EP0995817A1 - Thermal barrier coating system and methods - Google Patents

Abstract

Disclosed is a noble metal bond coat of a thermalbarrier coating system useful for enhancing adhesion ofa ceramic topcoat to a superalloy substrate. The bondcoat includes about 10 to 30 weight percent aluminum,about 2 to 60 weight percent noble metal, between traceamounts and about 3 weight percent of a reactive elementselected from the group consisting of yttrium,zirconium, hafnium, scandium, all the lanthanides, andmixtures thereof, and balance selected from the groupconsisting of nickel, cobalt, iron and mixtures thereof,wherein the bond coat is further characterized byabsence of added chromium. One method includes plasmaspraying a prealloyed powder of the bond coatcomposition on the substrate followed by aluminaformation and ceramic topcoat deposition. Uses includethermal barrier coating systems on gas turbine enginehot section components such as turbine blade and vaneairfoils, combustors, and exhaust nozzles.

Description

Related Applications
• This application is a continuation-in-part application ofU.S. Ser. No. 08/597,841 filed February 7, 1996.
Technical Field
• The present invention relates to protective coatings formetallic articles and more particularly to an improved bond coatof a ceramic thermal barrier coating system for superalloysubstrates.
Background Information
• During gas turbine engine operation, hot section componentssuch as turbine blade and vane airfoils, combustors, and exhaustnozzles are subject to oxidizing and corrosive, high temperaturecombustion effluent gas. Because these components often aresubjected concurrently to high magnitude thermally andmechanically induced stress, the art has developed a variety oftechniques in the design and manufacture of these components toensure maintenance of structural and metallurgical integritythroughout the operating range of the engine. For example,components typically are manufactured from material compositionssuch as nickel- and cobalt-base superalloys having desirableproperties at elevated, operating range temperatures. In thecase of turbine airfoils, the selected alloy generally is formedby casting. For enhanced high temperature strength, grainstructure advantageously may be controlled during solidificationof the casting to produce a directionally solidified or singlecrystal form, thereby providing greater strength for a givenalloy composition.
• In addition to component strength enhancement by selectionof alloy composition and control of the casting process, bothinternal and external cooling schemes are employed extensively to maintain component temperatures below critical levels. Tailoredfilm cooling of external surfaces and sophisticated turbulentflow cooling of serpentine shaped internal cavities in the castairfoils are routinely utilized in advanced gas turbine enginedesigns, respectively, to decrease the thermal energy input tothe component and reduce the temperature rise thereof.
• Despite efforts to optimize these varied approaches, bothalone and in combination, advanced gas turbine engine designefficiency is limited by the inability of the hot sectioncomponents to achieve acceptable operating lives under increasedmechanical and thermal loading. An additional method employed bythose skilled in the art of gas turbine engine design is the useof a relatively thin ceramic insulative outer layer on surfacesexposed to the effluent gas flow. This facilitates componentoperation at greater operating temperatures. These coatings,generally referred to in the industry as thermal barrier coatingsor TBCs, effectively shield the metallic substrate of thecomponent from temperature extremes. By reducing the thermalenergy input to the component, higher combustion effluent gastemperatures and/or more efficient use of cooling flows arerealized, with a resultant increase in engine operatingefficiency.
• Ceramic coatings are prone to delamination at or near theceramic/substrate interface due to differences in coefficients ofthermal expansion between the relatively brittle ceramic and themore ductile superalloy substrate. Subsequently, the ceramic mayspall or separate from the component surface. This failuremechanism is aggravated and accelerated under conditions ofthermal cycling inherent in gas turbine engine operation. Inorder to prevent premature failure of the ceramic, methods ofproviding strain tolerant ceramic coatings have been developed.Certain moderate service applications employ porous or pre-crackedceramic layers. In more harsh operating environments,such as those found in advanced gas turbine engines, the artexploits strain tolerant open columnar ceramic crystal structures, such as those described in U.S. Pat. No. 4,321,311 toStrangman. Substantial attention also has been directed to theuse of an intermediate or bond coat layer disposed between thesubstrate and the ceramic layer. The bond coat employs acomposition designed both to enhance the chemical bond strengthbetween the ceramic and metal substrate as well as to serve as aprotective coating in the event of premature ceramic topcoatloss.
• There are presently two primary classes of bond coatcompositions conventionally employed in multilayered TBC systemsof this type, each exhibiting inherent deficiencies which inhibittheir useful life. One type of metallic bond coat typicallyspecified by gas turbine engine designers is referred to asMCrAlY alloy, where M is iron, cobalt, nickel, or mixturesthereof. The other major constituents, namely chromium,aluminum, and yttrium, are represented by their elementalsymbols. In coating a superalloy substrate, the MCrAlY bond coatfirst is applied to the substrate by a method such as physicalvapor deposition ("PVD") or low pressure plasma spraying. TheMCrAlY class of alloys are characteristically very resistant tooxidation at the elevated temperatures experienced by hot sectioncomponents due to their ability to form a thin adherentprotective external film of aluminum oxide or alumina. Inaddition to providing protection, the alumina film also providesa chemically compatible surface on which to grow the insulativeceramic topcoat. As known by those having skill in the art, theceramic topcoat most commonly employ zirconium oxide or zirconia,either partially or fully stabilized through the addition ofoxides of yttrium, magnesium, or calcium. By growing an opencolumnar structured stabilized zirconia on the alumina film, themultilayered coating exhibits improved integrity under cyclicthermal conditions over ceramic coatings disposed directly on themetallic substrate, thereby providing the intended insulativeprotection to the underlying component over an extended period.
