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7/21/2019 Surface Engineering Part 1(1)
1/71
Surface Engineering Part 1Surface Engineering Part 1
Dr Zhu LiuDr Zhu Liu
Corrosion and Protection CentreCorrosion and Protection Centre
School of MaterialsSchool of Materials
The University of ManchesterThe University of Manchester
MATS 64532: Surface Engineering and Materials DesignMATS 64532: Surface Engineering and Materials Design
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Lecture ArrangementLecture Arrangement
L1 Surface Engineering I Introduction to SE
Hot-dipping Electroplating Chemical plating Thermal spray Overlay coating
Cold spray Diffusion coating
L3 Surface Engineering III
Introduction to LSE Heat-treatment Surface re-melting Surface alloying Cladding Shock-peening
L2 Surface Engineering IIhysical vapour deposition Chemical vapour deposition Ion implantation Sol-gel coating
L4 Surface Engineering I
Surface te!turing Surface modification of
polymers Surface finishing
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Definiti!n !f Surface EngineeringDefiniti!n !f Surface Engineering
1" Treatment of the surface and near-surface regions of
a material to allo# the surface to perform functions
that are distinct from those functions demanded from
the $ulk of the material %defined $y &S' hand$ook(
2" The design of a surface and a su$strate together as a
system) to give a cost effective performance of #hich
neither is capa$le alone %defined $y CEST(
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Surface EngineeringSurface Engineering
Surface "r!#ertiesSurface "r!#erties
I!roved corrosion resistanceI!roved corrosion resistance
I!roved "ear resistanceI!roved "ear resistance
I!roved theral insulationI!roved theral insulation
I!roved high#te!erature o$idation resistanceI!roved high#te!erature o$idation resistance
I!roved adhesion %for !olyers&I!roved adhesion %for !olyers&
I!roved "etta'ility %'oth !olyers and etals&I!roved "etta'ility %'oth !olyers and etals&
Ec!n!mic $enefitsEc!n!mic $enefits
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%lassificati!n !f Surface Engineering
%&anging surface
metallurg'
%&anging surface
c&emistr'
Adding
c!atings
I!n
im#lantati!n
Laser surface
all!'ing
Localised surface
hardening* +lame)
Induction) Laser and
Electron-$eam
,apid solidification*
Laser surface melting
Shot peeningLaser shock peening
Hot-dip coatingElectroplatingChemical platingDiffusion coatingThermal sprayOverlay coatingCold sprayhysical vapour
depositionChemical vapour
depositionSol-gelLaser Cladding
%&anging surface
structure
Laser surface
te(turing
)#atterning
)structuring
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(or a!!lications of corrosion !rotection)(or a!!lications of corrosion !rotection)
Metallic coatingsMetallic coatings
*nodic coating*nodic coating
Cathodic coatingCathodic coating
+eutral coating+eutral coating
Ceraic, !olyer coatingsCeraic, !olyer coatings
%lassificati!n !f %!atings%lassificati!n !f %!atings
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An!dicAn!dic%!atings%!atings
Coating material is less no'elthan the su$strate to $e protected"
Cathodic or sacrificial protection
& $arrier to the environment
E!amples* inc-coatings on steel .galvanised steel/
(eatures) at $reaks or pores
or cut edges the coating
corrodes preferentially andprovides sacrificial protection
to the underlying steel
su$strate"http*00###"gordonengland"co"uk0corrosion"htm
http://www.gordonengland.co.uk/corrosion.htmhttp://www.gordonengland.co.uk/corrosion.htmhttp://www.gordonengland.co.uk/corrosion.htm7/21/2019 Surface Engineering Part 1(1)
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%at&!dic%at&!dic%!atings%!atings
Coatings is ore no'leto the su$strate to $e protected
E!amples* nickel-coatings on steel) copper-coating on steel
Liitation) the coating
must provide a complete$arrier to the su$strate from
the environment1 other#ise)
at $reaks) pores or cut edges
the steel #ill corrode)
causing severe pitting
$ecause the anodes #ill $e
small #hile the cathode area
is much larger"
http*00###"gordonengland"co"uk0corrosion"htm
http://www.gordonengland.co.uk/corrosion.htmhttp://www.gordonengland.co.uk/corrosion.htmhttp://www.gordonengland.co.uk/corrosion.htm7/21/2019 Surface Engineering Part 1(1)
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Arrangement !f metals in *al+anic SeriesArrangement !f metals in *al+anic Series
rotection of car$on steels*rotection of car$on steels*
An!dic c!atingsAn!dic c!atings
2n2n&l .e!ceptional cases/&l .e!ceptional cases/
CdCd
'g .too reactive/'g .too reactive/
%at&!dic c!atings%at&!dic c!atings
TinTin
3ickel3ickel
CopperCopper
Corrosive End %*nodic or less no'le&
------------------------------------------------
Magnesium
,inc
Aluminium%admium
Ir!n)steel
Stainless steel -acti+e.
