Morphological and functional characterization of electroplated Ni-graphene composite coatings

Composite coatings (CECs) are providing a unique technological advantage for improving mechanical and tribological surface properties. Among different methods, electrodeposition is one of the most exploited to produce composite surface coatings on metal substrates. However, the process parameters affect graphene distribution, coating morphology and performance. This paper investigates how different deposition conditions influence the inclusion of large Graphene nanoplates (GnPs) in a Nickel matrix and the coating morphology and tribological performance. Set the process condition such as electrical parameters and the galvanic bath, the work focuses on the stirring rate effect. To this end, Ni-GnP coatings were obtained by a laboratory setup and evaluated through surface profilometry, SEM characterization and a dry-sliding linear reciprocating wear test. The results highlight the influence of stirring on coating uniformity. The low stirring rate allows larger particles to be embedded, which are not thoroughly covered; however, they act as a solid lubricant and reduce the friction coefficient.


Introduction
Graphene is a single-plane film of carbon atoms in sp2 hybrid orbitals, which shows low electrical and thermal resistivity but also outstanding strength [1][2][3]; due to its lightweight, mechanical strength, and chemical inertness, graphene is one of the most attractive materials for a composite coating [4].There are many graphene-related materials exploited as matrix reinforcement both for polymeric and metallic materials like, for example, fullerenes [5] carbon black [6], graphite [7], carbon nanotube, (CNT) [8] graphene oxide (GO) [9] and reduced graphene oxide (rGO) [10].
Recently, graphene has been applied in the galvanic deposition of Ni-Gr composite coatings; Szeptycka [11] increased the wear and abrasion resistance of pure Ni, while Chen [12] succeeded in improving a steel substrate hardness; also, its application led to improvement in the anti-corrosion of mild steel increasing the reduction potential to nobler value [13].Electroplating is widespread used in industries to produce metal layers since it is simple, stable and cheap as it is a very mature process, especially for Nickel [14][15][16][17]; the structure of the Gr-Me composite is expected to increase the metal performance as a function of the Gr-Me proportion and prevent the dislocation from moving through the metal lattice [18].
Among the graphene fillers available, GnPs have a low production cost, are easily storable and can be safely managed; therefore, it is precious to reinforce conventional structural engineering, enabling them with new functions [18].However, too large particles may affect their embedding within the metal structure and the ion reduction.Indeed, the coating's morphology and performance depend on process parameters and filler dimensions, which also affect the graphene flake distribution within the matrix [19,4].Many studies investigate the effect of different parameters such as bath composition, pH, surfactants, current density and bath temperature [20][21][22][23].The present paper aims to investigate the effect of the stirring rate on the co-deposition of Nickel and GnP; in particular, the GnP exploited is characterized by a large lateral dimension, which hinders the flake's inclusion in the metal matrix.After a preliminary literature review, a galvanic process was implemented to co-deposit Ni and GnP and two different bath stirring rates were investigated.The resultant coatings were tested to give a qualitative analysis of their surface in addition to tribological performance.The results show how the stirring deeply affects the coating morphology and performance.

Materials and Methods
The depositions were conducted on 38×30 cm2 steel substrates, which composition is given in Table 1.The galvanic solution used for the samples is the Watts bath, based on nickel sulphate and chloride, boric acid and surfactants to increase the dispersion of GnP in water.In the bath used for the nickelgraphene codeposition, on the other hand, an amount of GnP of 1 g/l was added.The experimental setup involves the realisation of the coatings downstream of an electrolytic degreasing and pickling preparation process.The first step of the process requires the deposition of a 5 µm pure nickel layer, following which a 10 µm nickel and graphene coating is applied.The galvanic co-deposition solution is sonicated for 40 minutes to ensure the absence of graphene clusters.In both cases, pure nickel electrodes are used as sacrificial anodes, while the substrate is cathodically polarised.The deposition occurs inside a 500 ml beaker, with the solution at 40°.Two scenarios were investigated, in which the coating is realised with the same current density but different stirring rates (Ni-GnP_low and Ni-GnP_high), to investigate the stirring effect on the co-deposition process.
Coatings were analysed using a Talysurf CLI 2000 3D profilometer, which enabled the acquisition of 3×3 mm2 three-dimensional maps with a resolution and profile spacing of 2 µm.Scratch tests were also carried out with a CSM micro-hardness tester using a Rockwell diamond drill with a 200 µm tip; the load is varied linearly up to 30N for a distance of 3 mm running at a speed of 1 mm/min.
Finally, dry sliding linear reciprocating tests were performed using a CSM tribometer through a 6 mm diameter steel ball as a counterpart; the test was performed with a sliding speed of 5 cm/s with a load of 1 N up to a sliding distance of 60 mm.

