**6.2. Ni–P composite coatings**

Ni–P alloy coatings exhibit refined microstructure, high hardness, and good corrosion resistance and high wear resistance under moderate conditions [50]. The good properties displayed by these coatings owe to the formation of stable phases such as Ni3P that leads to precipitation strengthening. However, their exposure to heat treatment conditions makes them brittle and thus rendering the coatings unfit for wear applications. The hardness of the alloy has also been reported to rise with increasing phosphorus content of up to 8 wt.% and decreases beyond that [51]. The incorporation of nano-carbon was reported to induce a positive potential shift of 60 mV to the Ni–P matrix by ref. [50]. The authors also found that the composite coatings possessed higher potentials and low current density after heat treatment than the non-heattreated Ni–P and Ni–P–C coatings. The formation of thermodynamic stable phases through the recrystallization of rich phosphorus Ni–P alloy is responsible for the super corrosion resistance exhibited by the heat-treated composites. The hardness property of Ni–P–Al2O3 was found to be improved when both P and Al2O3 content in the deposit were higher [51]. This result obtained by the authors show that phosphorus and second-phase particles content in the deposit have significant influence on the mechanical properties of Ni–P composite coatings. The addition of SiC particles in Ni–P matrix reduced the roughness of the coatings [52].