Tin
/ic0el
%!##er
Stainless steel -#assi+e.
*!ld
Protective End %Cathodic or ost no'le&
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!tdi##ed %!atings!tdi##ed %!atings
Definiti!n* & metallic coating o$tained $y dipping the su$strate
metal into a molten metal"4 coating #ith lo#er melting temperature
Stages:
5/ re-treatment* degreasing) descaling and rinsing
6/ +lu!ing* #etting
7/ Dipping*
8/ ost-treatment* annealing) or rolling) or rapid coling
T'#ical &!tdi##ing #r!cesses:
9alvanied steel
http*00###"youtu$e"com0#atch:v;c6n?hS$s
http://www.youtube.com/watch?v=c2J07n5hSbshttp://www.youtube.com/watch?v=c2J07n5hSbs7/21/2019 Surface Engineering Part 1(1)
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T'#ical Surface A##earance !f *al+anied SteelT'#ical Surface A##earance !f *al+anied Steel
Surface appearance* 5==@ 2n A $right) polycrystalline structure #ith
visi$le crystalline grains of inc) some times several centimetres in
sie"
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T'#ical Micr!structure !f *al+anied SteelT'#ical Micr!structure !f *al+anied Steel
utla'er -23 m.* 5==@ 2nMetallurgical !nding* various 2n-+e intermetallics
,n7e intermetallics is rittleA limited additional shaping
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t&er ,incased %!atingst&er ,incased %!atings
To improve f!rmailit' and ductilit'of coatings $y addition of other elements into the molten inc $ath"
8*alfan8 -,n59Al.:
Continuous coil-coating) much improved ductility
Better corrosion resistance than 2n coating
Similar function of sacrificial protection
8*al+alume -,n559Al.:
Continuous coil-coating) much improved ductility
Even $etter corrosion resistance than 2n-?@&l coatingLimited sacrificial protection
8*al+anneal8:
+ull 2n-+e intermetallic layer $y heat-treatment
Similar function of sacrificial protection
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%!rr!si!n "r!tecti!n !f *al+anied Steels%!rr!si!n "r!tecti!n !f *al+anied Steels
,n
Steel
,n #r!+ides:
"&'sical arrier .corrosion rate of 2n
is ?-5=@ that of steel/* hen e!posed
to the atmosphere) inc reacts #ith
o!ygen to form inc o!ide) #hich further reacts #ith #ater
molecules in the air to form inc hydro!ide" +inally inc hydro!ide reacts #ith car$on dio!ide in the atmosphere to
yield a thin) impermea$le) tenacious and uite insolu$le dull
grey layer of inc car$onate #hich adheres e!tremely #ell to
the underlying inc) so protecting it from further corrosion"
%at&!dic #r!tecti!n: 2inc coatings prevent corrosion of the
protected metal $y forming a physical $arrier) and $y acting as a
sacrificial anode if this $arrier is damaged"
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9alvanied Steel
9uard ,ail System
9alvanied
steel pipe
9alvanied
Steel $ucket
A##licati!ns !f *al+anied SteelsA##licati!ns !f *al+anied Steels
Lifetime of galvanied steel* up to ?= years
E!tended lifetime $y duple! system* up to >? years"
http://www.galvanizeit.org/resources/files/AGA%20PDFs/Duplex%20Systems.pdfhttp://www.galvanizeit.org/resources/files/AGA%20PDFs/Duplex%20Systems.pdfhttp://www.galvanizeit.org/resources/files/AGA%20PDFs/Duplex%20Systems.pdf7/21/2019 Surface Engineering Part 1(1)
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Electr!de#!siti!n)Electr!#lating
rinciples A ho# it #orks
Consist of* anode) cathode)
aueous-metal solution and
po#er supplier
Corrosion process in reverse
E!ample* 3i plating
&node* 3i 3i6E
6e-
Cathode* 3i6E 6e-3i
.6HE 6e-H6/
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Surface "re#arati!n and Electr!#latingSurface "re#arati!n and Electr!#lating
ProcedureDegreasing
Etching0chemical pre-treatment
ashing0rinsing
re-plating or other surface treatmentashing0rinsing
lating
ashing0rinsing
ost-treatments .annealing) polishing) passivating) etc"/
I!ortance of surface cleanliness) In real #orld) surface
preparation is more crucial than plating to ensure good
adhesion and surface uality
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Electr!de#!siti!n)Electr!#lating
Structure of !lating
Epita!ial gro#th for a fe#hundred atomic layer
.clean and o!ide-free surface/
Coating thic-nessroportional to the current density
and length of deposition time) leading
to one of the maFors dra#$acks in
electroplating A ina$ility of achieving uniform deposition forirregular shaped components
4 Thro"ing !o"er) a'ility of a !lating solution to !roduce a
unifor etal distri'ution on an irregular sha!ed o'.ect/
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Electr!de#!siti!n)Electr!#lating
Ma.or dra"'ac-s
oor thickness uniformity on comple! components Hydrogen em$rittlement for steels
3ot applica$le to insulating su$strates
Possi'le environental concerns "ith !lating 'aths
