Comparing the Effect of Low-Temperature Plasma on Cleaning and Polishing Coins with a Chemical Method and the Reversed Effect on the Plasma Properties

The main goal of this work is to use a simplified idea to provide a scientifically accurate plasma device at a reasonable cost and benefit to society. It is an attempt to present designing and building one plasma system that can operate as three plasma experiments using the same instruments. These experiments are glow discharge, magnetron, and sputtering. This study focused on working the device as a DC glow discharge argon plasma device by a positive electrode made of a copper rod. The Paschen curve was measured at different distances between the cathode and the anode (4, 6, 8, 10, and 12 cm). From these results, the operating conditions of argon gas pressure and voltage that will produce plasma at each distance were determined. The investigated pressures were \(1 \times {10 }^{-1}\) $1\times {10}^{-1}$ and \(2 \times {10 }^{-1}\) $2\times {10}^{-1}$ mbar, while the operating voltage ranged from 110 to 160 V. The generated plasma was used to clean and polish the surface of two Egyptian fifty-piaster coins (still in circulation) at different exposure times as a practical application for the designed plasma device. After that, the results will be compared with cleaning and polishing a coin by a chemical method. The observed cleaning and polishing by the plasma is better and resists the environmental effect more than the chemical polishing method. This is related to the plasma treatment of the surface of the metal, while the chemical replaces the dirty layer as a result of reaction with it without treating the metal surface itself. The reversed effect of the sample (coins) on the plasma parameters (voltage and current) during its exposure time is considered one of the most novel included in this work. Besides, the temperature increases of the chamber surface during the working time were recorded.