• While such MCrAlY-based TBC systems have been shown todemonstrate improved life over systems lacking the MCrAlY bondcoat altogether, ceramic topcoat spalling and failure continue tooccur, albeit after a greater number of thermal cycles. It hasgenerally been accepted that the failure mechanism is related todiffusion of substrate alloy constituents through the MCrAlY bondcoat layer. Because the MCrAlY alloys are primarily of the solidsolution type, they offer little resistance to diffusion ofelements from the underlying superalloy substrate which aredetrimental to interfacial bond strength. Upon reaching theintermediate alumina layer, the presence of the diffusedconstituents causes deterioration of the MCrAlY/alumina bondstrength and acceleration in the growth rate of the alumina filmwith resultant deterioration and failure of the ceramic topcoat.
• Another type of metallic bond coat routinely specified bythose skilled in the art includes a class of materials known asaluminides. These are popular compositions for gas turbineengine components and include nickel, cobalt, and iron modifiedaluminides as well as platinum modified aluminides. Generally,aluminides are intermediate phases or intermetallic compoundswith physical, chemical, and mechanical properties substantiallydifferent from the more conventional MCrAlY bond coats. Asdiscussed hereinbelow, some aluminide compositions are known tobe useful coatings in and of themselves for protecting iron-,cobalt-, and nickel-base alloys from oxidation and corrosion;however, some aluminides may be used as bond coats for ceramictopcoats in TBC systems.
• The aluminide-based TBC system is similar to the MCrAlY-basedTBC system insofar as the aluminide bond coat is firstformed on the substrate surface by conventional diffusionprocesses such as pack cementation as described by Duderstadt etal. in U.S. Pat. No. 5,238,752 and Strangman in published U.K.Patent Application GB 2,285,632A, the disclosures of which areincorporated herein by reference. The aluminide coated componentalso has a surface composition which readily forms a protective alumina film when oxidized. A ceramic topcoat of conventionalcomposition and structure, as described hereinabove, completesthe TBC system.
• As with the MCrAlY-based TBC, the weak link in thealuminide-based TBC is the strength of the bond between thealuminide bond coat and the intermediate alumina layer. However,instead of degrading as a function of diffusional instability asin MCrAlY-based TBC systems, the aluminide/alumina bond isinherently relatively weak. The failure mechanism for theseconventional aluminide diffusion coatings is the repeatedformation, spalling, and reformation of the alumina film underthermal cycling conditions of typical gas turbine engine service,with the eventual depletion of aluminum in the aluminide bondcoat below a critical concentration.
• In spite of the operational deficiencies of the aluminide-basedTBC systems, there are several recognized advantages ofaluminide-based TBC systems over MCrAlY-based TBC systems. Forexample, aluminide bond coats typically are applied by lower costprocesses and do not include the expensive strategic constituentchromium required in MCrAlY bond coats. Further, the aluminidebond coats have a substantially higher melting point and lowerdensity. These are important considerations when coating partssuch as turbine blade airfoils which operate at high temperaturesand high rotational speeds. More important, however, aluminidebond coats exhibit substantially lower solubility for thesubstrate alloy solute elements, thereby retarding theirdiffusion in service to the critically importantaluminide/alumina interfacial bond.
• Major deficiencies associated with aluminide bond coats arerelated to-the inherent metallurgical characteristics resultingfrom creation of the bond coat by diffusion. Production of aconventional aluminide bond coat relies upon reaction of thesubstrate alloy with aluminum from an aluminum rich gaseoussource and interdiffusion with the metallic substrate. Thegaseous aluminum source may be produced by any of a variety of conventional methods. For example, Duderstadt et al. discussesproduction of an aluminide bond coat on a nickel- or cobalt-basesuperalloy substrate preferably by the pack cementation method.According to this method, aluminum from an aluminum halide gas inthe pack mixture reacts and interdiffuses with the substratesurface over time at elevated temperature. Strangman discussesproduction of aluminide bond coats by reacting a nickel-, iron-,or cobalt-base superalloy component substrate with an aluminumrich vapor at elevated temperature. Strangman refers exclusivelyto the term "diffusion aluminide" as characteristic of theresultant bond coat. This characterization accuratelycorresponds to the method of aluminide bond coat production,namely by diffusion. As a result of the diffusion method, thealuminide bond coat contains nickel, iron, or cobalt from thesubstrate of the component being coated, depending on the primaryconstituent of the superalloy substrate. Further, many of thebase alloying elements of the substrate which are ultimatelydetrimental to TBC system integrity are necessarily alsocontained in the reaction product aluminide forming on thecomponent surface. These alloying elements therefore are presentin the aluminide bond coat as produced, and are available toaffect detrimentally the alumina film that eventually formsthereon.
• Another significant deficiency of aluminide bond coats isrelated to the aluminide composition as it affects adherence ofthe alumina film or scale. Strangman discloses the addition ofsilicon, hafnium,.platinum, and oxides particles such as alumina,yttria, and hafnia to the aluminide composition to improvealumina film adherence. However, the beneficial effects of theseelements are offset, at least partially, by the presence of thebase alloy elements previously described which are detrimental tosatisfactory alumina adherence.
• As stated hereinabove, some aluminide compositions are usednot as bond coats in TBC systems, but rather solely as protectivecoatings without ceramic topcoats. In such applications, a goal of those skilled in the art is to protect the underlying articlesubstrate from chemically aggressive effluent gases by retardingenvironmental deterioration of the substrate alloy due toaccelerated oxidation and hot corrosion. There has been researchconducted and patents granted on the beneficial effects ofreactive element additions, primarily to aluminide coatings usedsolely for coating purposes. For example, U.S. Pat. No.4,835,011 to Olson et al., the disclosure of which isincorporated herein by reference, describes a method of forming adiffusion aluminide coating on a nickel- or cobalt-basesuperalloy by heating the article to be coated in the presence ofa powder mixture containing an alloy or mixture of aluminum,yttrium, and one or more of chromium, nickel, cobalt, silicon,and titanium; a halide activator such as cobalt iodide; and aninert filler such as yttrium oxide. Reference may also be madeto an article entitled "Hot Corrosion of Yttrium-modifiedAluminide Coatings," Materials Science and Engineering, A121(1989) pp. 387-389, in which the researchers discuss improved hotcorrosion resistance of aluminide coatings when modified withyttrium. Further, in NASA Technical Memorandum 101408, entitled"The Effect of 0.1 Atomic Percent Zirconium on the CyclicOxidation Behavior of β-NiAl for 3000 Hours at 1200 °C," C.A.Barrett describes the beneficial effects of zirconium on thecyclic oxidation resistance of nickel aluminide. None of thesereferences disclose or contemplate use of any of thesecompositions as a bond coat in a ceramic TBC system.
