should be matched to the micro structure, and a surface of high quality is needed to take full advantage of the latest generationof ASP steels. This is because the largest defect in the volume under high mechanical stress sets the maximum strength of the material. For high performance materials, withhomogeneous and fine microstructure, such as ASP grades, the largest defect is often a surface defect (grooves, tensile residual stresses etc) whereas for HSS made byconventional casting technique, or PM-HSS of low cleanness, the largest defect is often found within the body of the material.

6

Bend

ing st

reng

th (M

Pa)

Ground surface and high performancematerial (ASP 2023)

High quality surface and high performancematerial (ASP 2023)

Ground surface and conventional HSS (M2) High quality surface and conventional HSS (M2)

200 µm

200 µm

200 µm

200 µm

6000

4000

2000

0


Surface Quality Basics

The surface quality of a tool is the result of thedifferent processing steps during the manufactureof the tool, such as heat-treatment, grinding, finalsurface preparation, etc.

The final performance of the tool is stronglydependent on the surface quality of the tool. Thesurface quality is achieved from a combination ofthe:

• Geometrical properties of the surface: surfaceroughness, presence of surface defects such asscratches

• Structural, mechanical and chemical alterationsof the material close to the surface compared tothe bulk material: phase trans formation, heat-affected zone, carburisation, decarburisation,oxidation, residual stresses, plastic deformationetc.

• Other defects such as burr.

Surface roughnessSurface roughness refers to the fine irregularities(peaks and valleys) formed on the surface by thetool manufacturing process. The most commonstatistical description of the surface roughness isthe roughness average Ra, which is obtained bysliding a stylus tip over the surface, measuring the

surface profile and calculating the averagedeviation from the mean line. It should be pointedout that Ra values are just a rough description ofthe surface roughness as it does not take intoaccount the possibility of directionality (likegrinding grooves) and the fact that differentsurface profiles may have the same Ra. Therefore,one often encounters examples of material withsimilar Ra values but different surface relatedproperties.

A common measure of the surface roughness is the Ra-value, which is a measure of the average deviation from the mean line in a specific direction. Other examples arethe Ry, which is the maximum deviation in a specific length l, and Rz, which is the average of the five highest peaks and the five deepest valleys in a specific length l.

Bending strength vs. surface roughness for standard (65 HRC) and high per formance(70 HRC) material. High hardness inevitably implies a lower fracture toughness(KIc), however, the same high level of strength is achievable for both types ofmaterials, provided that the surface of the high performance material is preparedwith a sufficiently high surface finish.

7

Ra

Ry

m

l

7

6

5

4

3

2

1

00 0.5 1.0 1.5 2.0

Surface roughness, Ra (µm)

Bend

ing st

reng

th (G

Pa) 65 HRC

70 HRC


Structural alterationsToo high surface temperature during toolmanufacturing can result in structural alterationsof the material closest to the surface. This may be,for instance, phase transformations, softening, etc.These alterations are generally more difficult todetect and to characterise than the surfaceroughness but may have a dramatic influence onthe overall properties of the tool. Grinding, heattreatment, Electric Discharge Machining (EDM)etc., are examples of techniques which can alterthe structure of the material immediately adjacentto the surface.

For high speed steels, temperatures between 500-900 °C at the surface result in an over tempering(softening of the material), temperatures between900-1200 °C in a rehardening (phase trans -formation as austenite is formed) whereas yethigher temperatures result in melting of thesurface. The cooling, following removal from thehigh temperature, is generally rapid as it is oftenonly a few hundred micrometers of the surfacewhich is heated during surface preparation. Fast

cooling from hardening or melting temperatures ofHSS results in the formation of very brittle non-tempered martensite. In this brittle martensite,cracks propagate easily giving a material of lowtoughness and strength.

Chemical alterationsVariation of chemical composition within thematerial is another type of structural alteration.Typically for high speed steels, if heat-treatedincorrectly, is a depletion of carbon at the surface(decarburisation), which results in a softening ofthe surface. This may be due to high temperaturesduring surface preparation in air or heat treatmentwith poor control of the atmosphere or media(often in salt baths).

Related also to decarburisation is the formation ofa thick oxide layer at the surface or even oxidepenetration. This happens if the atmospherecontains oxygen and the surface is at an elevatedtemperature, but can also occur in salt bath heattreatment.

Different surface homogeneity but similar Ra values of two otherwise identical ASP 2023 tool materials. In this particular example the lower Ra value of the material to the leftcould lead one to believe wrongly that this material should be less influenced by the surface, and hence have a higher strength, than the material to the right. Evidently, Ra valuesalone should be used with caution when judging the surface finish.

Ground surface (Ra = 0.12 µm), 5000 MPa bending strength

Homogeneous surface (Ra = 0.18 µm), 6000 MPa bending strength

50 µm 50 µm

8


Residual stressesResidual stresses at the surface of a tool may be ofboth compressive and tensile type or somecombination of the two. Residual stresses oftensile type may be revealed as cracks afteretching, whereas compressive stresses andmeasurements of stress profiles call for the use ofmore involved techniques such as X-raydiffraction.

Tools under high mechanical loads, such as ingrinding, peening, turning etc., often showcompressive stresses in the surface. If themechanical load is not too high, these compressivestresses may have a slightly positive and beneficialeffect on the performance of the tool.

Complicated stress profiles in high speed steels may result when the temperature is sufficiently high to initiate an austenitic phase transformation at the surface. I Heat is generated at the surface resulting in a phase transformation but also expansion and hence compressive stresses at the surface.

