| To meet the demand of wave-transparent materials with good heat-resistance,low dielectric constant and well ablation resistance,unidirectional and 2.5D Si3N4f/SiO2composites were fabricated by sol-gel method using silica sol as precursor for the radome and antenna window use in high mach number aircrafts.Thermal resistance of Si3N4 fiber,inorganic process of silica sol and the strengthening and toughing of silica matrix were systematically investigated.The influence of fabrication parameters on the microstructure and mechanical properties of the composites were carefully investigated.The analysis of mechanical behavior and failure mechanism of the composites at high temperatures were carried out.The thermophysical properties,dielectric properties and plasma ablation resistance of the composites were also profoundly characterized.The thermal resistance of Si3N4 fibers was studied.Si3N4 fiber showed high thermal stability in nitrogen atmosphere.After heat treatment at 1500℃for 3 hours,Si3N4 fiber remained amorphous and the strength retention rate was up to 49%.After heat treatment at 1600℃,Si3N4 fiber underwent severe pulverization and obvious crystallization ofα-Si3N4,and the tensile strength was completely lost.In air environment,Si3N4 fiber experienced high-temperature oxidation.After heat treatment at 1100℃for 3h,the retention rate of fiber strength reached 95%.However,when the temperature was higher than 1200℃,superficial oxidation of Si3N4 fiber occurred and the tensile strength of fibers was completely lost.Besides,the activation energy of the oxidation of Si3N4 fiber was about 135.5 k J·mol-1.The effects of preparation temperature and atmosphere on the crystallization,densification and mechanical properties of silica sol derived SiO2 matrix were investigated.The initial crystallization temperature of SiO2 was around 1200～1300℃.Further increasing the heat treatment temperature or extending the heat treatment time significantly improved the crystallization degree of SiO2.Besides,air condition could also promote the crystallization of SiO2.In addition,heat treatment at the temperature above 1100℃can significantly improve the density of SiO2 matrix.The relative density,micro-hardness and micro-modulus of SiO2 bulks prepared at 1400℃reached up to 99%,2.23 GPa and 34.6 GPa,respectively,but the flexural strength and elastic modulus were only 13.8 MPa and 2.3 GPa,respectively.High sintering temperature would lead to crystallization of silica and thus trigger the generation of microcrack,which is considered as one of the major reasons for the low flexural strength of SiO2 bulks.The toughing and strengthening influence of chopped Si3N4 fiber and BN second phase on the SiO2 matrix were studied.The addition of chopped Si3N4 fibers was detrimental to the densification of SiO2 matrix,and had no significant effect on the mechanical properties of porous SiO2 matrix.Comparatively,the introduction of BN greatly improved the density and strength of SiO2,and the crystallization of SiO2 could be retarded.The flexural strength of SiO2-BN composite prepared at 1000℃reached up to 66.0 MPa,and such composite could keep amorphous up to 1400℃.The reaction between borazine and the surface of SiO2 matrix was the underlying reason for the significant improvement of mechanical properties and the effective inhibition of crystallization at high temperatures.UD Si3N4f/SiO2 composites were fabricated by filament winding and sol-gel method.The effects of fiber content and preparation parameters on the mechanical properties of composites were studied.The fiber content was preferred to be 37%.The composites prepared at 800～1000℃showed well comprehensive properties,with the density and porosity of about 1.68 g?cm-3 and 25%,and the flexural strength and fracture toughness at around 166.5～174.0 MPa and 7.0～7.2 MPa·m1/2,respectively.If further increasing the preparation temperature,the flexural strength of composites would linearly decrease to around 50.0 MPa.In addition,when bending tested at 900℃,such composites experienced an improvement of mechanical properties,with a maximum strength of up to 255.9 MPa,which was 47%higher than that at room temperature.However,after a long-time oxidation at above 1200℃,the mechanical properties of composites would be significantly degraded,and the lowest flexural strength was only about 50.0 MPa.The release of residual stress at the fiber-matrix interface was the internal reason for the increase of flexural strength at 900℃,while the superficial oxidation of silicon nitride fibers and the increase of fiber-matrix interface bonding were the direct reasons for the degradation at 1200℃.The influence of preparation temperature on the mechanical properties of 2.5D Si3N4f/SiO2 composites was studied.The effects of CVD BN coating and CVD Py C coating on interfacial modification of composites were also investigated.2.5D Si3N4f/SiO2composites prepared at 1000℃performed well in mechanical properties,with the flexural strength,fracture toughness and interlaminar shear strength reaching 73.3MPa,2.1 MPa·m1/2 and 15.9 MPa,respectively.BN coating could not significantly improve the mechanical properties of the composite sintered at 1200℃,and there was no obvious fiber pull-out in the fracture section of composites.Py C coating could protect Si3N4 fiber from damage,but it also leaded to weak fiber-matrix interface bonding.Although the composite with Py C coating exhibited a ductile fracture mode,its flexural strength was less than 20.0 MPa.Furthermore,the introduction of BN second phase through PIP process could significantly improve the mechanical properties of composites.The maximum flexural strength of 2.5D Si3N4f/Py Cc/SiO2-BN composite prepared at1200℃reached about 77.3 MPa.The thermal conductivity,thermal expansion coefficient,dielectric property and plasma ablation resistance of 2.5D Si3N4f/SiO2 composites were studied.For the composites prepared at 1000℃,the thermal conductivity was about 0.682 W?m-1?K-1.The coefficients of thermal expansion at 200～1100℃were located at（2.5～4.5）×10-6 K-1.The average dielectric constant and loss tangent at 200～1200℃were 3.78 and 7.11×10-3（12GHz）,respectively.The linear ablation rate and mass ablation rate were 0.0732 mm?s-1and 0.56×10-2 g?s-1,respectively.During the process of plasma ablation,SiO2 would be melted,vaporized and decomposed,while Si3N4 fiber mainly experienced crystallization,pulverization and high-temperature decomposition.Because the thermal resistance of Si3N4 fiber is better than that of SiO2 fiber,the ablation performance of Si3N4f/SiO2composite was much better than that of SiO2f/SiO2 composite.In addition,by introducing BN second phase into SiO2 matrix,the ablative resistance of composites could be further improved.The linear and mass ablative rates of 2.5D Si3N4f/SiO2-BN composites were only 0.0214 mm?s-1 and 0.10×10-2 g?s-1,respectively. |
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