Mechanical Properties of Oxide Glasses at Constraint Gradients

如果必须将任何材料属性命名为最相关的，选择可能会落到机械行为上。这一点尤其适用于眼镜，因为实际应用(通常可用)和理论强度相差一个或多个数量级，因此，改进的潜力似乎最耐人寻味。这里的瓶颈是，一方面严重缺乏对结构组成和力学性能之间相互作用的根本了解，另一方面则缺乏潜在的增韧策略。缺陷，主要是表面缺陷及其产生的原因被认为是造成理论和实践之间巨大差异的主要原因。因此，今天的经验增韧过程主要集中在工程特定的表面结构上，主要依赖于残余压缩的产生。在本项目中，将共同处理这两个问题--结构-性质关系的理解和定量、表面定向增韧策略。拟议的项目将专注于将拓扑约束理论扩展和实验应用于成分连接处的玻璃形成系统，其中一个或多个结构参数已知会发生急剧变化。作为技术上相关的模型体系，已选择了混合网络前硼硅酸盐、铝硅酸盐和可选的钛酸盐玻璃，在这些玻璃中，已知的硼和铝的配位在相对较窄的组成区域发生了急剧变化。除了这两个体系外，还将考虑具有单一网络形成物(随网络改性剂含量改变配位)和磷酸盐玻璃(在共价网络和完全解聚的离子状态之间的过渡)的碲酸盐玻璃。对于这些系统中的每一个，将与SPP 1594的其他参与者合作，对弹性特性、硬度、韧性、脆性和固有强度的依赖性进行实验评估，并与中短尺的相关结构参数相关联。关注急剧结构梯度的组成--“极端组成”--有两个动机：在这样的组成领域，我们期望在确定玻璃机械性能的拓扑设计的本构定律方面具有最高的准确性。其次，该方法将进一步开发表面工程工具(特别是阳离子和阴离子交换)，其中可以在表面产生专用的拓扑梯度，扩展了化学增韧的经典观点。第一个资助期的主要目标是发现与力学性质有关的主要拓扑约束，并推导出与温度有关的函数，这将验证和扩展经典约束理论。预计在与SPP 1594的平行活动协同下，这些功能可以被推广，以弥合金属和无机氧化物系统之间的知识差距。在筹备第二个资助期时，将考虑在玻璃表面上建立这种拓扑梯度的工程方法，从而产生机械约束的具体梯度。

If any materials property had to be named the most relevant, the choice would probably fall to mechanical behavior. This holds especially for glasses, where practical (commonly available) and theoretical strength differs by one or more orders of magnitude and, hence, the potential for improvement appears most intriguing. Here, the bottleneck is the significant lack of fundamental understanding of the interplay between structural constitution and mechanical properties on the one side, and potential toughening strategies on the other side. Defects, primarily surface defects and their initiation are known as the primary reason for the large difference between theory and praxis. Today's empirical toughening procedures therefore focus on engineering specific surface architectures, mostly relying on the generation of residual compression. In the present project, both problems - the understanding of structure - property relations and quantitative, surface-targeting toughening strategies will be treated jointly. The proposed project will focus on extending and experimentally applying the theory of topological constraints to glass forming systems at compositional joints, where one or more structural parameters are known to change sharply. As technologically relevant model systems, mixed-network-former borosilicate, aluminosilicate and, optionally, titanate glasses have been chosen where boron and aluminum coordination, respectively, are known to sharply change in relatively narrow compositional regions. In addition to these two systems, tellurite glasses with a single network former (changing coordination as a function of network modifier content) and phosphate glasses (on the transition between covalent network and fully depolymerized, ionic state) will be considered. For each of those systems, the dependence of elastic properties, hardness, toughness, brittleness and - in cooperation with other participants of SPP 1594 - intrinsic strength will be assessed experimentally and related to relevant structural parameters on short and intermediate length scale. The focus on compositions at sharp structural gradients - "extreme compositions" - is motivated twofold: in such compositional areas, we expect the highest accuracy in identifying constitutive laws for the topological design of the mechanical properties of glasses. Secondly, the approach will enable further development of surface engineering tools (especially cation and anion exchange) where dedicated topological gradients can be generated at the surface, extending the classical view of chemical toughening. Primary objective of the first funding period is to perceive the dominant topological constraints with respect to the mechanical properties and to deduct temperature-dependent functions which will verify and extent the classical constraint theories. It is anticipated that in synergy with parallel activities of SPP 1594, these functions can be generalized to bridge the gap of knowledge between metallic and inorganic oxide systems. In preparation of the second funding period, engineering approaches to establish such topological gradients on glass surfaces and, hence, to create specific gradients of mechanical constraints will be considered.