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With graphene quantum dots (GQDs) of unique properties as the secondary phase additive, Ni-based nanocomposite coatings were prepared by supercritical electrodeposition technique. The effect of the addition of GQDs on the microstructure, microhardness, wear resistance, and corrosion resistance of the coatings under supercritical conditions was studied. Results show that the densification and homogenization occur in the coating microstructure after GQD addition. When the GQD content is 1.5 g/L, the surface morphology of the coating is more compact. X-ray diffraction analysis shows that the GQD addition can change the peak positions of (111), (200), and (222) nickel diffraction planes of the composite coatings, and the crystallographic preferred orientation appears in the (111) plane. The GQD addition greatly improves the properties of composite coatings. When the GQD content is 1.5 g/L, the coating microhardness is as high as 7381.4 MPa, which is nearly 980 MPa higher than that of the pure nickel coating. The cross-section area of the wear scar is 3336 μm², which is only 44% of that of the pure nickel coating. Tafel polarization test shows that the corrosion current density is 3.55×10^-6 A·cm^-2, which is 65% lower than that of the pure nickel coating (10.07×10^-6 A·cm^-2). The immersion corrosion tests of 150 h show that when the GQD content is 1.5 g/L, the optimal corrosion resistance occurs with the least pitting corrosion in the composite coating.