Correlations between residual stress and water diffusion in silica glass at low temperatures

Residual stress profiles in silica glass were measured after water diffusion treatment under 47.33 kPa (355 Torr) water vapor at 350 degrees C and 650 degrees C. Earlier, it was found that water solubility in silica glass exhibited peculiar time dependence: Solubility increased with time exceeding the normal water solubility expected from higher temperatures. Then, the water solubility decreased with time. It was hypothesized that the stress induced by water diffusion and its subsequent relaxation is responsible for the phenomenon. Residual surface stress generation in silica glass was found to correlate closely with surface hydroxyl concentration, systematically increasing until eventual surface stress relaxation results in stress decrease for treatments beyond 265 hours at 650 degrees C. This observation validates previous theories of time dependent diffusivity in silica glass.