Patent Publication Number: US-2016236994-A1

Title: Patterned abradable coatings and methods for the manufacture thereof

Description:
This application claims the benefit of U.S. Provisional Application No. 62/117,295 filed Feb. 17, 2015, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Abradable coatings on flowpath surfaces above the moving metal blade tips in a turbine engine can reduce over tip leakage and improve efficiencies. For example, a rotating part can erode a portion of a fixed abradable coating applied on an adjacent stationary part to form a seal having a very close tolerance. In one example application, an abradable seal can be used to minimize the clearance between blade tips and an inner wall of an opposed shroud, which can reduce leakage or guide leakage flow of a working fluid, such as steam or air, across the blade tips, and enhance turbine efficiency. 
     SUMMARY 
     As noted above, thermal barrier coat (TBC)/abradable coatings have been developed for traditional metal turbine shroud seals, which are also referred to herein as turbine blade track segments. However, ceramic matric composite (CMC) blade track segments require a ceramic environmental barrier coat (EBC)/TBC/abradable coating so the linear coefficients of thermal expansion (a) are better matched with the underlying substrate. During operation, a thermal mismatch results between a hot side of the abradable coating being scrubbed by a hot gas in the gas turbine flowpath and an opposed colder side of the abradable coating against the CMC segment. This thermal mismatch causes strains within the abradable coating as the hot side attempts to expand more than the cold side. High levels of these strains can cause the coating to crack and spall. Spallation can expose the underlying CMC component to excessive temperatures and can hurt turbine performance due to poorer efficiency due to increased tip clearance. 
     The exceeding low a of the CMC makes it difficult to find an abradable coating with a similar coefficient of thermal expansion. In addition, currently available ceramic coatings are not easy to apply and do not have a suitable balance of properties such as, for example, durability, life, TBC performance, and tip rub performance. 
     Porosity can be important in a traditional TBC/abradable coating for metal turbine blade track segments. Porosity reduces the thermal conductivity (improves TBC performance), while also improving tip rub performance. Porosity within the coating has traditionally been created while spraying the coating onto the component. However, only minimal levels of porosity can be created in a sprayed-on ceramic abradable coating, and such coatings are heavy, have high thermal conductivity, and poor tip rub performance. 
     In the present disclosure, in one embodiment porosity is created in a ceramic abradable coating by forming a pattern in a surface thereof 
     In one aspect, the present disclosure is directed to an article including an abradable ceramic coating, wherein at least portion of a major surface of the coating includes an array of depressions, wherein the depressions are arranged such that the coating has a porosity of about 5% to about 90%. 
     In another aspect, the present disclosure is directed to a method, including: forming an abradable ceramic coating on a CMC component; and machining a pattern of features into a surface of the abradable ceramic coating, wherein the features include an array of hemi spherically-shaped depressions. 
     In various embodiments, the ceramic abradable coatings described in this disclosure can have higher effective porosity compared to sprayed-on coatings, which can provide improved abradability. In various embodiments, the ceramic abradable coatings described in this disclosure can have higher density compared to sprayed-on coatings, which can provide improved erosion resistance. 
     Using spray techniques, it can be difficult to control the variation of porosity throughout the coating, which can lead to reduced minimum strength (less durable/life) and higher minimum thermal conductivity (poorer TBC performance). By forming a pattern into the surface, the variation in porosity can be more precisely controlled, which can result in improved durability/life, and better tip rub performance. 
     In various embodiments, the present disclosure provides a thicker abradable layer for a CMC component, which can increase the maximum tip rub capability of a blade track segment and can also provide increased thermal isolation (improved TBC performance). Compared to abradable coatings made with spray-coating techniques, the abradable coatings described herein can provide such enhanced properties at lower cost, and resulting parts can require reduced machining time to fit close tolerances. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic, overhead view of a pattern in a surface of an abradable ceramic coating. 
         FIG. 1B  is a schematic, cross-sectional view of the coating of  FIG. 1A . 
         FIGS. 1C-1E  are schematic, overhead views of patterns in a surface of an abradable ceramic coating. 
         FIG. 1F  is a perspective view of a portion of an abradable ceramic coating with a pattern in a surface thereof. 
