Comparison of tribological performances of plasma sprayed YSZ, YSZ/Ag, YSZ/MoO3 and YSZ/Ag/MoO3 coatings from 25 to 800 °C

YSZ, YSZ/Ag, YSZ/MoO3 and YSZ/Ag/MoO3 coatings were prepared on Inconel 718 alloys substrates by the atmospheric plasma spraying process. Their microstructures, mechanical and tribological properties were compared. The results show that the presence of soft Ag and dual addition of Ag and MoO3 exhibit enhanced impact resistance of YSZ coating, nevertheless the inclusion of MoO3 conversely deteriorates this property of YSZ coating. As referenced with YSZ coating, all composite coatings exhibit improved tribological properties from 25 to 800 °C. The presence of Ag in YSZ matrix provides effective function on reducing friction and wear at ambient temperatures, surprisingly YSZ/MoO3 composite coating demonstrate worse wear due to its poor mechanical strength. In contrast, the dual dopants of Ag and MoO3 are in favor of outstanding tribological performance. Characterization of the worn surface of YSZ/Ag coating reveals the enriched Ag island exists being responsible for its better frictional performance at ambient temperature. A distinct tribo-layer covers on the wear surface of YSZ/Ag/MoO3 coating at 800 °C, primarily consisting of Ag2MoO4, which ultimately contributes to reduced friction and improved wear resistance. The formation of Ag2MoO4 and its working mechanism are proposed for the tribological characteristics of YSZ/Ag/MoO3 coating over 500 °C.