Low-pressure hollow cathode plasma source carburizing of AISI 304L austenitic stainless steel at low temperature

A carburizing treatment of AISI 304L austenitic stainless steel was performed at a low temperature of 450 ? using the low-pressure hollow cathode plasma source technique. The hollow cathode plasma was produced in a cathodic cage made of stainless steel screen inlaid with a stainless steel tubes array. A pulsed high voltage power, with a peak voltage of 800 V and a pulse frequency of 30 kHz, was applied at a low gas pressure of 50 Pa. The austenitic stainless steel samples were carburized with a direct current bias of-200 V under the CH4/H-2 ratio changing in a range from 0.11 to 9, for a carburizing time of 4 h. Under the lower CH4/H-2 ratio of 0.11, a diamond-like carbon film of approximately 260 nm thick formed on the stainless steel surface. As the CH4/H-2 ratio increased to 0.67 and 1, both carburizing cases possessed a similar thickness of about 11 mu m, with the surface carbon concentration being 6 to 6.5 at.%. A layer of carbon supersaturated face-centered-cubic (gamma C) phase formed, together with carbon-rich (Fe,Cr)5C(2) carbide precipitation. Under the highest CH4/H(2 )ratio of 9, a single gamma C phase layer was obtained in a carburizing case of similar thickness, with a high surface carbon concentration of 7.5 at.%. The maximum surface hardness value of HV0.25 N 7.1 GPa occurred on the single gamma C phase layer. No pitting corrosion occurred on the single gamma C phase in the polarization curve. The higher passive film resistance and the lower donor and acceptor concentrations and flat band potential indicated that a more compact passive film is grown on the gamma C phase layer. The (Fe,Cr)5C(2 )carbide precipitates in the gamma C phase matrix degraded the passivity of the gamma C phase layer due to the reduced compactness of the passive film. A carbon transport competition mechanism between carbon adsorption onto the surface and carbon inward diffusion that was mainly dependent on the heavier CHn+ ion bombardment, was proposed to explain the transition with increasing CH4/H-2 ratio from carbon film deposition, to a carburizing case composed of ae single gamma C phase, with both increased hardness and higher corrosion resistance.