Indentation Curve Analysis for Pile-up, Sink-in and Tip-Blunting Effects in Sharp Indentations
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Indentation Curve Analysis for Pile-up, Sink-in and Tip-Blunting Effects in Sharp Indentations Yeol Choi1,2, Baik-Woo Lee1, Ho-Seung Lee1 and Dongil Kwon1 1. School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea. 2. Research Center, Frontics, Inc., Seoul 151-060, Korea ABSTRACT Hardness and elastic modulus can be derived from instrumented sharp indentation curves by considering the effects of materials pile-up and sink-in and tip blunting. In particular, this study quantifies pile-up or sink-in effects in determining contact area based on indentation-curve analysis. Two approaches, finite-element simulation and theoretical modeling, were used to describe the detailed contact morphologies. The ratio of contact depth to maximum indentation depth was proposed as a key indentation parameter and was found to be a material constant independent of indentation load. In addition, this parameter can be determined strictly in terms of indentation-curve parameters, such as loading and unloading slopes at maximum depth and indentation energy ratio. This curve-analysis method was verified by finite-element simulations and nanoindentation experiments. INTRODUCTION The nanoindentation technique has been widely used to evaluate mechanical properties of small-volume materials including thin films, coating layers and individual material phases [1-3]. In particular, it can obviate the many practical difficulties arising in measuring a residual contact impression directly at nanometer scale. Instead, the equipment records load and depth continuously and all the results, which are mainly hardness and elastic modulus, are evaluated only by analysis of the indentation load-depth curve. Oliver and Pharr suggested that contact depth can be determined by subtracting the elastic deflection depth at the contact perimeter from the maximum indentation depth through analysis of the indentation unloading curve [4]. This method has been adopted widely in the instrumented indentation field and has also been extended by numerous refinements [5-7]. There are, however, some practical difficulties in this method for obtaining mechanical properties at small scales. First, it cannot take into account pile-up or sink-in around the contact impression, and it overestimates hardness and elastic modulus of ductile materials since it deals only with elastic deflection under a reference plane [8]. Second, even though the indenter tip is not perfectly sharp due to manufacturing limitations and wear, the contact area is determined
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(a) (b) Figure 1. Schematic diagrams of (a) indentation load-depth curve and (b) indentation contact morphology including large pile-up and tip blunting during maximum loading. from contact depth assuming a perfectly sharp indenter. Although area calibration has been used to consider the tip-blunting effect with polynomial area function [4], doing this iteratively has significant practical limitations. This study introduces a new indentation-curve analysis that considers both pi
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