Surface and Coatings Technology 108109 (1998) 7379

Progress in coatings for gas turbine airfoils*G.W. Goward

40 Uncas Road, Clinton, CT 06413, USA

Abstract

The development of ever more efficient gas turbines has always been paced by the results of research and development in theconcurrent fields of design and materials technology. Improved structural design and airfoil cooling technology applied to higherstrength-at-temperature alloys cast by increasingly complex methods, and coated with steadily improved coating systems, have led toremarkably efficient turbine engines for aircraft propulsion and power generation. For first stage turbine blades, nickel-based superalloysin various wrought and cast forms, and augmented by coatings since the 1960s, have been singularly successful materials systems for thepast 50 yearsand still no real world substitutes are on the horizon. This paper traces the history of protective coatings for superalloyairfoils beginning with the simple aluminides, followed by modifications with silicon, chromium and platinum, then MCrAlY overlaycoatings, and finally the elegant electron beam vapor deposited ceramic thermal barrier coatings recently introduced to service. Thepublicly available results of several decades of research supporting these advances are highlighted. These include generic research onoxidation and hot corrosion mechanisms of superalloys and coatings, the intricacies of protective oxide adherence, mechanisms of lowtemperature (Type II) hot corrosion, and of aluminide coating formation and mechanical properties of alloycoating systems. With nopromising turbine materials beyond coated nickel-base superalloys apparent in the foreseeable future, continued progress will likely bemade by further refinement of control of thermally grown oxide adherence, and by more cost effective manufacturing processes forcontemporary types of protective coatings. 1998 Elsevier Science S.A. All rights reserved.

Keywords: Coatings; Gas turbines; Superalloys aluminides; MCrAlY; Thermal barrier

1. Introduction limitations references have been chosen so as to guidereaders to more detailed bibliographies and relevant photos

The efficiency of all types of gas turbine engines, and tables of data.aircraft, terrestrial and marine, is proportional to firing orturbine inlet temperature. Increases in temperature arefacilitated by improved structural design and airfoil cool- 2. Historicaling technology applied to higher strength-at-temperaturealloys cast by increasingly complex processes, and coated 2.1. Diffusion coatingswith steadily improved protection systems. First stageturbine blades, the most critical components of gas tur- The first public descriptions of pack cementationbines, made from nickel-base superalloys in various aluminizing were by Van Aller in a US patent filed in 1911wrought and cast forms, and augmented by coatings, have [1] and in a 1914 paper by Allison and Hawkins [2], all ofbeen singularly successful materials systems for the past the General Electric Research Laboratory. Metals were50 years. This paper will trace the history of turbine airfoil coated by embedding in a powder mixture of aluminum,coatings from simple and modified diffusion aluminides, sal ammoniac (NH Cl), and graphite, and heating the4through MCrAlY overlay systems, and finally, thermal assembly at 4508C (8428F) for 2 h. Later, Gilson, also atbarrier coatings. Highlights of several decades of research G.E., patented the use of alumina as inert filler [3]; thissupporting these advances will be presented. Attempts will was blended with a chloride and aluminum powder and thebe made to predict future advances in the science and mix used to calorize metals to render them inoxidiz-technology of superalloy coatings. Because of space able. Early uses included coating iron wire or ribbon

heating elements, and copper for power plant steam* condenser tubes. Ruder [4] summarized uses of aluminiz-Tel.: 11-1860-6691272; fax: 11-1860-203-4815039; e-mail:

sailbill35@aol.com ing in 1915, including coating steel furnace fixtures and

0257-8972/98/$ see front matter 1998 Elsevier Science S.A. All rights reserved.PI I : S0257-8972( 98 )00667-7

