Study On Mechanical Properties And Thermal Response Behavior Of ZrB₂ Based Ultra High Temperature Ceramics

The ultra-high temperature ceramics have become an important candidate for the thermal protection system of hypersonic vehicles due to their excellent high temperature comprehensive properties.Based on the analysis of the ultra-high temperature ceramics at domestic and abroad,the microstructure,mechanical properties and mechanical behaviors of ZrB₂ based UHTCs are simulated.The oxidation mechanism and failure behavior of ZrB₂ based UHTCs are explored,aiming at predicting the macro strength properties and the failure state of ablation of the ceramics.The mechanical behavior,thermal response and ablation behavior of ZrB₂based UHTCs were analyzed by finite element simulation,mechanical properties analysis and wind tunnel test.It provides theoretical and technical support for engineering application of ZrB₂ based ultra-high temperature ceramics.Pure phase ZrB₂ ultrahigh temperature ceramics,ZrB₂-20vol.%SiC multiphase ceramics and ZrB₂-SiC-G ceramics were prepared by ball milling and hot-pressing sintering.By analyzing the mechanical properties and fracture modes of ZrB₂ceramics,ZrB₂-20vol.%Si C multiphase ceramics and ZrB₂-20vol.%Si C-G ceramics,it is found that the fracture of the former two ceramics belong to typical brittle fracture,while the relative density of ZrB₂-20vol.%Si C ceramics is 99.8%,and the bending strength increases from 480MPa to 836±38MPa.The relative density of ZrB₂-20vol.%Si C-15vol.%G ceramic is 99.7%,and its bending strength is about 606MPa.The introduction of graphite can significantly alleviate the thermal residual stress,and the critical thermal shock temperature difference of ZrB₂-Si C is also increased from 300℃to 400℃,indicating that the addition of Si C ceramics and graphite soft phase can improve the thermal shock resistance and fracture toughness of ZrB₂ ultra-high temperature ceramics.The micro model of ZrB₂ based ultra-high temperature ceramics is established.On this basis,the model is solved by finite element method.The model can predict the micro scale performance calculation and analysis of ZrB₂ based ultra-high temperature ceramics,as well as the macro scale three-point bending and thermal shock properties.It can directly describe the macro scale three-point bending and thermal shock properties and analyze the influence of different hot-pressing directions on thermal shock properties.The thermal shock resistance is greatly improved by adding graphite sheet as reinforcement phase,and the advantage of thermal shock resistance is more prominent in high temperature differential thermal shock simulation.The simulated bending strength values are in good agreement with the experimental values,and the comparison errors are within 10%.This paper focuses on the study of macro morphology,thermal response and ablation resistance properties of ZrB₂ based ultra-high temperature ceramics under different conditions in wind tunnel test.The results show that the ablation behavior of ZrB₂ based ultra-high temperature ceramics mainly depends on the composition,component content and test conditions.At the same time,many factors such as thermal conductivity,volume specific heat capacity,catalytic efficiency,emissivity and oxidation characteristics play an important role in the thermal response of ZrB₂based ultra-high temperature ceramics.In addition,the surface temperature evolution and maximum surface temperature of ZrB₂ based ultra-high temperature ceramics are closely related to environmental parameters,oxidation reaction mode（i.e.active and/or passive oxidation）and physicochemical properties of oxidation products.ZrB₂based ultra-high temperature ceramics show good ablation resistance behavior compared with C/Si C materials under the conditions of heat flux 7.05MW/m²,gas flow enthalpy 27.7MJ/kg and stagnation point pressure 5.0KPa.The oxide layer formed in situ below 2300℃of ZrB₂ based ultra-high temperature ceramic material has good physicochemical stability.However,under the extremely high temperature environment for a long time（i.e.>2800℃）,the original spherical structure of ZrB₂based ultra-high temperature ceramics has changed into a flat shape,resulting in a significant increase in the radius of the sample and loss of its oxidant resistance protection.