Long-term amorphisation of C⁺ and N₂⁺ implanted layers on a uranium surface

Ion implantation of C⁺ or N2⁺ on uranium surfaces produces crystalline compounds (uranium carbides or nitrides, respectively), which have been shown to provide excellent protection against ambient corrosion. Some long-term changes of these implanted layers were detected that even though not affecting the protection ability, still are of fundamental interest. The most prominent change is the amorphisation of the carbide or nitride layers, which takes place after a few years of air exposure. This amorphisation did not result from the oxidation of the layer (the formed oxides are much thinner than the thickness of the implanted layers), but is still assisted by air exposure (i.e., the amorphisation rates of samples kept under vacuum are much lower than those exposed to the ambient atmosphere). Some additional long-term changes in the layer compositions (transitions of UN₂ to U₂N₃) and distributions (inward diffusion of the implanted ions) were also apparent. It is suggested that the presence of hydrogen in the implanted layer, originating from the dissociation of water molecules on the surface, may accelerate the amorphisation and the UN₂ to U₂N₃ transition due to the possible increase in the diffusion constant.