High Pressure And High Temperature Sintering Of Nanostructured B₆O-B₄C Superhard Composite

Boron sub-oxide (B₆O) is one of the boron-rich compounds, which is a new kind of super-hard material and attractive around the world in recent years. Boron sub-oxide (B₆O) has been verified to be as hard as cubic boron nitride (cBN), and as tough as diamond, which combines high hardness, low density and chemical inertness, high thermal stability, high thermal conductivity, and excellent wear resistance. It can form 5-10nm isotropic icosahedral multiply-twinned particles, making them useful as abrasives and for other high-wear applications.This work is based on a project of the Natural Science Foundation of China titled with " Nanostructured B₆O Superhard Composites: High-Pressure High-Temperature Sintering, Characerization, and Applications" (E020602). The work presents the technique details on one-step sintering of superhard B₆O-B₄C nano-structured composite, as well as the characterization of the high pressure and high temperature (HPHT) sintered samples.The cell assembly for high pressure sintering was optimized before the HPHT sintering experiments. The cell pressure is calibrated to be～5 GPa when the oil pressure of the press used in this work reaches to 75 MPa. The temperature of the high pressure cell is largely extended by using the optimized cell assembly, which can reach to a high temperature of 1800℃.We used a powder mixture of B, B₂O₃, and B₄C as starting materials. The powder mixture with a desired molar ratio was treated at 2. 5-5 GPa and 1,500-1, 900 K for 10-50 minutes. The HPHT treated samples were all well-sintered cylinder shape chunks, 8 mm in height and 7. 5 mm in diameter. After HPHT treatments, the recovered samples were polished and cleaned in hot water and acetone with an ultrasonic washer, and then investigated with x-ray diffraction (XRD), Vickers hardness test, ultrasonic measurements, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). XRD data of the sintered samples indicates that the crystallite size of B₆O in the composites increases with the sintering temperature from tens to hundreds of nanometers. The results are also confirmed by the TEM analysis.Hardness is a improtant index to super-hard materials. It was found that the B₆O-B₄C composite sintered at a mild pressure of～3 GPa can have a Vickers hardness of above 30 GPa. Under appropriate pressure and temperature (5 GPa, 1600℃) conditions, we could obtain the well-sintered compact with a higher hardness up to～34 GPa. The sintered B₆O compacts have a comparable hardness to the industrially manufactured PCBN materials, and can work for high-speed cutting of hardened steel. But the samples are found to be very brittle, and further works are needed to improve the performance of the B₆O compact.