**4. Conclusions**

Stainless steel films were fabricated via an electron beam physical vapour deposition method with starting electron beam power percentages of 3%–10%. The thin layers obtained with a controlled deposition rates of 0.05 Å/s and 0.16 Å/s have shown a uniform elemental distribution with a well-constructed film structure that covered the whole exposed area, while higher deposition rates have illustrated semi-detachments in the film structure. Furthermore, the closest film elemental content to SS 316L was achieved with a 0.05 Å/s, where higher deposition rates were seen to extend the maximum and minimum elemental limits of SS 316L. Surface topography of SS 316L before and after depositing 50 nm, 100 nm, and 150 nm films, using controlled 0.05 Å/s fabrication rate, was then examined. The results illustrated a reduction in structure height on the surface, MHS, RMSR, and average roughness from 87.3–204 nm, 291 nm, 12 nm, and 7.87 nm (uncoated substrate) to 35.9–83.7 nm, 120 nm, 6.86 nm, and 4.88 nm (150 nm coated substrate), respectively. It also showed, via the obtained Ssk values, that the degree of symmetry of the surface heights about the mean plane was improved by ~49.5% for the reference substrate after 150 nm film deposition. Surface wettability of the as-prepared samples were afterwards characterised with DIW's, of pH 4, 7, and 9, at a 20–60 ◦C liquid temperatures. The film thickness was seen to be inversely related to the liquid–surface CA, and hence the CA reduced with the increase in film thickness. Moreover, the rise in DIW's temperature has been shown to weaken the hydrophobic nature of the as-prepared substrates. It was also noticed that, unlike the DIW of pH 7, the liquids of pH 4 and 9 demonstrated some fluctuation in their CA data trend.

In summary, this article unlocks a new approach for depositing stainless steel thin films using an electron beam physical vapour deposition technique. The resulting film is ultrathin, uniform, conformal, and controllable. Moreover, an extension towards depositing different grades of stainless steel can be achieved by changing the composition of the evaporant source, based on exploratory experiments; and hence may facilitate a feasible route towards industrial usage of the process after further film properties investigation is provided (e.g., corrosivity, cohesion, hardness, and abrasiveness). Furthermore, as our approach is the first example of any stainless steel EB-PVD coating, the present work marks an important milestone in the future of stainless steel depositions on metallic surfaces and is expected to be beneficial to many applications such as medical equipment, automotive parts, and heat transfer devices.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/4/571/s1, Figure S1: (a1) SEM image of the 0.05 Å/s deposited film; (a2) SEM image of the 0.16 Å/s deposited film; (a3) SEM image of the 0.82 Å/s deposited film; (a4) SEM image of the 1.07 Å/s deposited film; (a5) SEM image of the 1.45 Å/s deposited film; (b1) SEM image of the as-received evaporant source before film deposition; (b2) Higher resolution SEM image of the as-received evaporant source before film deposition; (b3) SEM image of the as-received evaporant source after 0.05 Å/s film deposition; (b4) Higher resolution SEM image of the as-received evaporant source after 0.05 Å/s film deposition; Figure S2: (a) SEM image and its elemental maps of the characterized 150 nm deposited SS film at 0.05 Å/s on Cu substrate; (b) EDS X-ray spectrum of the elements, Figure S3: (a) AFM images and analysis of the uncoated substrate; (b) AFM images and analysis of the 50 nm coated substrate; (c) AFM images and analysis of the 100 nm coated substrate; (d) AFM images and analysis of the 150 nm coated substrate, Table S1: Contact angle measurements data.

**Author Contributions:** N.A. conceived and designed all the experiments, and manufactured the substrates. N.A. and A.A. carried out all the EB-PVD film deposition. F.A.-Z. conducted the AFM characterisation experiments and their analysis. J.A.T. and H.B. conducted the XRD and XRF measurements. A.A. and A.S. prepared and characterised the density, viscosity, and change in pH value of the working fluids. M.S. conducted the SEM and EDS characterisations experiments. N.A. and F.A.-Z. conducted and analysed the contact angle measurements. The manuscript was primarily written by N.A. using the inputs provided from all the authors.

**Funding:** This work was financially supported by the Kuwait Institute for Scientific Research (KISR) and Cranfield University.

**Acknowledgments:** We acknowledge the help provided by M. Sherif El-Eskandarany, the program manager of Nanotechnology and Advanced Materials Program at KISR, for his help and support throughout the conducted work.

**Conflicts of Interest:** The authors declare no conflict of interest.
