Thermally Sprayed Y2O3-Al2O3-SiO2 Coatings for High-Temperature Protection of SiC Ceramics

Abstract

The suitability of certain glass compositions in the Y2O3-Al2O3-SiO2 (YAS) system as protecting coatings for silicon carbide components has been prospected. One particular YAS composition was formulated considering its glass formation ability and subsequent crystallization during service. Round-shaped and homogeneous granules of the selected composition were prepared by spray drying the corresponding homogeneous oxide powder mixture. Glassy coatings (197 µm thick) were obtained by oxyacetylene flame spraying the YAS granules over SiC substrates, previously grit blasted and coated with a Si bond layer (56 µm thick). Bulk glass of the same composition was produced by the conventional glass casting method and used as reference material for comparative evaluation of the characteristic glass transition temperatures, crystallization behavior, mechanical, and thermal coating properties. The mechanical properties and thermal conductivity of the coating were lower than those of the bulk glass owing to its lower density, higher porosity, and characteristic lamellar structure. The crystallization of both bulk glass and coating occurred during isothermal treatments in air at 1100-1350 °C. Preliminary data on ablation tests at 900 °C using the oxyacetylene gun indicated that the YAS glassy coating was a viable protective shield for the SiC substrate during 150 s.