Patent Description:
Fiber-reinforced ceramic matrix composites (CMCs) are known and used for components that are exposed to high temperatures and corrosive conditions that can degrade other kinds of materials. Under such severe conditions, such as the operating conditions in aerospace applications, even ceramic materials are vulnerable to degradation. Over time, ceramic composites can form microcracks that further expose the ceramic material to oxygen or other corrosive elements, which form undesirable phases to the detriment of the properties of the ceramic matrix composite component.

Fibers can be coated with a layer of boron nitride to form a weak interface between the fibers and matrix material to enable desired composite characteristics. Boron nitride can sometimes form with a disordered structure that readily oxidizes to molten boria at high temperatures, exposing fibers to oxidation and causing fiber degradation. A protective layer of silicon carbide can be included in the coating system, but the roughness of such layers can lead to cracking which creates pathways for oxidant ingress to inner boron nitride layers, exposing the fiber to molten borosilicate. Therefore, coating materials with a greater oxidation resistance are desirable. In <CIT>, fibre coated with BN, Si-BN and PyC is described. In <CIT>, fibres coated with Si-doped BN or BN/Si-BN graded layers are described.

A coated fiber structure for use in a ceramic matrix composite comprises a fiber extending along a fiber axis and an interface coating arrangement applied to and circumscribing the fiber. The interface coating arrangement comprises: a first boron nitride layer extending coaxially about and in direct contact with the fiber, a first silicon-doped boron nitride layer extending coaxially about and in direct contact with the first boron nitride layer, a carbon layer extending coaxially about and in direct contact with the first silicon-doped boron nitride layer, a second boron nitride layer extending coaxially about and in direct contact with the carbon layer, and a second silicon-doped boron nitride layer extending coaxially about and in direct contact with the second boron nitride layer. A silicon content of the first silicon-doped boron nitride layer is higher than the silicon content of the second silicon-doped boron nitride layer.

A method of forming a ceramic matrix composite comprises forming a fibrous preform by arranging a plurality of ceramic fibers, depositing a first boron nitride layer on the plurality of ceramic fibers, depositing a first silicon-doped boron nitride layer on the first boron nitride layer, depositing a carbon layer on the first silicon-doped boron nitride layer, depositing a second boron nitride layer on the carbon layer, and depositing a second silicon-doped boron nitride layer on the second boron nitride layer. A silicon content of the first silicon-doped boron nitride layer is higher than the silicon content of the second silicon-doped boron nitride layer. The method further comprises depositing a silicon carbide matrix on the fibrous preform.

While the above-identified figures set forth one or more embodiments of the present disclosure, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features and components not specifically shown in the drawings.

This disclosure presents various fiber coating arrangements for improving mechanical, thermal, and/or oxidation resistance properties of CMCs.

<FIG> is a simplified cross-sectional illustration of CMC <NUM>, formed from coated fibers <NUM> (only one is represented in <FIG>) and matrix <NUM>. CMC <NUM> can be suitable for use in, for example, a gas turbine engine. Coated fiber <NUM> can include fiber <NUM> with interface coating arrangement <NUM>. Although not visible in <FIG>, fiber <NUM> is generally circumscribed by coating arrangement <NUM> and matrix <NUM>, and the various layers of coating arrangement <NUM> are coaxial with one another and fiber <NUM>. Fiber <NUM> can be formed from silicon carbide (SiC) or other suitable ceramic material. Multiple fibers <NUM> of the encompassing CMC <NUM> can be arranged in various woven or non-woven, unidirectional or multidirectional architectures. Matrix <NUM> can be formed from SiC or other suitable ceramic material.

