Tribological Behavior under Wide Temperature Range Cyclic Conditions and Regeneration Mechanism of High and Low Temperature Lubricating Phase of NiAl-Bi₂O₃ Coating after Heat Treatment

The plasma sprayed NiAl-Bi₂O₃ composite coating was heat treated under the condition of argon atmosphere of 800 ℃. Then, the friction and wear properties of the resulting heat treated NiAl-Bi₂O₃ composite coating in the temperature range from room temperature to 800 ℃ were evaluated by UMT-3 high temperature friction tester. By analyzing the evolution of composition and microstructure of NiAl-Bi₂O₃ composite coating before and after heat treatment, and deeply exploring the frictional surface/interface during the process of friction test at different temperatures, the adaptive regeneration mechanism of high and low temperature lubricating phases (NiBi, Bi₂O₃ and NiO) within the heat-treated NiAl-Bi₂O₃ coating was studied for the first time. Meanwhile, the tribological behavior of the heat-treated NiAl-Bi₂O₃ coating under temperature cycle test conditions of wide temperature range was analyzed in depth. The experimental results showed that the condition of heat treatment of 800 ℃ under argon atmosphere was able to promote the formation of dispersion-enhanced Al₂O₃ phase and intermetallic compound NiBi phase within the heat-treated NiAl-Bi₂O₃ composite coating. It was proved that the intermetallic compound NiBi had medium and low temperature lubricity in this study. At the same time, NiAl-Bi₂O₃ composite coating produced a significant reduction in friction and wear from room temperature to 800 °C through the heat treatment process. Especially when test temperature reached 400 ℃, the friction coefficient and wear rate had a dramatic reduction, where the friction coefficient was reduced from 0.39 before heat treatment to 0.28 after heat treatment, and the wear rate was reduced by a full order of magnitude from 35.56×10⁻⁵ mm³/(N·m) before heat treatment to 8.53 × 10⁻⁵ mm³/(N·m) after heat treatment. When the test temperature rose to 800 °C, the contact surface of the heat-treated coating generated high-temperature lubricating phase (Bi₂O₃ and NiO) again through the high-temperature friction oxidation. And then solid lubricants (Bi₂O₃ and NiO) and reinforced phase Al₂O₃ adaptively formed a continuous lubricating tribo-layer on the worn surface during the sliding process, and this played a significant synergistic lubrication and anti-wear role. As a result, the lubricating performance and wear resistance capability of the heat-treated NiAl-Bi₂O₃ coating under the conditions of wide temperature range and high and low temperature cycle condition were significantly improved. It could be concluded that after the heat treatment, there was a mechanism of lubricating phase regeneration and continuous tribo-layer formation on the worn surface of NiAl-Bi₂O₃ composite coating during the friction process. This mechanism was expected to further improve the service life of metal matrix composite coatings in a wide temperature range. This study had important practical significance for the development of new technologies of solid lubrication and anti-wear materials.