Highlights

Cracking behavior of various glasses under a 1 kilogram force load. 

a.) normal glass,  b.) intermediate glass, c.) anomalous glass

Vickers indentations under 1 kg load are shown. While many glasses exhibit extensive cracking, (a) and (c), new glass did not, (b). When a glass exhibits permanent deformation, it assumes a higher temperature structure. The cracking appears related to the changes of the glass properties associated with the higher temperature structure.

There are two types of oxide glasses. One type is normal glass and its typical example is soda-lime glass and the other type is anomalous glass and its typical example is silica glass. The former exhibits property change in one direction, e.g. a larger volume, when quenched from a higher melt temperature, while the latter exhibits property change in the opposite direction, e.g. smaller volume, when quenched from a higher temperature. Between these two types of glasses, there are intermediate glasses, whose properties do not change much with the cooling rate from the melts. These intermediate glasses were found highly crack-resistant. It is believed that the permanent deformation of the glasses is accompanied with the change of the glass structure to a higher temperature state. When the higher temperature structured glass produced by permanent deformation had the similar properties to the original glass, the cracking is less likely. For details, see the following paper. T.M. Gross, M. Tomozawa, A. Koike, J. Non-Crystalline Solids, 355 (2009) 563-568.