Modifying glass surfaces via internal diffusion

The surface chemistry and structure of iron-bearing silicate glasses have been modified by means of heat-treatment around the glass transition temperature under different gaseous media at ambient pressure. When the glasses are heat-treated in atmospheric air, oxidation of Fe2+ to Fe3+ occurs, which leads to outward diffusion (OD) of divalent cations (primarily Mg2+), i.e., diffusion from the interior of the glass to the surface, and thereby, to formation of an oxide surface nano-layer. In contrast, when the glasses are heat-treated in H2/N2 gas containing 10 vol.% H2, reduction of Fe3+ to Fe2+ occurs due to both the permeation and reducing ability of H2. In this case, OD of divalent ions also occurs, and hence, an oxide surface layer forms. However, such outward diffusion differs from that induced by the iron oxidation in terms of physical origin. The former is due to incorporation of the N3− ions in the network and their strong attraction to the modifying ions, whereas the latter is due to the requirement of the charge neutrality. The role of N3− in driving OD is verified by the composition profile of the surface layer of the glass treated in pure N2 gas. The OD exerts pronounced impacts on some properties such as hardness, chemical durability, and surface wettability.