Thermal Conductivity of the ZrO_2 Based Composite Coatings Prepared by Gas Tunnel Type Atmospheric Plasma Spraying

摘要

ZrO_2 based thermal barrier coatings prepared using plasma spraying are frequently modified by mixing some kinds of additives such Al_2O_3, in order to improve their oxygen ion resistance. Using gas tunnel type plasma spraying, characterized by higher plasma temperature and better stability than traditional arc plasma spraying, zirconia based composite coatings with various mixing ratios of alumina were used to generate graded-functional microstructures. Because of the plasma stability, the coating micro-structural parameters such as the thickness, mixing ratio of alumina and porosity were well reproduced by keeping the spraying parameter constant. A group of coating samples was prepared in this research to be almost the same thickness but different composition ratios. The surface and cross-sectional morphologies of the coatings were examined by SEM (Scanning Electron Microscope) and OM (Optical Microscopy). The coating thickness was measured and the alumina content ratio was assessed from the SEM photos, while the coating porosity (the porosity profile over the cross-section and the average porosity) was obtained from the image analysis of the optical photos. The coaling hardness was measured with a micro Vicker hardness tester and the thermal conductivity was measured based on the principle of steady heat flux between two parallel hot plates. The results showed that the addition of alumina increased the coating hardness and lowered the coating porosity, while not reducing the thermal resistance of the coating layers.