Beginning closest to fiber <NUM> and working outward toward matrix <NUM>, interface coating arrangement <NUM> includes inner boron nitride (BN) layer <NUM>, inner silicon-doped boron nitride (SiBN) layer <NUM>, carbon layer <NUM>, outer BN layer <NUM>, and outer SiBN layer <NUM>. As used herein, the terms "inner" and "outer" are relative to one another and fiber <NUM>, such that an outer layer (e.g., outer BN layer <NUM>) is positioned further from fiber <NUM> than an inner layer (e.g., inner BN layer <NUM>). Inner SiBN layer <NUM> can have a silicon content ranging from <NUM>% by weight (wt%) to <NUM> wt%, and in an exemplary embodiment, <NUM> wt%. Outer SiBN layer <NUM> can have a silicon content ranging from <NUM> wt% to <NUM> wt%, and in an exemplary embodiment, <NUM> wt%. Each of inner BN layer <NUM> and outer BN layer <NUM> can have a thickness ranging from <NUM> to <NUM>. Each of inner SiBN layer <NUM> and outer SiBN layer <NUM> can have a thickness ranging from <NUM> to <NUM>. Carbon layer <NUM> can have a thickness ranging from <NUM> to <NUM>. In general, the individual layers of coating arrangement <NUM> can have a generally uniform thickness, although variances can occur. Further, coating arrangement <NUM> can still be effective even if discontinuities exist within its individual layers.

In an operational environment (e.g., a gas turbine engine), cracks can form in matrix <NUM> and propagate towards fiber <NUM>. For example, cracks can penetrate matrix <NUM>, outer SiBN layer <NUM>, and outer BN layer <NUM>. Crack deflection can occur at outer BN layer <NUM> and/or carbon layer <NUM>, and in some cases, at the interface of carbon layer <NUM> and outer BN layer <NUM>. This can occur because carbon layer <NUM> can include aligned carbon, with planes oriented generally parallel to fiber <NUM>. Accordingly, cracks tend to deflect away from fiber <NUM> in the direction of these planes. Carbon layer <NUM> can further become debonded from inner SiBN layer <NUM>, leaving inner SiBN layer <NUM> partially exposed. Temperatures ranging from <NUM> to <NUM> can create oxidizing conditions, and outer BN layer <NUM> can begin to recede as boron within the layer oxidizes and volatilizes. Exposed to the oxidizing conditions, silicon within inner SiBN layer <NUM> can oxidize to form a glassy network of silica (SiO<NUM>), thus protecting inner BN layer <NUM> and fiber <NUM>. Silicon within outer SiBN layer <NUM> can similarly oxidize to form a layer of protective silica bridging/at least partially closing off any cracks extending through outer SiBN layer <NUM>, preventing further ingress of cracks and/or oxidants. Because of the relatively higher silicon content within inner SiBN layer <NUM>, the silicon tends to oxidize more slowly than the silicon within outer SiBN layer <NUM>, with its relatively lower silicon content. The relative rates of oxidation within inner SiBN layer <NUM> and outer SiBN layer <NUM> beneficially minimize oxidation and degradation of fiber <NUM>. For example, as noted above the relatively faster oxidation of outer SiBN layer <NUM> prevents further ingress of cracks and/or oxidants after the bridging silica forms, which protects inward layers (i.e., <NUM>-<NUM>) and fiber <NUM>. SiBN layer <NUM> remains intact for a longer period, with its relatively slower oxidation rate, increasing the duration of time before inner SiBN layer <NUM> is fully oxidized and volatilized in the operating environment.

<FIG> is a method flowchart illustrating steps <NUM>-<NUM> of method <NUM> for forming a CMC, such as CMC <NUM>, with interface coating arrangement <NUM>. At step <NUM>, a preform of multiple fibers <NUM> can be placed in tooling and/or a reaction furnace, and inner BN layer <NUM> with a thickness of <NUM> to <NUM> deposited on fibers <NUM> using chemical vapor infiltration (CVI). At step <NUM>, inner SiBN layer <NUM> with a thickness of <NUM> to <NUM> can be deposited over inner BN layer <NUM> using CVI. At step <NUM>, carbon layer <NUM> with a thickness of <NUM> to <NUM> can be deposited over inner SiBN layer <NUM> using CVI. At step <NUM>, outer BN layer <NUM> with a thickness of <NUM> to <NUM> can be deposited over carbon layer <NUM> using CVI. At step <NUM>, outer SiBN layer <NUM> with a thickness of <NUM> to <NUM> can be deposited over outer BN layer <NUM> using CVI. Each layer <NUM>-<NUM> can be generally amorphous and smooth, with a root mean square (RMS) roughness is less than <NUM>.

