Compositionally graded multi-principal-element alloy coating with hybrid amorphous-nanocrystalline structure by directional electrical explosion

Electrical explosion, characterized by ultra-fast atomization and quenching (dT/dt ∼10¹⁰–10¹² K/s) of the sample, is a unique approach for the one-step synthesis of nanoparticles or coatings. A dense μm-thick alloy coating made of AlCuTiMoWSn was manufactured via the pulsed-discharge-driven intertwined wire explosion in μs-timescale. The explosion products were guided by a nozzle, forming a thermal plasma jet spray (km/s) towards the substrate. The condensation and stacking of those elements on Si wafer appeared graded element distribution, on account of the discrepancy in electrical explosion dynamics of various metals and the jet-wall interaction effects. High-speed photography along with electro-physical diagnostics was applied to characterize the dynamics of the synthesizing process. Scanning electron microscopy and energy dispersive spectrometer were used to capture the microstructural and compositional features of the coating from the top/sectional views. The results indicate the discordance in electrical explosion causes various element abundance and phase states at different depths of the coating. Specifically, Sn is the first to explode and spray toward the Si wafer, then Al/Cu, and finally Ti/Mo/W. Morphology and x-ray diffraction results confirm the co-existence of nanocrystalline and amorphous phase for the alloy coating. In addition, the quenching, nucleation, and condensation are deeply associated with fluid dynamics such as turbulent mixing, near-wall vortex, etc. Also, the influence of nozzle configuration was discussed. This ingenious path could favor fabricating materials suited to applications demanding variable mechanical/electrical functions.