Continuous Microindentation of Passivated Surfaces in Surface Active Media
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CONTINUOUS MICROINDENTATION OF PASSIVATED SURFACES IN SURFACE ACTIVE MEDIA SHANKAR K. VENKATARAMAN*, HE HUANG*, DAVID L. KOHISTEDT* AND WILLIAM .W. GERBERICH* * Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. **Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455. ABSTRACT Continuous microindentation tests on electropolished, single crystal Fe(3wt%Si) and Ni surfaces with thin passivation layers exhibit sharp discontinuities in the load-displacement behavior. At the discontinuity, which occurs at a load of 1.8 mN for Fe(3wt%Si) and 0.1 mN for Ni, stresses below the indenter were close to the theoretical strength of the corresponding metal. Microindentation tests performed to loads less than the discontinuity point showed an elastic load-unload behavior. On removing the passive film with a NaCl or HCl solution, strengths are one to two orders of magnitude smaller in the presence of the chloride ions. On evaporation of the liquid, the load at the discontinuity returned to its initial value due to repassivation, indicative of a passivation oxide layer with varying thickness. This conclusion was verified by ellipsometry. Even though the elastic load-unload behavior on the passivated surface suggests elastic behavior, both elastic finite element and non-linear, elastic perfectly plastic finite element solutions strongly suggest that this might not be the case. Several suggestions as to the reasons for the deviation from continuum theory are presented. INTRODUCTION The mechanical strength of metals on a small scale increases with a decrease in the size [1]. This behavior holds not only for crystals grown specially in a small form, such as whiskers and evaporated thin films, but also in crystals prepared from the bulk. It was originally thought that whiskers possessed high strengths only because they were essentially dislocation free. However, various researchers have suggested that the ease or difficulty of generating dislocations either internally or at the surface and the pinning of existing dislocations by point defects could be responsible for the high strengths. Smith and Bowkett [2] have reported tensile strengths approaching the theoretical value in W, Al and Ni samples with cross-sectional diameters down to 0.05 jrm. In indentation hardness tests of very small volumes using field ion microscope tips as indenters, Gane and Bowden [3] observed strengths approaching the theoretical value in various metals. More recently, using continuous microindentation techniques and a diamond indenter, Venkataraman, Kohlstedt and Gerberich [41 have obtained theoretical strengths in Fe(3wt%Si) single crystals. One feature in common in these two studies was the presence of a thin passivation or other amorphous film between the indenter and the sample. These researchers have also shown that in the absence of the passivation or other amorphous film, the material strengths are much lower. Pethica and Tabor [51 studied the contact forces between a
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