Tribological Properties of Nickel-based Composite Coatings with the Addition of MoO₃-ZnO

Nickel-based composite coatings with the addition of oxides (MoO₃-ZnO) were prepared by atmospheric plasma spraying. The wear tests were carried out on an UMT-3 ball-on-disc high temperature tribometer at room temperature, 400, and 800 ℃, respectively. And the microstructures, phase compositions, and wear mechanisms were analyzed by SEM, EDS, XRD, and Raman. The tribological mechanisms on the view of oxidation and tribo-chemistry were determined by the microstructure and phase composition analysis of the worn surface of both composite coatings and corresponding counterpart balls. The XRD patterns of composite coatings showed the presence of α-ZnMoO₄ within composite coatings, indicating the solid reaction between MoO₃ and ZnO during the plasma spraying process. The XRD peaks of α-ZnMoO₄ in composite coatings were more intensive with the increasing content of MoO₃ and ZnO powders in feedstock powders. And the structure of MoO₃ and ZnO powders remained stable during the spraying process. The cross-sectional microstructure of composite coatings exhibited a lamellar structure with several pores and un-melted particles. The content of Mo and Zn element in the composite coatings increased with the increasing content of MoO₃ and ZnO powders in feedstock powders. The friction coefficients and wear rates of the composite coating were higher than that of the Ni–5%Al(mass fraction) metal matrix at room temperature and 400 ℃. Moreover, the lubrication and wear resistance of the composite coatings deteriorated with the increase of oxides content, the wear mechanism was mainly manifested as abrasive wear and adhesive wear. The high coefficient of friction at room temperature was related to the lack of effective lubricants in the coatings, whereas the added oxides were lubricious at high temperatures. The high wear rates of composite coatings at 400 ℃ was mainly due to the softening effect of materials and the wear before the formation of a stable oxide layer. At 800 ℃, the tribological properties can be effectively improved by the addition of MoO₃ and ZnO. In particular, the composite coating with the addition of 5% MoO₃ and 5% ZnO exhibited the lowest friction coefficient (0.28) and the wear rate 4.22×10⁻⁵ mm³/(N·m). And the wear rates of composite coatings increased with the increasing content of MoO₃ and ZnO powders in feedstock powders, which was related to the increasing peeling areas on the worn surface of the composite coatings. A protective tribo-layer was observed on both coatings and counterpart Al₂O₃ balls at 800 ℃. Furthermore, the elements of composite coatings (Mo and Zn) transferred to the worn surface of corresponding counterpart balls and were compacted to a relatively smooth film, which prevented the direct contact between the coatings and the Al₂O₃ ball, thus providing a low shearing interface and reduce the friction. The excellent high temperature tribological performance can be attributed to the synergistic effect of binary oxides (NiO, MoO₃, ZnO) and ternary oxides (ZnMoO₄ and NiMoO₄) on the worn surfaces of both composite coatings and counterpart Al₂O₃ balls.