Structure and properties of uranium oxide thin films deposited by pulsed dc magnetron sputtering

Crystalline uranium oxide thin films were deposited in an unbalanced magnetron sputtering system by sputtering from a depleted uranium target in an Ar + O2 mixture using middle frequency pulsed dc magnetron sputtering. The substrate temperature was constantly maintained at 500 °C. Different uranium oxide phases (including UO2−x, UO2, U3O7 and U3O8) were obtained by controlling the percentage of the O2 flow rate to the total gas flow rate (fO2) in the chamber. The crystal structure of the films was characterized using X-ray diffraction and the microstructure of the films was studied using transmission electron microscopy and atom probe tomography. When the fO2 was below 10%, the film contains a mixture of metallic uranium and UO2−x phases. As the fO2 was controlled in the range of 10-13%, UO2 films with a (2 2 0) preferential orientation were obtained. The oxide phase rapidly changed to a mixture of U3O7 and U3O8 as the fO2 was increased to the range of 15-18%. Further increasing the fO2 to 20% and above, polycrystalline U3O8 thin films with a (0 0 1) preferential orientation were formed. The hardness and Young's modulus of the uranium oxide films were evaluated using nanoindentation. The film containing a single UO2 phase exhibited the maximum hardness of 14.3 GPa and a Young's modulus of 195 GPa. The UO2 thin film also exhibited good thermal stability in that no phase change was observed after annealing at 600 °C in vacuum for 104 h.