Pack codeposition of Al and Cr to form diffusioncoatings resistant to high temperature oxidationand corrosion for c-TiAlZ. D. Xiang, S. R. Rose, and P. K. Datta

Thermochemical analyses were carried out for a series of pack powder mixtures formulated for codepositing Al withCr to form diffusion coatings on c-TiAl resistant to high temperature oxidation by the pack cementation process.Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures forcodepositing Al with Cr to form diffusion coatings with an adherent and coherent coating structure. The results ofthe thermochemical calculations performed indicated that codeposition of Al and Cr is possible with CrCl3.6H2Oand AlCl3 activated pack powders containing elemental Al and Cr as depositing sources. However, experimentalresults obtained at 1100°C revealed that CrCl3.6H2O is not suitable for use as an activator for codepositing Al withCr on c-TiAl. It caused a signi® cant degree of degradation indicated by weight losses instead of coating deposition tothe substrate. However, adherent coatings with excellent structural integrity consisting of an outer Cr doped TiAl3layer containing Al67Cr8Ti25 phase and an inner layer containing TiAl3 and TiAl2 phases were successfullycodeposited at 1100°C using pack powder mixtures activated by AlCl3. It is suggested that such coatings wereformed via a sequential deposition mechanism through inward diffusion of aluminium and chromium. Conditionsthat affect the pack codeposition process, and hence need to be carefully controlled, are discussed in relation tothe mechanism of the formation of diffusion coatings with an integral structure free from microcracking onc-TiAl. MST/5489

The authors are in the Advanced Materials Research Institute, School of Engineering, University of Northumbria at Newcastle,Ellison Building, Ellison Place, Newcastle upon Tyne, NE1 8ST, UK (z.xiang@unn.ac.uk).Manuscript received 16 April 2002;accepted 28 May 2002.# 2002 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.

Introduction

With excellent properties of low density and high strengthand modulus retention at elevated temperatures, the inter-metallic alloy c -TiAl has, in recent years, attracted consider-able research interest. Its enormous potential in criticalstrength applications has been investigated, especially inaerospace and power industries where high speci® c strengthand stiffness are required to enhance performance andoperating ef® ciency, critical to the ful® lment of ever increasingtechnical, economical and environmental requirements.Indeed, it is estimated that with the use of Ti ± Al basealloys, potential component weight savings of up to 70%can be achieved over conventional superalloys.1 ,2 However,in order to fully utilise its potential, ef® cient and effectivetechnologies must be developed that can ensure its longterm environmental stability in oxidative and corrosiveatmospheres at high temperatures up to 850°C withoutadversely affecting its mechanical integrity.1

Signi® cant progress has been made in microstructuremodi® cation and control through additions of ternaryalloying elements such as W, Nb, Ta, Mo, Cr and Si, whichgreatly improve its ductility at room temperature and creepresistance at high temperatures.2 ,3 Additions of such ele-ments can also increase moderately its high temperatureoxidation and corrosionresistance. Further alloying, althoughbene® cial for further enhancing its oxidation and corrosionresistance, especially with elements such as Cr and Si,usually leads to deterioration of its mechanical properties. Itis now generally recognised that deposition of coatings thatcan form dense, adherent and slow growing oxide scales onits surface offers the most promising approach to providingsolutions for its long term environmental stability at hightemperatures.2 ,3

A number of studies to develop coatings and coatingtechniques for protecting c -TiAl have been reported. In

recent reviews, the strategies to develop high temperaturedegradation resistant coatings for Ti ± Al based interme-tallics were discussed.4 ,5 In particular, coatings of the alloytype MCrAlY (M~Ni, Co) or TiCrAl, which are capable offorming stable Al2O3 scales at high temperatures, havedemonstrated promising application potential.5 ,6 These coat-ings are usually deposited on a substrate surface using variousthermal or plasma spray techniques and more recently usingan electron beam physical vapour deposition process (EB-PVD). However, these facilities not only require fairly highcapital investment but also impose great constraints onoperating ¯ exibility and productivity. More importantly,coatings produced often contain high levels of porosity,particularly in the case of thermal sprays, which can severelydegrade the long term effectiveness in protecting the sub-strate. A more ef® cient process is clearly required fordepositing protective coatings on c -TiAl.

