Tribological properties of plasma electrolytic oxide coatings on magnesium alloys

Plasma electrolytic oxidation (PEO) is a promising and environmentally friendly surface modification process free of Cr+6 and heavy metals. The process forms a protective and well adhered ceramic coating on valve metals such as Mg, Al and Ti. In this study, AZ91D and AM60B Mg alloys commonly used in automotive and aerospace industries were surface treated using an industrial scale PEO system in a KOH-Al2O3 based alkaline electrolyte. PEO coatings of two different nominal thicknesses, i.e. 10 mu m (K10) and 25 mu m (K25), were obtained. The microstructure and phase composition of the PEO coatings were analysed using scanning electron microscopy and X-ray diffraction respectively. Scratch tests were carried out to determine the adhesion strength of the PEO coatings. The tribological behaviour of PEO coatings under dry sliding conditions against 100Cr6 steel ball was investigated using the ball-on-plate test configuration. Test results indicated that the PEO coatings significantly enhanced the wear resistance of Mg alloys with increasing coating thickness. For uncoated Mg plates, abrasive and adhesive wear mechanisms acted together during sliding motion, while no predominant wear mechanism was seen for PEO coated alloys.