*3.2. Deposition Morphology*

Fabricated samples of 150 nm SS deposited on Cu substrates, which were synthesised by the EB-PVD starting electron beam powers of 3% to 10%, are shown in Figure 3. The SS thin film layer produced can be visually observed (Figure 3(b2–b6)), except for the corners where the adhesive tapes were placed to adjust the samples on the sample holder. The time required for achieving the film thickness varied from 500 min, at a fixed deposition rate of 0.05 Å/s, down to 17.25 min, via a 1.45 Å/s controlled evaporation rate. Samples were then characterised to determine the effect of the controlled deposition rates on the film and evaporant material (Figure 3(c1–c6)) morphologies, via the SEM and EDS analysis. The SEM characterisation of the 0.05 Å/s and 0.16 Å/s fabricated samples has demonstrated a well-constructed film structure as shown in Figure 3(a1,a2) (alternatively, Supplementary Figure S1(a1,a2)), whereas higher deposition rates have resulted in the development of non-uniform films with partial detachments from the surface as seen in Figure 3 (alternatively, Supplementary Figure S1(a3–a5)). Both observations can be linked to the phase of the SS film formed due to the elemental ratio of the deposit, where a well-constructed film structure indicates a ferritic phase and a semi-detached structure illustrates an austenitic phase [37]. In addition, analysing the evaporant source, before and after 0.05 Å/s deposition, has shown a local melt down in the material, due to the electron beam being focused on a single location at a low power (i.e., the beam is fixed and does not follow a movement path across the evaporant source), thus resulting in surface microstructural changes as shown in Figure 3(d1–d4) (alternatively, Supplementary Figure S1(b1–b4)). Further inspection of the samples using the EDS device (Figure 4) has illustrated that, unlike higher controlled deposit rates, the film fabricated via 0.05 Å/s was within the SS 316L elemental composition acceptable ranges, except for the chromium, manganese, and nickel, which showed partial divergences of +3.33%, +1.15%, and −5.24%, respectively. This indicates that the film is indeed SS but of a different category [38–40], whereas to obtain SS 316L thin films the evaporant source would require some modification in its composition (i.e., fabricate our own EB-PVD evaporant source). In addition, the film elemental distribution of the fixed 0.05 Å/s deposition was seen to be uniformly distributed

along the substrate surface as shown in Figure 5 (alternatively, Supplementary Figure S2a–b). It is important to note that due to the limitation of the EDS device used, elements of less than 0.5% of the composition mass could not be elementally mapped, as seen in the case of molybdenum. The EDS elemental composition percentages of the 150 nm SS deposited film at a fixed rate of 0.05 Å/s are tabulated in Table 2.

**Figure 3.** Characterisation of the Stainless steel evaporant source and deposited thin films, where (**a1**–**a5**) shows the SEM images of the 0.05–1.45 Å/s as-deposited films structure, (**b1**–**b6**) illustrates the copper substrates before and after SS deposition, (**c1**–**c6**) demonstrates the physical changes in evaporant source caused by different deposition rates, and (**d1**–**d4**) shows the SEM images of the evaporant source before and after 0.05 Å/s film deposition.

**Figure 4.** EDS characterisation of the chemical elemental percentages of the evaporant source (0% power), and the SS thin films deposited with a starting beam power of 3% up to 10%. The bars at the top and bottom of each data point indicate the maximum and minimum range of each element percentage of the stainless steel 316L composition and was attained from the XRF device installed database.

**Figure 5.** EDS elemental analysis, where (**a**) is the SEM image and its elemental maps of the characterised 150 nm deposited SS film at 0.05 Å/s on Cu substrate, and (**b**) demonstrates the EDS X-ray spectrum of the elements within the film shown in (**a**).

**Table 2.** EDS elemental composition percentage of the 150 nm SS fabricated film at 0.05 Å/s controlled deposition rate.