The pack cementation process is a versatile and econo-mical process normally used to deposit aluminide diffusioncoatings on nickelbase superalloys.7 ± 10 However, it is expectedthat this generic technology can be further developed toenable deposition of multiple element coatings on titaniumbase alloys including c -TiAl. In comparison with othertechniques for the deposition of coatings on metal alloysubstrates, the pack cementation process has the followingadvantages:

(i) high volume and economical deposition of diffu-sion coatings with easily controllable thickness upto 200 mm

(ii) simultaneous deposition of multiple elements(iii) superior adhesion between coating and substrate(iv) applicable to a wide range of shapes and sizes and

not subjected to line-of-sight restrictions(v) low environmental impact

(vi) low capital investment and low operating cost.Pack cementation is essentially an in situ chemical vapourdeposition process activated by halide salts such as NH4Cl,

DOI 10.1179/026708302225007817 Materials Science and Technology December 2002 Vol. 18 1479

NH4F or NaCl. The substrates to be coated are embeddedinto (in-pack process) or suspended above (out-pack process)a well mixed powder mixture consisting of pure or alloyeddepositing elements, a halide salt as an activator and aninert ® ller (normally Al2O3). The whole pack is then heatedto a high temperature at which the activator reacts with thedepositing elements to form a series of halide vapours. Inthe in-pack process, the substrate is in intimate contact witha loose pack powder mixture that allows the generatedhalide vapours to pass through to reach the substrate surface.The coating is formed by decomposition of these halidevapours on the substrate surface and subsequent solid statediffusion between the deposited elements and the substrate.

In a previous study, the conditions for depositing alumi-nium on c -TiAl to form aluminide coatings of TiAl3 andTiAl2 with a coherent structure free from microcracking bypack cementation were analysed using the thermochemicalanalysis technique and veri® ed by the results of experi-mental studies.1 1 The coatings produced were found to becapable of forming stable Al2O3 scales in air at temperaturesup to 850°C. The present investigation is part of an ongoingresearch programme to further apply the latest thermo-chemical analysis techniques to develop an easily control-lable pack cementation process capable of simultaneouslydepositing multiple elements on c -TiAl to form diffusioncoatings with de® ned thermochemical and mechanicalproperties. This paper reports the results of an initial inves-tigation, by means of thermochemical analysis in com-bination with experimental studies, into identifying packcompositions and coating conditions suitable for codeposit-ing Al with Cr to form diffusion coatings on c -TiAl via asingle step codeposition procedure. Introducing Cr into thesurface layer of c -TiAl is expected to increase the ductility ofthe surface layer and to provide enhanced protection notonly against high temperature oxidation but also againsthot corrosion.

Thermochemical analysis

In the pack process, halide vapours generated within thepack at high temperature critically in¯ uence the depositionprocess and hence the type of coatings formed on thesubstrate surface. The partial pressures of these halidevapours are determined by the composition of the packpowder mixture and the coating temperature. These thermo-dynamic parameters can be calculated with the assistance ofimproved computer software and database systems forthermochemical analysis. The results may then be used toidentify suitable pack parameters that need to be effectivelycontrolled to enable deposition, from the vapour phase, ofthe required elements with the proportions necessary for theformation of the intended diffusion coatings on c -TiAl. Inthis study, partial pressures of halide vapours for a range ofpack powder compositions were calculated using the Chem-Sage computer program in combination with the SGTEdatabase system. The results obtained were used to identifythe best possible coating conditions and pack powdermixtures for codepositing Al with Cr to form diffusioncoatings on c -TiAl. The program performs the calculationsbased on a Gibbs energy minimisation technique and themass conservation rule.1 2 For all the calculations under-taken in this study, the total pressure within the packs wasassumed to be one atmosphere.

