Carboborosilicide and oxide composite coatings on carbon materials

Abstract

kii, and N. S. Poltavtsev UDC 621.793.8 Complex carboborosilicide and oxide coatings on carbon materials prepared by vacuum activated diffusion impregnation predominantly through a liquid phase, vacuum fusion, and enamelling are studied. It is established that coating formation from the vapor phase hardly reduces the original porosity of carbon materials. The complex protective coatings developed that are formed by vacuum activated diffusion impregnation through a liquid phase lead to a reduction in the original porosity and gas permeability of carbon materials thereby increasing their high-temperature strength and heat resistance at elevated temperatures. It is shown that higher heat resistance in the temperature range 1800-2000°C is provided by multilayer coatings based on carbides and borides of silicon, titanium and zirconium, borosilicides of tungsten, hafnium, molybdenum, and also oxides of silicon, aluminum, zirconium, and hafnium. It is established that the protective properties of coatings depend markedly both on their composition, structure, preparation method and on the form of the original carbon materials.Carbon-carbon composite materials (CCCM) that exhibit the operating characteristics required for hightemperature structural materials are attracting greater attention of the developers of new machines and mechanisms in view of the possibility of expanding the field of their practical application. Currently CCCM are used in space rocket technology, in particular for preparing the nozzles of rocket engines and the edges of wings of equipment for repeated use. Their high heat-resistant properties are used extensively in high-temperature technology, i.e. in preparing heaters, support assemblies for high-temperature furnaces, equipment for compaction and other mechanisms. The considerable volume of CCCM used relates the preparation of brake disks for aircrafts. The required friction coefficient, comparatively low weight and a capacity to withstand large loads without melting and seizing play an important role in this case [1-3]. However, apart from good physicomechanical properties CCCM have comparatively low corrosion resistance at above 500°C and high porosity that limits expansion of the use of these materials in oxidizing atmospheres. Therefore a requirement arises for protecting carbon materials from high-temperature gas corrosion and reducing their porosity upon which to a considerable extent all of the physicochemical and mechanical properties of CCCM depend. Solution of this problem has been the subject of much work and questionable results have been obtained although currently it remains important. This is explained by the fact that protection of carbon composites from oxidation by applying anticorrosion coatings to them is connected with overcoming difficulties caused by the considerable porosity of carbon materials, the high volatility of their oxides, the nonuniform structure, low linear thermal expansion coefficient, and the anisotropy of physicomechanical properties [4,5]. We note that ...

Complex Protective Coatings for Graphite and Carbon-Carbon Composite Materials

“…The corrosion resistance of complex carbon-boron-silicide and oxide coatings on carbon-based materials in air at temperatures from 1773 to 2173 K has been investigated in [18] [19]. It should be noted once more feature of C/C composites that complicates the deposition of efficient protective coatings, i.e.…”