Hardness and deformation microstructures of nano-polycrystalline diamonds synthesized from various carbons under high pr
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T. Irifune Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan (Received 5 February 2007; accepted 16 May 2007)
Mechanical properties of high-purity nano-polycrystalline diamonds synthesized by direct conversion from graphite and various non-graphitic carbons under static high pressures and high temperatures were investigated by microindentation testing with a Knoop indenter and observation of microstructures around the indentations. Results of indentation hardness tests using a superhard synthetic diamond Knoop indenter showed that the polycrystalline diamond synthesized from graphite at 艌15 GPa and 2300–2500 °C (consisting of fine grains 10–30 nm in size and layered crystals) has very high Knoop hardness (Hk 艌 110 GPa), whereas the hardness of polycrystalline diamonds synthesized from non-graphitic carbons at 艌15 GPa and below 2000 °C (consisting only of single-nano grains 5–10 nm in size) are significantly lower (Hk ⳱ 70 to 90 GPa). Microstructure observations beneath the indentations of these nano-polycrystalline diamonds suggest that the existence of a lamellar structure and the bonding strength of the grain boundary play important roles in controlling the hardness of the polycrystalline diamond.
I. INTRODUCTION
Single-phase (binderless) polycrystalline diamond, in which small diamond grains are well sintered without any secondary phases, is an ideal superhard material with both high mechanical strength and excellent thermal stability. Recently, we successfully synthesized dense and well-sintered binderless polycrystalline diamond by directconversion sintering from high-purity graphite under static ultrahigh pressure and high temperature.1 The polycrystalline diamond consists of a very fine mixed texture of homogeneous granular structure (grain size: 10–30 nm) and lamellar structure without secondary phases2 and has considerably high hardness, equivalent to or even higher than that of single-crystal diamond.3 On the other hand, it is known that non-graphitic carbons, such as carbon black, glassy carbon, amorphous carbon, C60, and carbon nanotubes (CNTs) also directly convert to diamond under high pressure and high temperature.4–8 We found that single-phase polycrystalline diamond consisting only of very fine grains (less than 10 nm in size) can be obtained from those non-graphitic carbons at 1600–2000 °C under pressures higher than 15 GPa.9 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0295 J. Mater. Res., Vol. 22, No. 8, Aug 2007
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The differences in the mechanical behavior among those polycrystalline diamonds made from various forms of carbons are of great scientific interest. Few systematic experimental studies, however, have been carried out on the influence of the starting material or the microstructure on the mechanical properties of the polycrystalline diamonds. Study of the microstructure beneath an indentation of material allows us to obtain important information on the mechan
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