Hot corrosion-resistant coatings and components protected therewith

Abstract

A coating system on a superalloy or silicon-containing substrate of an article exposed to high temperatures. The coating system includes a coating layer that overlies the substrate and is susceptible to hot corrosion promoted by molten salt impurities. A corrosion barrier coating overlies the coating layer and contains at least one rare-earth oxide-containing compound that reacts with the molten salt impurities to form a dense, protective byproduct barrier layer.

Claims

1. A coating system on a substrate of an article, the coating system comprising: at least one coating layer overlying the substrate, the coating layer having a composition that is susceptible to hot corrosion promoted by molten salt impurities; and a corrosion barrier coating overlying the coating layer and wherein the corrosion barrier coating contains at least one rare-earth oxide-containing compound that reacts with the molten salt impurities to form a dense, protective byproduct barrier layer on the surface of the corrosion barrier coating, the at least one rare-earth oxide-containing compound being present in the corrosion barrier coating in an amount of at least 15 weight percent of the corrosion barrier coating wherein the rare-earth oxide-containing compound is comprised of a rare earth zirconate (RE.sub.2Zr.sub.2O.sub.7), a rare earth hafnate (RE.sub.2Hf.sub.2O.sub.7) or a mixture thereof wherein RE in the RE.sub.2Zr.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof and wherein RE in the RE.sub.2Hf.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof; wherein the substrate comprises a silicon-containing material selected from the group consisting of metal silicide alloys, metal matrix composites reinforced with any one or more of silicon carbide, silicon nitride, a silicide and/or silicon, composites having a matrix of silicon carbide, silicon nitride, a silicide and/or silicon, and composites with a silicon carbide, silicon nitride, silicide and/or silicon matrix reinforced with silicon carbide, silicon nitride, a silicide and/or silicon.

2. The coating system according to claim 1, further comprising at least one bondcoat between the substrate and the coating layer, the bondcoat comprising at least one of elemental silicon, silicon with one or more additional ceramic phases, and silicon alloys.

3. A coating system on a substrate of an article, the coating system comprising: at least one coating layer overlying the substrate, the coating layer having a composition that is susceptible to hot corrosion promoted by molten salt impurities; and a corrosion barrier coating overlying the coating layer and wherein the corrosion barrier coating contains at least one rare-earth oxide-containing compound that reacts with the molten salt impurities to form a dense, protective byproduct barrier layer on the surface of the corrosion barrier coating, the at least one rare-earth oxide-containing compound being present in the corrosion barrier coating in an amount of at least 15 weight percent of the corrosion barrier coating wherein the rare-earth oxide-containing compound is comprised of a rare earth zirconate (RE.sub.2Zr.sub.2O.sub.7), a rare earth hafnate (RE.sub.2Hf.sub.2O.sub.7) or a mixture thereof wherein RE in the RE.sub.2Zr.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof and wherein RE in the RE.sub.2Hf.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof; wherein the coating layer is an environmental barrier layer comprising silicates, alkaline-earth metal aluminosilicates and/or rare-earth metal silicates.

4. The coating system according to claim 3, further comprising at least one thermal barrier layer between the coating layer and the corrosion barrier coating.

5. The coating system according to claim 3, wherein the substrate comprises a metallic material selected from the group consisting of nickel-, cobalt- and iron-based superalloys.

6. The coating system according to claim 5, further comprising at least one bondcoat between the substrate and the coating layer, the bondcoat comprising an MCrAlX overlay coating (where M is iron, cobalt and/or nickel, and X is yttrium, a rare-earth metal, and/or reactive metal) or diffusion aluminide intermetallics.

7. The coating system according to claim 5, further comprising at least one thermal barrier layer between the coating layer and the corrosion barrier coating.

8. The coating system according to claim 3, wherein the corrosion barrier coating further comprises a thermal-insulating material comprising any one or more of stabilised zirconia, zirconate or perovskite material.

9. The coating system according to claim 3, wherein the corrosion barrier coating comprises, by weight, at least 30% of the at least one rare-earth oxide-containing compound.

10. The coating system according to claim 3, wherein the corrosion barrier coating comprises, by weight, about 50% to about 100% of the at least one rare-earth oxide-containing compound.

11. The coating system according to claim 3, further comprising a byproduct barrier layer on the surface of the corrosion barrier coating, the byproduct barrier layer defining an outermost surface layer of the coating system.