• The consequences of TBC system failure are tangible andcostly. Firstly, thermal operating margin must be factored intothe design of the gas turbine engine to preclude overtemperatureand failure of hot section components. By limiting combustionparameters to less than stoichiometric, the realizable efficiencyof the engine is reduced, with increase in fuel consumption aswell as levels of unburnt hydrocarbons and other pollutants.Further, baseline engine operating parameters are premised on the existence of uniform ceramic topcoats, and ceramic topcoat lifeis often significantly less than underlying component life. Thismeans engines must be removed from service for maintenance atpredetermined intervals, based, for example, on operating hoursor thermal cycles. Combustor, turbine, and exhaust modules aredisassembled and the coated parts removed, stripped, inspectedand recoated. Significant costs are attributable to aircraft andengine unavailability. Further, substantial direct costs areassociated with labor, tooling, and materials required to remove,recoat, and reinstall the affected hardware. Yet furtherunscheduled engine removals are forced whenever borescopeinspection of the internal configuration of the engine revealsTBC system degradation beyond predetermined field service limits,further disrupting operations and increasing support costs.
Summary of the Invention
• According to a first embodiment of the invention, animproved TBC system and methods of application are disclosed,primarily for use on nickel- and cobalt-base superalloy articles,such as hot section components of gas turbine engines, as well asfor use on iron-base superalloy articles. Superalloys aregenerally defined as a class of metallic alloys suitable for highstrength, high temperature applications and which have enhancedoxidation resistance. A superalloy substrate is first coatedwith a bond coat having an MAlY composition where M is nickel,cobalt, iron, or combinations thereof. An intermediate layer ofalumina is formed on the MAlY bond coat and a ceramic topcoat isapplied overall. As used herein, the chemical symbol "Y"signifies the use of reactive elements such as yttrium. Also, asused herein, the term "alumina" signifies predominantly aluminumoxide, which may be altered by the presence of reactive elementsto contain, for example, yttrium or zirconium oxides. Thealumina layer may also be referred to as a thermally grown oxidelayer or TGO layer.
• The bond strength or adherence between the MAlY bond coatand alumina film is enhanced over conventional aluminide andMCrAlY bond coats by substantially restricting the composition ofthe bond coat to between about 10 to 30 weight percent aluminum,between trace amounts and about 3 weight percent yttrium or otherreactive element such as zirconium, hafnium, scandium, or any ofthe lanthanides (i.e. atomic number 57-71, inclusive) eitheralone or in mixtures thereof, and balance selected from nickel,cobalt and iron, either alone or in mixtures thereof. Byspecifically excluding chromium from the bond coat in theaforementioned compositional ranges, in combination with theincrease in aluminum content, diffusional stability of thechromium-free MAlY bond coat is significantly improved overconventional MCrAlY bond coats. As a result, the inventionprovides a substantial reduction in diffusion of substrate alloyconstituents through the MAlY bond coat, and maintenance of a strong MAlY/alumina bond, resistant to degradation as a functionof time at elevated temperature, with a concomitant enhancementin ceramic topcoat integrity.
• A further benefit from the exclusion of chromium from thebond coat relates to the high vapor pressure of chromium andchromium oxidation products. At intermediate operationaltemperatures, for example between about 700°C and about 950°C,the beneficial effects of chromium for sulfidation or hotcorrosion resistance typically dominate detrimental effects;however, at higher service temperatures, pure oxidationresistance and thermal protection are dominant goals of thoseskilled in the art. It is in this operating range, whereadvanced TBC systems are required to perform, that high chromiumcontent in the bond coat can be detrimental.
• Yet further, the MAlY/alumina bond is stronger than that ofa conventional modified aluminide/alumina bond. In addition, thegrowth rate of the alumina film is reduced by the presence ofyttrium or other reactive element and the combined effectexhibits improvement over conventional aluminide-based TBCsystems.
• According to another embodiment of the invention, animproved TBC system and methods of application are disclosed,primarily for use on nickel- and cobalt-base superalloy articlessuch as hot section components of gas turbine engines, as well asfor use on iron-base superalloy articles. A superalloy substrateis first coated with a noble metal bond coat having a compositionincluding aluminum, one or more noble metals, and one or morereactive elements, with the balance being nickel, cobalt, iron,or combinations thereof. An intermediate layer of alumina isformed on the noble metal bond coat and a ceramic topcoat isapplied overall. As used herein, the term "noble metal" refersto inactive or inert, corrosion resistant elements, namelyruthenium, rhodium, palladium, silver, osmium, iridium, platinum,and gold.
• The bond strength or adherence between the noble metal bondcoat and alumina film is enhanced over conventional aluminide andMCrAlY bond coats by substantially restricting the composition ofthe bond coat to between about 10 to 30 weight percent aluminum,between about 2 and 60 weight percent noble metal, between traceamounts and about 3 weight percent yttrium or other reactiveelement such as zirconium, hafnium, scandium, or any of thelanthanides, either alone or in mixtures thereof, and balanceselected from nickel, cobalt and iron, either alone or inmixtures thereof.
