(528a) Particle ALD of Tungsten Nitride As a Hydrogen Environmental Barrier Coating for Nuclear Space Propulsion
Ultrathin WN films were deposited by atomic layer deposition (ALD) on zirconia nanoparticles and yttria stabilized zirconia (YSZ) micropowders using a fluidized bed reactor. The as-deposited film contained tungsten (W) in a variety of oxidation states, but was shown to dissociate nitrogen and form a crystallized W film upon annealing. The filmâs intended use as a hydrogen barrier coating motivated the investigation of hydrogen diffusion in W using density functional theory (DFT) and differential thermal analysis (DTA). The diffusion pathway was determined from the lowest energy adatom site and the hydrogen charge state at various sites along the pathway were calculated. DTA was used to investigate the efficacy of the W/WN films in preventing hydrogen attack experimentally. The temperature at which hydrogen reacted with the sample increased with film thickness, thereby indicating that the film inhibited this reaction. However, the existence of a hydrogen reaction peak in the thicker film indicated a material with greater than 2.0 eV energy barrier is necessary for a substantial decrease in hydrogen interaction with the substrate. This is the first study for ALD of WN on particles from bis(t-butylimido)bis(dimethylamino)tungsten(VI) and the use of WN ALD films as environmental barrier coatings (EBCs) for high-temperature H2 above 1000oC.