Studies of technological parameters influencing the nitriding behavior of spray-formed Al alloys

Abstract

Despite their typically high Si content of up to 30ma.-%, spray-formed Al alloys exhibit excellent nitridability in plasma processes. This results from the exclusive primary solidified and homogeneously dispersed small Si precipitates in the base material that occur due to the specific production process. Previous studies reported on the influence of the general technological parameters during plasma nitriding, such as temperature, time and gas composition, as well as chemical composition, of spray-formed Al alloys on their nitriding behavior. However, in connection with progress in automation of process control, such as, for example, in the case of steels, a significant influence of the adaptive control of the plasma duty cycle has been detected. In cases where the specific nitriding technology of direct current (DC) plasma nitriding is used with in situ sputter cleaning, the plasma duty cycle determines different processes that occur on the sample surface.The scope of this report covers both the influence of the duty plasma cycle and the initial surface characteristics, including the specimen shape. Despite equivalent general nitriding conditions (temperature, time, voltage), over time the plasma duty cycle and the target value of plasma duty lead to different thicknesses and coverage of the nitride layer. For a better understanding of the nitriding mechanism, analyses of interrupted nitriding tests were carried out. The results of plasma nitriding have been characterized by examining metallographic cross sections with optical microscopy, surface coverage by scanning electron microscopy (SEM), as well as measuring aluminum nitride layer thickness with different methods. Mechanical properties were studied using microindentation. Adhesion was tested by scratch tests.