• The bond strength between the noble metal bond coat and thealumina film is substantially enhanced over conventional TBCsystems, in part, due to the presence of the reactive elements.The noble metal bond coat composition and the reactive elementstherein interfere with the diffusion mechanism of constituentsfrom the substrate alloy through the bond coat. Accordingly,diffusion of the constituents into the alumina layer is reduced,as is the growth rate of the alumina layer. Additionally, crackinitiation and propagation, which tends to occur at the interfaceof the bond coat and alumina layer, is reduced, so that cycliclife of the TBC system is improved. This effect is due to oxideprecipitates of the reactive elements, which are present as dopesat the bond coat/alumina layer interface. Mechanisms useful forexplaining the beneficial influence of the oxide precipitates aredescribed by authors such as E. Orowan in AIME Publication"Dislocations in Metals" (1954) at page 69 and by Kelly and Finein "Werkstofftechnik Metalle I" (1992) edited by O. Knotek and E.Lugscheider (Vorlesungsumdruck fur die Vertieferrichtung,Werkstofftechnik, 1992) at pages 2.14 and 2.15, the disclosuresof which are herein incorporated by reference. The formation ofoxide precipitates at the interface can be the result of thecoating process employed to produce the TBC system.Alternatively or additionally, the formation of oxideprecipitates can result from use of the coated article in theengine.
• Various methods may be employed to apply the improved MAlYbond coat to a superalloy article substrate. For example, in afirst method similar to that employed to apply a conventionalbond coat, yttrium and/or other reactive element first isdeposited on the substrate using electron beam PVD followed bygas phase or pack cementation aluminizing. Although this methodhas the deficiencies previously described with respect todiffusion aluminides, the presence of yttrium or other reactiveelement markedly improves the oxide scale adherence for theaforementioned reasons. Simple physical or chemical vapordeposition of reactive elements on the surface of a conventionalaluminide coating could be effected to bring about the desiredsurface composition modification. Alternatively, in preferredembodiment methods, ion PVD or sputtering may be employed to coatthe substrate using a prealloyed MAlY cathode. Yet anotherpreferred method employs vacuum or low pressure plasma sprayingof prealloyed MAlY powder onto the substrate. One additionalmethod involves the deposition of nickel and simultaneousdeposition of an aluminum yttrium alloy powder. In all threepreferred methods, the coated component is subsequently subjectedto a thermal processing cycle to metallurgically bond the coatingto the component surface and in the last example tocompositionally homogenize the coating. Typically, a heattreatment in vacuum for approximately two hours at about 1080°Cwould be employed to effect the metallurgical bond. According tothe preferred methods, the MAlY bond coat thus produced at mostcontain traces of alloying constituents from the superalloysubstrate, because the composition of the MAlY bond coat isestablished prior to its application onto the substrate surfaceand because its application does not exploit a diffusion reactioninvolving the substrate. Accordingly, the bond coat issubstantially chromium-free. Some minor amount of chromium mightdiffuse into the bond coat over an extended period at operationaltemperatures. The anticipated detrimental effects of this areconsidered minor as compared with other coating alternatives, due both to the inconsequential amount of diffused chromium and theextraordinarily long time period required relative to the usefullife of the underlying component. In all of the methods, aluminagrowth on the deposited MAlY bond coat and application of theceramic topcoat may be accomplished by conventional methods.
• Various methods may also be employed to apply the noblemetal bond coat to a superalloy article substrate. For example,according to a first method, the noble metal bond coat may beapplied by PVD, for example electron beam PVD, using one sourceor multiple sources containing aluminum, one or a mixture ofnoble metals, and one or a mixture of reactive elements.Alternatively, the noble metal bond coat may be applied bythermal spraying techniques such as plasma spraying of a powdercontaining aluminum, one or a mixture of noble metals, and one ora mixture of reactive elements. Yet further, the noble metalbond coat can be applied by a combination of methods, using amultiple step approach, for example by depositing one or amixture of noble metals and one or a mixture of reactive elementsin a single step or separate steps. The deposition of theselayers may be accomplished in any order, followed by depositionof the aluminum.
• According to any of these methods, the coated component issubsequently subjected to a thermal processing cycle tometallurgically bond the coating to the component surface and/orto compositionally homogenize the coating. Typically, a heattreatment in vacuum for approximately two hours at about 1080°Cwould be employed to effect the metallurgical bond. According tothese methods, the noble metal bond coat thus produced at mostcontain traces of alloying constituents from the superalloysubstrate, because the composition of the noble metal bond coatis established prior to its application onto the substratesurface and because its application does not exploit a diffusionreaction involving the substrate. Accordingly, the bond coat issubstantially chromium-free. Some minor amount of chromium mightdiffuse into the bond coat over an extended period at operational temperatures. The anticipated detrimental effects of this areconsidered minor as compared with other coating alternatives, dueboth to the inconsequential amount of diffused chromium and theextraordinarily long time period required relative to the usefullife of the underlying component. In all of the methods, aluminagrowth on the deposited noble metal bond coat and application ofthe ceramic topcoat may be accomplished by conventional methods.
• For any of the MAlY or noble metal bond coats, the ceramictopcoat may have multiple layers, adjacent layers of which havegenerally columnar grain microstructures with different grainorientation directions, as disclosed in U.S. Ser. No. 08/987,354filed December 9, 1997, and entitled "Thermal Barrier CoatingCeramic Structure," the disclosure of which is hereinincorporated by reference in its entirety.
Brief Description of the Drawings
• The invention, in accordance with preferred and exemplaryembodiments, together with further advantages thereof, is moreparticularly described in the following detailed descriptiontaken in conjunction with the accompanying drawings in which:
• FIG. 1 is a schematic, cross-sectional view of a typical gasturbine engine depicting hot section components suitable forapplication of a TBC system in accordance with a preferredembodiment of the present invention;
• FIG. 2A is an enlarged schematic, cross-sectional view of aportion of a superalloy article coated with a TBC system inaccordance with a preferred embodiment of the present invention;
