Thermal Shock Performance Of ZrB₂-SiC-based Ultra High Temperature Ceramics

ZrB₂-based ultra-high-temperature ceramics are a great potential material in high-temperature structure areas due to their excellent properties.Howerer,the relatively poor thermal shock resistance and damage tolerance of these materials limit their application for engineering.To overcome the issues mentioned above,the researchers have contributed to much attention for improving the thermal shock resistance,and have achieved a certain amount of research results.The traditional methods of assessing the thermal shock resistance of materials are based on rectangular samples（3mm×4mm×36mm in high,width and length,respectively）.However,this kind of shape specimens are easily produce stress concentration at edage of the specimens,which effects the result of the thermal shock resistance.To minimize the effects of stress concentration on the material,cylindrical specimen（4 mm in diameter,length of 34 mm）is a good choice.Besides,the effects of shape and size of the speciments on thermal shock resistance were also investigated.Firstly,the effects of shape,i.e,cylindrical and rectangular speciment,and size（4mm,6mm,10 mm in diameter）on thermal shock resistance for Zr B 2-20 vol.% SiC（ZS）ceramics were investigated.Using cylindrical speciments not only the thermal shock fracture resistance was significantly improved,but also the thermal shock damage resisitance.Moreover,the thermal shock risistance of ZS ceramic was decreased with the increase diameter of the speciments.In addition,the effect of pre-oxidation temperature on the mechanical and thermal sh ock resistance was also conducted.The results point that the initial strength and thermal shock resistance are greatly increased after pre-oxidation.The surface of specimen were analyzed by Scanning Electronic Microscopy（SEM）.Secondly,the effects of shape and size on thermal shock resistance for ZrB₂-20 vol.%SiC-15 vol.%G（ZS15G）and ZrB₂-20 vol.%SiC-20 vol.%G（ZS20G）ceramics were also investigated.The critical themperature of thermal shock was increased due to the cylindrical speciments reduced the stress concentration.Besides,the size effect has also have a great influence on the thermal shock for the ZS15 G ceramic.In addition,the directions of hot pressing also have effect on the thermal shock resistance of ZS15 G ceramicsdue to the anisotropic of mec hanical properties and thermal physical properties.In this paper,the effects of pre-oxidation temperature on thermal shock resistance of ZS15 G and ZS20 G ceramics were studied.Increasing the pre-oxidation temperature,the initial strength and thermal shock resistance are improved.Specifically,the mechanical properties and thermal shock resistance of ZS20 G ceramics increased when the pre-oxidation time ranged from 30 to 60 min at 1400℃.Finally,the distribution of thermal field and stress field which ca used by the shape or size of ZS and ZSG ceramics during the process of thermal shock was calculated by finite element methods.Not only the distribution and the value of sress of the different shape and size were conducted,but also the effect of the directions of hot pressing on thermal shock resistance.The calculated values are consistent with the experimental results.