II The surface cools and shrinks and thus tensile stresses are initiated.

III An austenitic transformation to martensite, which results in an expansion of the outermost part of the surface and consequently compressive stresses in this part ofthe material.

As seen, the final stress profile is complicated with both high tensile and compressive stresses and a steep stress gradient in between.This surface layer will provide an ideal place for fracture initiation as the untempered martensite is brittle and the surface is under high tensile stresses with steep stressgradients. It is therefore very im portant to remove surface layers of untempered martensite before the tool is put into use.

Heating Cooling Martensitic transformation– + – + – + – + – +

Surface

Heataffectedlayer

Bulkmaterial

I II III

9

‘+’ is for tensile and ‘–’ is for compressive stresses.


BurrBurr is formed by plastic deformation of thematerial during machining at elevated mechanicalloads and/or temperatures in both cutting andgrinding operations. Burr formation is morefrequent when dull tools are used and this calls forthe use of, for instance, sharp grinding wheels.Burr on a tool implies that the surface finish of thework material will be poor. In addition, the burrwill eventually break off and leave fracturesurfaces of low surface finish which may act asinitiation points for further fracture. The negativeinfluences of burr is amplified if the tools arecoated.The removal of burr by fracture in this caseresults in fracture surfaces which are non-coatedresulting in accelerated wear at these parts of thetool. To minimise burr formation, one should usea sharp tool or decrease the forces duringmachining. Alternatively, manual deburring, sandblasting, and water deburring can be used.

In this example a tool with burr at the tool edges has been coated. Eventually, theburr will break off, which will result in a non-coated fracture area which willexperience accelerated wear and also act as an ideal point for crack initiation.

30 µm

10

Residual stresses are found at the surface of most ground tools. Compressivestresses may have a positive influence on the tool performance whereas tensilestresses, owing to heavy grinding, decreases tool life and can cause cracking of thesurface

Comp

ressi

ve

Te

nsile

A

B

A = heavy grindingB = light grinding

0.1 0.2Depth below surface (mm)

Tool surfaces exposed to fast temperaturegradients often show tensile stresses owing to theinitial heating of the surface followed by a coolingand heat transfer to the bulk of the tool. If theheating of the surface is sufficiently high, phasetransformations may take place which can resultin complex combinations of tensile and com -pressive stresses.


Today, there is a wide range of different techniquesfor tool manufacturing. The resulting surfaceconditions vary greatly from method to methodowing to the very different characteristics of themanufacturing techniques. This in turn impliesthat performance and mechanical properties oftools depend also on the type and quality of themanufacturing technique. In this chapter we willexplore some of the most common techniquesrelated to tool manufacturing and their influenceon tool performance. The high hardness of toolmaterials sets certain constraints on manufac -turing and this limits the choices. In this part, onlytechniques appropriate for tool materials will becovered.

EDMElectrical Discharge Machining (EDM) is one ofthe few methods available to machine highhardness materials. EDM removes material byspark discharges which raise the temperature highenough to melt or even vaporise the uppermostlayer of the work piece. The spark discharges aregenerated from a solid tool-shaped electrode, as indie sinker EDM, or from a continuously movingwire, as in electrical discharge wire cutting(EDWC). The major disadvantage with EDM isthe high temperature at the surface of the workpiece, which gives rise to melting, resolidificationand subsequently rehardening of the work piecesurface. The rehardened surface is very brittle andmay contain additional defects such as surfacecracks and porosity. For that reason, a posttreatment of the EDM machined surface is alwaysneeded for high performance tools in order toremove the heat affected layer. The post treatmentmay include grinding, tempering and/or shotblasting.

Influences of tool manufacturing techniques on the surface condition

High performance, high hardness tool materials are sensitive to surface quality. In the present case this is illustrated by a large variation in bend strength between materialprepared using different tool manufacturing techniques.

BEND STRENGTH VS. SURFACES FOR ASP 2023 AT 66 HRC

7000

6000

5000

4000

3000

2000

1000

0

Bend

stre

ngth

(MPa

)

Homoge

neous

surfac

e, Ra= 0.2

µm

Homoge

neous

surfac

e, Ra= 0.2

µm, co

ated

Ground,

R a= 0.2

µm

Ground,

R a= 0.2

µm, c

oated

Wire-EDM 10

µm*

Ground

paralle

ll, Ra= 2

µm

Ground

perpen

dicular,

R a= 2

µm

Vacuum

heat t

reatment

*

Salt ba

th heat

treatm

ent*

Nitriding

(25 µ

m)*

11

*No further treatment of the surface


An uppermost melted and resolidified layer, often referred to as the “white layer”, is always found after EDM-machining. The thickness of the layer can vary owing to, for instance,the discharge energy. For HSS, this uppermost layer should be removed as it contains very brittle non-tempered martensite which will lower the properties and the performanceof the tool.

Depending upon the type of EDM machining and strength of thedischarge energy, the final surface quality and hence the finalmechanical properties of the ASP steel will vary. From the figureto the right one may conclude that die sinker EDM has moresevere influence on surface quality than wire cutting EDM(EDWC) and that increased discharge energy decreases thesurface quality and hence the bend strength.

In Electric Discharge Machining (EDM), the temperature at thesurface layer can be very high.

In this example, the temperature has been high enough to meltthe outermost area of the surface, further down in the materialto form a rehardening zone and finally cause an over temperedzone.

This heat affected zone, which in total stretches some hundredµm down in the material, has a severe influence on themechanical properties as the resolidification cracks in the meltedzone provide excellent fracture initiation points, the rehardeningzone contains very brittle untempered martensite and theovertempered zone gives a softening of the tool.

