Fabrication of ZrN Barrier Coatings for U-Mo Microspheres Via Fluidized Bed Chemical Vapor Deposition Using a Metalorganic Precursor

A fluidized bed chemical vapor deposition (FB-CVD) system with an inverted reactor was used to develop barrier coatings of zirconium nitride on the surface of U 8 wt% Mo microspheres in order to examine the resulting coating structure and composition, as well as to evaluate the effects of certain process parameters on the coating properties. The process utilized the metal-organic precursor tetrakis(dimethylamino)zirconium heated to 51°C ± 2°C. Time-dependent studies analyzed the coating produced after 2 to 8 days of operation at 100 mL/min precursor carrier flow rate and 500 mL/min fluidization flow rate. Flow-rate–dependent studies produced coated samples using a total flow rate of 600 mL/min, with the precursor carrier flow rate ranging from 100 to 300 mL/min and the fluidization flow rate adjusted accordingly. Ultrahigh-purity argon was used for the precursor carrier and fluidization gas. The CVD reaction was carried out at 280°C ± 10°C with precursor transport tubes heated to 60°C to 75°C. The zirconium-based coatings were manufactured up to 2.2 ± 0.3 µm thick after 2 days of deposition. The estimated coating thickness was not significantly impacted by extending the duration of the deposition process or increasing the precursor carrier gas flow rate. Imaging of the microsphere cross sections provided evidence of uranium oxide, zirconium oxide, and zirconium nitride layers formed within the coating, with nitrogen content becoming more dominant farther from the bulk U-Mo surface. Certain modifications to the process would improve the results, including installing the system inside an inert atmosphere to limit oxygen contamination and higher resolution analyses to better determine coating structure, and finally, performance of a kinetic study of the coating process, particularly over the first 24 h in which the majority of the deposition was suspected to have occurred by enabling partial powder removal during FB-CVD operation to analyze samples from different points within a single experiment.