Strength increase of silica glass fibers by surface stress relaxation: A new mechanical strengthening method

Pristine silica glass fiber is well known to become mechanically weaker when heat-treated in the presence of water vapor. However, the same fiber was found to become stronger if heat-treated while held under a sub-critical tensile stress at a temperature far below the glass transition temperature. The added strength of the stress-treated fiber was nearly equal to the applied tensile stress. This added strength was attributed to the formation of a compressive stress layer on the surface of the glass, created by a surface stress relaxation process that occurred while being held under the tensile stress. The presence of the surface compressive stress was confirmed by observing the bending of the fiber when 1) a tensile-stressed fiber was sliced and 2) a bending-stressed fiber was released from the stress. In the present paper we demonstrate that even though heat-treatment of a silica glass fiber in water vapor weakens the glass, a tensile stress application during the heat-treatment can increase the strength of silica glass fibers. Silica glass fibers with estimated strengths of ~ 7-8 GPa were produced, exceeding that of other fibers previously reported to have a maximum strength of ~ 5.5 GPa at room temperature in air. This new glass strengthening method does not require glass to be of a minimum thickness, as in tempering, or a glass containing alkali ions, as in ion-exchange.