Environmental barrier coating

An article includes a ceramic-based substrate and a barrier layer on the ceramic-based substrate. The barrier layer includes a matrix phase and gettering particles in the matrix phase. The gettering particles with an aspect ratio greater than one are aligned such that a maximum dimension of the gettering particles extends along an axis that is generally parallel to the substrate. The barrier layer includes a dispersion of diffusive particles in the matrix phase. A composite material and a method of applying a barrier layer to a substrate are also disclosed.

BACKGROUND

This disclosure relates to composite articles, such as those used in gas turbine engines. Components, such as gas turbine engine components, may be subjected to high temperatures, corrosive and oxidative conditions, and elevated stress levels. In order to improve the thermal and/or oxidative stability, the component may include a protective barrier coating.

SUMMARY

An article according to an exemplary embodiment of this disclosure, among other possible things includes a ceramic-based substrate and a barrier layer on the ceramic-based substrate. The barrier layer includes a matrix phase and gettering particles in the matrix phase. The gettering particles with an aspect ratio greater than one are aligned such that a maximum dimension of the gettering particles extends along an axis that is generally parallel to the substrate. The barrier layer includes a dispersion of diffusive particles in the matrix phase.

In a further example of the foregoing, the diffusive particles include at least one of barium-magnesium alumino-silicate particles, barium strontium aluminum silicate particles, magnesium silicate particles, alkaline earth aluminum silicate particles, yttrium aluminum silicate particles, ytterbium aluminum silicate particles, and rare earth metal aluminum silicate particles.

In a further example of any of the foregoing, the gettering particles include at least one of silicon oxycarbide (SiOC) particles, silicon carbide (SiC) particles, silicon nitride (Si3N4) particles, silicon oxycarbonitride (SiOCN) particles, silicon aluminum oxynitride (SiAlON) particles, and silicon boron oxycarbonitride (SiBOCN) particles.

In a further example of any of the foregoing, the gettering particles are silicon oxycarbide particles, and the diffusive particles are barium-magnesium alumino-silicate particles.

In a further example of any of the foregoing, the barium-magnesium alumino-silicate particles have an average maximum dimension that is smaller than an average maximum dimension of the silicon oxycarbide particles.

In a further example of any of the foregoing, at least 50% of the gettering particles have an aspect ratio greater than one.

In a further example of any of the foregoing, at least 25% of the gettering particles have an aspect ratio greater than three.

In a further example of any of the foregoing, the article comprises a distinct intermediate layer between the barrier layer and the ceramic-based substrate. The distinct intermediate layer includes an intermediate layer matrix of and a dispersion of intermediate layer gettering particles in the intermediate layer matrix.

In a further example of any of the foregoing, the article comprises a ceramic-based top coat on the barrier layer.

A composite material according to an exemplary embodiment of this disclosure, among other possible things includes a matrix of SiO2, a dispersion of gettering particles in the matrix, and a dispersion of diffusive particles in the matrix. At least 50% of the gettering particles have an aspect ratio greater than one and at least 25% of the gettering particles have an aspect ratio greater than three.

In a further example of the foregoing, the gettering particles include at least one of oxycarbide (SiOC) particles, silicon carbide (SiC) particles, and silicon nitride (Si3N4) particles, silicon oxycarbonitride (SiOCN) particles, silicon aluminum oxynitride (SiAlON) particles, and silicon boron oxycarbonitride (SiBOCN) particles, and wherein the diffusive particles include at least one of barium-magnesium alumino-silicate particles, barium strontium aluminum silicate particles, magnesium silicate particles, alkaline earth aluminum silicate particles, yttrium aluminum silicate particles, and ytterbium aluminum silicate particles.

In a further example of any of the foregoing, the gettering particles are silicon oxycarbide particles and the diffusive particles are barium-magnesium alumino-silicate particles.

In a further example of any of the foregoing, the gettering particles are reactive with respect to oxidant particles. The oxidant particles include at least one of water and oxygen.

