High Temperature Deformation of AlN/CrN Multilayers using Nanoindentation
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High Temperature Deformation of AlN/CrN Multilayers using Nanoindentation F. Giuliani, A. Goruppa1, S.J. Lloyd, D. Teer1 and W.J. Clegg Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK. 1 Teer Coatings Ltd., West Stone House, Berry Hill Industrial Estate, Droitwich, Worcestershire, WR9 9AS, UK. ABSTRACT The aim of this paper is to investigate the deformation behaviour of an AlN/CrN multilayer with wavelengths varying from 6-400 nm, deposited on M42 tool steel and how this varies as the temperature is increased to 500 °C. These measurements have been correlated with changes in deformation patterns using cross-sectional transmission electron microscopy, where sections have been prepared directly through the indents. INTRODUCTION It is sometimes found that multilayered ceramic coatings can be harder than either of their component phases and this has been attributed to a variety of mechanisms such as changes in dislocation line energy due to differences in elastic modulus or lattice parameter between the phases [1,2,3]. However, there is a growing body of evidence to suggest that these enhancements are often relatively small and might be due to the structures of the individual layers being different in the multilayers to that on the monolith [4]. Here we examine the structures of a multilayered AlN/CrN coating with that obtained in monolithic coatings and compare the resulting deformation behaviour using nanoindentation at temperatures varying from room temperature and 500 oC. EXPERIMENTAL METHOD The samples were deposited by reactive sputter-deposition from Al and Cr targets in a closed field unbalanced magnetron reactor [5] on to an M42 tool steel substrates (20 mm × 25 mm), with a metal bonding layer approximately 350 nm thick. For the multilayers and the CrN monoliths the metal was Cr while for the AlN monolith it was Al. On top of this a layer 350 nm in thickness was deposited in which there was a gradual transition between metal and metal nitride before the multilayer began. Measurements of the hardness and elastic modulus of the coatings were made using loads ranging from 2-100 mN (Nanotest 600, Micro Materials, Wrexham, UK) with a Berkovich diamond indenter. The analysis following Oliver and Pharr [6] was used. For measurements at elevated temperature the specimen was mounted on a heated stage, allowing indents to be made at temperatures up to 500 °C. The indenter was heated by holding it against the sample surface for 2 minutes. Indents were made in a line and spaced 20 µm apart to ensure their plastic zones did not overlap. To show the deformation around the indent, electron transparent cross-sections were prepared by focused ion beam milling a ‘stair case’ either side of the area of interest to produce a final thin area of approximately 100-200 nm thick and 20 µm square. This was then removed from the rest of the sample and placed on a fine mesh Cu grid [7] and examined in a microscope (Philips, CM30) operating at 300 kV.
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