Patent Description:
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 a dense, hermetic seal 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.

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, CO<NUM>) or nitrous (i.e., NO, NO<NUM>, etc.) oxide gases cannot escape out through the dense EBC and thus, blisters form. The use of a silicon bond coat has been the solution to this blistering problem to date. The silicon bond coat 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 coat limits the upper temperature of operation for the EBC because the melting point of silicon metal is relatively low. In use, a thermally grown oxide (TGO) layer of silicon oxide forms on the top surface of the silicon metal bond coat of a multilayer EBC system. This silicon oxide scale remains amorphous at temperatures of <NUM> or lower, sometimes even at temperatures of <NUM> or lower, although this property is also dependent on the time the bond coat is exposed to this temperature. At higher temperatures, or when minor amounts of steam penetrate through the EBC to the bond coat, the silicon oxide scale crystallizes (e.g., into cristoblate), which undergoes phase transition accompanied by large volume change on cooling. The volume change leads to EBC coating spall.

As such, it is desirable to improve the properties of a silicon bond coat in the EBC to achieve a higher operational temperature limit for the EBC.

<CIT> discloses an article comprising a substrate comprising a silicon- bearing ceramic matrix composite; and a layer disposed over the substrate, wherein the layer comprises silicon and a dopant, the dopant comprising aluminum.

<CIT> relates to a protective coating composition for carbon articles comprising <NUM>-<NUM> wt % of molybdenum disilicide, <NUM>-<NUM> wt% of silicon oxide, <NUM>-<NUM> wt% of boron oxide and <NUM>-<NUM> wt% of alumina.

<CIT> relates to bond layers for ceramic or ceramic matrix composite substrates.

<CIT> describes solvent based slurry compositions for making environmental barrier coatings.

A composition is generally provided that includes a silicon-containing material of silicon metal and/or a silicide and a boron-doped refractory compound, as defined in the appended claim <NUM>, with <NUM>% to about <NUM>% by volume of the boron-doped refractory compound (e.g., about <NUM>% to about <NUM>% by volume). In one embodiment, a bond coating on a surface of a ceramic component is generally provided with the bond coating including such a composition, with the silicon-containing material is silicon metal.

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:.

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, "Ln" refers to a rare earth element or a mixture of rare earth elements. More specifically, the "Ln" refers to the rare earth elements of 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.

As used herein, the term "substantially free" means no more than an insignificant trace amount present and encompasses completely free (e.g., <NUM> molar % up to <NUM> molar %).

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.

A composition is generally provided that includes a silicon-containing material of silicon and/or a silicide and a boron-doped refractory compound as defined in the appended claim <NUM>. Generally, the composition includes about <NUM>% to <NUM>% by volume of the boron-doped refractory compound, such as about <NUM>% to about <NUM>% by volume.

In one embodiment, the silicon-containing material and a boron-doped refractory compound, as defined in the appended claim <NUM>, are continuous phases that are intertwined with each other. For example, the silicon-containing material and the boron-doped refractory compound are intertwined continuous phases having about <NUM>% to about <NUM>% by volume of the boron-doped refractory compound, such as about <NUM>% to about <NUM>% by volume (e.g., about <NUM>% to about <NUM>% by volume of the boron-doped refractory compound). For example, the composition can include the boron-doped refractory compound phase of about <NUM>% by volume to about <NUM>% by volume, with the balance being the silicon containing compound.

In another embodiment, the boron-doped refractory compound, as defined in the appended claim <NUM>, forms a plurality of discrete phases dispersed within the silicon-containing material (e.g., within a continuous phase of the silicon-containing material). In such an embodiment, the composition includes about <NUM>% to about <NUM>% by volume of the boron-doped refractory compound, such as about <NUM>% to about <NUM>% by volume (e.g., about <NUM>% to about <NUM>% by volume of the boron-doped refractory compound).

For example, the boron-doped refractory compound can include a compound having the formula:.

where Ln comprises Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof; x is <NUM> to <NUM> (e.g., x is up to about <NUM>, such as about <NUM> to about <NUM>).

where Ln comprises Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof; x is <NUM> to <NUM> (e.g., x is up to about <NUM>, such as about <NUM> to about <NUM>); M comprises Ga, In, Al, Fe, or a combination thereof; y is <NUM> to <NUM> (e.g., y is up to about <NUM>, such as about <NUM> to about <NUM>, about <NUM> to about <NUM>, or about <NUM> to about <NUM>); and x + y is greater than <NUM>. In one embodiment, both x and y are greater than <NUM>.

