Buildup and relaxation of stress in chemically strengthened glass

• Studied stress buildup/relaxation during ion exchange in soda lime glass
• MD analysis shows that fast structural relaxations occur over pico/nanoseconds.
• Estimated biaxial surface compression would be ~ 1.5 GPa at 723 K.
• Slow network relaxation generates a stress-free volume smaller than as-melted volume.
• The “dilation anomaly” in glass chemical strengthening concepts is largely resolved.

Cooper–Krohn analysis of stress generation in soda lime silicate (“SLS”) glasses during ion exchange strengthening suggests that the glass should develop 2.5 to 3 GPa surface compression (biaxial stress), though only ~ 600 MPa values are observed experimentally. In this paper, we suggest that this “network dilation anomaly” is largely resolved. MD simulations were carried out to monitor volume and pressure changes after ion-swap. Surface compression in an SLS glass ion-exchanged as a function of temperature and time in KNO3 bath was measured to follow the viscous relaxation. We show that biaxial surface compression builds initially to the expected ~ 2.5 GPa; thereafter fast secondary structural (“β”) relaxations, whose time constants span picoseconds to nanoseconds, cause the surface compression to relax rapidly to ~ 1.5 GPa at the ion exchange temperature. Stress relaxes thereafter due to “α” primary structural relaxations associated with viscous flow. Recovery occurs to a transition state which is somewhat short of the expected glass volume. Full volume is recovered at higher temperatures due to increased flow.