Investigation of Wear Properties and Corrosion Resistance of Electrodeposited Ni-P-PTFE Nanocomposite Coating on Mg-Li alloy

Ni-P-polytetrafluorethylene nanocomposite coatings based on Mg-Li alloy were successfully prepared by utilizing electrodeposition method. The nanocomposite coatings were investigated employing scanning electron microscopy, atomic force microscopy, and X-ray diffraction. The microstructure and composition of the coating were studied by friction, wear and corrosion tests. The experimental results reveal that when the concentration of PTFE is 3 g / L, deposition temperature is 70 °C, current density is 2 A/dm2, and pH of the plating solution is approximately 5.5, a uniform and dense Ni-P-PTFE nanocomposite coating is formed on Magnesium alloy substrate, and it exhibited a low coefficient of friction (∼1.5) and superior abrasion resistance. The co-deposition of PTFE particles can effectively reduce the friction coefficient and wear rate, and the wear mechanism is characteristic cutting wear. Under artificial seawater conditions, the experiments have demonstrated that the nanocomposite material possesses a corrosion current density of 3.9×10-5 A/cm2.


Introduction
Owing to the rapid development of industry, science, and technology, Magnesium alloys have the advantages of high strength, strong noise reduction ability, excellent electromagnetic shielding and radiation resistance [1,2], and have been widely used in marine construction projects.However, the harshness and complexity of the marine multi-factor service environment result in higher requirements for a comprehensive performance of the materials.A single-component material can no longer fulfill this practical application, and it has become inevitable to develop materials with excellent comprehensive performance and multi-component synergy.Metal composite materials overcome the limitations of single metals as engineering materials.In comparison with traditional metal materials, metal composite materials can not only enhance the mechanical strength of the metal matrix but also significantly reduce the thermal expansion coefficient of the metal materials.
With further development of research on metal composite materials in recent years, nanoparticles with good chemical stability have been added to further improve the metal composites considering the importance of composite structure and performance applications.Therefore, nanocomposite coatings (such as Ni-P-MOF [3], CeO2@GO-Ni-P [4], and Ni-P/nano-SiC [5]) prepared by electrodeposition on Mg alloy have become research hotspots for materials scientists.The use of electrodeposition methods for the preparation of metal matrix composites and their application to corrosion and wear resistance is very important.[6][7][8].Meanwhile, the low hardness of Mg-Li alloy will undoubtedly lead to its low mechanical properties and poor wear resistance [9].Moving parts made of Mg-Li alloy require good corrosion resistance, as well as excellent mechanical and tribological properties.
In this study, Mg-Li alloy (LZ91) alloys are used as the substrate, Ni-P as the transition layer, and polytetrafluorethylene as the solid lubricating particles.The surface morphologies, composition, wetting, corrosion and tribological behaviors of the composite coating were investigated, and the mechanism of friction and corrosion were analyzed and discussed.

Preparation of Ni-P-PTFE Nanocomposite Coating
3 cm 2 Mg-Li alloys are used as the cathode, a 6 cm 2 nickel plate (99.99% purity) as the soluble anode.The distance between the two electrodes was 2 cm.During electrodeposition, a DC stabilized power supply (DH1719A-5; Beijing Dahua Radio Instrument Factory, Beijing, China) was used to generate a DC current between the nickel plate and Magnesium alloy sheet, changing the experimental conditions, and determining the best preparation conditions for Ni-P-PTFE nanocomposite coating.The composition of the plating solution and preparation conditions of the nano-plated layer are shown in table 1. Prior to the experiment, ultrasonic power of 250 W was applied to sonicate the plating solution for 15 minutes in order to distribute the PTFE particles evenly.The specific process is as follows: degreasing-acid, washing-alkali, washing-immersion and nickel-pre-nickel plating.

Tribological Properties Analysis
The tribological properties of Ni-P coatings and Ni-P-PTFE nanocomposite coatings were obtained using Retc friction tester.The experiment used a reciprocating sliding method.The sliding speed was 8 mm/s.The normal load was 100 gf.The friction coefficient was continuously and automatically recorded as a function of time.

Surface Topography Analysis
The microstructures of Ni-P coating and Ni-P-PTFE nanocomposite coating were observed separately using SEM (JSM-5600LV).The crystalline structure of the as-prepared samples were analyzed using powder X-ray diffraction (XRD) with Cu-Ka radiation.

Corrosion Performance
A potentiostat/galvanostat corrosion measurement system (CHI660D; Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) was used to study the electrochemical corrosion behavior of the samples.Polarization measurements were performed in an etched cell containing 250 mL of a 3.5 wt% NaCl solution.The exposed area was 1 cm 2 .Potential polarization curves were measured after achieving a stable open-circuit voltage.The potential range was between +3 and -3 V with a scan rate of 0.2 mV/s.

Different Preparation Conditions
To further understand the effects of PTFE content, plating temperature, solution pH, and current density on tribological properties with nanocomposite coatings, relevant experiments were performed.
During the electrodeposition process, a single-factor orthogonal method is applied.One of the conditions is changed, and the other experimental conditions are maintained to prepare a metal-based nanocomposite coating, and the tribological properties are characterized, as shown in figure 1.When searching for the optimal PTFE content, the current should be 2 A/dm 2 .As seen in figure 1(a), when the PTFE content increases gradually, the friction coefficient of the composite coating layer decreases initially and then increases.The minimum value of the COF is about 0.16 at a particle content of 3 g/l.However, when the content of PTFE in the solution reaches a certain amount, severe particle aggregation occurs, which may hinder the co-deposition of PTFE particles and generate the coarse and loose structure.When changing the experimental conditions of the current density, the content of nanoparticles in the solution is 3 g/L.The experimental results are shown in figure 1(c) and (d).This phenomenon may be owing to the increase in the overpotential caused by the increase in the current density.Moreover, the electrostatic attraction of the adsorbed particles is enhanced, making the particles more easily captured.This may also be due to an increase in the metal deposition rate and particle transfer speed near the cathode owing to the high current density.This results in a further increase in the matrix metal deposition rate, thereby reducing the amount of particle recombination.

