Real-time observation of non-equilibrium liquid condensate confined at tensile crack tips in oxide glasses

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. So

Stress-enhanced ion diffusion at the vicinity of a crack tip as evidenced by atomic force microscopy in silicate glasses

The slow advance of a crack in soda-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 sub-micrometric 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]

Effect of stress gradient at the vicinity of a crack tip on ionic diffusion in silicate glasses : an AFM study

International audienceThe slow advance of a crack in sodo-silicate glasses was studied at nanometer scale by in-situ atomic force microscopy (AFM) in a well-controlled atmosphere (N2 and H2O). An enhanced diffusion of sodium ions in the stress-gradient field at the sub-micrometric vicinity of the crack tip was revealed through several effects: growth of nodules in height images, changes in the AFM tip–sample energy dissipation as detected in phase images. Ex-situ chemical micro-analyses completed the AFM measurements. The nodules patterns revealed a dewetting phenomenon evidenced by “breath figures”, i.e. analog to the fogging that occurs when a vapour condenses onto a 'cold' surface [D. Beysens et al., Phys. Rev. Let. 57, 1433 (1986)]. These experimental results were explained by a two-step process: i) a fast migration of sodium ions towards the fracture surfaces as proposed by Langford et al. [J. Mat. Res. 6, 1358 (1991)], 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. Raman and SIMS studies revealed that nodules – for samples studied after exposition to common air - are preferentially covered by an organic overlayer of a carboxylate salt with a long aliphatic chain. The catalytic role of sodium ions in that chemical process is suspected

Corrigendum to: Composition dependence of indentation deformation and indentation cracking in glass (vol 61, pg 5949, 2013)

International audienceThe authors regret that a typographic error has slipped into one of the numerical values of the coefficients needed to get a sigmoidal curve fitting using Eq. (11). In sub-section 3.2 on page 5958 of the original paper, the βR coefficient should read 0.0008 instead of 0.008

Evolution of the elastic modulus of Zr–Cu–Al BMGs during annealing treatment and crystallization: Role of Zr/Cu ratio

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Composition dependence of indentation deformation and indentation cracking in glass

International audienceAbstract Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson's ratio (ν), whereas shear is favored at large ν. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15 ⩽ ν ⩽ 0.20; Semi-Resilient, for 0.20 ⩽ ν ⩽ 0.25; and Easily-Damaged for 0.25  <  ν  <  0.30. Radial-median cracks occur at low load (⩽50 mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for ν (∼0.22 depending on the Young's modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small

FORMATION AND EVOLUTION OF A CONFINED LIQUID CONDENSATE AT THE CRACK TIP IN GLASSES

Crack propagation in oxide glasses at low crack velocities is controlled by stress corrosion. Proper knowledge of the crack tip chemical environment is thus crucial to understand the slow fracture process of these materials. The formation of a liquid condensate in the confined area of the crack tip is theoretically expected and is classically mentioned to explain specific behavior during the crack propagation. Since this condensate is of the nanometer scale and cannot be traced post mortem, it is extremely difficult to observe. This paper reports the experimental evidences of the presence of a liquid condensate at the tip of a crack propagating by stress corrosion in silica glass. The observation has been performed in situ under carefully controlled atmosphere through phase imaging by atomic force microscopy (AFM). The evolution of the condensate has been followed as a function of the relative humidity. The impact of those evidences of the liquid condensate will be discussed and information on its size or evolution kinetic will be reported

Correlation between mechanical and structural properties as a function of temperature within the TeO2–TiO2–ZnO ternary system

International audienceIn situ Raman spectroscopy, Resonance Frequency and Damping Analysis (RFDA) setup and Associated Raman Brillouin Calorimeter (ARABICA) setup experiments as a function of temperature were conducted on the (100-x)TeO2-5TiO2-xZnO (x=15; 17.5; 20; 22.5; 25) glass system. Structural and mechanical properties showed non-conventional properties evolution as a function of ZnO content which are managed by the modification of ZnO polyhedra. Before glass transition temperature (Tg), the evolution of the Boson peak (BP) frequencies and the elastic properties are in accordance with the idea of the relaxation of the strains inside the glass; after Tg, their evolution shows the transition toward a viscous liquid. BP and mechanical properties study evidences that as nano-heterogeneous domains size increases at a faster rate, the mechanical properties will decrease at a slower rate. This point will be helpful for choosing appropriate composition for optical fiber shaping

Fracture toughness, fracture energy and slow crack growth of glass as investigated by the Single-Edge Precracked Beam (SEPB) and Chevron-Notched Beam (CNB) methods

International audienceWe show that the Single-Edge Precracked Beam (SEPB) test is not only suitable to the determination of the fracture toughness (KIc) of glass, but also offers a unique opportunity to assess the slow crack growth behavior in a single experiment lasting for few minutes. Besides, we found that it is possible to get either a stable or an unstable final fracture regime (pre-cracked specimen) depending on the testing parameters, and that the unstable case is preferable for the estimation of KIc. The “pop-in” precrack was found mostly to close completely once the load was suppressed on the bridge-flexure device. This led to a reopening event on the loading curves. It is noteworthy that all these original observations were made possible thanks to the design of a very stiff testing apparatus (6.7 MN m−1) allowing for a cross-head speed as small as 0.01 μm s−1. Results obtained on four grades of commercially available glasses are compared to those stemming from Vickers indentation cracking and chevron notched experiments. © 2017 Acta Materialia Inc

Properties, structure and crystallization study of germano-gallate glasses in the Ga2O3-GeO2-BaO-K2O system

Mid-infrared transparent germano-gallate glasses (with gallium oxide as main component) have been studied following the composition law (100-x)[48.3GaO3/2-32.2GeO2-19.5BaO]-xKO1/2 (with x = 0, 5, 10, 13, 17 mol%). All prepared glasses are transparent from approximately 280 nm up to 5.5 μm, with a measured linear refractive index and density decreasing respectively from 1.72 to 1.67 (at 1538 nm) and 4.79 to 4.34 g/cm3 with increasing potassium oxide content. As expected, the increase of potassium oxide content also results in a decrease of the glass transition temperature (698 to 671 °C) and the viscosity in the softening regime whereas the thermal expansion coefficient increases (4.8 to 10.3 × 10−6 K−1). These results are related to the depolymerization of the 3D interconnected glass network which has been studied by Raman spectroscopy. Large thermal stability criteria (difference between onset of crystallization and glass transition temperatures) ranging from 182 to 199 °C have been measured by DSC for the compositions containing potassium oxide, suggesting strong potential for fiber drawing from glass preforms. The 40.1GaO3/2-26.7GeO2-16.2BaO-17KO1/2 glass composition (in cation mol.%, which corresponds to 28Ga2O3-37GeO2-23BaO-12K2O in mol.%) was then selected for further investigation. Its mechanical properties (Young and shear moduli, Poisson's ratio, hardness) were measured, showing values close to those reported for well-known barium gallo-germanate (BGG) glasses. A thorough crystallization study was then carried out using DSC (Marotta's method), X-Ray diffraction and electron microscopies (TEM and SEM) to better understand the unexpected surface crystallization issues observed during the glass preform fiber drawing process. The potential of using such germano-gallate glasses to produce mid-infrared optical fiber of high mechanical strength is discussed.Initiative d'excellence de l'Université de BordeauxImPRession laser de fibres exOtiques Multi-MaTEriau