Surface damage resistance of gel-derived oxycarbide glasses: Hardness, toughness, and scratchability

Gel-derived oxycarbide glasses have atomic network structures similar to th at of vitreous silica glass but with carbon-rich regions consisting of CSi4 tetrahedra and C-Si-O bonds finely dispersed in the glass. Therefore, oxyc arbide glasses exhibit the so-called "anomalous" hardness behavior, similar to silica-rich glasses, with a substantial densification-strain component beneath the indenter. However, the role of carbon is twofold: on the one ha nd, the covalently bonded carbon atoms slightly affect the behavior, simila r to the way network modifiers affect the behavior of silicate glasses, and favor a "normal" indentation behavior; and on the other hand, the free car bon, forming turbostratic graphite domains, provides easy crack initiation sites and low-energy fracture paths. Almost concentric shear steps and micr ocracks, which follow the turbostratic graphite domains, are observed after indentation. The ultimate coalescence of the microcracks produces Hertzian -tv pe cone cracks.