Influence of molybdenum composition in chromium oxide-based coatings on their tribological behavior

Plasma-sprayed Cr₂O₃-based coatings containing molybdenum were studied to gain a better understanding of the influence of molybdenum composition in the coatings on their tribological behavior. Cr₂O₃-Mo composite powders (Cr₂O₃-5 wt.% Mo and -20 wt.% Mo) 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 at 450 °C under dry sliding conditions. Measurement of friction coefficient was made in association with the sliding cycle. The physical characteristics of worn surfaces was investigated by scanning electron microscopy and chemical composition of the coating surfaces was analyzed using an X-ray photoelectron spectrometer and an X-ray diffractometer. The study showed that the friction coefficient of coatings containing molybdenum were lower than those composed of Cr₂O₃ alone at both test temperatures. The addition of molybdenum in the Cr₂O₃ coatings significantly improved the tribological performance of the coatings at 450 °C. Cr₂O₃-Mo composite coatings tested at 450 °C exhibited considerably lower friction coefficient than those tested at room temperature. Dispersed smooth films were formed in the worn surface for all coatings containing molybdenum. 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 plasma-sprayed coatings. The chemical composition of these films varied depending on the test temperature. XPS analysis of the smooth surface films indicated that MoO₃ composition was dominantly formed as Mo species. MoO₃ composition in the smooth film appears to be most favorable in reducing the friction.