A study of electrodepositing Sn–SiC composite coatings with good welding performances

Abstract

Sn–SiC composite coatings with a good welding performance were electrodeposited in a bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ methyl sulfonic acid (MSA), 10 mg L⁻¹ OP-21, 30 g L⁻¹ SiC nano-particles and 50 mg L⁻¹ sodium dodecyl sulfate (SDS), at an inclination angle of the plating tank θ of 45°, a temperature of 20 °C and a cathode–current density of 3 A dm⁻² for 15 minutes. The incorporation of SiC nano-particles in the Sn deposits refines the crystalline grains. The solder spreading performance and Sn whiskers of the Sn–SiC coatings composited with 1.0–2.3 wt% SiC are better and smaller than that of the pure Sn coating, respectively. Only when the intercalation of SiC nano-particles into the Sn grain boundary is firm and homogeneous could coatings with a good welding performance be obtained.

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1. Introduction

1.1 The necessity of developing lead-free coatings

The rapid development of electronic packaging technology, the high power and increasing miniaturization of electronic components is increasing the demands on the properties of encapsulating materials. At the same time, the key to electronic packaging technology is to guarantee the reliability of solder joints.^1–3 After the joints between solderable coatings and solders, the migration of metal atoms and the formation of intermetallic compounds can reduce the reliability of solder joints because of the material itself.⁴ Traditional Sn–Pb alloy solder has excellent performance, but the toxicity of Pb restricts its application. Various countries have passed legislation to prohibit the application of hazardous materials containing Pb in microelectronics and other industrial fields. In the early 1990s, the USA introduced the famous “Reid” bill in order to prohibit and reduce the application of Pb.⁵ At the beginning of this decade, Japan was the first to start the lead-free electronic packaging process, stipulating that retailers had a duty to retrieve abandoned facilities from businesses and consumers, and promote lead-free process.⁶ Under the drive of lead-free process all over the world,⁷ China put “Measures for Administration of the Pollution Control of Electronic Information Products” into force on March 1, 2007. As a substitute for Sn–Pb alloy solder, eutectic Sn–Ag–Cu solder has a low melting point, good wettability, high solderability and other advantages, so it is widely used in electronic packaging.^8–10 However, there are no cheap lead-free coatings which have excellent properties to match the solder at present.

1.2 Comparison of the lead-free coatings developed

Sn-based binary alloys which have been developed, such as Sn–Ag, Sn–Cu, Sn–Zn, Sn–Bi and so on, usually have the deficiency of brittleness and are easily oxidized.^11–14 The quasi eutectic Sn–Ag–Cu ternary alloy coating has good performance and good compatibility with Sn–Ag–Cu solder, but its plating solution is complex, so it is hard for industrialization.¹⁵ Pure Sn coating is characterized by a low cost and simple producing process, and it has recently been paid attention to again. However, tin whiskers are easy to grow on a pure Sn coating, and the intermetallic compounds can easily form between a pure Sn coating and copper matrix, which results in the invalidation of the welding spot.^14,16–18 Therefore, a pure Sn coating can't fully meet the requirements of reliability. In addition, the melting point of the pure Sn coating is 231 °C, which is 50 °C higher than the traditional Sn–Pb alloy coating and 13 °C higher than the eutectic Sn–Ag–Cu alloy coating, which makes the brazing welding process more difficult.⁴

1.3 Metal matrix nano-composite coatings

One of the effective ways to improve the metallic material performance is by adjustment of the metal structure to extreme conditions. On the one hand, it is thought that the metal grain boundary is the weak link. In order to reduce or even eliminate the grain boundary, single crystal and amorphous alloys have been developed.^19,20 On the other hand, polycrystalline grains have been refined to the order of magnitude of nanometers (usually below 100 nm, typically a value at about 10 nm), along with the increase of the volume of grain boundary.^21–23 The surface nano-crystallization of the metallic material is expected to improve the structure inhomogeneity of the soldered joint including the weld joint, heat affected zone and basis material, and to increase the mechanical properties of the welded joint.

