Preparation and tribological properties study of a novel self-lubricating Alumina-based composite coating

A self-lubricating composite coating Al₂O₃/C was designed and sprayed via atmospheric plasma spraying (APS), in order to endow the alumina coating with certain lubricating properties under room temperature conditions. The raw feeding powders of boehmite-alumina-graphite (AlO(OH)/Al₂O₃/C) with particle sizes of 30–50 µm were prepared through hydroxylation treatment, physical stirring, and spray granulation process. The tribological properties of the Al₂O₃/C composite coating were systematically studied using a CSM friction tester. The prepared Al₂O₃/C composite coating exhibited fewer defects, lower coefficient of friction (COF), and slighter adhesion wear degree compared to single-component Al₂O₃ coating. The graphite lubrication film was discovered on both the worn surface of the composite coatings and the friction pair after friction. The existence of the graphite lubrication film decreased the COF, and the reduction extent depended on the load, with a maximum reduction of 55 %. The composite coatings presented the best tribological properties when sliding against Si₃N₄ balls at a higher load (5 N) because of the diverse wear mechanisms and the influence of graphite lubrication film, resulting in a reduction in the COF and wear rate by approximately 48 % and 11 times respectively. In addition, part of γ-Al₂O₃ in the Al₂O₃/C composite coating transformed into α-Al₂O₃ after the friction test. But the Al₂O₃ coating remained phase stable, with no phase change occurring before and after the friction. The transformation from the γ-Al₂O₃ to the α-Al₂O₃ phase was positive as for the tribological performance. It could reduce the COF as well as improving the thermal conductivity and mechanical properties of the coating.