Real hardness of high-purity ultra-fine nano-polycrystalline diamond synthesized by direct conversion sintering under HPHT

摘要

yy Nano-polycrystalline diamond (NPD) with different grain sizes (10-1000 nm) was synthesized under ultrahigh-pressure and high-temperature conditions (16 GPa, 2200-2300 degrees C) starting from high-purity pyrolytic graphite having different crystallinity. The impurity content of these NPDs was 1 ppm or less, suggesting that the grain boundary of the structure has few impurities and the intergranular strength is high. These hardness character. istics were evaluated in detail by the indentation method using the super-hard (> 140 GPa) Knoop indenter prepared from synthetic type IM diamond. On ultra-fine NPD (UF-NPD) consisting of very small diamond grains of around 10 nm, it was found that the elastic recovery of the unloaded indentation impression is large and the length of the direction of the Knoop long diagonal also shrinks. Due to the shrinkage of indentation, the Knoop hardness of this UF-NPD erroneously yielded a high hardness value apparently exceeding 200 GPa. Detailed observations using high resolution FE-SEM around the unloaded Knoop indentations formed on the OF-NPD revealed line-like impressions (striations) along the outside length of the long diagonal as traces of contact with the Knoop indenter. Considering this striation length, the accurate hardness value of the UF-NPD was about 130 GPa. On the other hand the large elastic recovery of the indentation suggests that the OF-NPD has high critical tensile stress. Hertz fracture tests revealed that the crack initiation resistance (fracture strength) of NPD is improved by further grain refinement and high purification.