Microstructure and Mechanical Behaviors of Nano-polycrystalline Diamonds Synthesized by Direct Conversion Sintering unde
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0987-PP03-01
Microstructure and Mechanical Behaviors of Nano-Polycrystalline Diamonds Synthesized by Direct Conversion Sintering under HPHT Hitoshi Sumiya1 and Tetsuo Irifune2 1 Electronics & Materials R&D Laboratories, Sumitomo Electric Industries, 1-1-1, Koyakita, Itami, Hyogo, 664-0016, Japan 2 Geodynamics Research Center, Ehime University, 2-5, Bunkyo-cho, Matsuyama, 790-8577, Japan ABSTRACT High-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 nongraphitic 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 (7090 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.
INTRODUCTION Recently, we have successfully synthesized dense and high-purity polycrystalline diamonds by indirect heating of high-purity graphite under static ultrahigh pressure and high temperature ( ≧2300 °C at 15 GPa). [1]. This polycrystalline diamond directly synthesized from graphite has a mixed texture comprising a homogeneous fine structure (grain size: 10-30 nm, formed in a diffusion process) and a lamellar structure oriented in the direction (formed in a martensitic process) [2]. The polycrystalline diamond made from graphite has a considerably high hardness equivalent to or even higher than that in the (001) directions of the synthetic high-purity (type IIa) diamond crystal and obviously higher than type I diamond crystals [3]. On the other hand, single-phase polycrystalline diamond can also be synthesized from non-graphitic carbons, such as carbon black, glassy carbon, amorphous carbon, C60 and carbon nanotubes (CNT) by the direct conversion method under temperatures of more than 1600 °C at 15 GPa. We found that the polycrystalline diamond consisting only of very fine particles (less than 10 nm in size, formed only in the diffusion process) can be obtained from non-graphitic carbons at 16002000 °C under more than pressures of 15 GPa [4]. The differences in the mechanical behavior between those polycrystalline diamonds made from graphite and from non-graphitic carbons are of deep scientific interest. In this article, we describe the results of investigations of the sub
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