Microstructure and Mechanical Behaviors of Nano-polycrystalline Diamonds Synthesized by Direct Conversion Sintering under HPHT

Abstract

AbstractHigh-purity nano-polycrystalline diamonds have been synthesized by direct conversion from graphite and various non-graphitic carbons under static high pressures and high temperatures. The polycrystalline diamond synthesized from graphite at ≥15 GPa and 2300-2600 °C has a mixed texture comprising a homogeneous fine structure (particle size: 10-30 nm, formed in a diffusion process) and a lamellar structure (formed in a martensitic process), and has a very high Knoop hardness of 120-145 GPa. In contrast, the polycrystalline diamonds made from the non-graphitic carbons at ≥15 GPa and 1600-2000 °C have a single texture consisting of a very fine homogeneous structure (5-10 nm, formed in a diffusion process) without a lamellar structure. The hardness values of the nano-polycrystalline diamonds made from non-graphitic carbons (70-90 GPa) are significantly lower than that of polycrystalline diamond made from graphite. The investigation of the microstructure beneath the indentation of these nano-polycrystalline diamonds revealed that the existence of the lamellar structure and the bonding strength of the grain boundary have a decisive effect on the hardness.