         FIG. 2  is a schematic cross-sectional view of a tool used to create a pattern in a surface of a green ceramic article. 
         FIG. 3  is a schematic, cross-sectional view of a pattern of grooves in a surface of a patterned abradable coating. 
         FIGS. 4-8  are schematic, overhead views of patterns of blocks in a surface of a patterned abradable coating. 
         FIG. 9  is a schematic, partial side view of a rotating turbine engine component and an adjacent stationary component including patterned abradable coatings. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a schematic diagram of a non-limiting example embodiment of a pattern including an array of surface features  80  on at least a portion of a surface  82  of a ceramic abradable coating  81 . The surface features may occupy all or a portion of the surface  82  of the coating  81 . 
     In various non-limiting embodiments, the ceramic abradable coating  81  can include aluminum nitride, aluminum diboride, boron carbide, aluminum oxide, mullite, zirconium oxide, carbon, silicon carbide, silicon nitride, transition metal nitrides, transition metal borides, rare earth (RE) oxides, and mixtures and combinations thereof. In some embodiments, the ceramic abradable coating  81  includes at least one silicate, which in this application refers to a synthetic or naturally-occurring compound including silicon and oxygen. Suitable silicates include, but are not limited to, rare earth (RE) disilicates, RE monosilicates, barium strontium aluminum silicate, and mixtures and combinations thereof 
     Referring again to  FIG. 1A , the array  80  includes a regular pattern of pocket-like depressions  84  that have shapes, sizes and patterns selected to control, for example, the porosity of the coating  81 , the flow of a fluid over the surface  82 , or both, while minimizing thermal stresses and stress concentrations in the coating  81 . In the embodiment of  FIG. 1A , the array  80  includes linear, parallel rows  86 A,  86 B and linear, parallel columns  88 A,  88 B of depressions  84 . In rows  86 A,  86 B, the depressions  84  are a distance r apart, have a diameter x, and the depressions  84  of an adjacent row  86 B are offset a distance r/2 relative to the depressions in the row  86 A. In columns  88 A,  88 B, the depressions  84  are a distance r apart, and the depressions  84  of an adjacent column  88 B are offset a distance r/2 relative to the depressions in the column  88 A. 
     In various embodiments, the area of the individual depressions  84  should be sufficiently large, and the depressions should occupy a sufficiently large area of the surface  82 , to resist thermal mismatches between opposed sides of the coating. This reduction in thermal mismatch can relieve thermal stresses within a hot surface of the coating, which in some embodiments can reduce coating spallation. Reduction in thermal mismatches can also allow a thicker coating to be applied on a substrate without resulting in excessive spallation and coating loss. In various embodiments, a thicker coating is desirable to increase maximum tip rub capability and provide enhanced thermal and/or environmental isolation. 
     In some embodiments, the area occupied by the individual depressions  84  should be sufficiently small and occupy a sufficiently small area of the surface  82  to maintain or improve turbine performance by controlling flow across a tip of a rotating part that engages the coating. In some embodiments, the size of the depressions is small relative to the thickness of the tip of the rotating part that engages the coating, which can maintain or improve turbine performance by restricting the flow across the tip of the blade via a series of fluid expansions and/or contractions. 
     In some embodiments, which are not intended to be limiting, the depressions  84  can be aligned to create solid, unbroken ridges that extend from one side of a component to an opposite side thereof. Such unbroken ridges can allow a path for a fluid to travel across a tip of a rotating component that engages the coating  81  (such as, for example, a blade tip), which in some embodiments can diminish turbine efficiency. In addition, while not wishing to be bound by any theory, presently available evidence indicates that unbroken ridges in the surface  82  of the coating  81  can lower thermal stress compared to a solid abradable coating, but in some embodiments coatings with long unbroken ridges can still have a thermal mismatch between opposed sides. 
     In a presently preferred embodiment, the depressions  84  should be alternating and relatively close together as shown in  FIG. 1A , which minimizes the number of straight, unbroken ridges in the coating  81 . Again, while not wishing to be bound by any theory, presently available evidence indicates that alternating patterns of closely-spaced depressions  84  that create fewer straight, unbroken ridges in the surface  82  can more effectively relieve thermal strains in the coating  81 . In addition, in some embodiments alternating patterns of depressions  84  can minimize fluid leakage between the rotating component and the coating  81 . 