74 G.W. Goward / Surface and Coatings Technology 108 109 (1998) 73 79

nickel combustion screens. These pioneers correctly attrib- 2.2. Overlay coatingsuted the enhanced oxidation resistance of coated metals tothe selective formation of alumina scales. In about 1942 Pratt and Whitney initiated a program in the late 1960sAnselm Franz in Germany [5] used aluminized low alloy to develop coatings with compositions nominally indepen-steels for combustion chambers, and possibly blades, in the dent of substrates, and with capabilities for tailoring to theJumo 004 engine to avoid the use of short supplies of wide range of requirements of gas turbine applications.nickel and chromium in highly alloyed steels. The engine Research on the effect of reactive elements in improvingpowered the Messerschmidt ME 262 fighter, which entered oxide adherence led to exploitation of the phenomenon inservice in Germany in mid-1944. In the post-war period, new coating compositions. In 1937 Griffiths and Pfeiladditional development and uses of the process up to the patented [27] the use of active element additions, cerium,first practical use of pack cementation aluminizing of mischmetal, etc., to heater element alloys to enhancecobalt-base gas turbine vane airfoils in 1957 [6] are not resistance to thermal cycling. In 1964 an alloy with thewell documented. The first aluminizing of nickel-base composition Fe25%Cr4%Al1.0%Y emerged fromturbine blades may have been by hot dip processes at G.E. nuclear programs [28]. Pratt and Whitney researchAllison [7] and Curtiss Wright in about 1952. Pratt and confirmed that the alloy exhibited exceptional aluminaWhitney introduced nickel-base blades aluminized by a adherence in cyclic oxidation. A model compositionslurry-fusion process in about 1963 [8]. Since about 1970 FeCrAlY [29], with 1015% aluminum, was applied as amost vane and blade coatings have been applied by pack coating to nickel-base superalloys by electron beam phys-cementation and the more recently developed out-of- ical vapor deposition (EBPVD)thickness about 125 mmcontact or chemical vapor deposition (CVD) processes (5 mils)in a cooperative program with Airco Temescal[911]. The surfaces of internal cooling passages are Corp. The coating was not practically useful at highcoated by slurry aluminizing [12], or more effectively, by temperatures because nickel in the substrate alloys reactedforced flow gas phase aluminizing [11,13] and by vacuum with aluminum in the coating to form a layer of NiAlpulse aluminizing [14]. between the coating and the alloy. Next in the series was

Kelley described his invention of pack cementation CoCrAlY [30], which in special composition ranges, haschromizing of steels in 1923 [15]. One of the first uses was useful hot corrosion and oxidation resistance in somefor the protection of steam turbine buckets. Samuel and applications. Within these ranges, however, it did not meetLockington [16] published a comprehensive review of ductility requirements for airfoils in high-performancechromize coatings on steel in 19511952, and Drewett commercial and military engines. Since NiCrAlY has[17] another in 1951. In 1953 Gibson patented aluminizing limited hot corrosion resistance a compromise was soughtof chromized steels with aluminum paints and heat treat- by adding cobalt. This led to a NiCoCrAlY coating ofment to improve high-temperature oxidation resistance exceptional ductility within a useful range of compositions[18]. Time of first use of chromizing of gas turbine airfoils [31]. The nominal composition Ni23%Co15%Cris obscureit may have been in Europe for protection of 12.5%Al0.5%Y had adequate oxidation and hot corrosionindustrial gas turbine airfoils in the early 1960s [19]. Such properties and proved to be satisfactory for protection ofapplications continue for protection of later stage airfoils in blade airfoils in then contemporary Pratt and Whitneyboth ground and aircraft engines. Chromizing, followed by commercial aircraft engines. A follow on with additions ofaluminizing is used on early stage turbine airfoils in many silicon and hafnium [32] is still used on early stage turbineolder aircraft engines. From the 1970s on, developments in airfoils, either alone or as a thermal barrier coatingthe field of diffusion coatings include modification of bondcoat, in Pratt and Whitney commercial engines.aluminide coatings with chromium [20], silicon [21], and Throughout the industry many more complex NiCoCrAlYplatinum [22]. In the 1990s, aluminide coatings have been compositions containing additions of tantalum, tungsten,recognized as useful bond coats for some types of thermal titanium, niobium, rhenium, zirconium, etc., singly or inbarrier coatings [23]. While contemporary commercial, and combination, have been developed for specific purposes.some military gas turbines usually have more advanced Many of these are for protection of early stage airfoils incoating systems, e.g. platinum aluminides, MCrAlYs, etc., industrial turbines. Initial emphasis was on improvedon first stage blades, it is estimated that greater than 80% resistance to hot corrosion, including the low-temperatureof all coated airfoils are coated by pack cementation or the Type II variety, by the use of up to 40% chromium [33].more recently developed out-of-contact aluminizing and/or The higher temperatures in newer industrial turbines fueledchromizing processes. Incorporation of so-called active with natural gas have caused more emphasis to be placedelements in diffusion aluminide coatings to enhance adher- on oxidation resistance and some coatings are now aug-ence of protective alumina appears to have been successful mented by over-aluminizing [34]. There are now at leaston an experimental basis [24], but no such coatings are in 40 patented variations of the original MCrAlY concept.commercial production. References [25,26] provide more Manufacture of overlay coatings by EBPVD dominateddetails on the above items up until 1995. through the 1970s. Low-pressure plasma spray (LPPS),

