## 1 Introduction  
Thermal spraying is a deposition technique within the field of surface engineering. Various sub-processes can be used to apply coatings with thicknesses ranging from 10 \(\upmu\)m to several millimeters. The substrate undergoes minimal thermal stress, so no structural transformations are expected in materials such as steel. A well-known process is High Velocity Oxy-Fuel (HVOF) spraying, which is also used in repair applications [1, 2, 3]. This method enables the production of corrosion- and wear-resistant coatings with particularly low porosity, often below 1.0% [1, 2, 4, 5]. Thermal spraying, especially HVOF, is often the focus of research involving simulations to investigate multiphase flow problems. The aim is to improve the cost-effectiveness and efficiency of the process [4, 6]. Several studies have shown that the relatively low flame temperature in the HVOF process, compared to other coating processes, combined with the high kinetic energy, results in improved coating properties. One such improvement is reduced oxide formation during deposition, positively affecting the surface composition of the coating [5, 7, 8]. Cermet coatings such as WC-Co or Cr\({}_{3}\)C\({}_{2}\)-NiCr are often used as an alternative to hard chrome coatings [9; 10; 11]. Both cermet coatings offer properties comparable to those of hard chrome coatings, or even significantly better properties, such as corrosion resistance. WC-based coatings exhibit high wear resistance, primarily due to their high hardness, which typically exceeds 1000 HV. When combined with Co or Co-Cr they also demonstrate excellent toughness and ductility while maintaining high strength due to the combination of a hard phase and a metallic binder matrix [1; 3; 4; 12; 13]. A typical feature of WC-based coatings is their high density. For thermally sprayed WC-Co materials this lies around 15-16 g/cm\({}^{3}\). These coatings are generally suitable for use at temperatures up to around 450 \({}^{\circ}\)C. Beyond this, tungsten carbides demonstrate inadequate thermal stability [9; 14; 15; 16; 17; 18]. Excessive thermal stress during deposition, for example, can lead to decardurization (the formation of W\({}_{2}\)C) or oxidation, increasing the heterogeneity of the coating [4]. Operation at higher temperatures (>450 \({}^{\circ}\)C) further increases the risk of surface oxidation [4; 19]. The suitable particle sizes for HVOF spraying vary significantly depending on the application. In general, this technique cannot be used to process nanoparticles [2]. WC-Co-Cr coatings also demonstrate excellent corrosion resistance. Studies are often conducted in environments resembling seawater (3.5% NaCl, presence of Cl\({}^{-}\)), where the chromium content significantly improves corrosion resistance compared to WC-Co [14; 20; 21; 22]. Thanks to their high hardness and outstanding wear and corrosion resistance, WC coatings are widely used in demanding fields such as aerospace, automotive engineering, internal coating of cylinders, and pump manufacturing [12; 23; 24]. However, the use of cobalt-based materials raises environmental and health concerns. Current scientific findings indicate that cobalt may be carcinogenic, which is why alternatives are being sought, and the use of cobalt should be minimized wherever possible [25; 26; 27; 28]. In addition to challenging extraction conditions--for example, cobalt deposits are mostly found on the ocean floor--there is also significant economic dependence on countries outside the EU. Around 70% of the world's cobalt supply originates in the Democratic Republic of Congo, while most tungsten is produced in China or sourced from Chinese reserves [29; 30; 31; 32]. As economically recoverable amounts of these materials are very limited--cobalt, for instance, constitutes only about 0.001% of the Earth's crust--and since they are often mined in politically unstable regions, there is a high supply risk. For these reasons, cobalt and tungsten are classified as critical raw materials (CRMs) [27; 28].