Microhardness and flow mechanisms in fused silica

Abstract

The fundamental purpose of this investigation has been to develop a conceptual understanding of the hardness of glass, especially fused silica. Up until now, hardness has been treated primarily from an empirical viewpoint, with the subsequent result that hardness has little physical significance. In order to formulate hardness into a useful material property it is first necessary to understand the physical process occurring during indentation.

With this information, hardness of fused silica as determined by the Vickers Pyramid Indentor is defined as a resistance to densification.

From interference photographs of fracture-free, Vickers indentations made with a 1000 gram load, it was determined that an insignificant amount of fused silica is piled-up around the indentations. On this basis it is concluded that the mechanism of indentation of fused silica is densification. The nature of this densification has been investigated through annealing studies carried out on indentations. This suggested that a portion of the densification probably occurs by a reversible molecular entanglement mechanism. Material transport during indentation has been shown to occur primarily in a direction vertically downward into the specimen, rather than radially.

A critical review of the pertinent literature on hardness of glass has been included to point out some of the more serious errors which have been made and to indicate the background and current thinking of glass hardness