WO2017146726A1

WO2017146726A1 - Ceramic matrix composite material with enhanced thermal protection

Info

Publication number: WO2017146726A1
Application number: PCT/US2016/019789
Authority: WO
Inventors: Gary B. Merrill; Jr. Jonathan E. SHIPPER
Original assignee: Siemens Aktiengesellschaft
Priority date: 2016-02-26
Filing date: 2016-02-26
Publication date: 2017-08-31

Abstract

There is provided a component (10, 110, 110a, 210) comprising a body (12, 112) comprising a ceramic matrix composite material (14, 114), a top surface (16, 116), a bottom surface (18, 118), and a through thickness (20, 120) between the top surface (16, 116) and the bottom surface (18, 118). A plurality of first grooves (24) and second grooves (26) extend from the top surface (16, 116) down into the through thickness (20, 120) of the body (12, 112) in a predetermined pattern (50). A dopant material (135) may also be provided which enhances the thermal resistance properties of the ceramic matrix composite material (14, 114) may be further be included in the component (10, 110, 110a, 210).

Description

CERAMIC MATRIX COMPOSITE MATERIAL WITH ENHANCED THERMAL

PROTECTION

FIELD OF THE INVENTION

The present invention relates to high temperature materials, and in particular to components comprising a ceramic matrix composite (CMC) material with enhanced thermal protection features, including grooves formed in the CMC material and/or a gradient of a dopant material incorporated therein. BACKGROUND OF THE INVENTION

Gas turbines comprise a casing or cylinder for housing a compressor section, a combustion section, and a turbine section. A supply of air is compressed in the compressor section and directed into the combustion section. The compressed air enters a combustion inlet and is mixed with fuel. The air/fuel mixture is then combusted to produce high temperature and high pressure gas. This working gas then travels past the combustor transition and into the turbine section of the turbine.

The turbine section comprises rows of vanes which direct the working gas to the airfoil portions of the turbine blades. The working gas travels through the turbine section, causing the turbine blades to rotate, thereby turning the rotor. The rotor is also attached to the compressor section, thereby turning the compressor and also an electrical generator for producing electricity. A high efficiency of a combustion turbine is achieved by heating the gas flowing through the combustion section to as high a temperature as is practical. The hot gas, however, may degrade various turbine components such as the combustor, transition ducts, vanes, ring segments, and turbine blades that it passes when flowing through the turbine.

For this reason, strategies have been developed to protect such components from extreme temperatures, such as the development and selection of high temperature materials adapted to withstand these extreme temperatures and cooling strategies to keep the components adequately cooled during operation. For example, state of the art nickel-based superalloys with additional protective coatings are now commonly used for hot gas path components of gas turbines. However, in view of the substantial and longstanding development in the area of superalloys, it figures to be extremely difficult to further increase the temperature capability of superalloys.

Accordingly, ceramic matrix composite (CMC) materials have been developed. Such materials typically include a fiber-reinforced ceramic matrix. When utilized in turbine component, CMC materials offer the potential for higher operating temperatures than do the aforementioned superalloy materials due to the inherent nature of ceramic materials. If a CMC material is used within certain turbine components, this capability may be translated into a reduced cooling requirement that, in turn, may result in higher power, greater efficiency, and/or reduced emissions for the associated gas turbine. Notwithstanding the above, CMC materials still have operating temperature limitations due to degradation of the ceramic fibers of the CMC material. For this reason, protective thermal barrier coatings, e.g., friable graded insulation, have been developed and applied to the CMC materials in an attempt to increase operating limits of the CMC materials. However, such coatings require additional material, processing time, and are prone to delamination from the CMC material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a cross-sectional view of a component comprising a CMC material having grooves of differing depths formed therein in accordance with an aspect of the present invention.

FIG. 2 is a cross-sectional view of a component comprising a CMC material comprising a gradient of a dopant material in accordance with an aspect of the present invention.

FIG. 3 is a cross-sectional view of a component comprising three thickness regions having differing amounts of a dopant material in accordance with an aspect of the present invention.

