Stress-enhanced ion diffusion at the vicinity of a crack tip as evidenced by atomic force microscopy in silicate glasses

The slow advance of a crack in sodo-silicate glasses was studied at nanometer scale by in situ and real time atomic force microscopy (AFM) in a well controlled atmosphere. An enhanced diffusion of sodium ions in the stress gradient field at the submicrometric vicinity of the crack tip was revealed through several effects: growth of nodules in AFM height images, changes in the AFM tip/sample energy dissipation. The nodules patterns revealed a dewetting phenomenon evidenced by 'breath figures'. Complementary chemical micro-analyses were done. These experimental results were explained by a two-step process: i) a fast migration (typical time: few milliseconds) of sodium ions towards the fracture surfaces as proposed by Langford et al. [J. Mat. Res. 6 (1991) 1358], ii) a slow backwards diffusion of the cations as evidenced in these AFM experiments (typical time: few minutes). Measurements of the diffusion coefficient of that relaxing process were done at room temperature. Our results strengthen the theoretical concept of a near-surface structural relaxation due to the stress-gradient at the vicinity of the crack tip. The inhomogeneous migration of sodium ions might be a direct experimental evidence of the presence of sodium rich channels in the silicate structure [A. Meyer et al., Phys. Rev. Let. 93 (2004) 027801].