G.W. Goward / Surface and Coatings Technology 108 109 (1998) 73 79 75

and lately the Union Carbide (Praxair) shrouded plasma obtainable by varying the activities of source material [25],torch, are now favored for more complex formulations. this simple classification has found broad use in theHigh-velocity oxy-fuel (HVOF) deposition is also being coating community. Research by Janssen and Rieck [41]tested. and Shankar and Seigle [42] quantified these unique

diffusion mechanisms. Shankar and Seigle demonstratedwith coatings that diffusion of nickel predominates in

2.3. Thermal barrier coatings nickel-rich NiAl; rates of diffusion of nickel and aluminumare equal at about 51 at.% of aluminum; and aluminum

The authors first experience with thermal barrier coat- diffusion predominates in aluminum-rich NiAl as observedings was with a system for protection of a TD nickel by Goward and Boone. Work by Walsh [43], Levine andtransition duct in a military engine in 1970. A NiCr bond Caves [44] and Seigle and co-workers [26,45,46] on thecoat and MgOZrO top coat, both applied by air plasma thermodynamics and kinetics of pack aluminizing provided2spray, doubled the thermal fatigue life of the duct. Sub- a firm foundation for further technological progress. Guptasequent use of the coatings on burner liners and develop- and Seigle [47] reported on fundamentals for the selectionment work at Pratt and Whitney and NASA, led to the use of the most effective activators for aluminizing andof MCrAlY bond coats [35,36] and topcoats of zirconia chromizing, and refined details of the kinetics of coatingpartially stabilized with 8% yttria [37]. Major programs processes. Johnson and Komarek [48] published data onwere initiated in the late 1970s to solve top coat spalling activity coefficients of aluminum in NiAl, CoAl andproblems with the goal of using insulating properties to FeAl alloys which aids in the selection of source com-lower temperature and cyclic thermal stresses of turbine positions for pack cementation and out-of-contact pro-airfoils. Control of crack formation and substrate tempera- cesses. Rapp and co-worker [24] explored thermodynamicture during coating application provided beneficially mi- details for processes for co-deposition of other elements,cro-cracked and/or segmented structures in the ceramic chromium, silicon, yttrium, hafnium, etc., with aluminum.layer [38]. The improved coatings are useful for vane With regard to mechanisms of protection and degra-platforms and/or airfoils but durability sufficient for use dation, Pettit defined mechanisms of oxidation of nickelon blades proved elusive. A breakthrough came in the late aluminum [49] and nickelchromiumaluminum alloys1970s [39] when it was demonstrated that columnar, low [50]. Research at NASA [51] established the importance ofmodulus zirconia structures applied by EBPVD had ther- interdiffusion of aluminides with base alloys in decreasingmal cyclic lives of the order of a factor of 10 over the best coating life. Aldred [52] disclosed that small amounts ofplasma-sprayed coatings. The early work used EBPVD hafnium in a substrate alloy aided adherence of protectiveNiCoCrAlY as bond coats but either low-pressure plasma- alumina on coatings. Research on hot corrosion [53]sprayed NiCoCrAlY types [36], or diffusion aluminide helped to define which substrate elements would benefit orcoatings [23], modified with platinum, are now in use. decrease the hot corrosion resistance of aluminide coat-Considering that these coatings can lower the temperature ings. Godlewska and Godlewski [54] explored the hotof a cooled blade by up to 1708C (3068F), while simul- corrosion behavior of chromium modified aluminide coat-taneously reducing cyclic thermal strains, the importance ings and found that the best coatings contained chromium,of this technology cannot be underestimated. from chromizing, in the outer layer of inward diffusion

