Preparation and characterization of atmospheric plasma-sprayed NiCr/Cr3C2-BaF2•CaF2 composite coating

NiCr alloy-coated BaF2 center dot CaF2 eutectic and Cr3C2 powders were respectively prepared by both pressurized hydrogen reduction and solid state alloying technology. Using this NiCr/Cr3C2-BaF2 center dot CaF2 composite powder, a derived coating was produced by atmospheric plasma spray (APS) technology. Microstructures and phase compositions of the powders, as well as the deposited coating, were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The friction and wear behavior of the coatings from ambient temperature to 800 degrees C was evaluated using a ball-on-disk tribometer. The results show that the NiCr/Cr3C2-BaF2 center dot CaF2 composite coating exhibited low porosity, high microhardness and high cohesive strength, which result in good friction reduction and excellent anti-wear ability at elevated temperatures up to 800 degrees C. The friction coefficient of NiCr/Cr3C2-BaF2 center dot CaF2 coating decreases with increasing temperature. The wear rates of both coating and couple balls are significantly lower at temperatures above 500 degrees C than those tested at room temperature. From the investigation of worn surfaces, it was concluded that brittle fracture and delamination were the dominant wear mechanisms of the coatings at low temperature. A transition stage from brittle to plastic state with decreased shear strength was observed for BaF2 center dot CaF2 eutectic at high temperatures, which resulted in the formation of a continuous lubricating layer in the wear track above 500 degrees C The excellent mechanical properties of the coating were partially attributed to the protection of NiCr layer of the composite powders which decreased the oxidation. decarburization and ablation of Cr3C2-BaF2 center dot CaF2 during spraying. (C) 2009 Elsevier B.V. All rights reserved.