Study On High-temperature Thermal Shock Resistance And Thermal Shock Mechanism Of Alumina Ceramics

Alumina ceramics have excellent characteristics such as high melting point,high mechanical strength,good oxidation resistance,chemical corrosion resistance,and good chemical stability,and are typical representatives of high-temperature structural ceramic materials.For Al₂O₃ structural ceramics used in high-temperature applications,they are often subjected to complex thermal shock environments during their service,which poses a serious challenge to their high-temperature thermal shock resistance.It is of great theoretical importance to study the high-temperature strength and high-temperature thermal shock resistance of Al₂O₃ ceramics and to analyze the thermal shock damage and failure mechanism in complex high-temperature environments.In this work,we investigate the high-temperature thermal shock resistance of Al₂O₃ ceramics at different high-temperature thermal shock temperature intervals and various cooling rates by using compressed air quenching,analyze the effect of surface brittle-to-ductile transition on the high-temperature thermal shock resistance of Al₂O₃ceramics,and illustrate the mechanism of surface brittle-to-ductile transition behavior on the high-temperature thermal shock resistance of Al₂O₃ ceramics.The main researches and results of this work are as follows:（1）The effects of particle size and sintering temperature on the mechanical and thermal properties of Al₂O₃ ceramics were investigated by measuring the mechanical and thermal properties related to thermal shock resistance of Al₂O₃ ceramics.The densities and mechanical properties of Al₂O₃ ceramics increased significantly with the decrease of raw material size and the increase of sintering temperature.The average coefficient of thermal expansion of Al₂O₃ ceramics was 8.16-8.30×10^-6 K^-1,and thermal conductivity of Al₂O₃ ceramics was 26.8-29.4 W/m·K.At a raw material particle size of 0.5μm and a sintering temperature of 1590℃,the Al₂O₃ ceramics exhibited the optimal comprehensive mechanical properties with bending strength,elastic modulus,Vickers hardness,and fracture toughness of 375.7 MPa,381.6 GPa,18.17 GPa,and 3.45 MPa·m^1/2,respectively.（2）The high-temperature thermal shock resistance of Al₂O₃ ceramics was systematically evaluated by different thermal shock temperature intervals and cooling rates.When the thermal shock final temperature was 1000℃,the residual strength of Al₂O₃ ceramics after high-temperature thermal shock was high and reached more than70%of the original strength;while when the thermal shock final temperature was700°C,the retained strength of Al₂O₃ ceramics after high-temperature thermal shock was only about 40%.When the thermal shock temperature difference is 700℃and the cooling rate is 200℃/s,the residual strength rate of Al₂O₃ ceramics is about 40%;as the cooling rate is reduced to 100℃/s,the residual strength of Al₂O₃ ceramics reaches more than 70%of the original strength;and when the cooling rate is further reduced to70℃/s,the strength retention rate of Al₂O₃ ceramics is more than 80%,which exhibits excellent high temperature thermal shock resistance performance.（3）The effect of brittle-to-ductile transition on high-temperature thermal shock resistance of Al₂O₃ ceramics was studied.The high-temperature brittle-to-ductile transition temperature of Al₂O₃ ceramics is around 1000℃.When the thermal shock final temperature is above the brittle-to-ductile transition temperature,Al₂O₃ ceramics maintain high residual strength;while when the thermal shock final temperature decreases below the brittle-to-ductile transition temperature,the residual strength of Al₂O₃ ceramics decreases rapidly.The high residual strength of Al₂O₃ ceramics after high temperature thermal shock is mainly attributed to the surface brittle-to-ductile transition behavior,and its mechanism is the blunting of surface cracks and the stress relaxation of crack tip to reduce the thermal stress.