Method of forming a coating resistant to deposits and coating formed thereby

[0002] Not applicable

[0003] Not applicable

[0004] (1) Field of the Invention

[0005] This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method of forming a thermal barrier coating, in which the resulting coating is resistant to infiltration by contaminants present in a high temperature operating environment.

[0006] (2) Description of the Related Art

[0007] Hot section components of gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Air plasma spraying (APS) has the advantages of relatively low equipment costs and ease of application and masking, while TBC's employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures can be produced using other atomic and molecular vapor processes.

[0008] To be effective, a TBC must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (A1₂O₃) layer or scale that adheres the TBC to the bond coat.

[0009] The service life of a TBC system is typically limited by a spallation event brought on by thermal fatigue. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can occur as a result of the TBC structure becoming densified with deposits that form on the TBC during gas turbine engine operation. Notable constituents of these deposits include such oxides as calcia, magnesia, alumina and silica, which when present together at elevated temperatures form a compound referred to herein as CMAS. CMAS has a relatively low melting eutectic (about 1190° C.) that when molten is able to infiltrate the hotter regions of a TBC, where it resolidifies during cooling. During thermal cycling, the CTE mismatch between CMAS and the TBC promotes spallation, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively. This process of CMAS infiltration and resulting stresses during heating and cooling cycles is illustrated in FIG. 1.

[0010] Various studies have been performed to find coating materials that are resistant to infiltration by CMAS. Notable examples are U.S. Pat. Nos. 5,660,885, 5,871,820 and 5,914,189 to Hasz et al., which disclose three types of coatings to protect a TBC from CMAS-related damage. The protective coatings are classified as being impermeable, sacrificial or non-wetting to CMAS. Impermeable coatings are defined as inhibiting infiltration of molten CMAS, and include silica, tantala, scandia, alumina, hafnia, zirconia, calcium zirconate, spinels, carbides, nitrides, suicides, and noble metals such as platinum. Sacrificial coatings are said to react with CMAS to increase the melting temperature or the viscosity of CMAS, thereby inhibiting infiltration. Suitable sacrificial coating materials include silica, scandia, alumina, calcium zirconate, spinels, magnesia, calcia and chromia. As its name implies, a non-wetting coating is non-wetting to molten CMAS, with suitable materials including silica, hafnia, zirconia, beryllium oxide, lanthana, carbides, nitrides, suicides, and noble metals such as platinum. According to the Hasz et al. patents, an impermeable coating or a sacrificial coating is deposited directly on the TBC, and may be followed by a layer of impermeable coating (if a sacrificial coating was deposited first), sacrificial coating (if the impermeable coating was deposited first), or non-wetting coating. If used, the non-wetting coating is the outermost coating of the protective coating system. Whether used alone or in combination, the impermeable, sacrificial and non-wetting coatings are deposited as discrete layers on top of the TBC.

[0011] While the coating systems disclosed by Hasz et al. are effective in protecting a TBC from damage resulting from CMAS infiltration, further improvements would be desirable. In particular, infiltration of columnar TBC deposited by physical vapor deposition is not directly addressed by Hasz et al., yet is of considerable interest to the aerospace industry.

[0012] The present invention generally provides a thermal barrier coating (TBC) system and method for forming the coating system on a component suitable for use in a high-temperature environment, such as the hot section of a gas turbine engine. The invention is particularly directed to a method of forming a columnar TBC system to be resistant to infiltration by CMAS and other potential high-temperature contaminants.

[0013] The method of this invention generally entails forming a TBC on the surface of a component so that the TBC has at least an outer portion that is resistant to infiltration by CMAS. The thermal barrier coating is formed by evaporating at least one ceramic source material within a coating chamber of a physical vapor deposition apparatus. A first ceramic composition and a second ceramic composition are then co-deposited by physical vapor deposition so that the entire thermal barrier coating has columnar grains and at least the outer portion of the thermal barrier coating is a mixture of the first and second ceramic compositions. An inner portion of the TBC may be deposited to contain only the first ceramic composition, after which the outer portion of the TBC is deposited to contain the mixture of the first and second ceramic compositions. The outer portion is preferably a continuation of the inner portion, i.e., a change in composition may occur within the TBC between the inner and outer portions, but the TBC is otherwise continuous and not characterized by discrete inner and outer coatings. The second ceramic composition serves to increase the resistance of the outer portion of the thermal barrier coating to infiltration by molten CMAS. More particularly, the second ceramic composition is preferably capable of interacting with molten CMAS, forming a compound with a melting temperature that is significantly higher than CMAS. As a result, the reaction product of CMAS and the second ceramic composition resolidifies before it can fully infiltrate the TBC. The outer portion may also contain at least one platinum-group metal that further inhibits infiltration by molten CMAS. The platinum-group metal maybe co-deposited with the first and second ceramic compositions, or deposited before the thermal barrier coating and then diffused into the outer portion as a result of the parameters employed in the deposition process.

