Microstructural Evolution and Wear Resistance Enhancement of Electron-Beam-Fabricated Nickel Titanium Coatings on Titanium by Heat Treatment

We previously proposed a novel method for fabricating NiTi coatings on pure Ti surfaces using electron-beam multi-track scanning to improve wear resistance. However, electron-beam scanning coatings exhibit spatial inhomogeneity in phase and elemental distributions, which is suggested to decrease wear resistance. In the present study, we investigated the effects of heat treatment on reducing inhomogeneity and improving the performance of the coatings. As a result, the homogeneity of the electron beam (EB) scanned coatings was significantly enhanced after annealing (900°C, 10 min), and then the Ni4Ti3 phase precipitated in the annealed EB-scanned coatings after aging (400°C, 60 min). The results of nanoindentation show that the hardness/indentation modulus (H/E) ratio and depth recovery ratio (ηd,h) of the Ti-rich EB-scanned coatings increased by 27% and 21%, respectively, after annealing and aging. The H/E ratio and ηd,h of the Ni-rich EB-scanned coating improved by 8% and 14%, respectively, after annealing and aging. The wear test results demonstrate that the wear rates of the two coatings decreased by 52% and 26% after annealing and aging, respectively, approaching the wear performance of bulk Nitinol.