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Volume 6, Issue 4 (September 2017)
Special Issue Paper
Controlling Plasma Nitriding of Ferrous Alloys
(Received 14 April 2016; accepted 21 November 2016)
Published Online: 07 September 2017
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The control of plasma nitriding relies on the ability to effect the transfer of nitrogen species into the cathode. Those species are either N+ ions implanted into the cathode or adsorbed nascent nitrogen atoms, coming from decomposed N2+ and NHx+ radicals or from FeN clusters of sputtered iron atoms deposited on its surface. The latter contribute also to the formation of the dusty plasma, which may have negative consequences when processing complex geometry parts, especially those made of stainless steels. Control of the sputtering intensity is accomplished by using modern pulse plasma generators, allowing for a variable duty cycle, as well as for the setting of voltage and/or current at a specific level. Therefore, proper balancing of the energy input between plasma and the furnace radiation in hot-wall systems is extremely important. Plasma density is responsible for the formation of the active nitrogen species and has a direct effect on the thickness of the compound layer (CL), or depth of the diffusion layer (DL), if CL is not formed. Flow rate of the gases containing nitrogen has no effect on their absorption by the cathode in a very broad range and has no effect on formation of a nitrided layer, although precise control of the flow rate may have some benefits because the absorption rate of nitrogen changes during the cycle affecting characteristics of glow discharge. Partial pressure of nitrogen has a direct effect on the amount of nitrogen in the cathode surface and thickness of the compound layer. The best applications for plasma nitriding include stainless-steel products processed in a very broad range of temperatures, sintered PM products, parts requiring partial treatment, stamping dies, and many others.
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Title Controlling Plasma Nitriding of Ferrous Alloys