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Surface-initiated microstructure formation in glass ceramics

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Synthesis and Properties of Functional Materials
Term from 2017 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 382920141
 
Final Report Year 2022

Final Report Abstract

Using EBSD as primary analysis technique, surface-initiated microstructure formation in glass ceramics was studied extensively within this project. The focus was on preferential orientation caused by oriented nucleation, as well as growth-related effects. Novel analytical approaches and sample preparation techniques were applied to glass ceramics which were subject to highly-defined surface preparation and annealing conditions. As the main result of the project, we found that a strong orientation tendency appears for systems with a strong crystal anisotropy in anisotropic environments. Isotropic crystals would not provide any driving force of crystal orientation even in anisotropic environments. This statement is supported by the observation that this texture is evident even during early growth of separated crystals of ≈ 700 nm in size, not changing during further crystal growth. Hence, crystal orientation seems to result from oriented crystal nucleation rather than from crystal-crystal interactions during early growth or growth-selection phenomena. Based on these results, a theoretical approach to describe oriented nucleation was developed. The interfacial tension was identified as a possible cause of the orientation, and theoretical calculations were carried out to estimate the interfacial tension glass-crystal, taking into account different orientations (faces) of the crystals. For this approach, the bond energy related to different crystal faces was estimated using calculated average crystal fracture surface energies derived from diatomic bond energy. We assume that a minimum energy surface structure will occur during crystal growth, which is equal to the minimum energy cut-surface of this orientation. Therefore, the crystal face with the highest interfacial energy is wetted by the melt, whereas low energy faces can be oriented parallel to the surface. This model was developed for the isochemical surface crystallization of diopside but seems to hold also for other cases like STS-fresnoite, SrAl-silicate crystals, YAG, and cordierite. It fails, however, for BTS-fresnoite. However, during further growth of crystals oriented at nucleation in systems with strong anisotropy, changes in texture may occur. These can be caused, for example, by increasing stresses around the growing crystals. The chemistry surrounding the crystal also plays a significant role, especially in non-stoichiometric systems. This change in orientation was extensively discussed with the Mercator Fellows, and will be implemented in crystal-growth models of Prof. Tonchev. For crystal growth at lower viscosities, also a change of the primarily orientation of the crystals can occur by rotation of the surface crystals caused by interaction with neighbored crystals. Novel sample preparation techniques, aimed at allowing the generation of 3D datasets were developed. Sectioning of a samples with depth-dependent microstructure using initial notches for further, depth-dependent EBSD analysis was successfully demonstrated. In particular, it was shown that smooth surface sections with lowest levels of ion-beam induced amorphization can be created very efficiently. The preparation of XRM pillars was also successfully demonstrated, both in cross-sectional and in-plane geometry.

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