*dvantages)Lo# temperature treatment .G 5== C/ A no distortion and
no metallurgical changesCoatings are dense #ith e!cellent adhesion to the su$strate
3o technical limitation to coating thickness - thick layers possi$leSelective deposition possi$le $y applying masks&pplica$le to a #ide range of metal su$strates
Lo# cost
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A##licati!ns !f S#ecific %!ating S'stemsA##licati!ns !f S#ecific %!ating S'stems
+ic-el !lating* corrosion protection) undercoat forelectroplating chromium for decorative purpose of
automotive) consumer products and office furniture
Chroiu !lating) Chromium easily passivated A cannot $e
used as solu$le anode" Decorative .tarnish-resistant/) hard)#ear-resistance coatings .al#ays need nickel as undercoat/"
3ote a pro$lem #ith cracks and pores #ithin the chromium
coating) leading to limited corrosion protection"
Cadiu and Zinc !lating) Cadmium for marineenvironment) and inc .G m/ for industrial environment"
Tin !lating* for non-aggressive environment) tin coating
.="?-5 m/ is reuired) such as tin plated steel cans"
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A##licati!ns !f S#ecific %!ating S'stems
Micr! and /an!c!m#!site %!atings'etal 'atri! Composite .''C/ A Ceramic particles
.car$ides) o!ides/ in metal matri! .cermet/
Sie of ceramic particles* a fe# micron) su$micron) or nano-"
Ceramic particles suspendingin an electrolyte
Co-deposit ceramic particlesinto metal matri!
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A##licati!n E(am#les !f Electr!de#!siti!nA##licati!n E(am#les !f Electr!de#!siti!n
Mild steel c!re and
electr!#lated ;it& %u
9old plating
olished electroplated chrome or multi-layer Environmental concerns
Chromium Coating for
Superior 'etalrotection
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%&emical "lating
Cheical !lating* deposition of metal on su$strate
#ithout applying e!ternal current"
T"o fors
-1. Immersi!n #latingCu6E +e ; +e6E Cu
-2. Electr!less #lating
3i6E
6HO6-
; 3i H6 6O6-
Mec&anisms* Electrochemical in nature) involving
$oth o!idation and reduction reactions
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Immersi!n "lating
Immersion plating
O!idation* 2n 2n6 6e-) anodic) E= ; ="> J
,eduction* Cu6 6e-Cu) cathodic) E=; ="78 J
------------------------------------------------------------------------------
Overall reaction* 2n Cu62n6 Cu E= ; 5"5 J
& simply displacement reaction $et#een an anodic su$strate
and a more no$le metal ion in solution #ith the deposition
reaction driven $y local dissolution of the su$strate
Limited plating thickness .5m/* Displacement processcontinues until almost the entire su$strate is covered #ith
copper"
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Electr!less "lating
Electroless plating
,eduction* 3i6E 6e-3i E=;-6? mJ
O!idation* H6O6- H6O H6O7- 6HE 6e- E=; ?= mJ
------------------------------------------------------------------------------
3i6E
H6O6-
H6O 3i H6O7-
6HE
E=
; 6? mJ
0 Sustaina'le o$idation reaction
&utocatalytic mechanism
lating thicker coating layers
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%&emical %!ating)Electr!less #lating%&emical %!ating)Electr!less #lating
*dvantages)
Lo# temperature treatment 'ore corrosion resistant than electrodeposited Cr
Can coat comple! shapes uniformly
Hard particles can $e incorporated to increase Hv
T+E can $e incorporated to reduce friction
Can coat most metals and some insulators
Disadvantages'ore e!pensive than electroplated Cr
eattreatment
is needed t! de+el!# !#timum #r!#erties
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T&ermal S#ra' %!atingsT&ermal S#ra' %!atings
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T&ermal S#ra' %!atingsT&ermal S#ra' %!atings
A c!mm!n feature !f all t&ermal s#ra' c!atings is t&eir
lenticular !r lamellar grain structureresulting fr!m t&e
ra#id s!lidificati!n !f small gl!ules? flattened fr!m
stri0ing a c!ld surface at &ig& +el!cities
http*00###"gordonengland"co"uk0
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T&ermal S#ra' %!atings: $!nding
T&ermal S#ra' %!ating $!nding Mec&anisms:
1. Mec&anical 0e'ing !r interl!c0ing
6/ Diffusion $onding or 'etallurgical $onding
7act!rs affecting !nding and suse@uent uild u# !f
c!ating:
5/Cleanliness1 6/Surface area 7/Surface topography or
profile1 8/Temperature . thermal energy/1 ?/Time .reaction
rates K cooling rates etc" /1 /Jelocity . kinetic energy /1 >/
hysical K chemical properties1 /hysical K chemical
reactions"
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T&ermal S#ra' %!atings
%!ating structure
Lamellar !r flattened grains? !(ide inclusi!ns!utlining t&e grains !r #article !undaries
"!r!sit': 1259
L!; im#act energ' - unmelted #articles ) l!;
+el!cit' .
