Study on the properties and fracture mode of pure polycrystalline cubic boron nitride with different particle sizes

Abstract

In this paper, pure polycrystalline cubic boron nitride (cBN) without additives was synthesized by a direct sintering method at 7.3 GPa and 2100 °C using industrial cBN powders with different particle sizes. High-pressure experiments without heating verified that the actual pressure of the cBN particles in the cavity was lower than that of the fine-grained samples during the sintering of coarse-grained samples. The best hardness (4900 HV2) and density (3.342 g/cm2) of sintered bodies were obtained when using fine-grained (2–4 μm) particles. The fracture modes of fine-grained and coarse-grained sintered bodies were investigated using scanning electron microscopy, and the microstructures of intra-grain and grain boundaries were systematically studied using high-resolution transmission electron microscopy. It was concluded that the main fracture mode of the fine-grained sample was transgranular fracture, while that of the coarse-grained sample was intergranular fracture. Moreover, a large number of dislocations and high-density twins were generated in the cBN grains under high temperature and high pressure, which improved the hardness of samples and caused transgranular fracture in fine-grained (2–4 μm) samples. This study of the fracture mode of industrial pure polycrystalline cBN is important to enable its use as a high-precision machining tool material.