Micromechanisms and mechanics of ultra-mild wear in Al–Si alloys M. Chen a , X. Meng-Burany a , T.A. Perry b , A.T. Alpas a, * a NSERC/General Motors of Canada Industrial Research Chair, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, Ont., Canada N9B 3P4 b General Motors Research and Development Center, 30500 Mound Road, Warren, MI 48090-9055, USA Received 22 June 2008; received in revised form 23 July 2008; accepted 23 July 2008 Available online 1 September 2008 Abstract The micromechanisms that control the sliding wear behavior of Al–Si alloys tested under ultra-mild wear conditions were character- ized using optical profilometry and electron microscopy, and analyzed by means of a contact mechanics model. A series of damage events occurred to the chemically etched surfaces of an Al–25% Si alloy, subjected to boundary-lubricated contact during pin-on-disk tests, in the following sequence: the fracture and/or sinking-in of silicon particles into the aluminum matrix accompanied by the formation of aluminum pile-ups; aluminum wear (rapid wear period) and the development of an oil residue layer on the contact surface. Compared to an Al–11% Si with similar silicon morphology but lower hardness, the Al–25% Si delayed aluminum wear more effectively. Once an oil residue layer supported by a subsurface microstructure consisting of ultra-fine aluminum grains was formed, both alloys displayed decreasing wear rates. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Al–Si alloys; Ultra-mild wear; Wear mechanisms; Microstructure; Surface damage 1. Introduction Al–Si alloys are increasingly being utilized in the manu- facture of lightweight engine components in an effort to build fuel-efficient vehicles [1,2]. The use of a hypereutectic A390 Al–Si alloy (17 wt.% Si) in Chevrolet Vega engine block castings in the 1970s provided an early example of Al–Si automotive components operating in sliding motion [3]. The chemically etched surfaces of this alloy, with its large silicon particles supporting the contact stresses exerted by the piston rings, demonstrated suitable wear resistance for this application without the need for cast iron liners [3]. The casting of hypereutectic alloys, however, is challenging and the presence of a high percentage of pri- mary silicon particles in the microstructure adversely affects machinability [4–6]. Various manufacturing meth- ods, including squeeze casting, powder metallurgy and spray forming [7–9], have been employed to reduce the pri- mary silicon particle size and improve particle distribution. An Al–Si alloy with a composition of Al–25 wt.% Si, 4 wt.% Cu, 1 wt.% Mg was produced using a spray forming technique [10,11]. The high percentage of small, uniformly distributed silicon particles found in this alloy is considered to provide a favorable microstructure for wear-resistant cylinder liners [12]. A laboratory study performed on a spray-formed Al–20 wt.% Si determined that, in the load range considered, the dry sliding wear behavior of this alloy falls within the mild wear regime [13], with higher wear resistance compared to a conventionally cast alloy with a similar composition but with coarser silicon particle microstructure. Piston–cylinder bore assemblies normally run under lubricated conditions, where wear rates should not exceed a few nanometers per hour (or approximately 0.1 A ˚ per cycle [14]). The very small wear rates observed in the cylin- der bore define a new wear regime, which is different from higher rate wear regimes (mild and severe wear) not only in the rate of material removal but also the mechanisms of 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.07.043 * Corresponding author. Tel.: +1 519 253 3000x2602; fax: +1 519 973 7085. E-mail address: aalpas@uwindsor.ca (A.T. Alpas). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 5605–5616