Modification of copper surface by nanosecond-pulse diffuse discharges at atmospheric pressure

Nanosecond-pulse diffuse discharges can generate non-thermal plasmas with high power density and high electron energy at atmospheric pressure. In order to study the surface modification of metal surface using diffuse discharges, a diffuse discharge with a tube-to-plane electrode geometry was produced to find the optimal diffuse discharge parameters. The discharge was excited by an MPC-50D nanosecond-pulse power with a rise time of about 150 ns and full width at half maximum (FWHM) of about 300 ns in atmospheric pressure air, and then it was used for Cu surface modification. The experimental results showed that the diffuse discharge intensity, instantaneous power and deposition energy increased with the pulse repetition frequency (PRF). Moreover, the best performance of diffuse discharge occurred when the applied voltage was 31 kV, the PRF was 800 Hz and the gap distance was 3 cm. Furthermore, emission spectrum showed that the second positive system of N₂ (C→B, 0-0) and the first negative system of N₂⁺ (B→X, 0-0) were detected in air. The results of copper surface modification showed that plasma treatment would generate 0.5 μm-diameter melt holes on the copper surface. Both the hydrophilic and surface energy were significantly enhanced after the plasma treatment and reached a saturated value after 90 s. In addition, micro-hardness measurements showed that the surface hardness increased by about 26.5% after 480 s treatment.