Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

The deformation and crystallization of Li2O.2SiO(2) and Li2O.1.6SiO(2) glass fibers subjected to a bending stress were measured as a function of time over the temperature range similar to50 to similar to150degreesC below the glass transition temperature (T-g). The glass fibers can be permanently deformed at temperatures about 100degreesC below T-g, and they crystallize significantly at temperatures close to, but below T-g, about 150degreesC lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams-Watt (KWW) approximation), rather than a single exponential model. The activation energy for stress relaxation, E-s, for the glass fibers ranges between 175 and 195 kJ/mol, which is considerably smaller than the activation energy for viscous flow, E-eta (similar to400 kJ/mol) near T-g for these glasses at normal, stress-free condition. It is suspected that a viscosity relaxation mechanism could be responsible for permanent deformation and crystallization of the glass fibers below T-g. (C) 2004 Kluwer Academic Publishers.