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The goal of this article is to explore the dynamic mechanical behavior of annealed and chemically strengthened glass used in transparent laminated structures for buildings, armor and aerospace applications. Static compression (SC) tests were first conducted using the universal testing machine followed by dynamic compression (DC) tests at an average strain rate of 650 s−1 and 350 s−1 using modified Split Hopkinson pressure bar (SHPB) for annealed and strengthened glass, respectively. In DC tests, high-speed photography and flashlights were made synchronous with a loading pulse to spot the damage/crack initiation, propagation, and fracture process in both types of glasses. Static tests data concerning the compressive strength revealed that annealed glass (AG) showed strain rate sensitivity, in contrast to this chemically strengthened glass (CSG) showed no substantial strain-rate sensitivity. In DC tests compared to SC tests, it was found that both types of glass are rate sensitive and compressive strength remarkably increased at high strain-rate loading. The glass fracture process for both AG & CSG is also analyzed through high-speed imagery recorded during DC tests. The crack initiation, propagation, and glass debris are also discussed to explicate the failure mechanism of glass specimens.

Cooper–Krohn analysis of stress generation in soda lime silicate (“SLS”) glasses during ion exchange strengthening suggests that the glass should develop 2.5 to 3 GPa surface compression (biaxial stress), though only ~ 600 MPa values are observed experimentally. In this paper, we suggest that this “network dilation anomaly” is largely resolved. MD simulations were carried out to monitor volume and pressure changes after ion-swap. Surface compression in an SLS glass ion-exchanged as a function of temperature and time in KNO 3 bath was measured to follow the viscous relaxation. We show that biaxial surface compression builds initially to the expected ~ 2.5 GPa; thereafter fast secondary structural (“β”) relaxations, whose time constants span picoseconds to nanoseconds, cause the surface compression to relax rapidly to ~ 1.5 GPa at the ion exchange temperature. Stress relaxes thereafter due to “α” primary structural relaxations associated with viscous flow. Recovery occurs to a transition state which is somewhat short of the expected glass volume. Full volume is recovered at higher temperatures due to increased flow.

Three-point bending tests on unstrengthened annealed glass (AG) and chemically strengthened glass (CSG) were conducted under quasi-static and dynamic loading conditions to assess the flexural strength and failure characteristics with the help of a high-speed camera. The flexural strength of CSG was higher than AG due to residual compressive stress. At higher loading speed, the flexural strength of both AG and CSG specimens was increased and the strength dispersion was decreased. Additionally, the flexural strength of AG and CSG notched specimens were explored and results revealed that the flexural strength decreased remarkably due to stress concentration effect. However, the flexural strength of CSG specimens was still much higher than AG specimens for the same notch depth. Numerical simulation of the notched CSG specimen showed that the stress near the notch region redistributes due to the strengthened layer, making it under compression before loading. Also, fractography analysis was discussed to characterize the crack pattern and deformation mechanism.

Chemically strengthened glass is one of the key enablers of the modern consumer electronics industry. Molecular dynamics simulations offer an opportunity to build enhanced fundamental understanding of the ion exchange process used for chemical strengthening. In this work, we apply molecular modeling techniques to investigate several aspects of ion-exchanged glasses that are either inaccessible through experiment or too difficult to probe through experiments alone. In particular, we use atomistic modeling to study the dependence of the linear network dilation coefficient on the concentration of alkali ions exchanged. We also calculate the evolution of elastic moduli as a result of the ion exchange process. Both of these quantities—the network dilation coefficient and the Young's modulus—are critical parameters for determining the final compressive stress profile of the chemically strengthened glass. Finally, we revisit the issue of structural differences between as-melted and ion-exchanged glasses in terms of local atomic number density around alkali ions. The results of this work point to the unique structures attainable through ion exchange that cannot be achieved through melt-quenching alone.

