Role of silicon on the tribological performance of Al-based automotive alloys and the effect of used motor oil

Abstract

A wear test in a used motor oil sliding environment was performed on Al-based automotive alloys with Silicon doped in various levels. Where a pin-on-disc wear testing equipment was used at a normal pressure of 1.53 MPa and a sliding speed of 0.51 m/s, kept constant. For comparison of the wear performance, dry and fresh motor oil sliding environments were also considered. The results showed that as silicon content was increased in the alloys, the wear rate decreased up to the eutectic composition, followed by an increase for all the environments. It was mainly for higher levels of Si-rich intermetallic Mg2Si precipitates in the α-aluminum matrix and made the alloys’ strength superior, in addition to increased wear resistance. In the post eutectic composition, primary silicon particles which are coarse and polyhedral appeared weakening the matrix. The coefficient of friction also decreased because of the higher hardness and the Si particles' employment as solid lubricants. In a dry environment, the wear rate and friction coefficient were much greater for their direct contact but lower under motor oil due to the reduced roughness caused by the sealing effects of the contact surfaces. Conversely, in oil environment, the opposite phenomenon was observed where a higher coefficient of friction was added to the alloy because the oil formed a thin film working as a lubricant between the contact surfaces which controlled the wear properties. Used oil demonstrates some degree of higher wear rate along with friction coefficient due to heavy and harmful chemical compounds in it. Examined by optical microscopy and SEM analysis, worn surfaces have shown that Si added alloy improved wear resistance through mild and smooth abrasive grooves filled with oxides in dry sliding conditions. In case of oil sliding environment smooth surfaces are created by the resistance of the oil film to the direct contact between the surfaces.