Mechanical properties of polycrystalline cBN obtained from pyrolytic gBN by direct transformation technique

High-purity polycrystalline cBN (pcBN) has been obtained from bulk pyrolytic graphite-like BN at a pressure of about 8 GPa and temperatures of 2500 and 2800 K. The size of grains in the sample produced at 2500 K ranged from 100 to 400 nm. The formation of the fine-grained structure has given rise to an increase in both hardness and fracture toughness of pure pcBN when compared with cBN single crystals (s.c.), the elastic modulus is close in value to the elastic modulus of s.c. High mechanical characteristics of the sample produced at 2500 K are caused by a grain small size and pure dense grain boundaries of the recrystallization origin. The increase in the synthesis temperature to 2800 K results in an abrupt decrease in hardness because of the grain size growth in the course of the collective recrystallization.