Laser synthesis and microstructure of micro- and nano-structured WC reinforced Co-based cladding layers on titanium alloy

Micro- and nano-structured WC reinforced Co-based cladding layers on Ti6Al4V substrates were synthesized by laser cladding. The structure, phase composition, and wear behavior were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and friction-wear tests. High-angle annular dark field-energy dispersive spectroscopy (HAADF-EDS), transmission electron microscopy (TEM) and lattice images confirmed the XRD and SEM results. The interfaces exhibited good metallurgical bonding between the cladding layer and substrate owing to their high chemical affinity for each other. The cladding layer was composed of α(α′)-titanium, eutectics of CoTi2, β-titanium, TiC, eutectics of TiCTiB2, VC (Cr3C2), (Ti, W)C1−x, W, TiB2, and Co3W3C phases formed by the dissolution of WC. VC inhibited grain growth more effectively than Cr3C2 or the combined effects of VC and Cr3C2. The wear resistance of the micro-/nano-WC cladding layers was markedly improved compared with those of Ti6Al4V. Nano-WC tended to convert into tungsten, however, the cladding layers easily became hard and brittle owing to aggregates of carbides or oxides. This effect resulted from interfacial boundaries formed by major differences at the α-Ti/α-Ti (Cr3C2, TiB2) interfaces in terms of the chemical and structural contributions to the surface energy.