Effects Of Alkali And Alkaline Earth Additions On The Mechanical Properties Of Silicate Glasses

Author : Aubrey Fry
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (14 download)

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Download or read book Field Strength Effect on the Elastoplastic and Mechanical Properties of Single Modifier Aluminoborosilicate Glasses written by Aubrey Fry and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The theoretical strength of glass far exceeds its measurable strength. The limiting factor is glass's sensitivity to surface flaws, largely due to handling. Silicate glasses are the largest commercially used glass family because they are highly versatile and can be designed with many enhanced properties. Decades worth of patents and scientific literature discuss intrinsic and extrinsic strengthening mechanisms that compensate for glass's sensitivity to surface damage, including thermal tempering, ion exchange, ceramming, lamination, etc. The focus of this dissertation is to explore the impact of modifier field strength on the mechanical response of aluminoborosilicate glasses designed to have intrinsic damage resistance. By exploring the strengthening mechanisms at the atomic level, it is possible to gain a deeper understanding of how the glass composition governs physical properties and its mechanical response. Based on the principle of topological adaptability, aluminoborosilicate (ABS) glasses can be designed with intrinsically high damage resistance if they incorporate three-coordinated boron atoms. Using a careful selection of network modifying ions, they can also be amenable for chemical strengthening. This dissertation provides a comprehensive investigation of the effect of field strength (FS) on intrinsic damage resistance considering three monovalent and three divalent modifiers in ABS glasses. Various aspects of mechanical properties were analyzed as a function of network modifier (NWM) field strength and the magnitude of the applied stress. Results from this work help to (i) reevaluate assumptions regarding glass structure, (ii) improve future mechanical testing procedures, and (iii) gain insights into the cracking behavior and deformation mechanisms in silicate glasses as a function of modifier field strength and applied stress. (1) Atomic Structure: The first major discovery from this work regards the atomic structure of high strain point, mixed former glasses. For the current ABS glasses, the ratio of network formers and the low concentration of modifier oxides, in addition to high processing temperatures, increases the entropic contribution to the glass structure such that the enthalpically driven modifier and network former (NWF) interactions are frustrated. Therefore, in such systems it cannot be assumed that all Al3+ cations will be four-coordinated even if [NWM] >= [Al2O3] (concentrations denoted by square brackets). (2) Hardness and Crack Resistance: The second major discovery from this work addresses the simultaneous increase of both hardness (H) and crack resistance (CR) as a function of modifier field strength. Historically, an increase in H is presumably marked by an increase in rigidity and the probability of crack initiation. This is not the case for the current compositions, which brings the assumption of the inverse relationship between H and CR into question, specifically for glasses that might not be classified as normal. (3) Plasticity: The third discovery of this work involves the nuanced relationship between H, CR, and plasticity. Plasticity (VP) increases with modifier field strength, similar to H and CR. These findings appear counterintuitive since hardness is a measure of resistance to plastic deformation. It is found that a balance of plastic deformation mechanisms, namely, shear flow and densification, allow for an increase in H, CR, and plasticity as a function of field strength. (4) Topological Adaptability: The fourth major finding from this dissertation involves the predominant assumptions regarding topological adaptability. The topological adaptability is mainly attributed to two mechanisms: (i) the reduction in free volume under an applied stress (e.g., a decrease in bond length or angle), and (ii) structural unit conversion under an applied stress (e.g., NWF coordination changes). The first attribute is quantified by a decrease in molar volume (VM) post indentation or compression, and the second attribute is enabled by an initially high concentration of three-coordinated boron ([B3]). It is generally accepted that CR increases as topological adaptability is enhanced. The ABS glasses in this study were designed to take advantage of both mechanisms, while changing the modifier FS. Although this work did not systematically explore the impact of varying VM and [B3] on CR, it did show that CR increased as VM decreased. While this work implies that the topological adaptability can be helped or hindered based on the modifier species, future work needs to be done to systematically monitor changes in VM and CR as a function of varying [B3] in a series of related ABS glasses. (5) Poisson's Ratio: The fifth discovery from this research is that the suggested link between Poisson's ratio and cracking behavior should be reexamined. The elastic modulus increases with field strength but the Poisson's ratio did not trend with any measured property, namely, the VM and CR. Poisson's ratio has been touted as holding the key to understand cracking behavior in oxide glasses, but this is not the case for the current compositions. This misconception is likely due to the large amount of research conducted on normal (e.g., soda-lime silicate) and anomalous (e.g., pure silica) glasses which tend to ignore the intermediate glasses or mixed former glasses. (6) Elastic to Plastic Ratio: The sixth finding of this research has a direct connection to application: the mechanical performance of glass under increasing stress. The elastic-to-plastic deformation ratio (VE/VP) increases as the applied load increases. For the current compositions, a higher load results in a greater elastic response, which may help to explain static vs dynamic fatigue phenomena where a glass appears to be "stronger" when tested at higher strain rates. (7) Fracture Toughness: The seventh major discovery of this work is that the fracture toughness (KIc) of the alkaline earth glasses increases as a function of modifier field strength. Predicting fracture toughness has eluded many researchers both in ceramic and glass sciences. KIc has been related to a variety of material properties, but there is no universal predictor for how a material will perform during a fracture toughness test, especially since property trends are often found to deviate when changing the length scale of the test. From this work, results indicate that the hardness (micro-scale), crack resistance (meso-scale), fracture toughness (meso- to macro-scale), and the elastic moduli (bulk) all increase in ABS glasses with increasing FS, which has not been shown before. Additionally, the material property that had the highest linear correlation with KIc was VE/VP, indicating that a volumetric recovery study may serve as a new micro-scale test that can help predict the fracture toughness performance of ABS glasses. The goal of this dissertation is to explore the impact of glass composition on the elastoplastic response and mechanical performance of six novel single modifier aluminoborosilicate glasses. This work finds that modifier FS has a distinct role influencing mechanical properties and cracking behavior, even at modifier concentrations of around 10 mole percent. By rethinking glass chemistry accounting for these effects, properties such as hardness, crack resistance, elastic modulus, and even fracture toughness can be improved.