The silica fiber reinforced silica and boron nitride-based composites(SiO2f/SiO2-BN) were prepared firstly via the sol-gel method and then the urea route,and the effects of oxidation treatment on the component,structure,mechanical and dielectric properties of the composites were investigated.The results show that the oxidation treatment at 450 °C will not impair the structure of boron nitride,and carbon is the main impurity with the excessive urea.The density of SiO2f/SiO2-BN composites is 1.81 g/cm3,and the flexural strength and elastic modulus are 113.9 MPa and 36.5 GPa,respectively.After oxidation treatment,the density varies to 1.80 g/cm3,and the flexural strength and elastic modulus are decreased to 58.9 MPa and 9.4 GPa,respectively.The mechanical properties of the composites are severely damaged,but they still exhibit a good toughness.The composites show excellent dielectric properties with the dielectric constant and loss tangent being 3.22 and 0.003 9,respectively,which indicates that the oxidation treatment is ineffective to improve the dielectric properties of SiO2f/SiO2-BN composites.
The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the a to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3＋Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The flexural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa.m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the flexural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3＋Al2O3 as additives, making it to possess better mechanical properties.
