Effects of humidity on the sliding wear properties of Zn-Ni alloy coatings

Zn-Ni alloy coatings were developed in the 1980's as a replacement for pure zinc coatings as a sacrificial coating for steel in the automotive industry due to their better corrosion resistance and hydrogen embrittlement properties. More recently, the aerospace industry has taken an interest in these coatings because cadmium, the sacrificial coatings used for steel in the aerospace industry, is toxic and carcinogenic in nature. Although Zn-Ni coatings are mainly used for corrosion protection, an interest in their tribological properties has arisen due to their high hardness. In this study, tribological tests were performed with a reciprocating pin-on-flat tribometer using a steel countersphere on a commercially electrodeposited Zn-14 wt%-Ni coating under relative humidity levels of 0, 20, 40 and 60% with a normal load of 3.5 N. Worn surfaces, debris and transfer films were examined for their morphology, chemistry and microstructure using electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy. Wear rates were also determined using non-contact profilometry. Tribological behaviour of the coatings under the same contact conditions but different humidity was found to differ, as an increase in humidity corresponded to a more stable coefficient of friction and lower wear rate. This is related to the formation of ZnO on the surface of the wear track which acts as a lubricating layer when humidity is present.