Clearly, it is very important to remove the entire heat affectedzone before the tool can be used.

EDM 4 µm EDM 9 µm EDM 18 µm

EDM EDM EDWC EDWC Groundfine fine medium fine Ra = 0.2 µm

0.07 J 0.02 J

400 600 800 1000 HV

5000

4000

3000

2000

1000

0

Melted andresolidified layer

Rehardened layer

Over temperedlayer

Unaffectedmaterial

20 µm

12

Bend

stre

ngth

(MPa

)


Surface finish is often worsened by salt bath heat treatment. This image showssevere oxide penetration.

Surface quality is, in most cases, better after vacuum than salt bath heat treatment.However, in this particular case, the tool has not been cleaned properly and oil atthe surface caused a carburisation of the surface during vacuum furnace heattreatment. Effective cleaning of the tool surface is important or the surface qualitymay be ruined.

Heat treatmentAll high speed steels are heat treated and the heattreatment operation can have a major influence onthe surface finish if not carried out properly. Heattreatment, or more specifically hardening, iscarried out either in vacuum or salt bath furnaces.Vacuum furnace heat treatment, if carried out in acorrect manner, should have a minor influence on

the surface quality. Even so, it is very important toclean the tool thoroughly prior to vacuum heattreatment, as dirt on the surface may give raise tocarburisation of the surface and thus a brittlesurface layer. Salt bath heat treatment is moreaggressive to the surface, and corrosion and oxidepenetration are commonly observed. Generally,surface related heat treatment problems are madeworse with increasing temperature. Often the toolis ground after heat treatment. However, surfacedefects introduced at hardening can go deep intothe material and are sometimes not removedcompletely by grinding.

GrindingGrinding is one of the most common methods usedin the manufacture of tools and their surfaces.When the grinding operation is carried outcarefully, its only major influence should be on thesurface roughness. However, grinding is very oftenforced for economic reasons such as higherproductivity, and this may cause burning of thesurface owing to too high a temperature at thesurface or to the generation of high residualstresses in the surface. Burning of the sample cangive rise to a soft surface owing to over tempering(surface temperatures between 500-900 °C) or abrittle surface owing to rehardening (surfacetemperatures above 900 °C). Both these examplesgive an increased risk of tool failure and hence areduced tool life. Burning of the surface can berevealed from the oxide layer that is often foundon a surface that has been at too high atemperature. In this case the colour of the oxidelayer may be used as an indicator to thetemperature. However, it is very important torealise that the heat affected zone stretches muchfurther down into the material than the thicknessof the oxide layer would indicate. So removing theoxide layer only will not result in a surface of goodquality.

20 µm

50 µm

13


Fracture is often initiated at grinding marks. In thisfigure it is shown by the influence of surfaceroughness on impact strength. However, it is thelargest grinding mark which sets the strength ofthe material. But a high Ra value generallyindicates an increased probability of large grindingmarks, and hence we observe a decrease inimpact strength with increasing Ra value.

A rough surface after grinding may lead to micro-chipping starting from grinding marks at the tooledge and consequently a reduced tool life owing toaccelerated tool wear. Also, fracturing of a tool isoften initiated at large grinding marks. Inconnection to this, one should stress that it isdangerous to judge the surface finish from ameasure of the average surface roughness only, for

instance an Ra value. It is the largest grinding markat that part of the material which is under highmechanical load, and not the average surfaceroughness, which is important for the strength ofthe tool material. So the Ra value can be used onlyas a measure of the probability for fracture at aspecific stress level and not as an absolute measuredirectly related to strength.

20

18

16

14

12

10

8

6

4

2

0

Impa

ct str

ength

(J)

F

200 µm

0 0.5 1.0 1.5 2.0Surface roughness, Ra (µm)

14

In real life, low impact strength owing to poor surface quality oftenimplies micro-chipping or even fracture of the tool edge. In the presentexample micro-chipping has started from grinding scratches at the tooledge in milling application.


The drill above to the left has beenground with improved coolingwhereas the one in the bottom hasbeen ground with a standard coolingnozzle. The non-optimum cooling hasresulted in tensile stresses at thesurface which, after etching, arerevealed as large surface cracks.

Burning of the surface during grinding may give rise to a rehardened zone of very brittle non-tempered martensite at the surface. In the present case with ASP 2030, this was revealedfrom an increased frequency of micro chipping for tools with burned surfaces in comparisonto tools with non-burned surfaces.

0.1

0.08

0.06

0.04

0.02

0

Micro

-chipp

ing (m

m2 )N B

Rehardened (hard but very brittle)

Overtempered (soft with low strength)

Original (hard with high strength)

B = Burned as in the figure to the left

N = Normal non-burned surface

0.4 mm

15

The grinding operation is of great importance for the perfo rmance oftools made of high hardness tool materials such as ASP steels. For thatreason, Erasteel has produced a brochure covering the essentials ofgrinding in general and with focus on the grinding of HSS, both con -ventional and ASP, in particular.


Techniques for deburring and surface finishingTo utilise fully the performance of HSS materials itis important to remove burrs from edges andsmooth working surfaces before using the tool. Ifburrs are not removed and the surface finish is notoptimised, there is a greater risk of edge chippingand crack initiation, which will lower the per -formance of the tool. The extent of deburring andsurface finishing required is dependent on theapplication.

Some of the most important deburring and surfacefinishing techniques are described below.

