The present invention generally relates to superalloys employed under service conditions involving extended exposures to high temperatures. More particularly, this invention is directed to a process for incorporating a carburized region beneath an aluminum-rich environmental coating on substrates formed of nickel-based superalloys prone to coating-induced metallurgical instability, wherein the carburized region stabilizes the microstructure of the substrate beneath the coating.
Certain components of gas turbine engines, particularly turbine blades, turbine vanes, and components of the combustor and augmentor, are susceptible to damage by oxidation and hot corrosion attack and are therefore protected by an environmental coating. If used in combination with a thermal barrier coating (TBC), the environmental coating is termed a bond coat and the combination of the TBC and environmental coating form what may be termed a TBC system. Environmental coatings in wide use include diffusion aluminide coatings formed by diffusing aluminum into the substrate to be protected, resulting in a coating on the substrate surface and a diffusion zone beneath the substrate surface. Examples are disclosed in U.S. Pat. Nos. 3,415,672, 3,540,878, 3,598,638, 3,617,360, 3,667,985, 3,677,789, 3,692,554, 3,819,338, 3,837,901, and 6,066,405. Other environmental coatings in use include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, rare earth metals, and/or reactive metals), and beta-phase (β) NiAl overlay coatings. Examples of the former are disclosed in commonly-assigned U.S. Pat. Nos. 5,043,138 and 5,316,866, and examples of the latter are disclosed in commonly-assigned U.S. Pat. Nos. 5,975,852, 6,153,313, 6,255,001, 6,291,084, and 6,620,524. The suitability of environmental coatings formed of NiAlPt to contain the gamma-prime phase (γ′-Ni3Al has also been considered, as disclosed in U.S. Patent Application Publication Nos. 2004/0229075 to Gleeson et al., 2006/0093801 to Darolia et al., and 2006/0093850 to Darolia et al.
Environmental coatings (with and without TBC) are being used in an increasing number of turbine applications, particularly on combustors, augmentors, turbine blades, turbine vanes, etc., of gas turbine engines. The material systems used for most turbine airfoil applications comprise a nickel-based superalloy as the substrate material, a platinum-modified diffusion aluminide (β-(Ni,Pt)Al) as the environmental coating (bond coat), and a zirconia-based ceramic as the TBC material. Yttria-stabilized zirconia (YSZ), with a typical yttria content in the range of about 4 to about 8 weight percent, is widely used as the ceramic material for TBC's. Common deposition processes include thermal spraying (particularly air plasma spraying) and physical vapor deposition (particularly electron-beam physical vapor deposition (EB-PVD)).
The above-noted environmental coating materials contain relatively high amounts of aluminum relative to the superalloys they protect, while superalloys contain various elements that are not present or are present in relatively small amounts in environmental coatings. During the deposition of an environmental coating, a primary diffusion zone of chemical mixing occurs to some degree between the coating and the superalloy substrate as a result of the concentration gradients of the constituents. Such a diffusion zone is particularly prominent in diffusion aluminide coatings. At elevated temperatures, further interdiffusion occurs as a result of solid-state diffusion across the substrate/coating interface. The migration of elements across this interface alters the chemical composition and microstructure of both the environmental coating and the substrate in the vicinity of the interface, causing what may be termed coating-induced metallurgical instability, sometimes deleterious results. For example, FIG. 2 represents a substrate region 20 of a nickel-based superalloy containing high levels, e.g., two weight percent or more, of refractory elements such as rhenium, chromium, tantalum, tungsten, and combinations thereof. The substrate region 20 is shown as being provided with a diffusion coating 22, such as an aluminide or a platinum (or other platinum group metal (PGM))-modified aluminide coating, which may optionally serve as a bond coat for a TBC (not shown). As represented in FIG. 2, a primary diffusion zone 24 is present in the substrate region 20 beneath the coating 22 as a result of the coating process. The diffusion zone 24 generally contains the beta (β-NiAl or β-(Ni,Pt)Al) matrix phase 26 of the coating 22 and refractory metal rich precipitation phases such as topologically close-packed (TCP) phases 28. The incidence of a moderate amount of the TCP phases 28 beneath the coating 22 is typically not detrimental. However, at elevated temperatures (including those during coating formation), further interdiffusion occurs as a result of solid-state diffusion across the substrate/coating interface. In particular, because of its high refractory metal content, a secondary reaction zone (SRZ) 30 is present beneath the diffusion zone 24. The SRZ 30 is characterized by a gamma/gamma-prime inversion relative to the substrate region 20, such that the SRZ 30 has a gamma prime (γ′-Ni3Al) matrix 32 containing gamma (γ-Ni) and TCP-phase needles 34, which tend to be aligned perpendicular to the substrate-coating interface. SRZ 30 beneath the diffusion zone 24 can degrade mechanical properties of the superalloy substrate 20 by reducing the load-bearing cross-section or by crack initiation along the high angle grain boundary between the SRZ 30 and the superalloy substrate 20.
