Environmental barrier coating with physical barrier layer for silicon-comprising materials

[0001] This invention was made with Government support under Contract No. N00421-00-3-0536 awarded by the Navy. The Government may have certain rights to the invention.

[0002] This invention relates to a coating system for protection and mitigation against environmental contaminants, in particular oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof, that can become deposited onto an article exposed to high-temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to an article comprising a substrate formed of a material comprising silicon, an environmental barrier layer overlying the substrate, and a physical barrier layer overlying an environmental barrier layer.

[0003] Higher operating temperatures for gas turbine engines are sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. While superalloys have found wide use for components in gas turbine engines, alternative materials have been proposed. Materials comprising silicon, particularly those with silicon carbide (SiC) as a matrix material and/or reinforcing material, have been considered for high temperature applications, such as combustor and other hot section components of gas turbine engines.

[0004] In many applications, a protective coating is beneficial for Si-comprising materials. For example, protection with a suitable thermal-insulating layer reduces the operating temperature and thermal gradient through the material. Additionally, such coatings may provide environmental protection by inhibiting the major mechanism for degradation of Si-comprising materials in a corrosive water-comprising environment, namely, the formation of volatile silicon hydroxide (Si(OH)₄) products. Consequently, besides low thermal conductivity, a thermal barrier coating system for a Si-comprising material should be stable in high temperature environments comprising water vapor. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the Si-comprising material, low permeability for oxidants, and chemical compatibility with the Si-comprising material and silica scale formed from oxidation. As a result, suitable protective coatings for gas turbine engine components formed of Si-comprising materials have a dual function, serving as a thermal barrier and simultaneously providing environmental protection. A coating system having this dual function is often termed a thermal/environmental barrier coating (T/EBC) system.

[0005] While various single-layer and multilayer T/EBC systems have been investigated, each has shortcomings relating to the above-noted requirements and properties for compatibility with Si-comprising materials. For example, a coating of zirconia partially or fully stabilized with yttria (YSZ) as a thermal barrier layer exhibits excellent environmental resistance by itself since it does not comprise silica. However, YSZ does not adhere well to Si-comprising materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/° C. for YSZ as compared to about 4.9 ppm/° C. for SiC/SiC composites). Mullite (3Al₂O₃.2SiO₂) has been proposed as a bond coat for YSZ on Si-comprising substrate materials to compensate for this difference in CTE (mullite has a CTE of about 5.5 ppm/° C.). However, mullite exhibits significant silica activity and volatilization at high temperatures if water vapor is present.

[0006] Barium-strontium-aluminosilicate (BSAS) coatings suitable for Si-comprising materials exposed to temperatures of up to 2400° F. (about 1315° C.) have also been proposed. BSAS provides excellent environmental protection and exhibits good thermal barrier properties due to its low thermal conductivity. However, for application temperatures approaching the melting temperature of BSAS (about 1700° C.), a BSAS protective coating requires a thermal-insulating top coat. The addition of such a top coat on a BSAS bond coat can significantly increase the overall thickness of the T/EBC system. As application temperatures increase beyond the thermal capability of a Si-comprising material (limited by a melting temperature of about 2560° F. (about 1404° C.) for silicon) and the surface temperatures increase (up to 3100° F., or about 1704° C.), still thicker coatings capable of withstanding higher thermal gradients are required. As coating thickness increases, strain energy due to the CTE mismatch between individual coating layers and the substrate also increases, which can cause debonding and spallation of the coating system. Application of a top layer by EB-PVD methods on components such as airfoils results in a top coat having a columnar strain-tolerant microstructure. This helps to reduce stress and partially release strain energy, rendering the T/EBC more durable. However, high surface temperatures can cause rapid sintering of the top coat, which leads to less of the strain-tolerant microstructure and the development of horizontal and through-thickness cracks.

