Initial and grow-up stages of material transfer on Arc-DLC coating in aluminum forming processes at high temperatures

The mechanisms of material transfer in aluminum forming processes at high temperatures have remained a contentious tribological issue. This study aims to investigate the transfer mechanisms in the initial and grow-up stages of 6082-T6 aluminum alloy against commercial Arc-DLC coating under lubricant-free forming conditions. The warm and hot upsetting sliding test (WHUST) was used as the tribological test. It was conducted with two sliding configurations: the full sliding test to study the evolution of aluminum transfer and the short sliding test with a 2-mm sliding distance to observe the initiation of aluminum transfer. Furthermore, the effect of different sliding speeds, 0.5 and 5.0 mm/s, and initial temperatures of the specimen, 300 °C–500 °C, on the phenomenon of aluminum transfer was examined. Experimental results found that the aluminum transfer on the Arc-DLC coating in the initial stage of all cases was mainly caused by mechanical plowing. However, in the grow-up stage, the aluminum transfer could be dominated by mechanical plowing and/or adhesive bonding, depending on contact conditions. The different transfer mechanisms caused variations in the coefficient of friction and surface characteristics on the friction track. It led to the skewness Ssk could be an indicator to differentiate the transfer mechanisms.