Crack healing in soda-lime silicate glass: the effect of water
Résumé
Surface cracks lead to the decrease of the glass lifetime. Consequently, the understanding of self-healing can give a method to increase the glass durability. In this study, the self-healing behavior of radial cracks generated by Vickers indentation on float glass is analyzed when heat treated at 620°C under various atmospheres (from 0 to 75% RH). Two main types of radial crack evolution are observed: (1) either very large crack morphological changes, or (2) a simple direct crack closure. We associate these two phenomena respectively to the effects of capillarity forces induced by surface energy minimization or to the effects of residual stress release. The type and rate of crack changes are shown to be highly dependent on (a) the humidity level of the furnace atmosphere but also on (b) the initial water content of the glass at the vicinity of the crack. The large crack morphological changes, (enlargement, shortening and rounding of radial cracks) observed in case (1), are attributed to smoothing of the crack tip through viscous flow driven by capillary forces. We suggest that the viscosity of matter surrounding cracks can be significantly reduced or increased by hydration/des-hydration of the sample from the surface, as a function of the water vapour pressure in the furnace. Changes in cracks morphology are then dependent on the changes in the dissolved water around the cracks, which are dependent on the water vapour of the atmosphere and on the initial water content, but which also depend on time and depth. It is noteworthy that these morphological changes also depend on capillary forces, i.e. on the curvature radius of the crack, in a viscoelastic medium. Direct crack closure, case (2), is only observed when the hydration level of the glass in which the crack is generated is very low. The initial viscosity quite elevated around the cracks and viscous flow driven by capillary forces is impeded. We assume that direct crack closure is then due to release of residual tensile stresses induced by indentation.