The disclosure relates to coatings for high-temperature mechanical systems, such as gas turbine engines, and more particularly to coatings including one or more mullite layers.
The components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, hot section components of gas turbine engines, e.g., turbine blades and/or vanes, exposed to hot gases in commercial aeronautical engines may experience surface temperatures of greater than 1,000° C. Furthermore, economic and environmental concerns, e.g., the desire for improved efficiency and reduced emissions, continue to drive the development of advanced gas turbine engines with higher gas inlet temperatures. As the turbine inlet temperature continues to increase, there is a demand for components capable of operating at such high temperatures.
Components of high-temperature mechanical systems may include ceramic and/or superalloy substrates. Coatings for such substrates continue to be developed to increase the operating capabilities of such components and may include thermal barrier coatings (TBC) and environmental barrier coatings (EBC). In some examples, thermal barrier coatings (TBC) may be applied to substrates to increase the temperature capability of a component, e.g., by insulating a substrate from a hot external environment. Further, environmental barrier coatings (EBC) may be applied to ceramic substrates, e.g., silicon-based ceramics, to provide environmental protection to the substrate. For example, an EBC may be applied to a silicon-based ceramic or ceramic composite substrate to protect against the recession of the ceramic substrate resulting from operation in the presence of water vapor in a high temperature combustion environment. In some cases, an EBC may also function as a TBC based on low thermal conductivity values of the EBC, although a separate compatible TBC may also be added to a substrate in addition to an EBC to further increase the temperature capability of a component.
In general, the disclosure relates to techniques for depositing one or more materials on a substrate to form layers that make up a coating. Such techniques may be applicable to the depositions of a coating on ceramic or ceramic composite substrates, which in most cases may function as an environmental barrier coating (EBC) e.g., when applied to ceramic components of high temperature mechanical systems. In some cases, an EBC may include a silicon bond layer, an intermediate layer containing mullite (3Al2O3.2SiO2) and an outer layer. The mullite-containing layer may be formed by depositing mullite powder on the ceramic substrate. In some embodiments, mullite powder may be deposited on a ceramic substrate via thermal spraying, e.g., air plasma spraying, to form a layer including the deposited mullite. The outer layer may be formed by depositing a second material on the mullite-containing layer. In some embodiments, the second material may include barium strontium aluminum silicate (BSAS) or one or more rare-earth silicates, such as Yb-monosilicate or Yb-disilicate.
According to some embodiments of the disclosure, mullite powder may be deposited on a ceramic substrate via thermal spraying to form a mullite-based layer of an EBC in a low temperature environment. For example, a ceramic substrate may be at a substantially uniform temperature of less than 50 degrees Celsius at least at the beginning of the mullite deposition process. Such a temperature limit may correspond to conditions achievable without providing supplemental heat to a ceramic substrate, as may be the case in a thermal spraying while the substrate is in a furnace and/or the use of back side heating during the mullite deposition process. Despite the low temperature environment in which the thermal spraying of the mullite powder takes place, the mullite layer formed from the deposition may perform substantially the same or even better than mullite layers formed by depositing mullite via thermal spraying in a higher temperature environment, e.g., those associated with the use of furnace deposition and/or back-side heating of the substrate.
As will be described in further detail below, mullite powder that has been manufactured by a fused or sinter plus crush process, e.g., rather than that manufactured by a spray granulation process, may allow for the thermal spraying of mullite powder in a low temperature environment to form a mullite layer that perform substantially the same or even better than mullite layers formed via thermal spraying of mullite powder in a higher temperature environment. For example, mullite powder manufactured by a fused plus crush or sinter plus crush process may be thermally sprayed on ceramic substrates starting at a time when the substrate temperatures is less than 50 degrees Celsius to form a mullite-containing layer that performs adequately at high temperatures without exhibiting substantial cracking or delamination during thermal cycling, even without heat treating the mullite layer after its formation.
