Structure-Property Relations of Mixed-Alkali and Ion-Exchange Silicate Glasses
Improvement of the mechanical properties of glass was undertaken by furthering understanding of the fundamental relationships between composition, structure and mechanical response. Glasses which were known to already have desirable mechanical properties were made and analysed in order to establish correlations between different properties. Ion exchange (IE), where a smaller ion is replaced by a larger ion without subsequent structural relaxation, causes surface compressive stresses which increase strength and scratch resistance significantly. Micro-Raman spectroscopy was employed to directly measure the volumetric negative strain (contraction) and compressive stress as a function of IE processing temperature by observing changes in Raman peaks correlated with Si-O bond lengths and Si-O-Si bond angles. From the Raman data, the strain of the glass network and consequent stress was calculated relative to several reference states. The reference state of relaxed, fully exchanged glass produced results which matched the complex experimental behaviour. The mechanical response of the IE layer was probed using nano-indentation. Stiffness and hardness were measured as a function of distance from the surface and IE temperature. Additionally, elastic recovery and resistance to plastic deformation were determined. Low IE temperatures (which the Raman results indicated contained the most compressive stress) were observed to improve mechanical properties more than higher IE temperatures, likely due to increased thermal relaxation. The mixed-modifier effect (MME), a deviation from additivity when two or more different types of modifying cations are combined, is known to exist in in static, dynamic and mechanical properties. Yet, the underlying mechanism of the MME is poorly understood, thus, a comprehensive study of several mixed-modifier glass series was undertaken to better elucidate the complex relationships between these three categories of properties. The most significant predictor of the MME was the valence(s) of the mixed cations. Furthermore, the MME in conductivity, packing fraction, bulk modulus, hardness and fracture toughness was related to relative cationic field strength and ionic radii. The MME in shear modulus, Young's modulus and Poisson's ratio was related to structural connectivity rather than the properties of the modifier cations.