Preparation and Properties of Pulsed Composite Coatings of Supercritical Graphene Quantum Dots

Abstract

Using graphene quantum dots with unique properties as the second phase additive and utilizing the high diffusion and transfer properties of supercritical fluids, Ni-based nanocomposite coatings were prepared by pulsed electrodeposition technology. The effects of the pulse duty cycle on the microstructure, mechanical properties, and corrosion resistance of composite coatings were investigated. The results showed that the graphene quantum dots are successfully embedded in the coatings, and under supercritical conditions, a suitable pulse duty cycle can improve the surface density and sphericity of the coatings. Raman spectroscopy and carbon-sulfur analyzer test indicated that supercritical conditions can improve the quality and content of graphene quantum dots in the coatings. The graphene quantum dots composite coating prepared when the pulse duty cycle is 0.3 has more excellent mechanical properties. Its microhardness is higher, and it has a smaller friction coefficient and wear scar cross-sectional area. Tafel polarization experiments indicated that under supercritical conditions, the corrosion current density of graphene quantum dots composite coating prepared when the pulse duty cycle is 0.3 is small, which is 1.286 × 10⁻⁵ A·cm⁻². The 120 h immersion corrosion study showed that no obvious corrosion occurs on the surface. Therefore, its corrosion resistance is more excellent.

1. Introduction

Material surface friction and wear [1] and corrosion are the two main reasons for the failure of mechanical parts [1,2]. In the current environment of rapid development of aerospace, automobile, and high-end equipment manufacturing industries, the requirements for wear resistance and corrosion resistance of mechanical parts in various fields are constantly increasing [3,4,5]. Sabyasachi Ghosh et al. reported that phase-separated PEDOT:PSS ornamented with reduced graphene oxide (rGO) nanosheets, deposited on the merino wool/nylon (W−N) composite textile. This developed fabric could perform as a new potentially scalable single product in intelligent smart garments, portable next-generation electronics, and the growing threat of EM pollution. They also reported a route to fabricate rGO/Ag decorated cotton fabric having high electrical conductivity and superior electromagnetic shielding efficiency [6,7]. Shivam Tiwari et al. reported that the fluorescent nitrogen-doped carbon dots which have been produced by a bottom-up wet-chemical method employing citric acid as a precursor could be used for specific applications in different research fields such as sensors, solar cells, biomedicine, and catalysis [8]. The use of electrodeposition technology [9] to deposit enhanced coatings on metal substrates has been widely used because of its high efficiency and economy. Among them, nickel nanocomposite coatings [10] are particularly widely used because of their high wear resistance and corrosion resistance.

As a new type of zero-dimensional carbon-based material [11,12], graphene quantum dots have a size of less than 100 nm; due to its size, it has obvious quantum confinement effect and sideband effect, which also makes graphene quantum dots have more emerging physicochemical properties (such as chemical inertness, excellent water solubility). Deposition of neutral particles (such as nitrides, carbides, etc.,) in the coatings can improve the grain growth process and effectively improve the mechanical and chemical properties of the coating [13]. Incorporating graphene quantum dots with excellent properties as a second phase additive into the nickel coating can effectively improve its mechanical properties [14] and corrosion resistance. Electrodeposition technology has the problem that the second phase additives are easy to agglomerate and affect the composite coatings. The study found that the supercritical fluid technology [15] can make the electrodeposition solution form an excellent emulsification system [16] through reasonable system temperature, pressure, and suitable surfactant. The emulsification system enables the electrodeposition system to have better mass transfer performance and can effectively separate the second additive, thereby improving the agglomeration of the second phase additive. Direct current (DC) electrodeposition is easy to generate pores, and the coatings have high residual stress. The study found that pulse electrodeposition [17] compared with DC electrodeposition, the prepared coatings have the advantages of low residual stress [18], smooth surface, and tight bonding between the coatings and the base metals. The pulse duty cycle controls the on-off time of the current and has a great influence on the cathode overpotential. A suitable pulse duty cycle can significantly improve the quality of coatings.

In previous studies, it was found that the properties of the coatings prepared by pulse electrodeposition was better than that of the coating prepared by DC electrodeposition. For example, in terms of microhardness, under the same conditions, pure Ni coatings prepared with better parameters are used. The microhardness of pure Ni coatings prepared by pulse electrodeposition is about 100 HV higher than that of pure Ni coatings prepared by DC electrodeposition. Moreover, it was found that supercritical technology has a significant effect on the dispersion of graphene quantum dots. In this work, graphene quantum dots were used as the second phase additive, and the Ni-based composite coating of graphene quantum dots was prepared by pulsed electrodeposition technology with the assistance of supercritical CO₂ fluid. Through preliminary research, it is preliminarily determined that the optimal current parameters are current density 6 A/dm²; frequency 2000 Hz; plating time 60 min; optimal supercritical condition parameters are pressure 10 MPa and temperature 50 ℃. By comparing the microstructure, mechanical properties, and corrosion resistance of coatings prepared with different pulse duty cycles, a more suitable pulse duty cycle was explored. To further help with the research and development of metal matrix nanocomposite coatings, they were enhanced by adding second phase additives by pulse electrodeposition as a processing method.

