Scanning tunneling microscope observations of the mirror region of silicate glass fracture surfaces
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Yoshihisa Watanabe Department of Materials Science and Engineering, National Defense Academy, Hashirimizu, Yokosuka, Kanagawa 239, Japan (Received 25 June 1993; accepted 17 September 1993)
We report scanning tunneling microscope images of gold-coated fracture surfaces of soda lime glass and fused silica in the mirror region. The scans show a variety of nanometer scale features that are attributed to fracture phenomena at this scale. We find considerable similarity to the structures observed in regions of extensive crack branching (e.g., "mist"). The density of these features increases as one progresses away from the crack origin toward the mirror-mist boundary. Comparisons are made between soda lime glass and fused silica, revealing differences in the local deformation behavior of these two materials. Self-similarity of the observed structures is probed by measurements of the fractal dimension, Df, of the surfaces created in soda lime glass near the mirror-mist boundary, where we observe 2.17 < Df < 2.40.
I. INTRODUCTION Many brittle materials display a characteristic pattern of mirror, mist, and hackle features surrounding the fracture origin.1 A scanning electron micrograph (SEM) of typical mirror, mist, and hackle features on the fracture surface of a simple silicate glass (Na 2 0: 3SiO2) is shown in Fig. 1. The mist and hackle features are attributed to the onset of localized crack branching when a critical stress intensity or fracture energy is reached along the growing crack front.2"5 The smallest mist features are generally too small for observation by SEM. The resolution of scanning tunneling microscopy (STM) can reveal nm-scale fractographic detail, even after the application of a metallic film to provide the necessary conductivity on insulating materials.6'7 We present STM observations of gold-coated silicate glass fracture surfaces in which the onset of localized crack branching on the nm scale is readily evident. These observations also show evidence for crack fingering prior to crack branching due to nonlinearities in the mechanical response of the silicate glasses. The silicate glasses are key model materials in the study of brittle fracture. Their combination of brittleness (arrested cracks in soda lime glass are believed to be atomically sharp),8'9 isotropic material properties, and nm-scale homogeneity greatly simplify descriptions of the fracture process. We report observations on fused silica and a typical soda lime glass. Fused silica has a relatively open network and tends to deform by changing its density, while soda lime glass has a relatively compact structure and tends to deform in shear. Further, 476
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 2, Feb 1994
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FIG. 1. SEM image of the region surrounding the origin of failure during three-point bend loading of a typical glass. The mirror, mist, and hackle regions are identified with the letters A, B, and C, respectively. 1994 Materials Research Society
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