A Simple Method of Tip Shape Calibration for Nano-Indentation Test of Thin Films

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A Simple Method of Tip Shape Calibration for Nano-Indentation Test of Thin Films Koichiro Hattori, Junhua Xu, Hidetoshi Nakano and Isao Kojima National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Umezono1-1-1, Tsukuba, Ibaraki 305-8563, Japan. ABSTRACT We have evaluated the hardness and elastic properties of thin films by using a simple procedure to calibrate the tip shape effect of the nano-indentation data. For the simplification, a truncated-shape approximation and linear fit are used to estimate the tip-shape and contact stiffness, respectively, substituting for polynomial area-function and power-law fit. The parameters used in the correction were determined by a fused silica and a single crystal silicon (100) surface. Different film/substrate systems are designed in order to assess these fitted parameters used in the correction. The transition behavior observed from the film to the substrate is well coincide with the other film thickness results, where the indentation depth above 50nm. INTRODUCTION Instrumented indentation techniques in nano-range have been interested in recent years both in quality control and in research with product development for characterizing the mechanical behavior of thin film coatings and also small volume of the materials. There are many tests, which are developed and commercially used, however, the values obtained by these machines have not good agreement with different machines. ISO/WD 14577-4[1] has been presented to standardize the use of instrumented indentation test for measuring the mechanical properties of coatings. The main difficulties of the instrumented indentation tests in nano-range is how to determine the each indenter tip-geometry as a contact area as a function of penetration depth and also to estimate the deformation of the machine frame. Generally the method developed by Oliver and Pharr [2] is used to determine the parameters. It is difficult to calibrate the wide indentation area by using single function if it can be express the roundness of the tip. Thus we use truncated area function to calibrate the tip-shape. It has a limitation at the shallow indentation depth, however, the limitation will be easily estimated by its behavior. In this report, we corrected the actual indenter shape by means of the truncate-shape area function, and applied it to measure the different thin film coating systems. THEORETICAL BACKGROUND We used the basic analytical relations to determine the contact areas from the measured loads and displacements of the indenter. It is assumed that the load-displacement relation during unloading is ideally elastic and is well described by elastic contact theory. The compliance of the contact between axisymmetric indenter and elastic solid is given by [2] dh dP

= Cm = C f + Cs = C f + h max

π 1 E r 2 A(hc )

(1)

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where measured compliance Cm is the inverse of the measured stiffness and the subscripts i and s denote the indenter and the sample, respectively. Here Er is the reduc