[0007] Under normal conditions of operation, coated turbine engine components can be susceptible to various types of damage, including erosion, oxidation, and attack from environmental contaminants. At the higher temperatures of engine operation, these environmental contaminants can adhere to the heated or hot coating surface and thus cause damage to the coating. For example, these environmental contaminants can form compositions that are liquid or molten at the higher operating temperatures of gas turbine engines. These molten contaminant compositions can dissolve the coating, or can infiltrate its porous structure, e.g., they can infiltrate the pores, channels or other cavities in the coating. Upon cooling, the infiltrated contaminants solidify and reduce the coating strain tolerance, thus initiating and propagating cracks that cause delamination, spalling and loss of the coating material either in whole or in part. The modified outer layer typically displays an increased CTE and/or increased stiffness that decreases compliance of the outer layer to the underlying environmental barrier layer.

[0008] The pores, channel or other cavities that are infiltrated by such molten environmental contaminants can be created by environmental damage, or even the normal wear and tear that results during the operation of the engine. Typically, the pores, channels or other cavities in the coating surface are the result of the processes by which the coating is deposited onto the underlying substrate. For example, coatings that are deposited by (air) plasma spray techniques tend to create a sponge-like porous structure of open pores in at least the surface of the coating. By contrast, coatings that are deposited by physical (e.g., chemical) vapor deposition techniques tend to create a porous structure comprising a series of columnar grooves, crevices or channels in at least the surface of the coating. This porous structure can be important in the ability of these coating to tolerate strains occurring during thermal cycling and to reduce stresses due to the differences between the coefficient of thermal expansion (CTE) of the coating and the CTE of the underlying substrate.

[0009] For turbine engine components having outer coatings with such porous surface structures, environmental contaminant compositions of particular concern are those comprising oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. See, for example, U.S. Pat. No. 5,660,885 (Hasz et al), issued Aug. 26, 1997, which describes these particular types of oxide environmental contaminant compositions. These oxides combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide oxide systems (Ca—Mg—Al—SiO), hereafter referred to as “CMAS.” The contaminant compositions may also comprise oxides of iron and titanium, and at lower temperatures, also sulfates of potassium, calcium and sodium. During normal engine operations, CMAS can become deposited on the coating surface, and can become liquid or molten at the higher temperatures of normal engine operation. Damage to the coating typically occurs when the molten CMAS infiltrates the porous surface structure of the coating. After infiltration and upon cooling, the molten CMAS solidifies within the porous structure. This solidified CMAS causes stresses to build within the coating, leading to partial or complete delamination and spalling of the coating material, and thus partial or complete loss of the protection provided to the underlying substrate of the component.

[0010] Accordingly, it would be desirable to protect environmental barrier coatings having a porous surface structure against the adverse effects of environmental contaminants. In particular, it would be desirable to be able to protect such environmental barrier coatings from the adverse effects of deposited CMAS.

[0011] In one aspect, the invention relates to an article comprising a substrate formed of a silicon-comprising material; an environmental barrier layer overlying the substrate; and a physical barrier layer overlying the environmental barrier layer, the physical barrier layer comprising zirconia or hafnia stabilized with an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof.

[0012] The invention also relates to a gas turbine engine component comprising a substrate formed of a silicon-comprising material and having an environmental barrier coating system on a surface thereof, the coating system comprising an environmental barrier layer overlying the substrate and having a thickness of from about 25 to about 500 micrometers; and a physical barrier layer as described above overlying the environmental barrier layer, the physical barrier layer having a thickness of from about 25 to about 500 micrometers.

[0013] The invention also relates to a method for preparing an environmental barrier coating system on a substrate formed of a silicon-comprising material, said method comprising:

• • [0014] a) forming an environmental barrier layer overlying the substrate and having a thickness of from about 25 to about 500 micrometers; and
  • [0015] b) forming a physical barrier layer overlying the environmental barrier layer, the physical barrier layer having a thickness of from about 25 to about 500 micrometers and comprising zirconia or hafnia stabilized with an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof.

[0017] As used herein, the term “comprising” means various compositions, compounds, components, layers, steps and the like can be conjointly employed in the present invention. Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of.”

[0018] All amounts, parts, ratios and percentages used herein are by weight unless otherwise specified.