     The shapes, sizes, depths and patterning of the depressions  84  can vary widely depending on the intended application. In some embodiments, the shapes, sizes (e.g. diameters), depths and arrangement of the depressions  84  may be the same over all or a portion of the surface  82 , which is referred to herein as a regular array. In other embodiments, at least one of the shapes, sizes, depths and arrangement of the depressions  84  differs over all or a portion of the surface  82 , which is referred to herein as an irregular array. In some irregular arrays, the shapes, sizes, depths and arrangement of the depressions varies randomly over the surface  82 , or particular types of depressions may be used in different areas of the surface  82 . In the embodiment of  FIG. 1A , the depressions  84  are hemispherically-shaped dimples similar to those found on the surface of a golf ball, although shapes such as pyramidal, conical, or portions of geodesic spheres made from triangular, tetrahedral, icosahedral or octahedral elements are possible. 
     Referring to  FIG. 1B , the cross-sectional profile  83  of the depressions  84  of  FIG. 1A  is semi-circular or parabolic, although other cross-sectional shapes are possible, such as square, rectangular, triangular or trapezoidal. Square or rectangular cross-sectional shapes create depressions with sharp sides and flat bottoms, which in some embodiments currently available evidence indicates would be less effective in minimizing thermal stresses in the coating  81 , so depressions with gradually sloping sides are generally preferred. In some embodiments, depression shapes with sharp sides can also be more difficult to accurately and consistently manufacture at a reasonable cost. 
     To ensure performance and survivability of the coating  81 , in some embodiments the depth d of the depressions  84  can be as deep as the maximum depth of the channel created when a rotating part (such as, for example, a turbine blade tip) engages the coating  81 . In some embodiments the depressions  84  have a depth d extending all the way through the coating  81  to an underlying layer  85  on which the coating  81  is applied. In other embodiments the depth d of the depressions  84  should be less than the thickness of the coating  81  such that a solid area of the coating remains adjacent to the underlying layer  85  to act as a thermal and/or environmental barrier region. In various embodiments, the thickness of the abradable coating  81  is about 0.01 inches (0.25 mm) to about 0.125 inches (3.175 mm), and the depth d of the depressions  84  should not exceed the thickness of the abradable coating  81 , but can be any thickness less than the thickness of the abradable coating  81 . 
     In various embodiments that are merely included as examples are not intended to be limiting, a regular array of depressions that are each substantially spherical and have a depth d that that is the same as, or substantially similar to, their diameter x, have been found to be useful. For example, if x is the diameter of a depression at the surface of the abradable coating, and r is the spacing distance between adjacent depressions, (see  FIG. 1A ), in various embodiments the x/r ratio can be about 0.1 to about 1, or about 0.25 to about 0.75, or about 0.4 to about 0.7, or about 0.5 to about 0.67. In various embodiments that are not intended to be limiting, the diameter x of the depressions  84  is typically less than about 0.25 inches (6.35 mm). 
       FIG. 1C  shows an example of a regular array of spherical depressions with an x/r ratio of about 0.5. At x/r ratios below  0 . 5 , there are straight lines of surface material in four different directions. In another example,  FIG. 1D  shows a regular array of spherical depressions with an x/r ratio of about 0.4. At x/r ratios of greater than about 0.5, there are only thin, straight lines of material along the surface in two directions)(+/−45°.  FIG. 1E  shows a regular array of spherical depressions with an x/r ratio of about 0.67. Ratios of x/r greater than about 0.67 have minimal distances between depressions, or the depressions begin to intersect. 
     As the x/r ratio increases, greater effective porosity is created in the abradable coating layer including the depressions. Again, as an example that is not intended to be limiting, if the depressions have a depth d that is approximately equal to their distance apart r, the effective coating porosity at an x/r ratio of 0.4 was about 17%, at an x/r ratio of 0.5 was about 26%, and at an x/r ratio of about 0.67 was about 47%. In various embodiments, suitable porosities can be about 5% to about 90%, or about 15% to about 80%, or about 25% to about 75%, or about 25% to about 50%, or about 25% to about 45% (all values are ±1%). 