In one embodiment, x is <NUM> and y is greater than <NUM>, which indicates that boron is doped onto the metal site of the refractory compound. For example, the boron-doped refractory compound can have, in one embodiment, a formula of:.

where Ln includes Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof; M includes Ga, In, Al, Fe, or a combination thereof; and y is greater than <NUM> to about <NUM> (e.g., about <NUM> ≤ y ≤ about <NUM>). For example, y can be about <NUM> to about <NUM>, such as about <NUM> ≤ y ≤ about <NUM>).

In another embodiment, the boron-doped refractory compound can include a compound having the formula:.

where Ln comprises La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof; x is <NUM> to <NUM> (e.g., up to about <NUM>, such as about <NUM> to about <NUM>); D is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof, with D being different than Ln (i.e., D is a different element or combination of elements than Ln); M comprises Ga, Al, or a combination thereof; A comprises Fe, In, or a combination thereof; n is <NUM> to about <NUM> (e.g., n is greater than <NUM> to about <NUM>); y is <NUM> to <NUM>; and x + y is greater than <NUM>. If D is La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof (i.e., having an atomic radius of Sm or larger), then z is <NUM> to less than <NUM> (e.g., <NUM> < z < <NUM>, such as <NUM> < z ≤ about <NUM>). However, if D is Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof (i.e., having an atomic radius that is smaller than Sm), then z is <NUM> to about <NUM> (e.g., <NUM> < z < <NUM>, such as <NUM> < z ≤ about <NUM>).

where Ln comprises La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof; D is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof, with D being different than Ln (i.e., D is a different element or combination of elements than Ln); M comprises Ga, Al, or a combination thereof; A comprises Fe, In, or a combination thereof; n is <NUM> to about <NUM> (e.g., n is greater than <NUM> to about <NUM>); and y is greater than <NUM> to about <NUM>. If D is La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof (i.e., having an atomic radius of Sm or larger), then z is <NUM> to less than <NUM> (e.g., <NUM> < z < <NUM>, such as <NUM> < z ≤ about <NUM>). However, if D is Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof (i.e., having an atomic radius that is smaller than Sm), then z is <NUM> to about <NUM> (e.g., <NUM> < z < <NUM>, such as <NUM> < z ≤ about <NUM>).

In one particular embodiment, z is also <NUM>. In such an embodiment, the boron-doped refractory compound can have, in one embodiment, a formula of:.

where Ln comprises La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof; M comprises Ga, Al, or a combination thereof; A comprises Fe, In, or a combination thereof; n is <NUM> to about <NUM> (e.g., n is greater than <NUM> to about <NUM>); and y is greater than <NUM> to about <NUM>.

In another embodiment, the boron is doped interstitially within any refractory compound, such as those above (with or without the boron).

Compositions containing a boron-doped refractory compound, such as described above, can be utilized for a silicon-based coating. As such, silicon-based coatings that include a boron-doped refractory compound are generally provided for use with environmental barrier coatings for ceramic components, along with their methods of formation. In particular embodiments, silicon-based bond coatings for environmental barrier coatings (EBCs) are generally provided for high temperature ceramic components, along with methods of its formation and use. In particular, the silicon-based bond coating includes a component containing a boron-doped refractory compound for preventing crystallization of a thermal growth oxide ("TGO") on silicon-based bond coating in an EBC, which in turn prevents spall of the coating caused by such crystallization of the TGO. That is, the introduction of boron (B) within the silicon-based bond coating keeps the TGO (i.e., the SiO) in an amorphous phase. Accordingly, the operating temperature of the silicon-based bond coating (and thus the TGO and EBC coating) can be increased. Additionally, the inclusion of B can inhibit and prevent crystallization of the TGO without greatly accelerating the growth rate of the TGO. Additionally, boron-doped refractory compounds have limited reaction with and/or solubility into in silicon oxide, which can limit the rate of oxide scale growth.

<FIG> show exemplary embodiments of a ceramic component <NUM> formed from a substrate <NUM> and a silicon-based layer 104a (<FIG>), 104b (<FIG>), respectively. Each of the silicon-based layers 104a, 104b includes a silicon-containing material and about <NUM>% to about <NUM>% of the boron-doped refractory compound, as discussed above.

Generally, the boron-doped refractory compound is unreactive with the composition of the silicon-based layer 104a (e.g., silicon metal). The silicon-based layer 104a may include the boron-doped refractory compound dispersed throughout the silicon-based layer 104a, such as in the form of discrete particulate phases or as a continuous grain boundary within the silicon-based layer 104a.

