Environmental barrier coating with thermal properties

Abstract

An article according to an exemplary embodiment of the present disclosure includes, among other things, a substrate and a barrier layer on the substrate. The barrier layer includes a bond coat that includes a matrix, diffusion particles disposed within the matrix, and gettering particles disposed within the matrix. At least about 10% of the gettering particles are in a crystalline phase. The article also includes a top coat. An article is also disclosed.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims the benefit of priority to U.S. Patent Application No. 17 / 849,180, filed on June 24, 2022, which is hereby incorporated by reference in its entirety.

Background Art

[0002] A gas turbine engine typically includes a fan section, a compressor section, a combustor section, and a turbine section. Air entering the compressor section is compressed and sent to the combustion section where it is mixed with fuel and ignited to produce a high - energy exhaust gas stream. The high - energy exhaust gas stream expands through the turbine section and drives the compressor section and the fan section. The compressor section typically includes a low - pressure and a high - pressure compressor, and the turbine section includes a low - pressure and a high - pressure turbine.

[0003] The present disclosure relates to composite articles such as those used in gas turbine engines, and methods of coating such articles. Components such as gas turbine engine parts may be exposed to high temperatures, corrosive and oxidative conditions, and high stress levels. To improve thermal stability and / or oxidation stability, the components may include a protective barrier coating.

Summary of the Invention

Means for Solving the Problems

[0004] An article according to an exemplary embodiment of the present disclosure includes, among other things, a substrate and a barrier layer on the substrate. The barrier layer includes a bond coat that includes a matrix, diffusion particles disposed within the matrix, and gettering particles disposed within the matrix. At least about 10% of the gettering particles are in a crystalline phase. The article also includes a top coat.

[0005] In the foregoing further example, the bond coat includes a first bond coat layer adjacent to the substrate and a second bond coat layer adjacent to the top coat.

[0006] In any of the foregoing further examples, substantially all of the getter particles in one of the first bond coat layer and the second bond coat layer are amorphous getter particles, and substantially all of the getter particles in the other of the first bond coat layer and the second bond coat layer are crystalline getter particles.

[0007] In any of the foregoing further examples, at least one of the first layer and the second layer includes a mixture of amorphous getter particles and crystalline getter particles.

[0008] In any of the foregoing further examples, each of the first layer and the second layer includes a matrix of about 5 to about 40 volume percent.

[0009] In any of the foregoing further examples, the first layer includes getter particles of balanced crystalline silicon carbide or silicon dioxide.

[0010] In any of the foregoing further examples, the second layer includes about 55 to 94 volume percent of crystalline silicon carbide or silicon dioxide getter particles.

[0011] In any of the foregoing further examples, the second layer includes about 55 to 94 volume percent of amorphous silicon carbide or silicon dioxide getter particles.

[0012] In any of the foregoing further examples, the second layer includes getter particles having an average maximum dimension of about 1 to about 75 microns.

[0013] In any of the foregoing further examples, the average diameter of the getter particles in the second layer is larger than the average diameter of the getter particles in the first layer.

[0014] In any of the further examples described above, the bond coat further includes a third bond coat layer. The first layer, the second layer, and the third layer are in a pattern in which layers substantially all containing crystalline getter particles and layers substantially all containing amorphous getter particles are alternately arranged.

[0015] In any of the further examples described above, the getter particles include a mixture of amorphous and crystalline getter particles.

[0016] In any of the further examples described above, substantially all of the getter particles are in a crystalline phase.

[0017] In any of the further examples described above, the getter particles are silicon carbide.

[0018] In any of the further examples described above, the getter particles are silicon oxycarbide.

[0019] An article according to an exemplary embodiment of the present disclosure includes, among other things, a ceramic matrix composite material substrate and a barrier layer on the substrate. The barrier layer includes a bond coat that includes a silicon dioxide matrix, diffusion particles disposed within the matrix, and getter particles of silicon carbide or silicon oxycarbide disposed within the matrix. At least about 10% of the getter particles are in a crystalline phase. The article also includes a top coat.

[0020] In the further examples described above, the bond coat includes a first bond coat layer adjacent to the substrate and a second bond coat layer adjacent to the top coat.

[0021] In any of the further examples described above, substantially all of the getter particles in one of the first bond coat layer and the second bond coat layer are amorphous getter particles, and substantially all of the getter particles in the other of the first bond coat layer and the second bond coat layer are crystalline getter particles.

