Resistive-nanoindentation: contact area monitoring by real-time electrical contact resistance measurement
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Research Letter
Resistive-nanoindentation: contact area monitoring by real-time electrical contact resistance measurement Solène Comby-Dassonneville, Fabien Volpi, and Guillaume Parry, Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMaP, 38000 Grenoble, France Didier Pellerin, Scientec/CSInstruments, 91940 Les Ulis, France Marc Verdier, Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMaP, 38000 Grenoble, France Address all correspondence to Fabien Volpi at [email protected] (Received 20 February 2019; accepted 28 May 2019)
Abstract In the past decades, efforts have been made to couple nanoindentation with resistive measurements in order to monitor the real-time contact area, as an alternative to the use of traditional analytical models. In this work, a novel and efficient stand-alone method is proposed to compute the contact area using resistive-nanoindentation of noble metals (bulk or thin films). This method relies on three steps: tip shape measurement, set-up calibration, application to the sample to be characterized. The procedure is applied to nanoindentation tests on a sample with film-on-elastic-substrate rheology and is successfully validated against experimental measurements of the contact area.
Introduction In the past three decades, efforts have been made to couple instrumented indentation with resistive measurements.[1] This development was driven by several motivations, such as the local monitoring of phase transformation,[2–8] the study of native oxide fracture,[9–11] the investigation of MEMS operation at small scales,[12,13] and the contact area computation during nanoindentation test.[14,15] The later point is of particular interest for the determination of the contact area Ac which is essential to compute both the sample Young’s modulus and the hardness from nanoindentation tests: ➢ The sample Young’s modulus is determined using the Sneddon’s relation[16]: 2 Sc = √ E ∗ Ac (1) p with Sc the contact stiffness (measured continuously during nanoindentation test) and E* the reduced modulus, expressed as: −1 1 − v2tip 1 − v2sample ∗ E = + (2) Etip Esample with vtip, vsample, Etip, Esample the tip and sample Poisson’s ratios and moduli, respectively. ➢ The sample hardness H is computed using Eq. (3), with L the load on the sample. H=
L Ac
(3)
The contact area Ac (defined as the projected area of the contact interface between the tip and the sample) is however complex to monitor, even for the simplest case of homogeneous semi-infinite specimens. This is because the contact area depends both on the tip geometry and on the contact depth hc [hc being the length of the tip effectively in contact with the sample (Supplementary Fig. S1)]. The tip geometry is either determined by AFM imaging or by using a calibration sample with well-known mechanical properties.[17] As shown in Supplementary Fig. S1, the contact depth hc depends on sample rheology: the sample can either sink-in or pile-up around the tip during nanoindentation. The contact depth hc then strongly differs from the total penetrati
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