FIG. 4 is a cross-sectional view of a component formed from CMC material comprising a gradient of a dopant material and grooves of differing depths formed therein in accordance with an aspect of the present invention FIG. 5 illustrates first grooves and second grooves as described in a grid pattern in accordance with an aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Aspects of the present invention address the deficiencies in prior art by providing solutions to further increase the operational temperature limits of components, e.g., gas turbine components, formed at least in part from CMC materials. In this way, a gas turbine, for example, incorporating components formed as described herein may produce higher power, greater efficiency, and/or reduced emissions relative to gas turbine engines without such components.

In accordance with one aspect, there is provided a component comprising a ceramic matrix composite (CMC) material that includes a plurality of first and second grooves formed therein which extend to differing depths within the CMC material. The first and second grooves allow for additional thermal cooling of the CMC material. In addition, the differing depths and the lack of a common plane between a terminal end of the first and second grooves in the through thickness of the CMC material may substantially prevent delamination of the CMC material. In certain embodiments, if a delamination defect is formed, then the grooves act as an arresting feature of that delamination. In certain embodiments, the grooves may define a grid pattern, and the extent of delamination may only be as large as the size of the grid which the grooves form.

In accordance with another aspect, the operational temperature of the CMC material may be enhanced additionally or independently by providing a gradient of a dopant material within the body of CMC material that increases the thermal resistance properties of the CMC material. Dopant materials may be added to the matrix material to improve the chemical stability of the CMC material, and reduce erosion and environmental degradation. In certain embodiments, elemental oxides (e.g., rare earth types) can be added in small amounts to improve the recession of the matrix material over a period of time. Dopant materials can also promote improved chemical resistance (to water vapor, for example) or improved erosion resistance (through increased matrix density, for example), or a combination of both. In an embodiment, the gradient comprises a greater concentration of the dopant material in a first thickness region compared to that of a second thickness region. In operation, the first thickness region may be exposed to higher temperatures relative to the second thickness region.

Now referring to the figures, FIG. 1 illustrates a component 10 having a body 12 (CMC body 12) comprising a ceramic matrix composite material 14. The body 12 comprises a top surface 16, a bottom surface 18, and a through thickness 20 extending between the top surface 16 and the bottom surface 18. For purposes of illustration only, the illustrated CMC body 12 comprises a plurality of fibers in the form of plies 22 to which ceramic material may be added/impregnated in the formation of the CMC material 14. In the embodiment of FIG. 1 , a plurality of first grooves 24 and a plurality of second grooves 26 are provided that extend from the top surface 16 down into the through thickness 20 of the body 12 in a pattern. I n an embodiment, the second grooves 26 have a longer length and extend deeper into the through thickness 20 compared to the first grooves 24. As can be appreciated with reference to FIG. 1 , since the first grooves 24 and the second grooves 26 do not have an end within the through thickness 20 which terminates in the same plane into the through thickness 20, the possibility of delamination of the CMC material 14, particularly at high temperatures, e.g., > 1200° C, is substantially reduced. In addition, the grooves 24, 26 may allow for reduced strain surface sintering of the CMC body 12.

The component 10 may comprise any desired component comprising the CMC material 14. In an embodiment, the component 1 0 may comprise a gas turbine component as is known in the art. In a particular embodiment, the component 10 may comprise an airfoil configured for use in a combustor turbine hot gas section. The component 10 may be a stationary part or a rotating part of a gas turbine, such as one of a transition duct, a blade, a vane, or the like. The CMC material 14 of the component 10 may comprise any suitable ceramic or ceramic matrix material that hosts a plurality of reinforcing fibers as is known in the art. In certain embodiments, the CMC material 14 may be anisotropic, at least in the sense that it can have different strength characteristics in different directions. It is appreciated that various factors, including material selection and fiber orientation, can affect the strength characteristics of a CMC material. In addition, the CMC material 14 may comprise oxide as well as non-oxide CMC materials. In an embodiment, the CMC material 14 comprises an oxide-oxide CMC material as is known in the art.

In a particular embodiment, the CMC material 14 may comprise a ceramic matrix (e.g., alumina) and the fibers may comprise an aluminosilicate composition consisting of alumina and silica (such as 3M's Nextel 720 high temperature oxide fibers). The fibers may be provided in various forms, such as a woven fabric, blankets, unidirectional tapes, and mats. In an embodiment, the CMC material 14 is formed from a plurality of plies of the fiber material, which may be at one or more stages be infused with a ceramic material and any other optional materials, and subjected to a suitable heat treatment, e.g., sintering, process. A variety of techniques are known in the art for making a CMC material and such techniques can be used in forming the CMC material 14 for use herein. In addition, exemplary CMC materials 14 are described in U.S.

