INDENTATION BEHAVIOR OF SODA-LIME SILICA GLASS, FUSED-SILICA, AND SINGLE-CRYSTAL QUARTZ AT LIQUID-NITROGEN TEMPERATURE

In an attempt to elucidate the processes involved in the formation of indentation impressions, Vickers hardness measurements have been made on soda-lime silica glass, fused silica, and crystalline quartz indent ed at room temperature and 77 K. The hardness of all three materials i ncreases by a factor of similar to 2.5 on cooling to liquid nitrogen t emperature. High-magnification SEM photographs revealed that the defor mation and cracking patterns of the glasses changed strikingly: no she ar lines were observed within the indentations, and ring cracking occu rred instead of radial/median cracking. In addition, cracking occurs a t much higher loads than at room temperature. The hardness results hav e been explained in terms of volume how (densification) rather than sh ear flow (viscous or plastic) for the glasses at low temperature. The quartz crystal, on the other hand, deformed plastically at both room t emperature and 77 K. Cracking differences result from changes in both flow and water activity.