Rare-Earth Thin-Film Deposition and Oxidation Study

Some experiments implemented on the Laser Megajoule facility (LMJ) require the use of the rare-earth (RE) elements, the lanthanides (57 < Z < 71). Rare-earth metals are known to be unstable under atmospheric conditions and some of them are extremely reactive with air. They may react with oxygen and humidity to form RE oxides. In the present work, we study the oxidation of different RE thin films (gadolinium, dysprosium, and praseodymium) prepared by physical vapor deposition. Energy-dispersion spectroscopy, scanning electron microscopy, Rutherford backscattering spectroscopy, and weight measurement are performed to characterize the corrosion mechanisms as a function of time and aging atmospheres (air, dry box, and vacuum). It appears that the oxidation kinetics depends on atomic number and microstructure of the films. Praseodymium coatings are very quickly corroded (in a few hours) when exposed to air and degrade to a yellow powder. Aluminum layers, used as a diffusion barrier, allow us to preserve praseodymium coatings over a period of several weeks when aging in a dry box. Gadolinium and dysprosium coatings (without a protective layer) are preserved from corrosion due to the formation of a passivation layer on their surface. Whatever Z, a dense microstructure permits us to limit the oxygen content and allows us to stabilize the residual stress.