Influence of surface pretreatment on case formation during gaseous nitriding

Abstract

The results of studying the influence of cathode sputtering on surface layer formation during gas nitriding will be presented in this paper. Specimens of iron have been cathode sputtered before nitriding in controlled ammonia–hydrogen and/or nitrogen gas mixtures. The cathode sputtering process was performed in a separated chamber in argon or nitrogen atmospheres. The thickness and microhardness of the compound case and internal diffusion zone and their structure were compared for specimens pre-treated and non-pre-treated before gas nitriding.The increased number of active centres caused by cathode sputtering has a very important influence on the kinetics of surface layers formation. The thickness measurements of nitride layers show the significant growth of thickness for all the ranges of atmospheres used. After cathode sputtering, a compound layer developed rapidly on the iron surface during the subsequent nitriding. After 10min, the closed compound case was noticed. This phenomenon has an important influence on the conditions of the diffusion zone formation. Its higher thickness achieved through cathode sputtering was also measured.The results were controlled by metalographic testings (scanning and light microscopy) and microhardness measurements. The phase compositions of the layers were evaluated by X-ray diffraction.