The corrosion of ceramic-matrix composites

Ceramic matrix composites could replace existing metals and alloys in aircraft, naval engine parts or heat exchanged systems because of their low density and high thermostability. These composites are promising materials for long-life applications if the metastable state of the composite is preserved during the synthesis and on working atmospheres without deletorious evolution of the fibre, the matrix and of the fibre-matrix interface. The review begins with a brief recall on the corrosion of ceramics able to be used in composites (SiC, Si3N4, AlN, BN, aluminosilicates, C). The main sources of corrosion in combustion environment (proton, sodium ion) are discussed. A comparison is made with long-term corrosion at room-temperature. Examples of the different corrosion mechanisms observed for Nicalon NLM202 fibre glass-ceramic or ceramic (LAS, CAS, celsian, cordierite, Al2TiO5, mullite, Nasicon) matrix composites developed at ONERA are presented : ion exchange, grain boundary dissolution, fluxing, favourable and unfavourable fibre-matrix reaction, enhanced corrosion by prior fibre-matrix reaction. (orig.)