The surface of a glass is expected to have structure and properties different from those of the bulk. The structural state of the silica glass surface is important in determining a variety of silica properties, such as the mechanical and chemical properties of optical fibers and the electrical and optical properties of thin silica films. In general, the structural state of a glass changes with time through the relaxation process. Our results indicate that the surface relaxation takes place much more rapidly than the bulk relaxation. Furthermore, unlike the bulk relaxation, the surface relaxation was promoted by water vapor and an applied tensile stress. It is suggested that the surface structural relaxation promoted by water vapor and tensile stress may be a mechanism of the mechanical fatigue of silica glasses. Two different modes of surface relaxation appear to exist under a constant external condition.
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