Evaluation of micromechanical behaviour of plasma electrolytic oxidation (PEO) coatings on Ti–6Al–4V J.M. Wheeler a, ⁎, C.A. Collier a , J.M. Paillard a , J.A. Curran b a Department of Materials Science and Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK b Keronite International Ltd, Granta Park, Great Abington, Cambridge CB21 6GP, UK abstract article info Article history: Received 30 July 2009 Accepted in revised form 1 April 2010 Available online 9 April 2010 Keywords: Plasma electrolytic oxidation (PEO) Micro-arc oxidation (MAO) Titanium Nanoindentation Scratch Multiple impact indentation This paper investigates the micromechanical behaviour and wear properties of coatings on a Ti–6Al–4V alloy generated using the plasma electrolytic oxidation (PEO) technique. Four different compositions of electrolyte were used: aluminate, phosphate, silicate, and mixed phosphate and silicate. The coatings' composition was characterised using X-ray diffraction and energy dispersive spectroscopy, and their morphologies were examined using SEM and optical interference profilometry. Following this, the micromechanical properties of the different coatings and the substrate alloy were examined using nanoindentation, nanoindentation scratch, nanoindentation impact, and modified grit blasting equipment. Correlations between these mechanical performance measures and observed structures are discussed. The aluminate-based coating, which contained a hard Al 2 TiO 5 phase, was found to outperform other candidate systems and gave a performance enhancement over the bare substrate. However, it appears to be prone to delamination. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Plasma electrolytic oxidation (PEO) involves the creation of relatively thick, oxide-based surface layers by oxidation of the substrate and/or deposition from the electrolyte. The high electrical fields generated across the growing oxide layer cause repeated local dielectric breakdown and plasma discharges which modify the structure of the layer. The coatings were developed primarily for wear-protection of aluminium [1–11]. Since it normally involves at least some conversion (oxidation) of the substrate, the interfacial adhesion tends to be superior to that of most deposited coatings [2,4]. The plasma discharges can result in high temperature conversion of the growing coating into crystalline phases such as corundum [12]. These phases confer higher hardness on the coating than the amorphous oxides grown during conventional anodising. PEO coat- ings also contain significant levels of surface-connected, fine-scale porosity [13,14] and, partly as a consequence of this, have a relatively low global stiffness [14–16] making them strain-tolerant. The combination of good interfacial adhesion, high hardness, surface-connected porosity (giving good lubricant retention) and high compliance confers excellent tribological performance on PEO coatings in many modes of wear. In general, the wear performance is inferior under erosive or impact loading, particularly at normal incidence [11]. Of course, this is expected with ceramic coatings, which tend to fracture under these conditions, whereas a metallic coating or substrate tends to undergo plastic deformation. The shape, size and velocity of the erodent particles are often relevant, with large, high-speed, angular particles, incident at glancing angles, normally favouring excavation of metallic material. While PEO coatings on aluminium and magnesium alloys are now in a relatively mature state, their development for use on titanium is still in its infancy. There has been some preliminary work in the area [17,18], but there are often problems of brittleness and relatively high levels of coarse porosity, possibly associated with gas evolution. There is nevertheless considerable interest in their development, particu- larly in the context of biomedical applications [19–23], and there have also been some reports concerning their microstructure [24] and their resistance to corrosion and wear [25–28]. Currently, they are not expected to enhance the wear resistance of the substrate, which is, of course, generally expected to be superior to that of Al and Mg alloys in any event. The current work examines four different types of PEO coatings on a commercial titanium alloy (Ti–6Al–4V) with respect to their morphol- ogy, composition, and the consequent micromechanical properties, including impact, scratch wear, and erosion behaviour. 2. Experimental procedures 2.1. Specimen production Coatings were produced on Ti–6Al–4V alloy, using a 10 kW, 50 Hz AC Keronite™ commercial PEO processing unit. Prior to coating, substrates of dimensions 50 × 30 × 1 mm were ground with 180 grit SiC paper and ultrasonically cleaned in acetone, followed by water. Surface & Coatings Technology 204 (2010) 3399–3409 ⁎ Corresponding author. E-mail address: jw476@cam.ac.uk (J.M. Wheeler). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.006 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat