Strengthening mechanism of laser cladding FeCoCrNiMo high-entropy alloy with different contents of CNTs

Laser cladding is a promising surface engineering technique characterized by high energy density and strong metallurgical bonding, enabling rapid solidification and controllable microstructural evolution in high-entropy alloy (HEA) coatings. However, conventional HEA claddings often show a limited balance between strength and ductility. In this study, FeCoCrNiMo HEA powders were deposited on a 316L stainless steel substrate to fabricate composite coatings reinforced with various contents of carbon nanotubes (CNTs). The influence of CNT incorporation on the phase constitution, microstructure, and strengthening mechanisms of the coatings was systematically investigated. During the cladding process, partial decomposition of CNTs occurred, and the released carbon reacted with Cr and Mo to generate finely dispersed (Cr, Mo)-rich M23C6 carbides. The addition of CNTs also introduced lattice distortion and promoted grain refinement, transforming the microstructure from columnar grains into a hybrid of refined dendritic and equiaxed structures. The resulting improvement in mechanical performance was attributed to multiple concurrent strengthening mechanisms, including grain refinement, carbide dispersion, dislocation pinning, and load transfer at the matrix/reinforcement interface. Coatings containing 0.5–1.5 wt.% CNTs exhibited uniform microstructures, sound metallurgical bonding, and remarkable increases in hardness and wear resistance. In contrast, excessive CNT content (2 wt.%) caused interfacial agglomeration and the formation of brittle phases, leading to microstructural heterogeneity and performance degradation.