Microstructures and Properties of In Situ Nonoxide-Boron Nitride (Nobn) Composites

The microstructures and properties of in situ synthesized nonoxide-boron nitride (Nobn) composites include SiC-BN, Si sub 3 N sub 4 -BN, AlN-BN, SiAlON-BN and AlON-BN composites were investigated. For in situ SiC-BN composites prepared by hot pressing, with the increase of BN content, Young's modulus decreased obviously, on the other hand, high level of bending strength demonstrated compared to that of BN composites prepared by conventional process. For pressureless sintered AlN-BN, AlON and SiAlON in situ composites based on the nitridation of AlB sub 2 , relatively high dense materials with relative density higher than 80% were obtained when the BN content was less than about 20 vol%. An interesting phenomenon was that the residual pores in the composites were almost in closed type. These Nobn composites showed high strength and low Young's modulus. With the content increase of BN, the densification behavior became poor and the residual pores became open channels. It was suggested that the residual pores near BN formed with the in situ growth of BN flakes were originally in closed type. With the content increase of BN to higher than about 20 vol%, a percolation network of BN formed and resulted the residual pores in open type. The high strength of the in situ Nobn composites was explained by the fine and homogeneous microstructures and/or closed type pores in the composites.