S&ad!;ing effects - unmelted #articles ) s#ra'angle .
S&rin0age and stress relie+e effects
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ari!us t&ermal s#ra' c!ating #r!cesses
L!;energ' #r!cesses:
%!musti!n ;ire t&ermal s#ra'
%!musti!n #!;der t&ermal s#ra'
Arc ;ire t&ermal s#ra'
ig&energ' #r!cesses:
"lasma t&ermal s#ra'
7 t&ermal s#ra'
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%!musti!n T&ermal S#ra' -7lame S#ra'.
%!ating Material in #!;der? !r ;ire f!rm
7lame -!('acet'lene !r &'dr!gen m!st c!mm!n.
7lame +el!cit': 5 m)s and Tem#erature ac&ie+ed B 3 C%
L!; #erf!rmance c!atings -&ig& #!r!sit'? #!!r !nding.
http*00###"gordonengland"co"uk0
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ig& el!cit' ('gen 7uel -7. T&ermal S#ra'
Similar t! c!musti!n t&ermal s#ra'? ut ;it& s#ecial
design !n t!rc&
7lame +el!cit' u# t! su#ers!nic +alues
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"lasma S#ra'
7!rmati!n !f #lasma
Material in t&e f!rm !f #!;der inected int! a +er' &ig&
tem#erature #lasma flame ra#idl' &eated and accelerated
t! a &ig& +el!cit'
ig& +el!cit' and &ig& tem#erature -u# t! 55 C%.
7/21/2019 Surface Engineering Part 1(1)
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%!m#aris!n !f ari!us T&ermal S#ra' "r!cesses
T&ermal
#r!cess
el!cit'
m)s
Tem#era
ture? C%
Ma(> s#ra'
rate? 0g)&
"!r!sit'?
9
$!nding
%!musti!n
#!;der
3 22 62 #!!r
7 6115 31 14 B >5 ;ellad&erent
"lasma
s#ra'
24 55 5 B 2 ;ell
ad&erent
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A##licati!ns !f T&ermal S#ra'ed Al %!atings
T&ermal s#ra' metallic c!atings? ;it& and ;it&!ut sealersT&ermal s#ra' metallic c!atings? ;it& and ;it&!ut sealersand t!#c!ats? as a means t! #re+ent c!rr!si!n !f steeland t!#c!ats? as a means t! #re+ent c!rr!si!n !f steel
surfaces>surfaces>
T'#es !f metallic c!atings included are #ure inc? #ureT'#es !f metallic c!atings included are #ure inc? #ure
aluminum? and inc)aluminum all!'? F59 inc)159aluminum? and inc)aluminum all!'? F59 inc)159aluminum ' ;eig&t>aluminum ' ;eig&t>
A##licati!nsA##licati!ns
ig& tem#erature resistantig& tem#erature resistant
E(cellent resistance t! #!lluted and marine en+ir!nmentsE(cellent resistance t! #!lluted and marine en+ir!nments
7!rms !(ide la'er7!rms !(ide la'er
Geduces c!rr!si!nGeduces c!rr!si!n
E l f T& l S % ti
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E(am#les !f T&ermal S#ra' %!atings
E(am#le 1:Metal Matri$ Co!osite %MMC& Coatings
''C coatings) comprised of a hard ceramic phase
em$edded in a metallic matri!) are increasingly $eing
applied for many industrial components in the petrochemical
industry) po#er generation and offshore applications toprovide cost effective protection against #ear and corrosion"
Ty!ical e$a!les)
C CoCrC 3iCrBSi
Cr6C7 3iCr
Ty!ical coating techniue) HJO+
E l f T& l S % ti
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E(am#les !f T&ermal S#ra' %!atings
3anoparticles cannot $e sprayed directly1
,econstitution of 3s into spherical micro-sied granules
3ano-C0Co feedstock po#der
Conventional C0Co feedstock po#der
Coating !ro!erties)
Hardness of n-C0Co
coating is higher than that ofthe conventional one"
Corrosion resistance of n-
C0Co coating is higher than
that of the conventional one"
Potential !ro'les)
De-car$uriation of Cdue to higher surface to
volume ratio"
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E(am#les !f T&ermal S#ra' %!atings
