Performance Optimization And Failure Behavior Of C/Mullite Composites From The Sol With High Solid Content

With a series of excellent properties such as low density,high temperature resistance,good shock resistance and natural oxidation resistance,mullite ceramics has a very broad application potential in the realm of high temperature thermal structure materials.However,the mechanical properties are not satisfactory,especially the low fracture toughness(2～3 MPa·m^1/2),which make it prone to catastrophic failure when subjected to force/thermal shock loading,limiting it wide application in the realm of high temperature thermal structure materials.In order to improve the toughness of mullite ceramics,according to the existing problems of research team,the study of fabrication,performance optimization and failure behavior of carbon fiber reinforced mullite（C/Mullite）composites from the Al₂O₃-SiO₂ sol with high solid content were performed by using the sol impregnation-drying-heat treatment（SIDH）process as the technical route in this paper.On the one hand,by optimizing the physicochemical properties of Al₂O₃-SiO₂ sol and the interfacial structure of the composites,improve the comprehensive performance of C/Mullite composites,on the other hand,the failure behavior of C/Mullite composites in different environments were systematically investigated.Additionally,the performance of the composites was optimized furtherly according to the understanding of failure mechanism.The details were concluded as follows:The physicochemical properties of Al₂O₃-SiO₂ sol were optimized and the C-Si C interfacial coating was introduced in the composites.In addition,the densification process of the C/Mullite composites was systematically investigated.The mullite grew on the surface of carbon fiber firstly,then grew from inside to outside of carbon fiber bundle during fabrication process.Finally,it grew on the surface of the composites and formed a“mullite coating”.Porosity of the composites decreased gradually and pores of the composites transformed from connected ones into isolated ones as the fabrication cycles increased.When the fabricated cycles of the composites exceeded 24,the microstructure and properties of the composites remain unchanged.At this moment,the flexural strength of the composites（318.4±16.1 MPa）was comparable to that of dense monolithic mullite ceramic,and the fracture toughness(13.6±2.6 MPa·m^1/2)was 4～6 times that of monolithic mullite ceramic.Furthermore,the room temperature mechanical properties and the thermal physical properties of the composites were systematically investigated.The failure behavior of C/Mullite composites in non-oxidizing（Ar and vacuum）environments was investigated,and the high-temperature stability was optimized based on the failure mechanism of the composites.The main factors of performance degradation of the composites in Ar（vacuum）environment were carbothermal reduction between carbon fibers/interfacial coating and residual SiO₂ in the mullite matrix,and the thermal stress from shrinkage of the mullite matrix.The activation temperature of carbothermal reduction of the composites were about 1500oC in Ar and 1400oC in vacuum,respectively.In fact,it is hard to enhance the level of mullitization of the Al₂O₃-SiO₂ sol at present.By optimizing the sintering characteristics of Al₂O₃-SiO₂ sol,the sintering shrinkage of Al₂O₃-SiO₂ gel was reduced,which effectively reduces the thermal stress of the composites during heat-treatment.Thus,the strength retention,modulus retention and fracture work retention of the composites were increased by 33.7%,22.5%and 51.9%after heat treatment at 1500oC in Ar,and the strength retention,modulus retention and fracture work retention of the composites were increased by 16.9%,17.0%and 53.8%after heat treatment at 1400oC in vacuum,respectively.The failure behavior of C/Mullite composites in oxidizing environments was investigated,and the oxidation resistance was optimized based on the failure mechanism of the composites.The oxidation failure mechanism of the composites at1300oC to 1500oC in air favors a diffusion-controlled,where the weight loss and mechanical property degradation of the composites after oxidation was determined by the oxidation level of the carbon fibers and the interfacial coating.The use of silica-rich（the atoms ratio of Al/Si was 1:2）Al₂O₃-SiO₂ sol at the later stage of fabrication can effectively improve the oxidation resistance of the composites,the strength retention and modulus retention of the composites were increased by 28.1%and 39.7%after oxidation at 1500oC in air,respectively.In addition,the properties evolution and failure behavior of the composites were investigated after oxidation at1300oC in air,water-oxygen and after thermal shock at 1300oC to 1500oC in air.The high temperature mechanical and fatigue properties of C/Mullite composites were characterized in real time.Since the thermal stress in the composites was effectively released in the high temperature environment,the carbon fibers could better exert their own excellent mechanical properties,which led to the good high temperature in-situ mechanical properties of the composites at 1200oC to 1400oC in air environment.At 1500oC,the fracture toughness of the composites significantly deteriorated due to the oxidation of the composites.The composites showed favorable high temperature fatigue behavior at 1200oC in vacuum.When the stress level did not exceed 75%,the composites did not fail after 10⁵ cycles fatigue loads.In addition,the composites showed no performance degradation after 10⁵ cycles fatigue loads at the stress level of 70%.