Strain-rate sensitivity of glasses

Although glasses are commonly thought of as ideal brittle materials, a local plastic deformation can be induced by the penetration of sharp objects, such as the indenter tips commonly employed for hardness testing. Since the first experimental verification of this phenomenon, considerable research effort has been devoted to the underlying microscopic mechanisms, which govern the indentation deformation of glasses, but also to associated properties of technological relevance, like crack initiation and defect tolerance, fracture toughness, brittleness or even scratchability. In early indentation studies, this has been achieved through a post-mortem topographic or spectroscopic analysis of the residual hardness imprints left on the glass surface. However, with the ongoing demand for a mechanical characterization at small-scales, advanced indentation protocols have been developed, which nowadays allow for a more comprehensive description of the fundamental deformation processes in amorphous materials and by extension, the design of more defect-tolerant glasses. But regardless of the numerous studies on the contact damage resistance of glasses, the time or rate dependence of the indentation deformation, commonly referred to as creep, still remains poorly understood. To overcome this issue, the influence of the imposed strain-rate on the hardness of a variety of glasses with covalent, ionic and metallic bonding character as well as varying degrees of network dimensionality and atomic packing density was characterized in a nanoindentation strain-rate jump test. Based on these results, a qualitative mechanistic description of the topological principles that determine the rate dependence of the glass hardness has been proposed.