**6.4. Ni–B composite coatings**

Co-deposition of nickel with boron leads to the formation of stable Ni3B or Ni2B phases which improve the hardness, thermal, and tribological properties of nickel coatings. Their exposure to heat treatment further enhances their hardness, resistance to wear degradation, and reduces friction coefficient. During heat treatment, grain coarsening is induced which weakens the hardness of the coatings. The mechanical weakening induced by grain growth is counteracted by the formation of hard and thermodynamically stable Ni3B particles, hardening the coatings [60, 61]. Increasing boron content favours the improvement of mechanical properties of the coatings, especially hardness and wear resistance [62]. However, Ni–B alloy coatings with high boron content possess corrosion resistance lower than that of the matrix (see Figure 6). Therefore, this effect of boron on the functional properties of the coatings renders alloys to be more suitable for applications where excellent mechanical properties are required.

**Figure 8.** Polarization curves of Ni and Ni–B alloy coatings [62]

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

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

ing speeds [55]

220 Electrodeposition of Composite Materials

The addition of diamond particles modifies the microstructural and hardness properties of Ni–B alloy but has no positive influence on the improvement of its corrosion resistance. However, the combined effects of TMAB (trimethylamine borane) and diamond nanoparticles have significant effect on Ni–B electrochemical behaviour even though it cannot improve the corrosion resistance of Ni matrix [8]. Sol-enhanced TiO2 nanoparticles yielded similar behav‐ iour with 12.5 ml/l of the particles increasing microhardness of Ni–B from 677 to 1061 HV. The improvement of hardness was also accompanied by reduction in friction coefficient and low volume wear loss [63]. The TiO2 sol leads to dispersion strengthening and fining of grains, hence the improvement in hardness and tribological behaviour. Therefore, the incorporation of second-phase particles in Ni–B alloy matrix has not proved to increase nickel coating corrosion resistance, but its mechanical and tribological properties.