E(am#le 2:E(am#le 2:Plasa s!rayed theral 'arrier coatingsPlasa s!rayed theral 'arrier coatings
Coating material* M6O7-2rO6 .56=-6?=
m/) plus metallic $ond-coat .?=-56= m/
Coating characteristics*orosity .interconnected/) microcracks
and layered-structure
High-temperature corrosion $ehaviour*
enetration of corrosive gas) fuel and
liuid through the inter-connected pores
to get contact #ith metallic su$strate A
o!idation A spallation of coatings
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A##licati!ns !f T&ermal S#ra' %!atings
HJO+ spraying Tungsten Car$ide 0 Co$alt Chromium Coating
.C05=Co8Cr/ onto ,oll for the aper 'anufacturing Industry
&tt#:));;;>'!utue>c!m);atc&=+H0T"$m&s3M
&tt#:));;;>'!utue>c!m);atc&=+H,g0SJF&J4
http://www.youtube.com/watch?v=k-T_PBmhs3Mhttp://www.youtube.com/watch?v=ZgkSY8hYbH4http://www.youtube.com/watch?v=ZgkSY8hYbH4http://www.youtube.com/watch?v=k-T_PBmhs3M7/21/2019 Surface Engineering Part 1(1)
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A##licati!n E(am#les !f Surface EngineeringA##licati!n E(am#les !f Surface Engineering
2as tur'ine 'lades2as tur'ine 'lades
Environment* High-temperature) corrosive"
Coatings* Ceramic coatings as thermal $arrier
Bond coat* 'Cr&lM alloy
http://doc.tms.org/servlet/ProductCatalog?container=JOM+2005+January7/21/2019 Surface Engineering Part 1(1)
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T'#ical A##licati!ns !f T&ermal S#ra' %!atings
"lasma s#ra': %eramic? metal?
caride c!atings t! s!l+e
industrial #r!lems and ;ear?
arasi!n? c!rr!si!n? t&ermal
arriers>
7: Ser+ing all industriesre@uiring e(treme ;ear resistant?
dense caride and metal c!atings
l % i
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+erla' %!atings+erla' %!atings
Melting!f a la'er !f material !n t&e sustrate t!
f!rm a c!ating
eating s!urce: ;elding t!rc&? arc? #lasma?
electr!neam>
$!nding: metallurgical fusi!n
Main a##licati!ns:
ardfacing: engine +al+e seats
%!rr!si!n #r!tecti!n
Ge#airing !f damaged c!m#!nents
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%!m#ared ;it& t&ermal s#ra' c!atings
%!m#aris!n +erla' c!atings T&ermal s#ra'
T&ic0ness 26 mm /!rmall' B 1
mm/umer !f
la'ers%an e multi
la'ers/!rmal single
la'er
$!nding 7usi!n Mec&anical
interl!c0ing
Micr!structure S!lidificati!n Laminar and#anca0e li0e
% ld S -Ki ti S .
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%!ld S#ra' -Kinetic S#ra'.
"!;der sie: 1 5"!;der sie: 1 5 mm
"!;der #article +el!cit':"!;der #article +el!cit': 45 1 m)s -su#ers!nic.45 1 m)s -su#ers!nic.
"!;der im#inging !n t&e sustrate in s!lid f!rm at"!;der im#inging !n t&e sustrate in s!lid f!rm at
tem#erature ;ell el!; melting tem#eraturetem#erature ;ell el!; melting tem#erature
!; %!ld s#ra' %!ating $uilds
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!; %!lds#ra' %!ating $uilds
Stage 5* Su$strate cratering and first
layer $uilt up of particles .initialcratering) deformation of su$strate)
spat formation) fracturing of surface
o!ides on metal particles and
su$strates and first particle layer
formation
Stage 6* article deformation and
realignment .multilayer coating
$uild-up/
Stage 7* 'etallic $ond formation
$et#een particle0particle and void
reduction
Stage 8* +urther densification and
#ork hardening of the coatingJan Steenkiste) T"H") et al) Surface and Coatings
Technology 5?8 .6==6/ 67>-6?6"
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Im#!rtant "arameters !f %!ld S#ra' "r!cess
A critical +alue !f #!;der #article +el!cit' -c.
