The Use of Atomic Force Microscopy to Study Crack Tips in Glass
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INTRODUCTION
THE invention of atomic force microscopy (AFM) in 1986[1] opened new opportunities to explore and characterize the structure of solid materials; the resolution of the instrument now approaches the molecular scale of the materials being examined. The technique is now widely used in a great number of applications.[2,3] In living matter, details of the cell membrane, nuclei, and SHELDON M. WIEDERHORN, NIST Associate, is with the National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, MD 20899-8520. Contact e-mail: sheldon.wiederhorn@ nist.gov JEAN-PIERRE GUIN, CNRS Researcher, is with the LARMAUR ERL-CNRS 6274, University of Rennes I, 35000 Rennes, France. THEO FETT, Guest Scientist, is with the Karlsruhe Institut fu¨r Technologie, Institut fu¨r Keramik im Maschinenbau, Haid-und-Neu-Strasse 7, 76131 Karlsruhe, Germany. Manuscript submitted March 15, 2010. Article published online December 7, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
proteins within these structures can now be elucidated and detailed models of their structure suggested.[4] In metallic and ceramic crystals, individual dislocations can now be imaged, as can cracks formed as a consequence of stress-corrosion cracking, fatigue, or impact loading with a foreign body. Thus, experimental results obtained by AFM have supplemented findings by other techniques, such as optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) and now provide a more complete picture of the structure and properties of all sorts of materials. Furthermore, by placing an electric charge or a magnetic dipole on the tip of the AFM probe, localized electric or magnetic fields generated by solids can also be examined and related to the microstructure of the solid.[2,3] AFM has also been very useful in elucidating the structure of crack tips in inorganic, nonmetallic glasses. VOLUME 42A, FEBRUARY 2011—267
Because of the sharpness of cracks in these materials, and the fact that glasses are not crystalline and generally do not conduct electrons, high-resolution techniques such as SEM and TEM are not as useful for evaluating crack tips in glasses as they are for crystalline materials. By using very sharp probes, 5- to 20-nm radius, highresolution AFM images of the surface can be obtained. The instrument can be used for an in-situ examination of propagating cracks that penetrate through to the free surface[5,6] and for a post mortem examination of fracture surfaces after the experiment has been completed.[7,8] In both kinds of experiments, the in-plane resolution is approximately 2 to 6 nm depending on the sharpness of the probe. The resolution normal to the fracture surface is highly dependent on the instrument noise, which is usually about 0.05 nm (rms) for commercial instruments. For an instrument with an acoustic hood, the resolution is about 3 times this value, or 0.15 nm. Thus, the AFM is a depth-sensitive instrument that yields a quantitative three-dimensional image of the surface, which TEM and SEM do
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