Surface cracks lead to the decrease of the glass lifetime. A better understanding of cracks self-healing can give a method to increase the glass durability. In a previous study [1], the self-healing behavior of radial cracks generated indentation in float silicate glass has been analyzed when heat treated above glass transition temperature. Two main types of radial crack evolution are observed: (1) either very large crack morphological changes, or (2) a simple direct crack closure. We associate these two phenomena respectively to the effects of capillarity forces induced by surface energy minimization or to the effects of residual stress release. The type of crack evolution is proposed to be mainly depending on glass viscosity: it favors or impedes morphological changes driven by capillarity forces. Similar studies are also performed in others glass types, such as borosilicates or tin surface of float glass. We observe that the glass composition is also important as it probably determines potential hydration. But to go deeper in the analysis, we also focus on the measurement of surface tension in the typical temperature range of cracks thermal healing. These measurements are performed on a "home-made" apparatus, with a small mobile furnace moving along a glass fiber. These measurements of surface tension combined with low temperature viscosity measurements should allow us to confirm our hypothesis of cracks morphological changes driven by capillarity forces. [1] R. Girard, A. Faivre, F. Despetis, " Influence of water on crack self-healing in soda-lime silicate glass" (accepted by J. Am. Ceram. Soc.).

Surface cracks lead to the decrease of the glass lifetime. Consequently, the understanding of self-healing can give a method to increase the glass durability. In this study, the self-healing behavior of radial cracks generated by Vickers indentation on float glass is analyzed when heat treated at 620°C under various atmospheres (from 0 to 75% RH). Two main types of radial crack evolution are observed: (1) either very large crack morphological changes, or (2) a simple direct crack closure. We associate these two phenomena respectively to the effects of capillarity forces induced by surface energy minimization or to the effects of residual stress release. The type and rate of crack changes are shown to be highly dependent on (a) the humidity level of the furnace atmosphere but also on (b) the initial water content of the glass at the vicinity of the crack. The large crack morphological changes, (enlargement, shortening and rounding of radial cracks) observed in case (1), are attributed to smoothing of the crack tip through viscous flow driven by capillary forces. We suggest that the viscosity of matter surrounding cracks can be significantly reduced or increased by hydration/des-hydration of the sample from the surface, as a function of the water vapour pressure in the furnace. Changes in cracks morphology are then dependent on the changes in the dissolved water around the cracks, which are dependent on the water vapour of the atmosphere and on the initial water content, but which also depend on time and depth. It is noteworthy that these morphological changes also depend on capillary forces, i.e. on the curvature radius of the crack, in a viscoelastic medium. Direct crack closure, case (2), is only observed when the hydration level of the glass in which the crack is generated is very low. The initial viscosity quite elevated around the cracks and viscous flow driven by capillary forces is impeded. We assume that direct crack closure is then due to release of residual tensile stresses induced by indentation.

Les fissures de surface diminuent fortement la durabilité des verres. Une meilleure compréhension des mécanismes de guérison des fissures par traitement thermique pourrait permettre d'améliorer la durabilité des verres. Nous avons récemment analysé l'autoguérison de fissures générées par indentation dans des verres de silicate type verre à vitre par des traitements thermiques au-dessus de la transition vitreuse[1]. Deux types principaux d'évolution des fissures ont été observés : (1) soit un changement significatif de morphologie des fissures conduisant à une restauration du matériaux, (2) soit une simple fermeture des fissures sans restauration. Le type d'évolution des fissures est principalement dépendant de la viscosité du verre : elle favorise ou empêche les changements induits par les forces capillaires. Nous avons également montré que la teneur en eau du verre et que le degré d'hygrométrie de l'atmosphère du four avait une influence prépondérante sur le type de mécanisme de modification des fissures, puisque ces paramètres influence beaucoup la viscosité. Pour compléter cette étude, nous nous intéressons maintenant à d'autres compositions verrières : un verre de borosilicate, un verre de phosphate, ainsi que la face " étain " du verre à vitre. Ces nouveaux résultats confirment les mêmes types de modification des fissures par traitement thermique, mais mettent en évidence que l'influence de l'atmosphère du traitement thermique est très dépendante de la composition du verre, comme on pouvait s'y attendre. Ces résultats montrent également que l'oxydation ou la réduction de certains ions va changer drastiquement les comportements des fissures lors de la guérison. L'étain, en particulier, favorise la réticulation du réseau en s'oxydant et modifie donc la viscosité localement. Par ailleurs, les résultats concernant le verre borosilicaté indiquent une dépendance très faible visà vis de l'atmosphère du traitement thermique. Pour compléter l'analyse, nous nous sommes intéressés à la mesure de la tension de surface avec un appareil développé dans notre laboratoire. Ces mesures accompagnées de mesures de viscosité basse température nous permettent de confirmer nos hypothèses sur le changement de morphologie des fissures conduit par les forces capillaires. [1] R. Girard, A. Faivre, F. Despetis, " Influence of water on crack self-healing in soda-lime silicate glass" J. Am. Ceram. Soc., 94 [8] 2402-2407 (2011).

