Tribological properties of MAO coatings on aluminum alloy AL25

The tribological properties of the oxide coatings were studied on the aluminum alloy AL25. The coatings were formed by microarc oxidation (MAO) in an alkaline-silicate electrolyte under cathodic-anodic conditions. The structure of the working layer of MAO coatings, their phase composition, and hardness were studied. The coatings were tested in friction pairs with cast iron SCh20 and alloy AL25 under oil lubrication. The friction coefficient was determined on the SMC-2 friction machine according to the ‘disc-pad’ scheme, the load varied from 0.2 to 2 kN, the nominal pressure − from 1 to 11 MPa. It was found that the MAO coating has a crystal structure with a phase composition including mullite (3Al2O3∙2SіO2) and high-temperature modifications of aluminum oxide (α-Al2O3 and γ-Al2O3); hardness ~ 10-12 GPa, and total porosity of 10-15 %. The friction coefficient of the pairs ‘grey cast iron - MAO coating’ and ‘MAO coating - MAO coating’ reaches 0.04 and 0.007 when lubricated with oil. The friction coefficient in the pair ‘MAO coating - MAO coating’ is characterized by the absence of tribochemical interaction of the contacting surfaces. The low sliding friction coefficient and high mechanical properties of the coatings make it possible to use MАO technology to increase the service life of tribotechnical parts (pistons, shafts, plain bearings, etc.).