Mass finishing

Abrasive tumbling is a general expression whichencompasses a wide variety of different massfinishing or deburring techniques using the samebasic principle. It is a low-pressure process per -formed by abrading and deforming the work piecesurface by placing it in a moving chamber withabrasive media in a compound, together with aliquid. Although the method is relatively slow,costs are low and operation simple. In addition,for some of the mass finishing techniquesextremely smooth surfaces can be achieved.

Abrasive tumbling deburrs the edges and im provesthe surface roughness by a sliding and rollingeffect of the abrasive media. This action usuallyresults in deburring of all edges and generates asub sequent improvement in surface finish. Inaddition, undesirable residual tensile stresses canbe lowered, eliminated or even changed into com -pressive stresses. A drawback of mass finishingtechniques is that the process affects all surfaces ofthe tool. It is not possible to give preferentialtreatment to specified areas. In addition, owing tothe rolling effect on the surface it is not alwaysrecommended to PVD coat tumbled surfaces. Thisis because the rolling effect may encapsulateporosity or media used in the finishing process,which could be detrimental to coating adhesion.

16

A close up of the cutting edge of a tap after grinding. Dirt, burrs and grindingscratches can be seen.

dirt

flute

land

forwardflank

flankface

flute

flute

10 µm

1 mm

100 µm


work piece. The shots will produce a slightsmoothening effect on the surface, as they have theability to deform as well as break burr. Themethod also has a cleaning effect as the steel shotsadhere easily to oxides and other contaminants onthe surface. In the case of sharp edges it isimportant to use soft steel grades which do notstrain harden, otherwise there is a risk of bucklingand deformation of edges. In such cases, it isrecommended to use plastic shots or to changetechnique to, for example, water jet deburring.

Particles for removal of materialMedia containing hard particles like aluminiumoxide and silicon carbide usually consist ofangular particles, which provide an abrasivecutting action on most work pieces. Theseparticles are used primarily to remove somematerial from the surface, typically less then amicron. The blasting has to be well controlled asuncontrolled blasting with abrasive media canquickly remove far too much material.

Brushing

Brushing operations can today be used for manydifferent purposes, i.e. deburring, cleaning, edgepreparation and polishing or texturising ofsurfaces.

For deburring of HSS it is possible to use carbonsteel wire, stainless steel wires and nonferrousmaterials like brass and nylon. Also vegetable

Blasting

Blasting with particles is used widely for removingburrs from edges, cleaning, smoothing of surfacesand removing sharpness of edges. Abrasive mediaused in blasting can be:

•

•

•

The rule of thumb is that a significantly hardermedia than the work piece will have a cuttingeffect and a softer media a more deforming andbreaking effect on burrs and peaks on surfaces.

In the case of applying a PVD coating on HSScomponents, it is important to deburr as well as toremove and smooth surfaces. Therefore it isrecommended to do a two stage blastingprocedure: first deburring by using coarser and,preferably, relatively soft particles and secondly,surface finishing using micro blasting with smalland much harder particles.

Particles for deburringToday glass beads, plastic shots and steel shots areused for deburring of HSS tools.

Glass beads can sometimes fracture and fragmentscan contaminate the surface as they are easilyembedded in the deformed surface layer. The ruleof thumb is that the pressure used when blastingwith glass beads should not exceed 4 bar in orderto lower the risk of fracturing the glass beads.

Plastic shots are used mainly for cleaning ofmoulds for injection moulding. Owing to the lowhardness and weight, plastic shots are also usedfor mild deburring of HSS tools. However, if burrsare relatively large then plastic shots will not beable to remove them completely.

Steel shots are mainly made of stainless steel orother soft steel grades, which are softer than the

17

Above is a cutting edge after edge preparation. Most edge and surface preparationtechniques do not remove grinding burns.

Soft or hard (from soft and gentle sodiumbicarbonate to extremely hard ceramic particles)

Round or angular (Angular particles cut. Roundparticles deburr and smoothen)

Big or small particles (few microns to 1/10 ofmillimetre in diameter)

Grinding burn

10 µm


fibres like “Tampico” may be suitable fordeburring. Brushes made of these materials canalso be used in combination with abrasives pastes.It is also possible to use nylon filaments containinghard particles like aluminium oxide or siliconcarbide for deburring or surface finishing. Theadvantages with these brushes are that theabrasive process can be run dry and be used to givepreferential preparation of edges.

Magnetic finishing

Magnet finishing is a technique, which can be usedfor surface finishing as well as for deburring ofHSS details. The work piece is placed in betweentwo magnet poles and the gap between the twopoles is filled with magnetic abrasive media. Eachgrain of the media is made up of an abrasive and amagnet component. The magnet component holdsthe powder in the magnet field while the abrasiveperforms the grinding or polishing action.

Smooth surfaces can be achieved in combinationwith edges free from burrs. The surface createdusing magnet finishing offers a good base for aPVD coating, provided that the rolling action ofthe abrasives have not been exaggerated, as thiscan give unnecessarily high stresses in the surface,which may, in turn, reduce the strength of theinterface between the PVD coating and the HSS.

SEM micrographs of a flute and an edge of a HSS drill with optimal surface and edge preparation. The surface roughness in the flute is approximately 0.08 µm Ra.

Abrasive Flow Machining (AFM)

AFM is a deburring, edge preparation andpolishing process, which involves extruding anabrasive semi-solid media through a work piecepassage. This process is suitable for holes andcomplex geometries and if mirror finish of the toolis required.