PACK POWDER MIXTURES FORCODEPOSITING Al AND CrThe pack powder mixtures for codepositing Al and Crto form diffusion coatings on metal substrate are normallyexpected to be made up of powders of a master alloy of Al

and Cr as a depositing source, halide salts as activators andalumina (Al2O3) as an inert ® ller. However, in a previousstudy, it was demonstrated that elemental Al and Cr couldbe used as depositing sources to achieve the requiredconditions for codepositing Al with Cr on nickel basesuperalloys.1 3 Therefore, a mixture of elemental Al and Crwas used in this study as a depositing source. Using individ-ual depositing elements as depositing sources instead oftheir alloys, as conventionally recommended, is not onlymore economical but also provides an easy way of adjustingpack powder compositions and hence their depositioncharacteristics.

At the coating temperatures, Al and Cr react with halidesalts to form a series of aluminium and chromium halidevapours. In order for the intended codeposition to occur,the vapour pressures of aluminium halides should ideally becontrolled over a range comparable to that of chromiumhalides at the speci® ed coating temperatures. This may beachieved by adjusting the aluminium and chromium contentin the pack and by carefully selecting a suitable halide salt asan activator.

SELECTION OF ACTIVATORSThe type of halide salt selected as activator is of particularimportance in determining the partial pressure distributionof the various halide vapours generated at the coatingtemperature and hence the depositing tendency of the packpowder mixture. Unstable salts such as NH4F and NH4Clare often used as activators in the pack cementation processfor their activation effectiveness.1 1 However, these saltsdecompose rapidly at high temperatures and can generateundesirably high pressure in the sealed retort or crucible,which imposes a safety risk unsuitable for a viable industrialprocess. Stable salts such as NaCl, CaCl2 , MgF2 , etc., arealso sometimes used as activators. Often, however, thesesalts are avoided by the aerospace industries largely dueto concerns that released alkaline or alkaline earth metalelements may be unintentionally deposited into thesubstrate surface at the coating temperature, which mayadversely affect the properties of the substrate. A preferredalternative would be to use stable halide salts that containonly the same metal elements as those to be deposited. Thus,the possible choice of halide activators for codepositing Alwith Cr would include AlF3 , CrF3 , CrCl3 .6H2O and AlCl3 .Since the chloride salts are considered more environmen-tally friendly than the ¯ uoride salts, in the present studyCrCl3 .6H2O and AlCl3 were evaluated as activators forcodepositing Al with Cr.

PACK MIXTURES ACTIVATED BY CrCl3.6H2OIn pack powder mixtures containing Al and Cr elementsand chloride salts, the major metal chloride vapour speciesgenerated at high temperatures are AlCl, AlCl2 , AlCl3 ,CrCl, CrCl2 and CrCl3 . Of these vapour species, AlCl andCrCl are the deposition species that are mainly responsiblefor both transporting and depositing Al and Cr on thesurface of the substrate. Other species may contribute to theprocess of transporting Al and Cr within the pack.However, at this stage, it is not clear how much they aredirectly involved with the process of releasing Al and Cronto the substrate surface. Nevertheless, it has been shownthat the deposition tendency of a pack powder mixturecan be analysed by examining the levels of partial pressuresof only AlCl and CrCl species generated at the coatingtemperature.1 3

Figure 1 shows a plot of the vapour pressures of AlCl andCrCl at 1000°C as a function of aluminium content for thepack series 5Cr ± xAl ± 4CrCl3 .6H2O ± (91 ± x)Al2O3 (wt-%).It can be seen that in order for this series to codeposit Alwith Cr, aluminium content in the pack should be controlled

1480 Xiang et al. Heat resistant coatings for c-TiAl

Materials Science and Technology December 2002 Vol. 18

in the range 1.7 ± 2 wt-%, ideally, in the vicinity of 1.9 wt-%,at which the vapour pressure curve of AlCl intersects that ofCrCl.