A metal–nanoparticle composite coating can be obtained by adding nanoparticles to the common plating solution and making the particles codeposit with the metal matrix, in which process the solution should be stirred all along. Nanoparticles have a small size effect, surface effect, quantum size effect and macroscopic quantum tunneling effect, which make the nano-composite coatings of the mesoscale show an excellent performance.^24–27 Szczygieł B. et al.²⁸ have prepared Ni–Al₂O₃ composite coatings by adding Al₂O₃ particles to be codeposited into a Watts bath. They found that the codeposition of Al₂O₃ particles with Ni disturbed the regular surface structure of the pure Ni coating and increased its microcrystallinity and surface roughness. The potentiodynamic tests showed that the corrosion resistance of the Ni–Al₂O₃ composite coating was better than that of the standard Ni coating. Vlasa A. et al.²⁹ investigated the electrodeposition of the Zn–TiO₂ nano-composite coatings on a steel substrate and their corrosion behavior. The results indicated that the composite coatings exhibited a higher corrosion resistance compared with pure Zn coatings and there was no linear dependence of their polarization resistance on TiO₂ concentration in the plating bath. Vaezi M. R. et al.³⁰ prepared Ni–SiC nano-composite coatings with different contents of SiC nano-particles. It was found that the morphology, micro-hardness and wear and corrosion resistance of the nano-composite coatings changed because of the codeposition of SiC nano-particles. The Ni–SiC nano-composite coatings with smaller crystalline grains had a higher micro-hardness and better wear resistance as compared to the Ni coating.

Preliminary evidence suggests that the codeposition of Al₂O₃ particles with a Sn–Ag–Cu alloy coating can restrain the growth of Sn whiskers effectively and improve the wettability of solder on its surface. This means that the nano-particles have development potential in increasing the solderability of coatings and the reliability of solder joints. The codeposition of nano-particulates will have an effect on the alloy coating's melting point, wettability, corrosion resistance and electrical conductivity. After the connection between solder and lead-free solderable coating containing nano-particles, the tensile strength, creep property and interface reaction of the welding spot will vary with the properties of the nano-particles.

1.4 Research content of the study

Nano-sized SiC particles were codeposited with Sn by electroplating from a methyl stannous sulfonate bath in the study. On the basis of the preliminarily optimized plating process, the dependence of the SiC nano-particles content in the deposits and surface morphology of the coatings on plating parameters, such as the SiC and SDS content in the plating bath, inclination angle of the plating tank θ, temperature of the plating bath and cathode–current density were studied, as well as their impacts on the welding performance of the coatings.

2. Experimental

2.1 Bath composition and process conditions

The basic bath was composed of 55 g L⁻¹ Sn(CH₃SO₃)₂ as the main salt, 160 g L⁻¹ MSA as the conductive solvent and coordination agent, and 10 mg L⁻¹ OP-21 as the emulsifying agent. Analytical reagents and distilled water were used to prepare the bath. The SiC nano-particles (99% purity, Kaier, Hefei, China) with an average size of 40 nm in diameter were added with a concentration in the range of 0–40 g L⁻¹ into the basic bath, as composite particles. The dispersing agent SDS was added into the basic bath, while its concentration was controlled in the range of 30–110 mg L⁻¹. Electrodeposition took place in the bath at the cathode–current density of 1–3 A dm⁻², bath temperature of 20–40 °C and plating tank's oblique angle θ of 30–90°.

In order to make the SiC nano-particles disperse evenly in the solution, the process of adding SiC into the bath was as follows: the weighing of a certain amount of SiC nano-particles into a beaker → adding a little of the basic plating solution prepared to wet the particles → adding SDS and stirring adequately for 30 minutes → transferring the above solution to the rest of the basic bath → allowing to settle → stirring for 30 minutes.

2.2 Experimental instruments

A plating tank with a certain angle of tilt was needed in this experiment, and the experimental set-up is shown in Fig. 1. A Sn plate (99% purity) of 75 mm × 45 mm was used as the anode, and a copper foil with a dimension of 20 mm × 20 mm or a copper sheet with a dimension of 30 mm × 25 mm was used as the cathode. The distance between the anode and the cathode was 50 mm. A WLS-1A digital constant current source produced by Puyang Scientific Instruments Institute supplied the energy for the electroplating. The stirring rate of the plating bath was controlled by a thermostatic magnetic mixer.

