Mixed alkali effect on Vickers hardness and cracking

Vickers indentation measurements were carried out on borate, silicate and aluminophosphate glasses, each series comprised of samples of different relative alkali ratios (Na/Na + Li). All the glass series exhibited nonlinear variations of hardness with relative alkali ratio, which was attributed to the reduced plastic flow of mixed alkali glasses. The mixed alkali effect was also present in the length of radial cracks although less strongly than in hardness. Using a semi-empirical model, the variations of residual stresses and fracture toughness were estimated.