2. Materials and Methods

2.1. Sample Preparation

The preparation formula of graphene quantum dots(GQDs) is shown in

Table 1. Formulation of graphene quantum dots.

Medicines/Reagents	Dosage/g
C6H8O7·H2O	0.21
CH4N2O	0.18
H2O	5

. Add citric acid (C₆H₈O₇·H₂O) and urea (CH₄N₂O) to 5 mL of deionized water (H₂O), stir until clear, then place in a polytetrafluoroethylene reactor, seal the reactor, and place it in an oven at 180 °C for 8 h. After the heat preservation, take out the reacted liquid, pour it into alcohol, put it into a centrifuge for centrifugation, wash the separated graphene quantum dots with alcohol, dry them, put them in a sealed bottle, and store them in a dry and dark place for future use.

The supercritical CO₂-assisted electrodeposition device is shown in Figure 1. The temperature measurement device, temperature control device, cooling system, and air pressure pump, respectively, meet the temperature and pressure requirements for supercritical electrodeposition work. In this experiment, the anode and cathode materials were 20 mm × 20 mm copper sheet and 25 mm × 25 mm nickel plate with a purity of 99.9%, and the distance between the cathode and anode was 20 mm. After rust removal, oil removal, oxide film removal, sandpaper grinding (1200–5000 mesh), and polishing for copper sheets, after rust removal, oil removal, and oxide film removal for nickel plates, use insulating glue to fix the copper sheet and the nickel plate on the working cathode and anode respectively, and ensure that the cathode and anode were well energized. The Chunlin ultrasonic cleaner and EMS-12 split magnetic stirrer were used to fully stir the plating solution and fully disperse the graphene quantum dots in the plating solution. TMN surfactant was added to the plating solution before plating. The formula of the bath and the process parameters of the coatings (Ni-GQDs- I, Ni-GQDs- II, Ni-GQDs- III, Ni-GQDs- IV and Ni) prepared by different preparation processes are shown in

Table 2. Bath formula.

Medicines	Dosage/(g·L−1)
NiSO4·6H2O	300.0
NiCl2·6H2O	30.0
H3BO3	35.0
C12H25NaO4S	0.2
TMN Surfactant	0.15

and

Table 3. Process parameters.

	Sample	Ni-GQDs- I	Ni-GQDs- II	Ni-GQDs- III	GQDs- IV	Ni
Process Parameters
The amount of graphene quantum dots added g/L		1.5	1.5	1.5	1.5	0
Duty cycle		0.1	0.3	0.5	0.3	0.3
Cathode current density/(A/dm2)		6	6	6	6	6
Frequency/Hz		2000	2000	2000	2000	2000
Plating time/min		60	60	60	60	60
Stirring speed/(r/min)		250	250	250	250	250
Pressure/MPa		10	10	10	normal temperature	10
Temperature/°C		50	50	50	normal pressure	50
Thickness/μm		24.1 ± 2.1	27.6 ± 1.6	30.2 ± 0.9	26.8 ± 3.1	32.6 ± 2.8

, respectively.

2.2. Analysis and Characterization

After the samples were ion-thinned by Gatan 691 ion-thinning instrument, the state of graphene quantum dots in the coating was observed by Talos F200× G2 high-resolution transmission electron microscope (Talos, Beijing, China), and the elements of the coating were analyzed by SDD super-X energy spectrometer (EDAX, Beijing, China). The microscopic morphology of the coating and the coating after immersion corrosion were observed by Sigma-500 scanning electron microscope (Zeiss, Oberkochen, Germany). The carbon content of the coating was analyzed by Like CS844 carbon and sulfur analyzer (LECO, Laboratory Equipment Corporaton, Saint Joseph, MI, USA). The chemical structure of the coating was determined by HR Evolution Raman spectrometer (HORIBA, Kyoto, Japan). The selected laser was He-Ne, the wavelength was 633 nm, the spot size was 1 μm, and the scanning range was 1000–3000 cm⁻¹. The nickel crystal preferred orientation of the coating was tested by X’PERT POWDER X-Ray diffractometer (Malvern, Shanghai, China)target was used, the scanning range was 20°–80°, and the step size was 0.0131303°.