The AFM technique consists of the machine, thework piece and the abrasive media. By choosingthe most appropriate abrasive media it is possibleto abrade the vast majority of materials usingAFM. AFM is a versatile and controllable finishingprocess because the work piece is held stationaryand the abrasive media is directed to, and oftenthrough, the passages of the tool to be finished bythe extrusion. The surface produced can be ofhighest quality and smoothness.

Water jet deburring

High-pressure jets (about 200 MPa), can be usedto deburr both metallic and non-metallic parts.The advantage of this method is that it leavessharp corners with no increased radius. As cycletimes are significantly longer for burrs of largersizes, the process is best suited for fine deburringapplications. The process can be automated easily.

18

20 µm100 µm


Deburring Area Pcs*/ Example of method of interest hour application

Mass finishing external cylindrical parts 60-120 machine elements, drills, mills

Blasting (glass beads) external edges, surface finish 60-120 punches, drills, taps, etc

Blasting (plastic shots) external edges 60-120 taps, precision parts, etc

Blasting (steel shots) external edges 60-120 larger machine elements, larger cutting tools, etc

Micro blasting external edges, surface finish 60-120 all kind of cutting tools, machine elements, etc

Brushing external edges, internal holes, blind features 60-120 machine elements, drills

Magnetic finishing high finish parts with small burrs 50-100 dies, complex parts, drills, taps, end mills

Abrasive flow machining intersecting holes, machining, high finish 1-10 dies, medical parts, etc irregular contours,

Water jet deburring external edges, thin burrs 50-250 plastic moulds, engines, etc

Electrochemical polishing high finish parts 20-200 precision parts, valves, etc

Thermal energy method external burrs and some internal thin burrs 50-100 automotive parts, gears, castings, aircraft components, etc

Electrochemical polishing (ECP)

In electrochemical polishing, a solution such asphosphoric acid is combined with an electricalcurrent to remove burrs. Burrs and other surfacepeaks attract the electrical power, resulting ingreater material removal on these areas than onplane surfaces.

This process is most often used in high volumeproduction for removing of small burrs onprecision parts.

A problem with ECP is uneven etching orpolishing of high alloyed materials like HSS.Different phases are affected differently by thepolishing solution. We only recommend ECP forASP 2012 and ASP 2017. If ECP is carried outsuccessfully the surface obtained is an extremelygood base for a PVD coating.

Thermal energy method

In the thermal energy method (TEM) the workpiece is placed in a gas filled chamber (pressurevessel). The gases are then ignited and the intenseheat (which last for only microseconds) burns orevaporates thin burrs and surface peaks. Usingoptimal conditions, only a small radius will be lefton the edge. The TEM method is ideal to deburrblind and intersecting holes and internal surfaces,which are difficult to reach, by other methods.

Although the temperature of a work piece seldomexceeds 150 °C, the temperature at the surface willbe much higher, often so high that re-hardeningwill occur. A thin oxide layer is often formed onthe surface. Therefore, it is recommended to com -bine TEM with, for example, micro blasting, inorder to remove oxide and damaged surfacematerial. This is especially important if the toolsare to be PVD coated subsequently, otherwise itwill probably cause problems with coatingadhesion.

* Estimated treatment of a HSS drill 8 mm diameter and length 100 mm using a standard equipment.

19


No. Technique Parameters

1 Grinding Normal tool grinding.

2 Blasting with a water A technique used for deburring.atomised iron powder

Particles: min 99.0 Fe, max 0.011 O, max 0.010 C, typical particlesize: 150-212 6.0%, 75-150 39%, 45-75 31%, -45 24%. Pressure: 3 bar.

Process time per tool: 30 sec.

3 Drag finishing (tumbling) A patented technique, Walther Trowal GmbH, used for deburring,polishing and edge preparation.

The tools are attached to a special fixture and “dragged” in a planetarymovement through a bed of grinding and polishing media.

Process time: 60 sec.

4 Brushing A technique used for deburring and edge preparation.

Brushing using abrasive nylon filaments, 320 mesh SICBrushing speed: approximately 250 m/min.

Processing time per tool: approx. 10 sec.

5 Magnet finishing A patented technique, KMM Oberflächenbearbeitung GmbH, used fordeburring, polishing and edge preparation.

The tools is rotated in a magnetic abrasive powder while itsimultaneously is held between two magnetic poles.

Process time per tool: approx. 60 sec.

6 Thermal energy machining A patented technique, Extrude Hone, used for deburring, polishingand edge preparation.

The tools are sealed in a pressurized chamber with a mixture of anexplosive gas and oxygen. The gas mixture is then ignited by a sparkplug, which creates an intense, rapid burst of heat. Burrs and flash,because of their high ration of surface area to mass, quickly rise toa temperature well above their auto-ignition point and burst into flames.

Process time per tool: approx. 10 sec.

Comparison of different edge preparation techniques

A number of different deburring techniques whereinvestigated for edge preparation of taps anddrills. The resulting edges on the drills can be seenin the photos on the right. Description of thetechniques and the parameters used are found

20

below. With other parameters and types of toolsthe results would be different. The methodscleaned the surface of loose particles and surfaceburr, but none of them improved the surfaceroughness in any significant way.


The corner of the cutting edge of the drill after edge preparations.

21

flute

flank face

1

3

5

2

4

6

margin withleading edge

100 µm

100 µm

100 µm

100 µm

100 µm

100 µm

Grinding (reference)

Drag finishing (tumbling)

Magnetic finishing

Blasting with iron powder

Brushing with abrasive nylon filament

Thermal energy machining


COATINGSThe function of PVD coatings in machining applications

When the tempering temperature of the HSS isexceeded, the hardness decreases quickly andrapid wear occurs.

