Structure and Anticorrosion, Friction, and Wear Characteristics of Pure Diamond-Like Carbon (DLC), Cr-DLC, and Cr-H-DLC Films on AZ91D Mg Alloy

This study evaluated the protection against corrosion and wear afforded to AZ91D Mg alloy by coatings of hydrogen-free and low-Cr-doped hydrogenated diamond-like carbon (DLC) films. The microstructure and corrosion resistance were evaluated using scanning electron microscopy, atomic force microscopy, Raman spectroscopy, polarization curves, and neutral salt spraying tests. Wear tests were performed to investigate the friction and wear behaviors of the samples against 9Cr18 in humid air, deionized water, and 3.5 wt.% NaCl solution using a reciprocating sliding test in the ball-on-disk mode. The results showed that the more compact Cr-H-DLC film improved the corrosion resistance of the Mg alloy, whereas the Cr-DLC film accelerated the corrosion in the 3.5 wt.% NaCl solution. All Mg alloy samples coated with DLC films exhibited low coefficient of friction (COF) values and smaller wear volumes compared with those of bare substrate in air, water, or NaCl solution. The Cr-DLC film presented the lowest COF and wear rate in air, but the worst corrosion protection in air and NaCl solution, whereas the converse was found for the Cr-H-DLC film. Unfortunately, all coated samples showed limited protection ability because of pore defects in the films, high galvanic potential between the substrate and the buffer layer or film, and high electrical conductivity, which caused severe tribocorrosion of the Mg alloy during the wear tests in NaCl solution. The corrosion protection ability of DLC films is key to the wear resistance protection of Mg alloys in water or NaCl solution.