**1. Introduction**

Stainless steels are passive alloys, which due to their chemical composition tend to form a thin oxide layer that inhibits the metal dissolution in corrosive environments [1]. Physical, mechanical, and anticorrosive properties of the alloy are highly related to its microstructure, where one or two phases (i.e., austenitic, ferritic, or both) may be formed [2]. Due to their unique properties, including adaptation to changes in solution salinity and pH level, these alloys are widely used in application areas such as construction and building [3], heat exchangers [4], and biomedicine [5]. In addition to these application areas, stainless steel (SS) in its powder form was reported to be used in fabricating nanofluids [6,7], which are heat transfer fluids; and surface coatings, via the cold spray deposition method [8]. Since the current trend in engineering industry, such as the automotive sector, is to rely on light construction materials (e.g., aluminium and its alloys) in order to reduce the overall weight of constructions and manufactured parts, hence SS surface coatings on metals are considered to be a promising solution for achieving this target while providing anticorrosion and wear resistance to the bulk material [9,10]. So far, all reported deposition procedures of SS films are seen as adaptation of cold gas dynamic spraying of premixed powders onto the surface [9,11], wire feedstock melt down on surfaces via electron beam solid freeforming (EB-SFF) [12,13], ionic sputtering of a target

source [14–21], thermal evaporation of a source and ionic bombardment of the particles by ion beam assisted deposition (IBAD) approach [22], and pulsed laser evaporation technique [23,24]. However, some of these routes can be incompatible for industrial usage because of the lack in precision of controlling the deposited layer thickness, the thin film can be associated with contamination, and the cold spray deposited particles and/or its coated surface can suffer from intensive plastic deformation. Furthermore, the aforementioned methods raise processability and cost concerns due to the large number of parameters involved in the coating procedure. For example, when employing cold gas dynamic spraying approach, parameters such as the nozzle dimensions, jet velocity, particles size, and particles impact temperature need to be considered cautiously before starting the process.

On the other hand, electron beam physical vapour deposition (EB-PVD), which is a high vacuum thermal coating technology, is considered to be a simple and relatively cheap process in which a focused high energy electron beam is directed towards melting an evaporant material inside a vacuumed chamber. The evaporating material is then condensed on the surface of a substrate or component to form the film layer [25]. The distinct advantages of this approach are the high deposition purity, enlarged coating area, precise film thickness, in-situ growth monitoring, and smoothness control [26]. In addition to the associated benefits, the aforementioned technique has proven its capability of depositing alloys, as demonstrated by Almeida et al. [27] with their MCrAlY film fabrication study. On the industrial scale, EB-PVD has been widely employed for coating materials, including SS bulk materials, but to the authors of this article's knowledge, has never been reported as the means to deposit SS thin films [25,28].

Herein, we demonstrate the deposition of SS thin films on metallic substrates using an EB-PVD approach. The present study, based on the conducted literature review, is the first reported EB-PVD process for forming SS films and does not aim to challenge other film fabrication processes but rather unlocks opportunities for new ways of depositing SS thin films. Furthermore, to illustrate the crucial role of the controlled deposition rates on the uniformity and elemental distribution within the fabricated thin layer, a comparison between the morphologies of the SS films, of 150 nm, coated on copper (Cu) substrates with fixed deposition rates of 0.05 Å/s to 1.45 Å/s was performed. The main reasons behind selecting Cu as the hosting substrate is due to: (1) the fact that Cu is not part of the forming elements of the SS 316L alloy and can therefore be easily diverted from the deposited film when elemental characterisations is performed, and (2) most commercial heat pipes are made of Cu, because of the material high thermal conductivity, but usually faces erosion damages from water flows [29]; so providing an insight into SS coatings on Cu would be desirable for the industry since it can help reduce such common phenomena with minimum degradation effects on the heat transfer properties of the bulk material. Moreover, since there is still a need for further investigation and clarification of the wettability behaviour of SS 316L surfaces and the effect of different parameters on their wetting phenomena [30], the impact of the 0.05 Å/s as-prepared films on the surface topography and water wettability behaviour was explored for 50, 100, and 150 nm SS layers coated on SS 316L substrates. The expected applications that can benefit from this study include, but are not limited to, medical equipment, automotive parts, and heat transfer devices.
