Effect of MoO3 Content on Ni3Al-Ag-MoO3 Composite Coating Microstructure and Tribological Properties

In order to provide effective solid lubrication to Ni3Al coating, 10 wt.% Ag and different amounts of MoO3 solid lubricant were mechanically mixed with the SHSed Ni3Al powder and sprayed HVOF. Microstructure, mechanical properties, and tribological behavior from 25 °C to 800 °C of the coatings were studied, and the basic wear mechanisms were explored and discussed as well. Results show that the hardness and adhesive bonding strength of the coatings are slightly decreased, while there is little effect on the microstructure and mechanical properties of the Ni3Al-based composite coating when the content of MoO3 additive in the feedstock powder is lower than 15 wt.%. The composite coating formed by feedstock powder containing 15 wt.% MoO3 additive also presents excellent anti-friction and anti-wear performance from 25 °C to 800 °C, especially at 800 °C, where a complete, smooth, and thicker lubricating film comprised of NiO, Al2O3, MoO3, and Ag2MoO4 was formed, which reduced the friction coefficient (COF) and wear rate (WR) to the lowest value of 0.36 and 6.03 × 10−5 mm3/(Nm), respectively. An excessive amount of MoO3 in the feedstock powder (20 wt.%) results in inferior interlayer bonding of the formed coating, and the coating is more prone to delamination and abrasive wear above 200 °C.