aluminide coatings. Wu et al. [55], using electrochemicalmolten sulfate salt techniques, showed that two-phase(PtAl in NiPtAl) platinum-modified aluminide coatings2

3. Science and technology base had good resistance to high temperature (Type I) but lesserresistance to low temperature (Type II) hot corrosion.

3.1. Diffusion coatings Barkalow and Pettit [56] found that a thin layer of PtAl2on the outer surface of such coatings provided much

In general, practical processes and applications long improved resistance to Type II hot corrosion.preceded development of a significant science and technol- With the discovery that diffusion aluminide coatings canogy base for these coatings. In 1971, research by Goward serve as bond coats for thermal barrier coatings [23], theand Boone [40] provided a qualitative description of goal is now to provide perfect resistance to spalling ofdiffusion mechanisms for the formation of aluminide alumina to allow use of prime reliant thermal barriercoatings on nickel-base superalloys. The results enabled coatings with the capability of decreasing blade airfoilclassification of the coatings as either inward diffusion cooling air flow to gain still higher turbine efficiency.types, based on the singular motion of aluminum in Ni Al Lower sulfur contents of diffusion coatings appear to2 3and high aluminum NiAl, or outward diffusion types, improve alumina adherence [57]. Understanding of thebased on the higher rate of diffusion of nickel in low effects of platinum and various forms of active metals andaluminum NiAl. While some microstructural variations are their oxides [58] will lead to further improvements in the

76 G.W. Goward / Surface and Coatings Technology 108 109 (1998) 73 79

use of diffusion aluminide systems as thermal barrier bond that the beginnings of logical mechanisms which recog-coats. nized the acidbase properties of molten Na SO began to2 4

emerge [53,66,67]. Molten Na SO containing dissolved2 43.2. Overlay coatings sulfur trioxide (SO ), from sulfur in fuels, is acidic,3

whereas that deficient in SO or high in oxide ion3Most research in this area has centered around the concentration, is basic. Dissolution of protective oxides on

