Polycrystalline cubic boron nitride prepared with cubic-hexagonal boron nitride under high pressure and high temperature

Abstract

Polycrystalline cubic boron nitride (PcBN) compacts, using the mixture of submicron cubic boron nitride (cBN) powder and hexagonal BN (hBN) powder as starting materials, were sintered at pressures of 6.5-10.0 GPa and temperature of 1750 • C without additives. In this paper, the sintering behavior and mechanical properties of samples were investigated. The XRD patterns of samples reveal that single cubic phase was observed when the sintering pressure exceeded 7.5 GPa and hBN contents ranged from 20 vol.% to 24 vol.%, which is ascribed to like-internal pressure generated at grain-to-grain contact under high pressure. Transmission electron microscopy (TEM) analysis shows that after high pressure and high temperature (HPHT) treatments, the submicron cBN grains abounded with high-density nanotwins and stacking faults, and this contributed to the outstanding mechanical properties of PcBN. The pure bulk PcBN that was obtained at 7.7 GPa/1750 • C possessed the outstanding properties, including a high Vickers hardness (∼ 61.5 GPa), thermal stability (∼ 1290 • C in air), and high density (∼ 3.46 g/cm 3 ).

Synthesis of cBN-hBN-SiCw Nanocomposite with Superior Hardness, Strength, and Toughness

“…The Vickers hardness, fracture toughness, and flexural strength of the cBN-hBN-SiC w nanocomposites with various SiC w content are presented in Figure 3 . Due to the formation of low-hardness hBN, the Vickers hardness of SiC0 (27.1 GPa) is lower than the value of the single cBN crystal and pure-phase polycrystalline cBN [ 5 , 6 , 7 , 8 , 9 ]. However, the fracture toughness of the nanocomposites is higher than that of the single cBN crystal (~2.8 MPa·m 1/2 ) [ 7 ].…”

“…In addition, the anisotropy of single cBN crystals brings inconvenience to subsequent processing and use. For the pure-phase polycrystalline cBN, whose mechanical properties even exceed those of single-crystal cBN, the successful synthesis of dense samples cannot be achieved without harsh pressure conditions (≥7.7 GPa), whether through the high-pressure phase transformation of hexagonal boron nitride (hBN), amorphous boron nitride, onion boron nitride (oBN) or the high-pressure sintering of cBN [ 5 , 6 , 7 , 8 , 9 ].…”