Preparation of Sol-Enhanced Ni–P–Al2O3 Nanocomposite Coating by Electrodeposition

A Ni–P–(sol)Al2O3 coating was prepared on the surface of Q235 steel by direct-current electrodeposition. This method was combined with sol-gel and electrodeposition technique, instead of the traditional nanopowder dispersion, to prepare highly dispersible oxide nanoparticle-reinforced composites. The effects of temperature, pH value, and current density and Al2O3 sol on the hardness of composite coating were investigated. The coating surface morphology and structure were characterized by scanning electron microscopy and energy dispersive spectroscopy, respectively. The corrosion resistance of coatings in the presence of intermediate layers was evaluated by electrochemical measurement in 3.5% NaCl solution by open-circuit potential measurement at room temperature. The hardness and wear resistance of the coating were measured by a microindentation instrument and friction wear machine, respectively. The results showed that Al2O3 sol can effectively improve Ni–P alloy coating structure and refine grain. When the bath temperature was 55°C, the pH value was 4.5, the amount of sol was 80mL/L, the current density was 1A/dm, and the hardness of the nanosol coating was 569HV. Compared with Ni–P, the friction coefficient increases slightly, but the wear rate was only 1:768 × 10−6 g · m−1. The corrosion resistance was also better than that of Ni–P coating.


Introduction
It is a challenge for traditional nickel coating, with the rapid development of modern industry, to meet the special requirements in some harsh conditions. In recent years, how to improve the comprehensive performance of coatings has become a hotspot [1]. Direct current electrodeposition refers to the electrochemical deposition process of metals or alloys from their compound aqueous solution, nonaqueous solution or molten salt. It is the basis of the process of metal electrolytic smelting, electrolytic refining, electroplating, and electroforming. These processes are carried out under certain electrolytes and operating conditions. The difficulty of metal electrodeposition and the shape of the deposit are related to the properties of the deposited metal and also depend on the composition of the electrolyte, pH value, temperature, current density, and other factors. Ni-based alloys show several attractive properties, such as its high hardness [2], toughness, and relatively good corrosion resistance in air. For these reasons, they become the first choice of protective coating materials. However, the phosphorous nickel alloy coating obtained by electroless Ni-P plating is prone to pinholes and other defects on the surface, and the porosity and morphology will directly affect the corrosion resistance of the nickel-phosphorus alloy coating [3]. The corrosion resistance of the coating decreases regardless of whether the penetrating pore is formed or not. Therefore, the researchers tried to add an appropriate amount of inert particles to the plating solution and deposit them with Ni-P to get more excellent composite coating [4]. At present, nano-SiC [5], TiO 2 [6][7][8], WC [9], SiO 2 [10,11], PTFE [12,13], ZrO 2 [14], and Al 2 O 3 [15] are common solid particles that can be added. As a widely used ceramic material, alumina is not only of high hardness but also easy to be combined with matrix [16][17][18][19]. Nanocomposite coating has higher hardness, abrasion resistance, friction reduction, and corrosion resistance than ordinary electroplating. Balarju et al. [20] found that nanoparticles had no effect on the chemical structure of the composite coatings obtained by adding nanoalumina and significantly improved the hardness, corrosion resistance, and wear resistance. However, the so-called composite coatings are prone to some problems. Because nanoparticles have a high energy surface and activity, these nanoparticles are unstable and easily agglomerated in the nickel plating bath without special surface modification. It is difficult to provide enough time for nanoparticles to deposit on the surface of the substrate even if they are stirred at high speed and for a long time. Appropriate dispersants and stabilizers must be added. This issue directly affects homogeneous quality and weakens the mechanical properties of coatings. Alumina sol can effectively avoid the agglomeration of nanoparticles in the coating matrix [21][22][23] and is rapidly and uniformly dispersed in the plating solution, which is easy to be doped, and the required external conditions are easy to be realized. Alumina sol used in this project, the preparation, and properties of Ni-P-(sol)Al 2 O 3 composite coatings were studied.

