Microstructure and tribological properties of laser-clad Ni–Cr/TiB2 composite coatings on copper with the addition of CaF2

In this study, Ni–Cr/TiB2 metal matrix composite (MMC) coatings with the addition of a little CaF2 (2 wt.%) were successfully fabricated on a Cr–Zr–Cu alloy substrate by laser cladding process with powder mixtures of Ni, Cr, TiB2 and CaF2 as the precursor materials. The MMC coatings were free of defects and the interfacial substructure between the MMC coatings and the copper substrate was epitaxial, with excellent bonding by the strong metallurgical interface. The microstructure, phase and tribological properties were investigated by means of optical microscopy (OM), X-ray diffraction (XRD) and scanning electron microscopy (SEM), as well as dry sliding wear test. Results show that the influence of TiB2 on the microstructure and tribological properties of the coatings was significant. The microstructure of the coatings was mainly composed of dendrites, cystiform-dendrites and particles. The dendritic microstructural features of the MMC coatings could be changed into particles by increasing TiB2 to 20 wt.%. The laser-clad Ni–Cr/TiB2 MMC coatings on copper with the addition of CaF2 exhibited higher microhardness and better wear resistance than pure copper substrate. The highest microhardness was up to 946 HV0.1 which was improved 8 times compared to the original substrate. The friction coefficient of the laser-clad Ni–Cr–20 wt.%TiB2–2 wt.%CaF2 coating was reduced significantly to about 0.24, and a relatively smooth wear surface could be observed.