     Further, as shown in  FIG. 1F , in some embodiments the array of depressions can extend all the way to an edge of a part, such that partial depressions remain near the edge. In some embodiments, this arrangement can further interrupt and/or eliminate the straight line of coating material at the edge. 
     Referring again to  FIG. 1B , in various exemplary embodiments the layer  85  underlying the abradable coating layer  81  can be a bond coat layer, a thermal barrier coating layer, an environmental barrier coating (EBC) layer, or a CMC component. 
     Suitable examples environmental barrier coatings include, but are not limited to, mullite; glass ceramics such as barium strontium alumina silicate (BaOx-SrO1-x-Al2O 3 -2SiO 2 ; BSAS), barium alumina silicate (BaO-Al 2 O 3 -2SiO 2 ; BAS), calcium alumina silicate (CaO-Al 2 O 3 -2SiO 2 ), strontium alumina silicate (SrO-Al 2 O 3 -2SiO 2 ; SAS), lithium alumina silicate (Li2O-Al 2 O 3 -2SiO 2 ; LAS) and magnesium alumina silicate (2MgO-2Al 2 O 3 -5SiO 2 ; MAS); rare earth silicates and the like. 
     Suitable examples of thermal barrier coatings, which may provide thermal insulation to the CMC substrate to lower the temperature experienced by the substrate, include, but are not limited to, insulative materials such as ceramic layers with zirconia or hafnia. The thermal barrier coating may optionally include other elements or compounds to modify a desired characteristic of the coating, such as, for example, phase stability, thermal conductivity, or the like. Exemplary additive elements or compounds include, for example, rare earth oxides. 
     In some embodiments, the surfaces of the abradable ceramic coatings including the depressions can optionally be treated (e.g., machined, polished, ground, cut, burnished, galled, drilled, or the like or a combination thereof) to achieve a desired dimension, surface morphology or chemistry. 
     The depressions  84  can be created in the surface  82  of the coating  81  by any suitable technique including, but not limited to, machining with a tool, laser sintering, water jet cutting, electrochemical machining (ECM), milling, and combinations thereof. 
     While the depressions  84  in  FIGS. 1A-F  are generally spherically shaped dimples, a wide variety of shaped depressions may be used. Examples include, but are not limited to, generally conically shaped depressions created by a pointed tool ( FIG. 2 ), arrays and patterns of V-shaped grooves ( FIG. 3 ), and various patterns such as shown in  FIGS. 4-8  below. 
     Referring now to  FIG. 2 , in one embodiment a surface  113  of a ceramic layer  114  on a green ceramic article  110  can be machined by contacting the ceramic layer  114  with a tool  130 . 
     In the schematic example of  FIG. 2 , the tool  130  includes punch elements  132  that, when moved in the direction of the arrow A, could create, for example, a regular or an irregular array of apertures  133  in all or a portion of the surface  113 . The punch elements  132  may be configured to create apertures  133  with a wide variety of shapes, depths and patterns when observed from above including, for example, circles, squares, diamonds, triangles, trapezoids, and combinations thereof. The apertures  133  may also have a wide variety of cross-sectional shapes including, for example, circles, parabolas, triangles, squares, and combinations thereof. In various exemplary embodiments, the apertures  133  can be shaped like hemispheres, cones, pyramids, and combinations thereof. In some embodiments, these arrays of apertures can minimize thermal stresses and stress concentrations in all or a portion of the surface  113 . Other non-mechanical techniques may be used to make the array of apertures  113  in the ceramic layer  114  including, for example, laser drilling or cutting, or chemical etching through a mask. 
     The green ceramic article  110  may be sintered using well known techniques to harden the machined ceramic layer  114  to form a patterned abradable coating. The resulting patterned abradable coating may optionally be further machined to modify the pattern thereon. 
     In another embodiment shown schematically in  FIG. 3 , the tool  130  of  FIG. 2  could be moved over the surface  113  along the direction of the arrow B to create a pattern or array  138  of V-grooved channels  140  in all or a portion of the surface  113 . In various embodiments, the grooves  140  could have a depth δ of about 0.01 inches (0.25 mm) to about 0.125 inches (3.175 mm), a width w of about 0.01 inches (0.25 mm) to about 0.125 inches (3.175 mm), a spacing x of about 0.01 inches (0.25 mm) to about 0.25 inch (6.35 mm), and any length l up to the entire length of the component. The angle α of the grooves  140  is generally less than about 135°. The grooves  140  can be oriented parallel to an edge  139  of the ceramic article  110 , or can be oriented at an angle θ with respect to the edge  139 , with θ ranging from about 0° to about 90°. The channels  140  could be straight as shown in  FIG. 3 , or in some embodiments could be curved or resemble wavy lines. 
