Suman Kalyan Das

This study explores the influence of cobalt content on the tribomechanical and corrosion properties of Ni–Co–P coatings deposited through reverse pulse electroplating. The ratio of Co salt concentration to the total Ni and Co salt concentration was varied in the electrolyte to achieve different Co levels within relatively low cobalt (≤36 wt%) regime. X-ray diffraction revealed crystallite sizes of 1–3 nm in the deposited layers suggesting a mixture of amorphous and nanocrystalline structure. Critical loads in scratch adhesion tests are found to be in between 10.5 N and 15.1 N implying adequate adhesive bonding between the deposits and the substrates. Microhardness and pin-on-disc tribo test results demonstrate a positive correlation between Co content and both hardness and wear resistance of the coatings. However, incorporating small amounts of Co (≤6.2 wt%) exhibited minimal impact on these properties. Interestingly, corrosion resistance in saline medium peaked at the second-highest Co level (26.2 wt% of Co), followed by a decrease with further increase of Co. These findings indicate an optimal Co concentration for achieving a balance between wear resistance and corrosion protection in Ni–Co–P coatings.

The present research investigates the properties of duplex electrodeposited Ni-P/Ni-W-P coatings consisting of Ni-P and Ni-W-P as inner layers alternately. This study aims to examine how various heat treatment conditions affect the mechanical, tribological, and electrochemical properties of the coatings.The results show that the heat treatment temperature and duration have a profound influence on the coating’s microstructure, which significantly affects its overall performance. Optimal microhardness and lower wear rates are observed at 400 °C temperature for 1 h duration. However, at 800 °C for 4 h, duplex electrodeposited coatings display cracks and increased oxide layers, negatively impacting mechanical, tribological, and corrosion properties. The Ni-W-P outer layer coating exhibited higher hardness and a lower wear rate compared to the Ni-P outer layer coatings. However, the assessment of corrosion behavior shows that Ni-P offers increased corrosion resistance, largely due to its ability to form a passive layer, which is further enhanced after heat treatment. When compared to their individual single-layer coatings, the present duplex coatings demonstrate superior overall performance.

The present research evaluates the tribological performance of different Ni-P based duplex autocatalytic coatings (electroless coatings). The study investigates coatings including Ni-P/Ni-W-P, Ni-P/Ni-Mo-P, and Ni-P/Ni-Cu-P, examining various factors such as heat treatment conditions, sample composition, and their influence on the coatings’ performance. The phase structure of the coatings is influenced by heat treatment parameters viz. temperature and duration as well as phosphorus content. Notably, at temperatures above 400 °C, all duplex coatings form a robust nickel phosphide phase, enhancing hardness and wear resistance which is almost double of that in as-deposited state in some cases. When subjected to a temperature of 400 °C for duration of 1 h, the Ni-P/Ni-W-P coating displays the highest microhardness (1440 HV_0.1) and the lowest wear rate (10.62 × 10⁻⁵. mm³. N⁻¹. m⁻¹) among all the duplex coatings. In all three duplex systems, the wear typically involves a combination of abrasive and adhesive mechanisms. Among the as-deposited coatings, the Ni-P/Ni-Cu-P demonstrates higher corrosion potential (E _corr −302 mV) and lower corrosion current (i _corr 1.83 × 10⁻⁵ mA cm⁻²) which imply superior corrosion resistance compared to the other duplex coatings. In the heat-treated state, the Ni-P/Ni-Mo-P duplex coating generally exhibits better corrosion resistance when compared to the other two types of duplex coatings.