Microstructural Regulation And Properties Of C_f/Si(Zr)OC Ceramic Matrix Composites

Continuous carbon fiber reinforced ceramic matrix composites(CFRCMC)is an ideal thermal protection material.Compared with traditional brittle ceramic insulating tile,CFRCMC has the advantages of high porosity,low density,toughness,high reliability and excellent high temperature stability.It will become an important candidate material for thermal protection system in hypersonic aircraft.In this paper,in order to meet the practical requirements of new generation hypersonic vehicle for new thermal insulation materials,the micro-structure control and properties of Cf/Si(Zr)OC ceramic matrix composites were systematically studied.The preparation method of in-situ growth Si Cnw reinforced Cf/Si OC ceramic matrix composites was proposed,and multicomponent Ta Si2 matrix coating was prepared on the surface of the composites.The oxidation resistance of gradient coatings was evaluated by oxidation ablation test.The preparation technology and properties of Si OC ceramic precursor and Cf/Si OC ceramic matrix composites were investigated.It was found that a new type of Si OC ceramic precursor could be synthesized by molecular design theory with methyl trimethoxysilane and dimethyl diethoxysilane as structural monomers.When the ratio of methyltrimethoxysilane to dimethyldiethoxysilane is 4:1,the highest yield of ceramics can reach 81.6%.Cf/Si OC ceramic matrix composites with different densities were prepared by precursor infiltration pyrolysis(PIP)process,their mechanical and thermophysical properties were studied.When the number of PIP process increases from one time to three times,the compressive strength of the composites increases from 0.99 MPa to 3.21 MPa,the thermal conductivity increases from 0.1566W·(m·K)-1 at 50? to 0.5643W·(m·K)-1 at 1200?,and the thermal expansion coefficient increases from 2.458×10-6/? at 200? to 2.843×10-6/? at 1200?.In the air environment,the weight loss rate of the composites decreased from 33.3% to 20.2%.In this paper,the preparation method of in-situ growth Si Cnw reinforced Cf/Si OC ceramic matrix composites is proposed.Using methyl trichlorosilane as precursor,Si Cnw was grown in situ on the surface of three-dimensional braided carbon fibers by chemical vapor infiltration process.Cf/Si Cnw/Si OC ceramic matrix composites were prepared and the growth mechanism of Si Cnw was revealed.Compared with Cf/Si OC ceramic matrix composites,their mechanical properties and thermophysical properties were improved.The compressive strength of Cf/Si Cnw/Si OC ceramic matrix composites increased from 2.13 MPa to 6.16 MPa when the number of PIP process increased from one to three times.Under the same thermal environment,the thermal conductivity of the composites decreases,and the specific heat capacity and thermal expansion coefficient increase.In the air environment,the weight loss rate of the composites decreased from 30.1% to 18.7%.In this paper,a new process for improving the thermal stability of composites by introducing Zr element into the Si OC precursor is proposed.Si Zr OC ceramic precursor was synthesized from polymethylhydrosiloxane and zirconium n-propanol.The polymerization mechanism was revealed.The results show that the high temperature stability of precursor ceramics is improved by introducing Zr into Si OC ceramics,and the yield of ceramics increases with the increase of Zr/Si ratio.The effect of temperature on the pyrolysis behavior of Si Zr OC ceramics was investigated.With the increase of pyrolysis temperature,the high temperature stability of Si Zr OC ceramics was enhanced.Cf/Si Zr OC ceramic matrix composites were prepared by PIP process.The mechanical properties and thermophysical properties of Cf/Si Zr OC ceramic matrix composites with different densities were studied.When the number of PIP process increased from one to four times,the compressive strength of the composites increased from 24.70 MPa to 53.54 MPa.The thermal conductivity increased with the increase times of PIP process and temperature,and the weight loss rate decreased from 43.30% to 11.32%.In this paper,Ta Si2-based gradient oxidation resistant coatings were prepared on the surface of Cf/Si Zr OC composites,which improved the oxidation resistance of the composites.The coating was prepared via high temperature sintering of borosilicate glass with high emissivity of Ta Si2 and Mo Si2 meanwhile toughening by Si C whisker.The static oxidation temperature is 1500? and the oxidation time is 20min?120min.After 120 minutes examination,the weight loss rate of the composite was 7.43%,which indicated that the coating had good oxidation resistance.The experimental results of oxygen-acetylene flame ablation show that the mass ablation rates of the samples are 5.31×10-5g/(s·cm2)and 5.60×10-5g/(s·cm2),and the linear ablation rates are 1.3×10-4mm/s and 4.1×10-4mm/s,when the surface ablation temperatures are 1400? and 1500? meanwhile the ablation time is 1000 s.When the surface ablation temperature is 1600?,the coating can withstand 200 s ablation.With the increase of heat flux,the borosilicate glass on the surface of the coating evaporates seriously,resulting in the formation of a great number of "ablation pits",resulting in a decrease in the density of the coating surface.The mass ablation rate and linear ablation rate are 1.96×10-4g/(s·cm2)and 4.7×10-3mm/s respectively after 1600?/800 s ablation.