Effects of additives on microstructure and properties of electrodeposited nanocrystalline Ni-Co alloy coatings of high cobalt content

Saccharin and 2-butin-1,4-diol (BD) were investigated as electrolyte additives to electrodeposit high quality nickel-cobalt alloys (78 +/- 2 at-%Co) as a protective surface coating on steel for tribological applications. The additives facilitated controlled electrodeposition of nanocrystalline coatings. The properties of the coatings investigated included surface morphology, grain size, crystalline texture and hardness. Tribological performance against a steel counterpart was studied via a reciprocating ball-on-disc apparatus. The coating microstrain could be manipulated from tensile to compressive and texture could be modified from (10 (1) over bar0) for hexagonal close packed (hcp) structure to (0002)(hcp)/(111) for face centred cubic (fcc) structure. The inhibition effect of absorbed species on electrodeposited nanocrystalline coatings is explained via analysis of grain size and texture. The coating from the bath with an optimised additive content had a high hardness (500 HV) due to its reduced grain size (11 +/- 1 nm) and improved tribological properties due to a high proportion of hcp structure.