Mechanical properties of sheet glass at high pressure during indentation experiments

Abstract

Viscous flow is responsible for permanent deformation of glasses above the transformation temperature (Tg). The results of macroscopic indentation experiments (maximum loading forces Fmax ≥ 2 N) with sharp indenters (Vickers or Berkovich pyramids) demonstrate this property by crack-free indents at T ≥ Tg. But it is also possible to make crack-free indents at room temperature (RT), well below Tg, if the load, the contact area and the deformation rate, respectively, are small enough, which is consistent with glass as a supercooled liquid but inconsistent with glass as a well-known ideally brittle material at RT The mechanisms of this permanent deformation of glasses by indentation near RT have not been understood completely up to now. For the analysis of the macroscopic indentation experiments with Vickers pyramids and spherical indenters, respectively, a viscoelastic deformation behaviour is assumed for the complete temperature range from RT to Tg. Therefore the rheological analysis of all experiments were performed with the help of a simple Maxwell model. The results of Vickers indentation experiments were compared with cylinder compression experiments and ball indentation experiments which were analysed by the same rheological principles.