Enhanced hardness of CVD diamond after high pressure and high-temperature treatments

The diamond thick layers prepared using chemical vapor deposition (CVD) methods have been treated at high pressures and high temperatures (HP-HT). It was found that the Vickers hardness of the HP-HT treated bulks was nearly two times as high as that of the starting CVD diamond samples. The hardness of the samples treated at 8GPa and 1800 degrees C is approximate to 80GPa at a loading force of 49N, close to that of the single-crystal diamond (approximate to 100GPa). In addition, the oxidation resistance of CVD diamond samples was also enhanced greatly after HP-HT treatments, and the diamondization of sp(2) bonded carbon in the CVD diamond samples could occur at 7-8GPa and 1600-1800 degrees C. The HP-HT conditions of diamondization in our works are much lower than the case of the graphite-to-diamond direct transformation without the presence of catalyst (>15GPa and 2000 degrees C).