The Effect of Hydrogen on Plasma Nitriding of Austenitic Stainless Steel: Kinetic Modeling
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INTRODUCTION
DESPITE very good corrosion resistance in many aggressive environments, austenitic stainless steels (ASSs) have low hardness and poor tribological properties, which may shorten the life of components, which are subjected to wear.[1,2] Plasma-assisted nitriding of ASS is an excellent method to improve their hardness, wear resistance, and fatigue strength without compromising their well-known corrosion resistance performance.[3–8] Such nitriding processes are performed at moderate temperature, which are necessary to obtain about 10 to 20 lm treatment depths in reasonable times without degradation of corrosion resistance. The moderate [i.e., lower than ~723 K (450 C)] temperatures suppress the formation of CrN precipitates and the depletion of Cr in the steel matrix, i.e., to prevent a reduction of the corrosion resistance. On the other hand, the lower treatment temperature results in reduced nitriding efficiency compared with higher temperature treatments.[8] Hence, it is very important to enhance the density of active nitriding species in the plasma. It is known that hydrogen addition to the nitrogen plasma enhances the nitriding rate.[9–18] However, despite the fact, that many experimental studies have analyzed steel surface nitriding processes using nitrogen–hydrogen plasma,[9–18] the role of hydrogen is still not well understood. Overview of literature[9–20] shows that the main and widely stated role of hydrogen on the depth (and also properties) of the nitrided layer is explained by the fact that hydrogen enhances nitrogen TERESA MOSKALIOVIENE, Lecturer, and ARVAIDAS GALDIKAS, Professor, are with the Physics Department, Kaunas University of Technology, Studentu˛ 50, 51368 Kaunas, Lithuania. Contact e-mails: [email protected], t.moskalioviene@ gmail.com Manuscript submitted May 11, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
diffusion by reducing the diffusion barrier formed by the oxygen adsorbed on the steel surface during nitriding process (oxygen is a common contamination element in plasma chambers, directly affecting the nitriding process[18]) and oxide coverage before the nitriding (natural oxide, whose thickness depends on the pre-history of the sample). For instance, in References 16 through 18, it was established that the chemisorption of oxygen on steel surface diminishes the nitrogen diffusion into the bulk of steel. Hydrogen can be used as a chemical-etching agent to prevent the deleterious effect of oxygen, i.e., to neutralize the effect of the diffusion barrier for nitrogen.[18,19] This process is known as the hydrogen etching mechanism in nitriding process which includes three main steps[18,19]: (1) hydrogen adsorption and dissociation; (2) water molecule formation; and (3) desorption, dragging away water from the steel surface. Moreover, exhaustive experimental study in Reference 17 proved that hydrogen has an important role on the nitriding process by increasing the density of active nitriding species in the plasma. Similar effect of hydrogen was reported in Reference 21, where
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