High-temperature tribological characteristics and lubrication mechanisms of NiCrAlY-MoO3/CoO composite coatings: An experimental and DFT computation study

In effort to reveal the high-temperature lubrication mechanisms of ternary oxides containing Mo/Co/Ni, the tribological characteristics of atmospheric plasma sprayed (APS) NiCrAlY-MoO 3 /CoO composite coatings at elevated temperatures were investigated. The high-temperature lubrication mechanisms of Co/Ni-molybdate formed on worn surfaces were analyzed through experimental analysis and density functional theory (DFT) calculations. The results demonstrate that the coefficient of friction (COF) of NiCrAlY coating after 30 % MoO 3 /CoO added decreased from 0.43 to 0.30 and the wear rate reduced from 5.00 × 10 −5 to 8.32 × 10 −6 mm 3 N −1  m −1 at 800 °C. This is ascribed to the synergistic lubrication effects of the layer crystal structured ternary oxides CoMoO 4 and NiMoO 4 , which are self-generated through tribo-chemical reactions on the worn surface at elevated temperatures. DFT calculations show that the binding energies of Co O (−1.05 eV) and Ni O (−0.61 eV) in the detected ternary oxides CoMoO 4 and NiMoO 4 are weaker than those of the corresponding bonds in binary oxides CoO (−1.99 eV) and NiO (−1.44 eV), and have potential to break more easily at high temperatures. Such layered structures with easy-to-break bonds of the ternary oxides containing Mo/Co/Ni provides excellent lubrication properties in the composite coatings.