In a further example of any of the foregoing, the gettering particles are reactive with respect to oxidant particles.

In a further example of any of the foregoing, the gettering particles with an aspect ratio greater than one are aligned such that a maximum dimension of the gettering particles extends along an axis that is generally perpendicular to a thickness dimension of the composite material.

A method of applying a barrier layer to a substrate according to an exemplary embodiment of this disclosure, among other possible things includes mixing diffusive particles, gettering particles, and matrix material in a carrier fluid to form a slurry. At least a portion of the gettering particles have a first dimension and a second dimension defined perpendicular to the first dimension. the portion includes at least 50% of the gettering particles. The method includes applying the slurry to a substrate, aligning the portion of the gettering particles such that the first dimension of the gettering particles extends along an axis that is generally parallel to the substrate, and curing the slurry.

In a further example of the foregoing, the aligning is performed while the slurry is in an uncured or partially cured state.

In a further example of any of the foregoing, the aligning includes heating or cooling the slurry.

In a further example of any of the foregoing, the applying includes forming the slurry into one of more tapes with a binder material and applying the one more tapes to the substrate.

In a further example of any of the foregoing, the curing includes removing the binder material.

DETAILED DESCRIPTION

The engine20in one example is a high-bypass geared aircraft engine. In a further example, the engine20bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture48is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine46has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine20bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor44, and the low pressure turbine46has a pressure ratio that is greater than about five 5:1. Low pressure turbine46pressure ratio is pressure measured prior to inlet of low pressure turbine46as related to the pressure at the outlet of the low pressure turbine46prior to an exhaust nozzle. The geared architecture48may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 and less than about 5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section22of the engine20is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFCT’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)]0.5. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 meters/second).

FIG.2Aschematically illustrates a representative portion of an example article100for the gas turbine engine20that includes a composite material102that is used as a barrier layer. The article100can be, for example, an airfoil in the compressor section24or turbine section28, a combustor liner panel in the combustor section26, a blade outer air seal, or other component that would benefit from the examples herein. In this example, the composite material102is used as an environmental barrier layer to protect an underlying substrate104from environmental conditions, as well as thermal conditions. As will be appreciated, the composite material102can be used as a stand-alone barrier layer, as an outermost/top coat with additional underlying layers, or in combination with other coating under- or over-layers, such as, but not limited to, ceramic-based topcoats.

The gettering particles108may form a network within the matrix106. For instance, silicon oxycarbide particles108have silicon, oxygen, and carbon in a covalently bonded network, as shown in the example network112inFIG.2B. The network112is amorphous and thus does not have long range crystalline structure. The illustrated network112is merely one example in which at least a portion of the silicon atoms are bonded to both O atoms and C atoms. As can be appreciated, the bonding of the network112will vary depending upon the atomic ratios of the Si, C, and O. In one example, the silicon oxycarbide particles108have a composition SiOxMzCy, where M is at least one metal, x<2, y>0, z<1, and x and z are non-zero. The metal can include aluminum, boron, transition metals, refractory metals, rare earth metals, alkaline earth metals or combinations thereof.

In one example, the composite material102includes, by volume, 1-30% of the diffusive particles110. In a more particular example, the composite material102includes, by volume, 1-10% of diffusive particles110. In a further example, the composite material102includes, by volume, 30-94% of the gettering particles108. In a particular example, the composite material includes, by volume, 60-90% of the gettering particles108. In one further example, the composite material102includes, by volume, 5-40% of the matrix106. In a further example, the composite material102includes, by volume, 1-30% of the diffusive particles110, 5-40% of the matrix106, and a balance of the gettering particles108.