Patent Nos. 8,058, 191 , 7,745,022, 7, 153,096; 7,093,359; and 6,733,907, the entirety of each of which is hereby incorporated by reference. As mentioned, the selection of materials may not be the only factor which governs the properties of the CMC material 14 as the fiber direction may also influence the mechanical strength of the material, for example. As such, the fibers for the CMC material 14 may have any suitable

orientation, such as those described in U.S. Patent No. 7, 153,096.

While not wishing to be bound by theory, it is believed that the grooves 24, 26 may relieve surface stresses during exposure to a hot gas path, thereby allowing for surface densification without delamination. Surface densification may provide a more erosion and environmentally robust surface which will increase the life and robustness of the CMC material 14, and thus the component 10.

The grooves 24, 26 may be of any particular size, shape, and dimension for the intended purpose of the component. In an embodiment, the first grooves 24 have a depth of from about 0.5 mm to about 1 .0 mm and a width from about 0.3 mm to about 1 .0 mm, and in a particular embodiment have a depth of from about 0.6 mm to about 0.8 mm and a width from about 0.5 mm to about 0.7 mm. In addition, the second grooves 26 may have a depth of from about 1 .0 mm to about 2.0 mm and a width from about 0.3 mm to about 1 .0 mm, and in a particular embodiment have a depth of from about 1 .4 mm to about 1 .6 mm and a width from about 0.5 mm to about 0.7 mm. It is understood, however, that the present invention is not so limited.

In addition, a depth of the first and second grooves 24, 26 may be of any desired ratio relative to one another. I n an embodiment, the second grooves 26 may be at least twice as long (e.g., extend into the through thickness 20 at least twice as much) as the first grooves 24. In certain embodiments, the second grooves 26 may extend into the through thickness 20 to an amount greater than 50 percent of the entire through thickness 20 of the body 12 while the first grooves 24 do not extend beyond 50 percent or half of the through thickness 20. In addition, it is appreciated that the grooves 24, 26 may comprise any desired cross-sectional profile. In an embodiment, one or both of the grooves 24, 26 comprise a polygonal shape in cross-section with an open end at the top surface 16. In another embodiment, one or both of the grooves 24, 26 may comprise a rounded cross-sectional profile. Further, the grooves 24, 26 may be provided at any suitable angle relative to the top surface 16.

In accordance with an aspect, the selection of the groove formation strategy, including but not limited to the angle, dimensions, number, and location of the grooves, may vary in accordance with the intended use of the component 10. By way of example, for a turbine blade made of a conventional superalloy with a conventional coating system like a metallic bond coat + 8YSZ thermal barrier coating (TBC), the grooves may be designed to prevent a critical defect larger than 7mm² from forming. In one aspect, a grid of grooves spaced 2 mm in a first direction and 2 mm in a second direction may be employed. In this way, a system could stand to lose TBC in one grid portion and still stay under that critical TBC loss value. In certain embodiments, the TBC loss value may be determined by calculating the spall size large enough to raise the temperature high enough to burn a hole in the superalloy in a full interval.

The grooves 24, 26 may be formed in the CMC body 12 in any suitable pattern. In an embodiment, the grooves 24, 26 may be produced in a grid pattern in the surface of the CMC at the depths and widths specified above. In this embodiment, the grid can be either aligned with the 0/90 direction of fiber weavings or may be disposed at a +/- 45 degree direction relative to the principal fiber direction. Additionally, more complex patterns can be applied to the surface of the CMC to mitigate any angular weakness. Such examples can be utilized to create multi-faceted grooves in the surface of the CMC such as triangular, hexagonal, or other polygonal geometries. Further, the size of the unit cell may be adjusted to suit a particular need such as a fillet radius or high curvature surface, such as a leading edge.

It is appreciated that the grooves 24, 26 may also be formed in the body 12 utilizing any suitable energy source or sources, such as a laser energy source or via a suitable machining process. In certain embodiments, a laser source is utilized and the laser source comprises a YAG laser having a wavelength of about 1 .6 microns, for example. In this way, the YAG laser source may serve as a finer cutting instrument than would a carbon dioxide laser which has a wavelength of about 10.1 microns, for example.

