Novel Plasma-Electrolytic-Oxidation Technique to Reduce Erosion and Increase Lifetime of ECR Components

The Department of Energy requires research to advance fundamental accelerator technology and its applications to nuclear-physics scientific research. Innovations are sought that will improve Nuclear-Physics Scientific User Facilities and the wider community’s experimental programs. Passivation techniques or other surface treatments to Electron-Cyclotron-Resonance components are of interest to reduce contamination from heavy-ion-induced erosion of alloys used in the Electron-Cyclotron Resonance source.Key properties for the surface passivation layer include: 1) high secondary electron emission, 2) low heavy-ion erosion rate, 3) excellent adhesion to the metal substrate, 4) smooth surface finish, 5) chemical, thermal, and vacuum stability, 6) thermally conductive surface, and 7) ability to coat inside-diameter surfaces. Our research objective is to demonstrate a novel conversion-coating technology, Plasma Electrolyte Oxidation, to convert value-alloy (e.g., 6061 Aluminum) surfaces to crystalline aluminum-oxide (alumina) and mullite that will exhibit lower sputter yield than either pure Aluminum or bare 6061 Aluminum. This unique high-voltage anodizing method produces adherent, hard crystalline-oxides on value metals / alloys (e.g., Al, Mg, Ti, Zr, Ta). The method is an immersion non-line-of-sight method enabling it to treat complex shapes, consumes part of the underlying substrate resulting in excellent oxide adhesion, produces a harder surface than conventional “hard” anodize, uses environmentally- friendly electrolyte solutions, and can be doped with nanoparticles to improve oxide toughness / durability and reduce surface roughness. Our team has already converted several Al alloys (e.g., 3003, 2024, 6061, 7075 Aluminum), Titanium alloys (e.g., pure Titanium and Ti6Al4V), and Zirconium-alloys. We have also added oxide (e.g., alumina, zirconia) and conductive carbide (e.g., silicon carbide, boron carbide) nanoparticles to the electrolyte solution to reduce surface finish and increase toughness of the growing crystalline oxide. From the literature, alumina and magnesium- oxide exhibit a 76.5% and 90% reduction, respectively, in erosion rate when exposed to a 600eV Argon-ion plasma compared to bare Aluminum and Magnesium. Therefore, it appears these oxides will offer a longer-lifetime surface for Electron-Cyclotron-Resonance components. In Phase I, we will coordinate with candidate Department of Energy and University end-users to define components, alloys of construction, and desired oxide thickness. Converted Aluminum- alloy coupons will be prepared with selected electrolytes along with specific nanoparticles for oxide toughening for subsequent characterization and testing. Surface characterizations will include: 1) tape-peel and tensile plug bond-adhesion measurements, 2) surface finish measurements using Laser Scanning Confocal Microscopy, 3) vacuum compatibility and Argon- ion erosion in a vacuum chamber (using weight change as the metric), 4) X-ray diffraction to determine the alumina, mullite phases formed, and 5) conversion-coating thickness, morphology, and composition using Scanning Electron Microscopy/Energy Dispersive Spectroscopy. Down- selection of the best Plasma Electrolyte Oxidation parameters will be made in Phase I for recommendations for additional characterization in Phase II. This will include shipment of Plasma Electrolyte Oxidation processed coupons (e.g., extraction electrode) to Department of Energy or University Laboratories for screening in actual plasma / ion environments to provide a more realistic assessment of erosion-rate, adhesion, and lifetime. After these more realistic exposures, subscale or actual components will be processed by Plasma Electrolyte Oxidation to demonstrate application to Electron-Cyclotron-Resonance components. We will also survey the Nuclear- Physics community to learn about additional application areas, such as electron-ion collider and superconducting radio-frequency cavities, among other suitable components. Other application areas include wear and corrosion-resistant surfaces for components used in recreational and transportation vehicles, agriculture equipment, and components for military vehicles.Keywords: plasma-electrolytic-oxidation, crystalline aluminum-oxide, conversion-immersion, non-line-of-sight, ion-erosion, surface-finish, adhesion, toughness, complex-shapesSummary of Members of Congress: Demonstration of erosion-resistant oxide coatings on the internal surface of Electron-Cyclotron-Resonance source chambers will reduce contamination of sources thereby increasing the performance, reliability, and increase component lifetime. A novel high-voltage immersion-conversion process, Plasma Electrolytic Oxidation (PEO), will be applied to the value alloy 6061 aluminum, to convert the alloy surface to a crystalline-oxide, which will offer lower erosion rate than bare aluminum, exhibit excellent adhesion, and corrosion resistance. The PEO techni