HXD-1000TMS/LCD digital microhardness tester (Yongheng Optical Instrument Manufacturing Co., Ltd, Shanghai, China) was used to test the hardness of the coating, and each sample selected 5 points at different positions for testing, and the average value of these 5 points is the hardness value of the sample. The Nanovea TRB friction (Nanovea, Irvine, CA, USA) and wear testing machine was used to test the wear resistance of the coating. The steel ball with a diameter of 6 mm was used as the abrasive piece. The speed was 120 r/min, the load was 5 N, and the test time was 10 min. The Nanovea PS50 optical profiler (Nanovea, Irvine, CA, USA) was used to scan the surface topography of the wear scar. The scanning area was 2 mm × 2 mm, the step size was 5 μm, and the scanning rate was 3.33 mm/s. The Nyquist spectrum, Tafel polarization curve, and Bode spectrum were tested on the CHI760E electrochemical workstation. The coatings were immersed in a 3.5 g/L NaCl solution for 120 h at normal temperature and pressure, rinsed with alcohol after soaking, and then observed by a scanning electron microscope for the microscopic morphology of the coatings.

3. Results

3.1. The Effect of Pulse Duty Cycle on the Microstructure of Graphene Quantum Dots Composite Coatings

3.1.1. The Effect of Pulse Duty Cycle on the Surface Morphology of Graphene Quantum Dots Composite Coatings

Figure 2 shows the TEM analysis of Ni- graphene quantum dots composite (GQDs) composite coating prepared under supercritical conditions with a pulse duty cycle of 0.3. Figure 2a,b are the TEM images. It can be seen from Figure 2a that the graphene quantum dots are uniformly distributed in the composite coating and are closely combined with the nickel grains. It can be seen from Figure 2b that in the TEM image of a larger magnification, the rhombic lattice can be clearly seen, which is a typical multi-layer graphene, indicating that the graphene quantum dots exist in the composite coating in the state of multi-layer graphene. Figure 2c shows the selected area electron diffraction (SAED) pattern. The diffraction ring is a polycrystalline diffraction ring, including (111), (200), and (220) crystal planes, indicating that the crystal structure of Ni in the composite coating is a face-centered cubic structure. Figure 2e is the EDS diagram at point a in Figure 2d, and

Table 4. EDS elements composition table at a.

Element	Atomic %	wt.%
C	40	12
Ni	60	88

shows the composition ratio at point a. It can be seen from Table 4 that the atomic proportions of carbon and nickel at point a are respectively 40% and 60% and the quality ratios are respectively 12% and 88%, indicating that there is carbon in the composite coating, and the graphene quantum dots are successfully embedded in the composite coating and combined between the nickel grains.

Figure 3 is 1000 times SEM topography images of the surface of Ni-GQDs composite coatings prepared with different pulse duty cycles. It can be seen from the Figure 3 that when the pulse duty cycle is 0.1, the surface density of the Ni-GQDs composite coating is not high, and there are many small holes. When the pulse duty cycle is 0.3, the Ni-GQDs composite coating has a smooth surface, high density, uniform particle size distribution, and good sphericity. When the pulse duty cycle increases to 0.5, the surface of Ni-GQDs composite coating has coarse grains, uneven particle size distribution, and the quality of surface decreases. In the absence of supercritical conditions, the surface of the Ni-GQDs composite coating has coarse grains, cracks, and poor surface flatness.

The reason for the above phenomenon is as follows. After the power supply is turned on, the cathode generates an overpotential, which generates an adsorption force for the metal cations mixed with graphene quantum dots in the plating solution, and the graphene quantum dots are deposited on the surface of the cathode together with the metal cations. As the pulse duty cycle increases, the current conduction time becomes longer. When the duty cycle is small, the current conduction time is short, and the cathode reaction time is short, resulting in a short nickel crystallization time, which cannot form good sphericity. While the moment of turn-on, the peak current density is much larger than the average current density, a large number of nickel ions are consumed instantaneously, an extreme concentration difference appears near the cathode, and hydrogen evolution occurs. Therefore, there are holes on the surface of the composite coating. When the pulse duty cycle increases to 0.3, the current on time and off time reach a balance, the cathode reaction time is appropriate, and the nickel ions have sufficient time to complete the crystallization, so the sphericity and surface density of the composite coating are improved. When the pulse duty ratio increases to 0.5, the graphene quantum dots near the cathode cannot be replenished in time due to the long on-time, so the inhibition of nickel grain growth is reduced, resulting in coarse grains and uneven particle size distribution. Under normal temperature and pressure, graphene quantum dots cannot be fully dispersed in the plating solution, and agglomeration will occur. Graphene quantum dots enter the composite coating in the form of agglomeration, resulting in excessive stress during crystallization, resulting in cracks and poor quality of surface.

