Mechanical strength and surface microstructure of partially crystallised glasses

The influence on the mechanical strength of lithia-silica glasses of heat-treatment in the temperature range (400 to 600° C), where the degree of crystallisation is small, has been studied. It was shown for a binary Li₂O-SiO₂ glass that heat-treatment led to a significant increase in the mechanical strength and that similar but less marked effects were observed for a Li₂O-SiO₂-P₂O₅ glass. From observations of the effects of abrasion upon the mechanical strength of the heat-treated glass it appeared that the strengthening effect arose from a change in surface structure of the glass. Examination by electron microscopy and X-ray diffraction revealed that a surface layer of lithium silicate crystals is formed during heat-treatment. The mechanical properties are discussed in relation to the surface structure of the glass.