Surface roughness effects on the displacement bursts observed in nanoindentation
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The distinctive finding of unstable displacement bursts of indent depth observed in nanoindentation is thought to be linked to the dislocation emission phenomenon. However, because of a lack of understanding of the surface physics of the contact area, the explanation of the instability mechanism remains vague. In this paper, an effect of the surface roughness on the displacement bursts was experimentally investigated using single-crystalline aluminum. The surface properties of samples with several degrees of roughness of less than 10 nm were measured using atomic force microscopy. The discussion focuses on the relations between critical values of the load at the displacement bursts and the burst width.
I. INTRODUCTION
Recent nanometer-scale experiments demonstrating a strong scale-dependency on the mechanical deformation field have established the new field of nanoplasticity. Among such sophisticated experimental techniques, nanoindentation has been recognized as the most appropriate method of material testing to quantify a characteristic length of the scale dependency1 and also to predict an ideal shear strength of dislocation nucleation.2 Taking advantage of the controllable micro-Newton-level indent load and the nanometer-level displacement resolution, it can accurately measure the mechanical response of the extremely localized but nonuniform stress and strain fields. Relevant to the latter prediction, the distinctive finding of unstable displacement bursts of the indent depth observed in the curve of indent load and depth is commonly thought to be linked to the dislocation emission phenomenon.3,4 However, its detailed mechanism remained unclear because of the nonlinear collective behavior of dislocations. Furthermore, a physical understanding of the vicinity of the contact area indented by the rigid indenter is lacking due to the inhomogenity of the surface properties.5 Recent molecular dynamics studies6,7 suggest that the existence of a surface step makes the load needed to nucleate the dislocations decrease significantly. To assist in the comprehension of the contact physics of nanoindentation, in this paper we describe an experiment showing the effect of the surface roughness on the
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 1, Jan 2004
http://journals.cambridge.org
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displacement bursts observed in single-crystalline aluminum. Samples with several degrees of roughness were prepared by electropolishing. Then, their roughness was measured using atomic force microscopy (AFM). II. SURFACE ROUGHNESS
A single-crystalline aluminum sample with a (001) crystallographic surface plane was provided for indentation tests conducted with a commercial NanoIndenter DCM-SA2 (MTS Corp.) using a Berkovich-type diamond indenter. The radius of curvature at the tip of indenter was roughly estimated as 50 nm in comparison with the isotropic linear elastic solution8 under assumption of the spherical punch. The surface of the 10 × 1
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