Coating for Turbine Blades

8/19/2019 Coating for Turbine Blades

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Coatings for Turbine Blades

T. Sourmail

Historical overviewOnly some 60 years after the invention of the jet engines, flying has become aconventional method of transportation. Once the exclusive privilege of the`swagger rich', it has become as much of a commonplace as a bus trip to thecity centre and in fact, the comfort and service onboard some of the cheapestairlines push the analogy further!.

"et, bac# in the early $%&0's, many were seeing jet powered flight as no morethan a `laboratory experiment' maybe in the same way as we may be, today,sceptical about future applications of the recent `scramjet'!.

(hese doubts were not unfounded) materials used in parts of the engine couldnot survive more than a few hundred hours at then relatively modesttemperatures.

*y the $%+0's however, jet fighters were first put in combat over orea. -t theend of the $%60's, commercial jets were accepted, and by the end of the $% 0's,the commercial aviation mar#et overtoo# the military one.

(he efficiency of commercial airliners has increased beyond all earlyexpectations, and while it would probably be unfair to single out one factor,improvements in engine materials certainly played a #ey role.


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Evolution in engine efficiency, after Pratt & Withney

/ncrease in operational temperature of turbine components. -fter chul1 et al , -ero. ci. (echn. 7)2003, p43 0.

5conomical and, today, environmental concerns continue to provide impetus for operating the engines at ever increasing temperatures, thereby improving thethermodynamic efficiency and reducing pollutant emissions.

/n its early years, the uest for higher temperatures was dominated by materialsand processes developments. (he apparition of superalloys in the early $%+0's,considerable amelioration in casting technologies and, in the $%60's, the coolingsystem for turbine blades were all major steps forward, each allowing theservice temperature to be increased by 20 o7 or more.

Over the past 20 30 years, alloy improvement, directional and single crystalsolidification have contributed significantly, but, arguably, the emphasis hasbeen shifted to coating systems which have allowed an increase of gastemperature of up to $$0 o7.

7oatings in gas turbine serve a variety of purposes, whether in jet engines,land based power generation turbines or marine engines. - first re uirement tooperate turbines at higher temperatures was, of course, improved strength.8nfortunately, these conditions also mean severe oxidation9corrosion problems,and to ma#e things worse, the improvement in mechanical properties of thebase alloys was made at the expense of environmental resistance.


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(he first purpose of coatings was therefore to palliate for the poor oxidationresistance of the base alloy aluminide, :t aluminide, ;7r-l"!. - second type of coatings applied to high temperature parts are #nown as thermal barriercoatings (*7!. (hese are ceramic coatings with very low thermal conductivity.


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pressure turbine /:(!, low pressure turbine ?:(!, and thepressure and temperature profiles along the engine./mage of the (rent 00 courtesy =olls =oyce :lc.


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(he different materials used in a =olls =oyce jet engine. /n blue, titanium isideal for its strength and density, but not at high temperatures, where it isreplaced by nic#el based superalloys red!. /n orange) steel used for the staticparts of the compressor./mage courtesy ;ichael 7erven#a, =olls =oyce

/n this case, 'low temperature' blades are made of titanium alloys, while hightemperature components use Bi base superalloys. (he most severe conditionsare met in the first row of the turbine. (he entry temperature is around $&00 o7.(emperatures are #ept lower at the surface of the blade because of the coolingsystem ceramic surface approaching $$00 o7!, and the thermal coat ta#esanother $ 200 o7 leading to a metal temperature in the vicinity of %30 o7.

Blade Degradation

(he high pressure turbine of a jet engine provides one of the most severeenvironment faced by man made materials. (o temperatures approaching thesubstrated melting point, one must add the considerable stresses caused byrotation at more than $0,000 rpm.

