Microstructure and thermal & mechanical properties of La2Zr2O7@YSZ composite ceramic

Abstract La2Zr2O7 used as a top coat material has low thermal conductivity and high stability at high temperature, but it also has a low fracture toughness, which limits its application. To improve the fracture toughness of La2Zr2O7, a La2Zr2O7@YSZ core–shell structured composite ceramic was designed and prepared. The morphology of the La2Zr2O7@YSZ composite ceramic was investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The resulting images show that the YSZ is coated on the surface of the La2Zr2O7. The phases were analyzed by X-ray diffraction (XRD), and the XRD patterns show that pyrochlore and fluorite structures coexist in the La2Zr2O7@YSZ composite material without any chemical reaction. Differential scanning calorimetry (DSC) was used to detect the heat change of the composite ceramic during heat treatment. The properties of the La2Zr2O7@YSZ composite ceramic, such as the thermal conductivity, coefficient of thermal expansion (CTE), and mechanical properties were investigated using a laser flash method, high-temperature dilatometer, and nano-hardness tests, respectively. The thermal conductivity of the composite ceramic is in the range of 1.7745–2.3076 W m−1 K-1 in the temperature regime of 200–1000 °C. The maximum CTE of the composite ceramic is 10.3 × 10−6/°C. Owing to the thin YSZ coating on the La2Zr2O7 surface, the hardness and Young's modulus of the composite ceramic are 8.17 GPa and 168.3 GPa, respectively. The nucleation and propagation of micro-cracks are investigated using a micro-hardness tester. Compared to La2Zr2O7, the micro-cracks in the composite ceramic are shorter and more tortuous. The weak interface between the YSZ and La2Zr2O7 results in the nucleation and tortuous propagation of micro-cracks, which depletes part of the energy and improves the fracture toughness of the composite ceramic. The results reveal that the La2Zr2O7@YSZ composite ceramic has good mechanical and thermophysical properties.