Microstructure and Hardness of cBN–Zr Composite

Abstract

This paper summarizes theoretical and experimental studies of cBN–Zr composites of cBN:Zr molar ratio equal 9:1. From theoretical calculations it follows that Zr reacts with cBN in the full range of pressure (1·3×10 −3–1×10 7 Pa) for temperatures 950 and 1750°C to form at two new phases: ZrB 2 and ZrN whose amount depends on the conditions of synthesis. Experimentally, composite based on cubic boron nitride (cBN) with zirconium addition used as a binding material was prepared by high pressure hot pressing. Samples before and after heat treatment were characterized using transmission electron microscope. The TEM showed that after sintering a thin zone of fine ZrB 2 and ZrN crystallites was formed at the cBN/Zr interface. After additional heat treatment the ZrN crystallites grew forming a layer up to 2 mm thick built from columnar grains of 0·1–0·2 mm in diameter. The ZrB 2 remained as a thin (0·1 to 0·2 μm) layer of very fine equiaxial crystallites between the ZrN and cBN. The hardness of such sintered material was 20 GPa/HK1, after additional heat treatment at 950°C for 1 h in vacuum of 3×10 −3 Pa the hardness increased up to 29 GPa/HK1.