Yttria-Fully Stabilized Zirconia Environmental Barrier Coatings for All-Oxide Ceramic Matrix Composites Fabricated by RF-Magnetron Sputtering

Abstract

All-oxide ceramic matrix composites are developed at DLR for various high-temperature applications, with special focus on thermal protection systems for advanced gas turbines. These alumina/mullite-based composites (“WHIPOX”, wound highly porous oxide) are inherently oxidation resistant, however high-temperature corrosion by combustion products, mainly water vapor and molten salts, must be prevented by employing additional environmental barrier coatings (EBCs). Zirconia (ZrO2) exhibits a superior stability against such high-temperature corrosion as compared to most refractory oxides. Therefore, in conjunction with its thermodynamic stability to alumina and mullite, zirconia is one of the most promising EBC materials for WHIPOX-type CMCs. Yttria-fully stabilized zirconia (Y-FSZ) coatings were deposited on WHIPOX-type CMCs by radio-frequency magnetron sputtering. As-sputtered Y-FSZ coatings are characterized by very fine-grained, narrow-standing columns which are the result of a high nucleation density in the early stages of deposition and a low deposition velocity. Upon subsequent heat-treatment, this microstructure is transformed to a homogeneous and virtually dense Y-FSZ layer. For successful deposition of thin and uniform FSZ-coatings on WHIPOX-type CMCs the application of suitable bond-coats is mandatory. Performance of magnetron-sputtered Y-FSZ EBCs in combustion environments is discussed in comparison to zirconia coatings produced by thermal spraying and electron-beam physical vapour deposition.