The action of hydrogen in low-pressure r.f.-plasma nitriding

Samples of AISI 316 austenitic stainless steel were nitrided in a low-pressure r.f. plasma using various mixtures of nitrogen and hydrogen with the aim of elucidating the action of hydrogen in plasma nitriding. Exposure of samples to a pure hydrogen discharge prior to treatment in a pure nitrogen discharge increases both the thickness of and the nitrogen concentration in the treated layer compared to an unexposed sample. However, treatment in an N2–25% H2 mixture gives even greater layer thickness and nitrogen content. Separate in-situ measurements of the secondary ionisation coefficient, using a pre-breakdown discharge with an AISI 316 sample as the central part of an AISI 316 cathode surface, show a marked increase in secondary electron emission after hydrogen plasma exposure, presumably due to oxide removal. This is direct evidence in support of previous conclusions that hydrogen acts to increase ionisation. The fact that nitriding with an N2–H2 mixture produces the best result indicates the action of a second effect in addition to the effects of increased ionisation. Strong evidence for this was provided by a treatment in which hydrogen was present for part of the nitriding step only. Finally, it is suggested that the increase in the thickness of the treated layer caused by the presence of hydrogen points to an action deep (∼μm) beneath the surface of the sample.