[0019] The present invention generally provides a coating system for a substrate formed of a silicon-comprising material, particularly for articles comprising such a substrate that are exposed to high temperatures, including the hostile thermal environment of a gas turbine engine. The substrate is typically formed of a material selected from the group consisting of silicon carbide; silicon nitride; composites having a matrix of at least one of silicon carbide, silicon nitride and silicon; and composites have at least one of a silicon carbide, silicon nitride and silicon matrix reinforced with at least one of silicon carbide, silicon nitride and silicon. Examples of such materials include those with a dispersion of silicon carbide, silicon carbide and/or silicon particles as a reinforcement material in a nonmetallic matrix, as well as those having a silicon carbide, silicon nitride and/or silicon-comprising matrix, and particularly composite materials that employ silicon carbide, silicon nitride and/or silicon as both the reinforcement and matrix materials (e.g., SiC/SiC ceramic matrix composites (CMC)).

[0020] The invention relates to an environmental barrier coating (EBC) system that exhibits improved mechanical integrity for high application temperatures that necessitate thick protective coatings, generally on the order of 250 microns or more. The EBC system comprises an environmental barrier layer that overlies the surface of the Si-comprising material, and a physical barrier layer comprising zirconia or hafnia stabilized with an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and a low CTE oxide selected from the group consisting of niobia and tantala, and mixtures thereof, overlying the environmental barrier layer. The physical barrier layer has a relatively low CTE close to that of the environmental barrier layer, and retains a relatively low CTE even after exposure to CMAS contaminants, and thus results in a more compliant environment barrier coating system. In addition, the physical barrier layer is more chemically inert than the environmental barrier layer and may prevent direct interaction of CMAS contaminants with the environmental barrier layer. The physical barrier layer may thus serve as a chemical barrier to protect the environmental barrier layer from CMAS contaminants exposed to its surface. The physical barrier layer may also serve as a thermal barrier layer that at the same time provides a CTE transition between the environmental barrier layer and any CMAS reaction products of the surface of the component.

[0021] The environmental barrier coating system of the present invention thus provides at least partial and up to complete protection and mitigation against the adverse effects of environmental contaminant compositions that can deposit on the surface of such coatings during normal turbine engine operation. In particular, the environmental barrier coating system provides at least partial and up to complete protection or mitigation against the adverse effects of CMAS deposits on such coating surfaces. As a result, these CMAS deposits are unable to infiltrate the normally porous surface structure of the environmental barrier coating, and thus cannot cause undesired partial (or complete) delamination and spalling of the coating.

[0022] In addition, the environmental barrier coating system of the present invention provides protection or mitigation, in whole or in part, against the infiltration of corrosive (e.g., alkali) environmental contaminant deposits. The coatings are also useful with worn or damaged coated (or uncoated) silicon-comprising substrates of turbine engine components so as to provide for these refurbished components protection and mitigation against the adverse effects of such environmental contaminate compositions. In addition to turbine engine components, the environmental barrier coatings of the present invention provide useful protection for silicon-comprising substrates of other articles that operate at, or are exposed, to high temperatures, as well as to such environmental contaminate compositions.

[0023] According to one embodiment of the invention, a compositionally-graded EBC system as described above provides both thermal and environmental protection to a Si-comprising substrate at temperatures up to about 2000° C., particularly when present at a total coating thickness of about 250 micrometers or more, as a result of exhibiting improved mechanical integrity as compared to other coating systems for Si-comprising materials.

[0024] The present invention is generally applicable to components that operate within environments characterized by relatively high temperatures, and are therefore subjected to severe thermal cycling and stresses, oxidation, and corrosion. Notable examples of such components include combustor components, high-pressure turbine vanes, and other hot section components of gas turbine engines. A surface region 12 of a hot section component 10 is represented in the FIGURE for purposes of illustrating the invention. The component 10, or at least the surface region 12 of the component 10, is formed of a silicon-comprising material such as a SiC/SiC CMC, although the invention is generally applicable to other materials comprising silicon in any form.

