Superhard and Elastic Carbon Nitride Thin Films Having Fullerenelike Microstructure

${\mathrm{CN}}_x$ films were found to have a distorted graphitelike microstructure consisting of buckled and curved basal planes using high-resolution electron microscopy. Nanoindentation showed an elastic recovery of 85%, and a hardness of $\sim 60$ GPa. Based on detailed quantum chemical calculations and photoelectron spectroscopy, the observed structure is proposed to consist of a network of buckled ${\mathrm{sp}}^2$-hybridized ${\mathrm{CN}}_x$ planes, cross-linked by ${\mathrm{sp}}^3$-hybridized bonds. The buckling is shown to be due to the incorporation of pentagons in the basal planes, giving rise to a fullerenelike microstructure.