3.1.2. The Effect of Different Preparation Conditions on the Structure and Composition of Graphene Quantum Dots Composite Coatings

Figure 4 shows the Raman spectra of Ni- graphene quantum dots (GQDs) composite coatings prepared under different preparation conditions and the Raman spectra of graphene quantum dots, and

Table 5. The positions and intensity values of D and G bands in the Raman spectrum.

Sample	D-Band Position (cm−1)	G-Band Position (cm−1)	D-Band Strength (ID)	G-Band Strength (IG)	ID/IG
GQDs	1336.48	1564.03	4106.60	4375.03	0.9386
Ni-GQDs- II	1330.22	1582.69	1465.57	1874.10	0.7820
Ni-GQDs- IV	1325.20	1583.01	1223.94	1206.47	1.0145

shows the positions of the D and G bands of the Raman spectrum, as well as the intensity value and the intensity ratio of the D peak and the G peak. The relative intensity of the D band is a reflection of the degree of disorder in the crystal structure, and the G band is caused by the in-plane vibration of sp² carbon atoms. They are the main characteristic peaks of graphene. Usually, the intensity ratio (I_D/I_G) of the D peak and the G peak is used to represent the defects inside the graphene. The better the quality of the graphene quantum dots are, the smaller the I_D/I_G value [19] well be. It can be seen from Figure 4 and Table 5 that the I_D/I_G value of Ni-GQDs- II composite coating is the smallest at 0.7820, followed by the I_D/I_G value of graphene quantum dots of 0.9386, and the I_D/I_G value of Ni-GQDs- IV composite coating is the highest, which is 1.0145. The I_D/I_G values of different Ni-GQDs composite coatings indicate that after supercritical electrodeposition, the defects and disorder of graphene quantum dots in Ni-GQDs composite coatings are relatively reduced, and the quality is improved. However, after electrodeposition at normal temperature and pressure, the quality of graphene quantum dots in the Ni-GQDs composite coating decreases instead.

Table 6. Results of the carbon content of composite coatings prepared by different preparation processes tested by the carbon-sulfur analyzer.

Sample	The Quality of the Sample (g)	C (wt.%)
Ni-GQDs- II	0.5051	1.10
Ni-GQDs- IV	0.5131	0.56

shows the results of the carbon content of Ni-GQDs composite coatings prepared by different preparation processes tested by a carbon-sulfur analyzer. It can be seen from Table 6 that the mass fraction of carbon in the Ni-GQDs- II composite coating is 1.10%, which is 196% of the Ni-GQDs- IV composite coating, indicating that the content of graphene quantum dots in the Ni-GQDs- II composite coating is nearly double that of the Ni-GQDs- IV composite coating.

The reasons for this phenomenon are as follows. Under normal temperature and pressure, the graphene quantum dots are agglomerated, and the agglomerated graphene quantum dots are large and difficult to enter the coating. However, under supercritical conditions, the graphene quantum dots are sufficiently discrete, and when the cathode nickel ions are reduced and crystallized, they can easily enter the coating.

3.1.3. Preferential Orientation of Nickel Crystals in Coatings Prepared under Different Preparation Conditions

Figure 5 shows the XRD patterns of the Ni- graphene quantum dots (GQDs) composite coatings prepared under different preparation conditions. The preliminary observation of the XRD patterns shows that the positions of the diffraction peaks of Ni of the composite coatings prepared by different preparation processes are all the same; that is, at 2θ = 45°, 52°, and 76°, the crystal planes [20] corresponding to each diffraction peak are (111), (200), and (220) in turn. It can be seen from the diffraction peaks corresponding to the crystal planes that the crystal structure of Ni in the coating is a face-centered cubic structure, which is consistent with the results obtained from the SAED image. The diffraction intensity of Ni-GQDs- II composite coating on (111) and (200) surfaces are higher than that of Ni-GQDs- IV composite coating, while the diffraction intensity on (220) surface is significantly lower than that of Ni-GQDs- IV composite coating, indicating that under supercritical conditions, the graphene quantum dots are fully dispersed, and there is no agglomeration phenomenon. The discrete state of the graphene quantum dots has an impact on the process of nickel-free crystallization, which can change the process of nickel nucleation and growth during electrodeposition. According to the diffraction peak width and the Scherrer formula [21]:

$$\mathrm{D}=\frac{\mathrm{K}\mathsf{\gamma }}{\mathrm{B}\cos \mathsf{\theta }}$$

(1)

the grain sizes of the coatings are shown in

Table 7. Coating grain sizes.