Commercially-known high strength superalloys that contain significant amounts of refractory elements (such as rhenium, chromium, tantalum, tungsten, hafnium, molybdenum, niobium, and zirconium) include gamma prime (γ′) precipitate-strengthened nickel-based superalloys such as MX4 (U.S. Pat. No. 5,482,789), René N6 (U.S. Pat. No. 5,455,120), CMSX-10, CMSX-12, and TMS-75. Significant efforts have been put forth to control SRZ in these and other superalloys. For example, commonly-assigned U.S. Pat. Nos. 5,334,263, 5,891,267, and 6,447,932 provide for direct carburizing or nitriding of a superalloy substrate to form stable carbides or nitrides that tie up the high level of refractory metals present near the surface. Other proposed approaches involve blocking the diffusion path of aluminum into the superalloy substrate with a diffusion barrier coating, examples of which include ruthenium-based coatings disclosed in commonly-assigned U.S. Pat. Nos. 6,306,524 to Spitsberg et al., 6,720,088 to Zhao et al., 6,746,782 to Zhao et al., and 6,921,586 to Zhao et al. Still other attempts involve coating the surface of a high rhenium superalloy with chromides or cobalt prior to aluminizing the surface, as disclosed in U.S. Pat. No. 6,080,246. Finally, U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses that a PGM-based coating diffused directly into a superalloy substrate can eliminate the need for a traditional aluminum-containing environmental coating and thereby avoid SRZ and TCP phase formation.
The ability to successfully inhibit SRZ formation by surface carburization was demonstrated in the above-noted U.S. Pat. Nos. 5,334,263 and 5,891,267. Surface carburization reacts TCP phase-forming elements (most notably rhenium, chromium, tantalum, and tungsten) with carbon to form submicron-sized carbides, to the extent that the incidence of TCP phases can be reduced and the microstructure of the substrate stabilized against formation of SRZ. FIG. 3 schematically represents a substrate region 20 (corresponding to that of FIG. 2) whose surface has been modified by carburization, and FIG. 4 contains an SEM photograph and a detail thereof showing a layer of submicron carbide precipitates formed below the surface of a nickel-based superalloy as a result of a carburization treatment. The submicron size of the carbide precipitates avoids any detrimental effect on fatigue as they are significantly smaller than other features that could lead to fatigue initiation (e.g., pores, eutectic phases, and cast-in carbides). FIG. 3 represents the effect of a carburization treatment as the elimination of the SRZ 30 and its gamma-prime matrix 32 and gamma and TCP-phase needles 34 beneath the diffusion zone 24 of FIG. 2, and the presence of carbide precipitates 36 within a carburized surface region 38 of the substrate 20 that coincides with or extends beneath the primary diffusion zone 24 of the diffusion coating 22.
Various processes exist for carburizing metal surfaces. Each generally involves the use of a carbon-rich source and an enclosure within which a substrate to be coated can be exposed to carbon atoms made available by the source over a period of time and at a sufficiently elevated temperature to enable the substrate to be enriched with carbon. The composition of the substrate determines the effect of the carburization process. For example, in U.S. Pat. No. 5,702,540, a vacuum gas carburization process is disclosed for carburizing a steel material, in which the carbon source is acetylene gas and the steel material is carburized in a vacuum furnace for the purpose of hardening its surface.
In the context of inhibiting SRZ formation in nickel-based superalloys that undergo a diffusion aluminide coating process, there appears to be a need to accurately and consistently control the depth of carburization. Too little carburization can be inadequate to inhibit SRZ formation, while too much carburization can adversely affect mechanical properties. The nominal carbide layer depth in a nickel-based superalloy protected by a diffusion aluminide coating is believed to approximately coincide with the depth of the aluminum-enriched diffusion zone beneath the coating following application of the coating and subsequent post-coating heat treatments. On this basis, for a diffusion aluminide coating formed by conventional diffusion processes, a preferred carburization depth is believed to be about 25 to about 100 micrometers below the substrate surface. However, in practice it has been difficult to consistently form carburized surface regions in nickel-based superalloys with depths within this range, and particularly with depths that approximately coincide with a known depth of a diffusion zone of a given diffusion coating. The ability to consistently control the carburization depth becomes particularly important for turbine components that have relatively thin walls and cross-sections and are therefore more sensitive to carburization depth variations. Excessive carburization can be particularly problematic at sharp features, such as the trailing edge of an airfoil where carburization occurs from three directions.