Tribological behaviour of oxide/graphite composite coatings deposited using electrolytic plasma process

The research was intended to deposit a low-friction coating on a cast Al–Si alloy substrate using an electrolytic plasma process (EPP) and then investigate tribological behaviour of the coating. The coating was designed to have a bi-layered structure consisting of an oxide underlayer and oxide/graphite lubricant composite top layer. A symmetrical AC power supply, operating at a relatively low voltage (less than 250 V), was used in the EPP treatment. Tribological properties of coatings with different thicknesses were tested using a pin-on-disc tribometer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize microstructures and morphologies of the coatings. It was found that an AC power could be successfully used to produce oxide and oxide/graphite composite coatings on aluminium alloys. Both thick (50 μm) oxide and oxide/graphite composite coatings had high wear. However, the oxide/graphite composite coating was superior to the single-layered oxide coating because it had a low coefficient of friction (0.22) and exhibited good compatibility with the steel counterface during dry tribological testing.