• FIG. 2B is an enlarged schematic, cross-sectional view of aportion of a superalloy article coated with a TBC system inaccordance with an alternative embodiment of the presentinvention;
• FIG. 3A is an enlarged schematic, cross-sectional view of aportion of a superalloy article coated with a TBC system inaccordance with another preferred embodiment of the presentinvention; and
• FIG. 3B is an enlarged schematic, cross-sectional view of aportion of a superalloy article coated with a TBC system inaccordance with another alternative embodiment of the presentinvention.
Mode(s) for Carrying Out the Invention
• Depicted in FIG. 1 is a schematic, cross-sectional view of atypical turbofangas turbine engine 10 depicting hot sectioncomponents, shown generally at 12, suitable for application of aMAlY-base or noble metal-base TBC system in accordance withpreferred embodiments of the present invention. As depicted, theengine 10 includes, in serial flow relation from inlet toexhaust, aninlet frame 14, a two stage low pressure compressor("LPC") orfan 16, a three stage high pressure compressor ("HPC")18, acombustor 20, a single stage high pressure turbine ("HPT")22, a two stage low pressure turbine ("LPT") 24, aturbine frame26, and anexhaust nozzle 28.
• Compressed air exiting theHPC 18 is mixed with fuel in thecombustor 20 and ignited. The high temperature, high energycombustion effluent passes through both theHPT 22 andLPT 24where energy is extracted to drive theHPC 18 andfan 16. Eachturbine stage, forexample HPT 22, includes a set ofstationaryturbine vanes 30 androtating turbine blades 32 disposed in theeffluent stream to optimize flow orientation and energyextraction. After passing through theturbine frame 24, whichsupports the rotating components of theengine 10, the effluentis mixed with the fan flow and passes through theexhaust nozzle28, producing a net force or thrust which propels theengine 10forward.
• Hot section components 12 exposed to the high temperature,corrosive combustion effluent may be coated with the MAlY ornoble metal bond coat TBC systems, in accordance with theteachings of this invention, to protect the superalloy substratefrom excessive temperature as well as oxidation during engineoperation.
• Referring now to FIG. 2A, depicted is an enlarged schematic,cross-sectional view of a portion of asuperalloy article 34,such as an airfoil wall of aturbine blade 32, coated with theMAlY bond coat TBC system in accordance with a preferredembodiment of the present invention. Schematic representation and relative thickness of each layer of the multilayered TBCsystems depicted in FIGS. 2A and 2B are meant for illustrativepurposes only and in no manner are intended to restrict the scopeof the invention.
• Thearticle 34 in FIG. 2A includes asubstrate 36, a portionof which is depicted. Thesubstrate 36 is preferably composed ofan iron-, nickel-, or cobalt-base superalloy; however, it iscontemplated that the MAlY bond coat of the present invention maybe suitable for use with any superalloy or other metallicsubstrates with which it may form an adequate bond. For purposesherein, adequate bond may be characterized as adherence equal orsuperior to that between other layers in the TBC system.
• Produced on thesubstrate 36 is aMAlY bond coat 38 having acomposition of about 10 to 30 weight percent aluminum, betweentrace amounts and about 3 weight percent of a reactive elementsuch as yttrium, zirconium, hafnium, scandium, or any of thelanthanides or mixtures thereof, and balance being nickel,cobalt, iron, or mixtures thereof. In a preferred composition,bond coat 38 includes about 20 to 22 weight percent aluminum,about 0.2 to 0.4 weight percent yttrium, and balance nickel. Inboth instances, chromium is purposely omitted from thebond coat38, although some inconsequential, inadvertent trace amount mightconceivably exist therein. Thebond coat 38 is preferablyproduced by means of low pressure or vacuum plasma spray using aprealloyed powder, rather than conventional diffusion methodssuch as pack cementation. An exemplary plasma spray method isdisclosed, for example, in U.S. Pat. No. Re. 33,876 to Goward etal., the disclosure of which is incorporated herein by reference.By using a plasma spray method, the composition of thebond coat38 may be controlled better and the migration of base alloyelements from thesubstrate 36, which might otherwise pose adetriment to bond coat/alumina adherence, may be reducedmarkedly. A relativelythin diffusion zone 40 inherently formsbetween thebond coat 38 andsubstrate 36, supporting the bondtherebetween.
• Due to the highly reactive nature of theMAlY bond coat 38during production of the TBC system, aluminum proximate theexposed outer surface of thebond coat 38 substantiallyinstantaneously oxidizes upon exposure to any oxygen or moisturecontaining environment at elevated temperature, resulting in athin layer of aluminum oxide oralumina 42. Such an oxidizedlayer may also be referred to as an alumina film or scale.Lastly, aceramic topcoat 44 is disposed on thealumina film 42to achieve the desired insulative properties of the TBC system.As depicted, the preferredceramic topcoat 44 has a columnarmicrostructure, substantially consistent with that disclosed inU.S. Pat. No. 4,321,311 to Strangman, the disclosure of which isincorporated herein by reference. The columnarceramic topcoat44 preferably is produced by electron beam PVD, although othertechniques consistent with the production of such columnarmicrostructure may be used as desired. An exemplary PVD methodand apparatus is disclosed in U.S. Pat. No. 4,880,614 toStrangman et al., the disclosure of which is incorporated hereinby reference. As mentioned hereinabove, a multilayered columnarceramic topcoat with at least two grain orientation directionsmay be employed.
• Referring now to FIG. 2B, depicted is an enlarged schematic,cross-sectional view of a portion of asuperalloy article 134coated with a TBC system in accordance with an alternativeembodiment of the present invention. Thearticle 134 includes asubstrate 136, preferably composed of an iron-, nickel-, orcobalt-base superalloy. However, it is contemplated that theMAlY bond coat of the present embodiment of the invention alsomay be suitable for use with any superalloy or other metallicsubstrates-with which it may form an adequate bond.
• Produced on thesubstrate 136 is aMAlY bond coat 138 havinga composition of about 10 to 30 weight percent aluminum, betweentrace amounts and about 3 weight percent of a reactive elementsuch as yttrium, zirconium, hafnium, scandium, or any of thelanthanides or mixtures thereof, and balance being nickel, cobalt, iron or mixtures thereof. In a preferred composition,bond coat 138 includes about 20 to 22 weight percent aluminum,about 0.25 to 0.4 weight percent yttrium, and balance nickel.Here again, chromium is purposely omitted from thebond coat 138.In this embodiment, thebond coat 138 is produced by firstapplying yttrium to thesubstrate 136 by any conventional method,such as electron beam PVD. Thereafter, theMAlY bond coat 138may be produced by gas phase aluminizing. According to thisprocess, thesuperalloy substrate 136 is reacted with an aluminumhalide gas at elevated temperature for a length of timesufficient to produce the desired bond coat thickness andcomposition in accordance with the aforementioned constituentranges. Clearly, the method is not restricted to gas phasealuminizing in that any source of aluminum may be employed tosupport the aluminizing step.
