Abstract
Alkali-borosilicate glasses are one of the most used types of glasses with a high commercially and technological importance. In order to optimize glasses for diverse applications the understanding of the correlation between the microscopic structure and the macroscopic properties is of central interest in material sciences. It has been found that the fracture response of borosilicate glasses can be influenced by changes in the network interconnectivity. In the NBS2 borosilicate glass system (74.0SiO2-20.7B2O3-4.3Na2O-1.0Al2O3 in mol%), two subnetworks are present, i.e. a silicate and a borate network. Increasing cooling rates while processing were found to increase interconnectivity between the networks which in turn improves the glasses fracture response and is accompanied by an increasing capacity for densification. In the present study the borosilicate glasses were systematically modified by addition of up to 4.0 mol% Al2O3. Changes in the connectivity and short to medium-range order of the glass structure are characterized using Raman and NMR spectroscopic techniques. Both the Raman and the 11B NMR results show that as the Al2O3 content increases, a proportion of fourfold-coordinated boron are converted to threefold-coordinated ones. Additional 27Al NMR experiments show that aluminum is dominantly present in four-fold coordination. Aluminum-tetrahedra are thus charge balanced by sodium ions and incorporated into the silicate network. Finally, the inherent glass structure and its network interconnectivity is linked to the mechanical glass response. Nanoindentation testing, where shear flow, inelastic densification as well as cracking can be present, was employed for this purpose. It was found that the glass softens with increasing Al2O3 levels, which enhances the fracture response of the borosilicate glass.
Highlights
Nowadays oxide glasses are important structural and functional components
With increasing the content of Al2O3 within the sodium borosilicate glass, the boroxol amount increases at the cost of BIV units
This tendency can be observed with both spectroscopical methods used in the present study, Raman (Figure 1) and NMR (Figure 3) spectroscopy
Summary
Nowadays oxide glasses are important structural and functional components. The variety of possible applications places different demands on the material. In low-R glasses, where R is smaller than 0.5, the fraction of four coordinated boron N4 can be approximated to equal to R (Möncke et al, 2003; Winterstein-Beckmann et al, 2013; Januchta et al, 2019b) In this range the R-value of 0.2 defines the line of boron oxide anomaly at which many material properties exhibit extrema (Möncke et al, 2003; WintersteinBeckmann et al, 2014a). Those features make borosilicate glasses a very interesting system to study the link between structural alterations to the mechanical response (Winterstein-Beckmann et al, 2014a,b; Malchow et al, 2015; Zehnder et al, 2017a,b). The fracture behavior is investigated using high load nanoindentation
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