Surface crystallization of low thermal expansion glass ceramics

Reference Presenter Authors
10-062 Michael Kracker Kracker, M.(Otto-Schott-Institut für Materialforschung); Thieme, C.(Fraunhofer Institute of Materials and Systems IMWS); Thieme, K.(Otto-Schott-Institut für Materialforschung); Höche, T.(Fraunhofer Institute of Materials and Systems IMWS); Rüssel, C.(Otto-Schott-Institut für Materialforschung); The transformation of a glass into a partially crystalline material with a defined thermal expansion is of great scientific interest. This is justified by the fact that high temperatures are required, which often lead to considerable problems during the cooling procedure due to the differences in thermal expansion of the host glass and formed crystal phase. Hence, crystalline phases with low thermal expansion are to be preferred. If the formed phase crystallizes predominantly from the surface of the glass, this behavior is of particular interest for science and technology. In contrast to the commonly used Li2O-Al2O3-SiO2 glass systems, a new crystalline solid solution phase with the composition Ba1-xSrxZn2Si2O7 was recently developed in order to achieve low thermal expansion and a strong tendency of surface crystallization. Unfortunately, due to the high anisotropy of the coefficients of thermal expansion, the grown surface layer is responsible for the destruction of the complete sample after heat treatment. It has been found that a few large crystals are formed which induce large stresses into the material. The hypothesis is to increase the number of surface nuclei in order to reduce the size of the crystals by growth selection. The study presents the effect of different surface coatings on the resulting microstructure of the formed surface crystal layers. Also, the influence on surface morphology was investigated and compared to the microstructure. Particular attention was paid to the formation of cracks caused by the resulting microstructure. The microstructure and phase formation were investigated by scanning electron microscopy, electron backscattering diffraction and X-ray diffraction.
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