Fabrication And Properties Of C_f/Al₂O₃-BN Ceramic Matrix Composites

Cf/Al2O3-BN composites are fabricated by vacuum hot-pressing method using carbon fiber dispersed homogeneously,α-Al2O3 and BN powder as starting materials. The volume ratio ofα-Al2O3 and BN is 3:1. The relationship between the carbon fiber length, sintering procedures and mechanical properties of the composites is studied by means of XRD, SEM as well as three point bending and single edge notched beam bending tests. Besides, influence of carbon fiber content on microstructure, mechanical properties, thermal expansion coefficient, thermal conductivity, thermal shock resistance and ablation resistance are investigated.The experimental results show that: when the Cf length are 4mm, 8mm, 12mm, respectively, flexural strength and fracture toughness of the 20vol.%Cf/Al2O3-BN composites increase with the increasing of Cf length. Furthermore, when the fiber length is 12mm (CLf) mixed with 5vol.% 1mm (CSf), the fiber may be distributed more homogeneously, so flexural strength and work of fracture have an improvement, 113.6 MPa, 1596.3 J/m2 respectively. So 12mm CLf+1mm CSf (CLf+CSf ) is believed the suitable length.The relative density of the 20vol.%(CLf+CSf)/Al2O3-BN(1vol.%SiO2/ matrix) hot-pressed at 1500℃,1550℃,1600℃,1650℃,respectively, is remarkably increased with the increase of sintering temperature, but flexural strength, fracture toughness and work of fracture are decreased. When the composite is sintered with10vol.%SiO2/matrix at 1500℃, relative density, flexural strength, fracture toughness and work of fracture all increased to some degree, and 95.3%,155.0 MPa,3.08 MPa·m1/2,2244.6J/m2, respectively, were obtained.(CLf+CSf)/Al2O3-BN(10vol.%SiO2/matrix) composites with different Cf volume content of 15%, 20%, 25%, 30% would be fabricated at 1500℃and flexural strength and fracture toughness increase at first and then decrease with increase of Cf content. When the content of Cf is 25%, flexural strength and fracture toughness of the composite reached the maximum values, 170.5 MPa and 3.27 MPa·m1/2, respectively. The work of fracture is increased continuously and reached a maximum value, 2883.6J/m2. Thermal expansion coefficients and thermal conductivities of the composites are decreased with the increase of carbon fiber content. The oxidation of the carbon fiber and thermal shock damage are the main factors that make flexure strength of the composites down after thermal shock. Cf/Al2O3-BN composites have an excellent ablation resistance under the O2-C2H2 ablation condition. Mechanical erosion and thermochemistry oxidation are the main ablation mechanisms.