Processing, Properties and Evaluation of Environmental Barrier Coatings for Oxide/Oxide Ceramic Matrix Composites

Abstract

Oxide/oxide ceramic matrix composites (CMCs) are key engineering materials for high-temperature applications in oxidizing atmospheres, e.g. for next generation gas turbine combustor walls. To date commercially manufactured oxide/oxide CMCs consist of alumina and mullite fibers and matrices. However, alumina and mullite are prone to high-temperature corrosion caused by hydroxylation and subsequent decomposition in water vapour rich combustion gases. The application of environmental barrier coatings (EBCs) is considered the only way to overcome the corrosion problem and hence increase lifetime of oxide/oxide combustor walls to an acceptable level. At DLR, the development of environmental barrier coatings for porous WHIPOX-type (wound highly porous oxide) CMCs has been a focused research topic over the past years. EBC materials must exhibit superior corrosion stability with respect to alumina and mullite. But also compatibility between EBC and CMC substrate requires a careful selection of EBC materials. Environmental barrier coatings have been deposited on WHIPOX-type CMC substrates by a variety of state-of-the-art deposition techniques including electron beam physical vapor deposition (EB-PVD), vacuum plasma spraying, magnetron sputtering and chemical vapor deposition. Additionally, low-cost coating of CMCs has been performed using conventional ceramic sintering technologies. A review of DLR’s recent R&D activities will be presented with focus on EB-PVD coated model combustor wall tiles currently being evaluated in a high pressure combustor cooling rig.