Claude Guillot

Glass Breaks like Metal, but at the Nanometer Scale

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.

Scaling exponents for fracture surfaces in homogeneous glass and glassy ceramics

We investigate the scaling properties of postmortem fracture surfaces in silica glass and glassy ceramics. In both cases, the 2D height-height correlation function is found to obey Family-Viseck scaling properties, but with two sets of critical exponents, in particular, a roughness exponent zeta approximately 0.75 in homogeneous glass and zeta approximately 0.4 in glassy ceramics. The ranges of length scales over which these two scalings are observed are shown to be below and above the size of the process zone, respectively. A model derived from linear elastic fracture mechanics in the quasistatic approximation succeeds to reproduce the scaling exponents observed in glassy ceramics. The critical exponents observed in homogeneous glass are conjectured to reflect the damage screening occurring for length scales below the size of the process zone.

Surface fracture of glassy materials as detected by real-time atomic force microscopy (AFM) experiments

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 (10 � 3 to 10 � 10 ms � 1 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 from nucleation, growth and coalescence of nanometric damage cavities inside the amorphous phase, which generate large velocity fluctuation. Implications of the existence of such a nano ductile fracture mode in glass are discussed. # 2003 Elsevier Science B.V. All rights reserved.

Crack fronts and damage in glass at the nanometre scale

We have studied the low-speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully controlled surrounding atmosphere. By using optical and atomic force microscopy techniques, we tracked, in real-time, the crack tip propagation at the nanometre scale over a wide velocity range (10−3–10−12m s−1 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 that involved, at the micrometric scale, in ductile fracture. The only difference is very probably due to the related length scales (nanometric instead of micrometric). The consequences of such a nano-ductile fracture mode observed at a temperature far below the glass transition temperature, Tg, in glass is also discussed.

Si(001) vicinal surface oxidation in O2: Angle-resolved Si 2p core-level study using synchroton radiation

Abstract The aim of this work is to determine if, after oxidation in pure O2, a vicinal surface Si(001) miscut 5° towards the [1 1 0] direction, exhibits a preferential SiO2 formation on the [110] ledges. In a first stage, combining XPS (X-ray photoelectron spectroscopy) and LEED (low-energy electron diffraction) available in an auxiliary chamber, we examine the cleaning conditions leading to a non-faceted stepped Si surface with dominant (2 × 1) single domains. This surface preparation is reproduced in the synchroton facility chamber where ARPR-single-resolved photoemission spectroscopy) is performed: after cleaning the Si surface and studying the starting situation (san- states, initial contamination), an ultra-thin oxide film (∼ 1.5 × 1015 oxidised Si atoms/cm2) is grown at 785–800°C, under Torr of oxygen. The absence of anisotropy in the ARPES emission of the Si 2p core levels taken at the minimum electron escape depth (hv=130eV), points to no extra-growth of the oxide on the ledges. The in-depth distribution of the oxide states seems to be homogeneous, within the sensitivity of the probe. On the other hand, after partial thermal desorption of the ultra-thin oxide film in vacuum, a strong anisotropy in the emission of electrons from SiO2 is seen in areas where oxide patches coexist with bare silicon. It is shown that this ensues from a significant step bunching and (111) faceting of the surface. A correlation is made with SEM (scanning electron microscopy) images of the surface. The problem of step bunching, inasmuch as it can alter the interpretation of shadowing effects, is emphasized.