In accordance with another aspect, the component 10 may further or

independently include an amount of a dopant material incorporated into the CMC material 14. While not wishing to be found by theory, it is believed that the dopant material increases the ability of the CMC material 14 to withstand high temperatures, e.g., > 1200° C. By way of example, FIG. 2 illustrates an embodiment of a component having an amount of a dopant material therein.

In particular, FIG. 2 illustrates a component 1 1 0 comprises a body 1 12 which includes a CMC material 1 14 as described above. The body 1 12 further includes a top surface 1 16 and a bottom surface 1 18. In this instance, the body 1 12 may define a first thickness region 1 15 and a second thickness region 125 in a through thickness 120 between the top surface 1 16 and the bottom surface 1 18. In accordance with an aspect of the invention, the body 1 12 comprises an amount of a dopant material 135

incorporated within the CMC material 1 14. The dopant material 135 may be provided in any suitable amount in the CMC material 1 14 as set forth below.

The dopant material 135 may comprise any suitable material known in the art that is effective to decrease a likelihood of delamination or any other structural damage to the CMC material 1 14 upon exposure to high operating temperatures, e.g., > 1200° C. In an embodiment, the dopant material 135 may be a member selected from the group consisting of zirconia, zirconia hafnate, yttrium hafnate, titanium oxide, iron oxide, aluminum oxide, and combinations thereof, and in a particular embodiment comprises yttrium hafnate.

In accordance with one aspect, the dopant material 135 may be provided in a gradient 132 within the CMC material 1 14. By "gradient," it is meant that the

concentration of the dopant material 135 may be different in at least one region of the through thickness 120 relative to at least one other region. For example, the dopant material 135 may comprise a higher concentration in the first thickness region 1 15 than in the second thickness region 125. In this way, the first thickness region 1 15 may have enhanced thermal properties in the regions of the component 1 10 which are exposed to the highest temperatures.

In certain embodiments, the gradient 132 may comprise three or more concentrations of the dopant material 135 within the body 1 12 between the top surface 1 16 and the bottom surface 1 18. In addition, in any embodiment described herein, the concentration of dopant material 135 may increase in a direction from the bottom surface 1 18 toward the top surface 1 16 to define the gradient 132. For example, as shown in FIG. 3, there is shown another embodiment of a component 1 10a having a body 1 12 that comprises at least a third thickness region 145 below a second thickness region 125 and a first thickness region 1 15. In an embodiment, the third thickness region 145 may be provided with a dopant material 135 content, which is less than that of the first thickness region 1 15 and the second thickness region 125. In this embodiment, the first thickness region 1 15 may also have a dopant concentration greater than the second thickness region 125.

In accordance with another aspect, the first thickness region 1 15, second thickness region 125, and remaining thickness regions, e.g., third thickness region 145, may each comprise any suitable portion of the through thickness 120 of the body 1 12, such as from about 10 to about 90% by size or volume of the through thickness 120. In a particular embodiment, the body 1 12 comprises the first thickness region 1 15 and the second thickness region 125, wherein the first thickness region 1 15 and the second thickness region 125 each represent respective halves of the through thickness 120. In another embodiment where three regions 1 15, 125, 145 are employed, the first 1 15, second 125, and third 145 thickness regions may each represent respective thirds of the through thickness 120 of the body 1 12. Further, it is appreciated that the border(s) of each region may extend parallel to the top surface 1 16 or the bottom surface 1 18.

It is appreciated that the gradient 132 may be provided independently of the grooves 24, 26 as described herein or in conjunction therewith. For purposes of illustration, FIG. 2 illustrated a component 1 10 having the gradient 132 of the dopant material 135, but with relatively uniform grooves 134 formed in the component 1 10 which extend from the top surface 1 16 down into the through thickness 120 of the body 1 12 in a predetermined pattern. In another embodiment, as shown in FIG. 4, it is appreciated that the any of high temperature enhancing features described herein may be combined in the same article. In particular, FIG. 4 illustrates a component 210 having a gradient 132 of the dopant material 135 in at least first thickness region 1 15 and a second thickness region 125 as described herein, as well as first grooves 24 and second grooves 26 as described herein.