• As is represented schematically in FIG. 2B, theMAlY bondcoat 138 includes both the aluminide coating and embedded yttriumrichintermetallic phase particles 148. Clearly, if theunderlying substrate 136 is a nickel-base alloy, then the coatingformed will be nickel aluminide. Similarly, if the substrate isa cobalt-base alloy, the coating formed will be cobalt aluminide.Further, if instead of first applying yttrium to thesubstrate136, zirconium, hafnium, scandium, or any of the lanthanides wereapplied in sufficiently high concentration, theresultantparticles 148 would be rich in the applied reactive element.
• As with the embodiment of FIG. 2A, a relativelythindiffusion zone 140 inherently forms between thebond coat 138 andsubstrate 136 supporting the bond therebetween. Thediffusionzone 140 may contain the diffused reactive element first applied.
• Due to the highly reactive nature of theMAlY bond coat 138during production of the TBC system in FIG. 2B, aluminumproximate the exposed outer surface of thebond coat 138substantially instantaneously oxidizes upon exposure to oxygen ormoisture containing environment at elevated temperature,resulting in a thin layer ofalumina 142. Lastly, aceramic topcoat 144 is disposed on thealumina film 142 to achieve thedesired insulative properties of the TBC system. As depicted,the preferredceramic topcoat 144 has a non-columnar but straintolerant morphology produced by plasma spraying techniques,although other conventional methods of application may beemployed as desired. The plasma sprayedceramic topcoat 144 maybe uniformly dense, or may exhibit controlled porosity asdepicted generally at 146, having a substantially nonporousexternal surface and increasing porosity proximate thealuminalayer 142.
• The average thickness of individual layers of themultilayered TBC systems depicted in FIGS. 2A and 2B may beselected by those skilled in the art to achieve a desiredinsulative result. In a typical application in agas turbineengine 10 or other harsh environment, the thickness ofbond coat38, 138 may be between about 40 and 120 microns; the thickness ofthealumina film 42, 142 between about 0.1 and 3 microns; and thethickness of theceramic topcoat 44, 144 between about 80 and 350microns. These ranges are exemplary. Values outside theseranges, alone or in combination, are considered within the scopeof the invention. In a preferred embodiment for an airfoil of agasturbine engine blade 32 orvane 30, the thickness ofbondcoat 38, 138 may be between about 50 and 80 to 90 microns; thatof thealumina film 42, 142 may be between about 0.5 and 1.5microns; and that of theceramic topcoat 44, 144 may be betweenabout 100 and 150 microns.
• Referring now to FIG. 3A, depicted is an enlarged schematic,cross-sectional view of a portion of asuperalloy article 234,such as an airfoil wall of a turbine blade 232, coated with thenoble metal bond coat TBC system in accordance with anotherpreferred embodiment of the present invention. Schematicrepresentation and relative thickness of each layer of themultilayered TBC systems depicted in FIGS. 3A and 3B are meantfor illustrative purposes only and in no manner are intended torestrict the scope of the invention.
• Thearticle 234 in FIG. 3A includes asubstrate 236, aportion of which is depicted. Thesubstrate 236 is preferablycomposed of an iron-, nickel-, or cobalt-base superalloy;however, it is contemplated that the noble metal bond coat of thepresent invention may be suitable for use with any superalloy orother metallic substrates with which it may form an adequatebond.
• Produced on thesubstrate 236 is a noblemetal bond coat 238having a composition of about 10 to 30 weight percent aluminum,about 2 to 60 weight percent noble metal, between trace amountsand about 3 weight percent of a reactive element such as yttrium,zirconium, hafnium, scandium, or any of the lanthanides ormixtures thereof, and balance being nickel, cobalt, iron, ormixtures thereof. In a preferred composition,bond coat 238includes about 20 to 25 weight percent aluminum, about 30 to 40weight percent platinum, about 0.2 to 0.4 weight percent yttrium,about 0.03 to 0.06 weight percent zirconium, and balance beingnickel, cobalt, iron, and mixtures thereof. In both instances,chromium is purposely omitted from thebond coat 238, althoughsome inconsequential, inadvertent trace amount might conceivablyexist therein. Thebond coat 238 is preferably produced by meansof low pressure or vacuum plasma spray using a prealloyed powdercontaining aluminum, one or a mixture of noble metals, and one ora mixture of reactive elements, rather than conventionaldiffusion methods such as pack cementation. By using a plasmaspray method, the composition of thebond coat 238 may becontrolled better and the migration of base alloy elements fromthesubstrate 236, which might otherwise pose a detriment to bondcoat/alumina adherence, may be reduced markedly. A relativelythin diffusion zone 240 inherently forms between thebond coat238 andsubstrate 236 supporting the bond therebetween.
• Due to the highly reactive nature of the noblemetal bondcoat 238 during production of the TBC system, aluminum proximatethe exposed outer surface of thebond coat 238 substantiallyinstantaneously oxidizes upon exposure to any oxygen or moisture containing environment at elevated temperature, resulting in athin layer of aluminum oxide oralumina 242. Lastly, aceramictopcoat 244 is disposed on thealumina film 242 to achieve thedesired insulative properties of the TBC system. As depicted,the preferredceramic topcoat 244 has a columnar microstructure,which may be produced by electron beam PVD, although othertechniques consistent with the production of such columnarmicrostructure may be used as desired. Alternatively, amultilayered columnar ceramic topcoat with at least two grainorientation directions may be employed.
• Referring now to FIG. 3B, depicted is an enlarged schematic,cross-sectional view of a portion of asuperalloy article 334coated with a TBC system in accordance with another alternativeembodiment of the present invention. Thearticle 334 includes asubstrate 336, preferably composed of an iron-, nickel-, orcobalt-base superalloy. However, it is contemplated that thenoble metal bond coat of the present embodiment of the inventionalso may be suitable for use with any superalloy or othermetallic substrates with which it may form an adequate bond.
