Ultra-mild wear of a hypereutectic Al–18.5 wt.% Si alloy

Material removal rate from an automotive engine bore surface should not exceed a few nanometers per running hour. This corresponds to the ultra-mild wear (UMW) conditions. Understanding the role of the microstructure on the wear mechanisms in the UMW regime is essential for the development of lightweight automotive engines. In this work, sliding wear tests were performed on a hypereutectic Al–Si alloy containing 18.5 wt.% Si under a light load of 0.5 N and boundary lubricated conditions corresponding to the UMW regime using a ball (AISI 52100 steel)-on-disc tribometer. Sample surfaces were chemically etched to expose silicon particles. After sliding to 6 × 105 cycles, no measurable mass loss was detected using an analytical balance with an accuracy of 10−4 g. Wear damage was limited to the contact surfaces of the silicon particles. The calculated maximum contact pressure applied on the protruded silicon particles was less than the matrix hardness of the alloy, which was consistent with the experimental observations that large silicon particles in this alloy carried the applied load and prevented plastic deformation of the matrix. A eutectic Al–12% Si alloy with a softer aluminum matrix was tested under the same conditions and showed more extensive damage. In this alloy, silicon particles became embedded into the aluminum matrix, which in turn formed pile-ups near the particles and consequently were exposed to damage by the counterface. In 18.5% Si, neither silicon particle sinking-in nor damage to aluminum matrix in the form of plastic deformation and wear was observed. Accordingly, Al–18.5% Si was effective in maintaining a minimal surface damage under UMW conditions.