[0022] In any of the foregoing further examples, at least one of the first layer and the second layer includes a mixture of amorphous gettering particles and crystalline gettering particles.

[0023] In any of the foregoing further examples, the second layer includes gettering particles having an average maximum dimension of about 1 to about 75 microns. The average diameter of the gettering particles in the second layer is greater than the average diameter of the gettering particles in the first layer.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0025] FIG. 1 schematically shows a gas turbine engine 20. The gas turbine engine 20 is disclosed herein as a two-spool turbofan generally incorporating a fan section 22, a compressor section 24, a combustor section 26, and a turbine section 28. The fan section 22 drives air along a bypass flow path B in a bypass duct defined within a housing 15 such as a fan case or nacelle, and also drives air along a core flow path C for communication to the compression and combustor sections 26 and then expansion through the turbine section 28. Although shown as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use in a two-spool turbofan since the present teachings may also be applied to other types of turbine engines including three-spool structures.

[0026] The exemplary engine 20 generally includes a low-speed spool 30 and a high-speed spool 32 mounted to rotate about the central longitudinal axis A of the engine relative to the engine static structure 36 via several bearing systems 38. It should be understood that the various bearing systems 38 may be provided alternatively or additionally at various positions, and the positions of the bearing systems 38 may be appropriately changed according to the application.

[0027] The low-speed spool 30 generally includes an inner shaft 40 that interconnects a first (or low-pressure) compressor 44 and a first (or low-pressure) turbine 46. The inner shaft 40 is connected to the fan 42 via a speed-changing mechanism shown as a gear structure 48 in the exemplary gas turbine engine 20 to drive the fan 42 at a lower speed than the low-speed spool 30. The high-speed spool 32 includes an outer shaft 50 that interconnects a second (or high-pressure) compressor 52 and a second (or high-pressure) turbine 54. The combustor 56 is disposed in the exemplary gas turbine 20 between the high-pressure compressor 52 and the high-pressure turbine 54. The intermediate turbine frame 57 of the engine static structure 36 can generally be disposed between the high-pressure turbine 54 and the low-pressure turbine 46. The intermediate turbine frame 57 further supports the bearing system 38 within the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate about the central longitudinal axis A of the engine, collinear with its longitudinal axis, via the bearing system 38.

[0028] The core airflow is compressed by the low-pressure compressor 44 and then the high-pressure compressor 52, mixed and combusted with fuel in the combustor 56, and then expands through the high-pressure turbine 54 and the low-pressure turbine 46. The intermediate turbine frame 57 includes airfoils 59 within the core airflow path C. Turbines 46, 54 rotationally drive their respective low-speed spools 30 and high-speed spools 32 in response to the expansion. It will be understood that the positions of the fan section 22, compressor section 24, combustor section 26, turbine section 28, and the fan drive gear system 48 can each be varied. For example, the gear system 48 may be disposed behind the low-pressure compressor, or behind the combustor section 26, or even behind the turbine section 28, and the fan 42 may be disposed in front of or behind the position of the gear system 48.

[0029] In one example, the engine 20 is a high-bypass geared aircraft engine. In a further example, the bypass ratio of the engine 20 can be greater than about 6, greater than about 10 in an exemplary embodiment, and 18.0 or less, or more precisely 16.0 or less. The gear structure 48 is a planetary gear train such as a planetary gear system or other gear system having a gear reduction ratio greater than about 2.3. The gear reduction ratio may be 4.0 or less. The low-pressure turbine 46 has a pressure ratio greater than about 5. The low-pressure turbine pressure ratio can be 13.0 or less, or more precisely 12.0 or less. In one disclosed embodiment, the bypass ratio of the engine 20 is greater than about 10 (10:1), the diameter of the fan is significantly larger than the diameter of the low-pressure compressor 44, and the pressure ratio of the low-pressure turbine 46 exceeds about 5 (5:1). The pressure ratio of the low-pressure turbine 46 is the pressure measured in front of the inlet of the low-pressure turbine 46 relative to the pressure at the outlet of the low-pressure turbine 46 before the exhaust nozzle. The gear structure 48 may be a planetary gear train such as a planetary gear system or other gear system having a gear reduction ratio greater than about 2.3:1 and less than about 5:1. However, it should be understood that the above parameters are merely illustrative of one embodiment of a geared engine, and the present invention is applicable to other gas turbine engines including direct-drive turbofans.

