Effects of nanocrystallized layer on the tribological properties of micro-arc oxidation coatings on 2618 aluminum alloy under high temperatures

A composite oxidized layer formed on a nanocrystallized 2618 aluminum alloy surface was prepared by a two-step method—the micro-arc oxidation (MAO) process and a pretreatment of the ultrasonic cold forging technology (UCFT). The tribological properties of samples were investigated under 25, 80, 160, and 200 °C by an MST-400 high temperature tribometer. The wear performance of MAO + UCFT aluminum alloy drilling rods was evaluated in the field drilling experiment. The morphologies and microstructures were measured by the scanning electron microcopy and optical microscope. The results show that UCFT treatment promotes the formation of α­Al2O3 phase, reduces the number of micro pores, and improves the hardness of MAO coatings. Both friction coefficients and wear rates of untreated and UCFT samples increase with the rise of temperature, but those of the UCFT samples are lower at the same temperature. The friction coefficients and wear rates of MAO and MAO + UCFT samples decrease with the rise of temperature, but those of the MAO + UCFT samples are lower at the same temperature. The main wear mechanism of untreated samples is severe adhesive wear, plastic deformation, and slight abrasive wear. Whereas, that of UCFT samples is some degree of adhesive wear, abrasive wear, and plastic deformation. Additionally, oxidative wear occurred on the untreated and UCFT samples. The main wear mechanism of MAO samples and MAO + UCFT samples is mild adhesive wear and abrasive wear.