Plasma Sputtering Behavior of Structured Materials
- Author(s): Li, Gary Zhi
- Advisor(s): Wirz, Richard E
- et al.
The energetic ion bombardment, or sputtering, of plasma devices is a universal phenomenon and challenge. In applications that require long lifetimes and high performance, such as electric propulsion or nuclear fusion, the plasma facing components must be designed to minimize sputtering. Next generation devices will require advanced materials that can reduce sputtering beyond what is currently possible with standard solid materials. To address this need, the objective of this dissertation is to investigate the plasma sputtering behavior of structured materials for long-term resilience and reduced contamination.
The Plasma interactions facility at UCLA is configured to perform sputtering measurements of materials with evolving surface morphologies. A focused argon plasma is directed to an electrically biased target for energetic ion bombardment, and a scanning quartz crystal microbalance is used to obtain time-resolved sputtering measurements. The first experiment in this dissertation exposes a novel micro-architectured molybdenum sample to 300 eV argon ions for 17 h. The total sputtering yield is initially significantly reduced compared to that of a flat sample, but is shown to approach the flat yield after 10 h as the surface features are eroded away. The discovery that featured surfaces can only provide a sputtering reduction for a limited time motivates the investigation of open-cell foams as a volumetrically structured material.
Two aluminum foams with 40 Pores Per Inch (PPI) and 10 PPI pore density are tested in the Pi facility and sputtering yield measurements are obtained. For the first time, the aluminum foams demonstrate a sustained reduction in sputtering yield, ranging from 40% to 60% compared to a flat surface over a 30 h exposure to 300 eV argon ions. Furthermore, the reduction in yield is found to be as large as 80% at lower ion energies. The measured yields prove that foams can maintain a reduced sputtering yield and that the plasma environment plays a major role in determining the foam sputtering behavior. A plasma-foam sputtering theory and analytical model are introduced to understand the experimental findings and study the key physical processes.
A metallic open-cell foam uniquely interacts with plasma depending on the ratio of the foam pore size to the plasma sheath thickness. A plasma-infused foam is defined as having pore sizes much larger than the sheath thickness, which will allow plasma to infuse the foam interior. In the opposite limit, a plasma-facing foam will have an external sheath that accelerates ions normal to the macroscopic surface. The angular sputtering profiles at low ion energies and reduced ion fluxes from plasma density gradients are shown to play a major role in reducing the sputtering yield. Additionally, the effective sputtering area is significantly increased for a plasma-infused foam, resulting in a larger yield relative to a foam with less plasma infusion. The key processes involved in reducing, or in some cases enhancing, the effective sputtering yield are found to be the plasma-foam sputtering behavior and the geometric recapture of sputtered particles.
The dynamic interplay between plasma and material processes is shown to govern plasma-foam sputtering behavior. In addition to excellent sputtering resistance, metallic foams offer attractive thermomechanical properties and the potential for interlayer transport of fluids for various plasma applications. The potential impact of plasma-foam sputtering for both theoretical investigation and practical application is discussed and multiple avenues of future work are recommended.