Co-sputtered amorphous Nb-Ta, Nb-Zr and Ta-Zr coatings for corrosion protection of cyclotron targets for [18F] production

Protective corrosion resistant coatings serve for decreasing the amount of ionic contaminants from Havar® entrance foils of the targets for [18F] production. The corrosion damage of coated entrance foils is caused mainly by the diffusion of highly reactive products of water radiolysis through the protective film toward Havar® substrate. Since amorphous metal alloys (metallic glasses) are well-known to perform a high corrosion resistance, the glass forming ability, microstructure and diffusion barrier efficiency of binary alloys containing chemically inert Nb, Ta, Zr were investigated. Nb-Ta, Nb-Zr and Ta-Zr films of different alloy composition and ∼1.5 μm thickness were co-deposited by magnetron sputtering. Diffusion barrier efficiency tests used reactive aluminum underlayer and protons of acid solution and gallium atoms at elevated temperature as diffusing particles. Though co-sputtered Nb-Ta and Nb-Zr alloy films of different contents were crystalline, Ta-Zr alloy was found to form dense amorphous microstructures in a range of composition with 30-73% atomic Ta. The diffusion barrier efficiency of Nb-Zr and Nb-Ta alloy coatings decreased with increase of Nb content. The diffusion barrier efficiency of sputtered Ta-Zr alloy coatings increased with the transition from nanocrystalline columnar microstructure to amorphous for coatings with 30-73 at.% Ta.