[0030] Because of the high bypass ratio, a significant amount of thrust is provided by the bypass flow B. The fan section 22 of the engine 20 is designed to cruise at specific flight conditions, typically at about Mach 0.8 and about 35,000 feet (10,668 meters). At a flight condition of Mach 0.8 and 35,000 feet (10,668 meters), when the engine is at its maximum fuel consumption (also referred to as "bucket cruise thrust specific fuel consumption ('TSFC')"), it is an industry standard parameter of the lbm of fuel burned divided by the lbf of thrust generated by the engine at its minimum point. The engine parameters described above and in this paragraph are measured under this condition unless otherwise specified. "Low fan pressure ratio" is the overall pressure ratio of only the fan blades without using a fan exit guide vane ("FEGV") system. According to one non-limiting embodiment, the low fan pressure ratio disclosed herein is less than about 1.45, or more precisely, 1.25 or more. "Low corrected fan tip speed" is the speed in feet / second at the actual fan tip divided by the industry standard temperature correction value of 0.5 [(Tram°R) / (518.7°R)]

[0031] Figure 2 schematically shows a representative portion of an exemplary article 100 for a gas turbine engine 20 that includes a composite material bond coat 102 that functions as a barrier layer. The article 100 can be, for example, an airfoil within a compressor section 24 or a turbine section 28, a combustor liner panel within a combustor section 26, a blade outer air seal, or other components that benefit from the examples herein. In this example, the bond coat 102 is used as an environmental barrier layer to protect the underlying substrate 104 from environmental and thermal conditions. As will be understood, the bond coat 102 can be used as a stand-alone barrier layer, as an outermost / top coat having additional underlying layers, or in combination with other coating underlying or overlying layers such as, but not limited to, a ceramic-based top coat.

[0032] The bond coat 102 includes a matrix 106, a dispersion of "gettering" particles 108, and a dispersion of diffusion particles 110. In one example, the matrix 106 can be silicon dioxide (SiO2). In one example, the gettering particles 108 are silicide particles such as silicon oxycarbide particles (SiOC) or molybdenum disilicide (MoSi2) particles 108, although other examples are contemplated. The gettering particles 108 can be, for example, molybdenum disilicide particles, tungsten disilicide particles, vanadium disilicide particles, niobium disilicide particles, silicon oxycarbide particles, silicon carbide (SiC) particles, silicon nitride (Si3N4) particles, silicon oxycarbonitride (SiOCN) particles, silicon aluminum oxynitride (SiAlON) particles, boron silicon oxycarbide (SiBOCN) particles, or combinations thereof. The diffusion particles 110 can be, for example, barium magnesium aluminosilicate (BMAS) particles, barium strontium aluminum silicate particles, magnesium silicate particles, calcium aluminosilicate (CAS) particles, alkaline earth aluminum silicate particles, yttrium aluminum silicate particles, ytterbium aluminum silicate particles, borosilicate particles, boron aluminum silicate particles, other rare earth metal aluminum silicate particles, or combinations thereof.

[0033] The bond coat 102 protects the underlying substrate 104 from oxygen and moisture. For example, the substrate 104 can be a ceramic-based substrate such as a silicon-containing ceramic material. One example thereof is silicon carbide. Other non-limiting examples include silicon nitride. Ceramic matrix composite (CMC) substrates 104 such as silicon carbide fibers within a silicon carbide matrix are also contemplated. These CMC substrates can be formed by melt infiltration, chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP), particle infiltration, or other known methods.

[0034] The getter particles 108 and the diffusion particles 110 function as a diffusion barrier for oxygen and moisture that limits the underlying substrate 104 from being exposed to oxygen and / or moisture from the surrounding environment. Without being bound by a particular theory, the diffusion particles 110 such as the BMAS particles 110 diffuse to the outer surface of the barrier layer on the opposite side of the substrate 104 and form a seal layer that seals the underlying substrate 104 from exposure to oxygen / moisture, thereby enhancing oxidation and moisture protection. Further, the cationic metal species of the diffusion particles 110 (e.g., BMAS particles, barium, magnesium, and aluminum) can diffuse into the getter particles 108 to enhance the oxidation stability of the getter material. Further, the diffusion behavior of the diffusion particles 110 can function to seal any microcracks that may form in the barrier layer. By sealing the microcracks, oxygen can be prevented from penetrating the barrier layer, and the oxidation resistance of the barrier layer is further improved. The getter particles 108 may react with oxidizing species such as oxygen and water that may diffuse into the bond coat 102. In this way, the getter particles 108 can reduce the possibility that these oxidizing species reach the substrate 104 and oxidize.

