Wear 273 (2011) 9–16 Contents lists available at ScienceDirect Wear jou rnal h om epage: www.elsevier.com/locate/wear Wear and frictional mechanisms of copper-based bearing alloys S. Equey ∗ , A. Houriet, S. Mischler Ecole Polytechnique Fédérale de Lausanne (EPFL), Tribology and Interfacial Chemistry Group, Station 12, CH-1015 Lausanne, Switzerland a r t i c l e i n f o Article history: Received 13 October 2010 Received in revised form 29 March 2011 Accepted 30 March 2011 Available online 7 April 2011 Keywords: Leaded bronze Wear mechanisms Third body Microstructure Roughness a b s t r a c t The present study investigated the role of alloy microstructure and surface roughness on wear and fric- tion behavior of leaded and unleaded tin bronzes. Ball-on-disk experiments were carried out under dry conditions with steel balls sliding against bronze disks. Scanning electron microscopy (SEM), energy dispersive electron microscopy (EDX), x-ray photoelectron spectroscopy (XPS) and Auger electron spec- troscopy were performed to analyze the sliding tracks and the ball scars. The wear reducing effect of lead was associated to the formation of an oxidized lead rich layer. When no such film formed on the leaded bronze wear was more severe than in case of unleaded bronze. Nevertheless, the presence of lead in the alloy was a necessary but not sufficient condition to obtain a smeared layer and thus a lubricating effect of the leaded alloy. Two mechanistic hypotheses were formulated to explain the formation of the smeared layer. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Leaded bronzes have been widely used as bulk material or as coating of steel pieces for applications as bearings, shafts or hydraulic pumps. Lead acts as an efficient solid lubricant in systems functioning under boundary or dry conditions. For environmen- tal and legal reasons, efforts are being made to remove lead from bearing alloys. Numbers of materials are already proposed as alter- native to leaded alloys, in particular metallic composites containing self-lubricant particles like graphite, MoS 2 or PTFE [1–4]. Increas- ing the wear performances of alternative materials to leaded bronze requires a scientific understanding of the relevant mechanisms, and in particular of the relationship between materials microstructure and tribological behavior. However the present understanding of the mechanisms by which lead improves the tribological properties of alloys is limited. Lead-base alloys, like babbit alloy, are used as soft coating for bearings. Upadhyaya et al. [5] compared the tribological properties of lead-base babbit alloys produced by casting and thermal spraying in relation with their microstructure. Due to the low melting point, lead was found smearing on the counterpart at an early stage of sliding process. This thin lubricating film efficiently protected the sliding parts against wear. Samples produced by thermal spraying showed a lower wear due to a smaller spacing between intermetal- lic particles. Dispersed lead particles are also used as solid lubricant in aluminum-based or copper-based alloys. Mohan et al. showed ∗ Corresponding author. E-mail address: sebastien.equey@epfl.ch (S. Equey). the effect of lead content in a stir-cast Al-Pb alloy [6]. An increase of the lead content led to a decrease of wear of up to 20 wt % Pb. Higher Pb concentration increased the wear due to easier crack propagation and to the removal of thick layer of lead. The effect of lead content on Cu-Pb alloys was studied by Pathak et al. [7]. Increasing the lead concentration up to 40 wt % led to a decrease in wear. The coefficient of friction also decreased up to 35 wt% Pb before increasing at higher lead content. Pandey and Prasad studied the effect of applied pressure and sliding velocity on zinc-based and copper-based alloys [8]. The highest wear rate of the copper-based alloy was attributed to its microcracking tendency. The cracks were mostly present at low sliding speed and did not form at higher velocity due to the higher frictional heating. Concerning leaded bronzes, the crucial role of third bodies in dry sliding has been previously observed with similar tribologi- cal conditions as in this study [9]. At low load, only small metal oxide particles were generated. Under higher contact pressure, lead-enriched larger flat debris were built up, decreasing the wear coefficient and the friction. Microstructure played an important role in the formation of the antifriction flat debris. Fewer but larger lead inclusions allowed building up the beneficial flakes under lower contact pressure. In this study, the role of the alloy microstructure and of the surface roughness on dry wear and friction of leaded bronze is investigated with the aim to better identify critical mecha- nisms. A CuSn10Pb10, with two dendritic microstructures and a lead-free bronze, CuSn8, were studied. The sliding tracks and the ball scars were analyzed with scanning electron microscopy (SEM), energy dispersive electron microscopy (EDX), x-ray pho- 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.03.030