Nanoindentation of silver-relations between hardness and dislocation structure
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W. C. Oliver Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 21 July 1988; accepted 22 September 1988) The depth dependence of hardness in a well-annealed single crystal of silver has been characterized in nanoindentation experiments. The work is based on similar experiments performed by Chen and Hendrickson, but extends their results to indent depths on the nanometer scale. The hardness is generally found to increase with decreasing depth, with a rather sharp increase observed at depths of less than 50 nm. Using etch pitting to reveal the surface dislocation structure after indentation, the sharp rise in hardness is found to be associated with the disappearance of dislocation rosette patterns and any signs of near-surface dislocation activity, thereby suggesting that very small scale indentation plasticity may take place by nondislocation mechanisms. However, order of magnitude calculations show that possible alternatives, specifically, diffusional mechanisms, are too slow to make significant contributions. It is suggested that for very small indents, either the surface dislocation debris is quickly annealed out before it can be observed or indentation plasticity is accommodated entirely by subsurface dislocation activity. I. INTRODUCTION Recent developments in small-scale hardness testing have now made it possible to make hardness indents in materials and derive meaningful mechanical property data from depths as small as a few nanometers.'" 4 This has largely been accomplished by instrumenting specialized hardness testing equipment with high resolution devices to monitor continuously the loads and displacements experienced by an indenter as an indent is made. In many instances, this allows for the determination of indent size and many material parameters like the hardness without actually imaging the indent.5 By proper analysis of the load-displacement data, it is also possible to determine a number of material properties which cannot normally be obtained using conventional microhardness testing; for example, the elastic modulus.5"7 While nanoindentation promises to become a useful analytical tool in the determination of the mechanical properties of thin films and small volumes, the degree to which its usefulness can be realized depends strongly on how well we can understand and model the mechanics and mechanisms by which very small-scale indents are produced. To date, most efforts have concentrated on the elastic aspects of nanoindentation. For example, the application of elastic punch theory to the unloading portion of indentation load-displacement data has proven to be quite useful in determination of elastic moduli. 5 6 However, seemingly little has been done to improve our understanding of the ways in which plastic deformation occurs during very small-scale indentation. The results of numerous indentation studies using conventional microhardness techniques have revealed that at 94
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J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
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