The Calibration of the Nanoindenter
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THE CALIBRATION OF THE NANOINDENTER N J MCCORMICK, M G GEE and D J HALL National Physical Laboratory, Queens Road, Teddington, Middlesex, TWII OLW, UK INTRODUCTION The nanoindenter has a tremendous potential for the measurement of mechanical properties of thin films and coatings. Information can be obtained about plastic, elastic and time dependent deformation behaviour1 ' 2. In some cases fracture behaviour of the coatings and the adhesion of the coating to the substrate can also be investigated 3 . Before accurate and reproducible measurements can be made the nanoindenter needs to be suitably calibrated. The magnitude of the uncertainties associated with calibration and their relevance to measurements are also required. Additionally it is important that a thorough understanding of the effect of thermal drift on the measurement process is developed. Traceability of calibration is also an important consideration. The primary areas where calibration is essential are displacement, load and shape of the indenter. DISPLACEMENT CALIBRATION The nanoindenter uses a parallel plate capacitive transducer to measure displacement and has a quoted resolution of 0.03nm (Figure 1), although the accuracy for the system was not given. In particular the technique that was used for calibrating the displacement system used a Newton's ring interferometer that used light from a Sodium lamp with a wavelength of 589 nm so that the calibration steps were of the order of 295 nm. This method seemed an inadequate way to calibrate a displacement transducer with sub-nanometre resolution, and so the NPL unpolarized nanometric interferometer, Figure 2, with nanometre resolution was used to improve the calibration. The calibration was carried out by mounting a 10mm diameter silvered glass mirror on the indenter l .E Permatent magnet shaft and placing the interferometer below this as shown in Figure 2. By using Indenter shaft a modified version of the Suspension springs supplied system software it was possible to move the indenter shaft a small distance and then take a reading of the new position "`*- "Displacement gauge from the interferometer after waiting 5 seconds for the displacement of the indenter shaft to stabilize. This process was repeated Diarnond indertor 20 times over the displacement range of t 13 micrometres corresponding Figure 1, Schematic diagram of indenting head of nanoindenter to ± 1 volt applied to the loading coil. The
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Mat. Res. Soc. Symp. Proc. Vol. 308. 01993 Materials Research Society
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Fixed Retro-reflector
splitter
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Moving Reflector on Indenter Shaft (shown as retro-reflector
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for clarity) I'
Photodetectors
"Reflector Figure 2, NPL unpolarised nanometric interferometer temperature and barometric pressure were logged during the calibration exercise but the maximum error that could occur from changes in the refractive index of air was less than 0.2%. Four runs were carried out moving the indenter in both directions and the slope and correlation coefficient for displacement measured by the na
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