Aging of ErT₂ thin films: ERD analysis and mechanical property changes

Rare earth tritide films evolve as tritium decays into ³He, which forms bubbles that influence long-term film stability in applications such as neutron generators. We followed the properties of model ErT₂ films as they aged using ERD analysis to monitor T and ³He profiles, nanoindentation testing for mechanical properties and transmission electron microscopy to characterize bubble growth. The profiles of T and ³He are separately measured in ERD using a ΔE - E detector, taking advantage of the differences in energy loss within the ΔE detector. The composition measured by ERD followed the expected build-up of ³He up to near critical release, where ³He begins to escape from the film as bubbles overlap. These measurements complement observations of the changing mechanical properties of these films, where the observed behavior divided into two regimes: a substantial increase in layer hardness but elasticity little changed over ∼18 months, followed by a decrease in elastic stiffness and a modest decrease in hardness over the final 24 months. The evolution of properties has been explained by a combination of dislocation pinning by the bubbles, elastic softening as the bubbles occupy an increasing fraction of the material, and details of bubble growth modes. The ERD measurements confirm that the changes in properties are due to changes in bubble morphology and not to changes in ³He or T content.