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.
