Microstructure and tribological properties of plasma-sprayed chromium oxide - molybdenum oxide compostie coatings

Plasma-sprayed composite coatings of Cr₂O₃-MoO₃ were studied to identify their microstructure and to understand the influence of molybdenum oxide composition in the coatings on their tribological behavior. Cr₂O₃-MoO₃ composite powders and Cr₂O₃ powders were fabricated using a spray-drying method and plasma-sprayed coatings of these powders were produced to evaluate their tribological performance. Wear tests were conducted with a reciprocating motion at room temperature and 450 °C under dry sliding condition. Measurement of friction coefficient was made in association with the sliding cycle. The physical characteristics of surfaces were investigated by scanning electron microscopy and chemical composition of the coating surfaces was analyzed using a X-ray photoelectron spectrometer and a transmission electron microscope (TEM). The study showed that the microhardness of coatings containing MoO₃ were higher than that composed of Cr₂O₃ alone. The result of TEM analysis indicated that chromium/molybdenum was precipitated in the grain and grain boundary. The friction coefficients of the MoO₃ -added coatings were lower than that without MoO₃ addition at both test temperatures. However, the addition of MoO₃ in the Cr₂O₃ coatings did not significantly improve the wear performance of the coatings at both test temperatures. Dispersed smooth films were formed in the worn surface for all coatings. These wear protecting layers, formed by plastic deformation of adhered and compacted debris particles to the surface, strongly influence the friction of the coatings as already observed by the authors with different plasmasprayed coatings. The chemical composition of these films varied depending on the test temperature.