active element effect. Lustman [59] summarized knowl- the acidic side is termed acidic fluxing and on the basicedge prior to 1950 on active element-doped nickel side basic fluxing.chromium alloys, and from cyclic oxidation tests, post- Zhang and Rapp [68] and others measured the solu-ulated that the Cr O scales spalled less because of bilities in Na SO of oxides important to surface protec-2 3 2 4pegging by active element oxides. Tien and Pettit [60], tion as functions of acidity, that is, activity of SO in the3came to similar conclusions from oxidation of FeCrAl molten medium. While not absolutely complete, workablealloys doped with yttrium and scandium. Later work by hot corrosion mechanisms based on these phenomena areGiggins and Pettit [61] continued to support the pegging now available to guide coating and superalloy develop-mechanism and also showed that active elements minimize ment. These include the recognition of so called goodvoid formation at the scale metal interface. They demon- elements, chromium, platinum, silicon, and bad elementsstrated that fine dispersions of active element oxides also molybdenum, tungsten, vanadium [53], to use or avoid inimprove alumina adherence to NiCrAl and CoCrAl. In development of hot corrosion resistant superalloys and1986, Ashary et al. [62] apparently abandoned pegging coatings. Some of these fit the acidbase theoryforexplanations, and postulated only that active elements may example MoO , WO and V O are acidic in molten3 3 2 5assist in the formation of strong chemical bonds between Na SO , and SiO is relatively insoluble in acidic media.2 4 2oxide scales and the metal substrates. In 1983 Ikeda et al. Logical explanations of the beneficial effects of platinum[63], and later Funkenbusch et al. [64], showed that small in coatings have not yet emerged. There is thus a workingamounts of impurities, such as sulfur, can segregate to the science and technology base to guide the amelioration ofaluminametal interface and lower the interface bond turbine corrosion in land and marine machines involvingstrength. It followed that active elements must react with intake air filtration, control of detrimental fuel impuritiestramp sulfur to prevent this segregation. Sulfur segregation (sulfur, vanadium, sodium, lead, phosphorus, etc.), and theeffects have since been confirmed by many independent use of improved alloys protected by more corrosioninvestigations. Pint [65], however, in a review of the resistant coatings such as CoCrAlY and platinum-modifiedsubject, contended that such effects are not consistent with aluminides.the fact that active element oxide dispersions also improve In spite of this knowledge, the discovery in 1975 ofscale adhesion. Pint also showed that active elements, such accelerated corrosion of CoCrAlY in a marine gas turbineas yttrium, diffuse through grain boundaries in alumina running at low power, and low airfoil metal temperatures,scales, and lower scale growth rates. Pint proposed a was an unpleasant surprise [69]. Although it had beendynamic segregation theory to explain lower scale anticipated by Cutler [67], who recognized that moltengrowth rates and improved scale adhesionthis involves sulfates, rendered acidic by SO from fuel combustion,3segregation of active element ions to scale boundaries and cause severe corrosion of boiler and superheater steels atto the oxidemetal interface. Thus, the search proceeds about 6008C (11128F), much research was done [56,70]unabated while the practical benefits of the active element before it was recognized that corrosion in turbines andeffect have been commercially exploited for over 50 years. boilers had much in common. The phenomenon is now

universally recognized by the definitions of low-tempera-3.3. Hot corrosion mechanisms ture Type II and high-temperature Type I hot corrosion.

Although Type II hot corrosion in aircraft gas turbines isThe importance of this phenomena within all gas rare, it has been observed by the author, it being easily

turbines warrants a separate, if brief, overview. It was identified by its unique microstructure [71]. It frequentlyrecognized in the early 1960s that surface wastage of occurs in engines on aircraft operated over coastal and sosuperalloy airfoils by hot corrosion was probably caused called island hopping routes. It should be widely recog-by deposition of sodium sulfate (Na SO ), a model but nized by now that chromium [33] and silicon [21] are2 4typical compound, formed by the reaction of sodium particularly beneficial in coatings [72] for protectionchloride, from e.g. sea salt, and sulfur oxide byproducts of against Type II corrosion. It should also be universallycombustion of sulfur-bearing fuels. Because microstruc- accepted that laboratory and burner rig tests involvingtures representative of the process contained sulfides, for deposition of Na SO , which do not control partial pres-2 4many years the process was called sufidation. For this sure of SO in the gas phase, are essentially useless for3reason much early work centered on the effects of sulfur evaluation of corrosion resistance of superalloys andbut it proved difficult to develop a coherent description of coatings. This is particularly true in the Type II tempera-the process on this basis alone. It was not until about 1969 ture range: 600 7508C (110013508F).

