Conception chimique de verres à propriétés mécaniques spécifiques

National audienceLes verres sont des matériaux fragiles. Cependant, la quantité aussi bien que la diversité des verres disponibles ne cessent de croître. A chaque période de la vie du verre des propriétés mécaniques spécifiques sont recherchées. Ces propriétés doivent permettre l'élaboration et la mise en forme par des procédés industriels et garantir une bonne tenue en service. Sous l'appellation "Verre" sont rassemblés des matériaux très différents qui ont en commun une organisation atomique dépourvue d'ordre à longue distance, avec un module d'Young (E) et un coefficient de Poisson (ν) s'étalant respectivement de 5 à 180 GPa et de 0,1 à 0,4 pour les verres inorganiques. A côté du rôle essentiel que jouent les propriétés élastiques pour le choix d'un matériau en construction et le calcul de structure, nous montrons dans cet exposé que les caractéristiques élastiques macroscopiques (E,ν) permettent de sonder les ordres à courte et moyenne distances existant dans la plupart des verres. En particulier, une excellente corrélation existe entre ν, la densité d'empilements (C g) et la dimensionalité du réseau vitreux. ν>0,25 indique que le verre est principalement constitué de chaînes et de feuillets (chalcogénures, verres silicatés riches en cations compensateurs et modificateurs de réseau), avec C g >0,56, le maximum étant atteint pour les verres métalliques (ν~0,4 et C g >0,7). Au contraire, ν<0,25 correspond à une grande réticulation et une organisation tri-dimensionnelle s'accompagnant d'une faible compacité. En outre, la dépendance des modules d'élasticité avec la température apporte un éclairage original sur la "fragilité" (au sens de C.A. Angell) des verres et sur le degré de coopérativité des mouvements des atomes à l'origine de la déformation

Elastic properties and atomic bonding character in metallic glasses

International audienceThe elastic properties of glasses from different metallic systems were studied in the light of the atomic packing density and bonding character. We found that the electronegativity mismatch (Δe−) between the host- and the major solute-elements provides a plausible explanation to the large variation observed for Poisson's ratio (ν) among metallic glasses (MGs) (from 0.28 for Fe-based to 0.43 for Pd-based MGs), notwithstanding a similar atomic packing efficiency (Cg). Besides, it is found that ductile MGs correspond to Δe− smaller than 0.5 and to a relatively steep atomic potential well. Ductility is, thus, favored in MGs exhibiting a weak bond directionality on average and opposing a strong resistance to volume change

Elastic properties and indentation cracking behavior of Na2O-TiO2-SiO2 glasses

International audienceThe effects of composition on indentation deformation and cracking behavior of Na2O-TiO2-SiO2 glasses were studied in the light of structural considerations and parameters such as the atomic packing density (Cg) and the network energy, using a combination of elastic measurements and micro-hardness indentation experiments. Na2O-TiO2-SiO2 glasses with titania contents of 4–10 mol% and sodium oxide contents of 10–25 mol% were prepared through a traditional melt-quench process. Indentation experiments were conducted using a Vickers indenter with loads ranging from 10 mN to 49 N. Critical loads for crack initiation and cracking patterns were systematically investigated and correlated with the elastic properties of glass. In this ternary system concerning a relatively large range of Poisson's ratio (ν), a minimum in critical crack initiation load was observed at a ν of 0.21–0.22. This study brings to light the unusual role of titanium in the glass network, which gives birth to peculiar trends in the structural and mechanical properties

Direct observation of the displacement field and microcracking in a glass by means of X-ray tomography during in situ Vickers indentation experiment

International audienceThe actual displacement field in a glass during an in-situ Vickers indentation experiment was determined by means of X-ray tomography, thanks to the addition of 4 vol % of X-ray absorbing particles, which acted as a speckle to further proceed through digital volume correlation. This displacement was found to agree well with the occurrence of densification beneath the contact area. The intensity of the densification contribution (Blister field proposed by Yoffe) was characterized and provides evidence for the significant contribution of densification to the mechanical fields. Densification accounts for 27% of the volume of the imprint for the studied glass, that is expected to be less sensitive to densification than amorphous silica or window glass. A major consequence is that indentation cracking methods for the evaluation of the fracture toughness, when they are based on volume conservation, as in the case of Hill-Eshelby plastic inclusion theory, are not suitable to glass. The onset for the formation of the subsurface lateral crack was also detected. The corresponding stress is z 14 GPa and is in agreement with the intrinsic glass strength

High temperature elasticity and viscosity of GexSe1-x glasses in the transition range

International audienceThe viscous-flow behavior and temperature dependence of the elastic moduli of chalcogenide glasses from the germanium-selenium system were studied by means of homemade high-temperature indentation setup and resonant-frequency technique (1-10 kHz), respectively, for temperatures between 0.8 and 1.2×Tg. The softening rates, both in the elastic and in the viscous-flow regimes, were correlated to network destructuration or reorganization events in the light of previously reported high-temperature neutron-scattering data. The concomitant change of Poisson's ratio (ν) and the thermodynamic parameters of the thermally activated viscous-flow process were characterized and provide a new basis for the understanding of the sources for the softening in the transition range. The temperature dependence of ν suggests weak changes of the network cross-linking degree at large Ge contents. On the contrary, in the case of a-Se, a steep fragmentation of the structural units is inferred from the ν(T) data, and the flow process is accompanied by a huge entropy change (activation entropy at saddle point). The entropy contribution at Tg (Tg ×dSa) represents more than 50% of the activation enthalpy for flow (dHa) and increases with the selenium content. Hence the free activation energy (dGa) is much smaller than apparent activation energy as derived from viscosity data