Beyond β-C3N4-fullerene-like carbon nitride:: A promising coating material

Even though the synthesis of super-hard-crystalline beta-C3N4 remains elusive, noncrystalline CNx compounds are of increasing importance owing to their competitive properties. Especially the fullerene-like allotrope of CNx exhibits outstanding elasticity in combination with low work of indentation. This new class of thin solid film materials is characterized by a microstructure of bent and intersecting basal planes. Substitutional incorporation of nitrogen into the predominantly sp(2) hybridized graphitic layer triggers the formation of curvature-inducing pentagons and interplanar cross-links at a much lower energy cost as compared to carbon-only materials. The term "fullerene-like" was coined to reflect the nanometer scale of curved structural units. Thus, fullerene-like CNx deforms by bond angle deflection and compression of the graphitic interplanar lattice spacing, whereas the superior strength of the sp(2) bonds inhibits plastic deformation giving the material an extremely resilient character. The orientation, radius of curvature of basal planes, and density of cross-linking can be adjusted by the synthesis conditions. Here, the existence of significant numbers of precursor molecules is a determining factor. The inherent resiliency of the material in combination with the carbon-based beneficial friction promises to give rise to numerous tribological applications. (c) 2007American Vacuum Society.