Effect of bio-fuel (E85) addition on lubricated sliding wear mechanisms of a eutectic Al–Si alloy

Abstract

The effect of addition of bio-fuel blend consisting of 85% ethanol and 15% gasoline by volume, E85, on lubricated sliding wear of a eutectic Al–12.6% Si alloy was investigated. The alloy was tested under the loading conditions that promoted ultra-mild wear (UMW), using a mixture composed of equal proportions of E85 fuel and synthetic engine oil, designated as E85/oil (1:1) blend. UMW occurred in three stages when the E85/oil (1:1) blend was used: initially wear was limited to the Si particles that subsequently fractured and/or sunk into the Al matrix; this was followed by a running-in period of higher wear of Al matrix, and then a low steady state wear stage was reached, due to the formation of a protective tribolayer on the sliding surface—called an oil-residue layer (ORL). The ORL was supported by embedded Si particles and a subsurface microstructure consisting of nanocrystalline Al grains that formed as a result of severe local plastic deformation during sliding. The composition of the ORL incorporated nanocrystalline regions of Al, Si, ZnS and ZnO surrounded by amorphous regions consisting of carbon and possibly phosphates. Compared to the ORL formed on samples tested using unmixed engine oil (without E85) the E85/oil (1:1) blend generated higher proportions of Zn, S and P compounds in the ORL. It was proposed that the hydroxyl groups in ethanol molecules facilitated ZDDP degradation, thus leading to an ORL that was richer in anti-wear compounds and consequently lower volumetric wear was observed when the E85/oil (1:1) blend was used.