     In various exemplary embodiments, a layer (not shown in  FIG. 3 ) can underlie the abradable coating such as, for example, a bond coat layer, a thermal barrier coating layer, an environmental barrier coating (EBC) layer, or a CMC component. 
       FIGS. 2-3  are just examples of tooling and patterns that could be formed in an abradable coating. Additional examples of suitable machining techniques used to form patterns in abradable coatings include, but are not limited to, patterned rollers, laser sintering, water jet cutting, electrochemical machining (ECM), and combinations thereof. 
     In various embodiments, a tool could be utilized to create any type or combination of patterns in a surface of an abradable coating that could stop, redirect or otherwise control fluid flow adjacent that surface, adjust the porosity of the coating, or both. The pattern in the surface of the abradable coating can be continuous or discontinuous, regular or irregular, and can occupy all or a portion of the surface. 
     For example, as shown schematically in  FIG. 4 , the pattern  238  in all or a portion of a surface  213  of an abradable coating  210  can include staggered, spaced-apart block-like regions  242 , and the blocks  242  can be oriented at an angle θ with respect to an edge  239 . 
     In another example shown in  FIG. 5 , the pattern  338  in the in all or a portion of a surface  313  of a patterned abradable coating  310  can include densely spaced, staggered block-like regions  342 . 
     In yet another example shown in  FIG. 6 , the pattern  438  in the in all or a portion of a surface  413  of an abradable coating  410  can include diamond-shaped blocks  442 . 
     In yet another example shown in  FIG. 7 , the pattern  538  in the in all or a portion of a surface  513  of an abradable coating  510  can include chevron-shaped blocks  542 . 
     Many other complex patterns are possible, including honeycomb-like patterns and the like. Combinations of patterns can also be used. For example, in one embodiment shown in  FIG. 8 , the pattern  638  in the in all or a portion of a surface  613  of an abradable coating  610  can include a first arrangement of blocks  642 A- 642 B along respective edges  639 A- 639 B, and an arrangement of diamond-shaped blocks  644  between the blocks  642 A- 642 B. 
     In each of the patterns shown in  FIGS. 2-8 , the surface features can protrude upward from the surface of the abradable coating to a height of about 0.01 inches (0.25 mm) to about 0.125 inches (3.175 mm), or can be depressed into the surface of the abradable coating at a depth of about 0.01 inches (0.25 mm) to about 0.125 inches (3.175 mm). 
     The patterned abradable coatings of the present disclosure can be used on any type of turbine engine components, both shrouded and un-shrouded. In these applications, the patterned abradable coating may produce any or all of the following effects: reduce swirling of the fluid over the surface, modify the direction of flow to enhance aerodynamic efficiency, control leakage flow, modify coating porosity, and the like. 
       FIG. 9  provides a non-limiting example of how a patterned abradable coating may be used on a surface of a stationary turbine engine part adjacent to a rotatable turbine engine part to control fluid flow over the surface of the stationary part. In the non-limiting example of  FIG. 9 , a shrouded bucket  910  is mounted on a rotor wheel axially between a pair of upstream and downstream nozzle vanes  912 ,  914 . The shrouded bucket  910  includes a tip shroud  916  formed with radially projecting axially spaced teeth  918 ,  920 ,  922 . The teeth  918 ,  920 ,  922  are arranged to interact with patterned abradable coating regions  924 ,  926 ,  928  on a surrounding stator shroud  930 . The regions  924 ,  926 ,  928  form abradable coating seals that may be applied to respective surfaces  932 ,  934 ,  936  of the stator shroud  930  to modify fluid flow over the surfaces  932 ,  934 ,  936 . As another example, abradable ceramic coating regions  950 ,  952  may be applied on the nozzle vanes  912 ,  914 . 
     Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.