Effect of synthesis temperature on hardness of carbon phases prepared from C60 and nanosized diamonds under pressure

Vicker's hardness and Raman scattering spectra have been studied for carbon phases prepared from C60 fullerene and nanosized diamonds at high temperatures and a pressure of 6 GPa. It was found that the hardness dependence on annealing temperature has a maximum near ∼1100 K for both fullerene and nanosized diamonds as initial materials. This temperature is only slightly higher than the temperature at which the C60 cage collapses, and appears to correspond to the termination of intercluster bonding in the case of nanosized diamonds. The hardness maximum is interpreted as a result of competition between an increase in intercluster/intercage bonding and local instability for graphitic-like ordering.