Cith today's jet engine operating temperatures, thermal barrier coating failureresults in melting of the blade. *ut even without reaching such catastrophic

failure, blades suffer from accelerated oxidation and, depending on the


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environment, hot corrosion. 7oatings can considerably enhance theoxidation9hot corrosion resistance of these components, as illustrated below)

(he result of 2+00 h low altitude sea flight service on anuncoated and Bi-l coated blade turbine blade, from 5s#ner,200&.

O idation is the reaction between the coating or in its absence, base alloy!with the oxidants present in the hot gases.Hot corrosion occurs from surface reactions with salts deposited from thevapour phase.

-s discussed in the next section, different service conditions result in differentdegradation mechanisms being predominant./n addition of oxidation and hot corrosion, coatings will evolve through diffusionwith the substrate alloy, as they are not in thermodynamic e uilibrium with thelatter.(his is of concern, not only because it may modify the carefully designedmechanical properties of the substrate, but also because loss of -l to thesubstrate reduce the oxidation life of the coating.

Bibliogra!h"

$. 5s#ner ;., :h< thesis, =oyal /nstitute of (echnology, toc#holm, 200&


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Coatings

(ypical coatings for high temperature applications involve an oxidation resistantcoating and a thermal barrier coating (*7!. (he oxidation resistant coating isalso called bond coat because it provides a layer on which the ceramic (*7 can

adhere.

/n many papers, (*7 is used to refer to the ensemble bond coatD(*7, in thefollowing, / will use (*7 only to refer to the ceramic thermal barrier.

/llustration of a typical coating system in a high pressure turbineblade. 7loc#wise, a (*7 coated high pressure turbine blade,view from top showing the cooling systems image courtesy;ichael 7erven#a, =olls =oyce :lc!, and schematic profiletemperature, note the drop of temperature close to the bladesurface due to the presence of a thin cooling air film 5;picture courtesy ;. :usch !.

-s noted in the introduction, the (*7 is oxygen transparent and therefore doesnot provide any oxydation resistance.

Bote) the following will ma#e extensive use of these abbreviations)

•

TGO )thermally grown oxide• TBC )thermal barrier coating

http://www.mspusch.de/Ing-Diplomarbeit/2_GeneralBackground.htmhttp://www.mspusch.de/Ing-Diplomarbeit/2_GeneralBackground.htm


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• #$S )air plasma spray• %$$S )low pressure plasma spray• &B$'D )electron beam physical vapour deposition

Bibliogra!h"

1. :adture et al., cience, ()* )2002, 2 0 2 &, '(hemal barrier coatings forgas turbine engine applications'.

Bond Coats+ ,ntroduction

(he following table compares the severity of the different surface relatedproblems for gas turbine applications)

o idation hotcorrosion interdiffusionthermalfatigue

#ircraftengines severe moderate severe severe

%and-based!ower

generationmoderate severe moderate light

Marineengines moderate severe light moderate

7omparison of problems for gas turbine applications, after E. . :ettit andA. C. Aoward, 7oatings for >igh (emperature -pplications, -pplied

cience :ublishers, $% 3

=ecent generations of superalloys for single crystal turbine blades containrelatively high percentages of refractory elements such as (a, C or =e which

enhance the high temperature mechanical properties 7hen, $%%4!.

(his, however, is done at the expense of 7r and -l. Aiven the severeenvironmental conditions in which the blades operate, the removal of theelements beneficial for oxidation resistance! implies even greater degradationproblems.

(o palliate for this lac# of appropriate oxidation.corrosion resistance, an externalcoating is applied to the blades. /ts purpose is to allow for the growth of aresistant oxide layer. Of all possible oxides, F -l 2O 3 offers excellent protectionand very low growth rates in a minority of cases, 7r oxides are preferred!. (hecomposition of the coating must therefore be chosen carefully so as to ensuregrowth of F -l 2O 3.


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Optimum coating composition in relation to oxidation and hotcorrosion resistance, after ;. chGt1e, 7orrosion and5nvironmental , 2000.

