C/SiC composites for high temperature structural applications. Part II: Oxidation protection systems

The fact which currently excludes the use of C/SiC composites in high tempe rature structural applications is the high oxidation rate of carbon fibres at temperatures higher than 450 degreesC. In this second part of the paper, a review of the different oxidation protection systems, including inhibito rs, surface modification of composites, coatings and previous infiltration of the substrates, has been carried out. The addition of inhibitors reduces the oxidation rate, but only up to 850 degreesC, while the surface modific ation of composites leads to thin coatings with poor thermal shock resistan ce. On the other hand, the external layers are the most usual method employ ed because allows combining different compositions and thicknesses. The mul tilayer coatings are especially interesting in applications with wide tempe rature range and thermal shocks requirements. The infiltration of substrate porosity improves slightly the oxidation resistance of C/SiC composites re ducing the oxygen accessibility to carbon fibres. However, the infiltration complements very well the oxidation protection performance of a coating sy stem at low temperature, since these normally present open cracks due to mi smatch between coating and substrate thermal expansion coefficients.