Preparation, characterization and properties of Cr-incorporated DLC films on magnesium alloy

Cr-incorporated diamond-like carbon (Cr-DLC) films were deposited on AZ31 magnesium alloy as protective coatings by a hybrid beams deposition system, which consists of a DC magnetron sputtering of Cr target (99.99%) and a linear ion source (LIS) supplied with CH4 precursor gas. The Cr concentration (from 2.34 to 31.5 at.%) in the films was controlled by varying the flow ratio of Ar/CH4. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the microstructure and composition of Cr-DLC films systematically. An electrochemical system and a ball-on-disk tribotester were applied to test the corrosion and tribological properties of the film on the AZ31 substrate, respectively. At low Cr doping (2.34 at.%), the film mainly exhibited the feature of amorphous carbon, while at high doping (31.5 at.%), chromium carbide crystalline phase occurred in the amorphous carbon matrix of the film. In this study, all the prepared Cr-DLC films showed higher adhesion to AZ31 than the DLC film. Especially for the film with low Cr doping (2.34 at.%), it owned the lowest internal stress and the highest adhesion to substrate among all the films. Furthermore, this film could also improve the wear resistance of magnesium alloy effectively. But, none of the films could improve the corrosion resistance of the magnesium alloy in 3.5 wt.% NaCl solution due to the existence of through-thickness defects in the films.

Research Highlights
► In the present paper, the Cr-DLC films were successfully deposited on magnesium alloy (AZ31), where the incorporated Cr concentration could be well controlled by varying the Ar/C2H2 flow ratio. The deposited Cr-DLC films showed a very good adhesive strength to the soft AZ31 substrate compared with the pure DLC film. Specifically, the most important results obtained here is that the Cr-DLC film with low Cr doping could significantly improve the wear resistance of the magnesium alloy, even it did slight contribution to the improvement of corrosion resistance of AZ31 alloy.
► The works described in this paper is considered as one of the pioneer studies in the research of Cr-DLC coatings used to modify the surface properties of magnesium alloys, as the reviewer’s comment.