LAUSR

To improve the service life of valve retainer in the engine exhaust system, the NiCoCrAlY alloy coating was fabricated on compacted graphite cast iron by laser cladding. Microstructure and high-temperature wear resistance of the coating were studied. The coating exhibits a refined dendritic structure. The M23C6 nano-precipitates with rod-shape are embedded the γ(Fe, Ni) solid solution dendrites, and the M7C3 nano-precipitates are distributed in the inter-dendrite region. The average nano-hardness of the coating and the substrate are measured as 4.08 GPa and 3.96 GPa, respectively. The wear morphologies and coefficient of friction variation reveal that the friction layer formed at 25°C and 600°C is a mechanical mixing layer and a composite layer, respectively. The coating possesses excellent wear resistance at 600°C due to the formation of a dense oxide film. The main component of the oxide film is identified as Fe2O3, Fe3O4, Al2O3, CrO2, CrO3 and Cr2O3.