Preparation of self-lubricating composite coatings through a micro-arc plasma oxidation with graphite in electrolyte solution

• Graphite particles were added to the MAO electrolyte.
• Carbon contained compositions were formed in the MAO ceramic layer.
• The friction and wear resistance were improved significantly.

In this study, low friction and wear resistant composite coatings were produced through the micro-arc oxidation (MAO) on 6061 aluminum alloy with graphite particles in alkali sodium silicate electrolyte solution. The ball-on-disk wear tests were performed in humid environment to investigate the friction and wear properties. The Al2O3, Al2OC, Al4C3, Al4O4C, and graphite phases were observed in a grown layer with graphite additives in electrolyte solution. The volcano-like microstructures were decreased in population and widened significantly in the sample prepared with the additive of graphite particles. For the sample prepared without graphite additives, the coating failure occurred after 1200 cycles, while the additive of graphite maintained the average of friction coefficient around 0.1, and reached a maximum of 0.12 after 15 000 cycles of wear test. The graphite and carbon containing lubricating phases trapped in the alumina matrix or crevices acted as micro/nano-reservoirs to maintain low friction and favor high endurance of the coating.