Atomic force microscope study of the topography of float glasses and a model to explain the bloom effect

Abstract

The topography and nanostructure of several technical borofloat and soda-lime-silicate float glasses were investigated by a high-resolution atomic force microscope (AFM). The irregular ripple pattern to be seen on as-received atmosphere and tin bath side surfaces had an average diameter of ≈ 60 nm, heights < 1 nm and root mean square (rms) roughnesses on (1 × 1) µm2 images of < 0.25 nm. Topographies obtained in the mirror region of fracture surfaces displayed a somewhat coarser nanostructure. It could further be assured that there are no specific differences between the interior and the edge of the float glass sample. Inhomogeneities like precipitates, crystals, phase separation or pores are not caused by the in-diffusion of tin into the float glass. After annealing the float glasses in air, several of them showed the already long known phenomenon of bloom, a greyish haze produced by a wrinkling of the tin bath glass surface. The borofloat glasses did not produce bloom under any condition. The same was true for the Fe2O3-rich green and blue glasses. Depending on sample dimensions and annealing conditions only the Fe2O3-poor clear float glasses developed a pronounced bloom effect. It is known that annealing of the glasses in air causes an oxidation of Sn2+ to Sn4+, which acts as a network former. This causes a change in glass properties near the surface. However, the precisely measured in-depth profiles of all relevant species in the nanometer and the micrometer regions of the float glasses showed that only in the ease of the Fe2O3-poor silicate float glasses a reversed Sn2+ diffusion from the interior to the surface is caused by the air annealing, forming a very high and steep tin (Sn4+) enrichment in a superficial layer with a thickness between 50 and 150 nm. In analogy to thin film technology a simplified model was developed and a free buckling length of ≈ 2.3 µm was estimated for the bloom surface, which is in reasonable agreement with the experimental finding. It was further shown that a sol-gel derived SiO2 coating of the bloom surface could enhance the optical transmission of the glass considerably.