2021 Volume 62 Issue 6 Pages 726-730
Cu–Al2O3 composite coatings were prepared in the use of pulse jet electrodeposition technology. Pulse current parameters including duty cycle and frequency were researched about their effects on the microstructure variation and Al2O3 nanoparticles content of the composite coating. The mechanical properties and anti-corrosion resistance of the composite coating were examined. The results show that properly decreasing the duty cycle and raising the pulse frequency are able to generate a compact and fine nanocrystalline microstructure and increase the incorporation of Al2O3 nanoparticles in the composite coating, which is beneficial to strengthen the coating properties. With the optimal parameter combination as duty cycle 30% and pulse frequency 11000 Hz, the co-deposited Al2O3 content in the composite coating reaches a maximum of 14.4 at%, with the highest hardness of 308 HV and tensile strength of 755 MPa. The composite coating with optimized pulse parameters also displays improved corrosion resistance.
Nanocrystalline microstructure of the coating prepared at pulse duty cycle 30%.
Jet electrodeposition (JE) is a new cyclic electrodeposition technology developed in recent years.1–8) Compared with the conventional immersing-type electrodeposition, JE is simple, efficient, and economical.2–4) As shown in Fig. 1, during the process, the electrolyte always keeps circulating and enables the real-time replenishment and updating of metal ions. Thus, the ion concentration near the cathode can be maintained to enhance the current density and form a refined material microstructure. In addition, JE has a strong scouring effect on the cathode surface (see Fig. 1(a)), dispersing nanoparticles in the electrolyte to prevent agglomeration. Therefore, JE has been proved special value for nano-composite electrodeposition application. At the same time, in the last decade, pulse current has been widely employed in the field of electrodeposition because of its effect for grain refinement and deposition uniformity.9–16) From the perspective of deposition quality, if incorporating JE with pulse current, the combined technology is expected to play a synergistic role for further refining the coating structure and improving the deposition quality of nanocomposite coatings. Among a variety of composite coatings already applied in engineering, Cu–Al2O3 nano-coating have been proved well performance. However, present research mainly focuses on its fabrication using direct current (DC) electrodeposition or DC JE. The fabrication using pulse jet electrodeposition (PJE) has been rarely studied and the influence of pulse parameters including pulse frequency and duty cycle on the microstructure evolution, mechanical properties as well as corrosion resistance is not very clear, also hindering further PJE development.
Jet electrodeposition schematic diagram including (a) jet distribution and (b) scanning deposition process.
During pulsed electrodeposition, the duty cycle of the pulse current is an important parameter. For a period or cycle, it is defined as the ratio of the on-time of the cathodic pulse current to the off-time between pulses,5) as shown in eq. (1):
| \begin{equation} \textit{Duty cycle} = T_{\text{on}}/(T_{\text{on}} + T_{\text{off}}) \end{equation} | (1) |
In this study, different kinds of duty cycles and frequencies of the applied pulse current were used to prepare Cu–Al2O3 nano-composite coating. The microstructure characterization including microstructure morphology variation, incorporation of nanoparticles has been conducted. The mechanical properties and corrosion resistance were also tested.
A PJE experimental-setup was operated, which comprise a computer numerical control platform, a nozzle device, a set of electrolyte circulating and speed control system, and a pulse power supply (DASHUN, SMD-2000, China). During the electrodeposition, a jet of electrolyte is sprayed from the anode cavity to the substrate and scans on the substrate to generate a deposit coating (see Fig. 1(b)).
The substrate material was stainless steel 1Cr18Ni9 (C-0.15 Cr-18 Si–1 Ni-9 in mass%). The electrolyte configuration is as copper sulfate 250 g/L, sulfuric acid 50 g/L, electrolyte temperature 30°C and pH value 4. The PJE conditions were listed in Table 1 as follows.
The microstructure, morphology and element content of the composite coating were observed by scanning electron microscopy (SEM) LEO-1530, equipped with energy dispersive spectroscopy (EDS). The hardness was detected with a HV-1000 type Vickers hardness tester. The composite coating’s tensile strength was tested on a thin film testing machine (Sansi, China). The corrosion test was carried out in a tank containing 5 L 10% hydrochloric acid solution by mass loss method. The samples were cut into 20 mm × 10 mm × 10 mm by wire electrical discharge machining (WEDM). After degreasing and cleaning, each sample was suspended in a corrosive solution to achieve a full corrosion. The mass loss was weighed using an electronic balance with precision 10−4 g, and the average value of three samples was taken for each result.
