Preparation and characterizations of nickel-based composite coatings with self-lubricating property at elevated temperatures

A series of nickel-based self-lubricating composite coatings containing Ag or Ag-Mo as solid lubricants were prepared by atmospheric plasma spraying (APS). Phase composition, microstructure and mechanical properties of these coatings were characterized. Tribological properties were evaluated using a ball-on-disk high temperature tribometer from room temperature to 800 °C in air. The results showed that the addition of Ag in the composite coating could effectively minimize the mismatch between the coating and substrate, and also increase the mechanical strength of the coating. The addition of Ag or Ag-Mo as solid lubricants effectively reduced the friction coefficient of the coatings especially under a temperature above 400 °C, and elevated the wear resistance property in the whole test temperature ranges. Raman spectra analysis on the wear track has confirmed the formation of silver molybdate and nickel molybdate through tribo-chemistry reaction above 400 °C. Both silver molybdate and nickel molybdate provided synergistic lubrication effect at high temperatures, which decreased friction coefficients and wear rates of the composite coatings.