Figure 2 illustrates the dependence of the partial pres-sures of AlCl and CrCl on temperature for a pack con-taining 1.88 wt-%Al in the series. It can be seen that thepartial pressures of these two vapour species are of the sameorder of magnitude in the temperature range 850 ± 1200°C,indicating that codeposition of Al and Cr in the pack ispossible within this temperature range. However, it shouldbe pointed out that, at any speci® ed coating temperature,the conditions under which codeposition within a pack cantake place are also in¯ uenced by the thermochemical pro-cess of releasing the depositing elements through decom-posing reactions at the substrate surface and the subsequentinterdiffusion process between the released elements and thesubstrates.14 For Ti ± Al and nickelbase alloys, the interdiffusionprocess following the release of elements is often reactive bynature, resulting in the formation of intermetallic compounds.

PACK MIXTURES ACTIVATED BY AlCl3Figure 3 shows a plot of vapour pressures of AlCl and CrClagainst temperature for a pack of composition 20Cr ±1.9Al ± 2AlCl3 ± 76.1Al2O3 (wt-%). It can be seen that thevapour pressure of AlCl is much higher than that of CrCl inthe temperature range studied, indicating that codepositionof Al and Cr is unlikely to occur in this pack. Thus, in orderto codeposit Al with Cr, the aluminium content in the packwill have to be much lower than 1.9 wt-%. However, itshould be pointed out that the pack chemistry can alsobe adjusted by varying the quantity of activator in thepack.

Experimental procedures

The c -TiAl substrate used in this study was supplied byABB with a nominal composition of Ti ± 31Al ± 8.6W (wt-%).The alloy rod of about 20 mm in diameter was sliced intobuttons with a thickness of about 2 mm. Surfaces of thebuttons were ground and polished to a 1200 grit ® nish.Samples were then degreased and weighed before placingthem in pack powders.

Pack powder mixtures were prepared by weighing outand mixing appropriate amounts of Al2 O3 , Al, and Crpowders and a halide salt. The particle sizes of Al2 O3 , Al,and Cr powders were less than 50 mm, 75 mm, and 45 mmrespectively. The halide salts used as activators wereCrCl3 .6H2O and AlCl3 (anhydrous). These chemicals wereground by hand using an agate mortar and pestle, but notsieved, before being weighed and added into the packpowders.

The packs were prepared by embedding the substrates inthe pack powders in a cylindrical alumina retort of 30 mmdiameter and 40 mm length. The retort was then sealed withan alumina lid and cement. The cement seal was cured for atleast 1 h at room temperature and then further cured at anoven temperature of about 80°C for at least 2 h. The packwas then loaded into an alumina tube furnace ® tted with gascirculation ® ttings. The furnace was circulated with argonand the temperature was raised to and held at 150°C for 2 hto facilitate further cure of the cement and to remove anymoisture from the pack. The furnace temperature was thenraised to a ® nal coating deposition temperature, normally1000°C or 1100°C, at a heating rate of 10°C min ­ 1 and washeld there for the required duration. The furnace was thencooled to room temperature at its natural rate by switchingoff the heating power supply while maintaining the argongas ¯ ow. The coating times reported were the holding timesat the coating temperatures.

The coated samples were characterised using X-ray dif-fraction (XRD) and optical microscopy techniques. Thecross-sections of the coatings were analysed using scanningelectron microscopy (SEM) equipped with energy dispersivespectroscopy (EDS) and backscattered electron imagingfacilities. The coating thickness was estimated from theEDS data.

Results and discussions

CODEPOSITION BY PACKS ACTIVATED BYCrCl3.6H2OIn a previous study, it was demonstrated that codepositingAl with Cr on nickel base superalloys is possible using packs

1 Vapour pressures of AlCl and CrCl as function of Alcontent in pack series 5Cr ± xAl ± 4CrCl3.6H2O ± (91 ± x)Al2O3

(wt-%) at 1000°C

2 Temperature dependence of vapour pressures of AlCland CrCl in pack 5Cr ± 1.9Al ± 4CrCl3.6H2O ± 89.1Al2O3

(wt-%)

