A Study On DC Glow Discharge Plasma Generated Inside A Metallic Tube And Its Preliminary Application

Improving the hardness, wear resistance, corrosion resistance of the inner surface ofmetallic tubes has potential application in industrial practice. We proposed a normal DC glowdischarge method for the inner surface nitriding of stainless steel tubes with inner diameter of10mm. A coaxial electrode structure was constructed by coaxially stretching a tungsten wireas an anode inside of stainless steel tube. Steady DC glow discharge plasma can be generatedin the stainless steel tube by applying a negative DC high voltage to the tube. Then the ions inthe plasma will be accelerated and then implanted into the inner surface by the applyinghigh-voltage. In this work, we present our study on:(1) Plasma generation.A coaxial electrodes was constructed by coaxiaUy stretching a tungsten wire inside themetallic tube. DC discharge plasma was generated inside the tube by applying a negative dchigh voltage to the tube. It was found that the length of the plasma depends on the applied dcvoltage, and increases linearly while the applied voltage increases. The voltage-currentcharacteristic of the discharge shows that the plasma generated inside the stainless steel tubeare normal DC glow discharge.(2) Diagnosis of the plasma by optical emission spectroscopy.The diagnosis of the plasma generated inside the stainless steel tube was performed byoptical emission spectroscopy method. The active species in the plasma was identified. Thedependence of the intensity of active species on gas ratios and pressure was studied. Theelectron excitation temperature of the plasma and vibrational temperature of nitrogenmolecular in the plasma was estimated by Boltzmann plot method. The dependence of theelectron excitation temperature and molecular vibrational temperature on the gas ratios andpressure was investigated.(3) The preliminary study on inner surface nitriding of AISI304 stainless steel tubes.The AISI 304 samples after nitriding by the plasma of Ar+N₂ mixture were analysizedby Auger electron spectrometer (AES), X-ray diffraction (XRD) and microhardness tester.The preliminary results show that the normal DC glow discharge method developed in presentwork can achieve inner surface nitriding of stainless steel tube with inner diameter of 10mmand length of 100mm.