Determining fact!rs f!r #!;der #article +el!cit':
gas tem#erature
#article sie
m!lecular ;eig&t !f gas
-L!;er gas m!lecular
;eig&t? t&e faster t&e#!;der #article +el!cit'.
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Micr!structural %&aracteristics !f
%!lds#ra' %!atings
T!# la'er:
#!r!usmicr!structure
Inner la'er:
densermicr!structure
Tam#ing effect
Ti on steel su$strate
Jan Steenkiste) T"H") et al) Surface and Coatings Technology 5?8 .6==6/ 67>-6?6"
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Diffusi!n %!atings
'etals and alloys are used at high-temperature environment"
Protection echanis* rotective elements lead to
formation of chromia) alumina and silica scales respectively)
for o!idation and corrosion protection"
Process) & protective element .or elements/) like%r? Al !rSi) is deposited on the surface of the $ase metal) and diffused
into the $ase metal at high temperature
Diffusion coatings !rocesses)
&luminiing1
Chromiing1
&$ove-the-pack or out-of-pack chemical vapour deposition
Aluminising
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Aluminising
"r!cess* carried out $y a pack cementation process"
"ac0* &l or &l alloy po#der)
halide salt .activator/) e"g" 3H8Cl) to form metal halide
#ith high vapor pressure
inert filler .alumina/Tem#erature re@uired* a$ove ?= to 55==C for several hours
ack
Su$strate to $e
coated
Aluminising
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Aluminising
%&emical reacti!ns and diffusi!n*
on heating* activator reacts #ith the metallic component of the
pack to form metal halides) e"g"
3H8Cl N 3H7 HCl
HCl &l N &lCl! H6
&lCl!has high vapour pressure #hich diffuse through the pack)
to form &l deposit on the su$strate) then diffuse into the
su$strate to form +e-&l .or 3i-&l/ intermetallics
T'#ical micr!structures !f aluminised car!n steels: layers
of -+e6&l?) P-+e&l and0or +e7&l along the depth"
Aluminising
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Aluminising
Acti+it' !f aluminising #r!cess depends on* the amount
and type of &l source alloy) the amount and type of
halide activator) the amount of inert o!ide and process
temperature"
7!r aluminising !f /i ase all!'s:
ig&acti+it' aluminising* high concentration ofaluminium-rich vapour at the surface of the part
and coatings gro# mainly $y in#ard diffusion of
aluminium
L!;acti+it' aluminising* the coatings are formedmainly $y out#ard diffusion of nickel1 the
formation of higher aluminide phases is suppressed"
L!;acti+it' aluminiing
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L!;acti+it' aluminiing
The pack contains aluminium alloy po#der " The concentration of aluminium-rich vapor is lo#" The coatings gro# mainly ' !ut;ard diffusi!n !f nic0el" The processing temperature is 5===-55?=C) i"e" the high end" 'icrostructure* 3i&l) no post-coating heat treatment reuired The coatings have coarse-grained columnar microstructures
#ith limited ductility" The coatings thickness is less than high-activity coating
thickness"
T&e effect !f sustrate materials !n aluminiing: e>g>
1./i
2./i%r -+ariati!n !f %r c!ntent.
L!;acti+it' aluminiing
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L!;acti+it' aluminiing
Aluminising !n #ure /i sustrate
Originalinterface Kirkendall
porosity
Nidiusion
Inclusions
5/ Coating gro#s out#ard
6/ Original surface
7/ 3i diffuses out#ard faster
than &l diffuses in#ard Aformation of Qirkendall
porosity at the interface"
8/ +ormation of inclusions $y
trapping pack particles in
the coating"
L!;acti+it' aluminiing
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L!;acti+it' aluminiing
Aluminising !n /i%r sustrate
Originalinterface Kirkendall
porosity
Nidiusion
Inclusions
,egion $elo# the original
surface*
5/ Depletion of 3i
6/ Enrichment of Cr
+or 3i-5=Cr alloy*
Cr is still in solid solution
+or 3i-6=Cr alloy*
Cr precipitates out of the solid
solution to form RCr .reduce
ductility/"
ig&acti+it' aluminiing
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ig& acti+it' aluminiing The pack contains pure aluminium po#der" The concentration of aluminium-rich vapour is high"
The coatings gro# mainly ' in;ard diffusi!n !f aluminium" The processing temperature is >== - ==C" The coatings is then heat-treated) typically 8h at 5=?=-55==C)
to convert $rittle aluminium-rich phases) such as -3i&l7and -
3i6&l
7) to P-3i&l"
High-activity aluminiing is more common than lo#-activity
aluminiing"
%!m#aris!n et;een l!; and &ig&
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# g
acti+it' aluminiing
%!m#aris!n !f t&e f!ll!;ing as#ects:
5" Original surface
6" Diffusion direction .3i) &l/
7" 'icrostructures* Qirkendall porosity) em$edded pack particles)
8" 'echanical properties
Aluminising /i%r su#erall!'
