Effects of Pre-Existing Surface Flaws on Chemical Strengthening of SLS Glass

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New York State College of Ceramics at Alfred University. Inamori School of Engineering.
The present study focused on the effect of surface flaws on ion-exchange strengthening of soda-lime silicate (SLS) glass. Flaws produced by Vickers indentations, scratching or particle impact were investigated. MOR data were determined and SEM techniques were used to obtain images of the ion-exchange profile around large flaws after ion-exchange and to determine if the ion-exchange medium penetrated and remained in the cracks. The resulting MOR data indicated that even deep indentation surface flaws strengthened during the initial treatments (1 hour at 400) reaching a maximum, then decreasing for additional treatment times, (16 hours) before again strengthening at long times. It was further shown that this effect is a function of flaw size. For indentation flaw depths less than about 25 microns, samples strengthen continuously with time. Additional experiments, including post-fracture SEM studies of the indentation crack surfaces were aimed at determining the origin of this effect. Results indicate that the anomalous MOR behavior was not caused by stress relaxations during the ion-exchange treatment or by contamination in the molten salt ion-exchange liquid within the crack, or by stress relaxation during the pre-ion-exchange heat treatment. However, the exact mechanism of the anomalous behavior was not determined. The SEM images proved that the ion-exchange medium does enter the flaws and allow ion-exchange to occur even near or at the crack tip. It is also shown that the ion-exchange medium does stay on the flaw surfaces after processing, cleaning and testing to failure. This may have important implications for the static fatigue behavior of chemically strengthened glass and suggests possible simple methods for reducing fatigue in all materials subject to the effect.
Thesis completed in partial fulfillment of the requirements for the degree of Master of Science in Materials Science and Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Glass--Mechanical properties, Ion exchange, Materials--Strengthening mechanisms