Comparative failure analysis of electrodes coated with TiB2-ZrB2 and TiB2-ZrB2/Ni layers

Electrodes with a monolithic TiB2-ZrB2 coating as well as those with a multilayered TiB2-ZrB2/Ni coating were used to resistance spot welding (RSW) Zn-coated steel, in order to investigate the failure behavior of the coatings during welding. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction analysis (XRD), and microhardness measurements were performed to characterize the microstructures of the coatings, the reactions of the electrodes with the Zn coating of the sheets, and the formation of the alloy layer. It was found that the protective coatings significantly improved the surface hardness of the RSW electrodes by inhibiting plastic deformation at their tips, thus slowing the erosion process. In addition, the ZrB2-TiB2-coated and ZrB2-TiB2/Ni-coated electrodes exhibited different failure mechanisms. A combination of the alloying effect and plastic deformation caused the final failure of the ZrB2-TiB2-coated electrode, while the ZrB2-TiB2/Ni coated electrode failed primarily owing to plastic deformation.