Ultrahard boron nitride material through a hybrid laser/waterjet based surface treatment

We report a dual phase boron nitride (BN) material composed of 50% cubic and 50% wurtzite phases that has the same level of hardness as polycrystalline diamond. The dual phase BN material was initially synthesized from high pressure and high temperature consolidation of powder materials and subsequently, a laser/waterjet heat treatment (LWH) was applied to the material surface. The LWH process consisted of heating the sample surface using a continuous wave CO2 laser beam followed by tandem waterjet quenching of the laser irradiated material. The indentation hardness of the as-synthesized material was measured to be nominally 37 GPa. After the heat treatment the indentation hardness increased to nominal values of 75 GPa reaching the hardness of polycrystalline diamond 65-80 GPa. Dispersive Raman spectroscopy, high-resolution scanning electron microscope (HRSEM) and surface grazing XRD were used to characterize the BN phase signatures, grain size changes and phase transitions in both as-synthesized and heat treated material. Comparison of the as-synthesized and heat treated material microstructure revealed that heat treatment resulted in microstructure that consists of large grains; surrounded with regions of nano-grains between larger grains and; formation of solid interlayer along the grain boundaries. The increase in hardness was observed for LWH processing at laser fluence below 35 J/mm(2), and LWH processing above this value resulted in spallation of BN material from the surface. Raman spectrums of the material processed below the laser fluence of 35 J/mm(2) indicated that there are minimal phase transitions in the material; however, above that fluence, BN transformed into hexagonal phase resulting in surface damage through spallation. A combination of amorphous phase formation at the grain boundaries and grain size refinement are suggested as the mechanisms responsible for the LWH processing induced hardness increase. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (Less)