Surface Morphology
The surface morphology of the Ni-P-PTFE nanocomposite coating prepared under the optimal process conditions, as shown in figure 2. In comparison with the Ni-P alloy coating, Ni-P-PTFE nanocomposite coating is bright black, and the surface is smooth and uniform.From the insets of figure 3, we can see that the static contact angles of the two different surfaces of the Ni-P composite coating and Ni-P-PTFE nanocomposite coating after modification with stearic acid are 157.6°and 160.50°, respectively.The result is similar to Daniel's research [10].Both composite coatings are superhydrophobic, showing the good corrosion resistance of the composite coatings.

Structure Analysis
Figure 3 shows the XRD results of Ni-P coating and Ni-P-PTFE composite coating.It can be seen from the figure that the diffraction peaks of Ni-P-PTFE nanocomposite coating show a diffraction peak of (111) and characteristic peak of PTFE, which is similar to previous research [11].Moreover, the diffraction peaks appear to be broadened.This indicates that the addition of PTFE makes the alloy crystals finer i.e., metal cores and crystal growth cause the deposition of composite coatings, and the addition of PTFE causes more nucleation.Moreover, the wear rate was reduced by 3 orders of magnitude.The abrasion resistance is significantly improved, which is mainly due to the lubrication and wear reduction of the PTFE particles in the coating.According to the theory of polymer friction, when a polymer interacts with a relatively hard anti-wear material, the polymer is usually transferred and a polymer film is formed from the hard anti-wear material surface..The formation of this layer of polymer film is closely associated with friction and wear.Once the film layer is formed, subsequent interaction can be performed between the polymer and film layer with similar materials to reduce the effects of lubrication and friction.At the same time, the interface bonding force between the film layer and hard anti-abrasive material is generally higher than the bonding strength of the polymer matrix itself, and the film layer may be further thickened.During the experiment, the co-deposited PTFE particles utilized the characteristics of the solid lubricant particles completely.

Corrosion
Resistance.Nanocomposite coatings prepared by electrodeposition are widely used as anticorrosive materials.Further, one of the most common methods for examining corrosion resistance is the Tafel curve method, which can quantitatively describe the corrosion resistance of nanocomposite coatings.As shown in figure 5, the same conditions, the corrosion potential of the Ni-P-PTFE nanocomposite coating is greater than that of the Ni-P alloy coating.The calculated nanocomposite film has a lower corrosion current density.Hence, under the same corrosion conditions, the nanocomposite coating has better corrosion resistance.The possible reason is that the addition of polytetrafluorethylene particles reduces the number of defects and pore size in the coating, inhibits the anode reaction during the corrosion of the matrix metal, and hinders the diffusion of corrosion points.During the corrosion process, the polytetrafluorethylene particles dispersed in the metal matrix may occupy the preferential corrosion position, inhibit the corrosion of the metal by the salt solution, and improve the corrosion resistance of the metal-based nanocomposite coating.Another method for visually characterizing corrosion resistance is electrochemical impedance spectroscopy.In this method, the magnitude of the capacitance arc radius represents the resistance.

Conclusion
The electrodeposition method was used to successfully prepare Ni-P-PTFE nanocomposite coatings based on Mg-Li alloy.The most suitable conditions determined for preparing the metal-based nanocomposite coatings in this experiment are: deposition temperature of 70 ℃, current density of 2 A/dm 2 , and pH of the plating solution of approximately 5.5.The co-deposition of PTFE particles widened X-ray diffraction peaks of the nanocomposite coating and refined the matrix metal grains.Under the condition of simulating seawater, the corrosion resistance of the composite coating was tested using a CHI660E electrochemical workstation.The test results showed that the nanocomposite coating obtained after doping PTFE with solid lubricating particles exhibited excellent corrosion resistance, which provides a good reference for the wide application of Mg-Li alloys.

Figure 1 .
Figure 1.Figure 1. Friction coefficient and wear rate of nanocomposite coatings under different experimental conditions.(a) Friction coefficient and (b) Wear loss with different contents of PTFE.(c) Friction coefficient and (d) Wear loss of different current densities.

Figure 2 .
Figure 2. SEM image of (a) Ni-P and (b) Ni-P-PTFE nanocomposite coating of composite plating.The insets are photomicrographs of the contact angle of the water droplets on the corresponding surface after modification with stearic acid.

Figure 3 .
Figure 3. XRD of Ni-P coating and Ni-P-PTFE nanocomposite coatings.3.4.Performance of Nanocomposite Coating 3.4.1.Tribological Property.The COF (~ 1.5) of the nanocomposite film prepared under this condition is much lower than that of the Ni-P coating and Magnesium substrate, as shown in figure 4.

Figure 4 .
Figure 4. Comparison of friction coefficient and wear rate of the substrate, Ni-P, and Ni-P-PTFE nano-composite coating.

Table 1 .
Composition of bath and experimental conditions.