Techniques for forming an EBC that includes a silicon bond layer applied between a ceramic substrate and a mullite layer are also described. Such a silicon bond layer may be formed by depositing appropriate silicon material on a ceramic substrate prior to the deposition of the mullite material via thermal spraying. In some embodiments, the silicon bond layer may undergo a high temperature heat treatment for a relatively short amount of time, e.g., approximately 1 hour at about 1200 degrees Celsius, prior to the deposition of the mullite material on the silicon bond layer via thermal spraying. Such a technique may promote adhesion between the silicon bond layer and the ceramic substrate to enhance the adhesion of the EBC system to the ceramic substrate.
In one embodiment, the disclosure is directed to a method of coating a substrate comprising depositing mullite on the substrate during a first time period via thermal spraying to form a first layer, the mullite comprising mullite powder formed via at least one of a fused plus crush or sinter plus crush process; and depositing a second material on the first layer to form a second layer, wherein the substrate is at a temperature less than approximately 50 degrees Celsius at approximately a beginning of the first time period.
In another embodiment, the disclosure is directed to a method of coating a substrate, the method comprising depositing silicon on the substrate to form a silicon layer; heat treating the silicon layer; depositing mullite on the silicon layer via thermal spraying to form a first layer subsequent to the heat treatment of the silicon layer, the mullite comprising mullite powder formed via at least one of a fused plus crush or sinter plus crush process; depositing a second material on the first layer to form an intermediate layer; and depositing a third material on the intermediate layer to form a third layer.
In another embodiment, the disclosure is directed to a method of coating a substrate, the method comprising depositing mullite on the substrate via thermal spraying to form a first layer; and depositing a second material on the first layer via thermal spraying to form a second layer, the second material comprising at least one of barium strontium aluminum silicate (BSAS) or a rare-earth silicate, wherein the substrate is at a temperature of less than 150 degrees Celsius when the second material is first deposited.
In another embodiment, the disclosure is directed to a method of coating a substrate, the method comprising depositing silicon on the substrate to form a silicon layer; heat treating the silicon layer; and depositing mullite on the substrate during a first time period via thermal spraying to form a first layer, the mullite comprising mullite powder formed via at least one of a fused plus crush or sinter plus crush process.
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.
In general, embodiments of the disclosure relate to techniques for applying coatings to a variety of substrates, such as, e.g., ceramic substrates. In some embodiments, the applied coatings may function as an EBC on ceramic substrates used for components of high temperature mechanical system to increase the operating capabilities of components of high temperature mechanical systems and extend the components durability. For example, an EBC may be applied to a silicon-based ceramic component to protect against recession of the component caused by the volatilization of silica scale by water vapor in the high temperature combustion environment.
As will be described below, an EBC may be a multilayer coating including an outer layer containing one or more rare-earth silicates and/or barium strontium aluminum silicate (BSAS) that may be bonded to the ceramic substrate via a mullite-containing layer, which may be referred as the mullite layer. In some cases, the EBC may include a silicon-based bond layer provided between the substrate and mullite layer to adhere the mullite layer to the substrate and to extend the life of the coating.
The mullite layer of an EBC may be formed by depositing mullite in the form of mullite powder on the surface of a substrate via a thermal spraying process, such as, e.g., plasma spraying. In some examples, the mullite powder used may be manufactured using a plasma spray granulation process. However, due to the relatively high phase instability of mullite manufactured by a plasma spray granulation process, the mullite layer formed via the thermal spraying process may include non-homogenized mullite in both amorphous and crystalline phases. If the mullite layer contains too high a concentration of amorphous phase mullite, then the environmental protection provided by the EBC to the substrate may be reduced over the life of substrate. For example, the amorphous non-homogenous mullite in a mullite layer may be prone to crystalline phase transformations during thermal cycling that induce volumetric changes within the mullite layer. In some cases, the volumetric changes may cause cracking and/or delamination of the mullite layer, which may reduce the extent of environmental protection provided by the EBC to the ceramic component.