[0014] The TBC system of this invention has an increased temperature capability as a result of the TBC having reduced vulnerability to spallation from contamination by CMAS and other potential contaminants. The TBC is not required to be made up of multiple discrete layers as proposed in the past, but instead preferably has a continuous structure with its outer portion enriched with the second ceramic composition and optionally a platinum-group metal. Such a continuous TBC structure can be obtained by first depositing the inner portion to contain the first ceramic composition, and then initiating deposition of the second ceramic composition while continuing to deposit the first ceramic composition, such that the TBC is deposited in a single deposition cycle. According to a preferred aspect of the invention, the same coating cycle is used to deposit the TBC, the platinum-group metal and a metallic bond coat on which the platinum-group metal is deposited, such that the entire coating process is performed in a single operation with a single coating apparatus. In this manner, the additional cost of incorporating the second ceramic composition and the platinum-group metal can be minimized.

[0015] Other objects and advantages of this invention will be better appreciated from the following detailed description.

[0016] FIG. 1 illustrates the process of CMAS infiltration and resulting stresses that occur during heating and cooling cycles of a thermal barrier coating having a columnar grain structure.

[0017] FIG. 2 is a perspective view of a high pressure turbine blade.

[0018] FIG. 3 is a cross-sectional view of the blade of FIG. 2 along line 3—3, and shows a thermal barrier coating system in accordance with this invention.

[0019] FIG. 4 schematically represents an electron beam physical vapor deposition apparatus of a type for use in depositing the thermal barrier coating system of FIG. 3.

[0020] FIG. 5 is a scanned image of a thermal barrier coating system in accordance with this invention.

[0021] The present invention will be described in reference to a high pressure turbine blade 10 shown in FIG. 2, though the invention is generally applicable to any component that operates within a thermally and chemically hostile environment. The blade 10 generally includes an airfoil 12 against which hot combustion gases are directed during operation of the gas turbine engine, and whose surfaces are therefore subjected to severe attack by oxidation, hot corrosion and erosion. The airfoil 12 is anchored to a turbine disk (not shown) with a dovetail 14 formed on a root section 16 of the blade 10. Cooling holes 18 are present in the airfoil 12 through which bleed air is forced to transfer heat from the blade 10.

[0022] The surface of the airfoil 12 is protected by a TBC system 20, represented in FIG. 3 as including a metallic bond coat 24 that overlies the surface of a substrate 22, the latter of which may be a superalloy and typically the base material of the blade 10. As widely practiced with TBC systems for components of gas turbine engines, the bond coat 24 is preferably an aluminum-rich composition, such as an overlay coating of an MCrAlX alloy or a diffusion coating such as a diffusion aluminide or a diffusion platinum aluminide, all of which are known in the art. Alternatively, overlay coatings of beta-phase nickel aluminide (βNiAl) intermetallic can be used as the bond coat 24, in accordance with commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, and U.S. Pat. No. 6,291,084 to Darolia et al. Such aluminum-rich bond coats develop an aluminum oxide (alumina) scale 28, which is grown by oxidation of the bond coat 24. The alumina scale 28 chemically bonds a thermal-insulating TBC 26 to the bond coat 24 and substrate 22. As with prior art TBC's, the TBC 26 of this invention is intended to be deposited to a thickness that is sufficient to provide the required thermal protection for the underlying substrate 22 and blade 10. A suitable thickness is generally on the order of about 75 to about 300 micrometers.

