Morphology, topography, and crystal rotation during surface crystallization of BaO/SrO/ZnO/SiO2 glass

The initial stages of surface crystallization of glass with the composition 8BaO·8SrO·34ZnO·50SiO2 (mol%) are studied with respect to crystal orientation and morphology. Therefore, different microscopic techniques, such as laser scanning microscopy and scanning electron microscopy in combination with electron backscatter diffraction, were used to study the crystal growth at the initial surface, i.e. cross sections were not prepared. The precipitating crystalline phase has the composition Ba1−xSrxZn2Si2O7 with x ≈ 0.5 and shows highly anisotropic thermal expansion properties. Various morphologies were found which can be correlated with different crystal orientations. During growth, the crystals show a strong tendency towards orientation changes which lead to an unusual and rough surface. This study also explains the crystallization mechanism, according to which crystals at first spread at the surface and afterwards grow into the volume of the glass. The orientation changes, the morphology of the crystals and the resulting topography were correlated.