Magnetron Sputtering of Tungsten and its Application in Core-Shell Fabrication

Abstract

Magnetron sputtering has developed as a common and reliable method for the fabrication of thin films due to its ability to readily deposit a wide range of materials. However, much like other physical vapor deposition techniques, it is a line-of-sight limited method, which means deposition can only occur on surfaces directly exposed to the sputtering plasma. A focus of recent studies has been to utilize new chamber designs to overcome this line-of-sight limitation for the fabrication of core-shell structures. This research focused on understanding the deposition of tungsten films on both traditional and powder substrates, to fabricate core-shell structures for application in nuclear thermal propulsion fuel elements. The fundamentals of tungsten film growth were examined by deposition on traditional planar substrates. The stress state, grain size, and crystal structure of these films were found to vary based on the deposition parameters with significant differences in properties occurring at low working pressures. After examining films on traditional substrates, a piston crank agitation system was utilized to transition towards the fabrication of core-shell structures. The level of agitation imparted on the powder bed was found to be critical in promoting ideal conformal coatings over the entire batch of particles. With successful core-shell fabrication achieved, the ability of those structures to improve nuclear thermal propulsion fuel elements was investigated. The deposition of tungsten shells on surrogate nuclear fuel particles was found to enhance their post-consolidation microstructures by improving the particle-particle separation. This separation is deemed necessary to improve the overall efficiency and limit fissile loss of the fuel elements. Furthermore, the addition of carbide nanoparticles was found to limit the grain growth of the fuel elements metallic matrix during high temperature environmental testing, which could result in improved material lifetime. Overall, this work provides a better understand of the magnetron sputtering of tungsten core-shell structures from the fundamentals to final application.