Processing-property relationship in ion-exchanged ESP (engineered stress profile) glasses

A novel two-step ion exchange process was recently proposed to produce Engineered Stress Profile (ESP) glass. Important characteristics of ESP glass include high strength, relatively low strength variability and high surface damage resistance. It has been found that the mechanical reliability of ESP glass is mainly dependent on the processing conditions. Therefore, the primary objective of the current thesis is to quantitatively study the relationship between the mechanical properties of ESP glasses and the ion exchange processing conditions. Based on this relationship, processing conditions can be determined for any particular requirement of mechanical behavior for ion exchanged glass.; To establish a property-processing relationship in ESP glasses, it is necessary to predict the stress profile in ion exchanged glass from the processing conditions. Since the residual stress profile in ion exchanged glass is mainly caused by the K/Na ion exchange and the stress relaxation, the diffusion process and the stress relaxation behavior of glass were studied. The K₂O concentration profiles in singe-step and two-step ion exchanged soda lime silicate (SLS) glasses were calculated and found to be in a good agreement with the measured concentration profiles. The uniaxial compressive stress relaxation behavior of the SLS glass in the current thesis at typical ion exchange temperatures was studied. Since the surface composition in ion exchanged glass is significantly different from the composition of untreated glass, this composition difference could cause significant difference in glass properties including viscosity and stress relaxation. Therefore, properties of glasses with different K/Na ratios were studied, and empirical equations were obtained to estimate glass properties from the glass composition.; Given the diffusion coefficient, surface concentration, composition-dependent dilation coefficient and stress relaxation data, residual stress profiles in ion-exchanged glasses could be calculated. These calculated stress profiles in single-step and two-step ion exchanged glasses have a reasonable agreement with the measured stress profiles using an optimized optical method.; In addition, a variable temperature ion exchange process was used to produce ESP glass with a lower production cost than that for two-step ion exchange process. However, similar mechanical properties can be achieved by a variable temperature ion exchange process. Strength and strength variability of ion-exchanged glasses were predicted from the measured stress profile.; Overall, an experimental methodology for predicting stress profiles in ion exchanged glasses is described. This methodology involves measuring the diffusion coefficient, stress relaxation behavior of SLS glass and other mixed alkali silicate glasses, calculating the stress profile in ion exchanged glass from the measured diffusivity data and stress relaxation data, and then comparing the calculated and experimentally measured stress profiles in glass.