Nonoxide–boron nitride composites: in situ synthesis, microstructure and properties

Hexagonal graphitic boron nitride (h-BN) composites show excellent corrosion and thermal shock resistance, good mechanical tolerance and machinability, especially Si3N4–BN and Sialon–BN composites; they have already been used as break rings for horizontal continuous casting of steel. However, the strength of the conventionally processed BN composites were remarkably degraded by the addition of BN due to the poor densification behavior and the existence of large BN flakes or agglomerates of BN flakes that acted as fracture flaws. This means that BN dispersoids with fine particle size and homogeneous distribution are the key factors to obtain high strength composites. By in situ process, such microstructural features can be realized. In this work, by using the proposed in situ reactions, synthesis, microstructures and properties of various in situ nonoxide-boron nitride (Nobn) composites including SiC–BN, Si3N4–BN, AlN–BN, Sialon–BN and Alon–BN composites were investigated. For some Nobn composite systems, due to the large volume expansion during the reaction processes, near-net shape sintering can be realized. For example, the sintering shrinkage of AlN-30 vol.% BN was 3.1% and that of Alon-21 vol.% BN was 4.2%. This will be an advantage for the fabrication of large and complicated products.