Inward Cationic Diffusion and Formation of Silica-Rich Surface Nanolayer of Glass

This paper reports a chemical approach for obtaining a silica-rich nanolayer on the surface of a vanadium-bearing silicate glass. The approach involves depletion of earth alkaline ions (Mg²⁺ and Ca²⁺) from the glass surface by means of inward diffusion of those ions, i.e., diffusion from the surface to the interior of the glass. The inward diffusion is induced by the reduction of V⁵⁺ to V⁴⁺ ions, when the glass is heat-treated in H₂/N₂ (1/99 v/v) at the glass transition temperature (T_g) for certain durations (t_a). During the reduction of vanadium by H₂, structurally bonded hydroxyl groups form and are incorporated into the glass structure. Both the V⁴⁺ and the hydroxyl contents increase with increasing t_a and hydrogen partial pressure. The inward diffusion enhances the hardness of the glass surface. The mechanism of the inward diffusion is suggested on the basis of a model describing the outward diffusion. The new approach provides a possibility to create a silica-rich nanolayer on glass surfaces by means of the inward diffusion process.