Microstructure, phase transition and tribological performances of Ni3Si-based self-lubricating composite coatings

Abstract

Ni3Si-based composite coatings with various contents of MoS2 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 MoS2. 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 15wt.% MoS2 and 10wt.% 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.