Influence of oxygen in a plasma nitriding process

The influence of oxygen on the nitride layer formation on sintered steels was studied in a plasma nitriding reactor as a function of the gas mixture and sample composition. The nitride layers were characterized by metallographic and electronic microscopy techniques. The thickness, composition and microstructure of the layer were determined for three gas mixtures (100% N2, 75% N2+25% H2 and 90% N2+10% H2) in plain sintered iron in Fe-1.5% Si sintered alloy. The plasma chemistry was studied by optical spectroscopy and mass spectrometry as a function of the oxygen concentration in the atmosphere in the range 0–4%. It was observed that for low oxygen concentrations (O2≲4%), the layer thickness remain practically unaltered for the mixtures containing hydrogen, whereas the layer obtained when H2 was not used is completely damaged if oxygen is present in the gas mixture. When the gas discharge is in a N2-H2 mixture, a loss of oxygen is detected by mass spectrometry. These results are correlated with the sample analysis, and the depletion of the oxygen is interpreted in terms of oxygen-hydrogen reactions. This revised version was published online in November 2006 with corrections to the Cover Date.