To reduce the amount of amorphous mullite in the mullite layer, the thermal spraying process may include heating the ceramic substrate to an elevated temperature within a closed environment (e.g., by heating the substrate within a furnace room maintained at approximately 1200 degrees Celsius or providing supplemental heat towards one or more surfaces of the substrate) and maintaining the substrate at the elevated temperature during the thermal spraying of the mullite powder, which has been manufactured by plasma spray granulation technique, to promote the deposition of crystalline mullite rather than amorphous mullite. Similarly, the substrate may also be maintained at an elevated temperature during the deposition of the material used to form the outer layer, e.g., one or more rare-earth silicates and/or BSAS, to provide for suitable performance of the outer layer and/or bonding to the mullite layer.
Additionally or alternatively, a substrate may be heat treated, e.g., at a temperature greater than 1200 degrees Celsius for greater than 10 hours, after one or more of these EBC layers have been formed on the substrate to homogenize/stabilize the crystalline phase mullite or transform amorphous mullite to crystalline mullite in the EBC prior to exposing the substrate to thermal cycling to increase the reliability of the EBC.
However, while the high temperature application and/or heat treatment of one or more layers of the EBC can in some cases address the phase instability issues, such a high temperature process requirements can dramatically increase the cost and relative complexity of applying an EBC to a ceramic component. Furthermore, such process requirements can decrease the flexibility of manufacturing and design of such components. For example, in such cases, the overall size and/or shape of a component that the EBC is being applied to may be limited by the relative dimensions of the heat controlled environment, such as the size of the high temperature furnace, required to maintain the component at an appropriate elevated temperature during the application of one or more EBC layers and/or heat treat a coated substrate.
As will be described in greater detail below, embodiments of the disclosure may include techniques for forming one or more layers of an EBC, including a mullite layer, on a substrate in a low temperature environment by depositing the respective material via thermal spraying without requiring extended high temperature heat treatment of the mullite-coated substrate. Despite the low temperature deposition of the materials that form the respective layers of the EBC, the EBC may perform the same or similar to that of an EBC with respective layer(s) that have been deposited via thermal spraying on the substrate at elevated temperatures. In particular, the mullite layer may be formed on a substrate by depositing mullite powder manufactured via a fused or sinter plus crush process, e.g., rather than a plasma spray granulation process, via thermal spraying, such as, e.g., air plasma spraying, even without substantially heating the substrate in conjunction with the deposition and/or heat treating the mullite-coated substrate. Despite the low temperature application, such a mullite layer may not exhibit substantial cracking or delamination from the substrate even after undergoing thermal cycling.
Substrate 12 may be a component of a high temperature mechanical system, such as, e.g., a hot section component of a gas turbine engine. Examples of such components may include, but are not limited to, turbine blades, blade tracks, combustion liners, and the like. Substrate 12 may include silicon-containing ceramics, such as, e.g., silicon carbide (SiC), silicon nitride (Si3N4), composites having a SiC or Si3N4 matrix, silicon oxynitride, and aluminum oxynitrides; an silicon containing metal alloy, such as molybdenum-silicon alloys (e.g., MoSi2) and niobium-silicon alloys (e.g., NbSi2); and an oxide-oxide ceramic.
Substrate 12 may include a matrix, such as, e.g., a ceramic matrix composite (CMC), which may include any useful ceramic matrix material, including, for example, silicon carbide, silicon nitride, alumina, silica, and the like. The matrix may further include any desired filler material, and the filler material may include a continuous reinforcement or a discontinuous reinforcement. For example, the matrix may be reinforced with ceramic fibers, whiskers, platelets and chopped or continuous fibers.
The filler composition, shape, size, and the like may be selected to provide the desired properties to the matrix. For example, in some embodiments, the filler material may be chosen to increase the toughness of a brittle ceramic matrix. In other embodiments, the filler may be chosen to provide a desired property to the matrix, such as thermal conductivity, electrical conductivity, thermal expansion, hardness, or the like.
In some embodiments, the filler composition may be the same as the matrix material. For example, a silicon carbide matrix may surround silicon carbide whiskers. In other embodiments, the filler material may include a different composition than the matrix, such as mullite fibers in an alumina matrix, or the like. In one embodiment, a CMC may include silicon carbide continuous fibers embedded in a silicon carbide matrix.
