Scratch-induced yielding and ductile fracture in silicate glasses probed by nanoindentation

Lateral nanoindentation provides access to the scratch hardness of glass surfaces. The specific sensitivity of the scratching experiment to surface mechanical properties can be enhanced when the local load at the tip apex is reduced. Here, we report on ramp-load scratch tests on a range of silicate glasses using a sphero-conical tip shape. Similar as with regular scratching experiments using sharp indenters, such tests create a sequence of micro-ductile, micro-cracking, and micro-abrasive regimes. Detailed investigation of the indenter displacement h and of the lateral force F_L as recorded in situ, however, reveals pronounced deviations in comparison to Vickers or Berkovich scratching experiments. Most notably, this includes an abrupt increase in both h and F_L at moderate normal load, marking the onset of ductile fracture, and a yield point at the transition from fully elastic deformation to the elastic-plastic regime at low load. For the range of examined silicate glasses, we find that structural cohesion controls yielding, whereas scratch-induced fracture and micro-abrasion are dominated by the volume density of bond energy.     

Measurement approaches to develop a fundamental understanding of scratch and mar resistance

Instrumented indentation and confocal microscopy were used to characterize the surface mechanical response of polymeric materials. Viscoelastic behavior was measured using instrumented indentation. A model based on the contact between a rigid probe and a viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two polymeric materials, epoxy and poly(methyl methacrylate) or PMMA. Scratch testing was performed on these materials with various probes under a variety of conditions, and confocal microscopy was used to characterize the resulting deformation. Relationships among viscoelastic behavior, scratch damage, and appearance are currently being explored using these methods along with finite element modeling. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13–14, 2004, in Philadelphia, PA.     

Surface Damage Resistance of Gel-Derived Oxycarbide Glasses: Hardness, Toughness, and Scratchability

Gel-derived oxycarbide glasses have atomic network structures similar to that of vitreous silica glass but with carbon-rich regions consisting of CSi₄ tetrahedra and C–Si–O bonds finely dispersed in the glass. Therefore, oxycarbide 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 hand, the covalently bonded carbon atoms slightly affect the behavior, similar to the way network modifiers affect the behavior of silicate glasses, and favor a normal indentation behavior; and on the other hand, the free carbon, forming turbostratic graphite domains, provides easy crack initiation sites and low-energy fracture paths. Almost concentric shear steps and microcracks, which follow the turbostratic graphite domains, are observed after indentation. The ultimate coalescence of the microcracks produces Hertzian-type cone cracks.