G.W. Goward / Surface and Coatings Technology 108 109 (1998) 73 79 77

3.4. Thermal barrier coatings 3.5. Mechanical properties of coatings

In contrast to the science base of diffusion and overlay Early work on mechanical properties focussed on pos-coatings, all of the early development work to improve the sible effects of coatings on superalloy properties such asdurability of these coatings was by empirical experimenta- stress rupture, and isothermal low and high cycle fatigue.tion and evaluation by testing in burner rigs and actual With the exception of high cycle fatigue debits [82], fewengines. Only in the last 510 years has research been real effects on these properties have been observed. Eveninitiated to understand thermal barrier systems to aid in the though some such debits have been observed in laboratorydevelopment of more durable coatings. Early improve- tests, this author knows of no high cycle fatigue bladements in durability involving the use of MCrAlY bond failures directly assignable to coatings. Indeed, real highcoats [35] were based on prior knowledge of the better cycle fatigue failures signal design problems, and areoxidation and hot corrosion resistance, and oxide adher- alleviated when the problems are corrected. Instances ofence of these coating alloys. Experimental work at NASA debits in stress rupture properties have been documented[37] showed the improved durability of partially stabilized for some aluminide coateds on nickel-base alloys, such aszirconia (8%Y O ZrO ) and Ruckle [38] demonstrated Udimet 520 and Rene 80 [83,84] which are particularly2 3 2the beneficial effects of micro-cracked and/or segmented dependent on grain boundary carbides for strength. Theceramic layers. The fact that EBPVD produces columnar debits are caused by depletion of carbon from M C grain23 6grain structures in MCrAlY coatings led Strangman [39] to boundary carbides by strong carbide formers, e.g. tan-explore the properties of columnar grained EBPVD zir- talum, tungsten, molybdenum, which concentrate in theconia-based thermal barrier coatings. Knowledge that diffusion zone by rejection from NiAl during the formationactive elements in nickel-base superalloys and/or platinum of that zone [40]. While the phenomenon is real inmodifications caused improved scale adherence in diffu- laboratory tests, no blade failures attributed to the effectssion aluminide coatings must have surely led to the use of have been reported in the literature. The author is aware ofthese coatings as bond coats [23,52]. The large beneficial failure of coated blades made from a similar alloybut iteffects possible in gas turbine efficiency by the use of was not documented in the open literature. The effect isthermal barriers, e.g. up to 1708C (3068F) lowering of minimal in stronger cast polycrystalline alloys and has notblade metal temperature, have sparked a resurgence in been observed in directionally solidified and, of course,research on all aspects of scale adherence and failure single-crystal alloys.mechanisms involved in loss of ceramic layers. It is Significant thermal fatigue problems are rare in un-anticipated that the extensive research now in motion will cooled airfoilsin some cases coatings increase thermalyield incremental improvements in scale adherence toward fatigue resistance of polycrystalline superalloys in tem-the goal of perfection required to attain prime reliant perature regions above about 8608C (14728F) [85]. This isstatus necessary to take full advantage of saved cooling air. attributed to coating protection of grain boundary crackThe latest summaries of some of this work are collected in initiation sites. With the advent of highly cooled blades andrecent TBC Workshop Proceedings [73,74]. Near perfect vanes, however, serious thermal fatigue problems emerged.oxide adherence is implied both in a recent patent [75], and Early in the use of diffusion aluminide coatings, it wasan announcement by Pratt and Whitney [76] that a new recognized that they were inherently brittlean expectedengine in development will use an advanced yttrium- property of body-centered cubic NiAl, the predominantcontaining superalloy to which the TBC is applied directly phase in the coatings. In 1970 the concept of transitionwithout the need for a bond coat. from brittle to ductile behavior of these coatings as

