The Compositional Variation of Microindentation Induced Densified and Plastic Deformation Volumes in Simple Silicate Glasses

The densification and plastic deformation occurring in glass subjected to microindentation are established as two independent deformation mechanisms, and thought to be intimately linked to the concept of hardness and crack nucleation (quantified by the load at which radial cracks nucleate at half the corners of a Vickers pyramid indent.) These two properties are commercially important as they correlate with e.g. resistance to surface damage and the ultimate strength of glass, yet little is known of their variation with glass composition. In the present work, we determine the densified and plastically deformed volumes under microindentation in simple silicate glasses by AFM imaging the indent before and after annealing around the glass transition temperature at which the densified volume rapidly relaxes. The results show that the densified volume is inversely proportional to the bulk modulus, lending credence to the empirical correlation between hardness and elastic moduli. The plastic deformation volume was not measurable up to a modifying oxide fraction of 20%, but was found to increase linearly with this fraction above 20%. As the plastically induced stress is thought to be the cause of radial cracking, this finding is an important guide for discovering new highly crack resistant silicate glasses.