(he two most widely used types of coatings are aluminides Bi-l or Bi 2 -l 3! andMCrAlY `emcrawlee', where ; is Ee and9or 7r! coatings. (he formers areobtained by surface enrichment by diffusion, the later by plasma spray or

5*:H


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concept of a coating whose life does no longer condition that of the blade, hasnot been achieved yet. Cith the increasing cost of the blades themselves, thepractise of coating renewal has developed. /n this operation, a gas turbine ista#en apart, each blade is dismantled, its surface cleaned and a new coatingapplied.

Cith, on one hand, the prospect of coating failure leading to catastrophic bladeloss, and on the other hand an extremely expensive maintenance process, it isno surprise that much research is dedicated to improving the evaluation ofremnant life.

Bibliogra!h"

$. 7hen @. >. et al., urf. 7oat. (echn., )( )$%%4, 6% 44, '


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coating of small parts.

(he -l Bi phase diagram.

:ac# cementation falls in the category of chemical vapour deposition. /n thisprocess, the components to be coated are immersed in a powder mixturecontaining -l 2O 3 and aluminium particles. -bout $ 2 wtK of ammonium halideactivators are added to this `pac#'. (his is then heated to temperatures around

00 $000 o7 in argon or > 2 atmosphere. -t these temperatures, aluminiumhalides form which diffuse through the pac# and react on the substrate todeposit -l metal.

(he activity of -l maintained at the surface of the substrate defines twocategories of deposition methods) low and high activity, also referred to asoutward and inward diffusion respectively.

/n cements with low aluminium contents low activity9outward!, the formation ofthe coating occurs mainly by Bi diffusion, and results in the direct formation of anic#el rich Bi-l layer. (he process re uires high temperature $000 $$00 o7!. /nservice, the interdiffusion with the substrate is very limited and the gradient of -lin J is low.

/n cements with high aluminium contents high activity9inward!, the coatingforms mainly by inward diffusion of aluminium and results in formation of Bi 2 -l 3 and possibly J Bi-l. -lumini1ing temperatures can be lower 400 %+0 o7!. (herecan be a high -l concentration gradient in the coating, and also significant

interdiffusion with the substrate during service. Eor these reasons, a diffusionheat treatment is generally given at $0+0 $$00 o7 to obtain a fully J layer.


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icrostructures of two types of aluminides coating on superalloy, left! highactivity"inward diffusion, right! low activity"outward diffusion, from . Es#ner, Ph$

thesis, %oyal nsitute of 'echnology, (toc#holm

(he structure and composition of the coating depends on the substrate,implying that coatings must be tailored for a given alloy. -luminide coatings lac#ductility below 4+0 o7. One of the major problems faced by aluminide coatingsis thermomechanical fatigue, as cyclic strains induced by temperature gradientsin the blades can lead to thermal fatigue crac#s.

,nfluence of Substrate

(he substrate composition strongly influences the final microstructure of thesystem, in a manner which also depends on the process.

/n low activity9outward diffusion coatings, the alloying elements present in thesubstrate will also tend to diffuse into the coating layer, to an extend limited bytheir solubility. /n high activity9inward diffusion coatings, they enter in solutionthe compound layer in formation, or as precipitates potentially forming duringthe treatment.

Of the elements present in the alloy, titanium is thought to be particularlydetrimental to the oxidation resistance of such coatings, as it leads to theformation of (iO 2 crystals which brea# the alumina layer.

-n typical microstructure of low activity aluminide coating is illustrated below.(he external 1one is typically -l rich J Bi-l, while the internal one is Bi rich.


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chematic illustration of aluminide coating obtained by lowactivity pac# cementation. -fter =. :ichoir, in >igh (emperature

-lloys for Aas (urbines, igh (emperature -lloys for Aas (urbines, igh (emperature :ropertiesand -pplications, ;.A. ;oc#ing et al., ?ongman cientific and (echnical,

;arlow.3. 7oc#ing @. ?. et al., urf. ci. (echn., /* )$% , 34 &4, '7omparativedurability of six coating systems on first stage turbine blades in theengines of a long range maritime patrol aircraft'.

