Patent Application
20160305004
The present disclosure generally relates to coated substrates and methods of producing the same, and more particularly relates to ceramic substrates with protective coatings and methods of producing the same.
Ceramic components present numerous benefits over metallic components in the high pressure module of gas turbine engines. For example, ceramics are higher temperature-capable than many nickel-based metallic components. Ceramics also possess relatively high strength at low weights as compared to conventional superalloys. The relative thermochemical and thermomechanical stability of certain ceramics at high temperatures can reduce or eliminate the need for cooling systems that may be required for comparable nickel-based superalloys, therefore, the use of ceramic components can reduce weight, improve efficiency, and reduce the complexity of turbine engines. As can be imagined, the advantageous properties of ceramic components make them attractive options for many uses other than the hot sections of turbine engines.
The hot sections of turbine engines are challenging environments that can degrade ceramic or metallic components over time. These hot sections, especially the high pressure turbine section, can include high temperatures, rapid temperature changes, engine vibration, erosion, corrosion from fuel additives and salts, oxidation, etc. An environmental barrier coating (EBC) system may be applied to a ceramic component to provide some protection from the harsh gas turbine engine conditions. The EBC may be specifically designed for the environment the component will be exposed to. In most cases, a ceramic component has a multilayer EBC system where each layer serves a different function. For example, a thermal protective layer may be added near the top of the multilayer system to provide thermal protection to the underlying layers and the substrate in cases of very high gas path temperatures, or large thermal gradients. In another example, an abrasive protective coating layer may be applied as a top layer for anticipated rub conditions. Other layers may primarily serve as oxidation/corrosion protection for the substrate.
In many cases, the EBC does not readily adhere to the ceramic substrate of the component. In such cases, the EBC may spall off during use. When spallation occurs, the protection afforded the substrate by the EBC is lost and the underlying ceramic component is exposed to the engine environment. This can shorten the service life of the component, and can increase the risk of failure during service. A bond coat layer may be applied between the EBC and the substrate to adhere the EBC to the substrate and therefore minimize the risk of spallation. However, the harsh conditions within the hot sections of turbine engines can detrimentally impact the service life of a bond coat, and a failing bond coat can result in spallation of the EBC.
Accordingly, it is desirable to provide bond coats and methods of forming bond coats that provide strong adhesion in adverse conditions. In addition, it is desirable to provide bond coats and methods of forming bond coats for ceramic substrates. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
Coated substrates and methods of producing the same are provided. In an exemplary embodiment, a method of coating a substrate includes brazing a first bond coat layer to the substrate, where the substrate includes a ceramic material. A second bond coat layer is plasma sprayed overlying the first bond coat layer to form a composite bond coat with the first bond coat layer positioned between the second bond coat layer and the substrate. An environmental barrier coating layer is applied overlying the second bond coat layer such that the first and second bond coat layers are positioned between the substrate and the environmental barrier coating layer.
A method for coating a substrate is provided in another embodiment. The method includes forming a composite bond coat overlying the substrate, where the substrate includes a ceramic material. The composite bond coat includes a first bond coat layer having a first surface roughness Ra1, and a second bond coat layer having a second surface roughness Ra2 greater than the first surface roughness Ra1. The surface roughness measured in Ra is an arithmetic average of an absolute value of vertical deviations from a mean line. An environmental barrier coating layer is applied to the composite bond coat such that the composite bond coat is positioned between the substrate and the environmental barrier coating layer.
A substrate is provided in yet another embodiment. A first bond coat layer overlies the substrate, where the substrate includes a silicon based ceramic material. The first bond coat layer includes from about 60 to about 95 mole percent silicon, and from about 5 to about 40 mole percent of a first additional material. The first additional material includes one or more of a metal and a silicide. The first bond coat layer has a first surface roughness. A second bond coat layer overlies the first bond coat layer such that the first bond coat layer is positioned between the substrate and the second bond coat layer. The second bond coat layer includes from about 60 to about 100 mole percent silicon, and has a second surface roughness greater than the first surface roughness. An environmental barrier coating layer overlies the second bond coat layer such that the first and second bond coat layers are positioned between the substrate and the environmental barrier coating layer.
The present embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:
The following detailed description is merely exemplary in nature and is not intended to limit the various embodiments or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description.
Bond coats are used to adhere an environmental barrier coating layer to a ceramic substrate. Adhesion is improved by greater surface roughness in many embodiments, so a bond coat with greater surface roughness may improve adhesion. Plasma sprayed bond coats often have a higher surface roughness than brazed bond coats, but the use of a brazed bond coat layer between a plasma sprayed bond coat layer and the ceramic substrate can improve the adhesion of the plasma sprayed layer to the substrate.