The components described herein may be formed via any suitable manufacturing process. Typically, a CMC material may formed by laying up a plurality of layers, e.g., plies, of a ceramic material, either in the form of pre-preg material or as a dry material. The plies may then be infused with a wet matrix material, which may comprise a ceramic material in a suitable carrier liquid (e.g., organic solvent), suspension, or slurry, to obtain a desired thickness and composition. The infused plies may then be cured and/or fired to provide the final desired product at a suitable temperature, e.g., from about 1 100° to 1300° C. In this instance, when a dopant material 135 is provided within a component 10, 1 10, 1 1 0a, 210 as described herein, it is appreciated that the dopant material 135 may be included in the wet matrix material or other suitable medium, and applied to the fiber material in order to include the dopant material in the desired concentration. When a gradient 132 is present, the gradient 132 may be formed by applying differing amounts of the dopant material 135 to one or more fiber layers to define regions having different dopant concentrations during manufacture of the component. In this way, after lay-up, curing, and thermal processing, a laminate CMC structure may be produced with an increasing amount of dopant material 135 from one surface (1 16 or 1 18) to an opposed surface (the other of 1 16 and 1 18) of the

component 1 10, 1 10a, 210. When grooves 24, 26 are provided as described herein, it is appreciated that the CMC body 12 may be formed as described above. Upon being formed, the body 12, 1 12, typically in the form of a sintered composite, may be subjected to surface scoring with a laser source that would effectively cut the grooves 24, 26, 134 into the CMC body 12, 1 12 in a predetermined pattern. As noted above, it is these grooves 24, 26, 134 which may relieve surface stresses during exposure to a hot gas path, thereby allowing for surface densification without delamination.

The dopant material 135 may be provided in any suitable concentration in the components described herein. By way of example only, in one embodiment, the dopant material 135 may comprise from > 0 to 5.0 wt % of the respective body 12, 1 12, and in a particular embodiment from > 0 to 3.0 wt %. In an embodiment where the body 1 12 comprises a first thickness region 1 15 and a second thickness region 125, the dopant material 135 may be provided in a concentration of from > 0 to about 3.0 wt % of the respective body 1 12 in the first thickness region 1 15 and a concentration of from about >0 to about 2.0 wt % of the total solids content of the body 1 12 in the second thickness region 125. In another embodiment where at least an additional third thickness region 145 is provided (FIG. 3), the dopant material 135 may be provided in an amount of from > 0 to 1 .0 wt % of the respective body 1 12 while the first and second regions 1 15, 125 have the concentrations mentioned above.

As mentioned, the grooves 134, 24, 36 described herein may be formed in the body of any of the components described herein in any suitable pattern across a top surface and/or bottom surface of the component. In an exemplary embodiment, as shown in FIG. 5, the body 1 12 of the component 1 10 may comprise the grooves 24, 26 in a predetermined pattern 50. In the embodiment shown, the predetermined pattern 50 comprises a grid pattern 52 having first grooves 24 extending in a first direction 54 and second grooves 26 extending in a second direction 56. Alternatively, the second grooves 26 could extend in the first direction 54 and the first grooves 24 could extend in the second direction 56. Further alternatively or additionally, as mentioned previously, more complex patterns can be applied to the surface of the body 1 12 to mitigate any angular weakness. Such examples can be utilized to create multi-faceted grooves in the surface of the CMC such as triangular, hexagonal , or other polygonal geometries. While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.

Claims

CLAI MS The invention claimed is:

1 . A component (1 0, 1 10, 1 10a, 210) comprising:

a body (12, 1 12) comprising a ceramic matrix composite material (14, 1 14), a top surface (16, 1 16), a bottom surface (1 8, 1 18), and a through thickness (20, 120) between the top surface (16, 1 16) and the bottom surface (18, 1 18); and

a plurality of first grooves (24) and second grooves (26) extending from the top surface (16, 1 16) down into the through thickness (20, 120) of the body (12, 1 12) in a predetermined pattern (50), wherein the second grooves (26) have a longer length and extend deeper into the through thickness (20, 120) relative to the first grooves (24).

2. The component (10, 1 10, 1 10a, 210) of claim 1 , wherein the body (12, 1 12) defines a first thickness region (1 15) and a second thickness region (125) in the through thickness (20, 120) between the top surface (16, 1 16) and the bottom surface (18, 1 18), wherein the body (12, 1 12) further comprises a gradient (132) of a dopant material (135) within the body (12, 1 12), and wherein the gradient (132) comprises a greater concentration of the dopant material (135) in the first thickness region (1 15) than in the second thickness region (125).