&. iva#umar =. et al., urf. ci. (echn., /7 )$% %, $3% $60, '>ightemperature coatings for gas turbine blades) a review'.

+.


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Bond Coats+ $t-#luminides

Bi-l coatings tends to suffer strongly from interdiffusion with the substrate,which results in formation of M' at the expense of J.

(he idea of introducing a diffusion barrier lead to the invention of the :taluminide coatings, which are obtained by similar methods as conventionalaluminide, but after electroplating the blade with :t. (he layer of :t deposited istypically of + $0 Nm chGt1e, 2000!.

*ecause the plating can increase the life of the blades by up to three timesiva#umar, $% %!, the cost of :t plating is easily compensated.

/nterestingly, it was found that :t additions not only did not provide a diffusionbarrier, but also enhanced -l diffusion 7hen, $%%4!. Chen applied on thesecond generation superalloy 7; I&, :t formed (7: topologically closepac#ed! phases with some elements of the substrate =e, C, ;o, 7r!.(he exact reasons for the beneficial effect of :t are not fully understood, but itwas found that :t improves oxide adherence and also contributes to better hotcorrosion resistance.

:t appears to partially substitute Bi in J Bi-l, and also to form :t-l 2 which isbelieved to act as an -l reservoir. /t has also been proposed that :t acts in asimilar manner as " in ;7r-l" coatings. /n these coatings, " combines with this greatly increase the coating life as is otherwise detrimental to theadherence of the oxide layer. (here is nevertheless no evidence of a similareffect of :t in J Bi-l coatings 5vans, 200$!.

urther reading$. im A. ;. et al., cripta ;ater., 2* )2002, & % &%+, '(he effect of the type

of thermal exposure on the durability of thermal barrier coatings'.2. 5vans -. A. et al., :rog. ;ater. ci., 2* )200$, +0+ ++3, ';echanisms

controlling the durability of thermal barrier coatings'.3. 7hen @. >. et al., urf. 7oat. (echn., )( )$%%4, 6% 44, 'igh

temperature coatings for gas turbine blades) a review'.+. ;. chGt1e ed., 7orrosion and 5nvironmental


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Overlay coatings as opposed to diffusion coatings, provide more independencefrom the substrate alloy, but also more flexibility in design as compositions canbe modified depending on the degradation mechanisms expected to prevail.(ypical ;7r-l" bond coats ;PEe,7o or Bi! contain at least & elements, whichmeans that coating methods such as pac# cementation are considerably more

difficult to use, as the activity of each element in the pac# would have to becontrolled carefully so as to obtain a coating of re uired composition.

(he presence of a significant amount of 7r give these coatings excellentcorrosion resistance combined with good oxidation resistance.

-lternative methods are therefore preferred, such as air plasma spray -: !,low pressure plasma spray ?:: !, or electron beam physical vapourdeposition 5*:H

Microstructure

;7r-l" coatings typically exhibit a two phase microstructure JDM. (he presenceof M increases the ductility of the coating thereby improving thermal fatigueresistance.

-s for J Bi-l coatings, high temperature exposure results in depletion of the -lboth to the (AO thermally grown oxide! and to the substrate by interdiffusion.

-s the amount of -l decreases, the J phase tends to dissolve. Eor this reason, itis often described as an aluminium reservoir, and coating life often measure interms of depletion of J.

chematic illustration of;7r-l" microstructure. -l diffusion to the oxide layer and the substrateresult in depletion of beta from both sides


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Com!osition and role of additions

(he M of ;7r-l" stands for either Bi or 7o, or a combination of both whenapplied to steels, it can also be Ee!, depending on the type of superalloy. 7obased appear to have superior resistance to corrosion.