[0025] As shown in the FIGURE, the surface region 12 of the component 10 is protected by a multi-layer EBC system 14. The coating system 14 provides environmental protection to the underlying surface region 12 as well as reduces the operating temperature of the component 10 and layers 16 and 20 of the coating system 14, thereby enabling the component 10 to survive within higher temperature environments than otherwise possible.

[0026] 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)₄) products. The diffusivity of oxidants in outer layers of the component 10 is generally very high. In order to protect the Si-comprising surface region 12, the coating system 14 comprises an environmental barrier layer 16 that exhibit low diffusivity to oxidants, e.g., oxygen and water vapor, to inhibit oxidation of the silicon carbide within the surface region 12, while also being sufficiently chemically and physically compatible with the surface region 12 to remain adherent to the region 12 under severe thermal conditions. The environmental barrier layer typically has a thickness of from about 25 to about 500 micrometers, more typically from about 75 to about 250 micrometers.

[0027] In one embodiment, the environmental barrier layer 16 comprises an alkaline earth metal aluminosilicate, wherein the alkaline earth metal is barium, strontium, or more typically a mixture thereof, such as BSAS. Suitable BSASs include those comprising from about 0.00 to about 1.00 moles BaO, from about 0.00 to about 1.00 moles SrO, from about 1.00 to about 2.00 moles Al₂O₃ and from about 0.10 to about 2.00 moles SiO₂. Usually, the BSASs have from about 0.00 to about 1.00 moles BaO, from about 0.00 to about 1.00 moles SrO, about 1.00 moles Al₂O₃ and about 2.00 moles SiO₂, wherein the combined moles of BaO and SrO is about 1.00 mole. Typically, the BSASs comprise from about 0.10 to about 0.90 moles (more typically from about 0.25 to about 0.75 moles) BaO, from about 0.10 to about 0.90 moles (more typically from about 0.25 to about 0.75 moles) SrO, about 1.00 moles Al₂O₃ and about 2.00 moles SiO₂, wherein the combined moles of BaO and SrO is about 1.00 moles. A particularly suitable BSAS comprises about 0.75 moles BaO, about 0.25 moles SrO, about 1.00 moles Al₂O₃ and about 2.00 moles SiO₂. See U.S. Pat. No. 6,387,456 (Eaton et al.), issued May 14, 200, especially column 3, lines 8-27, which is herein incorporated by reference.

[0028] A BSAS layer overlying the Si-comprising surface region 12 provides environmental protection and thermal barrier properties due to its low thermal conductivity. BSAS is able to serve as an environmental barrier to the underlying surface region 12, which would exhibit significant silica activity and volatilization if exposed to water vapor at high temperatures. As a result, the BSAS layer is able to inhibit the growth of an interfacial silica layer at the surface region 12 when the component 10 is exposed to the oxidizing environment of a gas turbine engine. In addition, BSAS is physically compliant with a SiC -comprising substrate, such as the surface region 12, and is relatively compatible with the Si-comprising surface region 12 in terms of CTE. A suitable thickness range for the BSAS layer is from about 75 to about 500 micrometers (about 0.003 to about 0.02 inches), depending on the particular application. In one embodiment, the environmental barrier layer has a substantially uniform composition of barium strontium aluminosilicate.

[0029] In another embodiment, the environmental barrier layer 16 may comprise mullite or mixtures thereof with yttrium silicate (e.g., Y₂O₃.SiO₂, 2Y₂O₃.3SiO₂, and Y₂O₃.2SiO₂) or an alkaline earth metal aluminosilicate as described above, such as BSAS. However, since mullite itself tends to crack as a result of thermal spray fabrication processing, an environmental barrier layer typically comprises from about 40% to about 80% by weight mullite and from about 20% to about 60% by weight BSAS, yttrium silicate, or calcium aluminosilicate. The above materials may be deposited or formed as one or more separate layers, and they may have a substantially uniform or compositionally graded composition.

