Patent Application
20190161416
The present invention generally relates to the use of environmental barrier coatings on ceramic components, along with methods of their formation and use.
Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. Still, with many hot gas path components constructed from super alloys, thermal barrier coatings (TBCs) can be utilized to insulate the components and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface, thus limiting the thermal exposure of the structural component.
While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed, such as ceramic matrix composite (CMC) materials. CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide protection against the corrosive gases in the hot combustion environment.
Silicon carbide and silicon nitride ceramics undergo oxidation in dry, high temperature environments. This oxidation produces a passive, silicon oxide scale on the surface of the material. In moist, high temperature environments containing water vapor, such as a turbine engine, both oxidation and recession occurs due to the formation of a passive silicon oxide scale and subsequent conversion of the silicon oxide to gaseous silicon hydroxide. To prevent recession in moist, high temperature environments, environmental barrier coatings (EBC's) are deposited onto silicon carbide and silicon nitride materials, including silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composites.
Currently, EBC materials are made out of rare earth silicate compounds. These materials seal out water vapor, preventing it from reaching the silicon oxide scale on the silicon carbide or silicon nitride surface, thereby preventing recession. Such materials cannot prevent oxygen penetration, however, which results in oxidation of the underlying substrate. Oxidation of the substrate yields a passive silicon oxide scale, along with the release of carbonaceous or nitrous oxide gas. The carbonaceous (i.e., CO, CO2) or nitrous (i.e., NO, NO2, etc.) oxide gases cannot escape out through the dense EBC and thus, blisters form. The use of a silicon bond coating has been the solution to this blistering problem to date. The silicon bond coating provides a layer that oxidizes (forming a passive silicon oxide layer beneath the EBC) without liberating a gaseous by-product.
However, the presence of a silicon bond coating limits the upper temperature of operation for the EBC because the melting point of silicon metal is relatively low. In use, the silicon bond coating melts at coating temperatures of about 1414° C., which is the melting point of silicon metal. Above these melting temperatures, the silicon bond coating may delaminate from the underlying substrate, effectively removing the bond coat and the EBC thereon. As such, it is desirable to obviate the need for a silicon bond coating in the EBC to achieve a higher operational temperature limit for the EBC.
Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
Coated components are generally provided that may include, in one embodiment, a ceramic substrate having a Si-treated layer surrounding a ceramic core and an environmental barrier coating on the Si-treated layer of the ceramic substrate. The ceramic core may include silicon carbide, and the Si-treated layer may be pretreated to tailor its surface's properties for inhibiting or delaying the formation of carbon oxides to upon exposure of the Si-treated layer to oxygen.
In one embodiment, the Si-treated layer is pretreated such that the environmental barrier coating is coated directly thereon without a silicon bond coating therebetween. For example, the environmental barrier coating may be directly on the Si-treated layer of the ceramic substrate
In particular embodiments, the ceramic core includes silicon carbide having a first ratio of silicon to carbon, with the Si-treated layer including silicon carbide having a second ratio of silicon to carbon, where the second ratio has a greater amount of silicon relative to the first ratio.
In one particular embodiment, the Si-treated layer is integral to the ceramic core and has a depth of about 750 μm or less (e.g., a depth of 25 μm to about 260 μm) from an outer surface of the Si-treated layer extending into the ceramic substrate toward the ceramic core.
In one embodiment, the Si-treated layer may be an applied layer on the ceramic core, such as having a thickness of about 1 μm to about 250 μm. In such an embodiment, a barrier layer may be between the ceramic core and the Si-treated layer, such as a barrier layer that includes a rare earth disilicate.
In one embodiment, a coated component may include a ceramic substrate having a Si-treated layer surrounding a ceramic core, where the ceramic core comprises silicon carbide having a first ratio of silicon to carbon, and where the Si-treated layer comprises silicon carbide having a second ratio of silicon to carbon, with the second ratio having a greater amount of silicon relative to the first ratio. The coated component may also include an environmental barrier coating on the Si-treated layer of the ceramic substrate.
