Preparation And Properties Of C/SiBCN Composites

With the development of aerospace industry,more and more severe service environment poses severe challenges to the high temperature properties and oxidation resistance of thermal structural materials.Therefore,it is urgent to develop a high temperature structural material with excellent high temperature resistance,oxidation resistance and good mechanical properties.C/SiBCN composites are expected to utilize the toughening effect of carbon fibers and the high temperature thermal stability and oxidation resistance of SiBCN matrix at high temperatures.In this paper,the PIP preparation process of C/SiBCN composites was studied.The high temperature thermal stability and oxidation resistance of C/SiBCN composites were investigated,and the carbon/matrix interface of composites was optimized.The pyrolysis characteristics of the precursor and the high temperature thermal stability and oxidation resistance of the SiBCN ceramics were investigated.The transformation of the precursor is completed at 800°C or higher.SiBCN ceramics remain amorphous at 1700°C,and a large number of Si C and Si₃N₄ crystals appear at1800°C and above.SiBCN ceramics did not show obvious oxidation at 1600°C and below.After static oxidation at 1700°C,a dense oxide layer composed of Si C and Si O₂was formed on the surface of the ceramic.The C/SiBCN composites were prepared by PIP process.The density of C/SiBCN composites was 1.64 g/cm³ with an open porosity of 5.86%.The C/SiBCN composite has a flexural strength of 324 MPa and a modulus of 56.9 GPa.The mechanical properties of C/SiBCN composites after heat treatment were studied.The flexural strength of the composites was 180 MPa after heat treatment at1500°C with a strength retention of 59%.When the heat treatment temperature is raised above 1600°C,the flexural strength of the composites is sharply attenuated to?50 MPa.This is because the fiber was damaged seriously due to interfacial reaction of Si₃N₄grains in the SiBCN matrix with carbon fiber at 1600°C and above.Therefore,the mechanical properties of the composites decreased.The oxidation resistance of C/SiBCN composites in static air was studied.With the increasing of oxidation temperature from 1200°C to 1600°C,the oxidation degree of the composites increased first and then decreased.After the composites were statically oxidized at 1400°C and 1500°C for 30 min,the mechanical properties of the composites were almost completely lost.The oxidation strength is maintained above100 MPa after oxidation at 1200°C and 1600°C for 60 min.The former is associated with a low oxidation rate and the latter is associated with the formation of a glassy oxide layer.The fiber/matrix interface of the C/SiBCN composites is optimized.The Py C coating and BN coating can improved the high temperature and oxidation resistance of C/SiBCN composites.The strength retention rate of the composites with a Py C interfacial coating thickness of 0.5?m after heat treatment at 1600°C for 2 h increased from 16.3%to 40.4%.After oxidation at 1400°C for 30 min,the flexural strength retention of the composite increased from?10.2%to 35.1%.