Study of tribological behavior of atmospheric plasma sprayed (Ni-5wt% Al)-Bi coating at room temperature to 800 °C

In this study, bismuth (Bi) was incorporated in a plasma sprayed Ni-5wt% Al coating. The results demonstrate that Bi reacts with ambient oxygen and the Ni matrix during spraying to produce δ-Bi2O3 and NiBi3. The lubricity of Bi and δ-Bi2O3 enables the coating to exhibit low friction at 25 °C. However, the soft properties of Bi, δ-Bi2O3, and NiBi3 reduce the hardness and low-temperature wear resistance of the coating. In particular, at 200 °C, the soft and brittle NiBi3 increases the wear rate of the coating. Above 400 °C, with the phase transition from NiBi3 to NiBi and intense tribo-oxidation reaction, the worn surface gradually forms a highly strong and tough tribo-layer, comprising NiBi, α-Bi2O3, δ-Bi2O3, NiO, Al2O3, and Bi24Al2O39. Consequently, friction and wear decrease as the temperature increases. By 800 °C, the coating achieves optimal friction reduction and exhibits an antiwear behavior. Moreover, the tribo-layer formed at 800 °C provides a continuous antiwear effect even when the temperature drops below 400 °C.