A study of the interactive effects of hybrid current modes on the tribological properties of a PEO (plasma electrolytic oxidation) coated AM60B Mg-alloy

Abstract

The application of magnesium alloys in the automotive industry has been limited by their inferior corrosion and wear properties. PEO (plasma electrolytic oxidation) is an electrochemical process that uses a non-hazardous aqueous electrolyte to oxidize the metal surfaces to form oxide coatings which impart high corrosion and wear resistance. In this study we investigated the effect of current mode (unipolar, bipolar or hybrid (combination of both)) on the corrosion and wear properties of PEO coatings formed on an AM60B magnesium alloy (mass fraction: Al 5.6–6.4%, Mn 0.26–0.4%, Zn≤0.2%, balance Mg). Changing the current mode produces changes in the PEO process characteristics, including the breakdown voltage and discharge events, both in terms of discharge intensity and density. The morphology and microstructure of the coatings were investigated using scanning electron microscopy (SEM). PEO-coated materials tested using a pin-on-disk tribometer under dry sliding conditions show a higher coefficient of friction (COF) but a lower wear rate than the uncoated alloy. It was also found that the coatings formed using a bipolar or hybrid (unipolar+bipolar) current mode showed lower COF values and wear rates due to their modified coating microstructure and surface morphology. Inclined impact–sliding wear tests, which allow the simulation of a combination of impact fatigue failure and sliding wear failure under repetitive high local loads, also demonstrated the superior tribological properties of coatings produced using either a bipolar or a hybrid current mode.