Journal of the American Ceramic Society, Vol.90, No.8, 2502-2509, 2007
Ion-exchanged glass laminates that exhibit a threshold strength
Glass laminates, fabricated to include 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 degrees-450 degrees C for periods of 3-72 h 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 interface and perpendicular to the laminate top surface. Critical strain energy release measurements of the bonded interface were used to optimize the bonding temperature/time to avoid significant relaxation of the stress produced by ion exchange. Stress profiles, determined via the wafer curvature measurement method, showed a residual compressive stress maximum of 328 MPa for an ion exchange temperature of 450 degrees 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 1 to 5 kg. In contrast 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.