Design and fabrication of graded cBN tool materials through high temperature high pressure method

Abstract

The production of graded cubic boron nitride (cBN) tool materials starting with cBN-TiC-Al-Co mixture and having a three-layer symmetrical graded structure were obtained by high temperature high pressure method. In the first part, the design model which proposed a component gradient of cBN varied from 4 vol% to 10 vol% accompanied with Al and Co varied from 2 vol% to 5 vol% between the surface and interior was presented. Afterwards, the rationality of the graded structures were verified by the thermal residual stress analysis in the composites. In the second part, the relationships between the mechanical properties and the thickness ratio, sintering temperature, holding time as well as the component gradient were investigated. The significant gradients of the elements of Al and Co were detected in the graded cBN composites. When the temperature converted from 1350 °C to 1550 °C, the liquid phase sintering and reaction process were confirmed, leading to the formation of AlN, AlB 2 and Co 2 B. The excellent samples with a thickness ratio of 0.3 and a cBN gradient of 6 vol% acquired a homogeneous microstructure with no obvious defects under a pressure of 5.8 GPa at 1500 °Cfor 10 min. The optimum mechanical properties were achieved a flexural strength of 710 MPa and a hardness of 36.5 GPa, which was higher than that of the monolithic material with a cBN content of 80 vol%. The crack deflection and bridging played the main role in the toughening mechanism, reflecting a significant contribution of the residual stress to the mechanical properties. • The graded structure was introduced to the cBN cutting tool material to improve the strength and toughness, acquiring a flexural strength of 710 MPa and a hardness of 36.5 GPa. • The residual stress induced by the graded structure was the main toughening and strengthening mechanism, characterized by crack bridging and deflection.