High temperature friction and wear behaviour of Ni–P–Ag–Al2O3 hybrid nanocomposite coating

 For many years, friction and wear have been paid much attention as the most important challenges in sliding parts. A number of studies have been carried out on tribological properties of conventional and composite coatings at room and high temperatures in the past decades. Dellacorte has outlined in a wear study with Ag and BaF₂/CaF₂ lubricant additives that the usual solid lubricants such as graphite and MoS₂ are degraded above y350uC, whereas advanced solid lubricants, e.g. metal oxide (NiO, MoO₃), inorganic fluorides (LiF₂, CaF₂, BaF₂) and soft noble metals (Ag, Au, Pt) can be used at elevated temperatures due to the high stability and low shear strength of solid lubricants. Codeposition of silver and alumina particles in electroless Ni–P plating led to the formation of a self lubricating Ni–P–Ag–Al₂O₃ coating. Entrapment of silver in the Ni–P matrix could modify the lubrication and wear resistance of the Ni–P coating at high temperature. The Ni–P–Ag coating had the best lubrication and wear resistance at high temperature sliding wear. Silver particles could decrease the fluctuations of friction coefficient in composite coatings at elevated temperature. Tribological properties of the Ni–P–Ag–Al₂O₃ coating were similar to those of the Ni–P–Ag composite coating. Both coatings demonstrated a ‘chameleon’ behaviour at sliding contact conditions due to the formation of self–lubricating thin film from the silver.Ni–P–Ag–Al₂O₃ hybrid nancomposite coatings had a high wear resistance and lubrication, in addition to high hardness, and thus could be a suitable candidate to provide a combination of optimum hardness and tribological properties at high temperature.

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