2020 Volume 61 Issue 4 Pages 795-800
Nanocrystalline copper coatings were jet electrodeposited using a square-wave pulse current with three duty cycles (30%, 50% and 70%) and a direct current condition. The effect of the duty cycle on the surface morphology, microstructure, grain growth and mechanical performance of the copper coatings was examined. The experimental results revealed that a decrease in the duty cycle significantly improved the coating surface morphology and microstructure. It was shown that a pulse current at a low duty cycle during jet electrodeposition effectively generated a nanocrystalline structure in the coating and improved the mechanical properties. At a low (30%) duty cycle during pulse current electrodeposition, coatings with a fine and smooth surface and dense microstructure were produced with a minimum grain size of 25 nm, microhardness of 2.37 GPa and tensile strength of 712 MPa.
Fig. 5 Coating microstructures at different conditions and duty cycles: (a) under DC and duty cycles of (b) 70%, (c) 50% and (d) 30%.
Nanomaterial fabrication is a suitable technique for producing nanostructured high-performance coatings. In recent decades, it has gained interest for the production of anticorrosion and protective coatings for automotive components.1–4) Researchers have paid substantial attention to finding economical and convenient methods to generate nanostructured coatings on a target material. However, the current nanomaterial processing technologies, such as laser alloying, plasma spraying and CVD, are generally costly, complicated, and require a long preparation process.5–7) Among high-performance coating techniques, jet electrodeposition is a newly developed and special electrodeposition technology.8–13) As shown in Fig. 1, as the electrolyte impinges on the cathode substrate in the form of a jet, a special flow field and electric field are generated and play a role in propelling metallic cations, commonly Cu2+ and Ni2+, that are travelling at high speed from the anode to the cathode.14–16) Compared with that produced by conventional electrodeposition, jet electrodeposition produces a nanocrystalline structure at a very high deposition rate, which positions jet electrodeposition as an efficient and effective nanocoating technology.
Diagram of jet electrodeposition process.
However, due to the edge effect,17–20) an uneven current is usually distributed on the jet electrodeposited surface. As the deposition process continues, the surface of the deposit gradually becomes coarse with growing nodules and defects. Since jet electrodeposition involves a high applied current density, the current is highly concentrated in the local deposition area, increasing the uneven current distribution and deteriorating the coating properties. In an effort to improve the deposit quality, new methods, such as pulse electrodeposition, are usually adopted to produce coatings with an improved performance. For instance, pulse electrodeposition of Ni coatings has been widely researched because it has been found to have an improved surface smoothness and reduced porosity with a high hardness and strength.21–25) During pulsed electrodeposition, the duty cycle of the pulse current (PC) 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, as shown in eq. (1) and Fig. 2:
| \begin{equation} \textit{Duty cycle} = T_{\text{on}}/(T_{\text{on}} + T_{\text{off}}) \end{equation} | (1) |
General pulse waveforms applied during electrodeposition.
In this work, jet electrodeposition was used to prepare nanocopper coatings with DC and PC conditions using different duty cycles. The influence of the pulse duty cycle on the deposit quality and coating performance was analysed to explore the correlations. The experimental results, including the surface morphology, cross-sectional microstructure, grain size, microhardness and tensile strength of the coatings as a function of the duty cycle, were examined.
As shown in Fig. 3(a), a jet electrodeposition apparatus was built to produce copper coatings. It fundamentally consisted of a computer controlled worktable, a spray nozzle and a pulse power supply (Dayu dual-pulse power supply, China). The nozzle scanned and sprayed the electrolyte on the substrate within a certain gap and for a number of times to form a deposit with a controllable thickness, as shown in Fig. 3(b). A copper rod was installed inside the nozzle cavity as an anode. The substrate comprised 304 stainless steel with dimensions of 80 mm × 20 mm × 2 mm. The electrolyte contained 250 g/l CuSO4·5H2O (Nanjing regent; 99% purities) and 50 g/l H2SO4 (Nanjing regent; 98% purities).
Experimental apparatus for the jet electrodeposition process.