• Produced on thesubstrate 336 is a noble metal bond coat 338having a composition of about 10 to 30 weight percent aluminum,about 2 to 60 weight percent noble metal, between trace amountsand about 3 weight percent of a reactive element such as yttrium,zirconium, hafnium, scandium, or any of the lanthanides ormixtures thereof, and balance being nickel, cobalt, iron, ormixtures thereof. In a preferred composition, bond coat 338includes about 20 to 25 weight percent aluminum, about 30 to 40weight percent platinum, about 0.2 to 0.4 weight percent yttrium,about 0.03 to 0.06 weight percent zirconium, and balance beingnickel, cobalt, iron, and mixtures thereof. Here again, chromiumis purposely omitted from the bond coat 338. In this embodiment,the bond coat 338 is produced by first applying one or morereactive elements to thesubstrate 336 by any conventionalmethod, such as electron beam or other PVD technique, or chemicalvapor deposition ("CVD"). Thereafter, the noble metal bond coat 338 may be produced by gas phase aluminizing. According to thisprocess, thesuperalloy substrate 336 is reacted with an aluminumhalide gas at elevated temperature for a length of timesufficient to produce the desired bond coat thickness andcomposition in accordance with the aforementioned constituentranges. Clearly, the method is not restricted to gas phasealuminizing in that any source of aluminum may be employed tosupport the aluminizing step. For example, alternative methodsinclude pack cementation, ion vapor deposition from either a packsource or any other aluminum bearing gas, electroplating, andelectrophoteric techniques.
• As is represented schematically in FIG. 3B, the noble metalbond coat 338 includes both the aluminide coating and embeddedreactive element richintermetallic phase particles 348.Clearly, if theunderlying substrate 336 is a nickel-base alloy,then the coating formed will be nickel aluminide. Similarly, ifthe substrate is a cobalt-base alloy, the coating formed will becobalt aluminide.
• As with the embodiment of FIG. 3A, a relativelythindiffusion zone 340 inherently forms between the bond coat 338 andsubstrate 336 supporting the bond therebetween. Thediffusionzone 340 may contain the diffused reactive element first applied.
• Due to the highly reactive nature of the noble metal bondcoat 338 during production of the TBC system in FIG. 3B, aluminumproximate the exposed outer surface of the bond coat 338substantially instantaneously oxidizes upon exposure to oxygen ormoisture containing environment at elevated temperature,resulting in a thin layer ofalumina 342. Lastly, aceramictopcoat 344 is disposed on thealumina film 342 to achieve thedesired insulative properties of the TBC system. As depicted,the preferredceramic topcoat 344 has a non-columnar but straintolerant morphology produced by plasma spraying techniques,although other conventional methods of application may beemployed as desired. The plasma sprayedceramic topcoat 344 maybe uniformly dense, or may exhibit controlled porosity as depicted generally at 346, having a substantially nonporousexternal surface and increasing porosity proximate thealuminalayer 342.
• The average thickness of individual layers of themultilayered TBC systems depicted in FIGS. 3A and 3B may beselected by those skilled in the art to achieve a desiredinsulative result. In a typical application in agas turbineengine 10 or other harsh environment, the thickness ofbond coat238, 338 may be between about 20 and 120 microns; the thicknessof thealumina film 242, 342 between about 0.1 and 3 microns; andthe thickness of theceramic topcoat 244, 344 between about 80and 350 microns. These ranges are exemplary. Values outsidethese ranges, alone or in combination, are considered within thescope of the invention. In a preferred embodiment for an airfoilof a gasturbine engine blade 32 orvane 30, the thickness ofbond coat 238, 338 may be between about 20 and 70 microns; thatof thealumina film 242, 342 may be between about 0.5 and 1.5microns; and that of theceramic topcoat 244, 344 may be betweenabout 100 and 150 microns.
• In one embodiment, the noble metal bond coat includes theequivalent of a noble metal layer having a thickness of about 2to 5 microns, a reactive element layer having a thickness of upto about 3 microns, and an aluminum layer having a thickness ofabout 30 to 60 microns, the balance being nickel, cobalt, iron,or mixtures thereof. The noble metal layer may be deposited byPVD or electroplating, the reactive element layer by PVD or CVD,and the aluminum layer by pack cementation, gas phase CVD, ionvapor deposition, electroplating, or electrophoteric techniques.Diffusion of the aluminum may occur either during or after thecoating process.
• As mentioned hereinabove, the noble metal bond coat can beapplied by a combination of methods, using a multi-step approach,such as depositing one or a mixture of noble metals followed bydepositing an aluminide compound of aluminum with one or amixture of reactive elements by pack cementation, gas phase CVD, ion vapor deposition, electroplating, or electrophoterictechniques.
• According to one embodiment, the noble metal bond coat maybe applied by depositing a layer of one or a mixture of reactiveelements by PVD and, following the deposition, diffusing thislayer into the substrate. Thereafter, a layer of one or amixture of noble metals is deposited by electroplating and,following the deposition, this layer is diffused into thesubstrate, if the process so requires. Lastly, the bond coat maybe aluminized using a vapor phase deposition technique asdiscussed hereinabove.
• Lastly, the ceramic topcoat may be a partially (e.g. 6 to 8weight percent) yttria stabilized zirconia coating with acolumnar structure which is deposited on top of the noble metalbond coat. The alumina layer, having a thickness of about 0.1 to0.4 microns can grow on the bond coat either before, during, orafter deposition of the ceramic topcoat, although growth of thealumina layer during ceramic deposition may be a preferredmethod.
• While there have been described herein what are to beconsidered exemplary and preferred embodiments of the presentinvention, other modifications of the invention will becomeapparent to those skilled in the art from the teachings herein.For example, the columnar ceramic topcoats of FIGS. 2A and 3Acould be applied over thebond coats 138, 338 depicted in FIGS.2B and 3B. Similarly, the plasma sprayed ceramic topcoats ofFIGS. 2B and 3B could be applied over the bond coats 38, 238depicted in FIGS. 2A and 3A. Additionally, any PVD method couldbe used to generate the MAlY and noble metal bond coats. It istherefore desired to be secured in the appended claims all suchmodifications as fall within the true spirit and scope of theinvention. Accordingly, what is desired to be secured by LettersPatent is the invention as defined and differentiated in thefollowing claims.

