The friction and wear properties of nanocrystalline cobalt (nc Co) with a grain size of 20±5 nm and a hardness of 503±13 HV were studied using a pin-on-disc tribometer. Tests performed under unlubricated sliding conditions in ambient air showed that large tribolayer area covered the nc Co’s wear track. The oxygen concentration of the tribolayer was higher than that formed on contact surfaces of microcrystalline cobalt (mc Co) with a grain size of 16±3 μm and a hardness of 299±8 HV tested under the same conditions, due to the higher tendency of nc Co for oxidation. Higher rate of oxidational wear in nc Co resulted in higher initial surface damage in this material compared to the mc Co. Once the tribolayer was formed on top of the contact surfaces, a steady-state wear regime prevailed, reducing the coefficient of friction (COF) and the wear rate in this sample.

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