Preparation of Nanocomposite Sol
Coating. The substrate specimen used in the experiment was a Q235 cold-rolled steel sheet with a size of 40 × 25 × 2 mm 3 . The substrate was polished with 400, 800, and 1000 grade SiC paper and then washed and rinsed with distilled water. Figure 1 shows the experimental setup of the electrodeposition of Ni-P-(sol)Al 2 O 3 nanocomposite coating. The applied current is provided by a high-frequency plating rectifier. The CS2350 electrochemical workstation is used to provide DC stabilized power supply. The HJ-5 constant temperature magnetic stirrer is used to control the bath temperature and magnetic stirring to ensure uniform solution and dispersion of nanoparticles. The plated part is placed in the middle of the two anode plates to achieve double-sided growth of the plating on the substrate. In addition, it is necessary to ensure that the two anode plates are parallel to the cathode substrate and the distance is kept at 25 mm to ensure the same plating quality on both sides of the plated part.
Workpieces are inevitably contaminated with oil during processing, storage, and transportation. The oil removal formula used in the experiment is shown in Table 1. Then, it is cleaned with deionized water and ultrasonic cleaning is carried out for 2 min.
Pickling is the process of removing oxide film, oxide scale, and rust on the metal surface after oil removal. Hydrochloric acid has strong solubility to metal oxides, slow dissolution to iron and steel matrix, clean surface after pickling, but its acid fog is big, which corrodes equipment. Sulphuric acid also has less corrosion to the matrix and less acid mist, but is prone to overcorrosion and hydrogen embrittlement. In this experiment, the mixture of 15 wt% nitric acid and 5 wt% phosphoric acid was used as a pickling solution.
The purpose of activation is to remove the very thin oxide layer on the surface of the matrix after pickling and to expose the matrix metal evenly so that the coating can grow uniformly on its surface. In this experiment, 5 wt% hydrochloric acid was used for activation. The activation time was 3 min, and the temperature was about 25°C at room temperature. After activation, ultrasonic cleaning was added for 1 minute, and then plating was carried out.  Composition and parameters of composite plating bath as shown in Table 2. Alumina sol is provided by the supplier of crystal Fire Technology Glass Co., Ltd. (http://www .jinghuoglass.cn). The alumina sol specifications were the alumina sol concentration was 20% and the average particle size was 60-70 nm. In order to ensure the quality of the coating, the distance between the two anodic pure nickel plates and the cathode substrate is 25 mm. Prior to the addition of this sol suspension into the bath, Ni-P coating was performed for ten minutes for better adhesion of coatings with the substrate. A reference specimen of plain Ni-P coating was also prepared for the comparative study. The plating bath was agitated using a magnetic stirrer at 180 rpm during the plating course.

Methods.
The Vickers microhardness measurements of the coatings were taken using a VMH-002 V microhardness tester at a load of 50 g for 15 s. The corresponding final values were reported as the average of five measurements. Friction and wear tests were carried out using ball-on-disk method by using a tribometer (MS-T3000 Instruments, China) at a normal load of 500 g rotation speed of 300 r/min under reciprocating sliding motion in a dry condition at the temperature 25-30°C and humidity 25% ± 10%. A bearing steel ball having a diameter of 3 mm was used as a counter sliding partner. The polarization curve of the anode was measured at room temperature using a CS2350 electrochemical workstation, and the corrosion resistance of the coating was evaluated by a Tafel curve. The coating was exposed to a scanning range of -2.0-1.0 V at a scanning rate of 1 mV/s in 3.5 wt% NaCl solution. The auxiliary electrode was a platinum electrode, and the reference electrode was a saturated calomel electrode. The surface and cross-section morphology and chemical composition of the coatings were analyzed by Quanta 200 scanning electron microscope (SEM) coupled with energydispersive X-ray spectroscope (EDS).

Effect of pH Value on Microhardness of Composite
Coating. Figure 2 shows the effect of bath pH value on Ni-P-(sol)Al 2 O 3 composite coating at current density 1A/dm 2 . The microhardness of sol composite coating first increases and then decreases with the increase of pH value. This is because the pH value has a critical impact on the nickel deposition process and the mechanical properties of the coating. Near the area with high H + concentration, the deposition rate is low due to the slow formation and growth of crystal nucleus, resulting in thin coating and no significant improvement in hardness. Similar situations have been reported in a similar pH bath [24,25]. Besides, it has been pointed out that the pH of the plating solution has an effect on the P content of the coating, that is, the hardness of the coating increases with the decrease of P content [26]. The increase of pH may decrease the P content in the coating. When the pH value of the plating solution reaches 4.5, the microhardness of the sol composite coating was 537 HV at the maximum. As the pH value continues to increase, the coating was mixed with nickel hydroxide and other impurities easily, resulting in rough coating brittle, hardness decline.