In the use of different pulse current parameters, it is observed there are two kinds of typical microstructures on the deposit coating in Fig. 2, including dense nanocrystalline, corresponding to the light colour area, and micro-pores, corresponding the shadow area. With the decrease of the duty cycle of pulse current, it can be found that the morphology of the deposition coating changes gradually from a rough and porous state to a fined and dense state. Meanwhile, the area individually for the dense nanocrystalline and micro-pore are also changing, appearing that the porous structure area is decreasing, while the dense nanocrystalline is increasing. In comparison, the microstructure of deposit produced by using duty cycle of 30% is more compact and uniform than that produced using duty cycle of 80% and 10%. As shown in the enlarged view of the Fig. 2(b), the deposit coating contains a few nanocrystalline, mostly sized in the range of 30–50 nm size.
Effect of pulse duty cycle on the coating microstructures at (a) 80%, (b) 30%, and (c) 10% at (pulse frequency 11000 Hz).
Under the condition of low duty cycle, the turn-on time (ton) of pulse is shortened and the turn-off time (toff) is prolonged. In fact, in the toff period, the ions in the solution near the cathode surface will diffuse sufficiently, beneficial to reduce the concentration polarization and to obtain fine-grained deposit coating. However, if an over-low duty cycle applied, the pulse peak current density will increase, which will have adverse effects on the coating, such as nodulation and coarse grains. Meanwhile, too small duty cycle has a negative impact on deposition efficiency. Therefore, an appropriate duty cycle is of great importance in PJE of nanocomposite coating.
Figure 3 shows the deposit coating microstructure in the range of pulse frequency from 2000 Hz to 11000 Hz. When the pulse frequency is between 2000–8000 Hz, the coating microstructure shows no evident change, basically showing a porous state (Fig. 3(a), (b)). When the pulse frequency set as 8000 Hz, micropores are gradually decreased and when the frequency comes 11000 Hz, the deposit coating becomes highly dense and consolidated (Fig. 3(c)). It also can be seen from the magnified image (Fig. 3(c) and (d)) that the deposited coating is composed of small spherical nanocrystalline particles, among which a variety of grain size can be observed at different positions. The grains in the centre area appears larger, with the size between 30–50 nm while the grains at the edge of the hole (dark area in Fig. 3(d)) has small size between 20–40 nm.
Effect of pulse frequency on the coating microstructures at (a) 2000 Hz, (b) 8000 Hz, (c) 11000 Hz and (d) Magnified view for 11000 Hz at (duty cycle 30%).
From the above experimental phenomena, it can be found that with the increase of pulse frequency, the deposit coating is further refined and consolidated with decreasing pores and increasing nanocrystalline, which effectively promotes the deposit quality and forms a nanocrystalline structure. The reason lies that as the pulse frequency increases, the pulse width decreases, and the pulse current has a short duration of action during electrodeposition, beneficial to the formation of crystal nuclei and at the same time can play a role in hindering the growth of crystal grains.16–19)
3.2 Effect of pulse duty cycle and frequency on the Al2O3 content in composite coatingAs shown in Fig. 4(a), when the duty cycle drops from 80% to 30% at frequency 11000 Hz, the Al2O3 content rise from 11.41% (atom%) to 14.43%, and then decreases to 12.76% as the duty cycle drop to 10%. Figure 4(b) also shows how the frequency affect content of Al2O3 particles in the composite coating at duty cycle 30%. When the current frequency increases from 2000 Hz to 8000 Hz, the content of Al2O3 particles increases sharply from 13.37% to 14.39% and then after a slight increase it reaches to 14.43%.
Effect of pulse current parameters including (a) duty cycle at frequency 11000 Hz and (b) frequency at duty cycle 30%, on the content of Al2O3 in the composite coating.
It is analysed that if a small duty cycle is applied, it will increase the pulse peak current density and the cathodic over potential, limiting the diffusion rate of Cu2+ and the metallic ions reduction to the cathode. Since the reduction of metallic ions in the composite coating is lower than those Al2O3 particles entering the composite coating in the same time, the content of Al2O3 in the coatings will increases with the decrease of duty cycle. Nevertheless, when the duty cycle is excessively decreased, the Al2O3 nanoparticles near the cathode may not replenish in time, decreasing the content of Al2O3 in the composite coating.