PVD coatings are very hard and chemically stablematerials, which give a relatively low frictionagainst most materials, provided that coating isdeposited on a smooth substrate. This decreasesthe cutting force compared to an uncoated toolconsiderably, for the same cutting data. The heatgeneration is consequently lower on the coatedtool surface.

The coating also act as a stable thermal barrierbetween the hot work material (chip) and the toolface, protecting the tool from the heat.

Both these effects imply in turn, that for the sameheat generation as in the uncoated case, the cuttingspeed can be increased without exceeding thetempering temperature. Usually the cutting speedcan be increased 2-3 times for the same tool life.

In addition the coatings are extremely wearresistant when sliding against other materials.

Coating types

There are a wide variety of PVD coatings on themarket. The most popular are nitride basedcombinations of the elements Ti, Al, Cr and C.Also, more advanced multilayered coatings can befound as well as coatings customised to combinewear resistance with a very low friction againstmost steels.

When choosing a PVD coating it is important toremember that a broad range of standard tools arenot subjected to the most extreme workingconditions, for example dry machining or highspeed machining. Accordingly these tools will notrequire high-performance coatings. Instead moretraditional and reliable coatings such as TiN orTiAlN can be used with excellent results. However,if the application is severe, utilisation of the bestcoatings on the market can, provided they areapplied optimally, give exceptionally good,predictable results and an extended number ofdetails produced.

Today’s most common PVD coatings and their most important properties and application area.

22

Coating Micro hardness Coefficient Residual Oxidation onset Typical applicationmaterial (HV 0.05) of friction stress (GPa) temperature (°C)

TiN 2200 0.4 -2.5 600 General purpose coating, injection moulding of plastic material

TiCN 3000 0.4 -3.5 500 For tools subjected to high mechanical load, e.g. milling andforming of steels

TiAlN 3000 0.4 -3.5 800-900 Today’s best general purpose coating

CrN 2200 0.4 -2.0 750 For cu-machining and injection moulding of plastic materials

WC/C 1000 0.2 -1.0 300 A low friction coating for machine elements

DLC Various DLC processes are available on the market. DLC coatings typically are used for precision components such as automotive parts like tappets and injection parts. DLC coating can posses hardness values from 1000 HV to 5000 HV and the friction against steels is normally 0.1-0.2

TiAlSiN 3600 0.4 not known 1100 High performance coating for dry machining or high speed machining of steels

AlCrN 3200 0.35 -3 1100 High performance coating for dry machining or high speed machining of steels

Multilayers Various multilayer processes are available on the market. Typically a multilayer is combined of existing coatings, e.g. TiN + TiAlN. Normally it is claimed that multilayer coatings add toughness to the coating


How to optimise the performance of coated tools?

Substrate material – material carrying the coatingIn addition to coating adhesion, the hardness atworking temperature of the substrate materialcarrying the PVD coating is the most importantparameter. The rule is that the higher the hardnessof the substrate the better the load carryingcapacity and hence support to the coating. Thesurfaces have to be extremely smooth if a newharder HSS grade or even if a harder coating is tobe utilised in your application, in order tomaintain toughness.

The high carbide content in HSS is beneficial notonly in terms of hardness but also to increase thewear resistance of the tool. This can be ofimportance where PVD begins to wear locally, asthe higher the wear resistance of the exposed HSSmaterial, the longer it will take before total failure

will occur. In addition, MC carbides also promotecoating adhesion since they are very hard and havea crystalline structure similar to that of most PVDcoatings.

The surface carrying the coatingA unique feature of PVD coatings is thecompressive residual stress accumulated in thecoating after deposition. For some coatings thiscan be as high as 5 GPa. Because of it, the coatingcan easily flake from sharp edges and aroundporosity and other surface defects. Porosity in thecoating can, for example be found when thecoating is deposited on a non-optimised substrate,i.e. a substrate which contains a large number ofinclusions, e.g. oxides. In the worst case, a hugeconcentrate of oxides present on the surface of thesubstrate can suppress surface deposition entirelyor in part.

The diagram above shows how the substrate hardness influences the coatingadhesion. A diamond is pressed against a coated surface with increasing load. Theload at which the coating breaks is recorded. The critical load also depends on otherfactors, like: type of coating, surface roughness, direction, and many otherparameters.

To put a coating on top of a rough tool surface is not a good idea. First of all, theroughness is not improved by the coating, as the coating is as thin as a few µm.Secondly, the coating may fall off at the peaks and in the valleys of the roughsurface, owing to high compressive stresses in the coating. These unprotected areaswill show accelerated wear and in addition provide starting points for fractureinitiation. Thus, the combination of high performance tool materials, such as ASPgrades, and modern high performance PVD coatings requires also high qualitysurface preparation.

20 µm

23

100

80

60

40

20

0

Critic

al loa

d (N)

500 700 900 1100Hardness of substrate (HV)

Increasing load


close to the aimed surface roughness and then usean appropriate technique of edge preparation (seeprevious chapter).

It is important to remember that the function ofthe coating depends not only on surface roughnessbut also the quality of the surface. For example, ifthe grinder has burned the surface and the depthof the remaining damaged zone exceeds onemicron, a weakening of the interface will takeplace. The same argument can be used for sparkeroded surfaces.

Since PVD coatings work best if they are appliedon clean surfaces of high performance materials, itis easy to understand that defects like inclusions ofe.g. oxides, burning as a consequence of excessivegrinding or process residues from polishing willhave a negative impact on coating adhesion. Someof these defects can be removed by micro blastingprior to deposition of the PVD coating. Forpolished surfaces, where micro blasting can not beused, the polishing procedure must be optimisedso that no residues are left in the surface. Forsoldered materials it is important to choose asoldering material that does not contain metalswith high vapour pressure, e.g. cadmium, lead orzinc.

