Mixed amorphous-nanocrystalline cobalt phosphorous by pulse plating

Co–P alloy coatings were electrodeposited from a Watts type base electrolyte using a galvanostatic pulse plating technique. Alloy deposits of P content as low as 1% were obtained operating at very high pulse current density and low duty cycle from an electrolyte with a small concentration of phosphorous acid, in the presence of a surfactant and a stress reliever. Alloy deposits with low P content – in the range 1–2% – had a peculiar mixed nanocrystalline-amorphous structure; a fully amorphous deposit was obtained when the P content was in excess of about 5–6%. A short and relatively low temperature annealing of the mixed structure triggered a strong enhancement of the hardness, in connection with the precipitation of a phosphide phase in a slightly regenerated nanocrystalline structure. The remarkable thermal stability of the transformed structure of the alloy was lost in conjunction with the on-set of the allotropic transformation from hcp to fcc-Co. A side effect of the hardening was the cracking of the layer and its obvious impact on the protective performance of the Co–P alloy deposit. Alloy deposits with low P content and a mixed nanocrystalline–amorphous structure hold promise as alternative to traditional electroplated hard coatings.