At step <NUM>, matrix <NUM> can be deposited over coating arrangement <NUM> of the preform. Matrix <NUM> can be deposited using CVI, which can be carried out until the resulting CMC (e.g., CMC <NUM>) has reached the desired residual porosity. Other techniques for matrix formation are contemplated herein, such as one or a combination of slurry infiltration, melt infiltration, and polymer infiltration and pyrolysis. Such techniques can supplement the CVI process. Protective coatings for the CMC (e.g., thermal barrier coatings, environmental barrier coatings, etc.) can optionally be applied after step <NUM>.

A CMC component formed with the disclosed fiber coating arrangements can be incorporated into aerospace, maritime, or industrial equipment, to name a few, non-limiting examples.

The fiber structure of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:.

In the above fiber structure, the silicon content of the first silicon-doped boron nitride layer can range from <NUM> wt% to <NUM> wt%, and the silicon content of the second silicon-doped boron nitride layer can range from <NUM> wt% to <NUM> wt%.

In any of the above fiber structures, the silicon content of the first silicon-doped boron nitride layer can be <NUM> wt%.

In any of the above fiber structures, the silicon content of the second silicon-doped boron nitride layer can be <NUM> wt%.

In any of the above fiber structures, a layer thickness of the first boron nitride layer can range from <NUM> to <NUM>.

In any of the above fiber structures, a layer thickness of the first silicon-doped boron nitride layer can range from <NUM> to <NUM>.

In any of the above fiber structures, a layer thickness of the carbon layer can range from <NUM> to <NUM>.

In any of the above fiber structures, a layer thickness of the second boron nitride layer can range from <NUM> to <NUM>.

In any of the above fiber structures, a layer thickness of the second silicon-doped boron nitride layer can range from <NUM> to <NUM>.

In any of the above fiber structures, each of the first boron nitride layer, the first silicon-doped boron nitride layer, the carbon layer, the second boron nitride layer, and the second silicon-doped boron nitride layer can have a root mean square roughness less than <NUM>.

In any of the above fiber structures, the fiber can be formed from silicon carbide.

A ceramic matrix composite can include a plurality of any of the above fiber structures, and a silicon carbide matrix formed upon the interface coating arrangement of the plurality of fiber structures.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:.

In the above method, each of the first boron nitride layer, the first silicon-doped boron nitride layer, the carbon layer, the second boron nitride layer, and the second silicon-doped boron nitride layer can be deposited using chemical vapor infiltration.

In any of the above methods, the silicon content of the first silicon-doped boron nitride layer can range from <NUM> wt% to <NUM> wt%, and the silicon content of the second silicon-doped boron nitride layer can range from <NUM> wt% to <NUM> wt%.

In any of the above methods, the silicon content of the first silicon-doped boron nitride layer can be <NUM> wt%, and the silicon content of the second silicon-doped boron nitride layer can be <NUM> wt%.

In any of the above methods, a layer thickness of each of the first boron nitride layer and the second boron nitride layer can range from <NUM> to <NUM>.

In any of the above methods, a layer thickness of each of the first silicon-doped boron nitride layer and the second silicon-doped boron nitride layer can range from <NUM> to <NUM>.

In any of the above methods, a layer thickness of the carbon layer can range from <NUM> to <NUM>.

In any of the above methods, the silicon carbide matrix can be deposited using chemical vapor infiltration.

Claim 1:
A coated fiber structure for use in a ceramic matrix composite, the coated fiber structure comprising:
a fiber extending along a fiber axis; and
an interface coating arrangement applied to and circumscribing the fiber, the interface coating arrangement comprising:
a first boron nitride layer extending coaxially about and in direct contact with the fiber;
a first silicon-doped boron nitride layer extending coaxially about and in direct contact with the first boron nitride layer;
a carbon layer extending coaxially about and in direct contact with the first silicon-doped boron nitride layer;
a second boron nitride layer extending coaxially about and in direct contact with the carbon layer; and
a second silicon-doped boron nitride layer extending coaxially about and in direct contact with the second boron nitride layer;
wherein a silicon content of the first silicon-doped boron nitride layer is higher than the silicon content of the second silicon-doped boron nitride layer.