The influence of initial lead implantation into niobium substrate on the adhesion of Pb layers obtained in cathodic arc and by magnetron sputtering

The procedure for preparing superconducting electron gun Nb-Pb photocathodes, composed of a lead layer deposited on the niobium surface should ensure their sufficient adhesion due to mandatory cleaning of the gun in ultrapure water under high pressure. It is a real challenge since Nb and Pb neither form solid solutions nor present good mutual wettability. So far adequate adhesion has not been achieved. We present a new approach to increase adhesion by preliminary lead implantation into niobium before depositing the final photoemissive Pb layers by magnetron sputtering and in a cathodic arc. The purpose of this study is to demonstrate the effectiveness of such a procedure. The scope of the article includes a description of experiments carried out on 10 trial Nb-Pb photocathodes, showing the impact of using pre-implanted niobium substrates on the adhesion strength of lead layers measured in scratch tests. The results of these tests are interpreted based on the nanoindentation of the transition zones between the layers and substrates. Increased adhesion was reached at a graded change in nanomechanical properties between film and substrate. The way to achieve such gradual transition is proposed for the future work. The presented results serve as a guideline in preparing superconducting Nb-Pb photocathodes.