Nanolayered Diamond Sintered Compact Obtained by Direct Conversion from Highly Oriented Graphite under High Pressure and High Temperature

Abstract

A new type of polycrystalline sintered diamond has been successfully synthesized by direct conversion from highly oriented pyrolytic graphite at 15 GPa and 2300 ∘ C. It is optically transparent and consists entirely of layered nanocrystals (50-100 nm thick) of cubic diamond, which are tightly bound to each other and have strong [111] preferred orientation along the stacking direction. This nanolayered diamond has excellent indentation hardness (∼114 GPa in Knoop scale) comparable to the highest values obtained from single crystalline diamond. Furthermore, it is expected to have significantly high wear resistance on both ends of cylindrical sintered compact, since the surfaces are terminated exclusively by the hardest {111} planes of the layered diamond nanocrystals.

Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material

“…Furthermore, an area centred around 18 GPa and 1,400 K in the pressure-temperature diagram for carbon was attributed to a phase called 'retrievable hexagonaltype diamond' 10 , which corresponds to the conditions where lonsdaleite has been reported 3,11,15 .…”

“…However, these exceptional properties have not been proven experimentally because of the inability to synthesize lonsdaleite as a pure phase. It has been reported to form during static compression of graphite 3,9,13,[15][16][17][18] ; highpressure-high-temperature treatment of powdered diamond, graphite and amorphous carbon 19 ; explosive detonation and shock compression of graphite 11,20 and diamond 21 ; and chemical vapour deposition of hydrocarbon gases 22 ; however, in all cases the synthesis product also contained cubic diamond, graphite or both.…”

“…Published powder X-ray diffraction (XRD) patterns of lonsdaleite show peaks of cubic diamond plus extra, very broad and poorly resolved maxima at 0.218, 0.193, 0.151 and 0.116 nm that have been indexed using a hexagonal unit cell [1][2][3]14,15,20 . However, these maxima either occur on the shoulders of diamond peaks (Fig.…”

“…1a,b) or match those of graphite, but well-resolved X-ray reflections for lonsdaleite have not been reported. Selectedarea electron diffraction (SAED), Raman, electron energyloss spectroscopy (EELS) and high-resolution transmission electron microscopy have also been used for identification of lonsdaleite [4][5][6][7]14,15,18,23,24 ; however, interpretation of data is ambiguous (Supplementary Note 1). In spite of the many diffraction and spectroscopic studies, unambiguous data that prove the existence of lonsdaleite as a distinct material have not been reported.…”

“…Results 'Lonsdaleite' diffraction features. The XRD patterns of Canyon Diablo diamonds and the synthetic material display the poorly resolved diffraction maxima attributed [1][2][3]14,15,19,20 to 'lonsdaleite' (Fig. 1a).…”