Fig. 1 Schematic diagram of the experimental set-up. Setting θ (30°–90°) as the inclination angle of the plating tank, i.e. the angle between the electrode and the horizontal plane.

The process of electrodeposition was as follows: affixing insulating tape on the reverse side of copper sheet (or copper foil) → burnishing → scrubbing with tap water → degreasing → scrubbing with tap water → scrubbing with distilled water → activating with 10% HCl (for 1–2 min) → scrubbing with tap water → scrubbing with distilled water → electroplating → scrubbing with tap water → scrubbing with distilled water → drying with cold air.

2.3 Performance measurement of the coatings

The surface morphologies of the coatings were examined by a FEI Quanta 200F scanning electron microscope (SEM). The contents of Si in the deposits were determined using the Kevex model energy dispersive X-ray spectroscopy (EDAX) system attached to the SEM, and they were converted to the quality percentage of the SiC nano-particles of the composite coatings.

The standard to compare the surface state of the coatings is shown in Table 1.

Table 1 Scoring criteria of the quality of appearance of the coatings

Appearance	Score
Black and rough	1–3
Dark and smooth	4–7
White and slippy	8–10

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The solder spreading experiment of the coatings on a copper matrix was carried out in a GZX-DHG electro-thermostatic blast oven. When the temperature reached 275 °C, the test blocks were put into the oven, on whose surface there was 0.2 g Sn–Ag–Cu lead-free solder, and the time was controlled by the spreading of solder on the Sn–SiC composite coatings. After the solder was spread out completely, the specimens were taken out and cooled to room temperature. Their spreading areas were measured and their appearances were observed to evaluate the wettability of the coatings.

Sn whisker somatotrophic experiments of the coatings were carried out in a GZX-DHG electro-thermostatic blast oven, and test blocks were placed in a sealed box, below which there was hot water. The temperature and humidity of the oven were set at 85 °C and 85 RH% respectively. After 400 hours, the surface morphologies of the test blocks were examined by SEM to determine if there was formation of Sn whiskers.

3. Results and discussion

3.1 Effect of SiC concentration in the plating bath

The electrolytic codeposition of the SiC particles with Sn was carried out using a plating bath prepared by a basic bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ MSA, 10 mg L⁻¹ OP-21, 0–40 g L⁻¹ SiC nano-particles and 50 mg L⁻¹ SDS. Fig. 2 shows the contents of the SiC nano-particles and the appearance score of the coatings obtained from the plating bath containing various SiC concentrations, where the plating process conditions included a cathode–current density of 3 A dm⁻² for 15 minutes, a bath temperature of 20 °C and θ of 45°.

Fig. 2 SiC nano-particle content and appearance score of the coatings obtained from the plating bath containing various SiC concentrations.

As shown in Fig. 2, the content of SiC nano-particles in the deposits increased at the initial stage and then decreased with increasing SiC concentration in the plating bath, and the quality of the appearance of the coatings deteriorated gradually. The quality percentage of the SiC nano-particles codeposited in the composite coatings reached a maxima of 2.31 wt%, when the concentration of SiC in the plating bath was 30 g L⁻¹. When the concentration of SiC nano-particles was lower than 30 g L⁻¹, the quality percentage of SiC in the composite coatings increased with the increase of SiC concentration, which may be due to the increase in the amount of SiC approaching the cathode surface with increasing the SiC concentration in the plating bath. Only those SiC nano-particles that clung to the cathode surface for a sufficient period of time might be codeposited into the Sn matrix successfully. Therefore, the incorporation of SiC particulates into the growing Sn matrix depended on both the rate of SiC particulates approaching the cathode surface and the rate of Sn deposition. Although the amount of SiC particulates approaching the cathode surface increased with increasing SiC concentration in the plating bath, the capacity of inserting SiC into the growing Sn matrix remained virtually the same. Consequently, this is the reason why the quality percentage of SiC reached its maximum in a bath containing 30 g L⁻¹ SiC particulates. A similar trend was reported for the codeposition of SiC particles with nickel by Lee et al.³¹

3.2 Effect of SDS concentration in the plating bath

The electrolytic codeposition of SiC particles with Sn was carried out using a plating bath prepared by the basic bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ MSA, 10 mg L⁻¹ OP-21, 30 g L⁻¹ SiC nano-particles and 30–110 mg L⁻¹ SDS. Fig. 3 shows the content of SiC nano-particles and appearance score of coatings obtained from the plating bath containing various SDS concentrations, where the plating process conditions were a cathode–current density of 3 A dm⁻² for 15 minutes, bath temperature of 20 °C and θ of 45°.

