Exploration of wear and slurry erosion mechanisms of laser clad CoCrFeNi + x (NbC) high entropy alloys composite coatings

Wear and slurry erosion are common problems in the operation of underwater components. Preventive maintenance can extend the service life of underwater components and increase the working efficiency. CoCrFeNi + x (NbC) (x = 5 wt%, 10 wt% and 15 wt%) high entropy alloys (HEAs) composite coatings were prepared on the surface of 316 stainless steel (SS) commonly acted as underwater components combined with laser cladding (LC). The microstructure, wear and slurry erosion behaviors of the coatings were investigated. During the solidification, NbC could serve as the nucleation site for non-uniform nucleation and play a pinning role, promoting the formation of fine equiaxed grains and refining the grains of the composite coatings greatly. The average grain size decreased from 52.19 µm of Nb-free coating to 12.85 µm of CoCrFeNi + 15 wt% NbC composite coating. NbC also promoted the formation of low angle grain boundaries (LAGBs) and geometrically necessary dislocations (GND), the LAGBs of CoCrFeNi + 15 wt% NbC composite cladding layer reached 63.4% with ρGND up to 1.51 × 1015 m−2. CoCrFeNi + 10w.% NbC coating achieved higher microhardness (450.8 ± 5 HV0.2) thanks to the combined action of various strengthening mechanisms. Simultaneously, it obtained stronger wear resistance, with a specific wear rate of 0.17 × 10−4 mm3/Nm. In addition to oxidative wear, abrasive wear and adhesive wear coexisted on its worn surface. In slurry erosion test, the cumulative mass loss of CoCrFeNi + 10 wt% NbC HEAs composite coating was the lowest in each cycle and grew more slowly due to its higher microhardness and uniform distribution of reinforcement phases. The erosion mode of 316 SS and CoCrFeNi coating was mainly ductile fracture, while NbC-containing composite coatings exhibited a failure mode combining brittleness and ductility.