TRIBOLOGICAL PROPERTIES OF HARD ANODIZED EN AW-4006 ALLOY USING VEGETABLE OILS AS LUBRICANT: A STATISTICAL INVESTIGATION ON THE FACTORS OF INFLUENCE

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 aluminium alloy superficially treated with three different hard anodizing processes. The effect of two vegetable lubricants (olive and soybean oil) was evaluated. 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. The studied surface treatments were a traditional hard anodizing and two innovative treatments in which the anodic oxide porosities were sealed with Ag+ ions. Two hydrothermal fixing methods were also applied. The 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. Loads equal to 5, 10 and 15 N were studied to investigate their influence on the resulting coefficient of friction. The statistical analysis, performed according to the Design of Experiments methodology, allowed to determine the most influencing factors on the resulting coefficient of friction. Based on the results, the type of surface treatment and applied load were factors with the strongest influence on the coefficient of friction measured during the wear test. Besides, unsaturation number calculation enabled to correlate the chemical composition of the considered vegetable oils with wear behaviour.