For the purpose of environmental friendliness, low cost and energy saving, atmospheric plasma-sprayed chromium oxide (Cr2O3) coatings, due to its high hardness and excellent chemical stability and machinability, can be used as anti-wear surface on mechanical components, such as piston rings, to prolong their service life. However, plasma-sprayed Cr2O3 coatings usually exhibit great brittleness and high porosity and consequently poor wear resistance. To solve these problems, Cr2O3 composite coatings based on doping modification have been developed to improve their wear resistance, by reducing porosity or improving toughness, and so on. Metal-oxide and non-oxide ceramics with high hardness, such as TiO2, Al2O3, MoO3, TiC/TiB, TiN, are commonly used as additives to improve the wear resistance of Cr2O3 coatings. In addition, metals can be used as toughening agents to improve the mechanical properties of ceramic coatings. Indeed, Sadri et al. have prepared Cr2O3-based coatings directly doped with metallic silver, which has low melting point for toughening and reducing the coating porosity. The molybdenum (Mo) itself has relatively higher hardness among metals, also used as an additive in the Cr2O3 coatings to significantly improve the tribological performance of the coatings. Celik et al. has reported that the Cr2O3/Mo layers fabricated by atmospheric plasma spraying (APS) possess good interlayer cohesion and lubrication. However, there are few studies on the co-effect of metal/ceramic co-doping on the Cr2O3 coatings prepared by APS. It is of great significance to study the co-incorporation effects of metal and ceramic on the wear properties of plasma-sprayed Cr2O3 coatings. Different additives (metals, ceramics, and metal/ceramic assembles) were incorporated into chromium oxide (Cr2O3) coatings to improve its wear resistance. The doping or co-doping effects were compared in terms of the microstructure, phase composition, microhardness and sliding wear properties of plasma-sprayed Cr2O3-7wt.% Mo, Cr2O3-7wt.% Nb2O5, Cr2O3-4wt.% MoO3-3wt.% Mo, and Cr2O3-4wt.% Nb2O5-3wt.% Mo coatings. Under the low sliding loads, the composite coatings mated with WC-Co counterparts have lower friction coefficients than those against Si3N4, which are inverse with their microhardnesses, but more obvious fatigue wear characteristics. Under the high sliding loads, Mo/Nb2O5 co-doped Cr2O3 coating (CNM) has the best wear resistance than other coatings, due to the delaying co-effects of crack formation and propagation on basis of the crack deflection and the toughening effects of Mo additives and the high hardness of Nb2O5 additives. As the reciprocating sliding loads exceed the critical stress of brittle Cr2O3-based coatings, the coating detachments occur, displaying obvious fatigue wear characteristics.
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