Abstract:The viscoplastic friction and nanostructure formation mechanism of laser-clad Co-based coating were studied by rotary friction between laser-clad Co-Cr-Ni-Mo coating and WC-Co rod. The friction coefficient, friction interface temperature and axial displacement—time curves during rotary friction process were measured. The results showed that all the curves firstly experienced rising stage and then steady stage. The rising stage corresponded to sliding friction while the steady stage corresponded to viscoplastic friction. After viscoplastic friction processing, three typical zones of viscoplastic deformation zone, thermo-mechanically affected zone, and original laser-clad zone can be observed successively from the friction surface to the interior. The viscoplastic deformation significantly crushed the network M₂₃C₇ phase in original laser-clad zone and made it dispersively distributed with equiaxial shape and in nano-scale. The viscoplastic zone, in width of 37-131 μm, is mainly characterized by refined M₂₃C₇ and α-Co phase with grain size bellow 50 nm, and even a small quantity of amorphous. Thus, the hardness of viscoplastic zone about HV997 was improved compared with the hardness of original laser-clad zone about HV600.

Key words: viscoplastic deformation; severe plastic deformation; laser cladding; Co-Cr-Ni-Mo coating; nanostructure; rotary friction