Effects of graphite nano-particle additions on dry sliding behaviour of plasma-electrolytic-oxidation-treated EV31A magnesium alloy against steel in air

This work aims at assessing the PEO (Plasma Electrolytic Oxidation) treatment conditions which may lead to an improved tribological behaviour for the Rare Earth-containing Mg alloy EV31A alloy, which is mainly used in aerospace, military and automotive applications. In particular, we investigated the influence of graphite nanoparticles addition and PEO treatment time (30-90 s) on microstructure, friction/wear and corrosion of layers obtained in phosphate/silicate electrolyte. The addition of graphite was investigated because it may limit the typical high friction of PEO/steel contacts. Since no hazardous additives (such as potassium fluoride) were added to this electrolyte, graphite was investigated also as a possible structure densifier. Dry sliding tests were carried out with a block-on-ring tribometer against AISI 52100 steel. The results showed that the addition of graphite decreased friction and wear, due to the formation of a compact, protective, low-shear transfer layer, consisting of iron oxides (due to mild tribo-oxidation of the countermaterial) and amorphous carbon (from the PEO layer). An intermediate treatment time of 60 s, combined with the addition of graphite, led to the growth of PEO layers with the best combination of thickness, compactness, roughness and microhardness, thus leading to the best tribological behaviour.