Change in Vickers hardness at the glass transition region for fragile and strong glasses

The temperature dependence of Vickers hardness between room temperature and around the glass transition temperature T_g in a vacuum for some glasses with different fragile characters was measured. The hardness at T/T_g<0.9 decreases gradually with increasing temperature, but the glasses show unique temperature dependences in the range 0.9<T/T_g<1.1 depending on their fragile character. A sharp decrease in hardness is observed at around T/T_g=0.9-1.0 for fragile Bi₂O₃- and TeO₂-based glasses, but 50CuO_x·50P₂O₅ and soda-lime silicate glasses with strong viscosity characteristics still have relatively large Vickers hardness even at T_g and the hardness drops to almost zero around T/T_g=1.05-1.1. It is suggested that for fragile glasses the β-relaxation has an important effect on the Vickers hardness change near the glass transition region, but for strong glasses it is the α-relaxation that governs the hardness near T_g.