Microstructure, nano-indentation distribution and wear evolution behavior of laser cladding CoCrFeNi high entropy alloy coating reinforced by carbon nanotubes

In this study, microstructure, nano-indentation hardness distribution and wear resistance characteristics of the laser cladding CoCrFeNi high entropy alloy coating enhanced with carbon nanotubes (CNTs) were investigated. By adding 1.0 wt% of CNTs as reinforcement, macro and micro morphologies of the composite coating were well maintained. The results showed that the protection of the original microstructure of the CoCrFeNi powder and CNTs during the mechanical mixing for preparing CoCrFeNi-CNTs composite powder was crucial for achieving the strengthening effect of CNTs on the matrix of laser cladding CoCrFeNi coating. The nano-indentation characteristics tested within each grain zone were all different. The highest level of nano-indentation hardness was found in planar grain, however, both the hardness and elastic modulus levels measured in the columnar dendrite zone were relatively the lowest, and thus this zone was the weak link. When wear load increased from 10 N to 30 N, debris and groove characteristics on worn surface were dramatically suppressed, the fluctuation range of the friction coefficient-time curve was significantly reduced, the average friction coefficient decreased, and the wear mechanism changed from a combination of abrasive wear and fatigue wear to a single fatigue wear. An unique carbide hardening layer, which was the key to improving the wear resistance of the CoCrFeNi-CNTs composite coating, was found to have deposited beneath the worn surface. A complex amorphous substructure composed of the oxide deposition layer, grain deposition layer and grain boundary deposition particle was proved in the carbide hardening layer.