**6.3. Ni–Co composite coatings**

improvement of functional properties depends not only on incorporated particles but also on

Refined microstructures exhibiting small and uniform grains are also characterized by high hardness and excellent wear resistance. The particles have pinning effect allowing little or no movement of dislocations. Co-deposition of nickel with SnO2 nanoparticles yielded coatings with high hardness, low friction, and better wear resistance [47]. The improvement of me‐ chanical properties was a function of the content of incorporated particles. Samples with the highest ceramic particle content exhibited the highest hardness values, lowest wear volume and friction coefficient. It can be seen from Table 5 that various incorporations yield different results. This is due to the fact that different ceramic materials exhibit their unique properties. Si3N4 particles have been reported to exhibit high hardness and self-lubricating properties; hence, their incorporation into a metallic matrix enhances its tribological characteristics [48]. Other particles that have been reported to possess self-lubricating properties include carbon nanotubes and molybdenum sulphide [45, 49]. The deposits that contain these particles exhibit

reduced coefficient of friction and high wear resistance as compared to the matrix.

**friction**

**Composite Wear loss mm3 Wear coefficient Average coefficient of**

different factors such as type of substrate and operating parameters.

**Figure 6.** Polarization curves of Ni and Ni–SiC nanocomposite coatings

218 Electrodeposition of Composite Materials

Ni–SiC 10.838 × 10–3 2.276 × 10–6 0.798 Ni–Si3N4 4.817 × 10–3 1.282 × 10–6 0.890 Ni–Al2O3 8.304 × 10–3 1.648 × 10–6 0.845

**Table 5.** Tribological properties of Ni–SiC, Ni–Si3N4, and Ni–Al2O3 composite coatings [46]

The evolution of surface morphology of Ni–Co alloy mainly depends on cobalt content in the coating. This inclusion of the metal induces the formation of fine and compact microstructure with globular crystallites [53]. The presence of cobalt in the nickel coatings enhances their corrosion and mechanical properties which are maintained even when the coatings are exposed to elevated temperatures [54]. The incorporation of nano-Si3N4 particles improved the microhardness and lowered the friction coefficient of Ni–Co matrix [48]. The increase in microhardness and reduction in friction coefficient were found to be dependent on the quantity of the reinforcement particles present in the coating. Higher Si3N4 content in the deposits gave yield to high microhardness and lower wear loss. Si3N4 particles possess self-lubricating properties and reduce the load bearing for the matrix. Variation of normal load and sliding speed has been reported to affect the tribological behaviour of Ni–Co–CNT composite coatings [55]. The friction coefficient decreased with increasing normal load and sliding speed. Carbon nanotubes form lubricious transfer layer on the wearing medium during sliding and thus reducing friction. This lubricious transfer layer forms as the result of wear debris that are generated during sliding and accumulate on the counterpart surface. The rate of transfer and accumulation of the lubricious layer is a function of temperature and rises due to the increasing normal load and sliding speed. The friction coefficients of Ni–Co alloy and Ni–CNT composite coatings are shown in Figure 7. Similar results have been obtained by [56]. Ni–Co–MWCNT films fabricated by DC, PC, and PRC electrodeposition techniques exhibited lower friction coefficient and wear rate at high sliding speed.

The inclusion of micro- and nano-sized SiC particles revealed that the presence of these particles in the deposits has a positive influence on the grain growth of Ni–Co matrix [57]. The micro- and nano-composite coatings exhibited small grain sizes and improved functional

**Figure 7.** Friction coefficient of (a) Ni–Co alloy and (b) Ni–Co–CNT composite coating under different loads and slid‐ ing speeds [55]

properties such as hardness and corrosion resistance. However, the improvement of properties depended also on the size of the particles with nanoparticles strengthened matrixes exhibiting the best enhancement. Incorporation of SiC nanoparticles into Ni–Co matrix also prevents erosion-enhanced corrosion in oils and slurry hydrotransport system [58]. The combined effect of the wear and corrosion resistance of the particles makes it difficult for the turbulence of the flowing slurry to erode the coating and thus minimizing the exposure of the active surface of the substrate to chemical attack. Ref. [59] studied the effect of fly ash on the corrosion resistance of Ni–Co matrix. The inclusion of the fly ash particles yielded deposits with high potentials and low corrosion current. The deposits also exhibited high hardness values as compared to the Ni–Co matrix, thus proving fly ash to possess better mechanical and electrochemical properties. While reinforcing of Ni–Co matrix with second-phase particles has a positive influence on the resultant deposits, the improvement of functional properties depends on the nature of reinforcement particles and process parameters.
