Indentation deformation mechanism in glass: Densification versus shear flow

Although the characteristic time constant for viscous relaxation of glass is so large at room temperature that viscous flow would be hardly detectable, a permanent deformation can be easily achieved at ambient temperature by applying a sharp contact loading--a Vickers indenter for instance--for few seconds only. We provide direct evidence for densification and volume conservative shear flow by means of atomic force microscopy topological analysis of the indentation profile and volume on as-quenched and densified specimens (pressure up to 25 GPa). We show that both possible mechanisms contribute to different extents depending on the glass composition. A major finding is that densification predominates in glasses with relatively low atomic packing density but that shear flow relays on once densification is achieved.