Low-pressure hollow cathode plasma source carburizing technique at low temperature

Abstract

A low-pressure hollow cathode plasma source carburizing was developed as a novel surface treatment technique based on the active screen plasma carburizing combined with hollow cathode sources. The cylindric cathodic cage with array holes was made of pure graphite, where the hollow cathode graphite tubes were inlaid on. A pulsed high voltage power with a peak voltage of 800 V and a pulse frequency of 30 kHz was supplied to ignite the hollow cathode discharge inside graphite tubes at a low pressure less than about 300 Pa using a gas mixture of CH 4 + 90%H 2 . The hollow cathode plasma source together with the pulsed high voltage power realized a high biasing current on the carburized samples at low pressure, and produced an enhanced heating except the thermal radiation by the cathodic cage. The graphite cathode as a carbon source besides the gas precursor provided an additional carbon potential. The AISI 304L austenitic stainless steel was carburized at a low temperature of 450 °C for a treatment time of 6 h with a bias of 200 V under the carburizing pressure of 50 Pa and 300 Pa, respectively. It was observed that the carburizing case at 50 Pa was about 13 μm with a higher surface carbon concentration of about 6.5 at.% and a higher hardness of about HV 0.25 N 8.5 GPa compared with that at 300 Pa. The carbon-expanded austenite (γ C ) phase was observed by the x-ray diffraction of both the carburizing cases. The metastable high carbon carbide Fe 5 C 2 only appeared at 50 Pa, which was attributed to high carbon potential at the lower pressure. Therefore, an increased carburizing efficiency at the low temperature was obtained by the low-pressure hollow cathode plasma source carburizing technique using a pulsed high voltage power. • Hollow cathode plasma source is utilized in low temperature plasma carburizing. • Pulsed high voltage is used in graphite hollow cathode discharge at low pressure. • High sample biasing current with enhanced heating is realized by plasma source. • Graphite cathode provides additional carbon potential besides the gas precursor. • Thick carburizing case with metastable carbide and high hardness is obtained.