[0030] Alternatively, the environmental barrier layer 16 may comprise a low CTE rare earth silicate material such as disclosed in U.S. Pat. No. 6,759,151, Lee, issued Jul. 6, 2004, incorporated herein by reference. Such rare earth silicates have a formula selected from the group consisting of (1) RE₂O₃.SiO₂, (2) 2RE₂O₃.3SiO₂, (3) RE₂O₃.2SiO₂ and combinations thereof RE is a rare earth element selected from the group consisting of Sc, Y, Dy, Ho, Er, Tm, Yb, Lu, Eu, Gd, Th and combinations thereof. Exemplary rare earth silicates are Sc₂SiO₅, Er₂SiO₅, Yb₂SiO₅, and Y₂SiO₅, and combinations thereof.

[0031] The above silicate materials may be deposited as a separate layer on the substrate, alone or as a mixture with other materials (e.g., mullite) as described above, or they may be formed on the substrate in contact with an optional silicon under-layer, as described below.

[0032] Overlying the environmental barrier layer 16 is a physical barrier layer 20 that has a relatively low CTE close to that of the environmental barrier layer. The physical barrier layer 20 promotes cyclic durability and thus the thermal and environmental protection provided by EBC system 14 to the Si-comprising surface region 12 over numerous thermal cycles and at elevated temperatures. The physical barrier layer 20 also serves to adhere the environmental barrier layer to any outer layers, such as optional top coat 18, while also preventing interactions between the environmental barrier layer 16 and any outer layer at high temperatures. The physical barrier layer comprises stabilized zirconia or stabilized hafnia, where the stabilizer is an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof. The zirconia or hafnia is typically stabilized with up to about 40 mole %, more typically up to about 20 mole %, of the metal oxide. The metal oxide is typically selected from the group consisting of magnesia, calcia, scandia, yttria, and ceria, and mixtures thereof, especially yttria. The physical barrier layer also comprises a low CTE oxide selected from the group consisting of niobia and tantala, and mixtures thereof

[0033] While not intending to be limited by theory, it is believed that niobia and tantala have a CTE lower than the stabilized zirconia and/or the stabilized hafnia present in the physical barrier layer, and that this lower CTE is beneficial for overall cyclic durability of the EBC system. The hafnia, niobia and tantala ions are also heavier than zirconia ions and are expected to have lower diffusion rates so that coatings comprising these materials are more resistant to sintering. For applications such as turbine engine airfoils, it is often desirable that the outer layer has good resistance to erosion by small particles passing through the engine. The outer layer should also be resistant to breakage due to impact of larger particles (e.g., pieces of plasma-sprayed YSZ particles coming off from the EBC in the combustor). Monoclinic and tetragonal hafnia crystal structures have higher fracture toughness and better erosion and impact resistance as compared to hafnia's cubic crystal structure. In the present invention, hafnia in the physical barrier layer may be stabilized with up to about 10 mole % of an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, to maintain the desired tetragonal crystal structure, or a mixture of monoclinic and tetragonal structures at lower amounts of the metal oxide stabilizer. At least about 0.5 mole % of the metal oxide stabilizer is desired to make the hafnia sprayable by plasma-spraying methods. It is also believed that monoclinic hafnia coatings have increased thermal conductivity. In physical barrier layers of the invention, hafnia is typically stabilized with from about 1 mole % to about 9 mole %, more typically with from about 2 mole % to about 8 mole %, of the above metal oxide. The metal oxide is typically selected from the group consisting of magnesia, calcia, scandia, yttria, and ceria, and mixtures thereof. In one embodiment, the physical barrier layer 20 comprises hafnia stabilized with yttria, e.g., about 7 weight percent yttria.

[0034] The physical barrier layer 20 typically comprises from about 10% to about 90%, more typically from about 10% to about 80%, of niobia, tantala, or mixtures thereof; from about 10% to about 90%, more typically from about 20% to about 80%, of zirconia, hafnia, or mixtures thereof; and from about 2% to about 20% of the metal oxide stabilizer; all on a molar basis. In one embodiment, the physical barrier layer comprises from about 40% to about 80% zirconia, hafnia, or mixtures thereof, from about 10% to about 60% of niobia, tantala, or mixtures thereof, and about 2% to about 20% yttria; all on a molar basis.