In one particular embodiment, the substrate <NUM> is formed from a CMC material (e.g., a silicon based, non-oxide ceramic matrix composite). As used herein, "CMCs" refers to silicon-containing, or oxide-oxide, matrix and reinforcing materials. As used herein, "monolithic ceramics" refers to materials without fiber reinforcement (e.g., having the matrix material only). Herein, CMCs and monolithic ceramics are collectively referred to as "ceramics.

Some examples of CMCs acceptable for use herein can include, but are not limited to, materials having a matrix and reinforcing fibers comprising non-oxide silicon-based materials such as silicon carbide, silicon nitride, silicon oxycarbides, silicon oxynitrides, and mixtures thereof. Examples include, but are not limited to, CMCs with silicon carbide matrix and silicon carbide fiber; silicon nitride matrix and silicon carbide fiber; and silicon carbide/silicon nitride matrix mixture and silicon carbide fiber. Furthermore, CMCs can have a matrix and reinforcing fibers comprised of oxide ceramics. Specifically, the oxide-oxide CMCs may be comprised of a matrix and reinforcing fibers comprising oxide-based materials such as aluminum oxide (Al<NUM>O<NUM>), silicon dioxide (SiO<NUM>), aluminosilicates, and mixtures thereof. Aluminosilicates can include crystalline materials such as mullite (3Al<NUM>O<NUM> 2SiO<NUM>), as well as glassy aluminosilicates.

In the embodiment of <FIG>, the substrate <NUM> defines a surface <NUM> having a coating <NUM> formed thereon. The coating <NUM> includes the silicon-based layer 104a and an environmental barrier coating <NUM>. In one particular embodiment, the silicon-based layer 104a is a bond coating, where the silicon containing material is silicon metal, a silicide (e.g., a rare earth silicide, a molybdenum silicide, a rhenium silicide, or mixtures thereof) or a mixture thereof. In one embodiment, a composition is generally provided that includes silicon metal and the boron-doped refractory compound, such as in the relative amounts described above. In an alternative embodiment, a composition is generally provided that includes a silicide (e.g., a rare earth silicide, a molybdenum silicide, a rhenium silicide, or a mixture thereof) and the boron-doped refractory compound, such as in the relative amounts described above (e.g., about <NUM>% to about <NUM>% by volume).

During use, a thermally grown oxide ("TGO") layer forms on the surface of the bond coating. For example, a layer of silicon oxide (sometimes referred to as "silicon oxide scale" or "silica scale") forms on a bond coating of silicon metal and/or a silicide. Referring to <FIG>, a thermally grown oxide layer <NUM> (e.g., silicon oxide) is shown directly on the silicon-based layer 104a (e.g., a bond coating with the silicon containing material being silicon metal and/or a silicide), as forms during exposure to oxygen (e.g., during manufacturing and/or use) of the component <NUM>. Due to the presence of boron in the boron-doped refractory compound within the silicon-based layer 104a, the thermally grown oxide layer <NUM> remains substantially amorphous at its operating temperature, with the "operating temperature" referring to the temperature of the thermally grown oxide layer <NUM>. For example, for silicon metal bond coatings, the TGO layer may remain amorphous at operating temperatures of about <NUM> or less (e.g., about <NUM> to about <NUM>), which is just below the melting point of the silicon-based bond coating (Si metal has a melting point of about <NUM>). In another example, for silicide bond coatings, the TGO layer may remain amorphous at operating temperatures of about <NUM> or less (e.g., about <NUM> to about <NUM>), which is just below the maximum use temperature of the CMC. Without wishing to be bound by any particular theory, it is believed that boron in the silicon-based layer 104a migrates into the thermally grown oxide layer <NUM> and inhibits crystallization of the thermally grown oxide layer (e.g., silicon oxide) that would otherwise occur at these temperatures.

In the embodiment shown in <FIG>, the silicon-based layer 104a is directly on the surface <NUM> without any layer therebetween. However, in other embodiments, one or more layers can be positioned between the silicon-based layer 104a and the surface <NUM>.

<FIG> shows another embodiment of a ceramic component <NUM> with the substrate <NUM> having an outer layer 104b that defines a surface <NUM> of the substrate <NUM>. That is, the outer layer 104b is integral with the substrate <NUM>. In this embodiment, the outer layer 104b is the silicon-based layer, and a coating <NUM> is on the surface <NUM>. The coating <NUM> may include an environmental barrier coating <NUM> and/or other layers (e.g., a bond coating, etc.). In one embodiment, the outer layer 104b can be a silicon-containing monolithic ceramic layer. For example, the outer layer 104b may include silicon carbide. In one embodiment, the substrate <NUM> may include the outer layer 104b (e.g., including silicon carbide as a monolithic ceramic layer) on a plurality of CMC plies forming the remaining portion of the substrate.

