Obtaining “hard” and “crack-resistant” glasses have always been of great important in glass science and glass technology. However, in most commercial glasses both properties are not compatible. In this work, colorless and transparent xAl2O3–(100–x)SiO2 glasses (30 ≤ x ≤ 60) were fabricated by the aerodynamic levitation technique. The elastic moduli and Vickers hardness monotonically increased with an increase in the atomic packing density as the Al2O3 content increased. Although a higher atomic packing density generally enhances crack formation in conventional oxide glasses, the indentation cracking resistance increased by approximately seven times with an increase in atomic packing density in binary Al2O3–SiO2 glasses. In particular, the composition of 60Al2O3•40SiO2 glass, which is identical to that of mullite, has extraordinary high cracking resistance with high elastic moduli and Vickers hardness. The results indicate that there exist aluminosilicate compositions that can produce hard and damage-tolerant glasses.

Glasses with high elastic moduli have been in demand for many years because the thickness of such glasses can be reduced while maintaining its strength. Moreover, thinner and lighter glasses are desired for the fabrication of windows in buildings and cars, cover glasses for smart-phones and substrates in Thin-Film Transistor (TFT) displays. In this work, we report a 54Al2O3-46Ta2O5 glass fabricated by aerodynamic levitation which possesses one of the highest elastic moduli and hardness for oxide glasses also displaying excellent optical properties. The glass was colorless and transparent in the visible region, and its refractive index nd was as high as 1.94. The measured Young’s modulus and Vickers hardness were 158.3 GPa and 9.1 GPa, respectively, which are comparable to the previously reported highest values for oxide glasses. Analysis made using 27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy revealed the presence of a significantly large fraction of high-coordinated Al in addition to four-coordinated Al in the glass. The high elastic modulus and hardness are attributed to both the large cationic field strength of Ta5+ ions and the large dissociation energies per unit volume of Al2O3 and Ta2O5.

Over the past decades, constitutive relations have been developed to compute the mechanical response of silicate glasses at the continuum length scale. They are now reliable enough that we can calculate indentation induced stress and strain fields and examine the impact of material parameters on indentation response, and especially hardness, pileup and stress fields. In contrast to a presently widespread assumption in the literature, we show that (shear) flow stress is the primary determinant of these properties, and that densification plays a secondary role in the indentation response of all the silicate glasses. This result applies even for large values of the densification at saturation because of the high ratio between effective volumetric yield stress (i.e. yield pressure) and flow stress. It is well-known that, depending upon composition, silicate glasses exhibit very different sensitivities to indentation cracking, although all other standard mechanical properties remain quite similar. We point out that material damage incurred through plastic shear flow, and especially shear flow instability and localization may well control crack initiation, which would resolve the paradox. Shear flow instability and damage has not been quantitatively investigated in detail in silicate glasses as yet, neither experimentally nor theoretically. However, we believe it is key to an in depth understanding of cracking resistance in silicate glasses.

In oxide glasses, “hardness” and “damage‐tolerance” are usually competing attributes. The main reason for this is that hard glasses usually have closely packed structures compared to those of damage‐resistant glasses. Recent progress on the mechanical properties of alumina‐rich glasses has thrown some light on how to obtain hard and damage‐tolerant glasses. In this work, the elastic properties and indentation behavior of a glass system based on (R = La, Sm, Gd, Er, Tm, Y), prepared by an aerodynamic levitation technique, was investigated. It was found that the rare‐earth ions exhibited a strong size effect on the mechanical properties of the glasses. The elastic moduli and hardness increased with the packing density and as the ionic radii of the rare‐earth ions decreased. In addition, the resistance to surface damage by indentation increased with larger rare‐earth cations. Our results show that the elastic properties and damage tolerance of rare‐earth aluminate glasses can be tuned depending on the ionic radii of rare‐earth ions.