f!r gas turine engines
%&r!mising
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g
"r!cess* carried out $y a pack cementation process"
"ac0* Cr po#der) ammonium chloride and inert alumina filler
Micr!structures:
L!;car!n steels: t&e c&r!mium de#!sits !n t&e surface t! f!rm a s!lid
s!luti!n all!' !f ferritic ir!nc&r!mium
Medium&ig& car!n steels: tend t! result in t&e f!rmati!n !f a la'er !f
c&r!miumir!n caride #re+enting %r diffusi!n #r!cess>
Tem#erature re@uired* typically to 55?=C for several hours1
eight@Chr
omium
Temperature)
oC??
56
=
58==
==
= 56
a/ $/
Distance into su$strate
eight @ Chromium
+e-Cr euili$rium diagram
R
te
A!+et&e#ac0 #r!cesses
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# #
Su$strate to $e coated is positioned a$ove the pack in a retort
9as-phase process &luminising or Chromising
Ad+antages !+er #ac0 #r!cesses:
a$ility to uniformly coat internal passages and holes
coating #ithout pack particle entrapment .no direct contact
#ith the pack/
$etter mechanical properties in terms of lo#er transition
temperature .3i&l/
ut!f#ac0 c&emical +a#!ur de#!siti!n
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# # #
%&amer* a gas mi!ture of aluminium chloride and hydrogen .&lCl7is
created e!ternally $y passing HCl or Cl6gas over &l/
%&emical reacti!ns and diffusi!n*
HCl 6&l --- 6&lCl7 7H61
6&lCl7 8&l --- &lCl
&lCl 3i 7H6 --- 3i&l HCl .g/
Tem#erature:5=== C for 8 hours
L!; aluminium acti+it'* microstructure
is similar to lo#-activity pack aluminising"
The use of molten aluminium is to increase
aluminium activity $y forming lo#er
chlorides of aluminium #hen passing the
inlet gas mi!ture over liuid aluminium"
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%!m#aris!n !f +a#!ur#&ase aluminising
;it& #ac0 cementati!n aluminising
a#!ur#&ase aluminiing is m!re c!stl'
In large ret!rts? +ariati!ns in t&e c!ncentrati!ns !f +a#!ur
#&ase reactants can result in n!nunif!rm c!ating t&ic0ness
a#!ur#&ase aluminiing is effecti+e f!r c!ating c!m#le(
s&a#ed #arts? and dust &aards ass!ciated ;it& &andling
large am!unts !f #!;der are a+!ided>
T'#ical A##licati!ns !f "ac0 %ementati!n
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'aFority of aero-engine tur$ine
$lade aerofoils are aluminised to
improve their resistance to hightemperature o!idation or surface
preparation prior to deposition of TBC"
M!dified Diffusi!n %!atings
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M!dified Diffusi!n %!atings
Adding %rint! aluminide c!atings t! im#r!+e resistance
t! T'#e II &!t c!rr!si!n? T !(idati!n and ductilit'> Deposition of Cr-enriched layer prior to aluminising and
then aluminising of chromium coated su$strate
ack containing &l and Cr po#ders
Adding Siint! aluminied c!atings t! im#r!+e resistance
t! T'#e II &!t c!rr!si!n? and &ig& tem#erature !(idati!n>
Deposition of metallic Si layer prior to aluminising
Silicon-containing coatings are suscepti$le to $asic flu!ing) i"e" $y 3a6O)
leading to Type I hot corrosion) $ut have good resistance to sulphidation" In
addition) it may $e prone to cracking during thermal cycling due to the formation
of $rittle intermetallics"
M!dified Diffusi!n %!atings
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M!dified Diffusi!n %!atings
Adding "tint! aluminised c!atings
1. t! im#r!+e resistance t! !(idati!n and t'#e I &!tc!rr!si!n ut 2.resistance t! t'#e II &!t c!rr!si!n is
marginall' etter t&an c!n+enti!nal aluminides> In additi!n?
