Effects of Ni-5wt.% Al/Bi2O3 addition and heat treatment on mechanical and tribological properties of atmospheric plasma sprayed Al2O3 coating

To solve the problem of high friction and wear of the plasma sprayed Al2O3 coating in a wide temperature range, two Al2O3-based composite coatings, namely Al2O3-(NiAl-Bi2O3) coating and Al2O3-(NiAl-Bi2O3)-H coating (heat treatment), were prepared by involving Ni-5wt.% Al and Bi2O3, and applying a simple heat treatment. Compared to the Al2O3 coating, two Al2O3-based composite coatings attained increased toughness and adhesive strength. In particular, heat treatment endowed Al2O3-(NiAl-Bi2O3)-H coating with improved compositional homogeneity and more NiBi intermetallic compound and α-Al2O3 phases. In terms of tribological behavior, when subjected to rubbing against the counterpart Al2O3 ball, Bi2O3 and NiBi were shown to have good lubricating properties, and they exerted friction-reducing and anti-wear effects in both Al2O3-based composite coatings at 25 and 400 °C, respectively. At 800 °C, under the action of thermo-mechanical coupling, a tribo-oxide layer composed of multiple oxides (Al2O3, Bi2O3, NiO and Bi2Al4O9) was generated on the worn surface of the two composite coatings, achieving the excellent lubrication and wear-resistance.