D.C. glow discharge in a gas under lowered pressure in ion nitriding of Armco iron

Abstract

For many years a d.c. glow discharge under lowered pressure in nitrogen or nitrogen and hydrogen atmospheres has been applied in the nitriding processes. Presently, there are two major proposals regarding the basic reaction mechanism of ion nitriding. The first model, mainly developed for ferrous alloys, assumes the formation of high iron nitrides (FeN and Fe2N) in the plasma, results from the reaction between active nitrogen ions and sputtered iron atoms [1, 2]. After condensation on the cathode surface, these nitrides decompose to lower iron nitrides and release free nitrogen that diffuses into the substrate. Diffusion is accelerated by the existence of this strong source. The second model proposes that ionic bombardment introduces vacancies and vacancy clusters and therefore increases nitrogen diffusion [3, 4]. The energy of particles in conventional ion nitriding is sufficient to produce a defect layer a few atomic layers thick that can act as a source for vacancies and interstitials. Even though most of these defects will be annihilated at the surface, a number of them will have the chance to migrate into the interior of the material and thus enhance the diffusion rate. It is apparent that, in either one of the above models, the number of ions in the plasma and their energy play an important or dominant role in the nitriding process. In this paper research results will be presented showing that under plasma d.c. glow discharge conditions a nitriding process occurs not only on the cathode, but also on the anode and substrates with plasma potential (isolated from the cathode and the anode) if only they reach the proper temperature. The temperature necessary for the nitriding process on the anode could be obtained in the apparatus for glow discharge treatment with the hot anode. In the apparatus the anode is additionally heated e.g. in a resistance furnace. In the Fig. 1 the schematic diagram of the hot anode apparatus used for glow discharge treatment is shown [5]. The apparatus design allows for realization of the thermochemical treatment of plasma in various regions of a d.c. glow discharge. The substrates placed directly at the cathode or at the anode have respectively the temperature and the potential of the cathode or the anode. The substrates, which during the treatment are to have the plasma potential, are placed at the insulator with ceramic labyrinth (Fig. 2) [6]. When the insulator is placed at the cathode, the treated substrate has the cathode temperature, though it is isolated from the cathode. When the insulator is placed at the anode, the treated substrate has the anode temperature and is also isolated from the anode. Samples of Armco iron (0.05% C) were exposed to nitriding processes in various regions of a d.c. glow discharge using the hot anode apparatus (Fig. 1). Parameters of the nitriding process are given in Table I. Tests of the phase composition of nitrided layers were carried out in X-ray diffractometer using CuKα radiation. Metallographic tests were conducted on perpendicular microsections using the optical microscope. In Table II results are listed of the phase analysis and metallographic test carried out on Armco iron substrates exposed to nitriding in various regions of a d.c.