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Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100 nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10^-9 to 10^-10 m/s). These observations are based on in-situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip., Comment: Accepted in JNCS. In press

The slow advance of a crack in sodo-silicate glasses was studied at nanometer scale by in situ and real time atomic force microscopy (AFM) in a well controlled atmosphere. An enhanced diffusion of sodium ions in the stress gradient field at the submicrometric vicinity of the crack tip was revealed through several effects: growth of nodules in AFM height images, changes in the AFM tip/sample energy dissipation. The nodules patterns revealed a dewetting phenomenon evidenced by 'breath figures'. Complementary chemical micro-analyses were done. These experimental results were explained by a two-step process: i) a fast migration (typical time: few milliseconds) of sodium ions towards the fracture surfaces as proposed by Langford et al. [J. Mat. Res. 6 (1991) 1358], ii) a slow backwards diffusion of the cations as evidenced in these AFM experiments (typical time: few minutes). Measurements of the diffusion coefficient of that relaxing process were done at room temperature. Our results strengthen the theoretical concept of a near-surface structural relaxation due to the stress-gradient at the vicinity of the crack tip. The inhomogeneous migration of sodium ions might be a direct experimental evidence of the presence of sodium rich channels in the silicate structure [A. Meyer et al., Phys. Rev. Let. 93 (2004) 027801]., Comment: 60 pages, 16 figures, 1 table, 91 references, accepted for publication in Journal of Non-Crystalline Solids (2006)

Since crack propagation in oxide materials at low crack velocities is partly determined by chemical corrosion, proper knowledge of the crack tip chemistry is crucial for understanding fracture in these materials. Such knowledge can be obtained only from in situ studies because the processes that occur in the highly confined environment of the crack tip are very different from those that take place at free surfaces, or that can be traced post mortem. We report the occurrence of hydrous liquid condensate between the two fracture surfaces in the vicinity of the tip of tensile cracks in silica. Observations are performed in real-time by means of atomic force microscopy (AFM) at continuously controlled crack velocities in the regime of stress corrosion. Condensate formation and changes in extent and shape are demonstrated for a wide range of macroscopic humidity at different crack speeds. Its liquid character is confirmed by the study of AFM phase-contrast data. It is believed that this evidence of a nanoscale liquid hydrous phase at the crack tip will enable novel insights in the chemistry of failure of oxide materials., Comment: 13 pages, 4 figures, to be published on J. Am. Cer. Soc

We have studied the low speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully mastered surrounding atmosphere. By using optical and atomic force microscopy (AFM) techniques we tracked in real-time the crack tip propagation at the nanometer scale on a wide velocity range (mm/s - pm/s and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments reveal also -for the first time- that the crack progresses through nucleation, growth and coalescence of nanometric damage cavities within the amorphous phase. This may explain the large fluctuations observed in the crack tip velocities for the smallest values. This behaviour is very similar to what is involved, at the micrometric scale, in ductile fracture. The only difference is very likely due to the related length scales (nanometric instead of micrometric). Consequences of such a nano-ductile fracture mode observed at a temperature far below the glass transition temperature in glass is finally discussed., Comment: 12 pages, 8 figures, submitted to Journal of Physics: Condensed Matter; Invited talk at Glass and Optical Materials Division Fall 2002 Meeting, Pittsburgh, Pa, USA

We studied the low speed fracture regime (0.1mm/s - 1nm/s) in different glassy materials (soda-lime glass, glass-ceramics) with variable but controlled length scale of heterogeneity. The chosen mechanical system enabled us to work in pure mode I (tensile) and at a fixed load on DCDC (double cleavage drilled compression) specimen. The internal residual stresses of studied samples were carefully relaxed by appropriate thermal treatment. By means of optical and atomic force (AFM) microscopy techniques fracture surfaces have been examined. We have shown for the first time that the crack front line underwent an out-of-plane oscillating behavior as a result of a reproducible sequence of instabilities. The wavelength of such a phenomenon is in the micrometer range and its amplitude in the nanometer range. These features were observed for different glassy materials providing that a typical length scale characterizing internal heterogeneities was lower than a threshold limit estimated to few nanometers. This effect is the first clear experimental evidence of crack path instabilities in the low speed regime in a uniaxial loading experiment. This phenomenon has been interpreted by referring to the stability criterion for a straight crack propagation as presented by Adda-Bedia et al. (Phys. Rev. Letters (1996) 76} p1497)., Comment: 16 pages, 10 figures, submitted to Journal of Non-Crystalline Solids

We have studied the low speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully mastered surrounding atmosphere. By using optical and atomic force (AFM) microscopy techniques we tracked in real-time the crack tip propagation at the nanometer scale on a wide velocity range (1 mm/s and 0.1 nm/s and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments revealed also -for the first time- that the crack advance proceeds through nucleation, growth and coalescence of nanometric damage cavities inside the amorphous phase, which generate large velocity fluctuations. The implications of the existence of such a nano-ductile fracture mode in glass are discussed., Comment: 6 pages, 5 figures, submitted to Applied surface Science

We report in situ Atomic Force Microscopy experiments which reveal the presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in glass. Their presence might explain the departure from linear elasticity observed in the vicinity of a crack tip in glass. Such a ductile fracture mechanism, widely observed in the case of metallic materials at the micrometer scale, might be also at the origin of the striking similarity of the morphologies of fracture surfaces of glass and metallic alloys at different length scales., Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett, few minor corrections, Fig. 1b changed