Microstructural Regulation And Mechanics And Oxidation Resistance Properties Of C_f/SiHfBOC Ceramic Matrix Composites

With the advent of hypersonic space vehicles,the requirements for thermal protection materials such as lightweight,toughening and oxidation resistance in engine combustion chamber and tail nozzle of space vehicles are gradually raised.Ceramic heat-proof materials,represented by ceramic heat-proof tiles,can hardly meet the increasing demand of hypersonic spacecraft's service environment temperature.High performance silicon precursor ceramics have excellent thermodynamic stability,a pioneer among the SiOC ceramic had been used in the hot end of aerospace aircraft components.But this kind of ceramic in the environment above 1300?can produce strong carbon thermal reduction,which seriously restrain the silicon-based precursor ceramic development of ceramic materials in the aerospace field.In view of this problem,this project took Methyltriethoxysilane as Sisource and C source,Tetrachloride hafnium as Hf source,Boric acid as B source,and prepared SiHfBOC precursor solutions with different B:Siand Hf:Siratios by sol-gel method and solvothermal method,and prepared SiBOC precursor solutions in a similar way.Through TG and SEM analysis,SiBOC precursor had the highest ceramic yield of 76.7%,when B:Siwas 0.6.SiHfBOC precursor had the highest ceramic yield of 85.8%,when B:Siwas 0.6 and Hf:Siwas 0.1.With the increase of pyrolysis temperature,the Hf O₂ grain size generated by phase separation during the reaction process was significantly coarsened,and the crystal structure was transformed from t-Hf O₂ to m-Hf O₂,which was consistent with Oswald maturation mechanism.At the same time,the graphitization degree of SiHfBOC multicomponent ceramics also increased,with the increase of temperature.Moreover,when Hf element exceeds a certain stoichiometry,the graphitization degree of SiHfBOC's multicomponent ceramics significantly decreases.Static oxidation experiments were conducted on SiBOC and SiHfBOC's multi-component ceramics in an air atmosphere of 1400?.The results showed that SiHfBOC's ceramics were superior to SiBOC's ceramics in thermal stability and oxidation resistance at high temperature.In summary,SiHfBOC precursor was selected to prepare ceramic matrix composites and its pyrolysis process conditions were determined.As an important reinforcement material,carbon fiber has excellent mechanical properties,but its oxidation resistance is poor.To prevent high temperature damage of carbon fiber,this paper firstly used CVI method to prepare SiC coating on the surface of carbon fiber.Then,SiHfBOC ceramic precursor was impregnated into the SiC coated C_f preform skeleton by the precursor impregnation pyrolysis method?PIP?,and the ideal SiC coating coated C_f/SiHfBOC ceramic matrix composite was prepared through multiple PIP cycles.This paper emphatically focused on the synthesis and bonding mechanism of SiBOC and SiHfBOC precursor,the impregnation mechanism of SiHfBOC precursor solution in the carbon fiber preform skeleton,and strengthening and toughening mechanism of composite during fracture process.The static oxidation resistance of C_f/SiHfBOC ceramic matrix composites were studied by controlling static oxidation temperature and time,and the mechanical properties of C_f/SiHfBOC ceramic matrix composites were investigated under different conditions.The experimental results showed that the density of C_f/SiHfBOC ceramic matrix composites increased very small after seven PIP cycles,and the compressive strength in x/y direction and z direction reached the maximum value of 77.56±8.56MPa and 40.03±5.48MPa,respectively.The toughening mechanism was mainly due to fiber debonding,fiber pulling out and fiber fracture.The products of C_f/SiHfBOC ceramic matrix composites in the static oxidation process mainly include:CO₂,CO,B₂O₃,SiO₂,Hf SiO₄,etc.The weight retention rate and compression strength retention rate after 120min oxidation at 1500?were 77.16%and 21.27%,respectively.C_f/SiHfBOC ceramic matrix composites have excellent oxidation resistance and mechanical properties.