Surface Stress Relaxation and Resulting Residual Stress in Glass: A New Mechanical Strengthening Mechanism of Glasses

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NON-TECHNICAL DESCRIPTION: Glasses have many important applications because they are transparent, low cost, light-weight, and can be produced in large quantities and rapidly. They have one drawback: a lack of high mechanical strength. There are some methods to make glasses stronger such as rapid cooling called tempering which is used to make stronger window glasses or the ion-exchange method which is used to make stronger touch screen for smart phones. However, these methods are not applicable to all glasses and cannot be used, for example, to make stronger silica glass optical fibers. A new glass strengthening method, which can be used for all types of glasses, is explored.TECHNICAL DETAILS: Water vapor in the atmosphere can affect the glass surface characteristics. When a glass is subjected to a tensile stress below the critical stress which can break the glass, at an appropriate temperature below the glass transition temperature, some of the surface tensile stress is relaxed by the influence of moisture. When the applied tensile stress is removed, the surface of the glass acquires a residual compressive stress. The resulting glass sample is stronger than before, because a greater tensile stress is required, in order to overcome the generated surface compressive stress, to break the glass. By clarifying mechanism by which water promotes surface stress relaxation and controls the kinetics of surface residual stress generation, an optimum temperature, time, tensile stress, and moisture content can be chosen to produce high strength glasses. Specifically, the magnitude of a sub-micrometer thick surface residual stress in silica glass optical fiber, produced by heating under a tensile stress and water vapor, can be estimated by using both the bending of sliced fibers as well as FTIR reflection spectroscopy and the result compared with the measured mechanical strength of the fibers. Furthermore, the proposed mechanism of glass strengthening can explain some long-standing mysteries in glass science. For example, a crack in a glass can grow under a low tensile stress. However, when the glass was first subjected to a low tensile stress, one which does not cause crack growth, and then subjected to the original tensile stress, the crack does not grow. Apparently, the glass became stronger by a sub-critical tensile stress application, but no convincing explanation exists at the present. In this research two graduate students and two undergraduates are trained on this research topic which combines exploration of the science and technology of glasses.