Sample	Preparation Conditions	D/Å
Ni-GQDs- II	supercritical conditions	182
Ni-GQDs- IV	normal temperature and pressure	322

, where K is 0.89 and γ is 1.54056 Å.

It can be seen from the Table 7 that the grain size of the Ni-GQDs- II composite coating is smaller than that of the Ni-GQDs- IV composite coating. The reasons for this phenomenon are as follows: at normal temperature and pressure, graphene quantum dots inevitably agglomerate. However, under supercritical conditions, the discrete degree of graphene quantum dots becomes higher, and the dispersion distribution of graphene quantum dots will hinder the continuous growth of nickel grains and inhibit the growth of coating grains. Therefore, the grain size of Ni-GQDs- II composite coating is smaller than that of Ni-GQDs- IV composite coating.

3.2. The Effect of Pulse Duty Cycle on the Properties of Graphene Quantum Dots Composite Coatings

3.2.1. The Effect of Pulse Duty Cycle on the Microhardness of Graphene Quantum Dots Composite Coatings

The effect of pulse duty cycles on the microhardness of Ni- graphene quantum dots (GQDs) composite coatings is shown in Figure 6. It can be seen from the Figure 6 that with the increase of the pulse duty cycle, the microhardness of the Ni-GQDs composite coatings increase first and then decrease, showing an inverted U shape. When the pulse duty cycle is 0.3, the microhardness of the prepared coating Ni-GQDs- II is the highest, reaching 837.6HV_0.2, which is respectively 111%, 114%, and 120% of the microhardness of the Ni-GQDs- I, Ni-GQDs- III, and Ni-GQDs- IV composite coatings. The microhardness of Ni-GQDs- IV composite coating prepared under normal temperature and pressure is the lowest, which is lower than that of all Ni-GQDs composite coatings prepared under supercritical conditions.

The reasons are as follows. With the increase of the pulse duty cycle, the cathode overpotential becomes larger, and the adsorption force of the nickel ions mixed with graphene quantum near the cathode becomes stronger. Therefore, the content of graphene quantum dots in the coating also increases, which provides more growth points for the crystallization of nickel ions. On the one hand, the nucleation of nickel ions is promoted. On the other hand, it also inhibits the growth of nickel grains, which makes the surface of the coating more compact, and sphericity also becomes better, so the microhardness is relatively higher. Excessive pulse duty cycle makes the on-time longer, and the graphene quantum dots near the cathode cannot be replenished in time. Therefore, extreme concentration difference appears near the cathode resulting in uneven particle size distribution and decreased microhardness. If the pulse duty cycle is too small, the on-time is not enough, resulting in insufficient growth of nickel grains, resulting in gaps between grain boundaries, so the microhardness is not high. The agglomeration problem of graphene quantum dots at normal temperature and pressure makes the surface density and sphericity of the composite coating poor, the grains are coarse, and the surface of the coating is prone to plastic deformation. Therefore, the hardness of the Ni-GQDs- IV composite coating is small. According to the Hall-Petch equation: the grain size is inversely proportional to the microhardness value, and the microhardness test result corresponds to the grain size above. According to the classical Archard’s law [22], the wear rate is inversely proportional to the microhardness of the material, so the Ni-GQDs- II composite coating also has the lowest wear rate.

3.2.2. The effect of Pulse Duty Cycle on Wear Resistance of Graphene Quantum Dots Composite Coatings

The friction coefficient curves, wear scar 3D images, and cross-sectional images of Ni- graphene quantum dots (GQDs) composite coatings prepared with different pulse duty cycles are shown in Figure 7 and Figure 8. The maximum depths of the wear scar, the cross-sectional areas of the wear scar, and the volumetric wear amounts of the coatings were calculated according to the cross-sectional views of the wear scar, as shown in

Table 8. The maximum depths of wear scars, wear scar cross-sectional areas, and volume wears of the composite coatings.

