## 2 Materials and Methods  
Inconel 718 nickel superalloy was selected as the substrate material in the form of cylindrical samples (\(\phi\)25.4 mm \(\times\) 38.1 mm long) according to the ASTM-C633 standard requirements [29]. Before depositing the bond coats, samples were grit blasted with corundum at a 45\({}^{\circ}\) incidence angle with a blasting distance of 100 mm and degreased with isopropanol. The VPA (vapour-phase aluminium) bond coat was deposited on the substrate using the above-the-pack technique with an average thickness of 60 \(\upmu\)m. The surface of the bond coats was prepared by rough grinding (320\(\#\) mesh silicon carbide sandpaper), grinding (SiC paper 500\(\#\)), or polishing (diamond suspensions with a particle size of 3 \(\upmu\)m) in order to differentiate the surface roughness. In addition, as-coated samples without any treatment were used for comparison. Subsequently, the bond coats were activated with shot pinning (400-600 \(\upmu\)m glass beads) and degreased with isopropanol. To address potential inconsistencies, all samples were subjected to a heat treatment at 1050 \({}^{\circ}\)C for 30 min before the deposition process. This step was designed to ensure complete stress relaxation and to eliminate any residual stresses induced by the different polishing materials. The 7\(\%\) yttria-stabilised zirconia (7 YSZ) ceramic top coat was deposited on each bond coat group by the EB-PVD process (Smart Coater, ALD Vacuum Technologies GmbH) with the specific parameters shown in Table 1. The SMART Coater features a single electron beam gun with a maximum output of 160 kW, capable of evaporating from two crucibles. These crucibles are arranged perpendicular to the sample rotation axis and aligned with the main telescope sting. During the run, the crucibles can be moved parallel to the main sting axis while evaporation continues to form the top coat. For this study, no movements of the samples were applied. Samples before and after the EB-PVD process can be seen in Figure 1. The EB-PVD coating equipment used in the current study is shown in Figure 2. The process parameters in each trial were as similar as possible to obtain coatings with properties that were closely identical. All top coat depositions in this study were performed at a substrate temperature of approximately 990 \({}^{\circ}\)C using single-source evaporation. The temperature range observed during the deposition process was within the optimal range for the deposition of the ceramic coating, specifically between 950 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C. This temperature range is well known to provide excellent conditions for depositing ceramic coatings. In each EB-PVD process, the sample holder was mounted in a middle rake position, centred above the evaporation pool (Figure 1b) to ensure the most uniform thickness possible. Ingots from the same manufacturer (Phoenix Coating Resources Inc, now a subsidiary of Saint-Gobain Coating Solutions, Worcester, MA, USA) with diameters of 62.5 to 62.9 mm were used. The average thickness of the prepared 7 YSZ coatings was approximately 175 \(\upmu\)m. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Trial Number** & **Samples** & **Emission** & **Pressure** & **Temperature** & **Feed Rate** & **Duration [s]** \\  & **Coated** & **Current [A]** & **[mbar]** & **[\({}^{\circ}\)C]** & **[mm/min]** & **Duration [s]** \\ \hline
01 & I, II, III & 3.02 & 0.00925 & 985.8 & 1.5 & 900 \\
02 & IV, V, VI & 3.02 & 0.00923 & 988.9 & 1.5 & 900 \\
03 & VII, VIII, IX & 3.01 & 0.00920 & 992.6 & 1.5 & 900 \\
04 & X, XI, XII & 3.02 & 0.00925 & 996.5 & 1.5 & 900 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: EB-PVD process trial parameters. A surface roughness measuring system (Wave System, Hommelwerke, Villingen-Schwenningen, Germany) was used to measure the surface of the bond coat after different treatments and the corresponding surface of the deposited 7 YSZ coating for each sample. The surface roughness values quoted are an average of 10 measurements for each coating.