Cr provides hot corrosion resistance, but the amount that can be added islimited by the effect it is expected to have on the substrate, and the formation of 7r rich phases in the coating.

#l content is typically around $0 $2 wtK. ince oxidation life is essentiallycontrolled by the availability of -l, it would be tempting to increases thealulminium content. >owever, this results in significant reduction of ductility forexample, iva#umar, $% %!.

;7r-l" also typically contain $ wtK yttrium 3 !, which enhances adherence ofthe oxide layer. /t was initially thought that "ttrium helped the formation of oxidepegs which helped anchor the oxide layer to the coating.>owever, it has been shown that there is little if any correlation meggil, $% 4!,and it is now believed that the main role of " is to combine with sulfur andprevent its segregation to the oxide layer, which is otherwise detrimental to itsadhesion.

-dditions of hafnium Hf ! play a similar role

(he effect of other additions has also been investigated Bicoll, $% 2!. /t wasfound that silicon Si ! significantly improved cyclic oxydation resistance,however it also decreases the melting point of the coating. + wtK are enough tolower the melting temperature to about $$&0 o7. (here is also evidence that itaffects phase stability. Eor cyclic oxidation at $000 o7, 2.+ wtK was found to bethe optimum content. Eurther additions were detrimental.

-dditions of rhenium 4e ! have been shown to improve isothermal or cyclicoxidation resistance, and thermal cycle fatigue 71ech et al., $%%&!.

-dditions of tantalum Ta ! can also increase the oxidation resistance.

Bibliogra!h"$. Bicoll -. =. et al., (hin olid Eilms, )5 )$%%2, 2$ 3&, '(he effect of alloying

additions on ; 7r -l " systems -n experimental study'.2. meggil @. ?., ;ater. ci. 5ng., 07 )$% 4, 26$ 26+, ' ome comments on

the role of yttrium in protective oxide scale adherence'.3. iva#umar =. et al., urf. 7oat. (echn., /7 )$% %, $3% $60, '>igh

temperature coatings for gas turbine blades) a review'.&. 71ech et al., urf. 7oat. (echn., *0 )$%%&, $4 2$, '/mprovement of

;7r-l" coatings by additions of rhenium'.


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+. =ichard 7. . et al., urf. 7oat. (echn., 0( )$%%6, %% $0%, '(he influencesof heat treatments and interdiffusion on the adhesion of plasma sprayedBi7r-l" coatings'.

Thermal Barrier Coatings

(he role of thermal barrier coatings (*7! is, as their name suggests, to providethermal insulation of the blade. - coating of about $ 200 Nm can reduce thetemperature by up to 200 7.

- (*7 can be used either)

• to reduce the need for blade cooling by about 36K!, while maintainingidentical creep life of the substrate.

• increase considerably the creep life of the blade while maintaining levelof blade cooling and therefore allowing the blade to operate at lowertemperature for an identical turbine entry temperature!.

(he benefits of (*7 in terms of engine efficiency are comparable to thosebrought be the replacement of directionally solidified alloys for single crystalones trangman, $% +!.

*ecause they allow surface temperatures higher than the melting point of thesubstrate, ensuring the integrity of (*7s is critical. 8nderstanding the

mechanisms of (*7 failure is an active area of research. (he failure is lin#edwith large residual compression in the underlying thermally grown oxide (AO!,but details of the mechanism only begin to be understood 5vans et al., 200$!.

Bibliogra!h"• trangman (. 5., (hin olid Eilms, 6(7 )$% +, %3 $0+, '(hermal barrier

coatings for turbine airfoils'.• 5vans -. A. et al., :rog. ;ater. ci., 2* )200$, +0+ ++3, ';echanisms

controlling the durability of thermal barrier coatings'.

TBC Materials

Eor a ceramic coating to have any chance not to spall at the first thermal cycle,it is critical that its thermal expansion be close to that of the substrate.Eor the coating to be of use, it must also exhibit a very low thermal conductivity.

