Effect of lithium-to-magnesium ratio in metaphosphate glasses on crack-tip condensation and sub-critical crack growth

Corner cracks were initiated in a series of lithium–magnesium-metaphosphate glasses with different Li-to-Mg ratios by performing Vickers indentation under humid conditions (RH = 50% and RH = 65%) and in dry nitrogen gas atmosphere (RH = 0.01%). Propagation of each crack was observed up to 240 h. Distinct condensation of water at the crack-tip up to 30 μm occurred in humid environments, which led to a decrease in open crack length, while in dry nitrogen gas atmosphere this phenomenon was absent. A square root of time dependence was observed for the crack-tip condensation (CTC) kinetics as well as for weight gain experiments on crack-free glass specimens. Slow crack growth and CTC were dependent on the Li-to-Mg ratio of the glasses and showed a negative deviation from a linear compositional trend. Deviation of CTC was largest for glasses of Li-to-Mg ratio > 2:1 indicating a mixed mobile ion effect beyond charge neutrality, while the fatigue resistance exhibited a maximum at concentrations of equal charge. On the other hand a positive deviation from additivity was found for hardness and Youngʹs modulus indicating increased resistance against elastic and plastic deformation for glass compositions of mixed Li+ and Mg2 + ions.