Preliminary Project (Presented at CTC on July 3rd, 2007 at ICG2007 in Strasbourg)

Oxide glasses are highly homogeneous and isotropic materials for scales larger than a few nanometers. When decreasing to scales in the nanometer range, the decrease of the homogeneity is accompanied by the progressive transition to a higher degree of order, which reaches crystal like structures at the molecular scale. This property makes continuum approaches well applicable down to submicrometric scales, but it becomes a tremendous obstacle when trying to relate phenomenological properties to the molecular structure and chemistry of glasses. The homogeneity down to nanometric scales is at the origin of the best glass properties, such as the elevated degree of transparence, but it is also partly the cause of a major limitation : brittleness ! Most techniques for structural analysis are best suited for measuring bulk properties and can recently reach micrometric precision. On the opposite side, the great insights made possible by the developments of molecular dynamics simulation are often limited to a very small number of molecules. It is of great importance to develop experimental techniques to characterize the physical properties of glasses in the nanometer range in order to access the fundamental mechanisms at the origin of glass properties and relate them to the modelling of glass properties as related to structural and chemical characteristics of oxide glasses. This activity can benefit of the continuous development of local probe microscopies which allow resolving heterogeneities in the physico-chemical properties at the submicrometer scale. One of the major goals of this activity would be to shed light in the mechanisms of slow crack propagation in glasses. In several cases, phenomenological equations such as Wiederhorn’s can explain the dependence of crack velocity on stress and environmental parameters. Yet fundamental mechanisms that occur at the tip are still debated and modelling of the mechanisms acting at the nanometer scale is necessary to relate the phenomenological parameters to the specific composition and structure of glasses. Recent studies have allowed to put into evidence the presence of ductile behaviour at the nanoscale, as well as the effects of stress induced ion migration in the neighbourhood of the crack tip. Both these effects are related to the interaction of water molecules with the glass network. Several studies are revealing the important role of water diffusion into the glass network in promoting structural relaxation and diffusion of alkali ions, yet the very important details of water speciation are not well understood and need more work to be transposed to explaining the mechanical alterations in the neighbourhood of advancing crack tips.

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