## 1 Introduction  
Thermal barrier coatings (TBCs) have been used for the protection of turbine engine hot section components for several decades [1]. The development of TBCs has helped improve the durability and high-temperature performance of turbine blades, thus improving the integrity of the system and increasing the efficiency of the engine [2]. Thermal barrier coatings typically consist of different layers of metallic bond coat and ceramic oxide top coat on a metallic substrate [3]. The top coat typically consists of YSZ (ytria-stabilised zirconia), composed of zirconia (ZrO\({}_{2}\)) partially stabilised with 6-8% by weight yttria (Y\({}_{2}\)O\({}_{3}\)). Although the YSZ layer has low thermal conductivity and provides thermal insulation to the components, the metallic bond coat improves the adhesion of the top coat and also provides protection against oxidation and corrosion of the metallic substrate [4]. Because the TBC is oxygen-transparent, a thermally grown oxide (TGO) layer develops between the bond coat and the ceramic top coat during service. The TGO provides protection to the underlying component against high-temperature oxidation and corrosion, but as it thickens, it is also the main source of strain that ultimately drives the spallation of the ceramic top coat [5]. The TGO layer is the main location of failure of TBCsystems [6] caused by thermal cyclic stresses generated during service due to the coefficient of thermal expansion mismatch at higher temperatures (1000-1200 \({}^{\circ}\)C) [7]. Electron beam-physical vapour deposition (EB-PVD) columnar coatings are the most widely used TBCs. EB-PVD involves the vapourisation of the coating material using an electron gun. The coatings deposited by EB-PVD have been favoured because their unique microstructure offers the advantage of good bonding between the coating and the substrate, low roughness of the coating leading to aerodynamically smooth surfaces, good erosion resistance [8], and, above all, superior tolerance to mechanical strain and thermal shock during thermal cycling at the high temperatures at which gas turbines operate [9]. In recent years, advances in materials science have led to the development of novel TBC materials that offer improved performance under extreme conditions. One significant breakthrough is the introduction of pyrochlore-structured materials as alternatives to YSZ [10]. These materials have been shown to possess higher thermal stability and lower thermal conductivity, which could potentially extend the life of turbine components at higher temperatures. Additionally, the use of multilayered TBC systems has gained attention [11]. These systems involve the application of multiple layers of different materials to achieve a gradient of properties that can more effectively manage thermal and mechanical stresses. High-Entropy Alloys (HEAs) have also been explored as potential bond coat materials in TBCs [12]. These alloys are composed of multiple principal elements that provide a unique balance of properties that can improve the overall performance of TBCs. HEAs are known for their exceptional mechanical strength, corrosion resistance, and stability at high temperatures, making them ideal candidates for challenging environments such as those found in gas turbines [13]. Therefore, it is extremely important to conduct research aimed at assessing the impact of the bond coat surface quality on the properties of columnar grain growth, not only in standard materials such as YSZ but also in new materials that will play a larger role in future aerospace applications. Conducting research on standard materials may prove crucial in preparing R&D efforts related to the implementation of EB-PVD processes on already quality-tested layers that ensure high adhesion of top coats made from new intermetallic or ceramic materials.