12. The coating system according to claim 3, wherein the article comprises a component of a gas turbine engine.

13. A coating system according claim 3 wherein RE in RE.sub.2Zr.sub.2O.sub.7 is Sc, Ce, Pr, Pm, Eu, Tb, Ho, Er, Tm, Yb, or Lu.

14. A coating system on a substrate of an article, the coating system comprising: at least one coating layer overlying the substrate, the coating layer having a composition that is susceptible to hot corrosion promoted by molten salt impurities; and a corrosion barrier coating overlying the coating layer and wherein the corrosion barrier coating contains at least one rare-earth oxide-containing compound that reacts with the molten salt impurities to form a dense, protective byproduct barrier layer on the surface of the corrosion barrier coating, the at least one rare-earth oxide-containing compound being present in the corrosion barrier coating in an amount of at least 15 weight percent of the corrosion barrier coating wherein the rare-earth oxide-containing compound is comprised of a rare earth zirconate (RE.sub.2Zr.sub.2O.sub.7), a rare earth hafnate (RE.sub.2Hf.sub.2O.sub.7) or a mixture thereof wherein RE in the RE.sub.2Zr.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof and wherein RE in the RE.sub.2Hf.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof; wherein the corrosion barrier coating further comprises rare-earth monosilicates (RE.sub.2SiO.sub.5) and/or rare-earth disilicates (RE.sub.2Si.sub.2O.sub.7) wherein RE in the RE.sub.2 SiO.sub.5 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof and wherein RE in the RE.sub.2 Si.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb Lu and combinations thereof.

15. A coating system on a substrate of an article, the coating system comprising: at least one coating layer overlying the substrate, the coating layer having a composition that is susceptible to hot corrosion promoted by molten salt impurities; and a corrosion barrier coating overlying the coating layer and wherein the corrosion barrier coating contains at least one rare-earth oxide-containing compound that reacts with the molten salt impurities to form a dense, protective byproduct barrier layer on the surface of the corrosion barrier coating, the at least one rare-earth oxide-containing compound being present in the corrosion barrier coating in an amount of at least 15 weight percent of the corrosion barrier coating wherein the rare-earth oxide-containing compound is comprised of a rare earth zirconate (RE.sub.2Zr.sub.2O.sub.7), a rare earth hafnate (RE.sub.2Hf.sub.2O.sub.7) or a mixture thereof wherein RE in the RE.sub.2Zr.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof and wherein RE in the RE.sub.2Hf.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof; wherein the coating system further comprises a byproduct barrier layer on the surface of the corrosion barrier coating, the byproduct barrier layer defining an outermost surface layer of the coating system, and wherein the byproduct barrier layer comprises any one or more of REVO.sub.4, RE.sub.2V.sub.2O.sub.7 and NaRE-SiO compounds wherein RE in each of REVO.sub.4, RE.sub.2V.sub.2O.sub.7 and NaRE-SiO compounds is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof.

16. A gas turbine engine component comprising: a substrate comprising a superalloy or silicon-containing material; at least one bondcoat on the substrate; a barrier layer overlying the bondcoat, the barrier layer having a composition that is susceptible to hot corrosion promoted by molten salt impurities; a corrosion barrier coating overlying the barrier layer and having at least 15 weight percent of at least one rare-earth oxide-containing compound that reacts with the molten salt impurities, wherein the at least one rare-earth oxide-containing compound is selected from the group consisting of rare-earth zirconates (RE.sub.2Zr.sub.2O.sub.7) and rare-earth hafniates (RE.sub.2Hf.sub.2O.sub.7) and mixtures thereof, wherein RE in the RE.sub.2Zr.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof and wherein RE in the RE.sub.2Hf.sub.2O.sub.7 is selected from the group consisting of Sc, Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof; and a dense and continuous protective byproduct barrier layer on the surface of the corrosion barrier coating and formed by the at least one rare-earth oxide-containing compound reacting with the molten salt impurities, the byproduct barrier layer defining an outermost surface layer of the component; wherein the byproduct barrier coating comprises any one or more of REVO.sub.4, RE.sub.2V.sub.2O.sub.7 and NaRE-SiO compounds wherein RE in each of REVO.sub.4, RE.sub.2V.sub.2O.sub.7 and NaRE-SiO compounds is selected from the group consisting of Sc, Ce, Pr, Pm, Sm, Eu, Tb, Ho, Er, Tm, Yb, Lu and combinations thereof.

