Viscous flow is responsible for permanent deformation of glasses above the transformation temperature (Tg). The results of macroscopic indentation experiments 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), 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.
