Surface-initiated microstructure formation in glass ceramics

到目前为止，表面成核和表面诱导织构形成的机理还很不清楚。相应的现象被假设性地讨论，甚至是有争议的，相关的研究仅限于极少数的玻璃。作为一个主要缺点，以前关于表面晶体定向生长的绝大多数工作没有考虑玻璃表面处理(如抛光缺陷或附着的颗粒和裂纹)或环境退火条件(真空或大气压)的可能影响，这两者都会影响晶体的形核和生长。此外，与表面结晶有关的晶体取向的观察很少集中在单独生长的晶体上。总之，到目前为止，还没有关于初始取向晶体生长或从定义的活性形核位置成核的系统研究，因此，本项目的主要目标是对微晶玻璃表面诱导微结构的形成机理有一个基本的了解。我们将回答这样一个问题，表面晶体的择优取向是定向成核的结果，还是由晶体生长早期作用的其他取向选择机制造成的。在这两种情况下，晶体取向可能是由玻璃表面本身的取向或主动表面成核缺陷的各向异性和取向引起的。应采用表面结晶玻璃实验表征的新技术来寻找上述问题的答案。基于玻璃样品表面的受控制备和退火，包括定义的活性成核缺陷的产生，以及随后的显微结构表征，特别是在分离的表面晶体的非常早期的生长阶段，将分析缺陷和初始表面晶体之间的取向关系。为了获得新的见解，将开发或应用一些全新的微结构诊断技术，包括化学对比XRM、基于激光的XRM样品制备和3D电子背散射衍射。这样，使用X射线显微镜作为关键技术，以高空间分辨率无损地表征相对较大的样品(但也与其他微结构诊断技术相补充，如基于透射电子显微镜的横截面样品的元素映射)，将获得对模型玻璃表面晶化过程中微结构演变的前所未有的洞察。

Up to now, the mechanisms of surface nucleation and surface-induced texture formation are far from being understood. Corresponding phenomena are discussed hypothetically or even controversial, and related studies are restricted to very few glasses. As a main drawback, the vast majority of previous works on oriented growth of surface crystals do not consider the possible effects of glass surface treatments (like polishing defects, oradhered particles and cracks) or ambient annealing conditions (vacuum or atmospheric pressure), which both can influence crystal nucleation and growth. Moreover, very few observations of crystal orientation related to surface crystallization were focused on separately growing crystals. In conclusion, up to now, no systematic studies on initially oriented crystal growth or nucleation from defined active nucleation sites have been pursued.Therefore, the main objective of the proposed project is to gain a basic understanding of the mechanisms of surface-induced microstructure formation in glass ceramics. We shall answer the question whether preferred orientation of surface crystals is the result of oriented nucleation, or caused by other orientation selection mechanisms acting during early crystal growth. In both cases, crystal orientation may be caused by the orientation of the glass surface itself or the anisotropy and orientation of active surface nucleation defects. New techniques of experimental characterization of surface-crystallized glasses shall be applied to find answers to the questions posed above. Based on a controlled preparation and annealing of the glass sample surfaces, that includes the defined creation of active nucleation defects, and followed by the microstructural characterization especially during the very early growth stages of separated surface crystals, the orientation relations between defect and initiated surface crystal will be analyzed. In order to gain new insights, a couple of entirely new microstructure diagnostics techniques, including chemically contrasted XRM, laser-based sample preparation for XRM, and 3D electron backscatter diffraction shall be developed or applied.This way, using X-ray microscopy as a pivotal technique for the non-destructive 3D characterization of relatively large samples at superior spatial resolution (but also complemented by other microstructure diagnostics techniques, e.g. transmission electron microscopy based elemental mappings at cross-sectioned samples), unprecedented insights into the microstructure evolution upon surface crystallization of model glasses will be gained.