thermal spraying coatings

8/16/2019 Thermal Spraying coatings

1/40

Thermal Spray Coatings


2/40


3/40

Density

•Composition

•Stiochiometry

•Structure

•Morphology•Thickness

•Adhesion

•Hardness

•Elastic modulers•Porocity

•Tensile strength

•Oxide content

•Surace inish

Coating characteristics


4/40


5/40


6/40


7/40


8/40


9/40


10/40


11/40

Flame Spraying!lame spraying is the oldest o the thermal spraying processes" A#ide $ariety o materials can %e sprayed %y this process& and the$ast ma'ority o components are sprayed manually" !lame spraying

uses the heat o com%ustion o a uel gas (usually acetylene orpropane) and oxygen mixture to melt the coating material& #hichcan %e ed into the spraying gun in t#o orms& either po#der orsolid #ire*rod" The t#o consuma%le types gi$e rise to the t#oprocess $ariants+ powder flame spraying and wire flamespraying.,n the case o the powder flame spraying process, po#der is eddirectly into the lame %y a stream o compressed air or inert gas(argon or nitrogen)" Alternati$ely& in some %asic systems& po#der isdra#n into the lame #ith air %y a $enturi eect& #hich is sustained%y the uel gas lo#" ,t is important that the po#der is heatedsuiciently as it passes through the lame" The carrier gas eedspo#der into the centre o an annular com%ustion lame #here it isheated and propelled to#ards the su%strate" A second outerannular gas no--le eeds a stream o compressed air around the

com%ustion lame& #hich accelerates the spray particles to#ardsthe su%strate and ocuses the lame",n the wire flame spraying process& the #ire eed rate and lamesettings must %e %alanced to produce continuous melting o the#ire and a ine particulate spray" The annular compressed air lo#atomises and accelerates the particles to#ards the su%strate"


12/40

The Plasma Spraying Process is %asically the spraying o molten or heatsotened material onto a surace to pro$ide a coating" Material in the orm opo#der is in'ected into a $ery high temperature plasma lame& #here it israpidly heated and accelerated to a high $elocity" The hot material impactson the su%strate surace and rapidly cools orming a coating" This process

carried out correctly is called a .cold process. (relati$e to the su%stratematerial %eing coated) as the su%strate temperature can %e kept lo# duringprocessing a$oiding damage& metallurgical changes and distortion to thesu%strate material"

The plasma gun comprises a copper anode and tungsten cathode& %oth o#hich are #ater cooled" Plasma gas (argon& nitrogen& hydrogen& helium)

lo#s around the cathode and through the anode #hich is shaped as aconstricting no--le" The plasma is initiated %y a high $oltage discharge#hich causes locali-ed ioni-ation and a conducti$e path or a DC arc to orm%et#een cathode and anode" The resistance heating rom the arc causesthe gas to reach extreme temperatures dissociates and ioni-e to orm aplasma" The plasma exits the anode no--le as a ree or neutral plasmalame (plasma #hich does not carry electric current) #hich is /uite dierentto the Plasma Transerred Arc coating process #here the arc extends to thesurace to %e coated" 0hen the plasma is sta%ili-ed ready or spraying theelectric arc extends do#n the no--le& instead o shorting out to the nearestedge o the anode no--le" This stretching o the arc is due to a thermal pincheect" Cold gas around the surace o the #ater cooled anode no--le %eingelectrically non1conducti$e constricts the plasma arc& raising its temperatureand $elocity" Po#der is ed into the plasma lame most commonly $ia anexternal po#der port mounted near the anode no--le exit" The po#der is so

rapidly heated and accelerated that spray distances can %e in the order o 23to 435 mm


13/40

Schematic of Flame Spraying System


14/40


15/40


16/40

Schematic of Arc Spraying System


17/40

Schematic of Plasma Spraying System


18/40

• Benefits of Thermal Spraying • Comprehensi$e choice o coating materials+ metals&

alloys& ceramics& cermets and car%ides"• Thick coatings can %e applied at high deposition rates"• Coatings are mechanically %onded to the su%strate6can

oten spray coating materials #hich are metallurgicallyincompati%le #ith the su%strate& e"g"& materials #ith ahigher melting point than the su%strate"

• Components can %e sprayed #ith little or no pre1 or post1heat treatment& and component distortion is minimal"

• Parts can %e re%uilt /uickly and at lo# cost& and usuallyat a raction o the price o a replacement"

