Nanohardness and transmission electron microscopy study of nitrogen-implanted aluminium

Nitrogen implantation into aluminium with the intent of forming AlN has been studied for many years. Its hardness suggests that the production of such a superficial layer in aluminium alloys may alter their mechanical properties in such a way that they could be used in new applications. Among the possible applications, one can cite wear in a plastic bottle injection moulding machine; mould prototypes are made in Al alloy, while moulds in production are made of steel because of their greater wear resistance. This paper is devoted to the study of the surface hardness of pure Al implanted with molecular nitrogen of 100 keV for wide ranges of doses and temperatures. Hardness vs. depth has been measured with a nanoindenter, and the results are supported by transmission electron microscopy measurements of the implanted layer. This latter technique reveals that nitrogen implantation leads to the formation of AlN precipitates which have a crystallographic relationship with the host aluminium matrix. Furthermore their size depends strongly on the implantation conditions. The hardness results are interpreted in terms of precipitate concentration and size.