Chemically strengthened glasses are widely used in applications requiring high scratch and damage resistance, such as cover glass for personal electronic devices. Chemical strengthening occurs by replacing the smaller alkali ions in the glass by larger alkali ions from a molten salt bath, whereby a surface layer with high compressive stress is installed. The mechanical properties of chemically strengthened glass strongly depend on the magnitude of the compressive stress, which in turn is proportional to the concentration of exchanged alkali ions. It has hitherto been assumed that the total amount of alkali ions in the glass is conserved during the ion exchange (e.g., in the Nernst–Planck model). In this work, we immerse alkali aluminosilicate glasses [30M 2 O · 10Al 2 O 3 · 60SiO 2 (mol%)] in MNO 3 salt baths containing identical alkali ions (where M = Li, Na, K). We demonstrate a non-conservation of the total amount of alkali ions in the glass during the ion exchange process. Generally we find that the smaller ions exhibit a net flux into the glass, while the larger ions demonstrate a net flux into the salt bath. Moreover, the total amount of alkali ions in the top surface layer is not conserved in sodium and potassium aluminosilicate glasses after ion exchange. We explain these findings based on the thermodynamic driving force difference in alkali oxides residing in glass or nitrate salt. The results are important for chemical strengthening applications, since the direction of flux and the non-conservation of alkali ions affect the final compressive stress of an ion exchanged glass.

The chemical strengthening of glass involves the stuffing of large ions into network sites previously occupied by smaller ions. Typically this involves an exchange of Li+ or Na+ ions in the glass for larger K+ ions from a molten salt bath. This swapping of ions creates compressive stress in the surface of the glass, significantly increasing the strength of the final glass product. The magnitude of this compressive stress is governed by the linear network dilation coefficient (LNDC), which defines the amount of linear strain per unit of ion substitution. However, the amount of strain attainable through ion exchange is much smaller compared to what is expected from as-melted versions of the same final glass composition. This effect, which we have termed the “network dilation anomaly,” is a result of the different local environment around the invading ion species in as-melted versus ion-exchanged glasses. A remaining question concerns the nature of the network strain due to ion stuffing. Using molecular dynamics simulations, we show that the strain induced by ion stuffing is entirely elastic. In other words, when a reverse ion exchange is performed to swap the original ions back into the glass, the initial volume of the as-melted glasses is entirely recovered. Moreover, we show that the local structural environment around the alkali ions is restored to the as-melted conditions. The elastic nature of ion stuffing demonstrates that the network dilation anomaly is not a result of plasticity, but rather a failure to achieve the full amount of expected elastic strain during ion exchange. The elasticity itself consists of both instantaneous and delayed contributions.

The ion exchange process for chemical strengthening of glass involves an inter-diffusion of alkali ions between the glass and a molten salt bath. In most commercial glasses, this comprises an exchange of Na+ ions in the glass for larger K+ ions from the salt bath. The stuffing of K+ ions into sites previously occupied by Na+ in the glass results in the formation of a compressive stress profile in the glass. In the absence of stress relaxation, this compressive stress profile largely follows the concentration profile of the K+ ions. Any subsequent migration of K+ ions within the glass would result in a change in this stress profile, which could lead to different mechanical behaviors of the glass. In particular, a reduction of the surface compressive stress could lead to a compromise in the retained strength of the glass, making it more susceptible to failure. Recent work has shown that chemically strengthened glass can be used as an effective substrate material for organic thin film transistors (TFTs), since the temperatures involved with organic TFT deposition are low enough to avoid any compromise in the compressive stress profile. This opens the possibility of fabricating high strength organic TFT displays. However, the question remains as to whether the exposure of the glass to an electric field may lead to the diffusion of the alkali ions and a corresponding alteration of the stress profile, which could compromise the strength of the organic TFT device. In this paper, we demonstrate that there is no change in the stress profile of Corning® Gorilla® Glass 3 after subjecting the glass to much higher voltage dc fields compared to the maximum field that would be exhibited in an organic TFT device. The stress profile is modified only after treating the glass at sufficiently high temperature, where alkali migration becomes thermally activated.

Chemical strengthening methods, for example those using the ion stuffing technique, often provide a greater degree of reinforcement than that obtainable by conventional thermal toughening methods. However, on fracturing, not very thin-walled objects disintegrate into fairly large, sharp and therefore dangerous splinters. A method is described by means of which the fracture behaviour can be improved without adversely affecting the high strength. The fragments produced on breakage of an object strengthened by ion stuffing can be rendered harmless by causing the sharp edges and points to powder finely and thus become blunt. This can be achieved by forming such stress profiles in the glass that the maximum compressive stress is found not at the surface but in a sub-surface layer. The requirements to be satisfied by such a stress profile and the methods of producing it are discussed.