Fig. 3 Contents of SiC and appearance score of coatings obtained from the plating bath containing various SDS concentrations.

As can be seen in Fig. 3, the content of SiC in the coatings increased at the initial stage and then decreased with increasing SDS concentration. The maximum of 2.31 wt% was reached when the bath contained 50 mg L⁻¹ SDS. The quality of appearance of the coatings did not change with the variation of SDS concentrations, which indicated that SDS had little effect on the deposition of Sn. In this study SDS was used as a dispersant, and at the same time SDS itself is one kind of anionic surfactant.^32–34 The increase of SDS concentration in the plating bath, on the one hand, made the SiC nano-particles disperse homogeneously, and on the other hand, prevented SiC particulates from approaching the cathode. These two dimensions together affected the contents of the SiC deposits. When the concentration of SDS in the plating bath was 30–50 mg L⁻¹, with increasing the SDS concentration the dispersion of SiC nano-particles was more homogeneous, while the wettability of SDS displayed unobviously, so the contents of SiC in the coatings increased. When the concentration of SDS was above 50 mg L⁻¹, as the SDS concentration increased, the dispersity of SiC particles in the plating bath became better. But because the wettability of SDS made the number of SiC particles approaching the cathode surface decrease, eventually the amount of SiC incorporating with the Sn matrix decreased.

3.3 Effect of the inclination angle of the plating tank θ

The electrolytic codeposition of SiC particles with Sn was carried out using a plating bath prepared by the basic bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ MSA, 10 mg L⁻¹ OP-21, 30 g L⁻¹ SiC nano-particles and 50 mg L⁻¹ SDS. Fig. 4 shows the SiC nano-particles content and appearance score of the coatings obtained in the plating tank with various θ, where the plating process conditions were a cathode–current density of 3 A dm⁻² for 15 minutes, bath temperature of 20 °C and θ of 30–90°.

Fig. 4 Contents of SiC and appearance score of the coatings obtained in the plating tank with various θ.

As can be seen in Fig. 4, the contents of SiC in the coatings reduced and the quality of the appearance of the coatings became gradually better with increasing the θ. When θ was 90°, which meant that the cathode and anode were hung vertically, the incorporation of SiC nano-particles was very low. When θ was 30°, the obtained distribution of SiC nano-particles in the coatings was heterogeneous, having an obvious agglomeration phenomenon, and the quality of the appearance of the coating was poor. This was because at a lower θ, SiC nano-particles in the plating bath accumulated on the cathode surface after free sedimentation, and agitation could not make them disperse. Finally SiC particulates codeposited with the Sn matrix as aggregates. The content of SiC in the deposits at 45° was second only to the maxima at 30°, and the SiC distribution in coatings at 45° was more uniform than that at 30°. This trend is in agreement with earlier observations of studying the preparation of a diamond–metal composite film by Fang.³⁵

3.4 Effect of bath temperature

The electrolytic codeposition of SiC particles with Sn was carried out using a plating bath prepared by a basic bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ MSA, 10 mg L⁻¹ OP-21, 30 g L⁻¹ SiC nano-particles and 50 mg L⁻¹ SDS. Fig. 5 shows SEM images of a pure Sn coating deposited at 20°C and Sn–SiC nano-composite coatings deposited at different temperatures: 20 °C, 30 °C and 40 °C whilst maintaining the other plating process conditions: cathode–current density of 3 A dm⁻² for 15 minutes and a θ of 45°.

Fig. 5 SEM images of (a) a pure Sn coating deposited at 20 °C and Sn–SiC nano-composite coatings deposited at different temperatures of (b) 20 °C, (c) 30 °C and (d) 40 °C.

At bath temperatures of 20 °C, 30 °C and 40 °C, the contents of SiC nano-particles in the coatings were 2.31 wt%, 2.10 wt% and 2.21 wt%, and the appearance scores of the coatings were 7, 7 and 6, respectively. As the bath temperature varied, both the content of SiC and the appearance scores of the coatings had small differences.