3 Temperature dependence of chloride vapour pressuresin pack 20Cr ± 1.9Al ± 2AlCl3 76.1Al2O3 (wt-%)

Xiang et al. Heat resistant coatings for c-TiAl 1481

Materials Science and Technology December 2002 Vol. 18

activated by CrCl3 .6H2O.1 3 Thus, in this study, the ® rstattempt to codeposit Al with Cr on c -TiAl was made usingCrCl3 .6H2O as an activator. According to the thermochem-ical calculation results presented in Fig. 1, in order to achievecodeposition conditions, the Al content in the pack shouldideally be controlled at about 1.9 wt-%. Therefore, a packmixture with a composition of 1.9Al ± 5Cr ± 3CrCl3 .6H2O ±90.1Al2O3 (wt-%) was prepared and the specimens werecoated at 1100°C for 8 h. However, after the pack coat-ing treatment, the specimens showed a considerable weightloss of nearly 2 wt-%, indicating substantial degradation ofthe specimens during the coating process. The specimensurface turned black and looked very rough, showing evid-ences of degradation instead of element deposition. Thisresult suggests that CrCl3 .6H2O is not a suitable activatorfor codepositing Al with Cr on c -TiAl. This is in sharpcontrast with the observation that CrCl3 .6H2O can be usedto deposit Al and to codeposit Al with Cr on nickel basesuperalloys, with the coating produced having an excellentsurface ® nish.7 ,1 3 The exact reasons for the degradation ofc -TiAl during the coating process are still under investiga-tion but it is almost certain that it was caused by thedeteriorating effect of the large volume of water vapourreleased from the activator at the coating temperature. Inthe presence of chlorine, water vapour would react with Cl2

to form HCl with a partial pressure high enough to causesubstantial degradation to c -TiAl, which is known to bemuch less oxidation and corrosion resistant than nickel basesuperalloys at elevated temperatures.

CODEPOSITION BY PACKS ACTIVATED BY AlCl3After observing the corrosive nature of CrCl3 .6H2Otowards the c -TiAl substrate, experimental efforts were

focused on codeposition with pack powder mixtures acti-vated by AlCl3, which contained no intrinsic water apartfrom a small quantity of moisture absorbed from the atmos-phere. A series of pack powder compositions were formulatedand prepared with Al content varying from 1 to 4 wt-%, Cr1 to 6 wt-%, AlCl3 1 to 4 wt-% with the balance beingAl2O3 . Samples were coated for 8 h at 1100°C. With carefuladjustment of pack compositions, it was found that code-position of Al and Cr on c -TiAl could indeed be achievedunder these coating conditions.

Figure 4 shows a cross-sectional SEM image and elementconcentration pro® les in the coating layer for a specimencoated in a pack activated by AlCl3 at 1100°C for 8 h. Afterpack treatment, the substrate showed a weight gain of about0.06 mg cm

­ 2. It can be seen that the coating consisted of

two major, visually clearly distinguishable layers. A thin toplayer with a thickness of about 2 mm was enriched with bothAl and Cr, suggesting that both Al and Cr were deposited inthis layer. To illustrate more clearly the Cr concentrationpro® le in the top coating layer, an enlarged section of the Crconcentration pro® le is shown in Fig. 5. It can be seen thatCr concentration in the top layer was about 4 wt-%, whichrepresents a signi® cant enrichment of Cr. An inner layerwith a thickness of about 24 mm was enriched only with Al,indicating deposition of only Al.

The coated surface looked very smooth with a metalwhitish grey colour. Close examination under an opticalmicroscope revealed no microcracks in the coated surface,con® rming the formation of a coherent and adherent coat-ing under the deposition conditions used.

Figure 6 shows an XRD spectrum measured from theas coated surface. The labelled peaks belong to individualidenti® ed phases and each of the unlabelled peaks belongsto two or more detected phases as a result of overlapping.It can be seen that the spectrum is very complex, showingcharacteristics of the presence of multiple phases in thesurface layer. The phases detected were Ti2 5Al6 7Cr8 , TiAl3 ,Ti5Al1 1 and TiAl2 . As stated earlier, the top coating layerwas enriched with both Al and Cr and the SEM imageshowed that it had the appearance of a uniform phase.Thus, it may be reasonable to suggest that the major phasein this top layer is essentially Cr doped TiAl3 . This would beconsistent with the general observation that TiAl3 is themajor phase in the top layer of aluminised c -TiAl.1 1 ,1 5 ,1 6

Nevertheless, further studies are needed to ascertain thephases present in the top layer of the coating.

