Impact of machining strategy on OFE copper for SRF applications

Abstract

Abstract. The study evaluates the impact of various milling strategies - on the surface and subsurface quality of an Oxygen-Free Electronic (OFE) copper substrate, and subsequently on the niobium (Nb) coating performance- for Superconducting Radio Frequency (SRF) cavities. Several cutting tools, cutting conditions and machining paths are compared, given their paramount influence in the final ‘SRF’ quality of the machined surfaces. The near surface region is evaluated by different means: surface roughness and shape accuracy are measured using a profilometer and Coordinate Measuring Machine (CMM) respectively; the thickness of the layer affected by machining on the OFE copper substrate is estimated by Focused Ion Beam Scanning Electron Microscopy (FIB - SEM). The dislocation density in the near surface layer is assessed by Grazing Incidence X-Ray Diffraction (GI-XRD) in both the OFE copper substrate and afterwards in the niobium coating. Main investigation results are discussed, and conclusions drawn for new guidelines on machining strategy and for future research developments.