*2.3. Stainless Steel Film Deposition*

The thin film production method used consists of fabrication at constant deposition rates, as commonly seen in literature [31], and is demonstrated in Figure 1. Eleven of the thirteen substrates were individually placed inside the electron beam physical vapour deposition (EB-PVD) device chamber after being tightly adjusted on the sample holder and were screwed vertically above the evaporation source. The remaining two stainless steel 316L substrates were kept as references for characterisation purposes. Stainless steel AISI 316L bearing balls, which were used as the deposition source, were placed in an 8 cm<sup>3</sup> graphite crucible located at the bottom of the EB-PVD chamber, thus having a fixed target-to-sample distance of 26 cm. The EB-PVD device chamber (40 cm inner diameter × 50 cm inner height) was then vacuumed to a pressure of 6 × <sup>10</sup>−<sup>6</sup> Torr to ensure the removal of all particle contaminations within it, and the film thickness was controlled via an INFICON SQC-310 electronic thickness monitor system (Bad Ragaz, Switzerland) connected to a sensor located inside the chamber. It is worth noting that there was no external heating or cooling applied to the substrates temperature during the film fabrication process. In the case of the copper substrates, the deposition source was evaporated at a set of starting power percentages after which the deposition rates were maintained so that a film layer of 150 nm thick can be achieved. The starting power percentages employed were of 3, 4, 6, 8, and 10%, and the maintained deposition rates were of 0.05, 0.16, 0.82, 1.07, and 1.45 Å/s, respectively. It is worth noting that power percentages less than 3% had no trace of evaporation and that power percentages higher than 12% are restricted by the manufacturer of the EB-PVD device due to safety concerns. As such, the range of power percentage was selected to be from 3% to 10%, with a maximum deviation of ±1% to sustain the deposition rate. As for the stainless steel 316L substrates, based on the elemental characterisation of the film coated on the copper substrates, the evaporation was selected to be at the lowest deposition rate (i.e., 0.05 Å/s) for a set of film thickness of 50, 100, and 150 nm. After the completion of each of the aforementioned particle deposition processes, the substrate was kept in the chamber for 4 h to cool down before removal from the EB-PVD chamber for further analysis.

**Figure 1.** Electron beam physical vapour deposition process, where (Stage 1) shows the schematic illustration of the device configuration, and (Stage 2) demonstrates the source evaporation and film formation process.
