All coatings, pure nickel, Ni-Al₂O₃, Ni-SiC and Ni-ZrO₂, were pulse electrodeposited with the expected thickness of ~10 µ m over steel substrate. The electrolyte composition consists of NiSO₄· 6H₂O (265 g/L), NiCl₂· 6H₂O (48 g/L) and H₃BO₃ (31 g/L). For composite coatings, the nanoparticles (~50 nm) with an amount of 20 g/L were ultrasonically dispersed in the electrolyte. The electrodeposition process parameters were kept constant as current density (3 A/dm²) and pulse on-off time (20-80 ms) with a duty cycle of 20%. A high-quality nickel sheet was used as an anode, and a steel circular disc of 80 mm diameter and 8.20 mm thick was used as a cathode.

Scanning electron microscopy (JSM-6010, JEOL) was used to analyse the microstructure of the deposits. The weight percentages of embedded particles were calculated by the energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) analysis was performed by using a D8 Advance (Bruker) with 2θ range of 20° -100° and with a step increment of 0.02° . For coating-substrate interfacial adhesion analysis, the specimens were embedded by using ATM OPAL 460 equipment as shown in Fig.1. The hardness and elastic modulus of the coatings were measured by using CSM microindentation tester (MHT) at a loading force of 300 mN.

Fig.1

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	Fig.1 a Subsections images (by using precision cutting method) of electrodeposited coated disc used for different tests, b 3D profile of the sample wear track after wear-resistant test

The wear resistance tests conducted by using a unidirectional ball-on-disc tribometer in distilled water at a rolling speed of 1 m/s and normal load of 45 N. The corresponding Hertzian contact pressure [32] can be estimated to be 1.75 GPa. The tribometer consists of an upper plated specimen and lower three balls (100Cr6) spaced at 120° . 3D profile of one sample’ s wear track is shown in Fig.1. The wear volume was calculated as V=AL, where A is the cross-sectional area of worn scar in mm² and L is the length of the worn scar in millmetres. A specific wear rate is determined as v=V/(Fl), where F is the load in N and l is the rolling distance in metres [33]. All wear tests were conducted at least two times to ensure repeatability and minimize uncertainty.

The electrochemical measurements were taken by using a system of three electrodes. The deposit part was used as a working electrode, and a platinum wire was used as counter electrode. The reference electrode was an Ag/AgCl. The electrochemical results were obtained at opening circuit potential (OCP) with scanning rate of 0.001 V/s.

The scratch tests were conducted DIN EN 1071-3 standard method by using a CSM REVETEST machine. The scratch was made with a length of 10 mm on each type of the coating with a sliding speed of 10 mm/min and increasing load from 0 to 100 N/min. The critical load value (L_c) is defined as the load value at which the first mode of failure was observed. The surface free energy was conducted using Owens and Wendt (OWRK) method considering water and diiodomethane as reference liquids of known surface energies [34].

Pure nickel and nickel composite coatings were electrodeposited under same pulse parameters exhibit shiny grey appearance, surface uniformity and good adherence to the mild steel substrate. The micrographs of pulse electrodeposited pure nickel and nickel composite of nanosized particles of alumina, silicon carbide and zirconium are shown in Fig.2. All the pulsed deposited coatings exhibit compact fine grain size. However, the grain size of co-deposited Ni-SiC exhibits the highest grain size when compared to the rest of the coatings which causes higher surface roughness parameters (R_a ~ 0.21 µ m; R_z ~ 1.38 µ m). The variation in average surface roughness (R_a) and the maximum height of the roughness (R_z ) values for deposited coatings are shown in Fig.3. The presence of agglomerated particles and pores on the Ni-ZrO₂ coatings resulted in higher surface roughness values (R_a ~ 0.20 µ m; R_z ~ 2.2 µ m). The surface roughness values were minimum (R_a ~ 0.05 µ m; R_z ~ 0.63 µ m) for Ni-alumina followed by pure nickel (R_a ~ 0.08 µ m; R_z ~ 0.73 µ m) due to finer surface morphology. The cross-sectional SEM micrographs of incorporating nanoparticle distributions of composite coatings are shown in Fig.4.

