The oxidation of porous carbon composite materials can be inhibited at relatively low temperatures by very thin discontinuous layers of oxygenated boron and phosphorus compounds that block the most active oxidation sites. However, coating internal surfaces and filling pores with coherent diffusion-limiting physical barriers are more effective methods for providing oxidation resistance over a wider temperature range. These internal coatings can be metals, hard ceramics, or glasses. Certain glasses are high in B2O3 or P2O5 wet carbon, and they are also to flow at low temperatures. These characteristics promote the formation of adherent and coherent coatings that can accommodate large strains and remain continuous during thermal cycling and the application of external stresses. Flow and wetting also produce external glass layers when the glasses are present in high concentrations near the surface of the CC composite.
Coatings have been made on the internal surfaces of CC composites by three techniques: liquid precursor impregnation, chemical modification of the carbon matrix, and the CVD. Liquid precursor impregnation is the most chemically comprehensive of the three because with this method a wide range of inorganic coatings can be made. Chemical modifications, however, result in coatings only when a mechanism is present for producing glass layers. Finally, the CVD process is most appropriate for producing nonoxides in the presence of carbon; it can be used to make internal coatings, chemically modify the carbon matrix, or completely replace the matrix.