Microstructure control and mechanical properties of cubic boron nitride nanocomposites sintered under high pressure and high temperature

Abstract

Fabrication process and mechanical properties of cubic boron nitride nanocomposites were investigated. Cubic boron nitride (cBN) is one of the hardest materials known, with the value of hardness next to that of diamond while possessing many excellent physical and chemical properties. Unlike diamond, cBN by virtue of its lower chemical affinity towards elements like Fe, Ni and Ti, is used for the machining of hard ferrous materials such as hardened alloy steels, tool steels and cast iron. Polycrystalline cubic boron nitride (PCBN) is fabricated by sintering cBN powder with special ceramic materials as binders. In recent years, PCBN tools have also found wide acceptance in the various industrial machining fields of difficult-to-cut materials, such as titanium alloys and nickel based superalloys. In machining high precision and high hardness components, hard turning of PCBN tools provides an alternative to conventional grinding process. Cubic boron nitride loses its oxidation resistance at $1200\degC$. Compounds of titanium such as TiN or TiC are often added to improve the thermal stability of cBN cutting tools. In general, cBN is mechanically mixed with binders or sintering agents, then sintered at very high pressures and high temperatures. Due to very high melting point of the binders added, sintering proceeds without forming a liquid phase. During the solid state sintering, it is difficult to diffuse the binding material over the entire surface of every cBN particle. This often results in an inhomogeneous microstructure that either has portions lacking binder additives or portions where binding materials are segregated. In this study, Fabrication process and characterization of nanocrystalline TiN coated cubic boron nitride were investigated. TiN was employed as the binder material. Coating of the binder material on the surface of each cBN particle was explored for the purpose of increasing the microstructural homogeneity and decreasing the cBN-cBN contact i...