Abrasion resistance of a Nb3Sn magnetron-sputtered coating on copper substrates for radio frequency superconducting cavities

The paper presents the first study of the abrasion resistance and degradation processes of magnetron-sputtered Nb3Sn coatings on electropolished (EP) or chemical polished (CP) copper substrates. The coating-substrate system is of interest for application in radio frequency superconducting cavities as an improvement on bulk niobium cavities. The microstructural and chemical properties of the substrates and coating were examined by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), electron backscatter diffraction, X-ray diffraction, and focused ion beam cross-sectioning. The abrasion resistance was investigated by scratch testing under progressive loads up to 200 N, after which the scratched surfaces were characterised by SEM/EDS to determine the failure mechanisms of the coatings. Several stages of coating damage were identified, including nodule compression, cracking, pitting, fragmentation, comminution and detachment. Nevertheless, substantial amounts of fragmented and embedded coating remained attached to the copper up to the highest load. The findings indicated similar abrasion resistance behaviour for the EP and CP conditions.