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

•Distinct crack-tip condensation (CTC) in Li–Mg metaphosphate glasses in humidity•Sub-critical crack growth (SCCG) under CTC conditions follows Paris' law.•Negative deviation from compositional additivity of the CTC and SCCG kinetics•Mixed mobile ion effect of mechanochemical properties

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 Young's modulus indicating increased resistance against elastic and plastic deformation for glass compositions of mixed Li+ and Mg2 + ions.