[0035] The bond coat 102 can be applied by known methods such as the slurry coating method.

[0036] The ceramic top coat 114 is in direct contact with the bond coat 102. As an example, the ceramic top coat 114 may include one or more layers of an oxide-based material. The oxide-based material can be, for example, a hafnium-based oxide or a yttrium-based oxide (such as hafnia, hafnium silicate, yttrium silicate, yttria-stabilized zirconia or gadolinia-stabilized zirconia), calcium aluminosilicate, mullite, barium strontium aluminosilicate, or a combination thereof, but is not limited to such oxides. The top coat 114 can be applied by known methods such as a spraying (such as air plasma spraying) method or a slurry coating method.

[0037] The top coat 114 and the bond coat 102 together form a barrier coating 116 for the substrate 104. The top coat 114 is the outermost layer of the barrier coating 116 and is exposed to the elements during use of the article 100.

[0038] As described above, the barrier coating 116, particularly the bond coat 102, provides environmental resistance to the article 100. However, it may also be advantageous to provide a barrier layer 116 with adjusted thermal properties. For example, when the article 100 is cooled mainly by internal cooling, e.g., when the article 100 is subjected to a flow of cooling air, the exterior of the article 100 is exposed to very high temperature conditions, while the interior of the article is subjected to the flow of cooling air, so a large temperature gradient may be formed across the thickness of the article 100 during use. The article 100 has the advantage that the temperature gradient is reduced by the thermal protection provided by the barrier coating 116, which means that a barrier coating 116 with a low thermal conductivity can be used. On the other hand, when the article 100 does not receive significant cooling, a barrier coating 116 with a high thermal conductivity may be preferred so that no temperature gradient occurs in the article 100. In other examples, a barrier coating 116 with a moderate thermal conductivity may be desirable. For example, when it is beneficial to minimize the temperature gradient received by the article 100 while maintaining the temperature of the article 100 below the maximum allowable operating temperature, a barrier coating 116 with a moderate thermal conductivity may be preferred.

[0039] In particular, the bond coat 102 consists mainly of getter particles 108. In certain examples, the bond coat 102 includes from about 60 to about 95 volume percent of getter particles 102, from about 5 to about 40 volume percent of matrix 106, and balance diffusion particles 110. Thus, by selecting getter particles 108 having the desired thermal properties, a barrier coating 116 having the desired thermal properties is provided. Thus, the bond coat 102 described herein includes from 0 to 100% crystalline getter particles 108 and from 0 to 100% amorphous getter particles 108, the amounts of which are selected according to the desired thermal properties of the barrier coating 116. That is, the bond coat 102 may include substantially all crystalline getter particles 108, substantially all amorphous getter particles 108, or a mixture of the two. In certain examples, the bond coat 102 includes more than about 10% crystalline getter particles 108.

[0040] As described above, two examples of the gettering particles 108 are silicon carbide particles and silicon oxycarbide particles. Amorphous silicon carbide and silicon oxycarbide have lower thermal conductivities than crystalline silicon carbide and silicon oxycarbide, respectively. Other gettering particles 108 having chemical properties similar to those of silicon carbide / silicon oxycarbide operate in the same manner. Therefore, the bond coat 102 mainly containing amorphous gettering particles 108 has a lower thermal conductivity as compared with the bond coat 102 mainly containing crystalline gettering particles. The bond coat 102 containing a mixture of amorphous and crystalline gettering particles 108 has a medium thermal conductivity. Thus, the thermal conductivity of the bond coat 102 can be selected by changing the relative amounts of the amorphous and crystalline gettering particles 108. The amorphous particles can be produced by any known method such as thermal decomposition of any suitable polymer. Similarly, the crystalline particles are produced by any known method such as the Acheson process and then classified to separate particles in a desired size range.

