Nano-indentation and surface hydration of silicate glasses

Pure silica is commonly used as a calibration material for nano-indentation as it can be assumed, for the conventionally studied indentation depths, that the surface hydration layer can be ignored. However for very low load indentations this assumption cannot be made. To gain some insight into the properties of such gel layers nano-indentation studies on the hydration layers on three silicate glasses that hydrate more and less slowly are performed. With the less durable glasses (a soda-silica composition and a high soda, low lime soda-lime-silica composition) the evolution of the mechanical properties of the hydration layers with time can be readily followed. The limiting stiffness values obtained for very low load indentation on pure silica are used to assess the potential effect of such a hydration layer on pure silica on nano-indentation calibration using pure silica.