Highly localized acoustic emission monitoring of nanoscale indentation contacts
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This study evaluated a novel approach for acoustic emission (AE) monitoring of nanoindentation. The technique utilizes a miniature AE sensor integrated into a calibrated diamond indenter tip on a commercial nanoindentation system. The evaluation focused on the yield-point phenomenon in W (100); MgO (100); and sapphire C (0001); R (1¯012); A (12¯10); and M (101¯0) single-crystal surfaces. The minimum amount of elastic energy release sufficient to produce AE signal detectable with the indenter tip sensor was nearly two orders of magnitude lower than the minimum energy level required for conventional AE sensors. Wave forms detected with the indenter tip sensor were independent of sample size. A linear relationship between released elastic energies and the corresponding AE energies was observed for all three evaluated materials. The scaling coefficient of the linear relationship was independent of indenter tip size/shape and indentation depth. The differences between the mechanisms of the initial stages of plasticity for the various crystallographic orientations of sapphire were reflected in the following aspects of AE activity: detection of a specific type of AE wave form that correlated to the presence of linear surface features near the indentation sites; AE signal associated with the yield point, consisting either of one or two distinct wave forms; and presence or absence of AE signals after the yield point. The possibility of plasticity onset in sapphire involving both slip and twinning is discussed.
I. INTRODUCTION
Detection of potential failure sites is a challenge to modern technologies. With component dimensions decreasing down to the nanoscale, the earliest stages of plasticity and/or fracture may result in failure. Combining nanoindentation and in situ imaging of tested areas1– 4 enables the evaluation of the initial stages of plastic deformation and fracture. However, in many cases, even the combination of these methods does not provide sufficient information to identify the exact mechanism of the deformation process at ultralow load contacts. Often, there is ambiguity even in highresolution post-test transmission electron microscopy (TEM) images of indented areas.5 Additional insight into the mechanisms of nanoscale contacts may be gained through acoustic emission (AE) monitoring. AE signals result either from the sudden release of elastic energy or from surface interactions such as friction and adhesion. Sudden energy release occurs during unstable crack growth, high-speed phase transformations, and plastic instabilities. Plastic instabilities
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J. Mater. Res., Vol. 18, No. 4, April 2003 Downloaded: 29 Mar 2015
include initiation of slip6 or twinning systems,7 activation of dislocation multiplication sources,8 or sudden acceleration of existing dislocations.9–11 In most cases both energy-releasing and surface-interaction processes contribute to detected AE signals. For instance, during fatigue crack growth, pulses of r
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