EBC 14 may include bond layer 16, which in the example of
Bond layer 16 may formed at any suitable thickness, such as a thickness that allows EBC 14 to provide environmental protection to article 10 as described herein. For example, in one embodiment, bond layer 16 may have thickness in the range of approximately 0.2 mils to approximately 5 mils. In some embodiments, bond layer 16 may have thickness in the range of approximately 0.5 mils to approximately 5 mils, such as approximately 2 mils to approximately 4 mils or approximately 1 mil to approximately 4 mils.
EBC 14 may also include intermediate layer 18, which may be provided on top of bond layer 16 and/or substrate 12. Intermediate layer 18 may include mullite manufactured by a fused or sinter plus crushed technique, at least some of which is in the crystalline phase, and may be formed by depositing mullite powder on substrate 12 via one or more suitable thermal spraying processes, including plasma spraying. In some embodiments, intermediate layer 18 may additionally include greater than approximately 50 percent by weight crystalline mullite, such as, e.g., approximately 60 percent by weight crystalline mullite or 80 percent by weight crystalline mullite. In some examples, intermediate layer 18 may include approximately 100 percent by weight crystalline mullite.
In some embodiments, the balance of intermediate layer 18 other than that of the mullite may include an amount of BSAS, e.g., in cases in which outer layer 20 includes BSAS. For example, intermediate layer 18 may include up to approximately 80 percent by weight BSAS, such as, e.g., approximately 40 percent by weight BSAS or approximately 20 percent by weight BSAS. In other embodiments, the balance of intermediate layer 18 may include an amount of rare-earth silicate, e.g., in cases in which outer layer 20 includes rare-earth silicate. For example, intermediate layer 18 may include up to approximately 80 percent by weight rare-earth silicate, such as, e.g., approximately 40 percent by weight rare-earth silicate or approximately 20 percent by weight rare-earth silicate.
The mullite material may be deposited on bond layer 16 and substrate 12 via a thermal spraying process to form intermediate layer 18 without maintaining or providing substrate 12 at an elevated temperature relative the thermal spraying process and/or heat treating intermediate layer 18 after the mullite has been thermally sprayed. Despite the relatively low temperature application, intermediate layer 18 may still perform the same or similar to that of a mullite layer deposited at high temperature and/or that has undergone heat treatment. In particular, the mullite layer formed via the deposition of mullite powder manufactured via a fused or sinter plus crush process may not exhibit undesirable volumetric changes during thermal cycling that may cause EBC 14 to fail, e.g., due to cracking or delamination, even though the substrate was not maintained at an elevated temperature during the thermally spraying process or heat treated after intermediate layer 18 was formed. In some embodiments, intermediate layer 18 may include greater than approximately 50 percent by weight crystalline mullite, such as, e.g., greater than approximately 80 percent by weight crystalline mullite, even without thermal spraying the mullite powder on substrate 12 via thermal spraying or heat treating substrate 12 after the formation of intermediate layer 18.
As previously mentioned, such a process may be enabled by using a mullite powder for thermal spraying that has been generated via a fused plus crush process and/or sinter plus crush process rather than via a plasma spray granulation process, for example. In some cases, mullite powder generated via a fused or sinter plus crushed process may allow for deposition of the mullite powder via thermal spraying at a relatively lower temperature than mullite powder generated via spray granulation. Although not wishing to be limited by theory, it is believed that mullite powder generated via a fused or sinter plus crush process may contain relatively high concentration of stoichiometric amorphous phase mullite in the powder material. Conversely, mullite powder generated via spray granulation may include a relatively high concentration of non-stoichiometric or non-homogenous phase mullite powder. The volumetric changes associated with conversion of stoichiometric amorphous phase mullite to crystalline mullite in a high temperature environment is less than the volumetric changes associated with the phase conversion of non-stoichiometric amorphous phase mullite due to increased homogenity. In some cases, the volumetric changes associated with the conversion of stoichiometric amorphous phase mullite may not be great enough to cause delamination of an EBC due to the thermal expansion of the mullite layer in high temperatures, while the volumetric changes associated with the conversion of non-stoichiometric amorphous phase mullite may be great enough to cause delamination of an EBC due to the thermal expansion of the mullite layer in high temperatures.
