Effect of hydrogen and oxygen on stainless steel nitriding

The influence of hydrogen and oxygen on stainless steel implanted by nitrogen low-energy ions is systematically studied. It is shown that hydrogen intervenes moderately in the process only when the oxygen partial pressure in the deposition chamber is relatively high. For very low-oxygen partial pressures, the energetic nitrogen molecules impinging on the substrate sputter the thin oxide layer formed on the substrate. This allows the growing of a rich nitrogen layer beneath the surface, improving the diffusing of the implanted atom deeper in the bulk material. For higher-oxygen partial pressures, the sputtering is ineffective, and an oxide layer partially covers the surface even in the presence of hydrogen. The maximum depth penetration of nitrogen depends on the degree of oxygen coverage, which is fairly well described by a Langmuir absorption isothermal. Hardness depth profiling is consistent with the existence of a diffusion barrier formed by the oxygen absorbed on the surface. In order to understand the role of hydrogen on the nitriding process, samples preimplanted with hydrogen were subsequently treated with nitrogen and the hardness depth profiling analyzed. These results may provide a clue about the practical consequences of oxygen and hydrogen on the nitriding process. (C) 2002 American Institute of Physics.