Cast iron plasma nitriding in N₂ /H₂ /Ar working gas: the role of auxiliary gases (H₂ /Ar) in the growing kinetics of compound layers

Present paper sets out to investigate the role of N₂ /H₂ /Ar working gas in plasma nitriding of nodular cast iron. The proportion of N₂ /H₂ /Ar proved to be a relevant parameter in the growing kinetics control of the γ'-Fe₄N e ε-Fe₃N compound layers. X-ray diffraction and optical microscopy were used in order to characterize the compound layers. Results indicate that there are no generation of compound layers if the treatment is performed through plasma of pure N₂ . However, even small percentages (~10%) of auxiliary gases (H₂/Ar), added to the working gas, are enough to generate compound layers as thick as 10µm (at 500 °C, and 3,0 h). In addition, it is possible to obtain γ'-Fe₄ N monophasic compound layer when the nitriding process is carried out in low proportion of nitrogen in the N₂ /H₂ /Ar working gas mixture.(author)