TRIBOLOGICAL PROPERTIES OF HARD ANODIZED EN AW-4006 ALUMINUM ALLOY: A COMPARISON BETWEEN DRY AND LUBRICATED CONDITIONS

Over the last decades, the strong regulations about pollution and environmental impact have favoured the introduction of eco-friendly lubricants as a good alternative to the classic mineral oils. This work concerns the tribological characterization in lubricated conditions of an EN AW-4006 aluminum alloy superficially treated with three different hard anodizing processes. In particular, a traditional and two innovative treatments, in which the anodic oxides porosities were sealed with Ag+ ions according with the two different hydrothermal fixing methods, are considered. Olive, soybean, peanut and sunflower oils were selected as vegetable and eco-friendly lubricants to study the wear behaviour of the surface treatments. Considering that the tribological behaviour is related to the adhesion of the chains of polar fatty acids to the metal surface, these oils were selected for their different contents of oleic and linoleic acids. Wear tests were conducted in ball-on-disk configuration with a 2 Hz constant frequency oscillatory motion and a maximum linear speed of 0.1 m/s. A 100Cr6 steel ball was used as counterbody material. Loads equal to 5, 10 and 15 N were chosen to investigate their influence on the resulting coefficient of friction (COF). After the wear tests, a deep characterization of the wear tracks was performed by means of scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) in order to identify the wear mechanisms. Moreover, a 3-D optical non-contact profilometer was used to evaluate the transversal area of the wear scars and to calculate the wear rate (WR). The results suggest that that both COF and WR values are strongly related to the unsaturation number (UN) of the lubricant.