Mechanism of the formation of a diamond nanocomposite during transformations of C-60 fullerite at high pressure

The results of an investigation of the transformation of C-60 fullerite to diamond under pressure through intermediate three-dimensionally polymerized and amorphous phases are reported. It is found that treatment of fullerite C-60 at pressures 12-14 GPa and temperatures similar to 1400 degrees C pro duces a nanocrystalline graphite-diamond composite with a concentration of the diamond component exceeding 50%. At lower temperatures (700-1200 degree s C) nanocomposites consisting of diamondlike (sp(3)) and graphitic (sp(2)) amorphous phases are formed. The nanocomposites obtained have extremely hi gh mechanical characteristics: hardness comparable to that of best diamond single crystals and fracture resistance two times greater than that of diam ond. Mechanisms leading to the transformation of C-60 fullerite into diamon d-based nanocomposites and the reasons for the high mechanical characterist ics of these nanocomposites are discussed. (C) 1999 American Institute of P hysics. [S0021-3640(99)01011-7].