Eor this purpose, yttria " 2O 3! stabilised 1irconia LrO 2! " L! is widely used. (headdition of + $+K yttria stabilises the 1irconia in its high temperature crystallineform, therefore avoiding phase transition in the range of service temperatures.


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Lirconia LrO 2! based ceramics satisfy both re uirements, with a thermalexpansion coefficient of $$ $3x$0 6 $ and a thermal conductivity of about 2.3C9 m. ! at $000 o7 for a fully dense material, this can be further reduced byintroducing porosity 5vans, 200$!

(he very low thermal conductivity of " L is due to the presence of a highconcentration of point defects which scatter lattice vibrations :adture, 2002!

(hermal barrier coatings can be obtained by different processes, such aselectron beam physical vapour deposition 5*:H

chematic microstructure of a thermal barrier coating (*7! obtained by airplasma spray -: !. (he structure provides much less strain resistance thanthe 5*:H< counterpart, but is preferred for abradable seals


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chematic microstructure of a thermal barrier coating (*7! obtained byelectron beam physical vapour deposition 5*:H

(*7 coatings deposited by -: or ?:: low pressure plasma spray! offer athermal conductivity significantly lower than that of a fully dense coating 0.% $C9 m. !!, as the boundaries and pores tend to lie parallel to the surface andtherefore perpendicular to the temperature gradient.*y contrast, the thermal conductivity of 5*:H< (*7 is not as low $. 2 C9

m. !!, and lowering it further would imply further benefits chult1, 2003!.5*:H< (*7s are nevertheless preferred becauce of the strain toleranceimparted by the microstructure.

/n jet engine operating conditions, the lifetime of (*7 coatings obtained by5*:H< has been reported to be between and $3 times longer than e uivalentsystem where the (*7 was deposited by plasma spray chul1, 2003!.>owever, industrial experience indicates that the superiority of 5* :H< (*7s isnot maintained in the operating conditions of land based gas turbine, in which

-: tends to give the best performances Cells, 200&!.

7urrent research includes attempts to tailor the microstructure of the (*7 toinclude benefits of both types of microstructures.

Bibliogra!h"$. :adture et al., cience, ()* )2002, 2 0 2 &, '(hemal barrier coatings for

gas turbine engine applications.2. 5vans -. A. et al., :rog. ;ater. ci., 2* )200$, +0+ ++3, ';echanisms

controlling the durability of thermal barrier coatings'.3. chul1 8., -ero. ci. (echn., 7)2003, 43 0, ' ome recent trends in

research and technology of advanced thermal barrier coatings'.&.


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De!osition $rocesses

(here is a large number of competing processes for coating surfaces, and onlya few of them will be presented in the next sections, which are the most relevantto the application of overlay coatings ;7r-l"! or (*7.

7oating processes can be broadly divided in methods by which atoms attachindividually to the surface, or methods which apply particles. Physical vapourdeposition :H

-n example of 7H< has been described earlier aluminides coatings by pac#cementation !

(he following sections give a very brief overview of the following processes)

• (hermal spraying) air plasma spray -: !, low pressure plasma spray?:: !, high velocity oxyfuel spray >HOE! etc .

• 5lectron beam :H< 5*:


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chematic microstructure of thermal spray coating, showingonly a few layers of particles.

(he adhesion of the coating depends on the cleanliness of the substratesurface, and its area a high surface roughness is desirable for good adhesion!,

on the velocity of the particles, etc .

(here are various types of thermal spraying methods, some of which are brieflyintroduced)

• lame s!ra"ing ) the first method employed uses an oxyacetylene flameabout 2400 Qdegc! to melt and project the coating, fed as wire, rod or

powder. (ypical particle velocity is about &0 m9s, porosity $0 $+ K,identical oxide content, for a deposition rate of $ to $0 #g9h.

