Microstructure, phase transition and tribological performances of Ni ₃Si-based self-lubricating composite coatings

Ni₃Si-based composite coatings with various contents of MoS ₂ and BaF₂/CaF₂ were prepared by hot-pressed sintering. The effects of MoS₂ and BaF₂/CaF₂ on mechanical and tribology properties of the coatings were evaluated in the temperature range of 25-800 °C, respectively. The results showed that MoS₂ decomposes to Mo₂S₃ during the fabricating process, and Ni₃₁Si₁₂ phase was formed when the hot-pressing temperature rising up to 1050 °C. The coatings had high friction coefficients below 200 °C because of the decomposition of MoS ₂. It is interesting to note that the Ni₃₁Si₁₂ phase in all of the coatings is fully transformed to Ni₃Si at 800 °C during the friction process. The coating containing 15 wt.% MoS ₂ and 10 wt.% BaF₂/CaF₂ exhibited the lowest friction coefficient when the temperature exceeded 400 °C. A glaze layer was formed on the worn surface at higher temperature. XRD results showed that the worn surface of the coating contained BaMoO₄, CaMoO₄, and NiMoO₄ at 800 °C, and these oxides act as effective lubricants at this temperature. The synergistic action of the oxides is responsible for the reduction of the friction.