[0041] In a specific example of the low thermal conductivity barrier layer 116 shown in FIG. 3, the bond coat 102 includes two layers 102a / 102b. The first layer 102a is disposed on the substrate 104, and the second layer 102b is disposed on the first layer 102a. The top coat 114 is disposed on the second layer 102b. In this example, both layers of 102a / 102b contain about 5 to about 40 volume percent of a matrix 106 such as silicon dioxide or amorphous silicon carbide gettering particles 108. The first layer 102a contains balanced amorphous silicon carbide gettering particles 108 having an average diameter of about 1 to 75 microns (0.04 to 2.9 mils). The second layer 102b contains about 55 to about 94 volume percent of amorphous silicon carbide gettering particles 108 having an average diameter of about 1 to 75 microns (0.04 to 2.9 mils) and balanced diffusion particles 110.

[0042] In a specific example, the average diameter of the gettering particles 108 in the second layer 102b is larger than the average diameter of the gettering particles 108 in the first layer 102a. Generally, the smaller the particles, the easier it is to conform to the adjacent surface, and the higher the adhesion of the particles to the surface. As the particles become smaller, the fillability also improves, and the porosity of the coating layer decreases. Furthermore, since the ratio of the surface area to the volume increases as the particles become smaller, the oxidant particles in the barrier coating 116 are more likely to react with the gettering particles 108 than with the substrate 104, providing an additional protective layer near the substrate 104. On the other hand, larger gettering particles 108 have a longer lifespan in the barrier coating 116 because there are many substances that can react with the oxidant to provide environmental protection as described above. Therefore, providing small gettering particles 108 in the first layer 102a improves the adhesion of the first layer 102a to the substrate 104 and provides a relatively low-porosity layer 102a, while providing large gettering particles 108 in the second layer 102b improves the environmental protection and lifespan of the layer 102b, which is more likely to encounter oxidant particles because it is outside the first layer 102a.

[0043] Another specific example of the barrier layer 116 having a low thermal conductivity is the same as the above specific example, except that amorphous silicon oxycarbide is used instead of amorphous silicon carbide.

[0044] In a specific example of the barrier layer 116 having a high thermal conductivity, the bond coat 102 includes two layers 102a / 102b as in the example of FIG. 3. In this example, both layers of 102a / 102b contain about 5 to about 40 volume percent of a matrix 106 such as silicon dioxide or crystalline silicon carbide gettering particles 108. The first layer 102a contains balanced crystalline silicon carbide gettering particles 108 having an average diameter of about 1 to 75 microns (0.04 to 2.9 mils). The second layer 102b contains about 55 to about 94 volume percent of crystalline silicon carbide gettering particles 108 having an average diameter of about 1 to 75 microns (0.04 to 2.9 mils) and balance diffusion particles 110. In a specific example, the average diameter of the gettering particles 108 in the second layer 102b is larger than the average diameter of the gettering particles 108 in the first layer 102a.

[0045] Another specific example of the barrier layer 116 having a high thermal conductivity is identical to the above specific example except that crystalline oxycarbide silicon is replaced with crystalline silicon carbide.

[0046] In a specific example of the barrier layer 116 having a medium thermal conductivity, the bond coat 102 includes two layers 102a / 102b as in the example of FIG. 3. In this example, both layers of 102a / 102b contain about 5 to about 40 volume percent of a matrix 106 such as silicon dioxide. The first layer 102a contains a balance of crystalline silicon carbide gettering particles 108 having an average diameter of about 1 to 75 microns (0.04 to 2.9 mils). The second layer 102b contains about 55 to about 94 volume percent of amorphous silicon carbide gettering particles 108 having an average diameter of about 1 to 75 microns (0.04 to 2.9 mils) and balance diffusion particles 110. In a specific example, the average diameter of the gettering particles 108 in the second layer 102b is larger than the average diameter of the gettering particles 108 in the first layer 102a. The combination of amorphous and crystalline gettering particles 108 provides a medium thermal conductivity as compared to the above examples that contain mainly (or solely) amorphous or mainly (or solely) crystalline gettering particles.

[0047] Another specific example of the barrier layer 116 having a medium thermal conductivity is identical to the above-described specific example except that the crystalline oxycarbide silicon is replaced by crystalline silicon carbide. Similarly, amorphous oxycarbide silicon can be replaced by amorphous silicon carbide.

