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Glass : a not so fragile solid

Glass is an amorphous material, i.e., non-crystalline, it is solid at temperatures below the so-called glass transition temperature (Tref) but viscous above this temperature (Log viscosity <12). Silicate glasses are everywhere around us, from the cake form in our kitchens, to the beakers of our laboratories, from the screens of our smartphones to the lavas emitted by the volcanoes.The deformation properties of these silicate glasses are of interest to geologists as well as to industrial compagnies (Saint-Gobain, Corning, Schott). When subjected to stresses, the silicate glass begins to deform viscoelastically : the deformation is reversible, even if it is not perfectly linear or symmetrical. If the deformation is too important, it goes directly to a brittle behavior and the glass breaks. Our preferred silicates (olivine, pyroxenes, quartz, peridotites), on the other hand, behave like viscoplastic solids. Their elasticity is very low, so that the deformation is rapidly irreversible and they can accommodate very large deformations before breaking. Obviously, these mechanical properties depend strongly on temperature and chemical composition, but also on pressure : the screen of a smartphone often resists to small accidents, but rarely to a big fall.

Linfeng Ding, PhD student in Mainz (with Boris Kaus) and partner in the European CREEP network, studies the quantification of the pressure effect on the deformation of a silicate glass. He came to spend a month in Montpellier to study experimentally the effect of pressure (100-200-300 MPa) on an industrial glass, the SCHOTT N-BK7 ™ with Sylvie Demouchy and Manuel Thieme (team Earth mantle and interfaces). (Photo caption : Manuel Thieme and Linfeng Ding preparing a press experiment Paterson). This glass is part of the family of borosilicates, invented by Friedrich Otto Schott. It is a silicate glass where part of the silicon is replaced by boron (B2O3=10 %wt.), Which gives it strong fire resistance ; The N-BK7 is therefore a pyrex-like glass. Not to be confused with the more fragile soda-lime glasses from which our wine glass, bottles and other flask are made off and which constitutes 90% of the production of glass.

The behavior of silicate glass under the effect of pressure reveals surprises and contrasted reactions : some glasses have their viscosity (which quantifies the capacity to deform under the effect of stress) increasing with pressure (molten olivine, diopside), while for others, the viscosity decreases (obsidian, basalt, andesite, dacite, highly polymerized glasses). The objective here was to know in which category SCHOTT N-BK7 ™ glass belong to. After more than 20 Paterson press experiments, at various pressures, temperatures (535-700 ° C) and strain rates, the data acquired by Linfeng show a net increase in viscosity with the confinement pressure.

Linfeng will come back in Montpellier in June for a new series of experiments, under static conditions this time, to make glass under pressure before making measurements of post-experimental densities, as well as to prepare an article with Sylvie and Manuel.

Photo caption : a cylinder of N-BK7 before deformation in the Paterson press.