Fig.2

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	Fig.2 Surface morphologies of pulse electrodeposited nickel-based composite coatings: a pure nickel, b Ni-Al2O3, c Ni-SiC, d Ni-ZrO2

Fig.3

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	Fig.3 Surface roughness parameters (Ra and Rz ) of pure nickel and nickel composite coatings

Fig.4

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	Fig.4 Cross-sectional SEM micrographs of incorporated nanoparticle distributions in the nickel-based composite coatings: a Ni-Al2O3, b Ni-SiC, c Ni-ZrO2

The weight percentage of reinforcement content was calculated through energy-dispersive spectroscopy to be 2.5-3 for the nickel composite coatings. The microhardness and elastic modulus properties of the nickel and nickel composite coatings, electrodeposited under same pulse conditions, but with different types of incorporated particles are shown in Fig.5. There was no significant difference observed in the hardness values between all types of the coatings which contradicts the previous finding. Previously, it has been reported that the hardness of the composite coatings was significantly improved due to the strengthening effects of incorporated hard particles. However, the reinforcement content of hard particles in that study was (8-12) wt% [19]. Generally, the increase in the weight percentage of incorporated hard particles into nickel matrix results in enhanced mechanical properties. However, in practice the hardness to elastic modulus ratio (H/E) is an important parameter in predicting the better mechanical properties of materials [11]. Thus, lower hardness of Ni-Al₂O₃ (375HV) and Ni-ZrO₂ (405HV) composites than pure nickel coating can be attributed to lower plasticity index ratio (H/E) and porous surface morphology, respectively. As shown in Fig.5, the Ni-SiC coating exhibits the higher hardness to elastic modulus (H/E) ratio, which is an indication of good wear resistance property. This is because that H/E ratio is a better parameter for predicting wear resistance property than hardness alone [35].

Fig.5

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	Fig.5 Hardness and elastic modulus values of pure nickel and nickel composite coatings

The X-ray diffraction (XRD) patterns for pure nickel and nickel-based composite (Ni-Al₂O₃, Ni-SiC and Ni-ZrO₂) coatings are shown in Fig.6. For all the coatings, the XRD patterns show typical high peaks for (111) and (200) crystal planes of nickel and also evidenced by other researchers [7, 9, 11]. There are visible changes in orientation between the four samples; in particular, the changes affect the (311) reflection, which grows more intense for the Ni-alumina where the (002) grows less intense. For the Ni-SiC, there is a visible more intense orientation indicated by the (022) reflection. Note that due to very low reinforcement content percentage of nanoparticles, the corresponding peaks could not be resolved [19]. The crystalline grain sizes were calculated by using XRD patterns and observed less than 50 nm for all coatings: pure Ni=39 nm, Ni-alumina=33 nm, Ni-SiC=34 nm and Ni-ZrO₂=30 nm. The average grain size slightly reduced with the addition of the nanoparticles into a nickel matrix. Many studies have shown that the reinforcement of the nanosized particles into Ni matrix reduces the average grain size of Ni matrix [8, 10, 36].

Fig.6

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	Fig.6 X-ray diffraction patterns from electrodeposited pure nickel and nickel-based composite (Ni-Al2O3, Ni-SiC and Ni-ZrO2) coatings

The wear rates of pure nickel and nickel composite coatings in water-lubricated rolling contacts are shown in Fig.7. The Ni-SiC showed the minimum wear rate followed by Ni-Al₂O₃ in water-lubricated contacts. There was no significant difference in wear rates of Ni-ZrO₂ and pure nickel electrodeposited coatings. For all coatings, the two-body abrasive wear and plastic deformation were observed due to the higher hardness of counter steel balls than coatings as shown in Fig.1. However, relatively severe plastic deformation in a form of continuous wide grooves in the case of pure nickel and nickel-ZrO₂ was an indication of poor wear resistance property. Post-test surface examinations of worn wear tracks were further investigated through SEM and are shown in Fig.8a-d and magnified view in Fig.8e-h. Similar severe plastic deformation appearances of the pure nickel and Ni-ZrO₂ led to the smoothening of the surface as shown in Fig.8e, h, while, in the analysis of worn Ni-Al₂O₃ and Ni-SiC, the surfaces microploughing and delamination wear mechanism were observed, respectively, as shown in Fig.8f, g. It may relate to SiC and Al₂O₃ particles pulling out of the matrix and contribute to three-body abrasion wear [33]. However, lower wear rate in Ni-SiC and Ni-Al₂O₃ composites indicates that SiC and Al₂O₃ hard particles are restricting severe plastic deformation in worn surfaces.

