Effect of HfO2 addition on the high-temperature oxidation behavior of Si-HfO2-based composites at 1300 ° C and 1400 ° C

Si-HfO2 composite has been proposed to be a novel bond coat material in environmental barrier coatings (EBCs) used at ultra-high temperatures. However, the oxidation behavior of Si-HfO2 especially the effect of HfO2 is still in debate. In this study, four Si-HfO2 materials with different Si-to-HfO2 ratios were designed and prepared by spark plasma sintering (SPS) method. The effect of HfO2 on high-temperature oxidation behavior of the Si-HfO2 composites at 1300 degrees C and 1400 degrees C was investigated. The results indicated that the as-prepared Si-HfO2 composites by SPS contained Si and HfO2 two phases, preventing the formation of SiO2 during preparation. After 100 h of isothermal oxidation at 1300 degrees C, a protective oxide layer composed of SiO2, HfO2 and HfSiO4 was formed on the surface and an intermediate product HfSi2 phase was observed due to the oxygen-deficient state under the oxide layer. While after 100 h of oxidation at 1400 degrees C, globular Si precipitated on the surface and HfO2 was found surrounding it. More HfO2 and reaction product HfSiO4 formed a skeleton structure which played a crucial role in the material's thermal stability. The detailed high-temperature oxidation mechanisms of Si-HfO2 composites with various Si-to-HfO2 ratios at 1300 degrees C and 1400 degrees C were finally discussed.