Some research has been performed on possible effects of temperature is increased was introduced [86]. Aluminidehot corrosion on the ceramics used as thermal barriers, first and some CoCrAlY coatings exhibited brittle to ductileby Barkalow and Pettit [77] and also by Jones et al. [78]. transitions at around 7008C (13008F). It was thereforeIn conditions most likely to be encountered in industrial anticipated that if thermalmechanical tensile strains wereturbines, destabilization of the preferred crystal structures large enough below this transition temperature, brittleoccur by leaching out of stabilizers, such as Y O , by cracking of coatings could occur, followed by crack2 3molten acidic sodium sulfate (Na SO SO ). This does propagation into base alloys. In a seminal paper, Linask2 4 3seem less likely to occur in industrial engines fired with [87] described the results of an analytical study usingclean natural gas at the higher temperatures required for fracture mechanics principles to model turbine airfoilimproved efficiency. cracking. It was shown that cracks initiated in a CoCrAlY

Because of the dependence of airfoil integrity on TBCs coating when tensile strains, due to thermalmechanicalthere has been considerable effort to develop life predic- stresses, peaked at about 200C (3928F) in a thermallytion systems [7981] to aid in airfoil design. Parallel cycled TF30 engine air cooled first stage blade. The strainsefforts for development of design systems, based on peaked upon engine deceleration, at well below the brittle-constitutive properties of TBCs, are subjects of several to-ductile transition temperature of the CoCrAlY coating,current programs [73]. and exceeded the value required to crack the coating in a

78 G.W. Goward / Surface and Coatings Technology 108 109 (1998) 73 79

brittle fracture mode. The cracks then propagated into the thermal barrier coatings on burner components signaledsubstrate alloy. The fracture mechanics approach and possibilities of the coatings to substantially extend turbinethermalmechanical fatigue (TMF) testing have been airfoil lives and/or improve engine efficiencies. MCrAlYfurther refined over a period of years as airfoil design tools compositions as bond coat under controlled ceramic struc-and as guides to coating and alloy development [88]. tures in plasma-deposited thermal barriers have durabilityNiCoCrAlY overlay coatings, formulated by adding critical sufficient for extending stationary airfoil service lives. Inamounts of cobalt to NiCrAlY, were specifically developed the late 1980s, electron beam vapor-deposited zirconiato have some ductility in the 3008C (6008F) temperature coatings over MCrAlY or diffusion aluminide bondcoatsrange. Improvement in thermal fatigue resistance of provided durability sufficient for use on rotating blades. ANiCoCrAlY-coated superalloys was confirmed with TMF complete understanding of oxide adherence is still elusivetesting and by subsequent engine tests [31,89]. Although and is one of the most important areas of research, for bothnot immune to TMF cracking in ductile modes, the coatings and superalloys, to provide further advances incoatings provide significant life extension over prior engine efficiency and service life. Research and develop-compositions. ment on coating processing for diffusion, overlay, and

Holmes and McClintock [90] and Busso and McClin- thermal barrier systems deserves equally strong support.tock [91] devised a useful laboratory thermal fatigue test As the millennium approaches the future for coatingusing induction heating and air-blast cooling of stepped science and technology is brighter than ever.disc specimens to induce controllable thermal strains incoated superalloys. Application of the method providedmuch useful information on crack initiation and propaga- Referencestion, along with interacting effects of coatingalloy inter-diffusion and oxidative degradation of diffusion aluminide [1] T. Van Aller, US Patent 1,155,974 (1915).[90] and NiCoCrAlY overlay coatings [91] on single- [2] H.B.C. Allison, L.A. Hawkins, General Electric Rev. 17 (1914)crystal nickel-base superalloys. 947951.

[3] E.G. Gilson, US Patent 1,091,057 (1914).Because thermal expansion mismatch between[4] E. Ruder, Trans. Am. Electrochem. Soc. 21 (1915) 253261.NiCoCrAlY and superalloys, and relatively low yield and[5] C.B. Meher-Homji, Mechanical Eng. September (1997) 8891.creep strength are important deficiencies of NiCoCrAlY in [6] R.P. Seelig, R.J. Steuber, High Temp.-High Press. 10 (1978) 207

thermal-mechanical fatigue, some efforts have been made 213.to improve these properties by additions of refractory [7] E.S. Nichols, J.A. Burger, D.K. Hanink, Mech. Eng. (1965) 5256.