Effect of Temperature on Microhardness of Composite
Coating. Figure 3 shows the effect of bath temperature on Ni-P-(sol)Al 2 O 3 composite coating at current density 1A/dm 2 and pH value 4.5. With the increase of temperature, the microhardness of the sol composite coating increases firstly and then decreases. This is explained by an increase in the kinetic driving force with the increase in temperature can lead to a higher nucleation rate, i.e., formation of fine grains. These fine grains effectively inhibited its own growth, and the hardness of the coating increased accordingly [27]. As the temperature continues to increase, the thermodynamic driving force of crystallization decreases with an increase in the critical size of the nucleus resulting to lower nucleus densities, i.e., formation of coarse grain [28,29]. The formation of such coarse grains is likely to make the structure loose and the hardness of the coating decreased.

Journal of Nanomaterials
This discrepancy may occur due to the fact that an increase in the bath temperature has two contradictory effects on the thermodynamic and kinetic driving force [27]. Moreover, as the temperature is too high, the viscosity of the plating solution decreases, the adhesion force of the cathode surface decreases [30,31], and the content of Al 2 O 3 in the coating decreases, and the microhardness decreases. As a result, the plating temperature should be controlled at about 55°C.

Effect of Current Density on Microhardness of Composite
Coating. Figure 4 shows the relationship between the current density and the hardness of the composite coating when the plating temperature is 55°C and the pH value is 4.5. As the current density increases, the microhardness of the coating gradually increases. When the current density is 1 A/dm 2 , the microhardness reaches the maximum, and the current density increases further, the hardness of the coating decreases inversely. This is because, with the increase of current density, the current efficiency increases and the content of Al 2 O 3 sol in the sedimentary layer are increased per unit time. Too high current density will cause the rate of Al 2 O 3 sol embedding into the composite coating to be slower than the deposition rate of matrix metal, thus, reducing the content of Al 2 O 3 in the composite coating and decreasing the microhardness of the composite coating [15]. Therefore, the appropriate current density should be 1 A/dm 2 .

Effect of Al 2 O 3 Sol on Microhardness of Composite
Coating. Figure 5 shows the influence of Al 2 O 3 sol dosage on the composite coating when the current density is 1 A/dm 2 , pH value is 4.5, and the bath temperature is 55°C. It is clear that by adding more nanosol to the bath, the hardness increases to a maximum value of 569 HV for Ni-P-(sol)Al 2 O 3 and then decreases. On one hand, Al 2 O 3 nanoparticles can act as a barrier against plastic deformation and increase microhardness of the coating by preventing the movement of dislocations [32,33]. On the other hand, nanoparticles improved the grain refinement of the matrix and hence favor the higher microhardness of nanocomposite coating ( Figure 6) [11]. When the amount of sol is 80 ml/L, the microhardness is 569 HV; when the amount of sol in the plating solution is less than 80 ml/L, as the amount of sol increases, the amount of Al 2 O 3 particles deposited on the surface of the substrate increases, and the increased chance of being trapped into the coating, so that the coating hardness increases [17]; When the content of Al 2 O 3 sol in the plating solution is too much, some Al 2 O 3 particles may sink to the bottom without participating in the growth of coating, resulting in uneven deposition of Al 2 O 3 particles in the composite coating (Table 3). In addition, high alumina concentration reduces the reduction efficiency of matrix metal, thus, reducing the microhardness of composite coating, inconsistent with the literature report. [34]. It can find that when the dosage of Al 2 O 3 sol is 80 ml/L, the coating microhardness is better.

Microstructure and Composition Analysis of Coating.
According to the above results, the current density 1 A/dm 2 , pH value 4.5, bath temperature 55°C, and sol dosage 80 ml/L were selected in this study to prepare Ni-P-(sol)Al 2 O 3 composite coating. Figures 7(a) and 7(b) are the micromorphologies of Ni-P alloy coating and Ni-P-(sol)Al 2 O 3 nanocomposite coating, respectively. Figure 8 demonstrates that the Ni-P-(sol)Al 2 O 3 alloy coating deposited with these conditions has good quality and a uniform and smooth surface texture without porosity. Figure 9 is the   Journal of Nanomaterials    Figure 8. It is found that the composite coating contains 3.87% Al, 12.39% P, and 83.74% Ni, which proves the fact that nanoalumina particles in sol enter into the composite coating. As can be seen from Figure 7, pure Ni-P alloy coating has uneven grain size, slight microporous defects, coarse grain size, and uneven surface. The reason may be that in the early electroplating stage, the nickel ion concentration is high and the deposition rate on the substrate surface is fast. The nucleated crystals grow rapidly, and the older crystals prevent the later nucleated crystals from growing. The above results in uneven distribution of crystal size and more surface defects [15]. After the addition of Al 2 O 3 sol, the grains are refined, the size is uniform, and the deposition is relatively dense. Because the addition of Al 2 O 3 sol increases the cathode polarization in the composite electrodeposition process, leading to the reduction of the nuclear potential of the crystal, which facilitates the formation of a new crystal nucleus of Ni 2+ . Solutes such as dispersants are contained in the sol, which can inhibit the agglomeration and growth of metal grains. In addition, the incorporation of nanoalumina changes the crystal growth orientation and morphology of the substrate surface significantly. The use of nanoalumina particles as the center of nuclear formation has an inhibitory effect on Ni crystal growth [35]. The particle distribution changes gradually from a scattered distribution to a uniform distribution, and the degree of particle dispersion decreases from large to small, which makes the coating more compact.

