Effect of heat treatment on the structural state and mechanical properties of polycrystals based on the superhard boron nitride modifications

Abstract

An investigation was carried out into the effect of thermal cycling on the structural state of polycrystals based on the high-density boron nitride modifications. It is shown that a two-phase material is characterized by a high level of macro- and microstresses, which control its brittle rupture behavior and mechanical properties. In particular, microcracking processes initiated by internal stresses after the application of external load promote stress relaxation at the tip of a propagating macrocrack, thereby imparting high fracture toughness (Kc ∼16–18 MN/m3/2) to the material. During heat treatment (performed in this work at T ⩾ 800°C) internal stress relaxation takes place, which is accompanied by a fall in fracture toughness to the level characteristic of unstressed single-phase sintering (Kc ∼10–14 MN/m3/2) and changes in hardness and rupturing stress.