The modification of surface properties of materials by nitrogen implantation has been widely investigated since more than 30 years. The main goal with this treatment is to produce nitrides or eventually carbonitrides which enhance the tribological properties of materials.

In this paper, we demonstrate that a non negligible part of the implanted nitrogen produces bubbles in the metals located essentially at the grain boundaries. Three examples of nitrogen implantation into metals are detailed. The first example consist of high dose implantation of nitrogen into iron in order to obtain the saturation and the formation of e-Fe₂N phase, which is the iron nitride containing the maximum amount of nitrogen. It has been demonstrated that more than 40% of nitrogen is observed [1] and that the 7% of nitrogen in excess compared to the e-Fe₂N phase is located in small bubbles. Another example showing the presence of N₂ bubbles is the anodic oxidation of aluminium nitride produced by nitrogen implantation into aluminium substrate. When the oxide reaches the implanted region, nitrogen bubbles are observed in the electrolyte [2]. The last example consists of simultaneous carbon and nitrogen implantation into copper. We observed by various techniques that nitrogen bubbles are also located in the grain boundaries of the copper matrix.

[1] G. Terwagne et al., Mat. Sc. Eng., 115 (1989) 25,