As can be seen in Fig. 5, the incorporation of SiC nano-particles into the composite coatings changed the structure of the Sn matrix, which became more fine-grained. Comparing images of the composite coatings, conclusions can be drawn as follows. When the temperature of the bath was 20 °C, the obtained coating was densest. When the temperature of the bath was 30 °C, the grain size was smallest, and the content of SiC nano-particles in the coatings was lowest. The Sn grains of coatings obtained at 30 °C and 40 °C had different levels of refinement, with a lower smoothness and higher roughness. This may be because that with increasing the temperature the stability of the electrolyte became poor and the Sn²⁺ ions were apt to be oxidized, which could affect the deposition pattern of the Sn matrix, consequently coatings with different sizes of Sn grains were obtained.

3.5 Effect of the cathode–current density

Electrolytic codeposition of the SiC particles with Sn was carried out using a plating bath prepared by the basic bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ MSA, 10 mg L⁻¹ OP-21, 30 g L⁻¹ SiC nano-particles and 50 mg L⁻¹ SDS. Fig. 6 shows SEM images of the Sn–SiC composite coatings deposited at different cathode–current densities of 1 A dm⁻², 3 A dm⁻² and 5 A dm⁻² with maintaining the other plating process conditions: a bath temperature of 20 °C for 45 minutes, 15 minutes and 9 minutes respectively and θ of 45°.

Fig. 6 SEM images of the Sn–SiC composite coatings deposited at different cathode–current densities of (a) 1 A dm⁻², (b) 3 A dm⁻², (c) 5 A dm⁻² and (d) a partially enlarged view of (c).

The contents of SiC nano-particles in the coatings deposited at a cathode–current density of 1 A dm⁻², 3 A dm⁻² and 5 A dm⁻² were 2.23 wt%, 2.31 wt% and 1.46 wt%, and the appearance scores of the coatings were 9, 7 and 5, respectively. When the current density was 3 A dm⁻², the content of SiC in the coatings was higher and the quality of the appearance of the coatings was better, as compared to the coatings deposited at 1 A dm⁻² and 5 A dm⁻². The coating prepared at 5 A dm⁻² had the lowest SiC quality percentage and the worst surface appearance.

Fig. 6 shows that the cathode–current density had a great influence on the crystallization of Sn. The crystalline form of Sn was irregular and similarly oblong, with a smooth surface at 1 A dm⁻². At 3 A dm⁻², the crystalline form of Sn exhibited an inerratic tetragonum with a smooth surface. When the current density was 5 A dm⁻², the smoothness of the surface coating of the composite significantly reduced with many protuberances and scallops, which led to a large surface area. This should benefit the incorporation of the SiC nano-particles, but the EDAX test results failed to be consistent with the inference. This may be because on one hand the protuberances and scallops went against the attachment of SiC, and on the other hand, with a high deposition rate of Sn at a high current density the number of SiC nano-particles reaching the cathode and codepositing with the matrix was limited. So the codepositon of SiC in the coatings obtained at 5 A dm⁻² was low. As can be seen in Fig. 6d, the SiC particles incorporated in the Sn grain boundaries as aggregates, with a diameter of 1.5 μm approximately. This suggested that a high current density was adverse to the dispersion of SiC in the coatings. Composite coatings deposited at 1 A dm⁻² were compact, which could codeposite more SiC nano-particles; composite coatings deposited at 3 A dm⁻² were uniform and compact, which made for the incorporation of SiC; the surface of the coatings deposited at 5 A dm⁻² was black, which was mainly because that such a high current density led to the part charring of the coating.

3.6 Solder spreading performance of the composite coatings

The solder spreading experiment of the coatings deposited at different bath compositions and process conditions was performed. Fig. 7 was drawn by recording the spreading area of the solders on the coatings, which was associated with the content of SiC nano-particles in the corresponding coatings. This figure only considers the contents of SiC in the coatings, and does not take into account the experimental conditions of depositing the coatings.

Fig. 7 The variation of solder spreading areas on the coatings with the contents of SiC in the Sn–SiC composite coatings.

