The mechanical behavior of a passivating surface under potentiostatic control
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The mechanical behavior of a passivating surface under potentiostatic control D. F. Bahr, J. C. Nelson, N. I. Tymiak, and W. W. Gerberich Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 23 September 1996; accepted 29 August 1997)
Continuous microindentation has been carried out on an iron –3% silicon single crystal in 1 M sulfuric acid. The ability of the material to support elastic loading is directly linked to the presence of thermally grown oxide films and passive films applied through potentiostatic control of the sample. When the passive film is removed, either by chemical or electrochemical means, the iron alloy can no longer sustain pressures on the order of the theoretical shear strength of iron. Instead, the metal behaves in a traditional elastic-plastic manner when no film is present. The oxide film at the edges of the indentation can sustain applied tensile stresses up to 1.2 GPa prior to failure. Indentation in materials undergoing dissolution must account for the rate of material removal over the remote surface and the resulting plastic deformation around the contact of the indentation.
I. INTRODUCTION
Many commonly used metals and alloys, such as iron, titanium, and nickel, form passive films when exposed to oxidizing environments. These passive films, which are often considered to be stable oxides, are exploited for their ability to reduce corrosion and oxidation by orders of magnitude. Electrochemical testing of these materials and their various alloys is very well established, and commonly is used to evaluate the corrosion resistance of a particular metal in a given environment. However, the mechanical properties of these passive films and the metal on which the passive film has formed have not been thoroughly explored. This is in part due to the small scale of the passive films, which are generally oxide films with thicknesses on the nanometer scale. In addition, the mechanical properties of the passive films are expected to vary with changes in structure and composition, as there may be layers of hydroxide or carbonaceous species present on the surfaces of these films. Since the structure and composition of the passive films on iron are still being studied using a variety of techniques including secondary ion mass spectroscopy,1 x-ray absorption near-edge structure,2 laser methods,3 and Raman spectroscopy,4 it will be difficult to correlate the mechanical behavior of the films to the structure. The recent surge in the evaluation of the mechanical properties of surfaces has raised some interesting questions regarding the role of passive films on the mechanical behavior of a metal.5–13 The first evidence of the ability of small volumes of material to resist plastic deformation until stresses reached the theoretical shear strength was observed by Gane and Bowden.5 Initial experiments of the behavior of an oxidized metal surface were made by Pethica and Tabor,6 who examined J. Mat
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