Electrodeposition of nickel-based composite coatings for tribological applications

Abstract

The purpose of this research is to study, evaluate, and compare the different electro deposition processes for producing nickel-based composite coatings for application in tribology, including: Ni/BMA luminescent coating, Ni-P/MoS2 and Ni-P/WS2 self-lubricating coatings.

In the first part, a new luminescent Ni coating containing an embedded, blue emitting rare-earth mixed metal oxide (BaMgAl11O17:Eu2+) BAM was electrodeposited successfully from an aqueous electrolyte. Two types of surfactants were utilised to investigate the effective co-deposition of these phosphors into the nickel matrix. The surfactants of non ionic PEG (polyethylene glycol) and cationic CTAB (cetyl trimethylammonium bromide) were observed to increase the phosphor contents in the deposit from zero to 4.6% and 11.5%, respectively. A mixture of these two surfactants produced the highest particle embedded coverage (15.6%). Systematic study of the hardness, corrosion, tribological and luminescent properties of the as-deposited coating was performed before using it for wear sensing.

In the second and third parts of this research, a self-lubricating Ni-P/MoS2 composite coating hasbeen designed and deposited on a mild steel substrate by electrodeposition. The effect of current density, electrolyte additive and MoS2 concentration on the structure of the coating have been investigated. The Ni-P/MoS2 coating exhibited a dramatic reduction in friction compared to an electrodeposited Ni-P coating, the coefficient of friction against a bearing steel ball in sliding wear being as low as 0.05. The worn surfaces and the wear debris were characterised by surface analysis techniques. The composite coating showed only slight wear and oxidation compared to the severe wear and oxidation observed in the pure Ni-P coating or the uncoated mild steel substrate. It has been speculated that MoS2 in the coating underwent shear stress-induced reorganization, fragmenting into fine crystallites and ultimately forming a smooth lubricating film.

In the final part, Ni-P/WS2 composite coatings have been electrodeposited from an aqueous bath containing suspension of WS2 nanoparticles (80 nm) and their tribological properties have been evaluated by a reciprocating test with bearing steel balls. Increasing the bath temperature did completely eliminate the cracks in coatings by reducing the internal stress and prolonged the duration of the low friction coefficient. By optimizing the combinations of the WS2 and CTAB concentrations in solution, the amount of the WS2 particles incorporated was greatly boosted, demonstrating a highest WS2 content of 4.8 wt%. Significantly, this Ni P/WS2 coating exhibited a greatly lower friction coefficient of 0.17 compared to single Ni-P (0.6).

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Identifiers

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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