History matters for glass hardness

Two objects of the same material—wooden blocks, say, or windowpanes—are commonly thought to be identical. Microscopically, of course, things are not always so straightforward. Researchers from Denmark, Poland, and the US now show that the route a glass takes to reach its final state can profoundly affect its macroscopic properties. Morten Smedskjaer of Aalborg University in Denmark and his colleagues used both experiments and simulations in a comparative study of aluminosilicate glasses that had identical composition but were brought to the same density via two routes: thermal annealing and high-pressure quenching in a nitrogen-gas chamber. The effects of annealing are well known, but the role of pressure history is less studied. When the researchers compared glasses of the same density and chemical makeup, those that had been annealed were significantly harder—as measured by indentation tests—than the pressure-quenched ones. The simulations reveal that both methods result in the same interatomic distances between nearest neighbors, but in the annealed glass the bond angles have a narrower distribution, which leads to greater short-range order. On the other hand, pressure quenching has a stronger influence on the medium-range order—the repetition distances for typical atomic arrangements—which has much less influence on the material’s hardness. Thus, as shown schematically here, thermal annealing and pressure-quenching treatments can be thought of as independent degrees of freedom, comparable to composition, in designing new industrial glasses. (M. M. Smedskjaer et al., J. Chem. Phys. 143, 164505, 2015, doi:10.1063/1.4934540 .)