Referring to an exemplary embodiment illustrated in
In an exemplary embodiment, the substrate 10 includes a silicon based ceramic material, including but not limited to silicon nitride, silicon carbide, silicon oxynitride, SiAlON materials (materials that include silicon, aluminum, oxygen, and nitrogen), and silicon dioxide. In some embodiments, all of the ceramic material of the substrate 10 is a silicon based ceramic, but in other embodiments the substrate 10 may include silicon based ceramics combined with other ceramic materials that do not include silicon. In various embodiments, the silicon based ceramic portion of the substrate 10 may be from about 50 to about 100 weight percent, or from about 80 to about 100 weight percent, or from about 99 to about 100 weight percent of the entire substrate 10.
A first bond coat layer 14 is brazed to the substrate 10 on the substrate surface 12. As used herein, the term “braze” means bringing a material to it liquidus point such that the material flows, then solidifying the material, so the first bond coat layer 14 is brought to its liquidus point and then solidified onto the substrate 10 in the brazing process. The first bond coat layer 14 may be positioned on the substrate surface 12 before brazing, such as with a tape (as illustrated in
The coefficient of thermal expansion (CTE) of the first bond coat layer 14 may be close to the CTE of the substrate 10 in embodiments where the substrate will experience temperature extremes during use, because similar CTE's for the first bond coat layer 14 and the substrate 10 can reduce spalling do to the first bond coat layer 14 expanding or contracting at a different rate than the substrate 10 with temperature changes. In an exemplary embodiment, the CTE of the first bond coat layer 14 is within about 5 percent of the CTE of the substrate 10, or within about 3 percent, or within about 1 percent in alternate embodiments. The percent difference in the CTE is determined by finding the absolute value of the difference between the first bond coat layer CTE and the substrate CTE, and dividing that absolute value by the CTE of the substrate 10.
In embodiments where the substrate 10 includes silicon based ceramics, the first bond coat layer 14 may also include silicon to improve compatibility. In an exemplary embodiment, the first bond coat layer 14 includes from about 60 mole percent to about 95 mole percent silicon, or from about 70 mole percent to about 90 mole percent silicon, or from about 80 mole percent to about 90 mole percent silicon in alternate embodiments. The first bond coat layer 14 also includes a first additional material. The first additional material is from about 5 to about 40 mole percent, or from about 10 to about 30 mole percent, or from about 10 to about 20 mole percent of the first bond coat layer 14 in various embodiments. The first additional material may be one or more of a metal and a silicide, and the first additional material and the silicon of the first bond coat layer 14 may be at the eutectic point such that all components melt at the same temperature. References herein to a material being “at” the eutectic point means the material composition is within about 2 percent of the eutectic point, so if a the eutectic point were at 50 percent of component A and 50 percent of component B, combinations ranging from 48 to 52 percent of each component would be considered “at” the eutectic point. Combinations at the eutectic point may facilitate brazing due to simultaneous melting, but combinations at the eutectic point are not required and in some embodiments the first additional material and the silicon are combined at concentrations other than at the eutectic point (hyper- or hypoeutectic.) The first additional material may include several different metals and/or silicides, but in an exemplary embodiment the first additional material is selected from the group consisting of a transition metal silicide, or combinations transition metal silicides. Transition metals include elements in the d-block of the periodic table, as well as elements in the f-block of the periodic table.
The first bond coat layer 14 is brazed to the substrate 10 by heating. In an exemplary embodiment, the first bond coat layer 14 has a melting point of from about 1,300 degrees centigrade (° C.) to about 1,400° C. The brazing process includes melting or softening the first bond coat layer 14 by applying heat 16 at a temperature of from about 1,300° C. to about 1,400° C., such as in a furnace or other heating device. The substrate 10 and the first bond coat layer 14 may be positioned in an atmosphere that is free of oxidizing agents during the brazing process. As used herein, an atmosphere is “free of oxidizing agents” if oxidizing agents are present at a partial pressure of about 0.0001 atmospheres or less. Oxidizing agents include, but are not limited to, oxygen, carbon monoxide, and water. During the brazing process, the substrate 10 and first bond coat layer 14 may be heated in a vacuum, or in the presence of an inert gas such as helium, argon, or nitrogen.