3.resistance t! c'clic !(idati!n is #!!r as "t aluminides are
less ductile t&an !rdinar' aluminides>
Depositing a layer of t .?-5= Um/) and follo#ed $y
diffusion heat treatment) and then aluminiing
Micr!structure: a matri! of nickel aluminide #ith t present
in solid solution %.3i)t/&l() and as intermetallic phases) .t&l6
or t6&l7near the outer surface/"VV continue
M!dified Diffusi!n %!atings
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g
"r!tecti!n mec&anism* to promote formation of pure alumina scale and reduce scale
gro#th rate to act as a phase sta$iliser and prevent the detrimentalconvertion from to W on thermal cycling to improve alumina scale adherence) including mechanical
keying) as a possi$le mechanism due to the formation of anirregular o!ide-alloy interface1 to improve self-healing of the alumina scale after modest
spalling to suppress interfacial void formation to tie up &l as t-&l compounds) thus lo#ering &l activity) in
turn) to reduce the driving force for the diffusion of 3i from the
su$strate into the coating1
4 During service) t&l6can transform to t&l to cause cracking and void formation"
7ailure !f Metallic c!atings
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g
Defects in t&e c!ating !r at t&e c!ating)sustrate interface
L!ss !f #r!tecti+e elements fr!m t&e c!ating in ser+ice:
$y spallation of scale"
$y vaporiation into a gaseous atmosphere) e"g" Cr6O7can
$e further o!idied a$ove 5===C to $ecome volatile CrO7"
$y dissolution into molten salt deposits* 'olten salt has
3a6O reacting #ith &l6O7 O6$ecome 3a&lO7.aluminate/
7ailure !f Metallic c!atings
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g
%!atingsustrate interacti!ns in ser+ice -interdiffusi!n.-c!m#!siti!nal difference et;een c!ating and sustrate
#r!+ides dri+ing f!rce at &ig& tem#erature.>
5/ +ormation of detrimental phases #ithin the coating) e"g"$rittle intermetallics reduces ductility of the coating"
6/ Dilution of the coating in the protective elements) e"g" 'o
and J from alloy su$strate migrate to the coating) leading
to reduction of resistance to hot corrosion"7/ Qirkendall porosity at the coating0su$strate interface
reduces the adhesive strength) leading to spallation of the
coatings"
7ailure !f Metallic c!atings
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g
%!atingsustrate interacti!ns in ser+ice -interdiffusi!n.:
8/ Loss of &l to the su$strate $y interdiffusion1 and Diffusion
of refractory elements from the superalloy into the coating)
leading to degradation of o!idation resistance of protective
aluminide coatings on high temperature superalloy
components"
%!ating -2 4 at9. Al
Su#erall!'-5 1 at9 Al.
Cr) Ta)) 'o
Al
7ailure !f Metallic c!atings
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E(am#le 1: Aluminied /ic0elase su#erall!'s
A$ 2 &!urs at 1!% in air>
$% 5 &!urs at 12!% in air
7ailure mec&anism:
&l6O7scale tends to spall easily in service1
Interdiffusion $et#een the coating and the su$strate
leads to dilution of the coating in aluminium" If &l is
less than a critical value) it is no longer protective"
3ote the effect of temperature in service"
g
&
B
C
Time
eightgain
7ailure !f Metallic c!atings
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E(am#le 2: %&r!mied car!n steels? n!te t&e effect !f %
L!; car!n steel l!ss !f c&r!mium due t!:
s#allati!n !f %r23scale? diffusi!n !f c&r!mium
int! t&e steel sustrate and +!latiliati!n !f %r3
ig& car!n steel f!rms a l!t !f %r23%6#reci#itates
during c&r!miing t&ese are an effecti+e s!urce !fc&r!mium? t&ere' increasing t&e time t! rea0a;a'!(idati!n t&at arises fr!m l!ss !f c&r!mium fr!m t&e
c!ating>
Im#r!+ing lifetimes !f c!ating)sustrate
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Inc!r#!rating a arrier la'er et;een c!ating
and sustrate: to reduce the rates of interdiffusion
$et#een coating and su$strate"
43i-Hf compound) t-Hf compound used at interface of
superalloy0aluminide coating
Inc!r#!rating a reacti+e element? suc& as J !r
%e? int! t&e c!ating: to improve scale adhesion on
thermal cycling .needs to understand the effect of
Diffusion
$arrier layer
&luminide coating
Superalloy
&lumina scale
Recommended