Tribological characteristics of electroless Ni–P alloys as an alternative to chromium coatings: infometric analysis

Ni–P coatings, with their high microhardness (HV) and wear resistance, have been proposed as a viable alternative to hard chromium. However, wear testing experiments on electroless Ni–P coatings were conducted under a variety of conditions, including test equipment, load, substrate materials and counterface materials, resulting in unreliable comparative evaluations of experimental results. Therefore, this article presents an analysis of experimental research on Ni–P and Cr coatings, as well as a statistical analysis of the results obtained under similar conditions. The results of simultaneous wear tests on Ni–P and Cr coatings published from 1958 to 2022 were used for evaluation and comparative analysis. The relative wear of Ni–P versus hard Cr was calculated from experimental data and analyzed using statistical methods such as Tukey’s statistical methods, nonparametric and parametric statistics. This review found that the wear performance of electroless nickel coatings generally does not match that of hard chromium coatings. The results of this analysis indicate that: (i) Ni–P coating can be used as a protective and hardening coating; (ii) the wear resistance of the electroless Ni–P coating, being 1.9 ± 0.4 times less, does not reach the same level of wear resistance as that of hard Cr; (iii) there is a tendency for the wear resistance of Ni–P to gradually approach the wear resistance of Cr; (iv) the dependence of the wear rate on the phosphorus content of the Ni–P alloy indicates that the minimum possible wear rate is achieved at a P content of about 4–7 wt%. The search for effective alternatives to hard chromium plating is ongoing, and this review has identified several areas where effective engineering solutions could be found.