Impact of pressure on the structure of glass and its material properties
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Introduction Structure-related physical properties of glassy materials can be altered significantly by the application of pressure,1−10 making it an important parameter for processing and characterization in glass science and engineering. It is therefore important to understand the mechanisms of compaction, which can be gradual, or abrupt as in so-called polyamorphic transformations.11,12 Unraveling the nature of structural change is a formidable task because of the complexity that originates from the atomic-scale disorder of glass and the experimental difficulties that are associated with the in situ investigation of materials under extreme conditions.13 Nevertheless, the combination of experiment with modern modeling methods is beginning to reveal the structure and related properties of glassy materials at high pressures.10,14 In this article, the focus is placed on silica glass because it is the basic constituent of silicate materials, and there is much information on its pressure-dependent structure and properties from both experiments and computer simulations. It is important to realize that the force applied in an experiment will not induce pure normal stress, unless a pressure-transmitting medium is employed to give hydrostatic conditions. More generally, the force acting on a surface will have components that are both perpendicular and parallel to that surface, thus inducing normal and shear stresses, respectively. The resultant deformation is either elastic or plastic, depending on whether
or not the material returns to its original shape when the load is removed.
Why is pressure important for glass? Stress in the gigapascal (GPa) regime is easily generated at the surface of glass by sharp-contact loading as in a Vickers hardness test, or when a sharp tip is moved laterally to induce a scratch.15 Materials with the ability to resist surface deformation are highly desirable for applications that include the cover glass for electronic devices, high-pressure windows, and glass for security and safety purposes. A successful strategy for strengthening the surface of glass is to place it under “chemical pressure.” Here, an ion-exchange process involving a molten salt is used to replace, for instance, the smaller Na+ cations in the surface region of a sodium aluminosilicate glass by larger K+ cations.16–18 In this way, the surface is placed under a compressive stress that can be ≥0.5 GPa. It is also desirable for glass to have a high fracture toughness (the ability to resist fracturing once a crack is formed via suppression of crack propagation). The impact of high pressure on the structure of glass and its related material properties is therefore of key importance for understanding the fundamentals of fracture mechanics at an atomistic level. The properties of glass can be tuned by using pressure to induce permanent densification, as illustrated in Figure 1a–b for the bulk modulus, K, and Poisson’s ratio, ν, of vitreous SiO2.8,10
Philip S. Salmon, University of Bath, UK; [email protected] Liping Huang, Re
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