Claims (19)

1. A bond coat for a thermal barrier coating system ona superalloy substrate, the thermal barrier coatingsystem including a ceramic topcoat, the bond coatcomprising:

   about 10 to about 30 weight percent aluminum;

   about 2 to about 60 weight percent noble metal;

   between trace amounts and about 3 weight percent ofa reactive element selected from the group consisting ofscandium, yttrium, zirconium, all lanthanides, hafnium,and mixtures thereof; and

   balance selected from the group consisting ofnickel, cobalt, iron, and mixtures thereof, wherein thebond coat is further characterized by absence of addedchromium.
2. A bond coat according to claim 1 comprising:

   about 20 to about 25 weight percent aluminum;

   about 30 to about 40 weight percent platinum;

   about 0.2 to about 0.4 weight percent yttrium;

   about 0.03 to about 0.06 weight percent zirconium;and

   balance selected from the group consisting ofnickel, cobalt, iron, and mixtures thereof.
3. A substantially chromium-free bond coat accordingto claim 1 consisting essentially of:

   about 10 to about 30 weight percent aluminum;

   about 2 to about 60 weight percent noble metal;

   between trace amounts and about 3 weight percent ofa reactive element selected from the group consisting ofscandium, yttrium, zirconium, all lanthanides, hafnium,and mixtures thereof; and

   balance selected from the group consisting of nickel, cobalt, iron, and mixtures thereof.
4. A bond coat according to claim 3 consistingessentially of:

   about 20 to about 25 weight percent aluminum;

   about 30 to about 40 weight percent platinum;

   about 0.2 to about 0.4 weight percent yttrium;

   about 0.03 to about 0.06 weight percent zirconium;and

   balance selected from the group consisting ofnickel, cobalt, iron, and mixtures thereof.
5. A thermal barrier coating system comprising:

   a bond coat as claimed in any one of claims 1 to 4;

   an alumina layer on the bond coat; and

   a ceramic topcoat on the alumina layer.
6. A coated article comprising:

   a superalloy substrate; and

   a thermal barrier coating system on the substrate,

   wherein the thermal barrier coating system is as claimedin claim 5.
7. A coated article according to claim 6 wherein thesuperalloy substrate comprises a superalloy selectedfrom the group of nickel-base superalloy, cobalt-basesuperalloy, and iron-base superalloy.
8. A coated article according to claim 6 or claim 7wherein the coated article comprises an engine part, atleast a portion of which is exposed to combustioneffluent during operation thereof.
9. A coated article according to claim 8 wherein theengine part is selected from the group consisting ofcombustors, turbine blades, turbine vanes, turbineframes, and exhaust nozzles.
10. A method of producing a thermal barrier coatingsystem on a superalloy substrate, the thermal barriercoating system including a ceramic topcoat, the methodcomprising the steps of:

    1) providing a superalloy substrate; and

    2) producing a bond coat on the substrate by:

    a) depositing a reactive element on thesubstrate, the reactive element selected from the groupconsisting of scandium, yttrium, zirconium, alllanthanides, and hafnium, and mixtures thereof; and

    b) thereafter reacting the substrate with analuminum source, wherein the bond coat producedcomprises:

    about 10 to about 30 weight percent aluminum;

    between trace amounts and about 3 weight percent ofthe reactive element; and

    balance selected from the group consisting ofnickel, cobalt, iron, and mixtures thereof, wherein thebond coat produced is further characterized by absenceof added chromium.
11. The invention according to claim 10, wherein thebond coat producing step further comprises the substepof depositing a noble metal on the substrate, such thatthe bond coat comprises about 2 to about 60 weightpercent noble metal.
12. The invention according to claim 10 furthercomprising the step of forming an alumina film on thebond coat.
13. The invention according to claim 12 furthercomprising the step of producing a ceramic topcoat onthe alumina film.
14. A method of producing a thermal barrier coatingsystem on a superalloy substrate, the thermal barrier coating including a ceramic topcoat, the methodcomprising the steps of:

    1) providing a superalloy substrate;

    2) producing a bond coat on the substrate by plasmaspraying a prealloyed powder, the bond coat comprising:

    about 10 to about 30 weight percent aluminum;

    about 2 to about 60 weight percent noble metal;

    between trace amounts and about 3 weight percent ofa reactive element selected from the group consisting ofscandium, yttrium, zirconium, all lanthanides, andhafnium, and mixtures thereof; and

    balance selected from the group consisting ofnickel, cobalt, iron, and mixtures thereof, wherein theprealloyed powder is further characterized by absence ofadded chromium.
15. The invention according to claim 14 furthercomprising the step of forming an alumina film on thebond coat.
16. The invention according to claim 15 furthercomprising the step of producing a ceramic topcoat onthe alumina film.
17. A method of producing a thermal barrier coatingsystem on a superalloy substrate, the thermal barriercoating system including a ceramic topcoat, the methodcomprising the steps of:

    1) providing a superalloy substrate; and

    2) producing a bond coat on the substrate by plasmaspraying a prealloyed powder, the bond coat comprising:

    20 to about 30 weight percent aluminum;

    between trace amounts and about 3 weight percent ofa reactive element selected from the group consisting ofscandium, yttrium, zirconium, all lanthanides, andhafnium, and mixtures thereof; and

    balance selected from the group consisting of nickel, cobalt, iron, and mixtures thereof, wherein theprealloyed powder is further characterized by absence ofadded chromium, the bond coat having a thickness lessthan 90 microns.
18. The invention according to claim 17 furthercomprising the step of forming an alumina film on thebond coat.
19. The invention according to claim 18 furthercomprising the step of producing a ceramic topcoat onthe alumina film.

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