Cathodic Cage Plasma Nitriding: An Innovative Technique

Cylindrical samples of AISI 1020, AISI 316, and AISI 420 steels, with different heights, were simultaneously treated by a new technique of ionic nitriding, entitled cathodic cage plasma nitriding (CCPN), in order to evaluate the efficiency of this technique to produce nitrided layers with better properties compared with those obtained using conventional ionic nitriding technique. This method is able to eliminate the edge effect in the samples, promoting a better uniformity of temperature, and consequently, a smaller variation of the thickness/height relation can be obtained. The compound layers were characterized by X-ray diffraction, optical microscopy, and microhardness test profile. The results were compared with the properties of samples obtained with the conventional nitriding, for the three steel types. It was verified that samples treated by CCPN process presented, at the same temperature, a better uniformity in the thickness and absence of the edge effect. 1. Introduction Conventional plasma nitriding has been employed a lot in the industry, due to several advantages in relation to other nitriding processes (gas nitriding and salt bath nitriding), such as higher gas economy and smaller processing time, since the diffusion velocity of nitrogen (N2) in plasma nitriding is higher [1]. This process is used to improve superficial properties like hardness, wear, and corrosion resistances, in order to increase the useful life of nitrided components. However, it presents some inconveniences like edge effect in the sample surface, hollow cathode effect, nonuniform temperature, and arcing during the treatment, especially in pieces with complex geometry [2, 3]. When samples with different dimensions are nitrided by conventional plasma process, they present a thermal gradient between the surface and sample holder. In samples with height of 10？mm, the difference of temperature between the top surface and sample holder can reach 373？K [4]. In this work, a new plasma nitriding technique entitled cathodic cage plasma nitriding (CCPN) based on multiple hollow cathode effects is used and the results compared with the conventional plasma nitriding. This technique presents some advantages compared to the conventional method, including process efficiency and the production of layers with uniform thickness. The samples are totally involved by a metallic cage, where a high cathodic potential is applied, so the plasma is not formed directly on the sample surface, but in the cathodic cage. A ceramic disk (In this work, machined with aluminum oxide) is placed on the