• 7y using a premium material or the thermal spraycoating& coated components can outli$e ne# parts"• Thermal spray coatings may %e applied %oth manually

and automatically"


19/40

• Flame Spraying (FLSP)+ !8SP #as the irst thermal spraying process" ,t use29:5 ;C (3555 ;!) oxyacetylene lame to melt the targets #hich may %epo#ders& rods& or #ires"

• Plasma Arc Spraying (PSP)+ Similar to lame spraying& PSP ho#e$erproduces 4:&:35 ;C (


20/40

Process Target Form Energy Source

Flame spraying (FLSP) powder, rod, wire Oxyacetylene flame

Plasma arc spraying (PSP) powder plasma gun

Electric arc spraying (EASP) wire (motor driven) electric arc

etonation gun (d!"un) Powderspar# ignition of explosive gas

gun

$ig%!velocity oxy&fuel ($'OF) powderoxygen, %ydrogen, fuel, eg,

met%ane, comustion c%amer


21/40

• !usion hard facing is a process %y #hich #eldmaterials& #ith superior properties than the su%strate&are applied to the su%strate" Oten& two layers total < 1 :mm (4*@ 1 4*2 in) thick are applied to reduce the suracehardness dilution o the relati$ely lo#1cost steelsu%strates in the expansi$e co%alt1%ase alloys"

• Hard acing processes are $ery useul or impro$ing#ear and corrosion resistance to selected areas omachinery& such as cutting edges o earth1mo$ingmachinery" Common hard acing techni/ues include arc&

torch& and other processes"


22/40

$ardfacings are good for su)strate materials t%at are suita)le for welding, suc% as low!

car)on steel( *%e most common %ardfacing materials are nic#el alloys and iron&c%romium

alloys used in wear resistance and %ig% stress a)rasion( +ommon materials for %ardfacingare listed as follows

Metal Alloy Purpose

+o)alt!)ase alloys wear and corrosion resistance

+opper!)ase alloys re)uilding worn mac%inery parts

-ron c%romium alloys %ig% stress a)rasion

.anganese steel wear application

/ic#el!)ase alloys metal!to!metal wear resistance

*ool steel tooling, wear application

*ungsten car)ide %ig% stress a)rasion


23/40

• Adhesion

• 0ith the lo#er energy processes o lame and arc spraying&adhesion to the su%strate is considered to %e largely mechanicaland is dependent on the #ork piece %eing $ery clean and suita%lyrough" ?oughening is carried out %y grit %lasting andoccasionally& rough machining"

• 0ith the higher energy processes& %ond strengths are higher%ecause o the higher impact $elocities" Adhesion is impro$edthrough disruption o oxide layers on the su%strate andem%edding o the particles into the su%strate" There is somee$idence to suggest that a certain proportion o diusion %ondingtakes place" Surace preparation %y cleaning and grit %lasting isextremely important"


24/40

• E$amples of applications

• Corrosion protection o structures and components (e"g"&

%ridges& oshore platorms& 8"P"=" %ottles) #ith aluminum or-inc coatings" Aluminum is more expensi$e& %ut has resistanceto acidic gaseous atmospheres (e"g"& associated #ith theproducts o ossil uel com%ustion)& as #ell as neutral solutions&e"g"& salt #ater" inc has resistance to alkaline corrosion"

• ?eclamation o #orn shats& particularly o %earing areas #ith

4


25/40

• Oxyuel (H>O!) Spraying• The most recent addition to the thermal spraying amily& high $elocity

oxyuel spraying has %ecome esta%lished as an alternati$e to theproprietary& detonation (D1=F) lame spraying and the lo#er $elocity& airplasma spraying processes or depositing #ear resistant tungsten car%ide1co%alt coatings" H>O! spraying diers rom con$entional lame spraying inthat the com%ustion process is internal& and the gas lo# ates and deli$erypressures are much higher than those in the atmospheric %urning lamespraying processes" The com%ination o high uel gas and oxygen lo# ratesand high pressure in the com%ustion cham%er leads to the generation o asupersonic lame #ith characteristic shock diamonds" !lame speeds o2555ms14 and particle $elocities o :55G55ms14 are claimed %y H>O!

e/uipment suppliers" A range o gaseous uels is currently used& includingpropylene& propane& hydrogen and acetylene"

