Low-pressure plasma-sprayed ZrO2-CaF2 composite coating for high temperature tribological applications

The microstructure and tribological behavior of low-pressure plasma-sprayed (LPPS) ZrO2-CaF2 composite coatings were studied. Optimum spray parameters were obtained to produce a less porous and strongly adherent ZrO2-CaF2 com posite through carefully selecting the powder feed rate, primary gas pressu re and spraying distance. The as-sprayed composite coating exhibited a typi cal lamellar structure of ZrO2 and CaF2 constituents, with a lot of microcr acks in the splats. The resolidified interfacial structure featured by fine columnar grains were observed at the boundaries of ZrO2 lamellae and were considered to have formed due to the local temperature and compositional va riations during the solidification process of the molten splats. Small amou nts 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, ZrO2(Y2O3) partic les, and independent Al2O3 and SiO2 particles located within the rough surf ace of the substrate. The ZrO2-CaF2 composite surface exhibited a distinct improvement in wear resistance and frictional characteristics in comparison to Y2O3-stabilized ZrO2 (YPSZ) coating at elevated temperatures. At 600 an d 700 degreesC, the composite exhibited a lower friction and wear than at r oom temperature, 400 and 800 degreesC. CaF2, acting as a solid lubricant at 600 degreesC, effectively reduces friction and wear. Different tribologica l behaviors were observed on the worn surfaces, with different microstructu ral features after the 600 degreesC wear test. In the individual ZrO2 splat s, microcracking and microfracture dropping led to material removal. Howeve r, in CaF2 splats smooth CaF2 surface films containing fine ZrO2 hard parti cles was formed to reduce the friction and wear. Brittle fracture and delam ination of ZrO2-CaF2 composite were demonstrated to be the dominant wear me chanisms at room temperature and 400 degreesC. Plastic deformation, the con tinuous formation of CaF2 transfer films, adhesive wear and viscous flow ap peared as the dominant wear mechanisms at the higher temperature used in th is investigation. (C) 2001 Elsevier Science B.V. All rights reserved.