Synthetic diamond is formed commercially using high-pressure, chemical-vapour-deposition and shock-wave processes, but these approaches have serious limitations owing to low production volumes and high costs. Recently suggested alternative methods of diamond growth include plasma activation, high pressures, exotic precursors or explosive mixtures, but they suffer from very low yield and are intrinsically limited to small volumes or thin films. Here we report the synthesis of nano- and micro-crystalline diamond-structured carbon, with cubic and hexagonal structure, by extracting silicon from silicon carbide in chlorine-containing gases at ambient pressure and temperatures not exceeding 1,000 degrees C. The presence of hydrogen in the gas mixture leads to a stable conversion of silicon carbide to diamond-structured carbon with an average crystallite size ranging from 5 to 10 nanometres. The linear reaction kinetics allows transformation to any depth, so that the whole silicon carbide sample can be converted to carbon. Nanocrystalline coatings of diamond-structured carbon produced by this route show promising mechanical properties, with hardness values in excess of 50 GPa and Young's moduli up to 800 GPa. Our approach should be applicable to large-scale production of crystalline diamond-structured carbon.

Synthesis of nano- and microcrystalline sp3-bonded carbon (diamond) with cubic and hexagonal structure by extraction of silicon from silicon carbide in chlorine-containing gases has been reported recently. This process is attractive because it can produce diamond at ambient pressure and temperatures below 1000 °C. No plasma or other high-energy activation is required, thus providing an opportunity for large-scale synthesis. However, the mechanism of diamond formation has not been previously analyzed. This work reports on the formation mechanisms of diamond as well as the transformation of diamond to graphite and onionlike carbon upon heating. Study of SiC/carbon interfaces showed that direct epitaxial growth of diamond on SiC is possible, in agreement with previous molecular-dynamics simulation. However, random nucleation of diamond from amorphous sp3-bonded carbon produced as the result of extraction of Si from SiC has also been demonstrated. It has been shown that the presence of hydrogen in the environment is not required for diamond synthesis. However, hydrogen can stabilize the nanocrystals and lead to the growth of thick diamond layers. If no hydrogen is added, diamond nanocrystals transform to graphite, forming carbon onions and other curved graphitic nanostructures.

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