The Effect of Using Hollow Cathode on the Tribological Behavior of Plasma Nitrided Layers.

Abstract

The shape and geometry of the component play an essential role in surface plasma treatments. Certain plasma conditions can activate the hollow cathode effect (HCE) in parts with cylindrical geometry and, consequently, intensify the discharge by increasing the current density [1]. Due to this fact, a local surface over-heating can occur and cause some structural and microstructural transformation, developing columnar nitride compound layers and the appearance of braunite [2]. In this work, the tribological properties of nitride compound layers with microstructural characteristics induced by the hollow cathode effect (HCE) were evaluated. Hollow cylinders of sintered iron (7,5 mm in inner diameter and 9 mm in height) were plasma nitrided in three different conditions in order to obtain: a columnar compound layer (550 °C, 90%N2 + 9%H2 +1%CH4); a compound layer with presence of braunite (570 °C, 30%N2 + 70%H2); a compact and homogeneous compound layer with no microstructural defects (540 °C, 30%N2 + 70%H2). Pressure, output voltage and treatment time were kept constant in all treatments. The layer microstructures were analyzed by optical microscopy (OM) and the phase identification was conducted by X-ray diffraction (XRD). Reciprocating dry sliding tests were carried out to evaluate wear resistance and friction coefficient. White light interferometry was used to evaluate the surface topography and measure the wear volumes. The wear rate is affected by microstructural characteristics of the nitride layers induced by the HCE. Columnar nitride compound layer and the presence of braunite are undesired characteristics to achieve good wear resistance. Further-more, the friction coefficient was not influenced.