Reaction-bonded Al2O3 coatings for Al2O3/Al2O3 CMCs

Kurzfassung

Ceramic matrix composites based on Al2O3 fibers and porous Al2O3 matrices are being developed for various applications in harsh environments. Thermo-chemical degradation of Al2O3 is associated with hydroxylation in hot H2O-rich atmospheres and subsequent volatilization of Al-hydroxide species. This kind of degradation can effectively be reduced by recession-resistant environmental barrier coatings (EBC). A key issue for CMC is the relative low mechanical stability of the porous matrices, therefore CMC surfaces are prone to selective erosion in high-velocity gas streams, e.g. during coating by thermal spraying technologies. But also application of CMC without EBC may be limited due to low surface stability. Reaction-bonded Al2O3 coatings can effectively homogenize and stabilize CMC surfaces. Dispersions of Al2O3 and Al are deposited on the surface and fired in air where Al is oxidized and bonds together with Al2O3 to the CMC surface. Such coatings can be used as bond-coatings for thermally sprayed top coatings. On the other hand, stand-alone RBAO coatings promise a simple and cost-effective solution for CMC applications where outstanding H2O-stability, i.e. EBC functionality is less important than e.g. erosion resistance. A key to successful processing of RBAO coatings is the careful selection of raw materials and process parameters adapted to the respective CMC substrate.