Wear 268 (2010) 1080–1090 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Pressure and thermally induced stages of wear in dry sliding of a steel ball against an aluminium–silicon alloy flat Anirban Mahato a , Thomas A. Perry b , Vikram Jayaram c , S.K. Biswas a,∗ a Mechanical Engineering Department, Indian Institute of Science, Bangalore-560012, India b Materials and Processes Lab, General Motors R & D, Warren, USA c Materials Engineering Department, Indian Institute of Science, Bangalore-560012, India article info Article history: Received 22 May 2009 Received in revised form 21 December 2009 Accepted 7 January 2010 Available online 15 January 2010 Keywords: Sliding wear Non-ferrous metals Electron microscopy Internal combustion engines Nanoindentation abstract Wear of etched near-eutectic aluminium–silicon alloy slid against a steel ball under ambient is explored. The sliding velocity is kept low (0.01 m/s) and the nominal contact pressure is varied in a 15–40 MPa range. Four stages of wear are identified; ultra mild wear, mild wear, severe wear and post severe oxidative wear. The first transition is controlled by the protrusions of silicon particles, projecting out of the aluminium alloy matrix. Once these protrusions disappear under pressure and sliding, oxida- tion and bulk energy dissipation mechanisms take over to institute transitions to other stages of wear. The phenomenological characteristics of wear stages are explored using a variety of techniques includ- ing nanoindentation, focused ion beam milling, electron microscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS) and optical interferometry. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Lightweight aluminium–silicon (Al–Si) alloy is an attractive cylinder material in automobile engines because it imparts good power to weight ratio to the engine. Piston reversal near the top dead centre however causes starvation of lubrication which leads to wear. Previous works [1–6] have reported wear rate as functions of alloy composition, normal pressure, sliding speed, sliding distance and environmental conditions. Based on the wear rate criterion, the wear of aluminium–silicon alloys may be divided into three distinct regimes – ultra mild wear (UMW), mild wear (MW) and severe wear (SW). Proper alloy design can delay the transition from the less detrimental UMW or MW to the more detrimental MW or SW regime, respectively. The wear resistance of the alloy has been improved by adding alloying elements [1,3,4,7,8], manufacturing the alloy by different routes [9–13], adding hard ceramic materi- als in the alloy [14–17] and performing heat treatment [14,18–20]. These strategies change the morphology and distribution of the -aluminium dendrites and the second phase silicon particles in the alloy matrix. The resulting improvement in toughness, ultimate tensile strength (UTS) and ductility, aid to increase the wear resis- tance. Previous work [21] has shown that an optimum content of ∗ Corresponding author. Tel.: +91 80 22932589; fax: +91 80 23600648. E-mail address: skbis@mecheng.iisc.ernet.in (S.K. Biswas). iron in the aluminium–silicon alloy together with strontium pro- duce refined iron-rich intermetallics in the matrix which increase the wear resistance. It was also found that the exposed height of silicon particles on aluminium surface gives better wear resistance than the as cast aluminium–silicon alloy [22]. The beneficial effect of the exposed silicon was explained by one of the present authors [23] using the Greenwood and Tripp model. Ultra mild wear regimes in lubricated sliding have shown sinking-in of silicon, pile up of aluminium matrix around the sil- icon particle and somewhat attenuated flow of matrix [24,25]. In the ultra mild wear (UMW) regime [26] the load is initially sup- ported by the exposed silicon particles whereas in the latter stages a stable smooth layer made up of oil residue and ultrafine alu- minium grains bear the load. The wear rate in the UMW regime is a few nanometers per hour [27]. A study [28] on real engine blocks using the radionuclide-technique revealed that in the ini- tial stage of wear silicon particles bear the load, this leads to local wear of those regions. In the latter stage, silicon particles together with other wear particles “sinter” [28] into the matrix and make a protective tribolayer. In the mild wear regime silicon particles in the subsurface are fragmented due to plastic flow [29,30] of the matrix. In addition a hard amorphous tribolayer [31] or mechanically mixed layer (MML) [1–6,32,33] forms on the surface and protects the surface from wear. Tribolayers/MMLs consist of Al, Si, Fe and oxygen [33]. It is found that low Fe containing alloy gives a homogenous, thin, 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.01.008