Study On Fabrication, Mechanical And Thermal Properties Of Dense Silicon Oxycarbide-based Ceramics

As a kind of widely used amorphous ceramics,SiOC-based ceramics have low preparation cost,excellent high-temperature stability and oxidation resistance.The composites formed with lightweight carbon fiber skeletongs can overcome the disadvantages of traditional ceramics such as high intrinsic brittleness,low toughness and poor damage resistance ability.The low thermal conductivity and excellent oxidation resistance of the SiOC-based ceramics brought by the amorphous structure enable their application in thermal protection system.However,the current researches mainly focus on properties of SiOC-based composites with fibers and braids and porous materials in aerospace application under extreme conditions,investigations on the performance of SiOC-based dense ceramics themselves are scarce.Hence,it is necessary to figure out the mechanical and thermal physical properties of the SiOC-based materials to explore the evolution law of thermal properties at high temperature.In this study,SiOC-based ceramics are used as the research object.First,by changing the mass ratio of two kinds of silane raw materials,the preparation process of SiOC-based precursors is optimized to possess good oxidation resistance under high temperature conditions.FT-IR indicates that the two silanes are completely reacted and the introduced B elements can be attached to the SiOC network skeleton.After the thermogravimetric analysis of the as-prepared SiOC-based materials,it was found that the yield of the ceramics with a mass ratio of 8:1 is higher than that of the precursors prepared with mass ratios of 4:1 and 5:1,which is 83.07%.With the introduction of B,the yield of SiBOC ceramics with mass ratio of 4:1 increased sharply from 67.28%to 81.48%,showing a dramatic increase.Subsequently,the microstructure of SiOC was tuned by altering the pyrolysis atmosphere.Both XPS and XRD analysis confirmed that using the mixture of argon and carbon dioxide or argon and water vapor as the pyrolysis atmosphere would be conducive to reduce the percentage of segregated carbon in the SiOC ceramics whereas promoted the phase separation at high temperature and the precipitation of ?-SiC nanocrystal.After the SiOC-based ceramic powders were obtained with pyrolysis and ball-milling process,the SiOC-based ceramics were densified by hot pressing sintering,pressureless sintering and spark plasma sintering with the variation of sintering temperature.The phase analysis found that the crystallinity of the hot pressing sintering was the most remarkable and the products were silicon dioxide and silicon carbide multi-phase ceramics;SPS sintering could remain the amorphous status thus becoming an ideal sintering way.The reason is that SPS sintering can avoid the crystallization of carbon thermal reduction after SiOC phase separation.When the temperature reaches 1700?,?-SiC nanoparticles become visible in XRD upon phase separation and crystallization with the size of 31.67 nm and the material is more compact compared with that obtained by hot pressing sintering.The addition of boron enables some SiO₂ amorphous phase to convert to SiC particles and promotes the formation of ?-SiC.However,the growth of the sintered neck is difficult to occur at the sintering temperature of 1500?,so there are a number of pores appeared on the resulted microstructure.The mechanical and thermal physical properties of SiOC-based ceramics were evaluated.As evidenced by Archimedes method,the SiOC density of the pressureless sintering was greater than that of SPS and hot pressing,while the SPS and hot pressing sintering materials were more compact.At temperature of 1700?,the mechanical properties of SPS sintering SiOC ceramics are excellent,with the flextural strength of 178.35 MPa and the fracture toughness of 2.50 MPa·m^1/2.The introduction of boron can improve the fracture toughness of SiOC,and the obvious particle nailed phenomenon can be found so that more energy is consumed,leading to an improved fracture toughness.The as-prepared SiOC ceramics have low thermal expansion(2.26×10^-6K^-1)and thermal conductivity?0.87-1.65W/?m·K??,which are governed by phase separation as long as the temperature exceeding 1000?.At1100?,the thermal expansion coefficient of SiOC ceramics reaches the maximum value of 4.69×10^-6K^-1.After that,the CTE gradually decreases with the increase of temperature.When the temperature rises from 1200 to 1500?,the thermal conductivity declines due to the change of internal structure caused by phase separation,and decreases from 1.63 W/?m·K?to 1.42W/?m·K?.