Fabrication And Mechanical Properties Of 2D-SiO_2f/SiO₂-BN Ceramic Matrix Composites

2D-SiO_2f/SiO₂-BN ceramic matrix composites were fabricated by liquid phase infiltration and sol-gel methods followed by hot-pressing using silica fiber woven cloth, silica sol, fused silica powder and BN powder as starting materials. The fabrication process of 2D-SiO_2f/SiO₂-BN and the crystallization behavior were studied by means of XRD, SEM and TG-DSC. The mechanical properties of 2D-SiO_2f/SiO₂-BN were measured by three point bending method and single edge notched beam (SENB).The experimental results show that when the pH values of the silica sol with high purity and high solid concentration reach about 6.03, the shortest gel time (6.33h) is achieved. When the heat treatment temperature come up to 850℃, the fiber woven cloth infiltrated with silica sol began to crystallize and the crystallization become serious at 900℃. The temperature for the change from gel to silica is around 403℃according TG-DSC result. The addition of polyacrylic acid (PAA) as an anionic dispersant changed the viscosity of SiO₂/BN slurry and formed steric hindrance related to the adsorbed long polymer chain on the particle surface. In order to prepare the slurry with high concentration and low viscosity, the concentration of PAA should be around 0.64wt%. The lowest viscosity is 0.24Pa·s. When the pH value comes to 6.9, the slurry has a lowest viscosity (0.22Pa·s). The hot-pressing temperature of 2D-SiO_2f/SiO₂-BN (GB series) composites is several hundreds degree lower than that of the general fused silica composites. The SEM photographs show the addition of silica sol has improved the infiltration effect of the SiO₂/BN slurry.The crystallization temperature of 2D-SiO_2f/SiO₂-BN (SBN series) prepared by liquid phase infiltration and sol-gel methods is 1300℃, which was 150℃higher than that of the composites fabricated by sol-gel method. The density of SBN series composites hot-pressed at 1200℃comes to 2.068g·cm-3. The flexural strength, fracture toughness and elastic modulus attains to the maximum values at 1200℃, which are 36.9MPa(Z), 35.7MPa(X-Y); 1.138MPa·m1/2(Z), 1.141MPa·m1/2 (X-Y); 23.17GPa(Z), 27.16GPa (X-Y), respectively. For the 2D-SiO_2f/SiO₂-BN (SBN series) composites, best mechanical properties are exhibited when the original solid...