• Although similar in principle& potentially signiicant details& such as po#dereed position& gas lo# rates and oxygen to uel ratio& are apparent %et#eeneach system" The H>O! process produces exceptionally high /ualitycermet coatings (e"g"& 0C1Co)& %ut it is no# also used to produce coatingso metals& alloys and ceramics" ot all H>O! systems are capa%le o

producing coatings rom higher melting point materials& e"g"& reractorymetals and ceramics" The capa%ility o the gun is dependent upon the rangeo uel gases used and the com%ustion cham%er design"

• A li/uid uel (kerosene) H>O! system& has 'ust %een launched& #hich iscapa%le o much higher deposition rates than the con$entional gas1uelledunits"


26/40

• Examples o Applications H>O! spraying is a $ery recent processde$elopment& yet the high /uality o the coatings produced atcompetiti$e cost has already seen its introduction in a num%er o$ery signiicant industries" Potential applications o$erlap #ithplasma and D1gun spraying& particularly or 0C1Co coatings"

• • Tungsten car%ide1co%alt coatings or retting #ear resistance on

aeroengine tur%ine components"• 0ear resistant co%alt alloys onto luid control $al$e seating areas"• Tungsten car%ide1co%alt coatings on gate $al$es"

• >arious coatings or printing rolls& including copper& alumina&chromia"

• iCr7Si coatings (unused) or glass plungers"• iCr coatings or high temperature oxidation*corrosion resistance"• Alumina and alumina1titania dielectric coatings"

• 7iocompati%le hydroxylapatite coatings or prostheses"


27/40

Schematic of !igh &elocity #$yfuel (!F)

Spraying System

•


28/40

Comparison of Thermal Spraying Processes and Coating Characteristics

Process

Particle

Velocity

(m/s)

Adhesion

(MPa)

!ide

Content

(")

Porosity

(")

#eposition

$ate

(%g/hr)

Typical

#eposit

Thic%ness

(mm)

Flame 01 23 41546 41546 4541 1(7541

Arc 411 41581 41571 6541 9591 1(7541

Plasma 7115811 715:1 458 453 456 1(757

$'OF 9115311 ;:1 457 457 456 1(757


29/40

Process Fuels that can &e used ther gases

$'OF Acetylene, %ydrogen, propylene, propane, orli=uid #erosenedepending on materialtype

Oxygen and argon

Arc spraying /ormallycompressed air utcan use nitrogen orargon

Flame spraying .ainly acetylene, utsometimes propanedepending on material

Oxygen

Plasma spraying Argon and%ydrogen


30/40


31/40

Detonation Thermal Spray 'oating Process


32/40

Factors Effecting The Thermal Spray 'oating Process


33/40

Arc Spraying ' Electric Arc (ire Thermal Spray Process


34/40


35/40


36/40

• Some Properties Thermally Sprayed Coatings can Pro$ide+

• Tri%ological (#ear& resistance)"

• Corrosion resistance"

• Heat resistance"

• Thermal %arrier"

• Electrical conducti$ity or resisti$ity• A%rada%le or a%rasi$e"

• Textured suraces"

• Catalyst and prosthetic properties&

• ?estoration o dimension"• Copying o intricate suraces"


37/40


38/40

• P#PETES• Compare coatings to their #rought or cast e/ui$alents+• P#PET*++++++,,'#AT-G+++++++,'AST%.#/G!T

• StrengthIIIIIIIIIlo# (31


39/40

Porosity

• This is present in most thermally sprayed coatings (except >PS& post heat treatedcoatings or used coatings)"• 4 to 23B porosity is normal %ut can %e urther manipulated %y changes in process

and materials"• Porosity can %e detrimental in coatings #ith respect to+• Corrosion 1 (sealing o coatings ad$ised)"• Machined inish"• Strength& macrohardness and #ear characteristics"

Porosity can 0e important 1ith respect to2• 8u%rication 1 porosity acts as reser$oir or lu%ricants"• ,ncreasing thermal %arrier properties"• ?educing stress le$els and increasing thickness limitations"• ,ncreasing shock resisting properties"

• A%rada%ility in clearance control coatings"• Applications in prosthetic de$ices and nucleate %oiling etc"


40/40

!F sprayed Tungsten 'ar0ide % 'hromium -ic3el 'oating

(.'%45'r6-i )

!F sprayed Tungsten 'ar0ide % 'hromium -ic3el

' ti (.'%45' 6-i )

thermal spraying coatings

Documents