[0023] Of particular interest to the present invention is the susceptibility of TBC materials to attack by CMAS. As discussed previously, CMAS is a relatively low melting eutectic that when molten is able to infiltrate columnar and porous TBC materials, and subsequently resolidify to promote spallation during thermal cycling. To reduce its vulnerability to spallation from contamination by CMAS and other potential contaminants, the TBC 26 is formed of a base ceramic material co-deposited with at least one additional ceramic material capable of interacting with CMAS. The TBC 26 shown in FIG. 3 is shown as comprising two zones—an inner portion 30 nearer the bond coat 24 and an outer portion 32 overlying the inner portion 30. As will be discussed in greater detail below, the inner and outer portions 30 and 32 of the TBC 26 are not discrete layers, but instead may differ in their compositions. According to a preferred aspect of the invention, both the inner and outer portions 30 and 32 are formed of the same base ceramic material, with at least the outer portion containing the additional ceramic material(s) and optionally a platinum-group metal that render the TBC 26 more resistant to infiltration by CMAS and other potential high-temperature contaminants. In terms of processing, high temperature capability and low thermal conductivity, a preferred base ceramic material for the TBC 26 is an yttria-stabilized zirconia (YSZ), a preferred composition being about 3 to about 8 weight percent yttria, though other ceramic materials could be used, such as nonstabilized zirconia, or zirconia partially or fully stabilized by magnesia, ceria, scandia or other oxides. The additional ceramic material(s) present in at least the outer portion 32 of the TBC 26 are preferably capable of interacting with molten CMAS to form a compound with a melting temperature that is significantly higher than CMAS, so that the reaction product of CMAS and the additional ceramic material does not melt and does not infiltrate the TBC 26. While those materials identified by Hasz et al. as “sacrificial layer” materials are potential candidates for the additional ceramic material of this invention, alumina has been investigated and shown to be effective as the additional ceramic material for this invention. When the alumina content of the outer portion 32 of the TBC 26 interacts with molten CMAS, one or more refractory phases form with higher melting temperatures than CMAS. In effect, the alumina content of CMAS is increased above the eutectic point, yielding a modified CMAS with a higher melting and/or crystallization temperature.

[0024] According to another preferred aspect of this invention, the TBC 26 of FIG. 3 is represented as having a strain-tolerant microstructure of columnar grains. As known in the art, such columnar microstructures can be achieved by depositing the TBC 26 using a physical vapor deposition (PVD) technique, such as EBPVD, though other PVD techniques could be used such as laser beam PVD, sputtering (e.g., magnetron), ion plasma, and cathodic arc deposition. EBPVD processes generally require the presence of an evaporation source of the desired coating composition, and an electron beam at an appropriate power level to create a vapor of the evaporation source in the presence of the surface to be coated. In the present invention, a single evaporation source and more preferably multiple evaporation sources are used to deposit the base ceramic material for the TBC 26 and the additional ceramic material(s) for the outer portion 32 of the TBC 26. FIG. 4 schematically represents an EBPVD coating apparatus 50, including a coating chamber 52 in which a component 76 is suspended for coating. The TBC 26 of this invention is represented as being deposited on the component 76 by melting and vaporizing two ingots 54 and 56 of the desired ceramic materials with a pair of electron beams 66 produced by electron beam guns 68. The intensities of the beams 66 are sufficient to produce vapor clouds 70 and 72 that mix within the coating chamber 52, and then contact and condense on the component 76 to form the TBC 26. As shown, the vapor clouds 70 and 72 evaporate from separate pools 62 and 64, respectively, of the molten coating materials contained within reservoirs formed by crucibles 58 that surround the upper ends of the ingots 54 and 56. As they are gradually consumed by the deposition process, the ingots 54 and 56 are incrementally fed into the chamber 52.

[0025] In depositing the TBC 26 to have inner and outer portions 30 and 32 with different compositions, the inner portion 30 is first deposited by evaporating only the ingot (e.g., 54 in FIG. 4) of the base ceramic material (e.g., YSZ). Once a desired thickness for the inner portion 30 is obtained, evaporation commences of the second ingot (e.g., 56 in FIG. 4) containing the desired additional ceramic material (e.g., alumina), so that the outer portion 32 of the TBC 26 contains a mixture of the base and additional ceramic materials. A suitable thickness for the inner portion 30 of the TBC 26 is on the order of about 50 to about 500 micrometers, more preferably about 75 to about 100 micrometers. A suitable thickness for the outer portion 32 of the TBC 26 is about 10 to about 50 micrometers, more preferably about 10 to about 25 micrometers, and a suitable composition for the outer portion 32 is about 2 to about 90 weight percent alumina, more preferably about 50 to about 75 weight percent alumina, the balance YSZ. Alternatively, the TBC 26 can be deposited to have a uniform composition throughout its inner and outer portions 30 and 32 by evaporating both ingots 54 and 56 simultaneously, or combining the base ceramic material (e.g., YSZ) and the additional ceramic material (e.g., alumina) in a single ingot, so that the entire TBC 26 contains a mixture of the base and additional ceramic materials.