Methods are also generally provided for coating a ceramic component on its outer surface, where the ceramic component comprises silicon carbide. In one embodiment, the method includes exposing the outer surface of the ceramic component to silicon vapor at a temperature of about 1400° C. to about 1800° C. to form a Si-treated layer.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figs., in which:
Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention.
Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.
As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components.
In the present disclosure, when a layer is being described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers, unless expressly stated to the contrary. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer.
Chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth. As used herein, “rare earth elements” encompass the elements scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), or mixtures thereof.
A coated component is generally provided that includes a ceramic substrate pretreated to tailor its surface's properties such that an environmental barrier coating (EBC) may be coated directly thereon without a silicon bond coating therebetween. For example, the material underlying the EBC may be pretreated to keep the carbon activity below that required to form carbon oxides (e.g., CO and/or CO2) gas bubbles upon exposure to oxygen at operational temperatures. Pretreatment is designed to delay the formation of carbon oxides gases during use, thus extending life.
In one embodiment, the ceramic substrate may have a silicon-treated (Si-treated) layer surrounding a ceramic core, with the Si-treated layer being pretreated to tailor its surface's properties for inhibiting the formation of carbon oxides to upon exposure of the Si-treated layer to oxygen. For example, both the ceramic core and the Si-treated layer may include silicon carbide, and may have the same or similar stoichiometric ratios of Si to C prior to treatment. Then, the ceramic substrate may be treated to increase the relative amount of Si in the Si-treated layer such that the Si-treated layer may have an increased stoichiometric ratio of Si to C after treatment. As such, the Si-treated layer may have a greater amount of Si when compared to the stoichiometric ratio of Si to C in the ceramic core. Due to the increased amount of Si in the outer layer (compared to the amount of Si in the ceramic core), the amount of carbon within the outer layer may be decreased to a level that inhibits the formation of carbon oxides upon exposure to oxygen at elevated temperatures for longer periods of time, thus extending life.
As stated, the ceramic core 14 includes a silicon carbide having a first ratio of silicon to carbon. For example, the first ratio of silicon to carbon may be in the range of about 0.97 to about 1.03, which may vary slightly throughout the ceramic core 14.
Referring to
Depending on the exposure conditions (e.g., time, temperature, pressure, and duration), the temperature and/or other conditions of the silicon vapor may be varied to control the Si to C ratio in the resulting Si-treated layer 16, as well as the depth of penetration of the enriched Si portion. Generally, the silicon vapor needs to have a temperature high enough such that it is a vapor at the exposure conditions. For instance, the silicon vapor and the substrate 12 may have a temperature of about 1400° C. to about 1800° C., preferably about 1600° C. to about 1750° C. (e.g., about 1600° C. to about 1700° C.). The silicon vapor may be generated in a wide variety of ways. One simple way to generate silicon vapor would be to heat silicon at a temperature below that of the surface of the substrate being heated in an argon environment and passing the argon containing silicon vapor over the substrate 12.
In certain embodiments, the Si-treated layer 16 formed integral to the ceramic core 14 may have a depth of about 750 μm or less, preferably about 25 μm to about 260 μm. Such a depth may be sufficient to protect the underlying ceramic core 14 from undesired oxidation during the intended use of the coated component 10.
Similar to the embodiment above, the applied layer 22 may be exposed to silicon vapor 20 at a temperature sufficient for forming the Si-treated layer 16 (as shown in
Referring to
No matter the configuration of the Si-treated layer 16 and the ceramic core 14, an EBC 18 is formed over the ceramic substrate 12 to form the coated component 10 with an increased operating temperature if compared to a similar component using a silicon bond coating. The EBC 18 may include any combination of one or more layers formed from materials selected from typical EBC or thermal barrier coating (“TBC”) layer chemistries, including but not limited to rare earth silicates (e.g., mono-silicates and di-silicates), aluminosilicates (e.g., mullite, barium strontium aluminosilicate (BSAS), rare earth aluminosilicates, etc.), hafnia, zirconia, stabilized hafnia, stabilized zirconia, rare earth hafnates, rare earth zirconates, rare earth gallium oxide, etc. The EBC 18 may be formed from a plurality of individual layers 19 having different chemistries to be directed at different types of layers that work together to protect the underlying ceramic core 14.
The coated components 10 of
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
