Probing the Strain Hardening Response of Small Wear Volumes with Nanoindentation
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INTRODUCTION
DETERMINATION of the stress-strain behavior at the nano- and microscales is challenging. Some techniques that have been developed include tensile straining of free-standing thin films and nanowires[1,2] as well as focused ion beam (FIB) machined micron-sized tensile bars.[3,4] These techniques are good for thin films and wires but require complicated machining (FIB or photolithographic methods) or probe manipulation in order to obtain a specimen. Another drawback is that these techniques are not suitable for worn materials. In metallic systems, sliding contact often induces plastic deformation below the contacting surfaces. This subject M.J. CORDILL, formerly Research Student with the Chemical Engineering and Materials Science Department, University of Minnesota, Minneapolis, MN 55455, is now Scientist with the Department of Material Physics, University of Leoben, and the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences, 8700 Leoben, Austria. Contact e-mail: megan.cordill@ oeaw.ac.at N.R. MOODY, Distinguished Member of the Technical Staff, is with Sandia National Laboratories, Livermore, CA 94550. J.M. JUNGK, formerly Research Student with the Chemical Engineering and Materials Science Department, University of Minnesota, is now with Sandia National Laboratories, Albuquerque, NM 87158. M.S. KENNEDY, formerly Research Student with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, is now Assistant Professor with the School of Materials Science and Engineering, Clemson University, Clemson, SC 29634. W.M. MOOK, formerly Research Student with the Chemical Engineering and Materials Science Department, University of Minnesota, is now a Postdoctoral Researcher with the EMPA, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanics of Materials and Nanostructures, CH-3602 Thun, Switzerland. S.V. PRASAD, Member of the Technical Staff, is with Sandia National Laboratories. D.F. BAHR, Professor, is with the School of Mechanical and Materials Engineering, Washington State University. W.W. GERBERICH, Professor, is with the Chemical Engineering and Materials Science Department, University of Minnesota. Manuscript submitted September 26, 2010. Article published online February 24, 2011 2226—VOLUME 42A, AUGUST 2011
of wear-induced subsurface deformation has been the focus of numerous studies[5–15] and is critical in determining the performance and reliability of microsystems since surface interactions dominate as machine scale is reduced. The regions below sliding contacts can be severely stressed and may give rise to acute gradients of plastic shear strain, which accumulates with continued sliding.[8–10] These strains can easily exceed 10 pct near the wear surface[11,12] and have been estimated to approach 100 pct.[13] Strains of these magnitudes can markedly change surface roughness, hardness, grain size, and texture but also decrease rapidly with distance from the surface, creating a gradient of microstr
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