Fretting Wear and Fretting Fatigue Properties of Electroless Nickel-Phosphorus Coatings

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Abstract
Fretting wear tests have been carried out on electroless coatings which have been plated onto low alloy steel substrates. A sphere on flat configuration was used for these experiments and the fretting amplitude was maintained at either 20 or 40 μm. Tests carried out in air at 20°C indicated that coatings containing 11–12% phosphorus in the as-deposited condition (500 HV) had lower wear rates than the underlying steel provided the coating thickness was >25μm. Heat treatment at 400°C, which raised the coating hardness to 1000HV, reduced the fretting wear rate at all thickness levels, 10, 25 and 40μm. Scanning electron microscope observations demonstrated that the wear process involved the formation of debris, its consolidation into a film and then cracking and exfoliation. Electroless nickel coatings in common with a number of coatings reduce the fatigue life of steel components by ~20%. The heat treatment of the coating to maximum hardness produces a further reduction in fatigue performance (~45%). The conjoint action of fretting and fatigue produces similar stress-cycle plots for coated and uncoated steels provided the coating is in the softer as-deposited form.