<FIG> shows a thermally grown oxide layer <NUM> (e.g., silicon oxide) directly on the silicon-based layer 104b (e.g., a bond coating with the silicon containing material being silicon metal), as forms during exposure to oxygen (e.g., during manufacturing and/or use) of the component <NUM>. Due to the presence of boron within the silicon-based layer 104b, the thermally grown oxide layer <NUM> remains substantially amorphous at the operating temperature of the thermally grown oxide layer <NUM>. Without wishing to be bound by any particular theory, it is believed that boron in the silicon-based layer 104b migrates into the thermally grown oxide layer <NUM> and inhibits crystallization of the thermally grown oxide layer (e.g., silicon oxide) that would otherwise occur at these temperatures.

As stated, a boron-doped refractory compound is included within the silicon-based layer 104a, 104b, no matter the particular positioning of the silicon-based layer <NUM> in the ceramic component <NUM>.

The environmental barrier coating <NUM> of <FIG> can include any combination of one or more layers formed from materials selected from typical EBC or TBC layer chemistries, including but not limited to rare earth silicates (mono- and di-silicates), mullite, barium strontium aluminosilicate (BSAS), hafnia, zirconia, stabilized hafnia, stabilized zirconia, rare earth hafnates, rare earth zirconates, rare earth gallates, etc..

The ceramic component <NUM> of <FIG> is particularly suitable for use as a component found in high temperature environments, such as those present in gas turbine engines, for example, combustor components, turbine blades, shrouds, nozzles, heat shields, and vanes. In particular, the turbine component can be a CMC component positioned within a hot gas flow path of the gas turbine such that the coating forms an environmental barrier coating on the component to protect the component within the gas turbine when exposed to the hot gas flow path.

<FIG> is a schematic cross-sectional view of a gas turbine engine in accordance with an exemplary embodiment of the present disclosure. More particularly, for the embodiment of <FIG>, the gas turbine engine is a high-bypass turbofan jet engine <NUM>, referred to herein as "turbofan engine <NUM>. " As shown in <FIG>, the turbofan engine <NUM> defines an axial direction A (extending parallel to a longitudinal centerline <NUM> provided for reference) and a radial direction R. In general, the turbofan <NUM> includes a fan section <NUM> and a core turbine engine <NUM> disposed downstream from the fan section <NUM>. Although described below with reference to a turbofan engine <NUM>, the present disclosure is applicable to turbomachinery in general, including turbojet, turboprop and turboshaft gas turbine engines, including industrial and marine gas turbine engines and auxiliary power units.

The exemplary core turbine engine <NUM> depicted generally includes a substantially tubular outer casing <NUM> that defines an annular inlet <NUM>. The outer casing <NUM> encases, in serial flow relationship, a compressor section including a booster or low pressure (LP) compressor <NUM> and a high pressure (HP) compressor <NUM>; a combustion section <NUM>; a turbine section including a high pressure (HP) turbine <NUM> and a low pressure (LP) turbine <NUM>; and a jet exhaust nozzle section <NUM>. A high pressure (HP) shaft or spool <NUM> drivingly connects the HP turbine <NUM> to the HP compressor <NUM>. A low pressure (LP) shaft or spool <NUM> drivingly connects the LP turbine <NUM> to the LP compressor <NUM>.

For the embodiment depicted, the fan section <NUM> includes a variable pitch fan <NUM> having a plurality of fan blades <NUM> coupled to a disk <NUM> in a spaced apart manner. As depicted, the fan blades <NUM> extend outwardly from disk <NUM> generally along the radial direction R. Each fan blade <NUM> is rotatable relative to the disk <NUM> about a pitch axis P by virtue of the fan blades <NUM> being operatively coupled to a suitable actuation member <NUM> configured to collectively vary the pitch of the fan blades <NUM> in unison. The fan blades <NUM>, disk <NUM>, and actuation member <NUM> are together rotatable about the longitudinal axis <NUM> by LP shaft <NUM> across an optional power gear box <NUM>. The power gear box <NUM> includes a plurality of gears for stepping down the rotational speed of the LP shaft <NUM> to a more efficient rotational fan speed.

Referring still to the exemplary embodiment of <FIG>, the disk <NUM> is covered by rotatable front nacelle <NUM> aerodynamically contoured to promote an airflow through the plurality of fan blades <NUM>. Additionally, the exemplary fan section <NUM> includes an annular fan casing or outer nacelle <NUM> that circumferentially surrounds the fan <NUM> and/or at least a portion of the core turbine engine <NUM>. It should be appreciated that the nacelle <NUM> may be configured to be supported relative to the core turbine engine <NUM> by a plurality of circumferentially-spaced outlet guide vanes <NUM>. Moreover, a downstream section <NUM> of the nacelle <NUM> may extend over an outer portion of the core turbine engine <NUM> so as to define a bypass airflow passage <NUM> therebetween.

