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The influence of normal impact and sliding on wear and surface damage at the lubricated interface between the top piston ring and the piston was investigated using a specially adapted tribometer. Standard production components were tested, the piston rings being nitrocarburized steel with a production lapped finish and the pistons a hypereutectic aluminium-silicon alloy, either tin-plated or hard anodized. Experiments were undertaken at loads, velocities and temperatures comparable with a modern automotive, four-stroke, gasoline engine, either dry or lubricated with a fully formulated SAE 10W30 engine oil. Lubricant starvation was found to be the most critical operational parameter leading to surface damage and wear. An important phenomenon observed was the tendency of piston material to transfer and adhere to the ring surface, so-called ‘microwelding’. Anodizing was shown to be a very effective treatment to resist microwelding, although it is regarded as an expensive solution by the automotive industry. The experimental observations are consistent with fatigue wear of the piston groove, followed by material transfer to the piston ring flank and accelerated wear during sliding.

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