Role of counterfaces with DLC and N-based coatings on frictional behaviour of AZ31 magnesium alloy subjected to plasma electrolytic oxidation (PEO) process

This work examines the role of counterface materials on the tribological behaviour of Mg alloys subjected to plasma electrolytic oxidation (PEO) process and focuses on reducing the sliding coefficient of friction (COF) values of PEO treated surfaces. Surfaces of AZ31 grade Mg alloy were subjected to a PEO treatment in a sodium metasilicate and potassium hydroxide containing electrolyte. Unlubricated sliding wear tests were conducted using counterfaces made of nitride based (N-based), TiN, TiCN, CrN and hydrogenated diamond-like carbon (H-DLC) coated as well as uncoated SAE 52100 grade bearing steel balls. MgO based coatings formed on the surfaces of samples during PEO process increased the wear resistance of AZ31, regardless of the type of counterface material used. Sliding the PEO treated surfaces against counterfaces made of uncoated steel and N-based coatings yielded high COF values of 0.6–0.8 that exceeded those of the uncoated AZ31 against the same counterfaces. During dry sliding of H-DLC counterface on the MgO coating, smooth and stable friction curves with a low steady state COF value of 0.13, were recorded. Therefore, a major drawback of PEO treatment that gives rise to surfaces with high COF could be addressed by running them against H-DLC coated counterfaces, a method that could be applied to development lightweight tribological components that are both sliding wear resistant and have low COF.