[0026] The optional platinum-group metal may be platinum, ruthenium, rhodium, palladium, osmium and iridium, with platinum being preferred. The platinum-group metal is preferably present in at least the outer portion 32 of the TBC 26 in an amount of about 5 to about 50 weight percent. It is permissible that the outer portion 32 consist essentially of the platinum-group metal and alumina, e.g., about 10 to about 50 weight percent platinum-group metal and the balance alumina. In the ranges stated, the platinum-group metal is believed to serve as a barrier to infiltration of CMAS into the TBC 26. According to one aspect of the invention, the platinum-group metal is present throughout the TBC 26 or may be present within only the outer portion 32 of the TBC 26, as a result of being co-deposited with the ceramic materials of the TBC 26. For this purpose, a separate ingot of the platinum-group metal would be provided and evaporated with an electron beam. Alternatively, the platinum-group metal is not introduced into the outer portion 32 of the TBC 26 through deposition directly with the TBC 26, but instead is introduced by diffusion from the bond coat 24. For this purpose, the bond coat 24 is either alloyed to contain the platinum-group metal, or is overcoated with a thin layer of the platinum-group metal. Deposition of the TBC 26 is then carried out in a manner that promotes diffusion of the platinum-group metal though the inner portion 30 of the TBC and into the outer portion 32 of the TBC 26, as will be discussed below. Whether co-deposited with the ceramic materials or deposited separately before deposition of the ceramic materials, the platinum-group metal is preferably deposited in the same coating apparatus used to deposit the TBC 26, e.g., the EBPVD apparatus 50 of FIG. 3, and during the same coating cycle. However, it is foreseeable that the platinum-group metal could be incorporated using other methods, such as being electroplated on the bond coat 24.

[0027] If the platinum-group metal is deposited as a layer on the bond coat 24, a suitable thickness for this layer is believed to be about 1 to about 10 micrometers. Alternatively, the desired platinum-group metal can be incorporated into a suitable bond coat material. In a preferred embodiment, the bond coat 24 is a beta-phase nickel aluminide overlay coating alloyed to contain platinum and chromium in accordance with commonly-assigned U.S. patent application Ser. No. 10/029,320, though other bond coat materials containing a platinum-group metal could be used, notably diffusion platinum aluminides. Once a suitable amount of platinum-group metal is present within or on the bond coat 24, deposition of the TBC 26 is immediately commenced. It is believed that diffusion of the platinum-group metal into the outer portion 32 of the TBC 26 can be promoted by maintaining the absolute pressure within the coating chamber 52 at low levels. A suitable absolute pressure is believed to be not higher than 10⁻⁴torr (about 1.3×10⁻⁴mbar), preferably in a range of about 10⁻⁵to 10⁻⁴torr (about 1.3×10⁻⁵to about 1.3×10⁻⁴mbar). Following deposition of the TBC 26, a heat treatment can be performed at a temperature of about 1800 to about 2100° F (about 980° C. to about 1150° C. to further promote diffusion of the platinum-group metal into the outer portion 32 of the TBC 26.

[0028] In an investigation leading to the present invention, testing was performed with four superalloy specimens coated with platinum aluminide bond coats deposited to thicknesses of about sixty micrometers. The specimens were formed of the nickel-base superalloy known as René N5, having a nominal composition, by weight, of about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2% Al, 5.0% W, 3.0%Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B, 0.01% Y, the balance nickel and incidental impurities. The specimens were placed in an EBPVD apparatus and a mixed TBC layer containing both 7% YSZ and alumina was deposited. The mixed layer of YSZ and alumina was obtained by simultaneously evaporating separate ingots of YSZ and alumina. The alumina contents for the mixed layers ranged from about 1.8 to about 4.0 weight percent. As evidenced by the microphotograph of FIG. 5, the resulting TBC's were characterized by a substantially continuous columnar microstructure, and not the discrete layers taught by Hasz et al.

[0029] The spallation life potentials of the specimens were then evaluated by furnace cycle testing (FCT) at about 2125° F. (about 1160° C.) with one hour cycles. Testing was terminated for a given specimen when approximately 20 percent or more of its TBC had spalled. At the conclusion of the test, the specimens exhibited FCT lives ranging from 300 to 340 cycles, for an average FCT life of about 320 cycles. A baseline average FCT life for specimens lacking the TBC system of this invention but otherwise identical to the specimens tested is about 238 cycles, evidencing an improvement of about 30 percent. From these results, it was concluded that TBC systems within the scope of this invention are capable of exhibiting improved thermal cycle fatigue life. While alumina levels of about two to about four weight percent were evaluated, it is believed that alumina (or another sacrificial material) at levels of at least five weight percent within at least the outer portion of a columnar TBC would be preferred to promote the resistance of the TBC to infiltration by CMAS and other potential high-temperature contaminants.

[0030] While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art, such as by substituting other TBC, bond coat and substrate materials, or by utilizing other methods to deposit and process the TBC materials. Accordingly, the scope of the invention is to be limited only by the following claims.