Sample	Preparation Conditions	Maximum Depths of Wear Scars/μm	Wear Scar Cross-Sectional Areas /μm2	Volume Wears/μm3
Ni-GQDs- I	supercritical conditions	12.9	4126	4.126 × 10−7
Ni-GQDs- II	supercritical conditions	9.16	2568	2.568 × 10−7
Ni-GQDs- III	supercritical conditions	15.4	4444	4.444 × 10−7
Ni-GODs- IV	normal temperature and pressure	12.4	4828	4.828 × 10−7

. It can be seen from Figure 7 that the friction coefficients of the Ni-GQDs composite coatings first increase with the increase of the friction distance, then decrease, and finally stabilize. The reasons are as follows, when the abrasive pieces just contact the Ni-GQDs composite coatings, the surfaces of the Ni-GQDs composite coatings have a certain degree of sphericity, and the surfaces are not smooth. Therefore, the friction coefficients rise first. When the abrasive pieces are pressed into the composite coatings for substantial shearing, due to the self-lubricating properties of graphene quantum dots, it plays a role in reducing friction during the friction processes, so the friction coefficients gradually decrease and then remain stable. The friction coefficients of the composite coatings are Ni-GQDs- II, Ni-GQDs- I, Ni-GQDs- III, and Ni-GQDs- IV in descending order. The friction factor of Ni-GQDs- II composite coating is the smallest, which is stable at about 0.55, and the friction factor of Ni-GQDs- IV composite coating is the largest, which is stable at about 0.78, so Ni-GQDs- II has better wear resistance.

As can be seen from Figure 8 and Table 8, the wear scar cross-sectional area and volume wear of the Ni-GQDs- II composite coating are the smallest, which are respectively 62%, 58%, and 53% of those of the Ni-GQDs- I, Ni-GQDs- III, and Ni-GQDs- IV composite coatings, and the wear scar depth of Ni-GQDs- II composite coating is the shallowest. The experimental results of friction coefficients, maximum depths of wear scar, cross-sectional areas of wear scar, and volume wears of Ni-GQDs composite coatings show that with the increase of pulse duty cycle, the wear resistances of Ni-GQDs composite coatings first increase and then decrease. The reasons are as follows. When the pulse duty cycle is low, the surface density of the Ni-GQDs composite coatings are not high, resulting in a gap between grain boundaries, and the transverse shear force and adhesion force generated during friction are small. When the pulse duty cycle is increased to 0.3, the content of graphene quantum dots in the composite coating is the largest, and the graphene quantum dots dispersed in the composite coating make the composite coating by the second phase additive dispersion strengthening [23], and the graphene quantum dots located at the grain boundaries hinder the slip of dislocations [24] so that the coating is strengthened by dislocations, thereby improving the wear resistance of the coating. When the pulse duty cycle is too large, the particle size distribution on the surface of the Ni-GQDs composite coatings are uneven, the surface flatness of the coating is poor, and the transverse shear force and adhesion force are generated during friction are reduced. Therefore, the wear resistance is decreased. Under normal temperature and pressure, graphene quantum dots appear agglomeration, and most of them enter the coating in the form of agglomeration. The density and surface flatness of the coating is poor, and the ability to resist plastic deformation is correspondingly poor. Therefore, compared with the Ni-GQDs composite coatings prepared under supercritical conditions, the wear resistance is the worst.

3.2.3. The Effect of Pulse Duty Cycle on Corrosion Resistance of Graphene Quantum Dots Composite Coatings

Electrochemical Corrosion Analysis

Figure 9a shows the Tafel polarization curves of Ni- graphene quantum dots (GQDs) composite coatings prepared with different pulse duty cycles and pure Ni in 3.5% NaCl solution. The main electrochemical parameters are extracted from the potentiodynamic polarization curves and are represented in

Table 9. Parameters of corrosion process extracted from potentiodynamic polarization curves.

Sample	Preparation Conditions	Ecorr/eV	Icorr/(10−5 A·cm−2)
Ni-GQDs- I	supercritical conditions	−272 ± 12	1.703 ± 0.112
Ni-GQDs- II	supercritical conditions	−271 ± 9	1.286 ± 0.087
Ni-GQDs- III	supercritical conditions	−228 ± 13	2.029 ± 0.136
Ni-GQDs- IV	normal temperature and pressure	−308 ± 11	5.244 ± 0.105
Ni	supercritical conditions	−260 ± 17	3.046 ± 0.167

. The corrosion potential of Ni-GODs composite coatings prepared under supercritical conditions is higher than that of Ni-GODs- IV composite coating prepared under normal temperature and pressure, and the corrosion potential increases with the increase of pulse duty cycle. The corrosion potential of pure Ni coatings is between that of Ni-GQDs- II and Ni-GQDs- III composite coatings. The corrosion current density of Ni-GODs- II composite coating is the lowest, which is 25% and 42% of that of Ni-GQDs- IV composite coating prepared under normal temperature and pressure and pure Ni coating, respectively. From the corrosion potential and corrosion current density of Ni-GODs composite coating and pure Ni coating, it can be seen that the Ni-GQDs- III composite coating with a high pulse duty cycle has the highest corrosion potential and takes the longest time to start corrosion; however, after corrosion occurs, the corrosion rate is faster. Although the corrosion potential of the Ni-GQDs- II composite coating is slightly lower than that of the Ni-GQDs- III composite coating, the corrosion current density is significantly lower than that of the other Ni-GQDs composite coatings, so the corrosion rate is the lowest. Therefore, the Ni-GQDs- II composite coating exhibits more excellent corrosion resistance.