The self-healing behavior of radial cracks generated by Vickers indentation in float glass is analyzed when heat treated at 620 degrees C under various atmospheres. Results evidence that two main driving forces influence radial crack evolution: release of residual stresses induced by initial indentation and capillary forces due to surface energy minimization. Depending on the viscosity level, viscous flow allows crack morphological changes driven by capillarity forces or not. Our results evidence that at 620 degrees C, the viscosity of the glass surrounding cracks can be significantly reduced by water diffusion and glass hydrolysis, or increased by glass des-hydration, as a function of the humidity level of the furnace atmosphere. Hydration and des-hydration of glass are shown to play a major role in the crack morphology changes during healing, respectively, favoring or impeding morphological changes driven by capillarity forces.

The knowledge of the internal structure of porous materials is of main importance to compute their physical properties. This article focuses on base-catalyzed and colloidal silica aerogels, which are fractal materials and we use an original method for the reconstruction of these aerogels from TEM images. The method used is iterative and leads to the same fractal dimension as the real material, computed from the internal two-point correlation function. Unlike the reconstruction of porous materials found in literature, our method is based on the distribution of matter and not of the porous network, and has the additional advantage of being only half statistical, i.e. only the coordinates in the z direction are statistically obtained while the coordinates x and y of the elements of matter are accurately obtained from TEM images of our samples

Success in designing self-healing materials has important consequences for material safety, product performance and enhanced fatigue lifetime. For example, solid oxide electrolyser and fuel cells development is hindered by a lack of long-term durability of seals caused by cracking induced by thermal expansion mismatch. In these applications, self healing glasses need to be developed to unsure a reasonable durability of the seal. It is consequently interesting to better understand and analyse the self healing behaviour of cracks in glass sealant, and more generally in oxide glasses. Although the process of crack opening has been widely studied, the driving forces controlling crack healing are not clearly identified [1,2]. Viscous flow is obviously an important parameter but environment has also been observed to play a crucial role. And it is also evident that residual stresses cannot be ignored. In this study we analyse by micro-indentation the healing of cracks generated in different silicate and borosilicate glasses. Healing thermal treatment is performed at temperatures both above and below Tg in humid and dry environment. Basically, we analyse the shape and the length of crack as a function of treatment's time. Our results confirm that temperature and thus viscosity plays a major role. But although thermal treatments were realized at the same viscosities for the different glasses and under the same atmosphere, our results clearly evidence that the healing rate depends on the glass composition, being faster in the borosilicate glass. Our results also evidence that water vapour enhanced cracks healing both in silicate and borosilicate glasses. In the case of borosilicate glass in humid environment, almost complete healing is observed even 20°C below Tg. For thermal treatment realized above Tg, we can observe a change in the shape of the crack, with a spheroidization. This leads us to the conclusion, that surface tension can not be neglected. In this perspective, we develop a specific device allowing us to measure surface tension. [1] B.A. Wilson, E.D. Case, J. of Materials Science 32 (1997) 3163-3175 [2] P. Hrma, W.T. Han, A.R. Cooper, J. of Non-Crystalline Solids 102 (1988) 88-94 [3] K. Kese, M. Tehler, B. Bergman, J. of the Eur. Cer. Soc. 26 (2006) 1003-1011