Beyond $β-C3N4$—Fullerene-like carbon nitride: A promising coating material

Even though the synthesis of super-hard-crystalline $β-C3N4$ 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 $sp2$ 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 $sp2$ 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.