The effect of N ₂ ⁺ and C ⁺ implantation on uranium hydride nucleation and growth kinetics

Hydrogen attack on uranium and uranium alloys may cause embrittlement and hydride formation that are undesirable in nuclear fuel technology. Implantation of the uranium surface by a high dose of energetic ions modifies the surface in a way that delays the hydrogen attack and slows the growth rate of the hydride. The implanted surfaces also exhibited better passivation to air oxidation. In the present study, 45 keV N ₂ ⁺ and C ⁺ ions with a dose of 6·10 ¹⁷ ions/cm ² were implanted (separately) in pure uranium. The incipient hydriding nucleation and growth kinetics of the implanted uranium samples were measured in a hot-stage microscopy system. The surface was continuously monitored, during the hydrogenation process, by a TV camera and recorded on videotape. The reaction was stopped, for various experiments, at different reaction steps by pumping the hydrogen out. SEM micrographs revealed, especially for the C ⁺ implanted samples, a morphology in which the hydride appears as blisters, seemingly under the implanted layer. The hot-stage micrographs were analyzed by image-analysis procedures yielding the nucleation and growth rates for the implanted vs. unimplanted specimens. Possible explanations are suggested for the passivation effects imparted by ion implantation.