AFM observation of diamond indenters after oxidation at elevated temperatures J.M. Wheeler, R.A. Oliver, T.W. Clyne ⁎ Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK abstract article info Article history: Received 17 January 2010 Received in revised form 12 June 2010 Accepted 8 July 2010 Available online 24 July 2010 Keywords: Indentation Oxidation Diamond Berkovich and AFM Use of diamond indenter tips at elevated temperatures can cause oxidation and thermomechanical damage, leading to changes in their topography. A Berkovich diamond indenter has been exposed to 450 °C in air, followed by 750 °C and 900 °C in 1 bar of static, commercial purity argon (30–45 ppm O 2 ). The effects of oxidation on the geometry of the indenter were investigated using atomic force microscopy. A Berkovich and a 10 μm tip radius conospheroidal indenter were also examined, after being subjected to 5 years of intermittent use at elevated temperatures (≤400 °C). Significant changes in tip topography were observed, suggesting that commercial purity argon may be an unsuitable atmosphere for high temperature indentation testing. Finally, a mechanism of oxidative etching, which may have potential as a method of sharpening indenters, is also reported. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Nanoindentation testing of materials at elevated temperatures is an increasingly active field of research [1–8]. Berkovich diamond indenters are often used for testing non-ferrous materials at temperatures up to ~400 °C and sapphire indenters for ferrous alloys and other materials at temperatures up to ~ 750 °C. Oxidation (erosion) of diamond (indenters) represents a significant problem. Over the temperature range of interest, the oxygen partial pressure required to make oxidation thermodynamically unfavourable is well below the attainable range, even in UHV systems. The key issue is therefore the kinetics of oxidation, which naturally accelerate as the temperature is raised. Diamond indenters are thus not normally used above 400 °C in air, due to relatively rapid oxidation [9,10]. This is a significant limitation, since many materials used in high temperature applications, such as cermets for tool bits, are harder than sapphire. Direct oxidation of diamond produces CO or CO 2 gas, which, in flowing oxygen or air, gives a constant etch rate. Oxidation can also lead to formation of a carbon layer on the surface of the diamond, presumably due to reaction with chemisorbed oxygen, and this layer is then converted to CO and/or CO 2 gas. These three processes compete at different pressures and temperatures, such that some conditions favour the formation of a thick amorphous carbon layer and others the direct oxidation of a clean diamond surface [9]. This carbon layer formation differs from graphitisation of diamond, which occurs at much higher temperatures (e.g. N 1200 °C) via a phase transformation [11]. All of these processes act to reduce the surface area to volume ratio, blunting conical and pyramidal indenter tips. Elevated temperature micro-hardness testing is often carried out with inert gas or vacuum systems being used to minimise diamond tip oxidation [12]. However, the scale of interest during nanoindentation testing is often appreciably finer, such that the exact topography of specimen and indenter need to be controlled and monitored much more precisely than for micro-indentation. Some dependence has been reported [9] of the oxidation rate on the crystallographic orientation of the exposed face at different oxygen partial pressures — see Fig. 1. In general, however, there is relatively little information available about the oxidative erosion of (natural) diamond in the form of relatively large single crystals. This is in contrast to the situation for various artificial layers and coatings of diamond and diamond-like carbon, which have been quite extensively studied [13,14]. However, the behaviour of such systems, which often have very fine grain sizes and a range of compositions and bond types, is not expected to be close to that of pure, single crystal diamond, of the type used as indenters. The surface topology, specifically the projected area function, of indenter tips is a critical concern for reliable nanoindentation testing [15]. Accurate determination of the area function of an indenter from load-displacement data can be a complicated procedure [16], and these methods for determining area functions yield little morphological data about the tip. Direct observation of tips is necessary for reliable observation of changes in tip morphology and dimensions, which can significantly affect indentation measurements. In the present investi- gation, atomic force microscopy is used to explore the effects of high temperature oxidation on Berkovich diamond indenters. Diamond & Related Materials 19 (2010) 1348–1353 ⁎ Corresponding author. E-mail address: twc10@cam.ac.uk (T.W. Clyne). URL: http://www.msm.cam.ac.uk/department/profiles/clyne.php (T.W. Clyne). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.07.004 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond