Network Dilatation and Relaxation in Chemically Strengthened Alkali Silicate Glasses
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Glass chemical strengthening is an enabling technology for smart phones, tablets, and other personal electronic devices providing both enhanced strength and abrasion resistance. These enhanced properties rely upon the glass structure that results from “stuffing” a larger alkali ion into a smaller host alkali site within the glass. Incompatible expansion between the stuffed glass layers and the underlying substrate generates beneficial surface compression. The structural changes that take place during chemical strengthening, which influence compression development and retention, and thus mechanical performance, are not well understood. The present study utilized two approaches to improve understanding of alkali stuffed glass structures: molecular dynamics simulations were used to examine structural changes in the initial stages of stuffing alkali accommodation and laboratory measurements of dimensional swelling were used to observe elastic-plastic processes associated with chemical strengthening. Molecular dynamics simulations of potassium stuffing in soda-lime silicate and sodium aluminosilicate glasses produced network dilatation similar to that expected from laboratory stress measurements of glasses chemically strengthened by specialty techniques. Volume expansion per quantity of stuffing ion was found to increase with a decrease in network cross-polymerization, which also trended with Poisson’s ratio. Selective-surface chemical strengthening was used to form edge swelling and step swelling arrangements from which dimensional changes were examined with an optical profilometer. A strain model incorporating elastic, deviatoric plastic (shear flow), and hydrostatic plastic (densification) contributions was evaluated using inputs of step swelling, stress profiles, chemical diffusion profiles, and maximum strain from molecular dynamics simulations. Plastic strain averaged throughout the chemically strengthened layers accounted for about 70% and 40% of the total strain for soda-lime silicate and sodium aluminosilicate, respectively. The ratio of deviatoric plastic strain to total strain was observed to increase with increasing chemical strengthening temperature. Maximum compressive stress produced by traditional chemical strengthening was found to be much lower than the total strain indicated. Early relaxation modes were proposed to bridge the gap between maximum compressive stress produced by specialty chemical strengthening techniques versus that obtained traditionally. These results indicate notable improvements to maximum compressive stress can potentially be achieved through prevention of the various forms of relaxation.
Advisory committee members: Alastair Cormack, Alexis Clare, Nathan Mellott. Dissertation completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Glass Science at the Inamori School of Engineering, New York State College of Ceramics at Alfred University