Interface engineering in mullite fiber/mullite matrix composites

Mullite fiber/mullite matrix composites are attractive because of their inherent oxidation resistance at high temperatures. Mullite has better creep resistance than alumina. However, chemical interactions between oxides are often very severe; with the result no gain is made over monolithic mullite in terms of toughness. Even in the absence of chemical bonding, a strong mechanical bond component may be present. This originates from radial compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. Thus, the microstructure and behavior of the interface region are the key factors in obtaining an effective control of damage in composites and enhancement of toughness. This body of work on mullite/mullite composites shows the feasibility of producing fully dense, tough oxide/oxide composites by interface engineering. Coatings such as BN alone or SiC/BN double coating function effectively for mullite fiber/mullite matrix composites in that they provide a nonbrittle fracture and increased work of fracture at room temperature. It would appear that for use at high temperatures in air, one needs to identify structural analogs of BN among oxides.