The barrier layer protects the underlying substrate104from oxygen and moisture. For example, the substrate104can be a ceramic-based substrate, such as a silicon-containing ceramic material. One example is silicon carbide. Another non-limiting example is silicon carbide fibers in a silicon carbide matrix. The gettering particles108and the diffusive particles110of the barrier layer function as an oxygen and moisture diffusion barrier to limit the exposure of the underlying substrate104to oxygen and/or moisture from the surrounding environment. Without being bound by any particular theory, the diffusive particles110enhance oxidation and moisture protection by diffusing to the outer surface of the barrier layer opposite of the substrate104and forming a sealing layer that seals the underlying substrate104from oxygen/moisture exposure. Additionally, for BMAS particles, the cationic metal species of the BMAS particles110(barium, magnesium, and aluminum) can diffuse into the gettering particles108to enhance oxidation stability of the gettering particle108material. Further, the diffusion behavior of the diffusive particles110may operate to seal any microcracks that could form in the barrier layer. Sealing the micro-cracks could prevent oxygen from infiltrating the barrier layer, which further enhances the oxidation resistance of the barrier layer. The gettering particles108are reactive with respect to oxidant particles, such as oxygen or water, that could diffuse into the barrier layer. In this way, the gettering particles could reduce the likelihood of those oxidant particles reaching and oxidizing the substrate104.

In one example, shown inFIG.3, at least some of the gettering particles108have an elongated shape, e.g., have an aspect ratio greater than one. The gettering particles108have a maximum dimension. Further, the gettering particles108have a dimension D1that is defined along a first axis L1of the gettering particles and a second dimension D2that is defined along a second axis L2generally perpendicular to the first axis. The dimensions D1and D2can be determined by any known imaging technique. L1is defined as the axis closest to parallel to the interface of the composite material102and the substrate104, while L2is defined as the axis closest to perpendicular to the interface of the composite material102and the substrate104. The aspect ratio, α, of the individual gettering particles108is defined as the ratio of a diameters α=D1/D2. In a particular example, at least 50% of the gettering particles108have an aspect ratio greater than one and at least 25% of the gettering particles108have an aspect ratio greater than three.

Furthermore, an average aspect ratio for the gettering particles108in the barrier layer can be determined from a sample of individual gettering particles108. In one example, the average aspect ratio of the gettering particles108is greater than one. In a further example, the average aspect ratio of the gettering particles108is between about 2 and 5.

In one example, the gettering particles108with an aspect ratio greater than one are aligned such that the axis L1is generally parallel to the surface of the substrate104(e.g., generally perpendicular to a thickness dimension of the barrier layer). “Generally parallel” means that an angle between the axis L1and the surface of the substrate104is less than about 20 degrees. “Generally perpendicular” means an angle between the axis L1and the thickness dimension of the barrier layer is between about 70 and 110 degrees.

As shown inFIG.3, oxidant particles that may diffuse into the barrier layer follow a path P. Due to the elongated shape of the gettering particles108, the path is tortuous, and longer than a straight path, reducing the likelihood of the oxidant particles from reaching the substrate104. Furthermore, the orientation of the gettering particles108increases the surface area of gettering particles108that faces the path P, increasing the effective surface area of gettering particles108for encountering and reacting with oxidant particles. This also reduces the likelihood that oxidant particles will reach the substrate104. Also, the shape and orientation of the gettering particles108increases the packing efficiency of the gettering particles, which generally reduces the presence of areas with relatively large distances between adjacent gettering particles108. This in turn reduces the presence of areas with reduced localized oxidation resistance promotes the formation of a more uniform barrier layer. For at least these reasons, the longevity and oxidation resistance of the barrier layer is improved by the shape and/or orientation of the gettering particles108described herein.

In one example, an average maximum dimension of the diffusive particles110is less than the average maximum dimension D1of the gettering particles108. In a particular example, the average maximum dimension D1of the gettering particles108is between about 30 and 70 microns.

FIG.4shows another example article200that includes the composite material102as a barrier layer arranged on the substrate104. In this example, the article200additionally includes a ceramic-based top coat114interfaced with the barrier layer. As an example, the ceramic-based top coat114can include one or more layers of an oxide-based material. The oxide-based material can be, for instance, hafnium-based oxides, yttrium-based oxides (such as hafnia, hafnium silicate, yttrium silicate, yttria stabilized zirconia or gadolinia stabilized zirconia), or combinations thereof, but is not limited to such oxides.

