The Hardness of Vitreous Silica

Abstract

THE hypothesis proposed by Sir George Beilby to account for the hardening of metals by cold-working, and accepted by most metallurgists in this country, assumes the production of a vitreous phase of the metal by the breaking down of the crystalline structure during extensive deformation. It requires that the vitreous modification of a substance should be harder than the crystalline. Direct evidence on this point has rarely been obtained. Silica, however, suggests itself as a suitable substance for such a test, both the crystalline forms and the under-cooled glass being readily obtained in a form suitable for mechanical tests. The hardness of silica was carefully studied by Auerbach, who found a surface of vitreous silica to be less hard than either of the principal faces of quartz. Most commercial silica glass is, however, so weakened by the presence of numerous gas bubbles that any grinding test is likely to give low results for rthe hardness.