Figure 9b shows the Nyquist spectra of Ni-GQDs composite coatings prepared with different pulse duty cycles and pure Ni in 3.5% NaCl solution. It can be seen from the Nyquist spectrum in Figure 9b that the radius of the capacitive reactance arc is as follows: Ni-GQDs- IV < Ni < Ni-GQDs- III < Ni-GQDs- I < Ni-GQDs- II. Therefore, with the increase of pulse duty cycle, the capacitive arc radius of Ni-GQDs composite coating first increases and then decreases, showing an inverted U shape. The capacitive arc radius of Ni-GQDs- IV composite coating prepared under normal temperature and pressure and pure Ni coating are smaller than that of Ni-GODs composite coatings prepared under supercritical conditions. It indicates that pure Ni coating and Ni-GQDs- IV composite coating prepared under normal temperature and pressure have smaller impedance response and lower corrosion resistance than Ni-GQDs composite coatings prepared under supercritical condition. The fitting parameters and equivalent circuit diagram of the impedance experiment are shown in

Table 10. Parameters obtained from the fitting of the experiment impedance spectra.

Sample	Preparation Conditions	Rs, Ohm·cm2	Y1, sn/(Ohm·cm2)	n1	Rp, k Ohm·cm2
Ni-GQDs- I	supercritical conditions	29.53 ± 1.25	6.734 × 10−5 ± 0.214	0.6992 ± 0.1223	2119 ± 32
Ni-GQDs- II	supercritical conditions	29.58 ± 1.22	6.619 × 10−5 ± 0.301	0.6966 ± 0.0914	3125 ± 23
Ni-GQDs- III	supercritical conditions	27.65 ± 0.42	1.236 × 10−4 ± 0.487	0.6203 ± 0.0801	861.1 ± 13
Ni-GQDs- IV	normal temperature and pressure	30.95 ± 0.92	1.04 × 10−3 ± 0.254	0.4994 ± 0.1503	337.8 ± 12
Ni	supercritical conditions	25.56 ± 1.50	1.32 × 10−4 ± 0.356	0.5841 ± 0.0765	887.5 ± 14

and Figure 9e, respectively. The equivalent circuit consists of solution resistance (Rs), constant phase element (CPE), and polarization resistance (Rp). The impedance of the CPE element is calculated by [25,26,27]:

$${\mathrm{Z}}_{\mathrm{CPE}}=\frac{1}{{\mathrm{Y}}_0(\mathrm{j}\mathsf{\omega }){}^{\mathrm{n}}}$$

(2)

where Y₀ is the CPE constant and n (0 ≤ n ≤ 1) is the mathematical factor, j is the imaginary unit, ω is an angular frequency, rad⁻¹. It can be seen from Table 10 that with the increase of the pulse duty cycle, the Rp value of the Ni-GQDs composite coating first increases and then decreases. The Rp value of the Ni-GQDs- II composite coating is the highest, which is 3.5 times and 9.25 times that of the pure Ni coating and the Ni-GQDs- IV composite coating prepared under normal temperature and pressure. Moreover, the CPE constant Y₁ fitted for Ni-GQDs- II composite coating has the smallest value, it shows that the Ni-GQDs- II composite coating has more excellent corrosion resistance.

It can be seen from Figure 9c of the Bode spectrum that the Ni-GQDs composite coatings and pure Ni coating have only one phase angle peak, which is consistent with only one capacitive arc in the composite coatings in the Nyquist spectrum. It can be seen from the Bode diagram in Figure 9d that the capacitive modulus of the Ni-GQDs- II composite coating is greater than that of the pure Ni coating and the Ni-GQDs- IV composite coating prepared under normal temperature and pressure. It is further shown that the Ni-GQDs- II composite coating has more excellent corrosion resistance.

