Measurement of progressive stress buildup during ion exchange in alkali aluminosilicate glass

Abstract

A mixed-alkali aluminosilicate glass (glass transition temperature, T g=460 ∘C) was ion exchanged at 375 ∘C in a fused silica cell in the hot-stage of a polarizing microscope. Buildup of stress-induced optical birefringence was measured as a function of the depth below the surface and time of exchange to 8 h. Surface compression increased steadily during this period. Upon rapidly cooling the specimen to room temperature, stresses decreased ∼20%, without a significant change in the thickness of the compression layer implying that the surface layer had a higher thermal contraction following the exchange. Differential thermal contraction of ion-exchanged layers could not have been the cause of either large differences between calculated stress magnitudes from those observed experimentally, or the observed “anomalous” tensile stress maximum. It is argued that the ion-exchanged layers are stuffed derivatives having physical properties different from those of the parent glass or the as-melted glasses of corresponding compositions.