Coating yttria stabilized zirconia powders by magnetron sputtering

This thesis describes the application of Physical Vapor Deposition (PVD) for coating powders, with the work motivated by the need to provide conformal coatings for nuclear fuel for use in Nuclear Thermal Propulsion (NTP). The coated material was tungsten, because of its high melting point and low neutron cross-section, yttria-stabilized zirconia (YSZ) was used for the nuclear powder surrogate. The coating was done in a rotating drum that held and moved the powders under a cylindrical cathode. The sphericity of the powders, to improve their flow in the drum, was achieved using a gravity-based plasma Powder Alloying Spheroidization (PAS) process. The particles were coated between 5.5 kWh to 40 kWh resulting in a coating thickness between approximately 70 nm to 540 nm. The coatings were found to have powdery morphology spheres resulting from the particle-to-particle collisions. To further understand the stress state of the deposited film, a series of 100 nm tungsten films were deposited at two different rates (0.05 and 0.2 nm/s) and three pressures (2, 5 and 10 mTorr). At the lowest pressure, regardless of rate, the films had a compressive stress state. Upon increasing the pressure for both rates, the residual stress was near zero. X-ray diffraction revealed that the nominally body centered cubic tungsten film adopted the A15 phase referred to as beta-tungsten.