Liquid precursor method
The liquid precursor method involves impregnating the porous CC composite substrate with organometallics, metal alkoxides, metal salt solutions, or colloidal suspensions. Drying, gelation, or chemical conversion produces solid layers on internal surfaces that can then be decomposed or reacted by heating to form ceramic coatings. Multiple impregnation and heat treatment cycles are needed to produce continuous coatings of significant thickness because of the normally low yield of the precursors. This method has been the subject of numerous patents for improving the oxidation resistance of porous carbon bodies in general and recently was applied to CC composites. The recent CC work has shown that by virtue of B2O3 glass formation, internal coatings rich in boron are particularly effective in enhancing oxidation performance. As described earlier, the liquid precursor method is used to coat carbon fibers and can be effective either before or after the fibers have been configured into a composite.
Matrix chemical modifications
Chemical modifications are made to the carbon matrix either by adding the elements or compounds as a powder to the resin or pitch matrix precursor, or by altering the chemistry of the matrix precursor on the molecular level. The powder method was originally developed to improve the oxidation resistance of synthetic graphites. Chemical modifications are most effective and produce coherent coatings only when a low-viscosity, wetting glass is formed initially or when exposed to oxygen. This modification is illustrated in figure 12, which shows a region of borate glass formed by the oxidation of powders that were added to the matrix of a CC composite. The glass was produced near the surface of the composite because of oxygen permeation through a crack in the external CVD SiC coating. The glass forms at the expense of matrix oxidation, then coats fibers, fill pores, and partitions off regions of attack to prevent rapid catastrophic oxidation. Glass that originates in the composite directly below the external coating can also migrate to fill the CTE mismatch cracks in the coating. Chemical modification by glass-former powder additions is currently the most widely used method for forming internal coatings.
Chemical vapor deposition
The CVD process within a porous body is termed chemical vapor infiltration (CVI). This process involves the diffusion of gases into the body and the decomposition or reaction of the gases in order to produce a solid product that is deposited on the pore walls and fibers. Chemical vapor infiltration is one of the principal densification techniques used in the fabrication of CC composites. It can also be used to chemically modify the carbon matrix and to produce discrete internal coatings on the carbon constituents. Unless the carbon fibers are previously coated for protection, the CVI deposit must be a nonoxide to prevent fiber degradation during processing.
Improvements in the oxidation resistance of carbon fiber composites were demonstrated when the carbon matrix was replaced almost entirely with CVI matrix materials such as SiC, TiC, and BN. From the standpoint of oxidation resistance, it clearly is logical to fully densify with the ceramic if the composite alterations resulting from the replacement of the carbon matrix with a ceramic can be accommodated. Chemical vapor deposition is previously described as a useful fiber coating method; CVI densification actually occurs by the buildup of coating on the fibers. This type of composite was exhibited excellent short-term oxidation resistance at temperatures up to 1500C. Over longer periods of time, oxygen permeation through pores and cracks in the matrix results in significant oxidation of the fibers, which demonstrates the need for effective external coatings.