• $lasma s!ra"ing ) similar but uses a ionised gas plasma to melt andpropel the coating powder!, temperature of the plasma can exceed$6,000 Qdegc, while the substrate itself will seldom heat beyond $+0Qdegc. :article velocity can reach 200 300 m9s, the porosity is reduced to+ $0 K, the oxide content to $ 3 K for deposition rates of $ + #g9h.

• High velocit" o "fuel >HOE!) spraying uses oxygen and hydrogen witha fuel gas such as methane, particle velocity ranges between 600 $000m9s, oxide and porosity are reduced to $ 2 K for a deposition rate similar to that of plasma spray.

- couple of interesting variants of the conventional plasma spraying are #nownas low pressure plasma spraying %$$S ! and vacuum plasma spray '$S !,which, as indicated by their names, are identical to plasma spraying except for itbeing done in inert gas at low pressure, or in vacuum. (his reduces slowing and


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cooling the particles by air, and avoids oxidation during spraying. (he traditionalmethod is usually referred to as #$S air plasma spray!.

chematic illustration plasma torch.

-: and ?:: are widely used to apply ;7r-l" coatings. (*7s can also beapplied by this method. /n the case of jet engine operation conditions, it hasbeen shown that the use of 5*:H< increases by 4 to $3 times the life of thecoating, therefore easily compensating for the higher cost of the latter process./n the case of land based power generation, -: (*7s last up to twice the lifeof 5*:H< coatings, although the reason is not clearly understood Cells,200&!.

Bibliogra!h"

- good part of the above information comes from)

$. Cells @., =C5 /nnogy, personal communication 2. (C/) flame spraying , >HOE spraying , arc spraying , plasma spraying .3. (here is also a good introduction to plasma spraying here .&. and some information here .

&lectron beam !h"sical va!our de!osition

-s indicated by its name, electron beam physical vapour deposition belongs tothe more general category of physical vapour deposition.

/n these methods, film growth is obtained by condensation of a vapour on thesubstrate. (he vapour can be produced by heating the consumable enough toobtain evaporation, or by mechanically #noc#ing the atoms off e.g. sputtering!.

/n 5* :H


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- simple 5*:H< process, the whole assembly would be undervacuum. =otation of the electron beam is obtained by amagnetic field perpendicular to the drawing.

5*:H< and :H< methods in general are semi-line-of-sight , and produce asurface that replicates the original one an initially smooth surface will result in asmooth coating!.


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efficiency and therefore higher temperatures at the expense of long life meansthat the operating conditions are changed, and the dominant degradationmechanisms may be different.

@et engines and land based power generation turbines provide different

environments. /n general, jet engines high pressure turbine blades >:(blades! are expected to last for about 30,000 h. Eor land based powergeneration, this figure can vary between +0,000 and 4+,000 h about % years!.>:( blades in jet engine will typically undergo one refurbishment strip coatingand re coat! throughout their life in power generation applications, one or tworefurbishments depending on the target life.

-t the time of writing 200&!, rough estimates of costs provided by =C5 /nnogy are power generation!)

• set of blades for >:() $.+ million gbp• cost of refurbishment) variable) 0.3 to $ million gbp.

(he >:( blades in jet engines mainly suffer from oxidation :t aluminidecoatings are preferred in these conditions and are commonly used to coat themain surface.

- jet engine >:( blade.:hoto courtesy . (in, =olls

=oyce 8(7 .

