Effects of Gas Mixture on Active Screen Plasma Nitriding and Post-Oxidation of a 4140 Steel

Plasma nitriding, followed by post-oxidation, is a surface treatment used to enhance the properties of steel parts. The primary objective of such treatments is to develop a magnetite-rich oxide phase on the previously nitrided surface, aiming to achieve increased wear and corrosion resistance. The active screen plasma treatment method offers an alternative to direct plasma treatments, where parts are shielded from direct exposure to active species. This approach helps prevent defects caused by electric arcing and reduces the risk of increased porosity. However, due to differences in kinetics between active screen and direct plasma methods, the parameters commonly used in direct plasma treatments are not suitable for achieving the desired outcomes with active screen plasma. In this study, we applied plasma nitriding and oxidation to AISI 4140 hardened and tempered steel workpieces using the active screen method, varying the O2:H2 ratio in the gas mixtures. Treated workpieces were subjected to reciprocating wear tests. Our findings demonstrate that for the active screen post-oxidation step to be successful, it is essential to increase the O2 content compared to the reference treatment parameters used for nonactive screen methods. The highest O2 content investigated (40 %) facilitated the development of magnetite oxides over the nitrided surface. This resulted in remarkable improvements in wear resistance and a favorable reduction in the friction coefficient during our tribological tests.