Intermediate layer 18 may be formed at any suitable thickness, such as a thickness that allows EBC 14 to provide environmental protection to article 10 as described herein. For example, one embodiment, intermediate layer 18 may have thickness in the range of approximately 1 mil to approximately 7 mils. In still another embodiment, intermediate layer 18 may have thickness in the range of approximately 4 mils to approximately 6 mils. In some embodiments in which EBC 14 includes bond layer 16 and intermediate layer 18, the ratio of layer thickness between bond layer 16 and intermediate layer 18 may range from approximately 0.1 to approximately 1.5 such as, e.g., from approximately 0.2 to approximately 1.0.
EBC 14 may also include outer layer 20. In the example of
The material selected for outer layer 20 may be deposited on intermediate layer 18 via a thermal spraying process to form outer layer 18. As will be described in further below with respect to
Outer layer 20 may be formed at any suitable thickness, such as a thickness that allows EBC 14 to provide environmental protection and thermal barrier to article 10 as described herein. For example, in one embodiment, outer layer 20 may have thickness in the range of approximately 2 mils to approximately 15 mils. In still another embodiment, outer layer 20 may have thickness in the range of approximately 4 mils to approximately 12 mils.
After bond layer 16 has been formed on substrate 12, mullite powder that has been manufactured via a fused plus crushed and/or sinter plush crush process may be deposited on bond layer 16 via thermal spraying to form intermediate layer 18(26). The deposition of the mullite powder on the substrate may begin even when the substrate is at a substantially uniform temperature of less than 50 degrees Celsius (26).
After intermediate layer 18 has been formed on substrate 12, the outer layer material, e.g., one or more rare earth silicates or BSAS, may be deposited on intermediate layer 18 via thermal spraying to form outer layer 18. Such a deposition process may also begin even when the substrate is at temperature of less than 50 degrees Celsius (28). In this manner, the respective layers 16, 18, 20 of EBC 14 may be applied to substrate 12 in a relatively low temperature air environment while still forming a suitable coating that provides environmental protection to substrate in a high temperature combustion environment.
As illustrated by the example of
Depending in part on the environment that the layer deposition process is undertaken, e.g., the natural temperature of the room at which the deposition process is undertaken, substrate 12 may be at a temperature less than approximately 50 degrees Celsius at the beginning of the deposition of one or more of layer 16, 18, and 20, which includes the beginning of the deposition of the mullite powder on the substrate (26), as previously described. For example, substrate 12 may be at a temperature of less than approximately 40 degrees Celsius at the beginning of the mullite powder deposition, such as, e.g., between approximately 15 degrees Celsius and approximately 35 degrees Celsius. In some embodiments, substrate 12 is not be substantially heated above the temperature of the surrounding space in which the thermal spraying of the layer material is being performed, wherein the ambient temperature of the surrounding space, e.g., the room in which the thermal spraying process is performed, is less than approximately 50 degrees Celsius. In some embodiments, the ambient temperature of the surrounding space may be between approximately 10 degrees Celsius and 40 degrees Celsius, such as, e.g., between approximately 15 degrees Celsius and 30 degrees Celsius.
