Effect of reverse pulse current density on microstructure and properties of supercritical Ni-GQDs nanocomposite coatings

In this study, Ni-GQDs nanocomposite coatings were prepared by double-pulse electrodeposition under supercritical CO₂ with graphene quantum dots (GQDs) as the second phase additive. The effects of supercritical CO₂ conditions and reverse pulse current density on microstructure, crystal orientation, grain size, GQDs quality, mechanical properties, and corrosion resistance of Ni-GQDs nanocomposite coatings were investigated. The results show that when the reverse pulse current density is 0.8 A /dm², the surface of Ni-GQDs-Ⅱ nanocomposite coating is compact and flat, GQDs is uniformly dispersed in the coating, and GQDs is closely bound to Ni grains. Compared with the coating prepared at normal temperature and pressure. The grain size of the Ni-GQDs-Ⅱ nanocomposite coating is 4.58 nm, and the grain size is reduced by 75.3 %. The quality of GQDs in the coating was improved. The coating hardness is 867.22 HV, which is significantly increased by 53.7 %. The roughness is 0.236 μm, which is significantly reduced by 37.2 %. The friction coefficient and volume wear were 0.262 and 3.395 × 10⁷ μm³, respectively, which were significantly reduced by 27.4 % and 57.9 %. After electrochemical corrosion, the self-corrosion voltage of the coating was −139 mV, and the self-corrosion current density was 3.19 × 10⁻⁷ A/cm². The self-corrosion voltage was significantly increased by 61.2 %, and the self-corrosion current density was significantly decreased by 71.2 %. The R_ct value and N_dl value of the coating are 31594.53 Ω·cm² and 0.862, respectively. Significantly increased by 226.2 % and 67.1 %, respectively. The coating has excellent mechanical properties and corrosion resistance.