Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses

Abstract

Alkali aluminoborate glasses have recently been shown to exhibit a high threshold for indentation cracking compared to other bulk oxide glasses. However, to enable the use of these materials in engineering applications, there is a need to improve their hardness by tuning the chemical composition. In this study, we substitute alkaline earth for alkali network-modifying species at fixed aluminoborate base glass composition and correlate it with changes in the structure, mechanical properties, and densification behavior. We find that the increase in field strength (i.e., the charge-to-size ratio) achieved by substituting alkaline earth oxide from BaO to MgO manifests itself in a monotonic increase in several properties, such as atomic packing density, glass-transition temperature, densification ability, indentation hardness, and crack resistance. Although the use of alkaline earth oxides as modifier enables higher hardness values (increasing from 2.0 GPa for Cs to 5.8 GPa for Mg), their crack resistance is generally lower than that of the corresponding alkali aluminoborate glasses. We discuss the origin of this compromise between hardness and crack resistance in terms of the ability of the glass networks to undergo structural transformations and self-adapt under stress. We show that the extent of volume densification scales linearly with the number of pressure-induced coordination number changes of B and Al.

Breaking the Limit of Micro‐Ductility in Oxide Glasses

“…Sci. As we have shown in our previous work, [14,25,26] aluminoborate glasses generally exhibit high resistance to indentation cracking, including the present Cs 2 O-Al 2 O 3 -B 2 O 3 composition. [24] In this work, we discover that a caesium aluminoborate glass exhibits superior crack resistance and micro-ductility when it is subjected to surface aging.…”

“…[33,34] This is particularly important for the aluminoborate glass, as the chemical durability of the reference silicate and aluminosilicate glasses (used for comparison) are much higher than that of the aluminoborate glass. [25,26] These observations demonstrate that exposure to humid air has a positive impact on the damage resistance of the present glass, unlike the behavior of other known oxide glasses. Figure 1 shows that increasing the sharpness of the indenter tip increases the probability for crack formation, in agreement with a previous study, [35] as the sharper tip leaves less room for densification and increases the residual stress.…”

“…[14,25,26] Their common feature is that the coordination numbers of the network-forming nuclei (B and Al) are adaptive to external stresses. [14,25,26] Their common feature is that the coordination numbers of the network-forming nuclei (B and Al) are adaptive to external stresses.…”

Competitive effects of free volume, rigidity, and self‐adaptivity on indentation response of silicoaluminoborate glasses

“…In this work, we build on the recent progress made in achieving high crack resistance in alkali aluminoborate glasses . Specifically, we discovered a melt‐quenchable lithium aluminoborate glass with very high resistance to indentation cracking, that is, Vickers indentation loads close to 2 kgf (~20 N) are needed to induce radial cracking .…”

“…High self‐adaptivity corresponds to large pressure‐driven changes in density and coordination numbers of the network formers . Future applications of the lithium aluminoborate glass are, however, limited by its relatively low hardness and chemical durability …”