17. The gas turbine engine component according to claim 16, wherein the corrosion barrier coating comprises by weight, at least 30% of the at least one rare-earth oxide-containing compound.

18. The gas turbine engine component according to claim 16, wherein the barrier layer comprises silicates, alkaline-earth metal aluminosilicates, rare-earth metal silicates and/or yttria-stabilized zirconia.

19. A gas turbine engine component according to claim 16 wherein RE in RE.sub.2Zr.sub.2O.sub.7 is Sc, Ce, Pr, Pm, Eu, Tb, Ho, Er, Tm, Yb or Lu.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically represents a cross-sectional view of a gas turbine engine component formed of a Si-containing material and protected by a thermal/environmental barrier coating system, and further provided with a corrosion barrier coating in accordance with this invention.

(2) FIG. 2 schematically represents a cross-sectional view of a gas turbine engine component formed of a superalloy material and protected by a thermal barrier coating (TBC) system, and further provided with a corrosion barrier coating in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) The present invention provides a rare-earth (RE) oxide-containing coating suitable for protecting superalloy components and silicon-containing components and their protective TBC and EBC systems (including thermal/environmental barrier coating (T/EBC) systems) from attack and destabilization by molten salts, such as those that form as the result of impurities present in low grade fuels. Notable examples of such salts include, but are not limited to, those that form from Na.sub.2SO.sub.4 and/or V.sub.2O.sub.5. Examples of superalloy components and silicon-containing components protected by TBC and EBC systems include combustor components, turbine blades and vanes, and other components within the hot gas flow path of gas turbine engines used in various industries, including the aircraft and power generation industries. Examples of superalloy materials include nickel-based, cobalt-based and iron-based alloys, and examples of silicon-containing materials include those with a dispersion of silicon carbide, silicon nitride, metal silicides (such as niobium and molybdenum silicides) and/or silicon reinforcement material in a metallic or nonmetallic matrix, as well as those having a silicon carbide, silicon nitride and/or silicon-containing matrix, and particularly composite materials that employ silicon carbide, silicon nitride, metal silicides (such as niobium and molybdenum silicides) and/or silicon as both the reinforcement and matrix materials (e.g., ceramic matrix composites (CMCs)). While the advantages of this invention will be described with reference to gas turbine engine components, the teachings of the invention are generally applicable to any component whose substrate and/or coating system is subject to attack by molten salts.

(4) A multilayer thermal/environmental barrier coating T/EBC system 14 is schematically represented in FIG. 1. As shown in FIG. 1, a substrate 12 of a silicon-containing component 10 is protected by the coating system 14, which optionally includes a thermal barrier coating (TBC) 18. The coating system 14 provides environmental protection to the underlying substrate 12 of the component 10, while the optional TBC 18 reduces the operating temperature of the component 10 and interior layers 16, 20, 22, and 24 of the coating system 14, thereby enabling the component 10 to survive within higher temperature environments than otherwise possible. While the coating system 14 is represented in FIG. 1 as containing each of the layers 16, 18, 20, 22 and 24, it will become apparent from the following discussion that one or more of these layers could be omitted from the coating system 14. As such, the coating system 14 of FIG. 1 represents one of a variety of different coating systems within the scope of the invention.

(5) The interior layer 22 of the coating system 14 represented in FIG. 1 will be referred to as an environmental barrier layer 22 that is adhered to the substrate 12 by a bondcoat 16 directly applied to the substrate 12. Other layers represented in FIG. 1 include a transition or intermediate layer 20 and a transitional layer 24. In accordance with a preferred aspect of this invention, the coating system 14 further includes a corrosion barrier coating 26 that is deposited as the outermost layer of the component 10. As will be discussed in more detail below, the corrosion barrier coating 26 prevents or at least inhibits corrosion attack of the interior layers 16, 20, 22, 24, and 18, and particularly the environmental barrier layer 22, by inhibiting the penetration of molten salts.