[0035] The physical barrier layer may have a substantially uniform composition. In one embodiment, the physical barrier layer is a substantially homogeneous mixture of yttria-stabilized zirconia (YSZ) and/or yttria-stabilized hafnia (YSH)), and niobia and/or tantala, with optional additional materials such as mullite and/or alumina, with the YSZ and/or YSH and the niobia and/or tantala comprising up to 90 weight percent of the layer 20. Alternatively, the layer 20 can be made up of discrete sublayers, each with a different composition. In one embodiment, the composition of the sublayer contacting the environmental layer typically comprises more niobia and/or tantala than the outermost sublayer, and the outermost sublayer is typically essentially YSZ and/or YSH. For example, the physical barrier layer may comprise sublayers, a first sublayer contacting the environmental barrier layer and comprising niobia and/or tantala, and a second sublayer in a direction away from the environmental barrier layer having a substantially uniform composition of yttria-stabilized zirconia and/or yttria-stabilized hafnia. One or more intermediate sublayers may be present in the physical barrier layer and have compositions that are intermediate those of the first and second sublayers.

[0036] According to another embodiment, the physical barrier layer 20 has a continuously changing composition, from essentially all YSZ and/or YSH in a direction away from the environmental barrier layer to more niobia and/or tantala adjacent the environmental barrier layer 16. In this embodiment, the layer 20 has a decreasing concentration of niobia and/or tantala and an increasing concentration of YSZ and/or YSH in a direction away from the environmental barrier layer 16. In combination, the higher concentration of niobia and/or tantala adjacent the environmental barrier layer 16 and the higher concentration of YSZ and/or YSH in a direction away from the environmental barrier layer serve to provide a gradually increasing CTE, with a minimum CTE adjacent the environmental barrier layer 16 and a maximum CTE in a direction away from the environmental barrier layer. In one example, the physical barrier layer is compositionally graded and has a decreasing concentration of niobia and/or tantala and an increasing concentration of yttria-stabilized zirconia and/or yttria-stabilized hafnia in a direction away from the environmental barrier layer, and consists essentially of yttria-stabilized zirconia and/or yttria-stabilized hafnia in a direction away from the environmental barrier layer.

[0037] A suitable thickness range for the physical barrier layer 20 is from about 25 to about 500 micrometers (about 0.001 to about 0.02 inches), typically from about 50 to about 250 micrometers, depending on the particular application and the thickness of the environmental barrier layer 16. High application temperatures, e.g., up to 2000° C., typically use thick protective coating systems, generally on the order of 250 micrometers or more. It is with such coating systems that the benefits of the physical barrier layer 20 become more apparent to improve the mechanical integrity of the coating system. The YSZ and/or YSH and the niobia and/or tantala constituents of this layer 20 provide an overall CTE closer to that of any CMAS reaction products on the outer surface of the component 10.

[0038] The component 10 may further comprise an optional silicon layer may be included between the environmental barrier layer 16 and the surface region 12. Such a silicon layer is useful to improve oxidation resistance of the surface region 12 and enhance bonding between the environmental barrier layer 16 and the surface region if the surface region comprises SiC or silicon nitride. A suitable thickness for the silicon layer is from about 12.5 to about 250 micrometers.

[0039] The component 10 may also comprise an optional top coat, such as top coat 18 shown in the FIGURE, overlying the physical barrier layer 20. Such an optional top coat may comprise yttria-stabilized zirconia, and may also comprise other ceramic materials such as various zirconias, particularly chemically stabilized zirconias, such as ceria-stabilized zirconias, calcia-stabilized zirconias, scandia-stabilized zirconias, magnesia-stabilized zirconias, india-stabilized zirconias, and ytterbia-stabilized zirconias, as well as mixtures of such stabilized zirconias. See, for example, Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol. 24, pp. 882-883 (1984), for a description of suitable zirconias. Suitable yttria-stabilized zirconias can comprise from about 1 to about 20% yttria (based on the combined weight of yttria and zirconia), and more typically from about 3 to about 10% (e.g., about 7%) yttria. These chemically stabilized zirconias can further include one or more of a second metal (e.g., a lanthanide or actinide) oxide such as dysprosia, erbia, europia, gadolinia, neodymia, praseodymia, urania, and hafnia to further reduce thermal conductivity of the thermal barrier coating. See U.S. Pat. No. 6,025,078 (Rickersby et al), issued Feb. 15, 2000 and U.S. Pat. No. 6,333,118 (Alperine et al), issued Dec. 21, 2001, both of which are incorporated by reference.

