Ion Exchanged, Glass Laminates that Exhibit a Threshold Strength

Glass laminates, fabricated to contain periodic thin layers containing biaxial compressive stresses, exhibit a threshold strength, i.e., a stress below which failure will not occur. Ion exchange treatments in KNO3 at 350 to 450 deg C for periods of 3 to 72 hrs were used to create residual compressive stresses at the surface of soda lime silicate glass sheets. Wafer direct bonding of the ion exchanged glass sheets resulted in glass laminates with thin layers of compressive stress adjacent to the glass interfaces. Critical strain energy release measurements of the bonded interface were used to optimize the bonding temperaturetime to avoid significant relaxation of the stress produced by ion exchange. Stress profiles, determined via the wafer curvature measurement method, showed a residual stress maximum of 328 MPa for an ion exchange temperature of 450 deg C. The threshold flexural strength of the ion exchanged glass laminates was determined to be 112 MPa after the introduction of indentation cracks with indent loads ranging from 1kg to 5kg. Contrary to similar ceramic laminates, where cracks either propagate across the compressive layer or bifurcate within the compressive layer, the cracks in the glass laminates were deflected along the interface between the bonded sheets.