Effect of sliding environment on dry sliding wear of as-cast eutectic Al–Si

Abstract

Wear resistance has been found to improve for Al–Si alloyswith increasing silicon content up to the eutectic compo-sition (12.6 at.% Si), after which it deteriorates [1–3].Wear in Al–Si alloys has been characterized into mild andsevere wear regimes. Mild wear occurs under conditions oflow normal loads, and an oxide layer that forms on thewear surface usually slows down the wear process. Severewear occurs under high normal loads and is often charac-terized by inclusions of the worn counterface, along withoxides, in the mechanically mixed surface tribolayers [4,5]. The results of Razavizadeh and Eyre [6] suggest thatthere is generally insufficient frictional heating on the wearsurface for general oxide formation. Instead, a mechanismof oxidation of asperities followed by fracture and com-paction of these oxidized asperities into the valleys on themetal wear surface has been suggested [7, 8]. During mildwear of Al–Si alloys, a smooth surface is generallyobserved with interspaced wear scar areas [1]. The pres-ence of material from the worn counterface embedded inthe wear pin has been observed on both smooth and scarredareas [7, 9, 10].Lower wear rates may be observed when harder coun-terfaces, such as diamond-like carbon (DLC) or yttria-stabilized zirconia, are used instead of the hardened steelsurfaces traditionally employed in wear tests, particularly ifthe hard counterface has a smooth surface [11]. Also, argontest environments have shown reduced wear rates ascompared to tests performed in air, with the wear ratesunder argon being as much as ten times lower in somecases [11, 12]. The increase in wear rates in atmosphereswith higher oxygen content has been attributed to thepresence of abrasive wear debris on the surface [12].In a recent paper [12], we studied the wear of as-casteutectic Al–Si against a yttria-stabilized zirconia counter-face using pin-on-disk tribotests in two different environ-ments, air and dry argon. The wear rate of the pins tested inair was more than twice that of those tested under argon.Similarly, the zirconia counterface showed much morewear for tests conducted in air. There was substantialsubsurface cracking in the pins during wear testing in air,but none was observed in pins tested under argon. Scanningtransmission electron microscope (STEM) examination ofthe near-surface region of the pins that had been worn in airshowed mechanically mixed regions with considerableamounts of both aluminum oxide and zirconium oxide—the aluminum oxide particles had evidently acted as abra-sive particles to remove material from the zirconia coun-terface. In contrast, the pins tested in argon showed littlezirconium oxide in the near-surface regions.In that study [12], it was unclear whether the moisturepresent in air made a significant contribution to the wearrate. Thus, the objective of the present study was to clarifythe influence of moisture in the wear environment and itseffects on the proposed wear mechanism of crack forma-tion and debris accumulation on wear surfaces. To that end,pin-on-disc wear tests were performed in a flowing dryoxygen environment using the same materials and condi-tions as in our previous study, and the resulting wear sur-faces were characterized in the same fashion [12]. Theresults, described below, are compared with those fromprevious tests conducted in air and in dry argon. It ispossible that the size of the Si particles may affect the wear