Strengthening of Class Fibers: 11, Ion Exchange *

Abstract

Soda‐alumina‐silica glass fibers were ion‐exchanged in potassium nitrate at 400°C; their strength was measured before and after exchange. The potassium concentration distribution and the optical retardation were also measured. The results for unetched fibers are consistent with the premise that the kinetics of exchange are controlled by diffusion with D ⋍×10 −11 cm 2 /s) and with the predictions of thermal stress analysis with a dilation coefficient of 2.8×10 −4 (wt fraction potassium) −1 . The existence of a broad maximum in the plot of strengthening of unetched fibers vs time may be explained by the existence of Griffith flaws about 2 μm deep in the unexchanged glass. A deleterious effect from ion exchange was noted with etched fibers, in which even a short (½ h) exchange caused significant weakening.