Low-pressure plasma-sprayed ZrO 2–CaF 2 composite coating for high temperature tribological applications

Abstract

The microstructure and tribological behavior of low-pressure plasma-sprayed (LPPS) ZrO 2–CaF 2 composite coatings were studied. Optimum spray parameters were obtained to produce a less porous and strongly adherent ZrO 2–CaF 2 composite through carefully selecting the powder feed rate, primary gas pressure and spraying distance. The as-sprayed composite coating exhibited a typical lamellar structure of ZrO 2 and CaF 2 constituents, with a lot of microcracks in the splats. The resolidified interfacial structure featured by fine columnar grains were observed at the boundaries of ZrO 2 lamellae and were considered to have formed due to the local temperature and compositional variations during the solidification process of the molten splats. Small amounts of discontinuous oxides distributed at the interface region between the coating and substrate were demonstrated to be a mixture of the complicated oxidized products of iron, chromium, nickel and calcium, ZrO 2(Y 2O 3) particles, and independent Al 2O 3 and SiO 2 particles located within the rough surface of the substrate. The ZrO 2–CaF 2 composite surface exhibited a distinct improvement in wear resistance and frictional characteristics in comparison to Y 2O 3-stabilized ZrO 2 (YPSZ) coating at elevated temperatures. At 600 and 700°C, the composite exhibited a lower friction and wear than at room temperature, 400 and 800°C. CaF 2, acting as a solid lubricant at 600°C, effectively reduces friction and wear. Different tribological behaviors were observed on the worn surfaces, with different microstructural features after the 600°C wear test. In the individual ZrO 2 splats, microcracking and microfracture dropping led to material removal. However, in CaF 2 splats smooth CaF 2 surface films containing fine ZrO 2 hard particles was formed to reduce the friction and wear. Brittle fracture and delamination of ZrO 2–CaF 2 composite were demonstrated to be the dominant wear mechanisms at room temperature and 400°C. Plastic deformation, the continuous formation of CaF 2 transfer films, adhesive wear and viscous flow appeared as the dominant wear mechanisms at the higher temperature used in this investigation.