DLC Films in Boric Acid Media for Corrosion and Friction Behavior Studies

The DLC films were prepared on 0Cr17Ni4Cu4Nb stainless steel (17-4 stainless steel) substrates by magnetron sputtering (MS) deposition. The corrosion resistance of the DLC films was assessed in boric acid media by testing their polarization curves at varying concentrations by an electrochemical workstation. To explore the mechanism of friction and wear of the thin films in aqueous boric acid solutions, friction and wear tests were executed using a ball-and-disk friction and wear tester in boric acid solutions with different concentrations. The obtained chemical products at the abrasion spots after friction were characterized by Raman, XRD and EDS analyses. The results of electrochemical experiments revealed that the corrosion current density of substrate was one order of magnitude higher compared to that of the DLC films in the boric acid solution. Additionally, the corrosion current density of the films increased as the boric acid concentration increased. Furthermore, the presence of DLC films enhanced the corrosion resistance of the substrate. The results of friction experiments showed that the wear mechanisms of both the DLC/Al₂O₃ ceramic ball and the DLC/GCr15 steel ball friction substitutes in boric acid medium were abrasive and adhesive wear. For the DLC/Al₂O₃ ceramic ball friction pairs, the friction coefficient and wear rate of the film system increased gradually with the increase of boric acid concentration. The lubrication mode between the DLC/Al₂O₃ friction pairs was fluid lubrication. On the one hand, the lubrication effect of the system was lower due to the lack of a stable transfer film at the wear spots of the Al₂O₃ ceramic balls. On the other hand, with the increase of the boric acid concentration, the Raman signals of the wear tracks showed a decrease in the value of I_D/I_G, which was due to the fact that a large amount of sp² hybridised carbon was dissolved in the solution causing a decrease in the content of the sp² hybrid bonds at the surface of the wear tracks and then an increase in abrasive wear between the friction pairs. The increase of abrasive wear between friction pairs caused higher friction coefficient and wear rate. For the DLC/GCr15 steel ball friction pairs, the friction coefficient and wear rate of the friction system decreased with the increase of boric acid concentration. In particular, the film showed 20% lower friction coefficient and 51% lower wear rate in 5000 mg/L boric acid solution compared to the friction in deionized water. The lubrication mode between this friction pairs was boundary lubrication, and the transfer film containing graphite, Fe₂B and B₂O₃ with high adhesion and lubricity was formed on the contact surface of the balls during the friction process. Thus, the balls and the film failed to come into direct contact with each other, which greatly reduced the abrasive wear and improved the lubrication performance between the friction pair leading to decreased friction coefficient and wear rate.