Fig.7

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	Fig.7 Comparison of the wear rates of pure nickel and nickel composite coatings in water-lubricated rolling contacts

Fig.8

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	Fig.8 SEM micrographs of worn surfaces of pure nickel and nickel composite coatings in water-lubricated rolling contacts; overall a-c, magnified d-f views

The potentiodynamic curves for the steel substrate and electrodeposited coatings are shown in Fig.9. As can be seen, the corrosion resistance potential for deposited coating is significantly higher than the smooth substrate surface. In comparison, the Ni-Al₂O₃ exhibits the higher corrosion resistance potential followed by pure nickel. This can be attributed to finer grain structure with the addition of Al₂O₃ particles and validates the previous findings [37, 38]. It is interesting to observe that the corrosion potential of Ni-SiC and Ni-ZrO₂ is lower than pure nickel and exhibits similar behaviour. The lower corrosion potential behaviour in nickel composite coatings incorporating SiC and ZrO₂ can be because of higher surface roughness (R_a) and maximum height of the roughness (R_z ) parameter values. The roughness valleys in these coatings can provide weak points for penetration of aggressive corrosive solution towards substrate and consequently cause lower pitting potential.

Fig.9

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	Fig.9 Potentiodynamic polarization curves for bare substrate, pure nickel and nickel composite of different nanoparticles in 3.5% NaCl solution

The adhesion performance of all coatings was investigated to evaluate their suitability in industrial application. The adhesion critical load values (L_c) measured in scratch tests for pure nickel and nickel composite coatings are compared in Fig.10. All coatings except Ni-ZrO₂ showed adhesion strength above the critical load of 30 N which is generally required as a minimum critical load value for industrial applications. Nickel composite of alumina nanoparticles showed the maximum L_c value of 79 N and lowest L_c value of 28 N and was observed in Ni-ZrO₂ composite coatings. Note that these coatings were deposited on mild steel substrate without any intermediate coating which can further improve adhesion strength. Coatings failure mechanism in scratch testing was observed through microscopy and is shown in Fig.11.

Fig.10

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	Fig.10 Critical loads for different types of nickel and nickel composite coatings according to DIN EN 1071-3

Fig.11

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	Fig.11 Optical microscopic examinations for failure mechanism according to DIN EN 1071-3 for nickel and nickel composite coatings

Figure 12 shows the polar and dispersive component of the surface free energy and water contact angle measurement results of all coatings. In all coatings, the higher dispersive component share reveals that the coatings mainly possess covalent bonds. Their disperse shares vary from 29 to 36 mN/m, and polar shares vary from 2 to 16 mN/m. It is noticeable that no significant differences were observed in wettability behaviour between pure nickel and nickel composite coating. However, the polar component of Ni-Al₂O₃ composite coating was remarkably reduced by addition of nanosized alumina particles. The reduced polar component in nickel alumina composite coatings results into the higher contact angle of water as compared to the rest of the coatings. The surfaces become less hydrophilic with reducing polar shares due to reduction in dipole interactions [39]. Furthermore, it is noticeable that lower surface energy of Ni-Al₂O₃ coating exhibits the highest adhesion to the substrate which is contrasting to previous investigations [31]. Their studies concluded that the high adhesion can be achieved with increasing high surface energies. However, they used PVD method for coating development which may be the reason for the present contradiction.

Fig.12

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	Fig.12 The comparison of the surface energy and changing wettability behaviour of the pure nickel and nickel composite coatings