Carbon,carbide and silicide coatings

The elements carbon and silicon are the significant constituents of the refractory coating materials discussed in this paper, namely B₄C, pyrocarbon, TiC, SiC, Si₃N₄ and MoSi₂. Because of their hardness and their chemical and oxidation resistance, these materials are of increasing interest as coatings on various substrates. However, their physical and chemical properties limit the number of suitable plating methods. Whilst the CVD technique is the most versatile, the sintering technique is a good alternative, especially if thicker coatings are required.The different coating materials are discussed in detail. The ways in which different coatings can be modified for special applications, e.g. isotropic pyrocarbon coatings for bioengineering purposes, are discussed. Pure silicon carbide and silicon nitride coatings become important as protection against internal oxidation and as barriers in carbon fibre reinforced composites.The problems arising from the simultaneous deposition of two elements to form a specific compound are demonstrated with the examples of SiC, Si₃N₄ and B₄C. The necessity of avoiding chemical reactions with the substrate during deposition is exemplified by the case of TiC coatings on carbon fibres.Finally the technique of preparing metal silicide coatings on refractory metals and carbon by plasma spraying and isostatic hot pressing as protection against oxidation is explained, and the modification of the coatings, in particular to improve thermal cycling behaviour, is discussed.