## 2 Materials and Methods
### Characterization Techniques  
The morphological and elemental analyses of the coatings were performed using a Quanta 250 scanning electron microscope (FEI Company, Eindhoven, The Netherlands) operating at 15 kV, equipped with an EDAX energy-dispersive X-ray spectrometer (EDAX, Mahwah, NJ, USA) for semi-quantitative compositional assessment. To avoid interference from the substrate, samples for SEM-EDS analysis were prepared on silicon substrates.  
The phase composition of the HEA coatings was analyzed using a Rigaku Ultima IV X-ray diffractometer (Rigaku Corporation, Akishima, Japan) equipped with parallel beam optics and a thin-film attachment. XRD scans were conducted over a 2\(\theta\) range of 20-100\({}^{\circ}\) at a scanning speed of 2\({}^{\circ}\)/min with a step size of 0.02\({}^{\circ}\) and a fixed incidence angle (\(\omega\)) of 0.5\({}^{\circ}\). The Cu K\(\alpha\) radiation source (\(\lambda\) = 1.5405 A) enabled detailed investigation of the crystalline phases and grain sizes in the deposited coatings.  
The coatings adhesion was assessed using a NANOVEA Scratch Tester (NANOVEA, Irvine, CA, USA) system with a 120\({}^{\circ}\) conical indenter and a 50 \(\upmu\)m tip. The indenter was applied with a load range of M0-M1250, measuring the critical load for coating delamination. The load of the indenter was progressive from 1 Newton to 10, with a 5 N/min loading rate for a distance of 5 mm.  
Vickers microhardness was measured using a Wilson VH3100 (Wilson, Chicago, IL, USA) hardness tester with a 0.05 kgf load held for 30 s. Five indentations were averaged for both the coated stainless steel substrates and target alloys, allowing for a comparative analysis between 5% and 10% Cu HEA coatings.  
Figure 1: Deposition of HEA by DC magnetron sputtering.  
The electrochemical corrosion studies were conducted at room temperature in an ASTM (American Society for Testing and Materials) corrosion cell with a volume of 1 L, equipped with a three-electrode setup: the working electrode, which was the material under analysis, with an exposed surface area of 1 cm\({}^{2}\); the reference electrode, an Ag/AgCl electrode in a 3 M KCl solution; and the counter electrode, a platinum wire. These three electrodes were connected to an Autolab PGSTAT 128N potentiostat/galvanostat (Methrom, Autolab Utrecht, The Netherlands) linked to a computer running NOVA v2.1 software. The electrolyte solution used was NaCl at a concentration of 3.5 g/L, in accordance with ASTM G61-86 standard (Reapproved 2003) [26]. Experiments were carried out after a 60 min immersion period of the electrodes in the electrolyte solution. The working procedure for the experiments involved linear polarization, which included determining the open circuit potential (OCP) of the working electrode, followed by linear sweep voltammetry (LSV) with a stepped profile in the potential range of \(-\)0.1 V to +0.1 V relative to the open circuit potential, at a sweep rate of 0.001 V/s. Corrosion analysis was performed through the automatic determination of corrosion parameters, providing valuable insights into the material's corrosion resistance under the specified test conditions.  
An AFM characterization was conducted to obtain nanoscale topography of the coatings. These studies were performed by means of AFM (, Bruker Nano GmbH, Berlin, Germany) operating with an image resolution of 512 \(\times\) 512 pixels. The images were collected in Quantitative Imaging (QI) mode, with pixel times of 6-18 ms for each pixel, using an Cr/Pt-coated silicon nitride cantilever (Multi75-G tip, Budget Sensors, Watsonville, CA, USA) with a nominal spring constant of 75 N m\({}^{-1}\). The software JPK Data Processing v6.4 (JPK Instruments AG, Berlin, Germany) was used for offline analysis of the topography and the film roughness. Image flattening was performed with the first order, least-square polynomial function, which removes the tilt and the vertical z-offset between line scans.