Microstructure, bonding properties and the basis of pinning effect of in-situ NbC reinforced Co50 composite coating by plasma cladding

One of the vital problems limiting the application of multi-layer cladding coating is that there appears a coarse grain structure at interface between different layers. In order to refine the grain and increase the bonding strength, Nb was added to Co50 composite coating in this study. The multi-layer Co50 composite coating was fabricated by plasma cladding. Microstructure was investigated by scanning electron microscopy (SEM) with energy dispersive spectrum (EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). And the effect of Nb on bonding strength was investigated using tensile test and first-principles calculations based on density functional plane-wave ultrasoft pseudopotential method. The results show that when Nb is added, a few in situ synthesized NbC with very small grain size appears on the grain boundaries, restraining those movements which results in the grain and microstructure refinement. Besides that, the strength bonding between different layers increases, which is attributed to the fine-grain strengthening and dispersion strengthening effect of NbC. Further, the strong Co-C covalent bonds in the NbC/Co interface and the Co-Co metallic bond in the internal coating might be why the NbC can serve as an obstacle and have a strong drag and pinning effect on the grain boundary according to the first principle calculations.