Slurry-based Protective Coatings for Oxide/Oxide Composites

Abstract

Ceramic matrix composites based on oxide fibers and porous oxide matrices are attractive materials for various applications in harsh environments. State-of-the art all-oxide CMC are based on alumina or mullite fibers and porous matrices. A key issue is the relative low mechanical stability of the porous oxide matrices, therefore CMC surfaces are prone to selective erosion in high-velocity gas stream. Moreover, thermo-chemical degradation of the constituents SiO2 and Al2O3 is associated with hydroxylation in hot H2O-rich atmospheres and subsequent volatilization of Si- or Al-hydroxide species. Degradation of CMC can effectively be reduced by protective coatings. Beyond classical coating techniques such as thermal spraying, a simple process based on deposition of slurry based coatings and subsequent consolidation by sintering is attractive for cost-efficient fabrication of CMC components. A serious drawback, however, is the low thermal stability of state-of-the art oxide fibers which requires short-term sintering of slurry-based coatings at temperatures not exceeding about 1300°C. This can be achieved by means of transient phases which bond particles and substrate at moderate temperatures. Since transient phases are reactive constituents, no residual non-crystalline phases affecting thermal stability are present after processing. Due to the rigid CMC substrates sintering of coatings is constrained, therefore excess volume shrinkage during processing must be avoided. As a consequence, slurry based coatings for all-oxide CMC exhibit generally a relative high porosity. Examples presented include slurry based coatings consisting of Al2O3, SiO2, Y2O3, and ZrO2.