Protective Coatings for Enhanced Performance of Oxide-Oxide Composites

Kurzfassung

Ceramic matrix composites based on oxide fibers and porous oxide matrices are materials developed for various applications in harsh environments. State-of-the art oxide/oxide CMC are based on alumina or mullite fibers and porous alumina matrices. Thermal-chemical degradation of the fibers and matrices is associated with sintering/grain growth, surface reactions with mineral dusts or molten salts, and hydroxylation/volatilization of Si- or Al-hydroxide species. All mechanisms are typically relevant for applications in combustion atmospheres. Moreover, the relatively low mechanical stability of the porous oxide matrices makes CMC surfaces prone to surface degradation by abrasion. Degradation of CMC can effectively be mitigated by functional and protective oxide coatings which can be deposited e.g. by thermal spraying or in the form of particle dispersions and subsequent consolidation by sintering. Thermal protection as well as improved chemical stability are provided by low-thermal conductivity and environmentally stable coating materials. Additional benefits of ceramic coatings can be improved mechanical stability, reduced gas permeability, or thermo-optical properties for enhanced performance of all-oxide CMC in special applications. On the other hand, typical coating deposition processes are associated with additional thermal loads or surface effects which may significantly alter properties of the CMC substrate. Various types of oxide coatings for oxide/oxide CMC and their specific functionalities are presented. The impact of coatings on the overall performance of coated CMC materials is discussed.