Surface preparationTo achieve all the benefits possible from a PVDcoating on a HSS material, a number of para me -ters must be checked prior to coating of the part.Firstly, all surfaces must be free from oil and/orgrease. This is normally no problem as the PVDcoating companies have processes, which take careof it. It is however a good practice to clean thetools directly after grinding as dried in dirt isdifficult to remove later on.

Burrs on cutting edges must be removed beforecoating deposition. Any remaining burr, howeverslight, is detrimental. The presence of even a verysmall burr will lower the strength of the cuttingedge, leading to micro chipping and consequentlycoating removal. The rule of thumb is to aim for azero burr height and if possible also apply a smallrounding of the cutting edge, in order tostrengthen the edge. The size of the roundingdepends on the application, but typically a 5 to 20µm rounding will give a significant improvement.

The smoother the surface of the HSS, the betterthe PVD coating will work as the load is betterdistributed on the surface. In order to obtainoptimal surfaces, it is a good idea to first grind

An uneven surface will locally give high loads on and underneath the surface. Thiswill lead to chipping of the substrate or the coating.

This tap has not been cleaned before the deburring operation. The dirt remains afterthe treatment.

24

1 mm


•

•

•

•

•

•

Preparation of PVD coated surfacesMany of today’s PVD coatings are deposited usingtechniques, which generate relatively roughsurfaces. The main reason for this is that thetechniques produce small droplets, which areentrapped in the coating, which normally increasethe surface roughness. It is important to note thatthe protruding droplets are as hard as the rest ofthe coating and must therefore be removed inorder to avoid that they act as a metal file in theapplication. Hence a so-called post treatment, i.e.a smoothening process of the coating, is oftendone by the coating companies to remove thedroplets. Such post treatment is today available atmost coating companies but is normally notoffered as a standard solution.

Manufacturing for best performance (deburring, surface topography, surface roughness)

There are some principles to follow for thesuccessful utilisation of PVD coated tools.In case of cutting and punching tools:

25

NitridingNitriding is used for the production of casehardened surface layers. The hardened surfacelayer is effective in reducing wear, both abrasivewear owing to the high hardness of the nitridedlayer, and also adhesive wear owing to the highchemical stability of the nitrided layer. A nitridedlayer can also improve the corrosion resistance ofa tool. However, on the negative side, the nitridedlayer is very brittle and provides an ideal site forcrack initiation and therefore a reduction in thetool edge strength. The brittleness of the nitridedsurface is due to nitride films found in grainboundaries of the HSS material and/or theoutermost layer of pure nitride - the so-called‘white layer’. Very often, surface cracks are foundin the nitrided layer and this is particularly true fortools for which the ‘white layer’ has not beenremoved. Thus, for high speed steel it isrecommended that the nitrided layer is no thickerthan a few micrometers.

Tool parts can be nitrided by various processessuch as gaseous mixtures, salt baths, plasmas etc.The characteristics of these methods are quitedifferent; for instance, the temperature at the toolsurface may vary from 400 to 600 °C. Generally,longer times and higher temperatures generate athicker nitrided layer and hence higher wearresistance. However, high temperature can alsoyield over-tempering, and hence softening, of thetool surface. Therefore, tools made of HSS withthick nitrided layers have significantly reducedmechanical properties and surface cracks, microchipping and fractures are often observed in suchtools.

Always aim for zero burr height at all cutting edgesand an edge radius of 5 – 20 microns.

Aim to achieve a surface roughness (Ra-value) below0.2 µm, preferably 0.1 µm.

No damaged material should remain just under thesurface, i.e. there should be “ideal” HSS material allthe way up to the surface.

Always protect PVD coated HSS from corrosion usingfor example environmentally friendly lubricants.

All PVD coated HSS tools should be handled with care,as edges are very sensitive to chipping.

In the case of forming tools larger edge radius areacceptable, although surface roughness (Ra-value)should be 0,1 µm or better.


Pictures of a nitrided zone. In the high magnification picture on the bottom thenitride films in the grain boundaries are seen as white lines. These nitride films makethe material extremely brittle and hence lower the strength of the tool considerably.

Section of a tool that has been peened. In this case the peening has been too hardand cracks at the surface of the tool can be observed. These cracks will act asfracture initiation sites and lower the strength of the tool.

In this case the marking has been carried out by stamping and placed at a part ofthe tool which is under high mechanical load. The mark has acted as an initiationpoint for fracture and lowered the total strength of the tool.

PeeningPeening is one of the final operations inproduction of, for instance, drills and is carriedout in order to reduce residual stresses and tostraighten the tool. However, it is important thatthe peening strokes are not too hard as it mayintroduce surface cracks and marks which willlower the strength of the tool.

300 µm

30 µm

300 µm

50 µm

26

MarkingPrior to final sale, the finished tool is often markedin some way. If the marking is made mechanically,e.g. stamping, the marks may easily act asinitiation points for fracture. Preferable, themarking should be made in a way which minimisesthe destructive influence on the surface, such aslaser marking, or be placed at a part of the toolwhich is under low mechanical stresses.


Technique Problem Possible cause Solution

Heat treatment Decarburisation Salt bath heat Add regeneration agent to thetreatment salt bath prior to heat treatment.

