The Effect Of Pre-oxidation On The Mechanical Properties And Thermal Shock Resistance Of ZrB₂-SiC-G Ultra High Temperature Ceramics

As one of non-ablative ultra-high temperature materials, ZrB₂-based ultra-hightemperature ceramics （UHTCs） which have excellent performance such as goodoxidation resistance and low density, have been widely used in nose-cone, wing leadingedge and other key parts of hypersonic vehicle. However, as one kind of ceramicmaterials, its shortcomings of brittleness and low toughness affect the strength andthermal shock resistance of the material. Taking the application environment ofhypersonic vehicle into account, there’s a urgent need to improve the performance ofmaterials.The material used in this study was ZrB₂-SiC-G ultra-high temperature ceramic.As the phenomenon of the different products and surface structures from the oxidationreactions of ZrB₂-based UHTCs at different temperatures for research background,combined with the thermal shock theory, using the test and analysis method ofthree-point bending test, scanning electronic microscope（SEM） and waterquenching-residual strength method, this paper analyses the effect of pre-oxidationtreatment on the mechanical properties and thermal shock resistance of the materials,furthermore, we explore the process condition of pre-oxidation treatment.First of all, we studied the effect of pre-oxidation temperature on the strength ofmaterials. In the pre-oxidation temperature regions of800¹100℃and1200¹500℃,the products of oxidation healing the cracks were respectively B₂O3and SiO2, the bestpre-oxidation temperatures were respectively900℃and1400℃, the strength wasrespectively increased by56MPa and24MPa. Then we studied the effect ofpre-oxidation time （15min²h） on the strength of materials. The samples pre-oxidizedfor1h performed best. Based on the research on the effect of pre-oxidation on themechanical properties, this paper further discussed the effect of pre-oxidation on thethermal shock resistance of materials. We studied the effect of pre-oxidationtemperature （1200¹500℃） on the thermal shock resistance of materials. With thepre-oxidation temperature increased, the thermal shock resistance of materials improved.The critical thermal shock temperature difference of non-pre-oxidative samples was374.9℃, and the ones of samples pre-oxidized respectively increased to538.16℃,538.09℃,575.11℃,659.31℃. We also studied the effect of pre-oxidation time（30min²h） on the thermal shock resistance of materials. With the pre-oxidation timeincreased, the thermal shock resistance of materials improved. At the same time, thetrends of the surface oxidation layer thickness increasing and the thermal shockresistance improving were consistent. The improvement of thermal shock resistance wasdue to the strength improvement and thermal barrier function of the surface oxidation layer. Combined with the study of the pre-oxidation effect on the strength, we can get aconclusion that when the pre-oxidation condition changed, the trends of strength andthermal shock resistance changing were different. The joint action of mechanical andthermal properties influenced the thermal shock resistance of materials. Finally,considering the effect of pre-oxidation treatment on the strength and thermal shockresistance of the materials, the best process condition of pre-oxidation treatment inpractical application is1400℃,1h.