Sr-fresnoite surface crystallisation in a 2SrO·TiO2·2.75 SiO2 glass studied by EBSD

Abstract

A glass of the composition 2SrO·TiO₂·2.75SiO₂ was melted. Cooled samples were polished and thermally annealed at 970 °C for 10 min to 20 h to achieve surface crystallisation of Sr₂TiSi₂O₈ fresnoite. At the surface, the crystals were immediately oriented with their crystallographic c-axes perpendicular to the surface. Crystal growth occurred in the form of similarly oriented structures. Very homogeneous structures were also observed but they fray into areas of higher orientation diversity at some distance from the surface. The crystallised areas were highly permeated by nanoscale inclusions of residual glass. Kinetic selection occurs and leads to a preferred orientation with the crystallographic c-axis tilted by about 43 ± 5° from the surface normal after about 300 μm. Crystal growth is not governed by the fastest growing crystallographic direction or the formation of a diffusion barrier, but rather by the ability to circumvent the residual glass.