Thermal Shock Failure Of ZrB₂ -SiC Based Ultra High Temperature Cereamic

As boride based ultra high temperature ceramic (UHTC) with high temperature resistance, eroded resistance, mechanical and chemical stability under high temperature, it becomes one of the most prospective materials for application in spaceflight, ultrasonic aircraft, and propulsion system of rocket. However, as brittleness of the material, catastrophic damage may be caused by thermal shock. It is necessary to improve thermal shock properties of UHTC, and characterize thermal shock properties of UHTC properly. Under this background, in this work, several aspects of problem for thermal shock are investigated as following:It can improve thermal shock property of the material by improving fracture property of the material. Therefore, this work started from factors affecting fracture property, and analyzed the effect of residual stress, microstructure, strain rate and stress concentration on fracture property of material, and it may provide help for improving thermal shock property of material. Result shows that thermal expansion of ZrB₂ grain can not be ignored, and grain boundary around abnormal large size ZrB₂ grain is easy to be damaged and even failure. Therefore, it is significant to control the size of ZrB₂ grain and uniformity of grain size distribution. Flake shape of graphite can be formed by adding C into UHTC, and it can improve crack resistance of UHTC. Crack resistance can also be improved by large size of ZrBB₂ grain, but the degree of the latter is relatively lower. By investigation of three-point bending experiment, it can be found that material strength is different with different strain rate, namely, strain rate has contributions to bending stress, and the stress values contributed by strain rate increases with increasing strain rate. By three-point bending experiment of UHTC with notch, it may decrease sensitivity of notch for UHTC with adding C, so it may decrease stress concentration status at the position of shape transition of parts.Thermal shock property of UHTC was investigated by quenching test, and two factors of microstructure and quenching environment affecting thermal shock property of UHTC were investigated. Result shows that as flake shape graphite can improve crack resistance of material, residual strength of the material with flake shape graphite is relatively higher. Material strength can be improved by decreasing grain size, so critical temperature difference was improved by decreasing grain size. However, as limited by process, material strength distribution of UHTC with smaller grain size was large, and stability of mechanical property for the material was worse. Density of surface crack is not the main factor affecting residual strength, and crack depth is in a stable range after critical quenching difference, so crack depth is the main factor. Heat transfer coefficients are different for UHTC with different quenching medium, so thermal shock properties of the same material are different when quenched into different medium. It was proved by finite element method that heat transfer coefficients and quenching temperature difference affect material failure by affecting surface heat transfer velocity. Namely, surface heat transfer velocity was the natural factor for controlling thermal shock failure.Thermal shock model considering dynamical behavior and crack propagation model were established. Crack depth of ZrB₂+20%SiC+5%AlN after quenching test was predicted by crack propagation model. Results show that dynamical thermal stress is much higher than quasi-static one, so effect of dynamical stress should be considered during quenching test. For non-penetrative surface crack, crack depth after quenching test was c<0.6H (H=1.5mm, half thickness of specimen), and it was the same with quenching test (0.2H2+20%SiC and ZrB₂B+20%SiC+10%C were characterized by the new equipment. Results show that heating up rate at local place may reach 500℃/s, and it can be realized for thermal shock at local place. It was implied by calculation that thermal stress caused by thermal expansion mismatch of ZrB₂ and SiC was much higher than that caused by temperature gradient in the experiment, but failure crack direction was controlled by the latter. As higher crack resistance of ZrB₂+20%SiC+10%C, the holding percentage of residual strength of the material may remain higher after one or several thermal shock experiment. Therefore, it is significant for the credibility of UHTC applied under thermal shock conditions to improve material fabrication process to decrease residual stress among matrix and second phase and to improve crack resistance.