The progress of formation of electroless Ni-B coatings is investigated on pure magnesium and AZ91D alloy. A non-chromate-based electroless treatment is employed that does not use prior chemical activation of the substrate. The surfaces of the substrates are examined at selected times of treatment using scanning electron microscopy, glow discharge optical emission spectroscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. Etching of the substrates develops an oxide layer on the surface of both substrates. The oxide is thicker on the magnesium than on the alloy. An enrichment of zinc between the bulk substrate and the oxide layer occurs in the case of the AZ91D alloy. Following the commencement of the electroless treatment, the amount of oxide on the substrates is reduced in comparison with the etched surfaces. The average rates of coating growth are similar on the two substrates, namely ~. 12 to 13. μm/h, although coating material is nucleated faster on the magnesium during the early stages of treatment. Wear tests reveal significantly improved wear resistance from application of the coatings, with comparatively small influences on the coefficient of friction.
- Magnesium alloys
- Sliding wear