Viscosity of glass below the transformation temperature

Abstract

Glass is an ideally brittle material at room temperature. The results of hardness indentation tests demonstrate this property by crack formation at the corners of the indent. But it is possible to avoid the crack formation if the load, the contact area and the deformation rate, respectively, are small enough. The mechanisms of this permanent deformation of brittle glasses by indentation close to room temperature have not been understood completely up to now. Viscous flow is responsible for permanent deformation of glasses above the transformation temperature (Tg). The same mechanism of deformation during indentation experiments is assumed in this work hypothetically, taking into account the very high stresses (GPa range) in the local volume around the indenter tip. This hypothesis of deformation at room temperature will be verified by independent experiments that work like viscometers in principle (e.g. cylinder compression experiments, ball indentation experiments). All experiments were analysed with the help of simple rheological models. Additionally some ball indents were analysed by topographical investigations to get first hints of viscoelastic deformation behaviour below Tg of glass.