Thermal shock resistance and failure analysis of plasma-sprayed thick 8YSZ-Al2O3 composite coatings

The effects of Al2O3 content and spraying power on bonding strength and thermal shock resistance of plasma-sprayed 8YSZ-Al2O3 composite coatings with thickness of similar to 900 mu m were investigated. Thermal shock resistance decreased obviously with increasing the Al2O3 content and test temperature, while bonding strength showed an increasing trend. Relatively, the effect of spraying power on thermal shock resistance and bonding strength was small. Moreover, the Young's modulus of coating surface was increased from 99.07GPa to 134.59GPa with increasing Al2O3 content from 10 wt% to 40 wt%, while the corresponding thermal expansion coefficient (CTE) decreased from 9.68 x 10(-6)/degrees C to 7.29 x 10(-6)/degrees C. Thus, residual stresses on the surface of composite coatings changed from tensile (+13.3 MPa) to compressive (-189.0 MPa), which would cause greater stresses gradient as well as strain energy in the direction of coating thickness. During thermal shock testing, thermal stress increased sharply with increasing the test temperature and Al2O3 content. Meanwhile, introduction of Al2O3 resulted in denser microstructure, which was detrimental for stress releasing. According to the results, the horizontal crack propagation in the interface area caused by thermal stress and volume contraction caused by amorphous phase recrystallization were considered to be the main factors for thermal shock failure.