Oxidation resistance and microstructure evolution of ZrB2-SiC-La2O3/SiC dual-layer coating on siliconized graphite at 1800 °C under low air pressures

The oxidation behaviors of a ZrB2-SiC-La2O3/SiC dual-layer coating on siliconized graphite at 1800 degrees C under low air pressures (50, 5 and 0.5 kPa) were investigated. The results showed that with the decrease of air pressure, the oxidation kinetics of the coated samples changed from parabolic weight gain to linear weight loss. A protective oxide scale consisted of ZrO2 and SiO2 with La dispersed was formed on the coating surface after oxidation in 50 kPa air. The oxide scale formed in 5 kPa air was full of bubbles. Only porous ZrO2 layer was left on the coating surface after oxidation in 0.5 kPa air. At 1800 degrees C, the active oxidation of SiC occurred and gaseous SiO formed at the coating/oxide interface. The surface volatilization of SiO became severe with the decrease of air pressure, resulting in the presence of non-protective oxide scale.