3.3 Effect of pulse duty cycle and frequency on the mechanical properties of composite coatingFigure 5 shows the mechanical properties of the composite coating with the pulse parameters. It can be seen from Fig. 5(a), with frequency 11000 Hz and as the duty cycle reducing from 80%, the coating hardness gradually increases and attains the maximum value 308 HV as duty cycle reduced to 30%, which is 34% higher than the hardness 228 HV obtained as the duty cycle 80% applied. However, as the duty cycle continues decreasing, the hardness begins to slowly decrease and attains to 255 HV as the duty cycle reduced to 10%. Figure 5(a) also shows a positive correlation between hardness and pulse frequency. When the pulse frequency is continuously increased in the range of 2000–11000 Hz at duty cycle 30%, the hardness of the coating increases from 227 HV to 308 HV. In addition, the maximum hardness of composite coating 308 HV is also 14% higher than the pure copper coating harness 270 HV, which is derived with the same duty cycle of 30% and frequency of 11000 Hz.
Effect of pulse duty cycle and frequency (a) on the microhardness (b) on the tensile strength.
Figure 5(b) shows the effect of duty cycle and frequency on the tensile strength. The influence of pulse current parameters on tensile strength is basically consistent with that on hardness. When a pulse current with duty cycle 30% and frequency 11000 Hz is applied, a maximum of tensile strength can be obtained as 755 MPa, which is also 6% higher than the pure copper coating tensile strength 712 MPa. The above mechanical performance comparisons are consistent with the varying coating morphology and phase structure in Fig. 2 and Fig. 3. In fact, the change of duty cycle and frequency has been proved closely related with the structure variation and the incorporation of nanoparticles in the composite coating. Properly reducing duty cycle and increasing pulse frequency can effectively increase nucleation, refine the coating structure, and bring more nano-particles into the composite coating, eventually strengthening the properties of the composite coating. Additionally, the above experimental results proves that the addition of nano alumina particles as reinforcing phase also helps to improve the mechanical properties of the composite coating.
3.4 Effect of pulse duty cycle and frequency on the anti-corrosion resistanceFigure 6(a) shows the corrosion kinetics curves of the matrix material 1Cr18Ni9 and other PJE processed samples. As shown in the corrosion rate curve, in the first 10 hours of corrosion, the curve appears relatively steep, indicating that each sample is in a rapid corrosion state. After that, the curve gradually slows down as the corrosion rate become decreased. Through comparison, it can be found that the corrosion rate of the substrate material 1Cr18Ni9 is much more rapid than those PJE processed samples. In addition, among the PJE processed samples, the sample with duty cycle of 80% has the fastest corrosion rate and the sample with a pulse duty cycle of 30% has the lowest corrosion rate. It also can be observed from Fig. 6(b) that as the frequency increases, the corrosion rate for the corresponding samples gradually decreases.
Effect of (a) pulse duty cycle and (b) frequency on the anti-corrosion resistance.
From the curve’s comparison, it is clear that the PJE sample with a duty cycle of 80% has the lowest anti-corrosion resistance, because of its large surface porosity intensifying penetration of corrosive liquid, which is also consistent with the microstructural observation in Fig. 2(a). And along with the corrosion process, the pore size gradually increases, and a corrosion couple between the copper coating and 1Cr18Ni9 matrix forms, promoting the corrosion rate. In contrast, the PJE sample with a duty cycle of 30% shows a relatively good anti-corrosion resistance, due to less porosity and grain refinement, which has been validated by microstructural observation. It is the refined and compact microstructure that presents a barrier on the corrosive media the infiltration. In addition, a higher content of Al2O3 nanoparticles in composite coating enables excellent chemical stability and well corrosion resistance, and these compound effects eventually improves the corrosion resistance of the coating. Regarding the corrosion failure mechanism of the coating, many researchers believe that the corrosive medium enters the coating along the pores, causing the metal matrix to corrode and material peeling.19,20) Generally, the key to improving the erosion of the deposited coating is to reduce the number of holes as the penetration path of the corrosive medium.21,22) Appropriately reducing the porosity can improve the corrosion resistance of the coating. In fact, the optimization of the duty cycle and frequency of the pulse current is conducive to reduce porosity and forms a dense microstructure, which has been proved beneficial to inhibit the corrosion medium from entering the interior of the coating.
By using PJE method, a group of Cu–Al2O3 composite coatings were prepared. The results show that properly decreasing the duty cycle and increasing the pulse frequency can affect the microstructure evolution and increase the incorporation of Al2O3 in the composite coating. The experimental parameters were optimized as duty cycle 30% and pulse frequency 11000 Hz, by which the co-deposition of Al2O3 of the composite coating attains a maximum of 14.43 at%. The hardness and tensile strength of the composite coating respectively obtains their maximum at 308 HV and 755 MPa. The composite coating with optimized pulse parameters displays improved corrosion resistance.
This research is finally supported by the National Natural Science Foundation of China (51305178), Jiang Su Natural Science Foundation (BK20181473), Xuzhou science and technology innovation project (KC19128).