(6) The major mechanism for degradation of silicon carbide (as well as silicon and other silicon compounds) in a corrosive environment is the formation of volatile silicon hydroxide (Si(OH).sub.4) products. Because the diffusivity of oxidants in materials typically suitable for use as the TBC 18 is generally very high, especially if the TBC 18 has a columnar grain structure resulting from deposition by PVD, the barrier layer 22, individually and preferably in combination with other layers of the coating system 14, exhibits low diffusivity to oxidants, such as water vapor, to inhibit the formation of deleterious crystalline oxide products within the bondcoat 16 and/or substrate 12. Preferred compositions for the barrier layer 22 are also chemically and physically compatible with the substrate 12 to remain adherent to the region 12 under severe thermal conditions. In accordance with the teachings of U.S. Pat. Nos. 5,496,644, 5,869,146, 6,254,935, 6,352,790, 6,365,288, 6,387,456, and 6,410,148, the relevant contents of which are incorporated herein by reference, suitable materials for the barrier layer 22 include alkaline-earth metal aluminosilicates, notable examples of which include calcium aluminosilicates, barium aluminosilicates, strontium aluminosilicates, and especially BSAS (as noted above). Alternatively or in addition, the barrier layer 22 may contain rare-earth silicates, for example, in accordance with U.S. Pat. Nos. 6,296,941, 6,312,763, 6,645,649 and 6,759,151. BSAS (and particularly stoichiometric BSAS) and other preferred compositions for the barrier layer 22 provide environmental protection for the Si-containing substrate 12 as well as the underlying layers 16 and 20, as discussed above. As a result, the barrier layer 22 is able to inhibit the growth of a deleterious crystalline silica layer at the substrate 12 when the component 10 is exposed to the oxidizing combustion environment of a gas turbine engine. In addition, preferred compositions for the barrier layer 22 exhibit good thermal barrier properties due to low thermal conductivity, are physically compliant with SiC-containing substrates such as the substrate 12, and are relatively compatible with the Si-containing substrate 12 in terms of CTE. A suitable thickness range for the barrier layer 22 is about 50 to about 250 micrometers.

(7) As noted above, the bondcoat 16 of the coating system 14 serves to adhere the environmental barrier layer 22 (and, therefore, the remaining layers 18, 20, 24 and 26) to the substrate 12. Preferred compositions for the bondcoat 16 contain silicon. For example, the bondcoat 16 may contain or consist of elemental silicon, silicon with one or more additional metal, intermetallic or ceramic phases (for example, silicon carbide and/or silicon nitride), and/or one or more silicon alloys (for example, silicon aluminum, silicon chromium, silicon magnesium, silicon calcium, silicon molybdenum and/or silicon titanium alloys). In accordance with commonly-assigned U.S. Pat. Nos. 6,299,988 and 6,630,200 to Wang et al., the inclusion of silicon in the layer 16 is useful to improve oxidation resistance of the substrate 12 and enhance bonding of the other layers 18, 20, 22, 24 and 26 to the substrate 12, particularly if the region 12 contains SiC or silicon nitride. A suitable thickness for the layer 16 is about 50 to about 125 micrometers.

(8) The intermediate layer 20 is useful in certain applications to promote the adhesion of the barrier layer 22 to the bondcoat 16 and the underlying substrate 12 of the component 10. Notable materials for the intermediate layer 20 include mullite and mixtures of mullite and an alkaline-earth aluminosilicate, for example, BSAS, or rare-earth silicates of formula RE.sub.2Si.sub.2O.sub.7. A suitable thickness range for the intermediate layer 20 is about 25 to about 75 micrometers depending on the particular application.

(9) The TBC 18 is employed to protect the underlying barrier layer 22 and the component 10 it covers from high operating temperatures. Hot gas path components such as buckets, nozzles, and shrouds in turbine engines burning liquid fuels are often protected by thermal barrier coatings (TBCs). Suitable materials for the TBC 18 include YSZ alone or with appropriate additions of other doping oxides capable of reducing the CTE of the TBC 18. Alternative materials for the TBC 18 include other ceramic materials known and proposed in the art for thermal barrier coatings, such as zirconate and perovskite materials. A suitable thickness range for the TBC 18 is about 25 to about 750 micrometers, depending on the particular application.

(10) The transition layer 24 is an optional layer of the coating system 14 that, if present, may be used to mitigate a CTE mismatch between the TBC 18 and the barrier layer 22, and/or inhibit reactions between the TBC 18 and barrier layer 22, for example, if the TBC 18 contains YSZ and the barrier layer 22 contains stoichiometric BSAS. If the TBC 18 contains YSZ and the barrier layer 22 contains BSAS, particularly suitable materials for the transitional layer 24 include mixtures of YSZ with alumina, mullite, and/or an alkaline-earth metal aluminosilicate, as taught in commonly-assigned U.S. Pat. No. 6,444,335 to Wang et al. Transition layer materials that have been proposed in the past also include rare-earth silicates of formula RE.sub.2Si.sub.2O.sub.7. Suitable thicknesses for the transition layer 24 will depend on the particular application, though thicknesses in a range of up to about 100 micrometers are typically adequate.

