Fabrication and characterization of Polycrystalline cubic boron nitride (PcBN) sintered with nanocrystalline TiN coated cBN powders

Abstract

Cubic boron nitride (cBN) is the second hardest material following diamond and polycrystalline cBN (PcBN) tools, unlike diamond, are suitable for machining hard ferrous materials such as hardened alloy steel, tool steels, cast irons and nickel/chromium cast iron and titanium alloys. They can also be used to machine very tough materials including Ni based, tungsten carbide and other ceramics. 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. On a commercial basis, 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, 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 in the final sintered compacts. A novel sol-gel process was applied to prepare nanocrystalline TiN coated cBN powders. High pressure high temperature (HPHT) sintered PcBN compacts prepared from TiN coated cBN powders exhibited better microstructural homogeneity than non-coated PcBN compacts. By using TiN coated cBN powders, TiN phase thickness was reduced and its homogeneity was improved remarkably in. This improved microstructural homogeneity was complimented by the superior mechanical properties such as hardness, TRS, compressive strength and cutting performance of the PcBN compacts with coated cBN. Hence, the developed coating process of cB...