Preparation and Tribological Properties of NiAl-TiO₂/Bi₂O₃ Nanocomposite Coatings in Wide Temperature Range

NiAl-TiO₂/Bi₂O₃ nanocomposite coatings were prepared by atmospheric plasma spraying, where nanostructured TiO₂/Bi₂O₃ feedstock powders were prepared by high-energy ball milling and spray drying. The influence of nanostructured TiO₂/Bi₂O₃ with different mass ratios on microstructure, mechanical and tribological properties of composite coatings was investigated. The results show that all composite coatings possessed dense microstructure and uniform distribution of the phases. The coatings by using nanostrctured TiO₂/Bi₂O₃ composite powders performed the typical bimodal microstructure and displayed higher adhesive strength to substrate (>40 MPa). The composite coating with addition of monolithic TiO₂ feedstock powders displayed poor tribological properties at low/mid temperatures. The combination of TiO₂ and Bi₂O₃ improved plasticity and tribological properties of the coatings at low/mid temperatures, and further enhanced the lubricious behavior of composite coatings at high temperatures. For its composite coatings, the wear rate were below 7×10^-5 mm³/(N·m) in the whole test temperatures and the friction coefficient was about 0.1 at 800 ℃. With the Bi₂O₃ content increasing, the hardness of composite coating decreased and resulted in higher wear rate. The sliding process promoted the formation of Bi₄Ti₃O₁₂ and NiTiO₃ at 800 ℃, which formed a continuous and complete glaze layer together with TiO₂ and NiO on worn surfaces and resulted in excellent tribological properties of composite coatings at high temperature.