[8] A.D. Joseph, US Patent 3,102,044 (1960).elements and/or oxide dispersions [9294]. Experimental[9] R.S. Parzukowski, Thin Solid Films 45 (1977) 349355.results seem promising but no practical applications have

[10] G. Gauje, R Morbioli, in: S.C. Singhal (Ed.), High Temperatureso far been reported. As previously described, thermalProtective Coatings, The Metallurgical Society of AIME, Atlanta,barrier coatings significantly reduce TMF strains in cooled GA, 1983, pp. 1326.

airfoils and it is therefore from improvements in durability [11] B.M. Warnes, D.C. Punola, Surf. Coat. Technol. 9495 (1997) 16.of these coatings that major improvements in thermal [12] P.M. Galmiche, US Patent 3,900,613 (1975).

[13] R.S. Parzuchowski, R.B Benden, US Patent 4,148,275 (1979).fatigue life result.[14] J.E. Restall, M.I. Wood, Mater. Sci. Tech. 2 (1986) 225231.[15] F.D. Kelley, Trans. Am. Electrochem. Soc. 43 (1923) 351370.[16] R.L. Samuel, M.L. Lockington, Metal Treatment and Drop Forging

4. Summary and conclusions 18 (1951) 354359, 407414, 440444, 495506, 543548; and 19(1952) 2732, 8185.

[17] R. Drewett, Anti-Corrosion Methods Mater. 16 (1951) 543548.The use of coatings on gas turbine airfoils, in concert[18] T. Gibson, US Patent 2,809,127 (1957).with improved alloys and design technology, has unfailing-[19] R. Burgel, Mater. Sci. Tech. 2 (1986) 302308.ly supported increases in either or both efficiency and [20] H.W. Brill-Edwards, US Patent 3,801,353 (1974).

service life in varieties of turbomachinery. Early coatings [21] R. Bauer, H. Grunling, Thin Solid Films 1 (1982) 320.were simple diffusion aluminides applied principally by [22] G. Lehnardt, H. Meinhardt, Electrodeposition Surf. Treatment 1

(1972) 189193.pack cementation and later by gas-phase processes. Addi-[23] T.E. Strangman, US Patent 5,514,482 (1996).tions of silicon, chromium and platinum improved the[24] R. Bianco, R.A. Rapp, in: K.H. Stern (Ed.), Metallurgical andservice lives of these coatings. Parallel research in oxida-

Ceramic Coatings, ch. 9, Chapman and Hall, London, 1993.tion and hot corrosion mechanisms, coating processing, [25] G.W. Goward, L. Cannon, J. Eng. Gas Turbines Power 110 (1998)and mechanical properties of coatingalloy systems have 150154.

[26] G.W. Goward, L.L. Seigle, ASM Handbook, vol. 5, Surface En-provided sound bases to guide the intelligent use of thegineering, 1994, pp. 611620.coatings over several decades. Research aimed at under-

[27] W.T. Griffiths, L.B. Pfeil, UK Patent 459 (1937) 848.standing protective oxide adherence supported the develop- [28] C.S. Wukusik, J.F. Collins, Mater. Res. Standards 4 (1964) 637ment of MCrAlY overlay coatings which offered the 646.ability to tailor properties for practical uses in a variety of [29] F.P. Talboom, J. Grafwallner, US Patent 3,542,5330 (1970).gas turbine applications. Early uses of zirconia-based [30] D.J. Evans, R.C. Elam, US Patent 3,676,085 (1972).

G.W. Goward / Surface and Coatings Technology 108 109 (1998) 73 79 79

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