In plasma cleaning applications, discharge parameters exert a decisive influence on the properties of plasma jets and cleaning performance. Power directly governs the electron temperature and energy density of plasma, which further affects the excitation and ionization of electrons within the plasma. Rising power elevates electron temperature and raises the concentration of excited particles, thereby boosting plasma activity and reactivity. In addition, cleaning duration acts as another critical parameter that controls the interaction time between plasma and the treated substrate surface. Accordingly, this paper mainly investigates the effects of power and cleaning duration on the treatment performance of rotary plasma cleaning.

Prior to experiments, sample surfaces were pre-cleaned with 99% industrial alcohol, followed by treatment using an atmospheric plasma cleaner, as illustrated in Figure 1.1. During cleaning, 20 specimens were sequentially placed on a movable platform with a travel distance of 12.5 cm and a moving speed of 1.25 cm/s. The discharge powers adopted in this experiment were 550 W and 750 W, while the cleaning durations were set to 0.5 min, 1.0 min, 1.5 min, 2.0 min, 2.5 min and 3.0 min. A water contact angle measuring instrument was used to test the water contact angles on stainless steel electrode surfaces before and after plasma cleaning.

Figure 1 Plasma cleaning of stainless steel materials

Figure 2.1 presents the variations in water contact angles on stainless steel electrode surfaces before and after air plasma treatment under different process parameters. Figure 2.1(a) shows the initial untreated state; Figures 2.1(b)–(g) display the evolving water contact angles on stainless steel surfaces under 550 W discharge power as cleaning time increases from 0.5 min to 3.0 min. Figures 2.1(h)–(m) correspond to the contact angle trends of electrode surfaces at 750 W discharge power. To visualize the contact angle variation more intuitively, raw data extracted from Figure 2.1 are plotted as line charts in Figure 2.2. For more accurate determination of the optimal cleaning duration, supplementary measurements at 0.25 min were conducted under 750 W discharge power.

The results indicate that under 550 W discharge power, the water contact angle on electrode surfaces drops from the initial 70.76° to a minimum value of 29.31° after 1.0 min of cleaning. With treatment time exceeding 1.0 min, the contact angle gradually climbs to 55.19°. At 750 W discharge power, the contact angle reaches its minimum of 34.12° after only 0.5 min of cleaning, followed by a rapid rise thereafter. The contact angle rises to 45.98° at 1.5 min and stabilizes after 2.5 min.

Figure 2.1 Water contact angles on stainless steel electrode surfaces before and after air plasma cleaning: (a) untreated; (b)–(g) 550 W discharge power; (h)–(m) 750 W discharge power

Figure 2.2 Variations in water contact angles on stainless steel electrode surfaces before and after air plasma cleaning: (a) 550 W; (b) 750 W

Overall, as plasma cleaning duration extends, the water contact angle on stainless steel surfaces first decreases and then increases, which demonstrates that the surface hydrophilicity of electrodes is initially enhanced and subsequently degraded. For discharge powers of 550 W and 750 W, the cleaning durations required to achieve the minimum contact angle are 1 min and 0.5 min respectively. Higher power increases plasma electron temperature and energy density, hence accelerating the surface treatment process.