As can be seen in Fig. 7, at the initial stage, the solder spreading area on the coatings showed a small increase with increasing the contents of SiC nano-particles codeposited with the Sn matrix. When the content of SiC was higher than 1.0 wt%, with the increase of SiC embedded in the coatings, the spreading area changed little among a certain scope. The spreading area reached its maximum when the quality percentage of SiC was 2.31 wt%, above which the spreading area decreased. Composite coatings with a quality percentage of SiC between 0.3 wt% and 2.3 wt% had a better solder spreading performance than the pure Sn coating, and coatings with a content of SiC above 2.3 wt% had poor apparent conditions and solder spreading performance.

3.7 Sn whisker growth resistance of composite coatings

Exactly controlling the temperature and humidity of the bake oven, a Sn whisker growth experiment was conducted on the coatings deposited at different bath compositions and process conditions. Comparing the growth of tin whiskers on the coating's surface, SEM images of coatings with different contents of SiC are shown in Fig. 8. This figure only considered the contents of SiC in the coatings, with no account for the experimental conditions during the depositon of the coatings.

Fig. 8 SEM morphologies of Sn whisker growth on the surface of the coatings deposited at different conditions with the following wt% contents of SiC in the coatings. (a) 0, (b) 0.32, (c) 0.40, (d) 1.20, (e) 1.46, (f) 2.10, (g) 2.21, (h) 2.23, (i) 2.37.

Fig. 8a indicates that the pure Sn coating had a whisker of 70 μm in length and 10 μm in width, which was apt to cause a short circuit and was very dangerous in electronic packaging. The coating in Fig. 8b with 0.32 wt% SiC also had a big Sn whisker, with a length of 80 μm and a width of 30 μm. With the increase of contents of SiC in the coatings, the size of Sn whisker lessened. When the contents of SiC was between 1.0 wt% and 2.3 wt%, the size of Sn whiskers was smaller than that of pure Sn prominently.

4. Conclusions

According to the analysis above, the following conclusions can be drawn.

(1) The compositions of electrolyte and the process conditions had effects on the contents of SiC nano-particles and surface morphology of the coatings. With the increase of SiC concentration in the plating bath, the contents of SiC in the coatings increased at the initial stage and then decreased. The maximum was reached when the SiC concentration was 30 g L⁻¹, and the change of the morphology of the coatings was small. The concentration of SDS had little impact on the coating's apparent condition, and the content of SiC nano-particles in the coatings reached a maximum when the SDS concentration was 50 mg L⁻¹. The bigger the inclination angle of the plating tank θ was, the lesser the quality percentage of SiC and the better the quality of the appearance of the coatings was. At θ of 45°, the content of SiC in the coatings ranked only second to the maximum. Increasing the temperature made the Sn matrix finely crystalline, and the quality percentage of SiC was above 2 wt% for each coating in this group. However the quality of the appearance of the coatings was worse when the temperature was too high. The coating was smoothest and the content of SiC was highest when the cathode–current density was 3 A dm⁻², compared to 1 and 5 A dm⁻². A too high or too low current density could refine the Sn crystalline grain, but was not good for the incorporation of SiC nano-particles.

(2) The contents of SiC nano-particles in the coatings had impacts on the solder spreading performance and the Sn whisker growth resistance of the coatings. Composite coatings with a quality percentage of SiC between 0.3 wt% and 2.3 wt% had a better solder spreading performance than a pure Sn coating. When the content of SiC in the coatings was between 1.0 wt% and 2.3 wt%, the coatings had smaller Sn whiskers than that of the pure Sn coating. Coatings with a good welding performance could be obtained, only when the intercalation of SiC nano-particles into the Sn grain boundary was firm and homogeneous.

(3) Due to the synthesizing effects of each parameter, it is recommended to prepare the Sn–SiC composite coatings with excellent weldability by plating in a bath containing 55 g L⁻¹ Sn(CH₃SO₃)₂, 160 g L⁻¹ MSA, 10 mg L⁻¹ OP-21, 30 g L⁻¹ SiC nano-particles and 50 mg L⁻¹ SDS, at a θ of 45°, a temperature of 20 °C, and a cathode–current density of 3 A dm⁻².

Acknowledgements

This work is supported by the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology.

Notes and references

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