Growth behavior and microstructure of oxide scales grown on WSi2 coating

Abstract

Growth behavior and microstructure of oxide scales formed on WSi2 coating by isothermal oxidation between 800 and 1300 degrees C were investigated. At and below 1000 degrees C, lamellar WO3 embedded in the amorphous SiO2 matrix was formed from the direct oxidation of WSi2 by inward diffusion of oxygen through short-circuit paths. At the intermediate temperatures between 1100 and 1200 degrees C, fastest oxidation rate was observed. The oxide scale consisted of the spheroidal WO3 particles embedded in the amorphous SiO2 matrix. WSi2 was initially oxidized to form W5Si3 and SiO2 followed by the oxidation of W5Si3 to form WO3 and SiO2 phases. At 1300 degrees C, oxide scale consisted of continuous cristobalite SiO2 and W5Si3 layer underneath, leading to the slowest isothermal oxidation behavior. It is concluded that the interplay of phase stability of WSi2, defect formation process, and viscosity of SiO2 gives rise to unique and complex oxidation behavior especially at intermediate temperatures. (C) 2007 Elsevier Ltd. All rights reserved.