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.

制备超导电子枪用铌-铅（Nb-Pb）光电阴极的工艺，需在铌（Niobium, Nb）表面沉积铅（Lead, Pb）层，且因必须采用高压超纯水对电子枪进行强制清洁，故需确保镀层具备足够的附着力。这一要求实则颇具挑战：铌与铅既无法形成固溶体，也不具备良好的相互润湿性。迄今为止，尚未实现足够的镀层附着力。本研究提出一种提升附着力的新方法：在通过磁控溅射（magnetron sputtering）与阴极电弧（cathodic arc）沉积最终的光电发射铅层之前，预先向铌基底中注入铅离子。本研究的目的在于验证该工艺的有效性。本文的研究范畴涵盖针对10个试制Nb-Pb光电阴极开展的实验，用以展示采用预注入铅的铌基底对铅层附着力强度（通过划痕试验（scratch test）测得）的影响。上述试验结果将通过对镀层与基底之间过渡区（transition zone）的纳米压痕（nanoindentation）测试进行分析阐释。当镀层与基底间的纳米力学性能呈梯度变化时，可实现附着力的提升。实现此类梯度过渡的具体路径，将在后续研究中予以探讨。本次研究所得结果可为超导Nb-Pb光电阴极的制备提供参考依据。