Possible Artefacts in Measurement of Hardness and Elastic Modulus on Superhard Coatings and the Verification of the Corr
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POSSIBLE ARTEFACTS IN MEASUREMENT OF HARDNESS AND ELASTIC MODULUS ON SUPERHARD COATINGS AND THE VERIFICATION OF THE CORRECTNESS OF THE DATA S. Veprek*, S. Mukherjeea, P. Karvankova, H.-D. Männling, J. L. He, J. Xu, J. Prochazka Institute for Chemistry of Inorganic Materials, Technical University Munich, Lichtenbergstr. 4, D-85747 Garching b. Munich, Germany, A. S. Argon Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, A. S. Li, Q. F. Fang Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China, S. Z. Li Qingdao University of Chemical Technology, Qingdao 266042, China, M. H. Manghnani, S, Tkachev, P. Zinin School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA a On leave from Bhabha Atomic Research Center, Mumbai, India ABSTRACT Measurements of the hardness and Young’s modulus of superhard coatings (HV≥40 GPa) by means of automated load-depth-sensing indentation technique can be subject to a number of errors that are discussed and exemplified here. Only load-independent values of hardness for loads larger than 30-50 mN can be considered reliable when the technique of Doerner and Nix (linear extrapolation of the unloading curve) is used to determine the corrected indentation depth. The results are compared with values of Vickers hardness calculated from the contact area of the remaining plastic deformation which was measured by means of calibrated scanning electron microscope. The values of Young’s modulus obtained from the indentation are close to the zero-pressure shear modulus of the coatings as measured by means of Vibrating Reed and surface Brillouin scattering techniques. INTRODUCTION Plastic indentation hardness is defined as the pressure under the indenter that causes the material to flow [1,2]. Thus, for materials which show little or no strain hardening the hardness H = L/AC must be independent of the applied load L (AC is the contact area between the indenter and the material). In the modern automated load-depth-sensing instruments the hardness (or more precisely the corrected indentation depth hcorr) and Young’s modulus are evaluated from the slope of the unloading curve using either a linear extrapolation of the unloading curve [3] or a power law fit [4]. These and other researchers (e.g. [5,6]) verified that this technique yields values of hardness and Young’s modulus in agreement with the results obtained by conventional techniques. Oliver and Pharr emphasized that for very hard materials, which show a large elastic recovery upon unloading, the values obtained by the load-depth-sensing technique may differ from those obtained in the conventional way [7]. This is due to the fact that for ultrahard materials with elastic recovery of ≥ 90 % [9,10] the shape of the unloading curve is dominated by the elastic response of the material/diamond indenter pair. This can be also understood when considering the “univesal hardness” HU=Lmax/C⋅hmax2 (hmax is the maximum inden
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