Microstructure and tribological properties of low-pressure plasma-sprayed ZrO2–CaF2–Ag2O composite coating at elevated temperature

Microstructure and tribological properties of low-pressure plasma-sprayed (LPPS) ZrO2–CaF2–Ag2O (ZFA) composite coating were studied by means of optical microscope, scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDXA), electron probe microanalyzer (EPMA) and high-temperature reciprocating wear tester. Coating selected for this investigation was 8 wt.% Y2O3 partially stabilized zirconia coating doped with CaF2 and Ag2O as the solid lubricants. The as-sprayed ZFA coating exhibits a typical lamellar structure of ZrO2 and CaF2 constituents, and some amounts of isolated Ag2O particles. ZFA coating possesses a distinct improvement in wear resistance and frictional characteristics as compared to yttria partially stabilized zirconia (YPSZ) coating at elevated temperatures. At 300–700°C, the ZFA coating exhibits lower friction and wear than at room temperature, 200 or 800°C. Ag2O and CaF2 acting as solid lubricants effectively at 300–400 and 600–700°C, respectively, reduce the friction of the composite. But with the increase of temperature up to 800°C, the severe adhesive sliding causes more material transfer and tearing-out of coating, and finally leads to a high friction and wear. The change or wear transition from a microcrack- or microfracture-controlled wear at room temperature and 200°C to plastic deformation- and adhesion-controlled wear at elevated temperatures is quite distinct and is accompanied by a great change in both friction coefficient and wear loss (such as those at 300 or 800°C).