CBN-TiH2 composites: chemical equilibria, microstructure and mechanical studies

This paper summarizes theoretical and experimental studies of cBN-TiH2 composites with cBN:TiH2 molar ratio equal to 9:1. From theoretical calculations it follows that TM, reacts with BN in the full range of pressure and temperature to form of two new phases: TiB2 and TiN, the amounts of which depend on the conditions of synthesis. Experimentally cBN-TiH2 composites were prepared by high pressure hot pressing, and the samples were subsequently heat treated. Samples before and after heat treatment were characterized using transmission electron microscopy. The pristine samples exhibit dense polycrystalline structure, and at the BN-binder interface a thin layer of fine TiB2 and TiN crystallites are visible. After heat treatment TiN crystallites grow and form a layer of equiaxial grains of 0.5-2 mu m diameter. Columnar TiB2 of 0.1-0.2 mu m diameter form as a thin layer between the TiN and BN crystallites. Heat treatment significantly increases the hardness and Young's modulus of the composites. (C) 1997 Elsevier Science S.A.