During operation of the turbofan engine <NUM>, a volume of air <NUM> enters the turbofan <NUM> through an associated inlet <NUM> of the nacelle <NUM> and/or fan section <NUM>. As the volume of air <NUM> passes across the fan blades <NUM>, a first portion of the air <NUM> as indicated by arrows <NUM> is directed or routed into the bypass airflow passage <NUM> and a second portion of the air <NUM> as indicated by arrow <NUM> is directed or routed into the LP compressor <NUM>. The ratio between the first portion of air <NUM> and the second portion of air <NUM> is commonly known as a bypass ratio. The pressure of the second portion of air <NUM> is then increased as it is routed through the high pressure (HP) compressor <NUM> and into the combustion section <NUM>, where it is mixed with fuel and burned to provide combustion gases <NUM>.

The combustion gases <NUM> are routed through the HP turbine <NUM> where a portion of thermal and/or kinetic energy from the combustion gases <NUM> is extracted via sequential stages of HP turbine stator vanes <NUM> that are coupled to the outer casing <NUM> and HP turbine rotor blades <NUM> that are coupled to the HP shaft or spool <NUM>, thus causing the HP shaft or spool <NUM> to rotate, thereby supporting operation of the HP compressor <NUM>. The combustion gases <NUM> are then routed through the LP turbine <NUM> where a second portion of thermal and kinetic energy is extracted from the combustion gases <NUM> via sequential stages of LP turbine stator vanes <NUM> that are coupled to the outer casing <NUM> and LP turbine rotor blades <NUM> that are coupled to the LP shaft or spool <NUM>, thus causing the LP shaft or spool <NUM> to rotate, thereby supporting operation of the LP compressor <NUM> and/or rotation of the fan <NUM>.

The combustion gases <NUM> are subsequently routed through the jet exhaust nozzle section <NUM> of the core turbine engine <NUM> to provide propulsive thrust. Simultaneously, the pressure of the first portion of air <NUM> is substantially increased as the first portion of air <NUM> is routed through the bypass airflow passage <NUM> before it is exhausted from a fan nozzle exhaust section <NUM> of the turbofan <NUM>, also providing propulsive thrust. The HP turbine <NUM>, the LP turbine <NUM>, and the jet exhaust nozzle section <NUM> at least partially define a hot gas path <NUM> for routing the combustion gases <NUM> through the core turbine engine <NUM>.

Methods are also generally provided for coating a ceramic component. In one embodiment, the method includes applying a bond coating directly on a surface of the ceramic component, where the bond coating comprises a silicon-containing material (e.g., silicon metal and/or a silicide) and a boron-doped refractory compound, such as described above.

Claim 1:
A composition, comprising: a silicon-containing material and a boron-doped refractory compound, wherein the silicon-containing material is silicon metal and/or a silicide, wherein the composition comprises <NUM>% to <NUM>% by volume of the boron-doped refractory compound, wherein the boron-doped refractory compound comprises
(a) a compound having the formula:

        Ln<NUM>-xBxSi<NUM>O<NUM>

where
Ln comprises Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof; and
x is <NUM> to <NUM>;
(b) a compound having the formula:

        Ln<NUM>-xBxSi<NUM>O<NUM>

where
Ln comprises Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof; and
x is <NUM> to <NUM>;
(c) a compound having the formula:

        Ln<NUM>-xBxM<NUM>-yByO<NUM>

where
Ln comprises Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof;
x is <NUM> to <NUM>;
M comprises Ga, In, Al, Fe, or a combination thereof;
y is <NUM> to <NUM>; and
x + y is greater than <NUM>; or
(d) a compound having the formula:

        Ln<NUM>-x-zBxDzM<NUM>-n-yAnByO<NUM>

where
Ln comprises La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof;
x is <NUM> to <NUM>;
D is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof, where:
D is not equal to Ln;
if D is La, Ce, Pr, Nd, Pm, Sm, or a mixture thereof, then z is <NUM> to less than <NUM>;
if D is Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a mixture thereof, then z is <NUM> to <NUM>
M comprises Ga, Al, or a combination thereof;
A comprises Fe, In, or a combination thereof;
n is <NUM> to <NUM>;
y is <NUM> to <NUM>; and
x + y is greater than <NUM>.