Surface crystallization of low thermal expansion Ba0.5Sr0.5Zn2Si2O7 from an 8 BaO·8 SrO·34 ZnO·50 SiO2 glass

Thermal treatment of a glass with the composition 8 BaO·8 SrO·34 ZnO·50 SiO2 has led to the crystallization of a Ba1−xSrxZn2Si2O7 solid solution. This solid solution has a very low or even negative thermal expansion. The glass system possesses a strong tendency towards surface crystallization, while bulk nucleation is negligible. The surface crystallization behavior was characterized using differential scanning calorimetry, X-ray diffraction, optical microscopy, and scanning electron microscopy including electron backscatter diffraction. The observed morphology strongly depends on the type of surface pretreatment. Two different surface qualities were compared: a polished surface and a surface obtained by cutting. The as-cut surface shows a significantly enhanced nucleation rate, which leads to smaller crystals caused by a growth selection near to the surface. The crystal orientation related to the inward growth starting from the surface was successfully investigated by EBSD, for the first time, using a recently reported crystal structure. The crystals show a preferred orientation of the c-axis perpendicular to the surface; this has a significant effect on the crack formation.