(11) During investigations leading to the present invention, uncoated Si-containing substrates and Si-containing substrates protected by EBC systems were evaluated for susceptibility to hot corrosive attack by deposits of molten salts over intermediate temperature ranges. As discussed previously, Type I sodium hot corrosion is caused by molten Na.sub.2SO.sub.4 deposits, and vanadium hot corrosion is caused by molten V.sub.2O.sub.5 deposits. The corrodants that initiate the corrosive attack can be present as impurities in the fuel combusted in a gas turbine engine. The detailed mechanism of hot corrosion varies with the nature of the material attacked and that of the corrodant, the temperature and time of exposure, deposition rate of the molten salt, and concentration of the deleterious cation (for example, sodium and/or vanadium) in the fuel. The uncoated Si-containing substrates and the Si-containing substrates protected by EBC systems were both shown to be very susceptible to corrosive attack. It was the determination of the investigations that CMCs confer no intrinsic advantage over their superalloy alternatives for turbines burning low-grade fuels containing salt-forming impurities. Since the TBC 18 as well as other layers 16, 20 and 22 of the coating system 14 may contain Si and/or complex aluminosilicates, they are susceptible to hot corrosion and are in need of further protective measures.

(12) According to a preferred aspect of the invention, to protect the underlying layers 16, 18, 20, 22 and 24 of the coating system 14 from hot corrosion, the outermost surface of the deposited coating system 14 is formed by the corrosion barrier coating 26, and the corrosion barrier coating 26 contains at least one rare-earth (RE) oxide-containing compound capable of reacting with one or more molten salts at elevated temperatures, for example, liquid salt corrodants that form from impurities such as Na.sub.2SO.sub.4, V.sub.2O.sub.5, K.sub.2SO.sub.4, and PbSO.sub.4 over a range of about 600 C. to about 1000 C. Furthermore, the reaction between the rare-earth oxide-containing compound and the molten salts must form a dense, protective byproduct barrier layer 28 on the surface of the corrosion barrier coating 26. The byproduct barrier layer 28 has a higher melting temperature (for example, about 1200 C. or higher) than the molten salt itself and prevents or retards penetration of further molten salt to the underlying structure.

(13) Particularly suitable rare-earth oxide-containing compounds include, but are not limited to, rare-earth oxides (RE.sub.2O.sub.3), rare-earth phosphates (REPO.sub.4), rare-earth zirconates (RE.sub.2Zr.sub.2O.sub.7), rare-earth hafnates (RE.sub.2Hf.sub.2O.sub.7), and/or other compounds containing a rare-earth oxide, nonlimiting examples of which include rare-earth monosilicates (RE.sub.2SiO.sub.5) and rare-earth disilicates (RE.sub.2Si.sub.2O.sub.7). In preferred embodiments of this invention, the rare-earth (RE) element is one or more of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or Lu. The one or more rare-earth oxide-containing compounds are present in the corrosion barrier coating 26 in an amount sufficient to form the byproduct barrier layer 28, represented in FIG. 1 as a continuous and outermost layer of the coating system 14 and the component 10. For this purpose, the one or more rare-earth oxide-containing compounds should constitute, by weight, at least 15%, for example, about 30% to about 100%, and more preferably about 50% to about 100% of the corrosion barrier coating 26. In addition to the rare-earth oxide-containing compounds, the corrosion barrier coating 26 may further contain conventional TBC materials, including stabilized zirconia and particularly yttria-stabilized zirconia (YSZ), and/or other known TBC materials, for example, other ceramic materials such as zirconates and perovskite materials. A suitable thickness for the corrosion barrier coating 26 is at least 50 micrometers, for example, about 50 micrometers to about 200 micrometers, and more preferably about 75 micrometers to about 125 micrometers, with preferred thicknesses depending on the particular application.