Results and discussion
The 3D maps in Figure 1 show that the morphology of the pure nickel film is smooth and homogeneous, in all respects comparable to that of the untreated steel substrate.In contrast, the deposition with low agitation presents an extremely inhomogeneous morphology; the map shows the presence of agglomerates on the surface with a homogeneous distribution in the investigated area.This morphology is in line with what is presented in the literature, for which the graphene presence in composite coatings is related to a high roughness due to the nano-particles present within the metal matrix.Higher stirring tends to reduce the presence of these clusters, generating a more compact surface, albeit one with less homogeneity than Ni and with some higher surface peaks.In particular, it can be noted that the component deposited with a low stirring rate shows large clusters spaced apart.By increasing the stirring rate, the coating, although presenting a porous structure, shows greater deposition homogeneity and smaller clusters.Indeed, graphene acts as a preferential site for nucleation by promoting the reduction of metal ions [17]; the combination of charge build-up on the asperities and the filler's high conductivity; therefore, it tends to form surface clusters with a size proportional to the reinforcement dimensions.By increasing the agitation of the bath, the increased centrifugal force pulls the heavier reinforcements away from the electric double layer and hinders the reduction of ions in solution; this combination then leads to the surface clusters reduction by diminishing the average particle size on the substrate and the ability to trap large particles.In fact, in the coatings produced with low stirring, exposed graphene particles can be seen, while with high stirring rates, they are more likely to be embedded within the nickel grains.
The scratch test results are shown in Figure 4.The pure nickel coating has a low penetration depth similar to the substrate.In addition, residual depth less than zero indicates the presence of a pile-up at the end of the track to confirm good adhesion.In contrast, functionalised coatings exhibit greater penetration due to their inherent porosity; in this sense, the more compact the film, the less penetration.The same applies to residual depth; in this case, however, the material is not accumulated at the end of the track.The Pd-Rd value, an elastic springback index, reaches the highest value for Ni and substrate; this behaviour indicates less damage to the sample, which recovers part of the imposed deformation.Instead, the low performance reported by composite films may be caused by the coating compaction and delamination.It is worth noting, however, that in the coating obtained with low mechanical agitation, the graphene particles manage to exert a lubricating action while maintaining a constant coefficient of friction regardless of the load exerted.
The scratch trails are represented in Figure 5.The as-built sample and the one deposited with Ni only show a compact trace with a slight pile-up at the end of the scratch.In contrast, one can see the removal of material from the trace sides for the sample with low agitation; the conical shape of the tip exerts shear stresses on the clusters, which are removed and induce a film delamination.Conversely, the samples achieved with high stirring suffers different effect; indeed, the coating is compacted by the action of the indenter or plastically deformed by moving the material to the sides of the trace, and the graphene particles cannot exert a lubricating action.
The wear tests in Figure 6 show similar results to those highlighted in the scratch test.The nondeposited sample appears to have the worst coefficient, hovering around 0.55.The nickel coating, although showing a peak at the beginning of the trace, settles and remains stable during the test at a value of 0.25.The composite coatings have opposite trends.In the case of high stirring, the increased agitation may reduce the quantity and size of the particles in the coating, thus reducing its tribological properties, which are worse than those of pure nickel; on the contrary, the large particles present with low mechanical agitation are able to interpose themselves between surface and counterpart.Once the film has been compacted, the compacted fragments and particles within the wear track act as a lubricant reservoir, allowing friction coefficients lower than the metal to be achieved and equal to about 0.15.

Conclusion
The article aims to present the effect of stirring on a nickel-graphene codeposition process using a laboratory setup.The results show how stirring the bath is able to push larger graphene particles away, resulting in a more homogeneous and less rough coating.In addition, at low agitation rates the graphene particles are partially uncovered as opposed to larger agitation levels.
The higher compactness results in better resistance to delamination, with the film compacting under the action of the indenter.However, the uncovered particles in the scenario with a low stirring rate act as a solid lubricant, promoting sliding and a low coefficient of friction.In contrast, the smaller particle size does not function as a lubricant.

Figure 1 .
Figure 1.Surface 3D maps of the different scenario.

Figure 2 .
Figure 2. Roughness parameter measured for the different samples.

Figure 3 .
Figure 3. SEM images at different magnification of pure Ni and Ni-GnP with different stirring rate.

Figure 6 .
Figure 6.Friction coefficient measured through dry sliding test.