Fretting wear behavior of Zn–Ni alloy coatings

Abstract

Cadmium coatings are used in the aerospace industry primarily as a corrosion resistant plating, but also in applications where tribological properties are also important. Due to the carcinogenic nature of Cd, many coatings have been proposed as replacements, with Zn–Ni being the leading candidate. In this study, we examine two Zn–Ni coatings with differences in their surface roughness and their content of through thickness defects, including pores and cracks. The morphological differences between the coatings had a noticeable effect on their fretting wear behavior.A customized tribometer, with a reciprocating rounded pin (AISI 440C steel) on flat (Zn–Ni) geometry, was used to perform fretting wear tests. The two morphologically different Zn–Ni coatings were tested at room temperature using ±70, 100 and 150μm displacements and 133N and 447N constant normal loads. The surface of the wear scar was analyzed using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy for changes in morphology and chemistry. Wear scar depth was measured from surface profiles obtained using confocal microscopy.Hysteresis fretting loops of the tests showed that for both coatings, at ±70μm displacement remains in no slip condition, at ±100μm in the mixed slip condition, and at ±150μm displacement remains in gross slip condition. Although the coatings had similar stick–slip behavior, the smoother coating has a slower progression of wear from the no slip to gross slip conditions than the rougher coatings. Also, differences in the wear scar morphologies are attributed to the differences in the coating morphologies, which resulted in different wear and velocity accommodation mechanisms.