The pH value of the electrolyte was maintained at 4 by adding dilute sulfuric acid or NaOH. Table 1 shows the experimental conditions. The electrodeposition lasted for 30 minutes. The duty cycle was varied during the electrodeposition process and was 30%, 50% and 70%. The frequency and average current density in the pulse used were 5000 Hz and 400 A/dm2, respectively. These parameters were optimized in previous experiments.
A scanning electron microscope (JSM5160LV Japan) was employed to examine the surface morphology, microstructure and grain size of the coatings. X-ray diffraction (XRD) spectrometry (Japan) was used to determine the phase composition and crystallite size. A Vickers hardness testing system (HVS-1000A, China) was used to measure the coating hardness. A universal mechanical tensile tester (Sansi China) was used to measure the tensile strength.
The effects of the duty cycle on the surface morphology of the deposited coatings are shown in Fig. 4(b)–(d), while Fig. 4(a) shows the coating prepared using a DC. The deposited surfaces have similar and different appearances. In Fig. 4(b)–(d), although different duty cycles were applied, the deposition surface appears flat and uniform. Compared with the particles in the coating prepared under DC conditions, the particles of the deposit surface are finer and evenly distributed. Additionally, as the duty cycle decreases, a fine and smooth surface morphology tends to be produced. The coating prepared with a 30% duty cycle is obviously finer, denser and smoother than that derived using a 70% duty cycle.
Surface morphology of the composite coatings under different conditions and duty cycles: (a) under DC and duty cycles of (b) 70%, (c) 50% and (d) 30%.
Figure 5 shows the effect of the duty cycle on the cross-sectional microstructure of the copper coatings. As shown in Fig. 5(a) for the coating prepared with DC conditions, the coating structure appears highly uneven and rough with large grains and pores. However, with a decrease in the pulse current duty cycle, as shown in Fig. 5(b)–(d), the coating microstructure gradually changes from coarse, porous and uneven in appearance to a fine and uniform microporous structure. In comparison, the microstructure of the coating with a duty cycle of 30% is much denser and more uniform than that using a duty cycle of 70%, and the pore area is obviously reduced, and the pores tend to be closed.
Coating microstructures at different conditions and duty cycles: (a) under DC and duty cycles of (b) 70%, (c) 50% and (d) 30%.
Magnified views of the coating microstructure are displayed in the upper right in Fig. 5(c) and Fig. 5(d). It can be observed that at a duty cycle of 50%, a large number of extremely small particles are arranged in a close and orderly manner in a honeycomb structure. The micropore size is mostly less than 1 µm with a particle size at the nanoscale. Additionally, at a duty cycle of 70%, the surface of the deposited layer is composed of nanocrystalline grains, most of which are between 30 and 50 nm.
The improvement in the surface morphology and microstructure of the deposited coating with a decrease in the duty cycle can be explained by the following interactions. At a given pulse period, if the duty cycle is reduced, the pulse interval time Toff is prolonged, and the supplementary period for the metal ion concentration in the pulse diffusion layer is also extended. Thus, the ion concentration near the cathode/solution interface area is fully recovered and enough ions are supplied to the reduction reaction area.9,22,26–28) Additionally, because there is no current for crystal growth during the pulse interval Toff, additional crystal nucleation occurs during the deposition process, refining and consolidating the deposited layer structure. On the other hand, as the cathodic current density is fixed, if the duty cycle decreases, the deposition rate also decreases and the deposition time is extended for coating depositions with a fixed thickness. Therefore, although reducing the pulse duty cycle has the effect of improving the deposition quality, it cannot be reduced without limitation in light its impact on the deposition rate. In addition, according to the experimental analysis, the duty cycle should not be decreased excessively, and 30% can be taken as an ideal parameter in the current experiment. If the duty cycle continues to be reduced, the deposition thickness per unit time is greatly decreased, and the deposition efficiency declines significantly because less material is deposited in the same processing period. Therefore, the necessary balance between the duty cycle and deposition efficiency needs to be fully considered.
