Ion exchange of mixed-alkali glasses

The effect of mixed-alkali composition on ion-exchange strengthening of glasses was studied. The partial substitution of potassium for sodium in a soda-lime-silicate host glass was found to increase the potassium penetration depth during ion exchange in molten potassium nitrate. A host glass with an alkali ratio of K:Na = 0.27 showed a 13% increase in potassium penetration depth for an eight-hour exchange at 510°C, as compared to the base soda-lime-silicate glass. However, the substitution of potassium caused a decrease in the strain point which resulted in appreciable stress relaxation during ion-exchange strengthening. Hence less compressive stress developed. This decrease was addressed by a partial substitution of alumina for silica in the host glass. The substitution of 3 mol% alumina increased the strain point ≈ 12°C and resulted in a decrease in the stress relaxation of the host glass. For high-temperature ion-exchange treatments, the addition of alumina was found to increase the potassium penetration depth of the mixed-alkali glasses. As a result of the increased potassium penetration depth and the decreased stress relaxation, the mixed-alkali glasses were found to have an increased resistance to abrasion by SiC, as measured by breaking strength.