However, with Al concentration decreasing from about76 wt-% to about 52 wt-% and Ti concentration increasingfrom about 19 wt-% to about 37 wt-%, it is almost certainthat the major phase in the inner layer varied from the Alrich TiAl3 to TiAl2 across the depth of this layer in a similar

a

(b)

4 Cross-sectional SEM image and element concentrationpro® les in coating layer for specimen coated inCrCl3.6H2O activated pack at 1100°C for 8 h

5 Enlarged section of Fig. 4 showing Cr concentrationpro® le in coating layer

1482 Xiang et al. Heat resistant coatings for c-TiAl

Materials Science and Technology December 2002 Vol. 18

manner to that observed in the aluminide coatings formedon c -TiAl by aluminising.1 1

The EDS data plotted in Fig. 4 showed that Al con-centration decreased and Ti concentration increased almostlinearly across the whole depth of the coating layer. Such agradual and continuous change in composition across thethickness of the coating no doubt provided a crucial thermalexpansion gradient, especially at the boundaries of differentlayers, that ensured integrity of the coating during cooling.Without such a gradient, microcracking would occur in thecoating due to the thermal stresses induced by mismatch ofthermal expansion coef® cients between the coating and sub-strate, as observed in the aluminide coatings produced byother investigators.1 5 ,1 6

The SEM image shown in Fig. 4 revealed that thethickness of the top layer was not uniform. The localisedthickening of the coating in areas at or close to the grainboundaries of the inner layer was probably caused by thehigher Cr diffusion rate in these areas. It can also be seenthat small precipitates were present and evenly distributedin the inner layer. These precipitates contained high concentra-tion of W, suggesting that the precipitation was caused bythe limited solubility of W in the TiAl3 and TiAl2 phases.

The observed two layer coating structure strongly sug-gests that codeposition took place in a sequential mannerduring the coating deposition process. In the initial stage,Al was the only element deposited and an aluminide coatinglayer containing TiAl3 and TiAl2 was formed via inwarddiffusion of Al. As the deposition process progressed, theactivity of Al in the pack, particularly in the vicinity of thesubstrate, decreased and that of Cr increased, creatingfavourable conditions for depositing Cr as well as Al. Thus,codeposition of Al and Cr occurred only at a later stage ofthe coating process. Such a sequential deposition mechan-ism is very similar to that observed in the process of code-positing Al with Cr on nickel base superalloys.1 3 However,in the latter case, only Cr was deposited in the second stageof the codeposition process.

It is thus demonstrated that AlCl3 can be used as anactivator to codeposit Al with Cr to form diffusion coatingson c -TiAl with an adherent and coherent two layer structurefree from microcracking. As a stable activator, AlCl3 hasclear advantages over unstable activators such as NH4F andNH4Cl. However, it must be pointed out that althoughAlCl3 is `stable’ at high temperatures it is unstable at roomtemperature. It can react violently with water and has astrong tendency to absorb a large quantity of moisturefrom the atmosphere. This can greatly alter its activatingproperties and hence the deposition chemistry of the packs.Activators that are stable both at room temperature and athigh temperatures clearly need to be identi® ed for an easily

controllable industrial process. In this respect, mixed acti-vators such as NaCl/NH4Cl or AlF3 /NH4Cl, the ratio ofwhich can be easily adjusted to modify the pack chemistry,could offer some promising opportunities in codepositingmultiple elements to form robust diffusion coating systemsfor c -TiAl.

It is expected that the Al and Cr codeposited coating willprovide effective protection for c -TiAl against high tempera-ture oxidation due to its ability to form a stable Al2O3

scale.5 The results and detailed analyses of an oxidationstudy on coatings produced at temperatures up to 850°Cwill be presented in a future paper.