(14) As previously noted, the presence of one or more rare-earth oxide-containing compounds in the corrosion barrier coating 26 is intended to promote reaction with molten salts at elevated temperatures, and particularly with those salts that may form as a result of impurities in low grade fuels that may be combusted in gas turbine engines. Suitable compositions for the byproduct barrier layer 28 formed by the corrosion barrier coating 26 include REVO.sub.4, RE.sub.2V.sub.2O.sub.7 and/or NaRE-SiO compounds, wherein RE is the rare-earth metal of the rare-earth oxide of the corrosion barrier coating 26. For example, a corrosion barrier coating 26 containing at least 15 weight percent of a rare-earth oxide such as Y.sub.2O.sub.3 (yttria) as the rare-earth oxide-containing compound is capable of reacting with V.sub.2O.sub.5 to form a YVO.sub.4 reaction product that has a melting temperature of about 1810 C. and forms the byproduct barrier layer 28, which then inhibits penetration of additional V.sub.2O.sub.5 into the interior layers 16, 20, 22, 24 and 18 of the coating system 14. Yttria is also believed to react with Na.sub.2O-containing molten salts, such as Na.sub.2SO.sub.4, to form NaYSiO compounds whose melting temperatures may be as high as about 1365 C. As such, a corrosion barrier coating 26 containing a sufficient amount of yttria and/or one or more similar rare-earth oxide-containing compounds should be capable of protecting or at least significantly extending the lives of gas turbine components exposed to molten salts at high temperatures.

(15) To promote its resistance to cracking or spallation and exposure of the underlying substrate 12 and coating layers 16, 20, 22, 24 and 18 to molten salts, the content of the one or more rare-earth oxide-containing compounds in the corrosion barrier coating 26 may be adjusted to increase its compliance and/or promote a coefficient of thermal expansion (CTE) that more closely matches that of the substrate it protects. For example, the corrosion barrier coating 26 may contain relatively large amounts of Y.sub.2Si.sub.2O.sub.7, whose CTE is approximately 5 ppm/ C. and compatible with that of typical CMCs. Alternatively or in addition, the one or more rare-earth oxide-containing compounds may be chosen on the basis of having a CTE that more closely matches that of the remainder of the corrosion barrier coating 26. For example, CTEs of pure RE.sub.2O.sub.3 compounds and REPO.sub.4 compounds are typically compatible with that of YSZ.

(16) The corrosion barrier coating 26 may be used alone or in combination with other methods for protecting the coating system 14 and its underlying substrate 12 from hot corrosion. For example, the corrosion barrier coating 26 may be used in combination with one or more Mg compounds added to a liquid fuel containing vanadium to produce inert, high melting point products such as Mg.sub.3V.sub.2O.sub.8. These products are known to immobilize vanadium-base impurities to inhibit vanadium-induced hot corrosion.

(17) As with prior art bond coats and environmental coatings, the layers 16, 18, 20, 22, and 24 of the coating system 14 as well as the corrosion barrier coating 26 can be individually deposited by thermal spray processes, for example, air and vacuum plasma spraying (APS and VPS, respectively), though it is foreseeable that deposition could be performed by other known techniques, such as chemical vapor deposition (CVD) and high velocity oxy-fuel (HVOF) processes. The corrosion barrier coating 26 can also be deposited by known techniques such as slurry coating and PVD techniques, the latter of which can be performed to obtain a columnar grain structure within one or more of the individual layers 16, 18, 20, 22, 24 and 26. A heat treatment may be performed after deposition of individual layers 16, 18, 20, 22, 24 and/or 26 to relieve residual stresses created during cooling from elevated deposition temperatures.

(18) As previously noted, the corrosion barrier coating 26 of this invention is also applicable to use on superalloy components that are susceptible to molten salt corrosion (hot corrosion). FIG. 2 represents such a component 30 as having a superalloy substrate 32 protected by a thermal barrier coating (TBC) system 34. As also shown in FIG. 2, the TBC system 34 includes a metallic bondcoat 36 and a ceramic topcoat 38. The bondcoat 36 is intended to provide environmental protection to the underlying substrate 32 of the component 30, while the topcoat 38 (optionally in combination with internal cooling of the component 30) reduces the operating temperature of the component 30, thereby enabling the component 30 to survive within higher temperature environments than otherwise possible. A variety of materials can be used for the topcoat 38, including yttria-stabilized zirconia (YSZ) compositions that have are widely used because of their high temperature capability, low thermal conductivity, and relative ease of deposition. Suitable materials for the bondcoat 36 include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, a rare-earth metal, and/or reactive metal), and oxidation-resistant diffusion coatings that may contain compounds such as aluminide intermetallics. The corrosion barrier coating 26 and the byproduct barrier layer 28 that it forms can be as described in reference to FIG. 1.

(19) While our 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. Accordingly, the scope of our invention is to be limited only by the following claims.