Despite their transformative role in our society, oxide glasses remain brittle. Although extrinsic postprocessing techniques can partially mitigate this drawback, they come with undesirable side effects. Alternatively, topological engineering offers an attractive option to enhance the intrinsic strength and damage resistance of glass. On the basis of this approach, we report here the discovery of a novel melt-quenched lithium aluminoborate glass featuring the highest crack resistance ever reported for a bulk oxide glass. Relying on combined mechanical and structural characterizations, we demonstrate that this unusual damage resistance originates from a significant self-adaptivity of the local atomic topology under stress, which, based on a selection of various oxide glasses, is shown to control crack resistance. This renders the lithium aluminoborate glass a promising candidate for engineering applications, such as ultrathin, yet ultrastrong, protective screens.

Discovery of Ultra-Crack-Resistant Oxide Glasses with Adaptive Networks

We report on the loading-rate dependence of localized plastic deformation in inorganic covalent, metallic, ionic and superionic glasses. For this, the strain-rate sensitivity is determined through instrumented nanoindentation in a load-controlled strain-rate jump test. Through relating the strain-rate sensitivity to the reduced temperature, the packing density, the network dimensionality and the average single bond strength of the system, a qualitative mechanistic description of the strain-mediating process is possible. A strong variability of strain-rate sensitivity is obtained only at intermediate values of packing density, network connectivity or bond strength, when other parameters such as chemical composition and specific structural arrangement are dominating the deformation process. On the other side, for high bond strength and connectivity or for high packing density, the strain-rate sensitivity of the considered glasses is always low, which is also confirmed through the dependence of strain-rate sensitivity on Poisson ratio. Here, only for glasses with a Poisson ratio of ~ 0.3–0.4 we observe a wide variability of the loading-rate dependence of local deformation. For higher or lower Poisson ratio, the observed dependence is always low: when the limiting factor in deformation is primarily network connectivity and bond strength or packing density, respectively, once an activation barrier is overcome, deformation is only weakly loading-rate-dependent. This is regardless of the height of the activation barrier. When approaching the glass transition temperature, high strain-rate sensitivity is observed only in glasses where non-Newtonian flow is expected also in the corresponding liquid.

Strain-rate sensitivity of glasses

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.

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Crack-resistant Al2O3-SiO2 glasses.

We provide a comprehensive description of the defect tolerance of sodium-borosilicate glasses upon sharp contact loading. This is motivated by the key role which is taken by this particular glass system in a wide variety of applications, ranging from electronic substrates, display covers and substrates for biomedical imaging and sensing to, e.g., radioactive waste vitrification. The present report covers the mechanical properties of glasses in the Na 2 O–B 2 O 3 –SiO 2 ternary over the broad range of compositions from pure SiO 2 to binary sodium-borates, and crossing the regions of various commercially relevant specialty borosilicate glasses, such as the multi-component Duran-, Pyrex- and BK7-type compositions and typical soda-lime silicate glasses, which are also included in this study. In terms of structure, the considered glasses may be separated into two groups, that is, one series which contains only bridging oxygen atoms, and another series which is designed with an increasing number of non-bridging oxygen ions. Elastic moduli, Poisson ratio, hardness as well as creep and crack resistance were evaluated, as well as the contribution of densification to the overall amount of indentation deformation. Correlations between the mechanical properties and structural characteristics of near- and mid-range order are discussed, from which we obtain a mechanistic view at the molecular reactions which govern the overall deformation reaction and, ultimately, contact cracking.

Plasticity, crack initiation and defect resistance in alkali-borosilicate glasses: From normal to anomalous behavior

This study investigates the mechanical response of sodium borosilicate (SBN) glasses as a function of their chemical composition Vickers's indentation tests provide an estimate of the material hardness ( H V ) and indentation fracture toughness ( K C VIF ) plus the amount of densification/shear flow processes Sodium content significantly impacts the glass behavior under a sharp indenter Low sodium glasses maintain high connected networks and low Poisson's ratios ( ν ) This entails significant densification processes during deformation Conversely, glasses with high sodium content, ie large ν , partake in a more depolymerized network favoring deformation by shear flow As a consequence, indentation patterns differ depending on the processes occurring Densification processes appear to hinder the formation of half-penny median–radial cracks Increasing ν favors shear flow and residual stresses enhance the development of half-penny median–radial cracks Hence, K C VIF decreases linearly with ν 

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R. Conradt

Papers

Composition dependence of indentation deformation and indentation cracking in glass