Surface modification of aluminum using ion nitriding and fluidized bed

Abstract

Recently, applications of aluminum (Al) use have extended into the area of the lightening of machine parts. However, the hardness of Al is lower than that of the steel mainly used for these machine parts at present. Therefore, it is necessary to carry out surface modifications of Al in order to use it in sliding parts. By heating in a nitrogen plasma, ion nitriding forms a nitride layer on the steel surface. Ion nitriding is widely used as a processing tool to increase the fatigue and wear resistance of machine parts of carbon or alloy steels, etc. However, a dense oxide film exists on the surface of Al, which prevents the formation of a nitride layer by ion nitriding. Therefore, it is difficult to apply ion nitriding to Al alloys. In this study, an aluminum nitride (AlN) layer was first formed on the surface of Al in a fluidized bed filled with alumina (Al 2O 3)/aluminum magnesium (Al–Mg) alloy powder, and ion nitriding was thus enabled. In addition, the influence of process parameters on the film thickness and hardness of the AlN layer was examined. After nitriding, constituents of the modified layer were identified by XRD, the structure and thickness were observed by optical microscopy, and the hardness was measured by a micro Vickers hardness tester. The processing time for all conditions was 18.0 ks. The thickness of the AlN modified layer was compared for three nitriding methods: fluidized bed only; gas nitriding after fluidized bed; and ion nitriding after fluidized bed. After nitriding, there are two layers on the substrate, one is the modified layer composed of AlN, and the other is a deposited layer composed of Al 2O 3 and AlN. With increasing time in the fluidized bed, the thickness of the modified and deposited layers increases. For the combined processes, the thickness of the modified layer is less than that carried out by fluidized-bed nitriding alone. In regions close to the deposited layer, AlN concentration and hardness in the modified layer are high. The thickness of the deposited layer increases only with increasing time in the fluidized bed. It is possible that only the thickness of the modified layer increases when gas or ion nitriding is used. In addition, the hardness of the modified layer formed by ion nitriding is higher than that formed by gas nitriding. The thickness of the modified layer formed by ion nitriding decreases, along with that formed by fluidized-bed nitriding.