Wear 271 (2011) 2673–2680 Contents lists available at ScienceDirect Wear jou rnal h om epage: www.elsevier.com/locate/wear Micro-tribology experiments on engineering coatings M.G. Gee ∗ , J.W. Nunn, A. Muniz-Piniella, L.P. Orkney National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK a r t i c l e i n f o Article history: Received 25 August 2010 Received in revised form 7 February 2011 Accepted 7 February 2011 Keywords: Mico-tribology Wear Friction Engineering coatings a b s t r a c t A new tribometer has been designed to carry out micro and nanoscale tribological experiments. The sys- tem has been designed for both use on the laboratory bench and in situ in a scanning electron microscope, although the preliminary experiments reported in this paper were carried out on the laboratory bench. Experiments have been carried out under a range of coatings to validate the testing system and exam- ine frictional performance and durability. The damage to the coatings that was caused by single and multiple pass experiments under a range of different loading conditions was evaluated using confo- cal microscopy, atomic force microscopy (AFM) topography measurement, and in some cases scanning electron microscopy (SEM) stereo reconstruction, as well as high resolution SEM. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction There has been considerable interest recently in the testing of micro- and nano-scale contacts. There are two drivers for this. The most obvious is to provide information on the performance of tri- bological contacts for the development of nano and micro devices. The second is to enable measurements to be made of the response of materials under conditions that are representative of typical asper- ity contacts in macroscale contacts. Responding to this need micro and nano tribology test systems have been developed recently [1,2]. Much of the recent work has been concerned with the use of atomic force microscopy in nano- tribology experiments with applied load up to about 100 mN. There are also a number of test systems that have been developed in recent years to cover the load range between AFM and macroscopic test systems [3,4]. These and similar test systems have resulted in a growing body of research that has been applied to the investigation of the micro- tribological behaviour of coatings and thin films, hard materials, and polymers [5–12]. An emphasis of this work has been to inves- tigate the friction at micro-scale contacts and its relationship to macroscale measurements, and also to look at the mechanisms of damage that take place at these small scale contacts. This paper describes a new tribometer for micro and nanoscale tribology experiments, and preliminary results from experiments carried out with it on a range of commercial coatings. ∗ Corresponding author. Tel.: +44 20 8943 6374. E-mail address: mark.gee@npl.co.uk (M.G. Gee). 2. Experimental 2.1. Design of test system The test system has been designed (Fig. 1) so that it can either be used as a bench top system or an SEM in situ system. Bench top experiments enable tests in air or other controlled atmospheres to be carried out. In situ experiments facilitate high resolution, near real time experiments where the build up of damage in samples can be explored with high resolution imaging and analysis carried out at frequent intervals during a multipass experiment without removal of the sample from the SEM. The requirement for SEM operation meant that a new design was needed to fulfil the size constraints to fit the system inside the SEM chamber whist still giving a relatively small working distance of 6 mm between the SEM polepiece and the sample surface. A key feature of the system is the flexure element that was designed [13] (Fig. 2): • to support the probe, • restrict the motion of the probe to the two required orthogonal directions, • generate the applied normal force, and • measure friction. The flexure element design was based on a 3 dimensional cage structure that prevented angled approach to the sample through the eight bar kinematic chain (bars c and d in Fig. 2a). The deflection of this flexure element is measured with capac- itance displacement probes, where deflection of this element normal to the sample surface (z direction in Fig. 2a) generates the 0043-1648/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.02.031