Corrosion Resistance of Nanocomposite Sol Coating.
Tafel polarization diagrams for the Electroplate Ni-P and Ni-P-(sol)Al 2 O 3 coatings in the as-plated conditions in the 3.5% NaCl solution are shown in Figure 10. Corrosion parameters such as corrosion potential (Ecorr) and the corrosion current density (Icorr) after calculation based on diagrams are presented in Table 4. The obtained data demonstrate that the addition of alumina nanoparticles has resulted in the tendency of the corrosion potential of the Ni-P-(sol)Al 2 O 3 composite coating toward more diminished. Also, corrosion current density in the composite coating is lower than that of the Ni-P. This is because Al 2 O 3 particles are uniformly distributed in the coating, and the coating is relatively compact, reducing the intergranular corrosion in the coating [17,36]. Owing to the low conductivity 3.7. Wear Resistance of Composite Sol Coating. Figure 11 displays the variation of coefficient of friction of Ni-P, Ni-P-(sol)Al 2 O 3 nanocomposite coatings recorded during the wear test. Wear is a constant and undesirable gradual decrease of the material at the surface during contacting with other materials. The wear mechanisms of the Ni-P coatings are mainly abrasive and adhesive [37]. The coefficient of friction is initiated with the lower values in all the samples and reached maximum value within about two minutes of a sliding test. The friction coefficient of each sample increases rapidly at the initial stage of friction coefficient measurement until reaching an approximately stationary state at greater distance. The premature variation of coefficient of friction in a dry wear process is often referred to as "running" or "break-in" and can be attributed to the formation and rupture of surface oxide films or changes in the geometry of the contact surface [38]. The coefficient of friction varied with the sliding distance in the range 0.1-0.3, values that coincide with reports from the literature [39]. As the friction process proceeds, the surface of the specimen becomes smoother and the friction coefficient tends to be stable due to the plastic deformation of the microprotrusions on the friction contact surface. It can be seen from Table 5 that the friction factor of Ni-P-(sol)Al 2 O 3 nanocomposite coating is greater than that of Ni-P coating, while the wear rate of the composite coating is lower than that of Ni-P coating. This is explained by the addition of alumina particles increased the roughness of the coating, partially disappearing the nodular structure but keeping a homogeneous and uniform distribution of the reinforcement [40]. In the process of adding alumina sol to the plating solution, alumina particles enter the coating and microprotuberance occurs inside the coating. These microprotrusions cause the friction coefficient to increase. In addition, Al 2 O 3 particles have high hardness and wear resistance, which can support the friction surface load during the friction process, reduce the wear of matrix alloys, and resist plastic deformation. Therefore, the wear resistance of Ni-P-(sol)Al 2 O 3 composite coating is higher than that of Ni-P coating.

Conclusions
Ni-P-(sol)Al 2 O 3 composite coating has been produced by the sol-gel and electrodeposition technique. High dispersion nanoalumina particle reinforced composites were prepared. The hardness and corrosion resistance of the coatings prepared by different processing parameters were investigated. The best conditions for electrodeposition of Ni-P-(sol)Al 2 O 3 composite coating are pH 4.5, temperature 55°C, current density 1 A/dm 2 , and the dosage of Al 2 O 3 sol are about 80 ml/L. Ni-P-(sol)Al 2 O 3 composite coating has finer and more uniform grains, denser deposition, and higher hardness than pure Ni-P coating. The corrosion current densities of Ni-P coating and Ni-P-(sol)Al 2 O 3 coating are 5:57 × 10 -4 A/cm 2 and 4:18 × 10 -4 A/cm 2 , respectively. Corrosion resistance in 3.5%NaCl solution was improved compared with Ni-P   Ni-P -0.