Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

Abstract

HfO2 doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO2-Si composite coatings with different HfO2 contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO2 structures. During steam oxidation, HfSiO4 phase was formed at the SiO2/HfO2 interface by solid-state reaction between them. The HfSiO4 or HfO2/HfSiO4 mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO2-Si coating with isolated HfO2 particles. However, the HfO2-Si coating with inter-connected HfO2 framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO2 framework structure played a dominating role in determining the steam oxidation resistance of the HfO2-Si coating, and the connected HfO2 framework and TGO network provided a rapid diffusion path for oxidants (H2O, O2− and OH−) and deteriorated the oxidation resistance.