Plasma nitriding design for aluminium and aluminium alloys

Abstract

Plasma nitriding for aluminium and aluminium alloys is a promising processing to improve the wear resistance for automotive parts. Normal plasma nitriding is characterised by three processes: presputtering, aluminium nitride nucleation and nitrided layer growth processes. N₂⁺ presputtering is used to effectively eliminate the preexisting oxide films of Al₂O₃, covering the surface of aluminium matrix. Relatively long incubation time is required for nucleation process to form AlN islands or nodules on its surface. In addition, formation rate becomes very slow owing to low nitrogen diffusion coefficient in the nitrided layer. Physical and chemical modification methods to this normal nitriding processing are proposed to accelerate the formation rate of nitrided layer. Refinement of grain size in the aluminium matrix increases the formation rate by enlarging grain boundary area as a diffusion path. Crystallographic coherency between TiN and AlN reflects on enhancement of nucleation process by coformation of TiN with AlN. Standing on the nitriding design by physical and chemical modification of inner nitriding mechanism, an alternative plasma nitriding is proposed as the third processing for copper bearing aluminium alloys. In this processing, reduction of duration for nucleation and acceleration of growth rate are attained with the aid of the precipitate, Al₂Cu. Crystallographic coherency between AlN and Al₂Cu is effective to enhance the formation of AlN nodules and islands. Solid state reaction between Al₂Cu and penetrating nitrogen is also significant to form the fine interfacial boundaries as a nitrogen diffusion path and to accelerate the formation rate of nitrided layer.