The reasons for the above phenomenon are as follows. The Ni-GQDs- IV composite coating prepared under normal temperature and pressure, due to the agglomeration of graphene quantum dots, has problems such as coarse grains, low surface density, and many cracks. Corrosive medium can easily penetrate into the composite coating, and the shielding effect of graphene quantum dots is not obvious, so the corrosion resistance is relatively weak. When the duty cycle is 0.3, the surface density of the coating is high, the particle size distribution is uniform, and the graphene quantum dots are evenly distributed in the composite coating. The graphene quantum dots evenly distributed in the coating layer can form a layer-by-layer protective film of graphene quantum dots. It is obvious that the addition of the graphene quantum dots play a major role in the improved corrosion resistance and they act as an inert physical barrier to the initiation and development of the corrosion attack. Therefore, the Ni-GQDs- II composite coating has more excellent corrosion resistance. Although the surface density of the pure Ni coating without graphene quantum dots is high, due to the uneven particle size distribution, the local protective film of the coating is relatively weak, and the corrosive medium can penetrate into the coating from a relatively weak position. In addition, a large number of microbatteries may be formed in the nickel matrix, with nickel as the anode and graphene quantum dots as the cathode. Such corrosive galvanic couples contribute to anodic polarization. Therefore, the Ni-GQDs- II composite coating has better corrosion resistance than the pure Ni coating.

Immersion Corrosion Analysis

Figure 10 shows the 1000× SEM surface topography of Ni- graphene quantum dots (GQDs) composite coatings prepared by different preparation processes and pure Ni coating after soaking in 3.5% NaCl solution for 120 h at normal temperature. Under the corrosion of Cl⁻ and other corrosive media, the Ni-GODs- IV composite coating has obvious pitting corrosion. The corrosion pits are deep and large in diameter, and serious pitting corrosion occurs. Although there are many corrosion pits on the surface of the pure Ni coating, the depth and diameter of the pure Ni coating are not deep compared with that of the Ni-GODs- IV composite coating. The surface of the Ni-GQDs- II composite coating was not corroded significantly. It is shown that the corrosion resistance of the Ni-GQDs- II composite coating prepared under supercritical conditions is better than that of the Ni-GODs- IV composite coating prepared under normal temperature and pressure and pure Ni coating.

The surface of the Ni-GQDs- II composite coating did not corrode significantly, which was due to the physical barrier formed by the graphene quantum dots evenly distributed in the Ni-GQDs- II composite coating, which inhibited the initiation and development of corrosion. Although graphene quantum dots also exist in the Ni-GODs- IV composite coating, due to the agglomeration and uneven distribution of graphene quantum dots, graphene quantum dots cannot form an effective physical barrier. Moreover, the surface quality of the Ni-GODs- IV composite coating is relatively low, so the corrosion resistance is poor, even inferior to that of the pure Ni coating.

4. Conclusions

• The graphene quantum dots (GQDs) are uniformly distributed in the composite coatings and tightly bound to the nickel grains. Under supercritical conditions, the Ni-GQDs- II composite coating prepared with a pulse duty cycle of 0.3 has denser surface, better sphericity, and higher surface quality. Supercritical conditions can change the preferred orientation of the grains of Ni-GQDs composite coatings, reduce the grain size and refine the grains of the coatings. The Ni-GQDs- IV prepared under normal temperature and pressure has coarse grains and cracks, and the surface quality is worse.
• The I_D/I_G value of the Ni-GQDs- II composite coating prepared under supercritical conditions is smaller. Supercritical conditions can reduce the defects and disorder of graphene quantum dots and improve the quality of graphene quantum dots in the coatings. The Ni-GQDs- II composite coating prepared under supercritical pulse conditions has the highest carbon content, and supercritical condition can increase the content of graphene quantum dots in the coating.
• The hardness of Ni-GQDs- II composite coating prepared under supercritical conditions with a pulse duty cycle of 0.3 is higher, which is 111%,114%, and 120% that of Ni-GQDs- I, Ni-GQDs- III, and Ni-GQDs- IV composite coatings, respectively. The wear scar cross-sectional area and volume wear of the Ni-GQDs- II composite coating are smaller, which are respectively 62%, 58%, and 53% of those of the Ni-GQDs- I, Ni-GQDs- III, and Ni-GQDs- IV composite coatings, and the Ni-GQDs- II composite coating has the shallower wear scar depth, showing more excellent wear resistance. The hardness and wear resistance of Ni-GQDs composite coatings prepared under supercritical conditions are better than those of Ni-GQDs composite coatings prepared under normal temperature and pressure. It shows that supercritical conditions can effectively improve the microhardness and wear resistance of Ni-GQDs composite coatings.
• Under supercritical conditions, the Ni-GQDs- II composite coating prepared with a pulse duty cycle of 0.3 has the lowest corrosion rate, the largest impedance value, and the largest capacitive modulus, so it has the best corrosion resistance. After 120 h immersion corrosion, the surface of the Ni-GQDs- II composite coating prepared under supercritical conditions was not significantly corroded. However, the surface of the Ni-GQDs- IV composite coating prepared under normal temperature and pressure has serious pitting corrosion. It indicates that supercritical conditions can effectively improve the corrosion resistance of Ni-GQDs composite coatings.