Despite the relatively low temperatures of the substrate during thermal spraying, intermediate layer 18 may contain a concentration of stoichiometric and/or homogenous amorphous and crystalline mullite phases such that cracking and/or delamination of intermediate layer 18 is prevented during thermal cycling. In this manner, one or more of layers 16, 18, 20 may be formed via thermal spraying of mullite powder without having to provide additional heat to substrate 12 to a temperature substantially greater than that of the space
While substrate 12 may be at a temperature of less than 50 degrees Celsius at the beginning of the deposition of the materials of layers 16, 18, and 20, it is recognized that the local temperature of certain portions of substrate 12 may be increased above approximately 50 degrees Celsius, including local temperature greater than or equal to that of 50 degrees, at periods throughout the overall deposition time of the respective layer material. For example, during thermal spraying of mullite powder on substrate 12 to form intermediate layer 18, the deposition surface of substrate 22 may reach temperatures greater than or equal to 50 degrees Celsius because of elevated temperature of the material being deposited on the surface of substrate 22. However, while the substrate temperature may increase above the temperature at the beginning of the mullite deposition, it is as a result from the heat provided by the thermal spraying process rather than by a supplemental source, as would be the case in a furnace room or with additional heat directed to substrate 12. Even with the additional heat from the thermal spraying process, in some embodiments, substrate 12 may be maintained at a temperature of less than 200 degrees Celsius, such as, for example, between approximately 15 degrees Celsius and approximately 200 degrees Celsius or between approximately 20 degrees Celsius and approximately 150 degrees Celsius, throughout the deposition of the mullite material via thermal spraying.
The beginning temperature of substrate 12 relative the deposition of one or more of the layer materials may be achieved via any suitable method. Substrate 12 may be at a temperature of less than approximately 50 degrees Celsius by simply allowing the temperature of substrate 12 to be substantially equal to that of the room temperature of the surrounding space, assuming that that surrounding space is less than approximately 50 degrees Celsius. This may allow EBC 12, and intermediate layer 18, in particular, to be formed on substrate 12 via a thermal spraying process without having to undertake any additional steps to heat substrate 12 during the deposition and/or heat treat substrate 12 after being coated with EBC 14
Similar to that of the example of
Unlike the example of
After undergoing diffusion heat treatment, mullite powder may be deposited on bond layer 16 via thermal spraying to form intermediate layer 18 (36) and outer layer material may be deposited on intermediate layer 18 via thermal spraying to form outer layer 20 (38), as described previously with respect to
The inclusion of the diffusion heat treatment step may serve to increase the adhesion of EBC 14 to substrate 12, especially in cases in which the coefficient of thermal expansion of the layers of EBC 14 are not the substantially the same. In some embodiment, substrate 12 and bond layer 16 may undergo diffusion heat treatment as described above when the coefficient of thermal expansions of layers 16, 18, 20 differ by more than 5 percent, such as e.g., more than 10 percent, to increase the adhesion of EBC 14 to substrate 12, even when intermediate layer 18 and outer layer 20 are formed by deposition of the respective materials via thermal spraying without maintaining substrate 12 at an elevated temperature, e.g., at a temperature greater than 50 degrees Celsius.
Unlike the embodiment shown in
The structure and composition of second intermediate layer 48 may vary, and may be selected based on one or more factors. With reference to
In some embodiments, second intermediate layer 48 may include at least one of alumina and/or mullite. The composition of second intermediate layer 48 may be selected based on the composition of first intermediate layer 46 and/or outer layer 50. For example, the composition of first intermediate layer 46 and outer layer 50 may generally dictate the coefficient of thermal expansion for the respective layers. Second intermediate layer 48 may include one or more components which allow for second intermediate layer 48 to have a coefficient of thermal expansion that is in between that of the coefficients of thermal expansion of first intermediate layer 46 and outer layer 50. In some embodiments, second intermediate layer 48 may have a coefficient of thermal expansion that is within approximately 10 percent or less than the coefficient of thermal expansion of first intermediate layer 46 and/or outer layer 50.
In this manner, the difference in thermal expansion between first intermediate layer 46 and outer layer 50 may be tempered by the thermal expansion of second intermediate layer 48 when configured as shown in
In some embodiments, second intermediate layer 48 may include one or more components of both first intermediate layer 46 and outer layer 50. For example, when first intermediate layer 46 includes mullite and outer layer 50 includes component ytterbium di-silicate, second intermediate layer 48 may include a mixture of mullite and component ytterbium di-silicate. The mixture may include an approximately equal amount of the components of first intermediate layer 46 and outer layer 50, or may include any other desired mixture or proportion of components from first intermediate layer 46 and outer layer 50.
