Underwater surface discharge characteristics and multi-physical effects with conductive coating load under fast electric pulse

Underwater pulsed discharge, where intense reactions between ionized gas and condensed-state water exist, can be a joint problem of both physics and chemistry. The study tries to build a comprehensive visualization of nanosecond-risetime discharge initiated by a conductive coating and its successive multi-physical effects. The scenario is established via a pair of thin-plate electrodes positioned on both sides of the coating, and diagnosed using high-speed backlight photography synchronized with electrical and optical measurements. For the sprayed Cu/Ag composite coating, the current density can achieve 20 A mm⁻² which is high enough to induce the surface ‘electrical explosion’ and breakup the conductive matrix within 500 ns. By increasing the discharge energy from 0.5 to 10 J, the explosion of coating can exhibit different discharge types as exploding wires. Adopting a thicker carbon foil or cermet sheet can reduce the current density and energy deposition rate, which converts the global explosion to partial ones, significantly increasing the lifetime. With the aid of the conductive coating, the breakdown delay diminishes, and hot plasma spots form in 100 ns due to non-uniform Joule heating of the pulsed current, which gradually evolve to a plasma bubble cluster above the lower-conductive coating (bypassing branch). Once the high-conductive plasma channel bridges two electrodes, it will be intensively heated (MW-level energy deposition rate) and rapidly expand, accompanied by underwater shock wave (10² kPa @30 cm) and bubble/cavity generation (20 mm maximum). Finally, microscopic characterization has been made, and the erosion morphology suggests typical arc erosion features (pits, cracks, etc) and nanoparticles condensation from evaporated materials.