In view of the excellent water lubrication performance of Si3N4, the composite coatings of Ni/Si3N4 with different Si3N4 particle contents were fabricated and the ultra-low friction coefficient of the coatings sliding against Si3N4 ball in water was expected. Experimental results manifested that only the Ni/Si3N4 composite coatings with a high particle content can achieve the ultra-low friction coefficient (0.008). The SEM and 3D profiler measurements indicated that the formation of consecutive and smooth contact interface is one of the essential conditions to achieve the ultra-low friction. Besides, the tribo-chemical product, silicon hydroxide, is another requirement. The major wear mechanism changes from the mechanically dominated wear during the running-in process to the tribo-chemical wear at the ultra-low frictional stage. Such result indicates that low friction is not a specific feature of Si3N4 and SiC self-mated pairs. This behavior of ceramic composite coating may develop a new approach for the low energy loss water lubrication systems.

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