Second intermediate layer 48 may be applied as a separate layer from first intermediate layer 46 and outer layer 50. For example, mullite powder may be deposited first via thermal spraying to form first intermediate layer 46, as described herein. The desired mixture of the first intermediate layer components, e.g., mullite, and the outer layer components may then be mixed and deposited on first intermediate layer 46 via thermal spraying to form second intermediate layer 48, followed by application of outer layer 50 on second intermediate layer 48, as previously described. Similar to that of outer layer 50 and first intermediate layer 46, second intermediate layer 48 may be initially deposited on first intermediate layer 46 when the temperature of substrate 40 is less than approximately 50 degrees Celsius.
Additionally, second intermediate layer 48 may include more than one sublayers (not shown). In some embodiments, a second intermediate layer having one or more sublayers may allow for the interface between first intermediate layer 46 and outer layer 50 to be compositionally graded. Such compositional grading may reduce the strain on the interface of outer layer 50 and first intermediate layer 46 during thermal cycling for cases in which first intermediate layer 46 and outer layer 50 exhibit different coefficients of thermal expansion. For example, the inclusion of second intermediate layer 48 having multiple sublayers that are compositionally graded may reduce the coefficient of thermal expansion gradient, or in other words, make the compositional transition from the first intermediate layer 46 to outer layer 50 more gradual, thus making the change of coefficients of thermal expansion more gradual. It may be understood that the more sublayers included in the second intermediate layer, the lower the interfacial stresses due to mismatches of coefficients of thermal expansion. The number of sub-layers in the transitional layer need not be limited, but may be chosen according to the desired properties of the article and the time and expense involved in producing the article.
Furthermore, a coating may also include a second intermediate layer that is not divided into sub-layers, but which includes a continuously graded composition. For example, the second intermediate layer may be compositionally most similar to the first intermediate layer at the first intermediate layer-transitional layer interface, and most similar to the outer layer at the outer layer-transitional layer interface, with a composition that continuously transitions from the first intermediate layer composition to the outer layer composition along the depth of the transitional layer.
In some embodiments, intermediate layer 48 may be a rare-earth silicate layer that transitions into another rare-earth silicate layer. For example, intermediate layer 48 may include Yb-disilicate while outer layer 50 may include Yb-monosilicate. The respective layers may be deposited as discrete layers or as functionally graded materials.
Some embodiments of the disclosure may relate to a method of coating a substrate consisting essentially of depositing mullite on the substrate via thermal spraying to form a first layer; and depositing a second material on the first layer to form a second layer, wherein the mullite comprises mullite powder formed via at least one of a fused plus crush or sinter plus crush process.
Some embodiments of the disclosure may relate to a method of coating a substrate consisting essentially of depositing silicon on the substrate to form a silicon layer, depositing mullite on the silicon layer via thermal spraying to form a first layer; and depositing a second material on the first layer to form a second layer, wherein the mullite comprises mullite powder formed via at least one of a fused plus crush or sinter plus crush process. Such a method may not include depositing the mullite layer at a relatively high temperature and/or heat treating the mullite layer, as previously described. In some examples, the method may further consist essentially of heat treating the silicon layer and substrate prior to depositing the mullite on the silicon layer.
Referring to
Referring to
Articles 52a and 52b were exposed to substantially the same steam thermal cycling in an environment of greater than 1800 degrees Fahrenheit and greater than 60% humidity. The steam thermal cycling involved exposing articles 52a and 52b to the described environment for greater than 100 cycles for a total of amount of time greater than 100 hours. Additionally, articles 52a and 52b were exposed to laser heat flux through EBCs 54a and 54b, respectively, above 1500 degrees Fahrenheit in conjunction with the steam thermal cycling, for greater than 5 hours. Such testing of articles 52a and 52b simulated engine operating conditions in a high temperature combustion environment in the presence of water vapor.
The cross-sectional photographs of
Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.
This application claims priority from U.S. Provisional Application Ser. No. 61/231,510 entitled “TECHNIQUES FOR DEPOSITING COATING ON CERAMIC SUBSTRATE,” filed Aug. 5, 2009, the entire content of which is incorporated herein by reference.
Number | Date | Country | |
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61231510 | Aug 2009 | US |