Friction characteristic of micro-arc oxidative Al2O3 coatings sliding against Si3N4 balls in various environments

The alumina ceramic coatings were prepared on 2024Al alloy by micro-arc oxidation (MAO) technique. The phase structure of the MAO Al2O3 coating was determined using X-ray diffraction. The thickness and micro-hardness of the MAO Al2O3 coatings was measured using eddy current thickness equipment and micro-hardness tester. The friction property of MAO Al2O3 coatings sliding against Si3N4 ceramic balls were investigated in air, water and oil by a ball-on-disk tribo-meter, and the worn surfaces of the MAO Al2O3 coatings were observed using scanning electron microscope (SEM). The results showed that the MAO Al2O3 coatings mainly contained α-Al2O3 and γ-Al2O3 phase. The micro-hardness of the polished MAO coatings was HV1740 ± 87. With an increase in normal load and sliding speed, the friction coefficient in air increased from 0.74 to 0.87, while decreased from 0.72 to 0.57 in water and 0.24 to 0.11 in oil. This indicates that the fluid lubrication could improve the friction behavior of the MAO Al2O3 coatings. The worn surfaces' observation indicated that the wear mechanism of the MAO Al2O3 coatings changed from abrasive wear in air to mix wear in water, and became microploughing wear in oil.