After brazing, the first bond coat layer 14 has a first surface roughness 20 that can be measured. In an exemplary embodiment, the first surface roughness 20 and other surface roughnesses described below are determined by the arithmetic average of the absolute value of vertical deviations from a mean line, where the mean line is the average surface level. In an exemplary embodiment, the first surface roughness Ra1 20 is measured as a roughness parameter Ra1. The surface roughness can be measured with a profilometer 22 or with other surface roughness measuring devices. In general, increasing the surface roughness improves adhesion. In some embodiments, the surface roughness of the first bond coat layer 14 after brazing is relatively smooth, so a rougher surface could improve adhesion for subsequent layers.
Referring to the exemplary embodiment in
The operating conditions of the second bond coat layer plasma spray device 34 can be modified to adjust some properties of the second bond coat layer 30. For example, if the small droplets are heated to higher temperature, the viscosity increases and the second bond coat layer 30 may be smoother than for small droplets at a lower temperature. In a similar manner, a smoother surface may be obtained by propelling the small droplets more rapidly so the small droplets flatten more on impact. In an exemplary embodiment, the second bond coat layer 30 is formed such that it has a second surface roughness 36 that is greater than the first surface roughness 20. For example, the second surface roughness 36 may be measured as a roughness parameter Ra2, as described above, where Ra2 is greater than the first surface roughness Ra1 20. The second surface roughness Ra2 36 may be about two or more times the first surface roughness Ra1 20, or about 4 or more times the first surface roughness Ra1 20, or about 5 or more times the first surface roughness Ra1 20 in various embodiments. The higher roughness value for the second bond coat layer 30 may improve adhesion for additional layers formed overlying the composite bond coat 32. The second surface roughness 36 may be measured with a profilometer 22 or other measuring device, as described above. The second bond coat layer 30 may also be applied at appropriate conditions to produce a desired thickness, such as from about 0.5 to about 20 mils thick, or from about 1 to about 10 mils thick, or from about 1 to about 5 mils thick in various embodiments.
The second bond coat layer 30 is compatible with the first bond coat layer 14 and the substrate 10, and may have a CTE within about 5 percent of the CTE of the substrate 10, or within about 3 percent, or within about 1 percent in alternate embodiments, as described above. In embodiments where the substrate 10 includes a silicon based ceramic, the second bond coat layer 30 may also include silicon, which may improve compatibility. In an exemplary embodiment, the second bond coat layer 30 includes from about 60 to about 100 mole percent silicon, so the second bond coat layer 30 may be essentially pure silicon in some embodiments. In alternate embodiments, the second bond coat layer 30 may include about 70 to about 100 mole percent silicon, or about 80 to 100 mole percent silicon. The second bond coat layer 30 may also include other components in some embodiments, such as a second additional material. The second bond coat layer 30 may optionally include a second additional material at from about 0 to about 40 mole percent, or from about 0 to about 30 mole percent, or from about 0 to about 20 mole percent in various embodiments. The second additional material may be one or more of a metal and a silicide, and the second additional material and the silicon of the second bond coat layer 30 may optionally be at the eutectic point, as described above. The second additional material may include several different metals and/or silicides, but in an exemplary embodiment the second additional material is selected from the group consisting of a transition metal silicide, or combinations transition metal silicides, as described above for the first bond coat layer 14.
An environmental barrier coating layer 40, referred to by the abbreviation EBC, is applied overlying the second bond coat layer 30. The EBC 40 is adhered to the substrate 10 by the composite bond coat 32. The EBC 40 may include a wide variety of materials in various embodiments, and the EBC 40 may be customized for specific environments that the substrate 10 will be exposed to. The EBC 40 may include 1 or more layers, and in embodiments with more than 1 layer the different layers may or may not have different compositions. For example, the EBC 40 may include an oxide ceramic, such as various silicates or other ceramics that may or may not include the silicon element. The EBC 40 may be applied as a plasma spray using an EBC plasma spray device 42, as illustrated, but the EBC 40 may be applied in a wide variety of other manners including but not limited to tape, paint, other spray techniques, dips, brazing, etc. The EBC 40 may optionally be heat treated after application, but the optional heat treatment process for the EBC 40 typically does not involve temperatures at or above the melting point of the first bond coat layer 14. The composite bond coat 32 may improve bonding of the EBC 40 to the ceramic substrate 10 compared to a bond coat that does not include a brazed layer and a plasma sprayed layer. The first bond coat layer 14 provides secure adhesion to the substrate 10, and the second bond coat layer 30 provides a rough surface to improve adhesion of the EBC 40.
While at least one embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the embodiment or embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described without departing from the scope as set forth in the appended claims and their legal equivalents.
This embodiment was made with Government support under contract W911W6-08-2-0001 awarded by the U.S. Army Aviation Applied Technology Directorate. The Government has certain rights in this embodiment.