FIG.5illustrates another example article300that is somewhat similar to the article200shown inFIG.4but includes a distinct intermediate layer316interposed between the barrier layer of the composite material102and the substrate104. In this example, the distinct intermediate layer316includes an intermediate layer matrix318of silicon dioxide and a dispersion of intermediate layer gettering particles320in the intermediate layer matrix318. The intermediate layer gettering particles320are similar to the gettering particles108in composition but, in this example, the intermediate layer silicon oxycarbide particles320have an average maximum dimension (D3) that is less than the average maximum dimension (D1) of the gettering particles108. The gettering particles320maybe spherical or elongated in shape. The relatively small intermediate layer gettering particles320provide a relatively low roughness for enhanced bonding with the underlying substrate104. The larger gettering particles108of the barrier layer provide enhanced blocking of oxygen/moisture diffusion. Thus, in combination, the barrier layer and intermediate layer316provide good adhesion and good oxidation/moisture resistance.

In one example, the intermediate layer316can include, by volume, 5-40% of the intermediate layer matrix318of silicon dioxide and a balance of the intermediate layer gettering particles320. In further examples, a portion of the diffusive particles110from the barrier layer can penetrate or diffuse into the intermediate layer316, during processing, during operation at high temperatures, or both. In a further example, a seal coat layer of SiO2, with or without BMAS particles, can be provided between the barrier layer and the intermediate layer316to provided adhesion and additional sealing. In further examples of any of the compositions disclosed herein, said compositions can include only the listed constituents. Additionally, in any of the examples disclosed herein, the matrix106and318can be continuous. The two-layer structure can also demonstrate good oxidation protection at 2000-2700° F. for 500 hours or longer as well as good adhesion with the ceramic-based top coat114.

The composite material102and/or intermediate layer316can be fabricated using a slurry coating method. The appropriate slurries can be prepared by mixing components, such as gettering particles, diffusive particles, and matrix material, such as powder of silicon dioxide or colloidal silica (Ludox) in a carrier fluid, such as water. The slurries can be mixed by agitation or ball milling and the resulting slurry can be deposited onto the underlying substrate104. The slurry can then be partially cured by drying at room temperature or at an elevated temperature to remove the carrier fluid. In one example, the slurry is dried and cured at about 100-300° C. for about 5-60 minutes. During the heating, cross-linking of the colloidal silica in the matrix material occurs. The green coating can then be fully cured by sintered at an elevated temperature in air for a selected amount of time. In one example, the sintering includes heating at 1500° C. or greater in an air environment for at least 1 hour.

The gettering particles108/320are oriented as discussed above during fabrication of the barrier layer. In one example, the orientation of gettering particles108/320can be achieved by applying a force to the slurry after the slurry is deposited on to the substrate104but while the slurry is in an uncured or partially cured state (e.g., the green state). The force can be applied in a direction perpendicular to the surface of the substrate104, by a press, for example. The application of the force can be paired with either heating or cooling the slurry. In another example, the orientation of gettering particles108/320can be achieved by rapidly freezing the slurry after the slurry is deposited on to the substrate104but while the slurry is in an uncured or partially cured state (e.g., the green state).

In another example, a tape casting process is used to fabricate the barrier layer. The slurry includes a binding agent and is formed into flexible tapes by partially curing the slurry into thin, flat sheets. The partial curing can include drying to the green state, as discussed above. During the formation of the flexible tapes, the gettering particles108/320are oriented by any of the methods discussed above. The binding agent facilitates the formation of tapes and provides adhesive properties to the tapes so that the tapes can be arranged on the substrate104. The slurry is then sintered as discussed above. The binding agent is removed from the slurry during the sintering.

Although the different examples are illustrated as having specific components, the examples of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from any of the embodiments in combination with features or components from any of the other embodiments.