[0048] As another specific example of the barrier layer 116 having a medium thermal conductivity, there is a multilayer bond coat 102 having two or more layers 102a / 102b. In particular, the exemplary barrier layer 116 can have 3 to 8 bond coat layers 102a / 102b. Each layer 102a / 102b can include crystalline gettering particles or amorphous gettering particles, or a combination of the two. In a specific example, each of the layers 102a / 102b has amorphous or crystalline gettering particles 108, and the crystalline gettering particles 108 and the amorphous gettering particles 108 are alternately arranged in the layers 102a / 102b. In another example, one or more of the layers 102a / 102b include a mixture of crystalline gettering particles and amorphous gettering particles.

[0049] The term "about" as used herein has its typical meaning in the art, but in a specific example, "about" can mean a deviation of up to 10% of the value described herein.

[0050] Although different examples have been described as having specific components, the examples of the present disclosure are not limited to these specific combinations. It is possible to combine a part of the components or features from any of the embodiments with the features or components from any of the other embodiments.

[0051] The foregoing description should be construed as illustrative and not in any limiting sense. Those skilled in the art will understand that certain modifications may fall within the scope of the present disclosure. For these reasons, the following claims should be considered to determine the true scope and content of the present disclosure.

Claims

1. A substrate, A barrier layer on the substrate, An article comprising: The barrier layer comprises: A bond coat including a matrix, diffusion particles disposed within the matrix, and gettering particles disposed within the matrix, with at least about 10% of the gettering particles in a crystalline phase, A top coat, The article.

2. The article according to claim 1, wherein the bond coat includes a first bond coat layer adjacent to the substrate and a second bond coat layer adjacent to the top coat.

3. The article according to claim 2, wherein substantially all of the gettering particles in one of the first and second bond coat layers are amorphous gettering particles, and substantially all of the gettering particles in the other of the first and second bond coat layers are crystalline gettering particles.

4. The article according to claim 2, wherein at least one of the first layer and the second layer includes a mixture of amorphous and crystalline gettering particles.

5. The article according to claim 2, wherein each of the first layer and the second layer includes from about 5 to about 40 volume percent of the matrix.

6. The article according to claim 5, wherein the first layer includes gettering particles of balanced crystalline silicon carbide or silicon dioxide.

7. The article according to claim 5, wherein the second layer includes from about 55 to 94 volume percent of gettering particles of crystalline silicon carbide or silicon dioxide.

8. The article according to claim 5, wherein the second layer includes from about 55 to 94 volume percent of gettering particles of amorphous silicon carbide or silicon dioxide.

9. The article according to claim 2, wherein the second layer includes gettering particles having an average maximum dimension of from about 1 to about 75 microns.

10. The article according to claim 9, wherein the average diameter of the gettering particles in the second layer is greater than the average diameter of the gettering particles in the first layer.

11. The article according to claim 2, wherein the bond coat further includes a third bond coat layer, and the first, second, and third layers are arranged alternately with a layer of substantially all crystalline gettering particles and a layer of substantially all amorphous gettering particles.

12. The article according to claim 1, wherein the gettering particles include a mixture of amorphous and crystalline gettering particles.

13. The article according to claim 1, wherein substantially all of the gettering particles are in a crystalline phase.

14. The article according to claim 1, wherein the gettering particles are silicon carbide.

15. The article according to claim 1, wherein the gettering particles are silicon oxycarbide.

16. An article comprising a ceramic matrix composite material substrate and a barrier layer on the substrate, wherein the barrier layer comprises a silicon dioxide matrix, diffusion particles disposed within the matrix, and gettering particles of silicon carbide or silicon oxycarbide disposed within the matrix, and at least about 10% of the gettering particles are in a crystalline phase, a bond coat, a top coat, and an article comprising the same.

17. The article according to claim 15, wherein the bond coat comprises a first bond coat layer adjacent to the substrate and a second bond coat layer adjacent to the top coat.

18. The article according to claim 17, wherein substantially all of the gettering particles in one of the first and second bond coat layers are amorphous gettering particles, and substantially all of the gettering particles in the other of the first and second bond coat layers are crystalline gettering particles.

19. The article according to claim 17, wherein at least one of the first and second layers comprises a mixture of amorphous and crystalline gettering particles.

20. The article according to claim 17, wherein the second layer comprises gettering particles having an average maximum dimension of about 1 to about 75 microns, and the average diameter of the gettering particles in the second layer is larger than that of the gettering particles in the first layer. ​

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