-lumimised jet engine >:( blade.:hoto courtesy . (in, =olls

=oyce 8(7 .

http://www.rweinnogy.com/index.asphttp://www.msm.cam.ac.uk/UTC/http://www.msm.cam.ac.uk/UTC/http://www.msm.cam.ac.uk/UTC/http://www.msm.cam.ac.uk/UTC/http://www.rweinnogy.com/index.asphttp://www.msm.cam.ac.uk/UTC/http://www.msm.cam.ac.uk/UTC/http://www.msm.cam.ac.uk/UTC/http://www.msm.cam.ac.uk/UTC/


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(hey are also used to coat the internal cooling chanels in power generationapplications, this choice being mostly dictated by the process as discussedearlier, aluminide coatings are typically obtained by pac# cementation, which isa non-line-of-sight process allowing the coating of complex geometry!./n power generation applications, the surface of the >:( blades typically

receives an ;7r-l" coating, which is best suited to provide the corrosionresistance re uired in these environments. (he composition of the bond coatcan be adapted depending on the operating conditions e.g. 7r content, additionof i etc .!

Other parts of the >:( blades in jet engines may however receive a differentcoating) the base and tips often receive an ;7r-l" coating as corrosion can bemore important at these locations.

(he conditions in the low pressure turbine are considerably less severe than inthe >:(. *lades are simply aluminised they are not cooled and do not receivea (*7 coatings. /n the case of land based gas turbine, they are not necessarilymade of superalloys, but the coating may contain additions to enhancecorrosion resistance.

#bradable Coatings

(his subject is slightly off topic and is only covered very very briefly. -bradablecoatings are used to reduce the clearance beetween the tip of the blade and thecasing, to an extent that precision machining cannot achieve. ;7r-l" or (*7coatings are used depending on the component temperature. Eor this purpose,a high porosity is usually desirable and thermal spraying is therefore ideal. (heuse of abradable coatings can result in up to $K improvment in efficiency.

%ow-C"cle atigue and Thermomechanicalatigue of Coated Su!erallo"s

Sv etlana Ste8ovic9 %in8:!ing ;niversit"

(he application of coatings to superalloys has mechanical side effects whichcan lead to the premature failure of coated components via low cycle fatigue

?7E! and thermo mechanical fatigue (;E!. (hese are often the limitingmodes of degradation in gas turbines.

;ismatch in the properties of the coating and the substrate alloy areresponsible for exacerbating the failure mechanism. (he mismatch may arisedue to differences in the thermal expansion coefficients, ductility, strength andelastic moduli, and can significantly reduce the fatigue life.

(he mechanical behaviour of the coated systems under the influence of ?7Eand (;E has been studied using experiments that simulate the operationalconditions of gas turbine parts. (he tests were performed on three nic#el basedsuperalloys, polycrystalline /B4%2 and two single crystal alloys designate7; I & and 7*, and four different coatings) an overlay coating -;


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Bi7o7r-l"(a!, a platinum modified diffusion coating =(22 and two innovativecoatings with BiC diffusion barrier /7$ and /73!. (he ?7E tests wereperformed at two temperatures, +00R7 and %00R7, and three strain ranges, $.0,$.2 and $.&K with = P $ and no hold time in laboratory air = is the ratio of theminimum strain to its maximum value!.


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the ductile brittle transition temperature and the slower propagation of crac#scaused by oxidation at the front of the crac# tip.

igure (+ 4T(( on single-cr"stal substrate9 5==>C

igure /+ 4T(( on single-cr"stal substrate9 )==>C


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(he O: (;E life of the coated specimens is shorter when compared to theuncoated specimens. -ll (;E tested specimens have shorter fatigue life than?7E tested specimens. /B4%2 gives shortest life both ?7E and (;E! of allcoated systems. (he O: (;E life of the coated 7; I & was longer than of thecoated 7* specimens. (he -;


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igure 5+ 4T(( on single-cr"stal substrate

#c8nowledgements

(he creation of this document was supported by the >igher 5ducation Eunding7ouncil for 5ngland, via the 8. . 7entre for ;aterials 5ducation.

$. ome of the above comes from discussions with @. Cells, =C5 /nnogyand . (in, =olls =oyce 8(7.

). -bradable 7oatings /ncrease Aas (urbine 5ngine 5fficiency .
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