3. The component (10, 1 10, 1 10a, 210) of claim 2, wherein the gradient (132) comprises two or more distinct concentrations of the dopant material (135), and wherein the dopant material (135) increases in concentration from the bottom surface (16, 1 16) toward the top surface (18, 1 18).

4. The component (10, 1 10, 1 10a, 210) of claim 2, wherein the dopant material (135) comprises a member selected from the group consisting of zirconia, zirconia hafnate, yttrium hafnate, titanium oxide, and combinations thereof.

5. The component (10, 1 10, 1 10a, 210) of claim 2, wherein the dopant material (135) comprises a concentration of > 0 to about 3.0 wt % of the body (12, 1 12) in the first thickness region (1 1 5) and a concentration of from > 0 to about2.0 wt % of the body (12, 1 12) in the second thickness region (125).

6. The component (10, 1 10, 1 10a, 210) of claim 1 , wherein the first grooves (24) have a depth of from about 0.6 mm to about 0.8 mm and a width from about 0.4 mm to about 0.6 mm, and wherein the second grooves (26) have a depth of from about 1 .4 mm to about 1 .6 mm and a width from about 0.4 mm to about 0.6 mm.

7. A component (1 0, 1 10, 1 10a, 210) comprising:

a body (12, 1 12) comprising a ceramic matrix composite material (14, 1 14), a top surface (16, 1 16), and a bottom surface (18, 1 18), the body defining a first thickness region (1 15) and a second thickness region (125) in a through thickness between the top surface (16, 1 16) and the bottom surface (18, 1 18);

a gradient (132) of a dopant material (135) within the body (12, 1 12), wherein the gradient (132) comprises a greater concentration of the dopant material (135) in the first thickness region (1 15) than in the second thickness region (125); and

a plurality of grooves (24, 26, 134) extending from the top surface (16, 1 16) down into the through thickness (20, 120) of the body (12, 1 12) in a predetermined pattern (50).

8. The component (10, 1 10, 1 10a, 210) of claim 7, wherein the gradient (132) comprises two or more distinct concentrations of the dopant material (135), and wherein the dopant material (135) increases in concentration from the bottom surface (18, 1 18) toward the top surface (16, 1 16).

9. The (10, 1 10, 1 10a, 210) of claim 7, wherein the dopant material (135) comprises a member selected from the group consisting of zirconia, zirconia hafnate, yttrium hafnate, titanium oxide, and combinations thereof.

10. The component (10, 1 10, 1 10a, 210) of claim 7, wherein the dopant material (135) comprises a concentration of from > 0 to about 3.0 wt % of the body (12,

1 12) in the first thickness region (1 15) and a concentration of from about >0 to about 2.0 wt % in the second thickness region (125).

1 1 . The component (10, 1 10, 1 10a, 210) of claim 7, wherein the plurality of grooves (24, 26, 134) comprise first grooves (24) and second grooves (26), and wherein the second grooves (26) have a longer length and extend deeper into the through thickness (20, 120) relative to the first grooves (24).

12. The component (10, 1 10, 1 10a, 210) of claim 1 1 , wherein the first grooves (24) have a depth of from about 0.6 mm to about 0.8 mm and a width from about 0.4 mm to about 0.6 mm, and wherein the second grooves (26) have a depth of from about 1 .4 mm to about 1 .6 mm and a width from about 0.4 mm to about 0.6 mm.

13. A component (1 0, 1 10, 1 10a, 210) comprising:

a body (12, 1 12) comprising a ceramic matrix composite material (14, 1 14), a top surface (16, 1 16), a bottom surface (1 8, 1 18), and a through thickness (20, 120) between the top surface (16, 1 16) and the bottom surface (18, 1 18);

14. The component (10, 1 10, 1 10a, 210) of claim 13, wherein the gradient (132) comprises two or more distinct concentrations of the dopant material (135), and wherein the dopant material (135) increases in concentration from the bottom surface (18, 1 18) toward the top surface (16, 1 16).

15. The component (10, 1 10, 1 10a, 210) of claim 13, wherein the dopant material (135) comprises a member selected from the group consisting of zirconia, zirconia hafnate, yttrium hafnate, cobalt oxide, titanium oxide, iron oxide, aluminum oxide, and combinations thereof.