ATOMISTIC SIMULATIONS OF BOROSILICATE GLASSESS: MECHANICAL RESPONSE UNDER DIFFERENT LOADING CONDITIONS
Open Access
- Author:
- Lee, Kuo Hao
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 30, 2021
- Committee Members:
- Seong Kim, Outside Unit & Field Member
Hojong Kim, Major Field Member
John Mauro, Chair & Dissertation Advisor
Ismaila Dabo, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- molecular dynamics
deformation
plastic flow
shear flow
borosilicate glass
densification
fracture toughness - Abstract:
- It has been challenging to understand the cracking behavior of oxide glasses under sharp contact due to the complicated stress state in the samples and compositional dependence of the mechanical behavior of glasses. Molecular dynamics (MD) is a powerful technique to study materials structure and properties. By taking advantage of its capability of providing the time-dependent trajectories of all atoms in the system, the in-situ observation of mechanical deformation can be achieved. In this research, we used MD simulations to study the mechanical response of two commercial multi-component borosilicate glasses, Borofloat®33 (Boro33) and N-BK7® (N-BK7), under different loading conditions to obtain a more complete picture of the deformation mechanism of the indented glass from the perspective of atomic scale. This dissertation can be divided into five parts. In the first part, the performances of two sets of classical interatomic potentials for borosilicate glasses were evaluated in terms of structural and elastic properties for the two glass compositions. The results were also compared with available experimental data. It was found that the potential by Wang et al. [M. Wang, N.M. Anoop Krishnan, B. Wang, M.M. Smedskjaer, J.C. Mauro, and M. Bauchy, J. Non. Cryst. Solids, 498 294–304 (2018)] provides a closer N4 value for Boro33 but underpredicts the N4 value for N-BK7. In contrast, the N4 value of N-BK7 using the potential of Deng and Du [L. Deng and J. Du, J. Am. Ceram. Soc., 102 [5] 2482–2505 (2019)] agrees well with the experimental data, but that of Boro33 is overpredicted. Our result also indicates that Wang’s potential gives a better prediction in the short-range structure, while Du’s potential provides a closer medium-range structure compared with the experimental data. Neither set of potentials is able to provide accurate predictions of elastic moduli. Wang’s potential predicts lower elastic modulus due to the underpredicted N4 value, whereas Du’s potential yields higher elastic modulus compared with the experimental values, resulting from its overpredicted N4 value. The second part presents the results from cold compression-decompression MD simulations of the borosilicate glasses. Our results suggest that the densification of these two borosilicate glasses involves different types of structural changes. The fraction of permanent densification can be correlated to the change in intermediate-range structure. By performing Voronoi analysis, the contributions to densification from different cation types in these two multicomponent borosilicate glasses were qualified. It was found that 3-coordinated cations facilitate the densification process, and higher-coordinated cations are relatively stable and can even show a slight expansion in their Voronoi volume. The third part describes the shear behaviors of the borosilicate glasses under different pressures. It was found that the addition of alkali ions lowers the yield stress and changes the pressure dependence of shear modulus. Moreover, shear-induced densification was observed in both glasses. The results show that the decreases of the oxygen-centered bond angle and the coordination number change of B are responsible for the density changes at low pressures, and the increase of 5-coordinated Si is the dominant mechanism for densification at high pressures. The atomic shear stress was calculated the results suggest that B is able to relax mechanical stress more easily under pressurized shear compared to other element types. By analyzing the nonaffine displacement of atoms, it was found that N-BK7 exhibits more localized plastic deformation compared to Boro33 at low pressures and the local rearrangements in both glasses become more homogeneous with increasing pressure. The mean squared nonaffine displacement curves show that alkali ions have the highest mobility induced by shear compared to the network formers and B is more mobile than Si for both glasses. It was also observed that plastic deformation tends to take place around boron atoms for Boro33, whereas it occurs in the alkali-rich regions for N-BK7, indicating that these two glasses have different atomic-scale deformation mechanisms. In the fourth part, the plasticity of the two glasses under tension was investigated by implementing a uniaxial tension test using MD simulations. A bond-switching mechanism is found to be responsible for the plastic response of both glasses and is governed by the increasing rate of non-bridging oxygen (NBO) production during the uniaxial tension. It was found that the amount of B4OSi4 linkages in the glass governs the stress drop after yielding, due to its higher tendency to create NBOs compared to Si4OSi4. Also, the initial existence of NBOs weakens the critical stress for breaking the B4-O bond in B4OSi4, which in turn lowers the yield strength of the glass. The last part describes the effects of pressure on elastic properties, surface energy, and fracture toughness (K_IC) of the borosilicate glasses. It was found that the impact on K_IC is mainly dominated by the change of Young’s modulus under pressure, which is proportional to the relative change in density. Between the two glasses under investigation, K_IC can be improved more effectively by the hot-compression process for Boro33, due to its higher concentration of 3-coordinated boron (B3), which facilitates densification via B3 to B4 conversion under compression.