Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process

In the paper, a modified Plasma Electrolytic Oxidation process (PEO) to produce ceramic coatings on Al alloys, suitable for tribological applications, is discussed. The process utilises bipolar current pulses in the kHz frequency range, providing better control over plasma discharges occurring at the sample surface. The coatings, formed on a 2024 series Al alloy, are characterized by means of optical microscopy, SEM, EDX, XRD and surface profilometry. Microhardness and scratch adhesion tests are performed to evaluate the coating mechanical performance. The effects of current pulse frequency on both the layer growth kinetics and the process energy efficiency are discussed. It is found that in the 1 to 3 kHz frequency range, the layer growth rate can be increased from 0.5 to 1 to 1.6 to 3.2 μm min−1 and the volume fraction of the porous outer layer can be reduced from 25 to 20% to 15 to 10% of the total layer thickness, compared to the conventional 50 Hz AC PEO process. The inner layer, despite a slight increase in porosity, preserves a relatively high hardness of 1200 to 1500 HK25 and good adhesion (LC2=60 N), which should be sufficient for many tribological applications.