Refractory materials which possess the ability to withstand high temperatures form important articles of commerce which are useful in many instances. The refractory materials, due to their hardness as well as resistance to chemical and oxidative processes, are useful as coatings on various substrates, for example protective coatings of silicon carbide, boron carbide, etc. On substrates such as steel, they may be used for rocket nozzles or in places where resistance to abrasion is important. Other uses for this type of material are found in gas turbines or other high temperature applications. In the past, the refractory materials have been deposited on substrates which are of relatively low surface area. In addition, the refractory materials which have been deposited on the substrate material are only deposited on the surface thereof, little or no deposition of the refractory material occurring within the microporous structure of the support. For example, U.S. Pat. No. 3,900,540 discloses a process for preparing a film of a refractory material comprising a substantially defect-free pyrolytic graphite. The pyrolytic graphite is deposited as a film on a substrate by introducing a pre-mixed gas comprising a mixture of a hydrocarbon gas and a diluent gas at a distance above the surface of the liquid substrate and when the hydrocarbon gas contacts the surface, it will decompose to form a pyrolytic graphite film on the surface of the substrate. Another U.S. Patent, namely, U.S. Pat. No. 4,271,235, discloses a method for preparing a polycrystalline silicon. Again, this patent teaches a surface coating comprising a solid substrate body having a base coating disposed on the surface of the substrate body, a layer of carbon disposed on the face of the base coating, and a polycrystalline silicon disposed over the carbon face. Under the conditions which are utilized to prepare these coatings, the resultant composite would contain, as hereinbefore set forth, a polycrystalline silicon or pyrolytic graphite only on the surface of the substrate. For example, U.S. Pat. No. 3,900,540 teaches that the deposition of the pyrolytic graphite source is effected at temperatures ranging from about 900.degree. to about 2100.degree. C., while when using other refractory materials to form a thin film such as boron, a temperature ranging from 600.degree. to about 1500.degree. C. is employed. However, the patent teaches that a limitation on the type of thin film refractory material that can be made by this method is that neither the reactants used to make the refractory material nor the product itself reacts with pyrolytic graphite. This statement appears to be in conflict with a statement set forth in a later portion of the specification that two distinct films of boron carbide and pyrolytic carbide have been discovered which are separated from the substrate surface and from each other.
As will hereinafter be shown in greater detail, it has now been discovered that refractory materials which possess a high degree of resistance to heat, oxidation, etc. may be deposited uniformly as a coating on the internal surfaces of microporous substrates which possess relatively high surface areas whereby the shape and dimensions as well as a substantial portion of the pore structure of the substrate material may be replicated.