Conclusions

Thermochemical analyses of a series of pack powdermixtures for depositing Al with Cr on c -TiAl by the packcementation process were carried out using a computerprogram ChemSage in combination with a database systemSGTE. Based on the results obtained, experimental studieswere undertaken to identify optimum pack powder mixturesfor codepositing Al with Cr to form diffusion coatings withan adherent and coherent structure free from microcrack-ing. It was demonstrated that thermochemical analysisprovided a useful tool in designing pack compositions andin selecting suitable halide salts as activators to enableprecise control of the deposition process and hence of themicrostructure of the coatings produced.

The results of thermochemical calculations for the packactivated by CrCl3 .6H2O indicated that codeposition of Aland Cr is possible using elemental Al and Cr as depositingsources. However, the results of coating deposition experi-ments suggested that CrCl3 .6H2 O is not a suitable activatorfor codepositing Al with Cr on c -TiAl. It caused substantialdegradation of the substrate.

The results of thermochemical calculations for packsactivated by AlCl3 suggested that in order to achieve code-position of Al and Cr the aluminium content in the packpowders should be much lower than 1.9 wt-%. The experi-mental results demonstrated that codeposition could indeedbe achieved at a coating temperatures of 1100°C using packpowder mixtures containing 1 to 4 wt-%Al, 1 to 6 wt-%Crand 1 to 4 wt-%AlCl3 , with the balance being Al2 O3 . Withcareful control of pack compositions and coating depositionconditions, a coating consisting of an outer layer containingAl6 7Cr8 Ti2 5 phase and an inner layer containing TiAl3 andTiAl2 phases was produced. The coating had an adherentand coherent structure free from microcracking. It is sug-gested that this coating is formed via a sequential depositionmechanism through inward diffusion of Al and Cr.

Acknowledgements

The authors wish to thank ABB, Switzerland, for supplyingthe substrate material for this study.

References

1. m. p. brady, w. j. brindley, j. l. smialek, and i. e. locci: JOM,November 1996, (11), 46 ± 50.

2. w. kaysser: Surf. Eng., 2001, 17, 305 ± 312.

3. p. r. bhowal, h. f. merrick, and d. e. larsen, jr: Mater. Sci.Eng., 1995, A192, (193), 685 ± 690.

4. p. k. datta, j. s. burnell gray, and k. natesan: in

`Intermetallic compounds: principles and practice, Vol. 3Process’ , (ed. J H Westbrook and R L Fleischer); 561 ± 588,2001, New York, John Wiley.

6 X-ray diffraction spectrum of coating formed by code-positing Al with Cr on c-TiAl at 1100°C for 8 h

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5. m. p. brady, b. gleeson, and i. g. wright: JOM, January 2000,(1), 16 ± 21.

6. d. w. mckee: Mater. Res. Soc. Proc., 1993, 288, 953.

7. z. d. xiang, j. s. burnell-gray, and p. k. datta: J. Mater. Sci.,2001, 36, 5673 ± 5682.

8. j. kipkemoi and d. tsipas: J. Mater. Sci., 1996, 31, 6247 ±

6250.9. w. d. costa, b. gleeson, and d. j. young: J. Electrochem. Soc.,

1994, 141, 2690 ± 2698.

10. r. bianco and r. a. rapp: J. Electrochem. Soc., 1993, 140,1181 ± 1190.

11. z. d. xiang, s. rose, and p. k. datta: Surf. Coat. Technol., 2002,

161, (2 ± 3), 286 ± 292.12. g. eriksson and k. hack: Metall. Trans. B, 1989, 21B, 1013 ± 1023.

13. z. d. xiang, j. s. burnell-gray, and p. k. datta: Surf. Eng.,2001, 17, 287 ± 294.

14. s. r. levine and r. m. caves: J. Electrochem. Soc., 1988, 121,

731 ± 741.15. t. c. munro and b. gleeson: Metall. Mater. Trans. A, 1996,

27A, 3761 ± 3771.

16. c. zhou, h. bin, and k. y. kim: Metall. Mater. Trans. A, 2000,

31A, 2391 ± 2394.

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