Carburisation Grease or oil at Cleaning of the tool prior tosurface in vacuum heat treatmentheat treatmentNon dissolved All regeneration agent must beregeneration agent in dissolved before the tools aresalt bath put into the salt bath

Oxidation Oxidising media Change to vacuum heat(salt or air) treatment

EDM Thick white Melting of the surface Reduced energy will give aowing to high energy thinner white layer.layer,in spark discharges Remove white layer by, forsurface cracks

instance, grinding or electro-chemical polishing.

Grinding* Residual stresses Too high load and / Decreased stock removal rateor temperature at the Larger grit size of wheelsurface More open dressing

Softer wheelBurrs Too high load Lower stock removal rate

Dull wheel Sharper (dressed) wheelBurning Too high temperature Improved cooling

at the surfacePoor surface finish Too coarse wheel Finer dressing

Too soft wheel Smaller grit sizeToo low wheel speed Use harder wheel

Decrease metal removing rateIncrease wheel speed

Loss of form Too soft wheel Harder wheelToo low wheel speed Decrease metal removal rate

increase wheel speed

Coating Coating fall off Poor surface roughness Improved grinding,Dirty surface see above.during use of(oxidation) Use of surface preparationtool

technique such as blasting,polishing, electrochemical polishing in order to reduce surface roughness and remove oxide layer.

Nitriding Micro-chipping or Insufficient strength Decreased thickness ofowing to thick brittle nitrided zonefracture of toolnitrided layerduring use

Marking Fracture initiated Marking acts as fracture Place the marking at a position initiation point where the stresses are lower.at marking

Use a less destructive technique,such as laser marking

* See also the educational brochure about grinding produced by Erasteel

Trouble shooting

27


Head OfficeTour Maine-Montparnasse 33, avenue du Maine 75 755 Paris Cedex 15FRANCETel: +33 (0) 1 45 38 63 00Fax: +33 (0) 1 45 38 63 30Email: [email protected]

SALES ADDRESSES

Boonton

Romeoville

São Paulo

Production Plants

Service Centers

Sales Offices

Paris

SheffieldWarrington

VikmanshyttanLångshyttan

SöderforsDüsseldorf

Chalon

Champagnole

Milano

Mumbai

Commentry

Barcelona

Tokyo

ShanghaiTianjin

Taipei

Seoul

NORTH AMERICAErasteel Inc. 95 Fulton StreetBoonton, N.J. 07005-1909USAToll free: (1) 800 969-6444Fax: +(1) 73 335 8420

Romeoville Services Center 1351 Enterprise Drive - Romeoville IL 60446 - USA Toll free:(1) 800 365 1152Fax: (1) 630 378 1965

SOUTH AMERICAEramet Latin AmericaAvenida Tucunaré 1086Tamboré - Barueri 06460-020 SPBRAZILTel: +(55)11 4689 6744Fax: +(55)11 4195 8408

FRANCEErasteel S.A.S.Tour Maine-Montparnasse 33, avenue du MaineF - 75755 Paris Cedex 15Tel: +(33)1 45 38 63 41Fax: +(33)1 45 38 63 50

ITALYEramet Italia s.r.l.Viale Leonardo da Vinci, 97200 90 Trezzano Sul Naviglio (Mi)Tel: +(39)02 48 42 611Fax: +(39)02 48 46 32 50

GERMANYErasteel GmbHHocksteiner Weg 30 D- 41189 Mönchengladbach Tel. +(49) 2166 9459 0 Fax +(49) 2166 9459 21

GREAT BRITAINEramet Alloys UK LTDUnits 4 & 5, President BuildingsSavile Street East Sheffield S4 7UQ Tel: +(44)114 261 0410Fax: +(44)114 261 7797

SCANDINAVIAErasteel Kloster ABSE-815 82 SöderforsSWEDENTel: +(46)293 543 00Fax: +(46)293 307 39

SPAINErasteel Commentry EspañaCami Font de les Canyes, 46Poligono industrial Can Petit08227 TerrassaTel: +(34)937 839 498Fax: +(34)937 852 214

ALL OTHER COUNTRIESErasteel Export SalesTour Maine-Montparnasse 33, avenue du MaineF - 75755 Paris Cedex 15FRANCETél: +(33)1 45 38 63 09Fax: +(33)1 45 38 63 30YOUR CONTACT

2010-04This document is for information only and does not create any binding contractual obligations.

JAPANEramet International S.A.S. Tokyo BranchJimbocho NK Building 9F2-7, Kanda Jimbocho - Chiyoda-KuTokyo 101-0051Tel: +(81)3 3265 3931Fax: +(81)3 3265 3932

CHINAEramet China Limited RM 01-02, 26/f Aurora Plaza No 99 Fucheng Road Pudong 200120Tel: +(86)21 61 00 61 61Fax: +(86)21 61 00 61 60

KOREAEramet Korea701 F7 Unik B/D,706-13Yeoksam-Dong, Gangnam-Gu,Seoul (135-918)Tel: +(82)-2 557 4813Fax: +(82)-2 557 4814

TAIWANEramet International S.A.S. Taiwan Branch12F-1C, No. 207 Tun Hwa North RoadTaipeiTel: +(886)22 545 1228Fax: +(886)22 545 1219

INDIAErasteel India Private Ltd709, Swastik ChambersSion Tromblay RoadChembur, Mumbai 400 071Tel: +(91)22 25 22 71 10Fax: +(91)22 25 23 29 61


valerie.perdu

Texte tapé à la machine

TB_ 04_EN_V0_2010

erasteel, surface, eng.2010-04:erasteel, surface, eng...surfaces influence of surface quality on...

Documents