Figure 6 displays the XRD pattern of the deposits with different duty cycles. The texture coefficient of each crystal surface is shown in Table 2. The peaks for the (111), (200) and (220) crystal planes are present, among which the (111) plane displays the strongest diffraction peak intensity, while the (220) plane has a relatively weak intensity. With an increase in the duty cycle, the intensity of the (111) plane increases at the initial stage and then weakens at the end, while the intensity of the (220) plane decreases at first and then increases. Overall, when the duty cycle is approximately 30%, the (111) crystal plane shows the lowest diffraction intensity compared with that of the other duty cycles herein. This leads to the experimental observation that for a duty cycle of 30%, the (111) crystal plane decreases in intensity, and all the crystal planes show balanced intensities in the diffraction pattern. This result is also consistent with the observation that the surface of the deposit is relatively uniform when the duty cycle is approximately 30%.
Preferred orientation in the composite coatings prepared with different duty cycles.
Figure 7 illustrates the effect of the duty cycle on the coating grain size. It can be found that in the duty cycle range from 30%–70%, as the duty cycle increases, the grain size of the coating increases gradually. The results also indicate that with a DC, the grain size increases to 60 nm, 129% larger than the minimum value of 25 nm at 30% duty cycle PC condition, and 67% larger than 33 nm at 70% duty cycle condition. This change not only indicates that a decrease in the duty cycle impacts the grain refinement but also confirms that the use of pulse square waves has a strong refining effect compared with the use of a DC.
Grain size of the deposited coatings with different duty cycles and conditions: (a) 30%, (b) 50%, and (c) 70% duty cycles and (d) DC condition.
Figure 8 shows the hardness and tensile strength test results for the coating produced under DC and PC conditions. In Fig. 8(a), with the DC condition, the deposit coating microhardness is only 1.73 GPa. In contrast, after application of a pulse current, as the duty cycle decreases from 70% to 30%, the microhardness increases from 2.12 GPa to the maximum value of 2.37 GPa, respectively, which is an increase of 11.8%. In Fig. 8(b), the tensile strength shows the same trend as the microhardness in Fig. 8(a). The maximum tensile strength of 712 MPa is still derived at a duty cycle of 30%.
Mechanical performance of the composite coating including (a) microhardness and (b) tensile strength.
Figure 9 displays four stress-strain curves for the specimen conducted under DC and PC conditions. The four curves below correspond to 30%, 50%, and 70% duty cycle conditions and DC condition. The top three curves correspond to pulse current conditions, which indicates that the maximum tensile strength is much higher than that obtained under the DC condition. When a 30% duty cycle is applied, the coating has the highest tensile strength herein, which is consistent with the results shown in Fig. 8(b). In addition, the curves of the coating prepared under pulse current conditions display typical plastic deformation and strain softening. In contrast, when the direct current is applied, there is no obvious plastic deformation stage, and fracture occurs when the stress reaches the maximum value.
Stress-strain curves of coatings produced at various duty cycles and under a DC condition.
The improvement in the microhardness and tensile strength is also consistent with the observed of microstructure and grain size in Fig. 5 and Fig. 7, respectively. This indicates that with a decrease in the duty cycle, the coating gradually becomes dense and uniform with refined grains, which improves the mechanical properties of the coating material. This means that with the adoption of a pulse current and a decrease in the duty cycle, the material strength and plasticity are also greatly improved. The results also show that the microhardness, tensile strength and yield strength of the coating prepared with a pulse current are better than those of the coating prepared by direct current.
A copper coating was prepared with the use of PC jet electrodeposition. A decrease in the duty cycle significantly improved the coating surface morphology and microstructure. The coating prepared with a 30% duty cycle had an optimum surface morphology and a dense microstructure and exhibited better deposition qualities than those prepared under a higher duty cycle pulse current and DC conditions.
A nanocrystalline copper coating prepared with the optimized parameters (duty cycle of 30%, current density of 400 A/dm2 and pulse frequency of 5000 Hz) attained a minimum grain size of 25 with a maximum microhardness of 2.37 GPa and a maximum tensile strength of 712 MPa. These improvements confirm that for the jet electrodeposition process, a pulse current at a low duty cycle effectively generates coatings with a nanocrystalline structure, fine grains and improved mechanical properties.
This work was supported by the National Natural Science Foundation of China (51305178), Jiang Su Natural Science Foundation (BK20181473), Xuzhou science and technology innovation project (KC19128), Topnotch Academic Programs Project of Jiangsu Higher Education Institutions (No. PPZY2015C251), and the Priority Discipline Construction Program of Jiangsu Province (No. 2016-9) supported this work.