[0040] The top coat may comprise hafnia stabilized with an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals, and mixtures thereof. The top coat is typically stabilized with up to about 40 mole %, more typically with up to 20 mole %, of such an oxide. While not intending to be limited by theory, it is believed that hafnia has a CTE lower than the yttria-stabilized zirconia often present in top coats, and that this lower CTE is beneficial for overall cyclic durability of the T/EBC system. Hafnia ions are also heavier than zirconia ions and are expected to have lower diffusion rates so that coatings comprising hafnia are more resistant to sintering. For applications such as turbine engine airfoils, it is often desirable that the top coat has good resistance to erosion by small particles passing through the engine. The top coat should also be resistant to breakage due to impact of larger particles (e.g., pieces of plasma-sprayed YSZ particles coming off from the TBC in the combustor). Monoclinic and tetragonal hafnia crystal structures have higher fracture toughness and better erosion and impact resistance as compared to hafnia's cubic crystal structure. In the present invention, hafnia in the top coat may be stabilized with up to about 10 mole % of the metal oxide to maintain the desired tetragonal crystal structure, or a mixture of monoclinic and tetragonal structures at lower amounts of the metal oxide stabilizer. At least about 0.5 mole % of the metal oxide stabilizer is desired to make the hafnia sprayable by plasma-spraying methods. It is also believed that monoclinic hafnia coatings have increased thermal conductivity. In the top coats of the invention, hafnia is typically stabilized with from about 1 mole % to about 9 mole %, more typically with from about 2 mole % to about 8 mole %, of the above metal oxide. The metal oxide is typically selected from the group consisting of magnesia, calcia, scandia, yttria, and ceria, and mixtures thereof. In one embodiment, the top coat comprises hafnia stabilized with yttria, e.g., about 7 weight percent yttria.

[0041] The top coat typically comprises from about 10% to about 90%, more typically from about 30% to about 80%, of hafnia and from about 10% to about 90%, more typically from about 20% to about 80%, of zirconia, all on a molar basis. In one embodiment, the top coat comprises from about 50% to about 70% zirconia, from about 20% to about 50% hafnia, and from about 2% to about 20% yttria, all on a molar basis.

[0042] As with conventional bond coats and environmental coatings, the environmental barrier layer 16 and the physical barrier layer 20 can be individually deposited by 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). The optional top coat can also be deposited by known techniques, including plasma spraying and physical vapor deposition (PVD) techniques. Thereafter, a heat treatment may be performed after deposition of the individual layers 16 and 20, and/or the optional top coat, to relieve residual stresses created during cooling from elevated deposition temperatures.

[0043] The invention thus also provides a method for preparing an environmental barrier coating on a substrate formed of a silicon-comprising material, said method comprising:

• • [0044] a) forming an environmental barrier layer overlying the substrate and having a thickness of from about 25 to about 500 micrometers; and
  • [0045] b) forming a physical barrier layer on the environmental barrier layer, the physical barrier layer having a thickness of from about 25 to about 500 micrometer and comprising zirconia or hafnia stabilized with an oxide of a metal selected from the group consisting of magnesium, calcium, scandium, yttrium, and lanthanide metals; and a low CTE oxide selected from the group consisting of niobia and tantala; and mixtures thereof.

[0046] The present invention is particularly useful in providing thermal and environmental protection for Si-comprising materials in newly manufactured articles. However, the invention is also useful in providing such protection for refurbished worn or damaged articles, or in providing such protection for articles that did not originally have a EBC system.

[0047] While specific embodiments of the method of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the present invention as defined in the appended claims.