1. Introduction
Aerospace, power generation, heavy machinery, and other industries all use thermal spray (TS) techniques extensively for coating applications. In these procedures, feedstock—materials that have been heated or melted—are sprayed onto a surface to create a coating. However, different process variables have a direct impact on the characteristics of the coated materials.[
1,
2,
3]. A thicker coating forms on the substrate as the number of passes during the TS process is increased, and a denser microstructure is produced when the coating's porosity decreases with increased particle temperature[
4,
5]. Additionally, the flow of oxygen and fuel reduces the coating's porosity. Greater oxygen and acetylene fuel rates create a rise in particle velocity, this lead to an increase in both the pressure inside the combustion chamber and the degree of contact [
6]. To prevent coating problems, process variables must be carefully managed.
An inferior base material can be swapped out with a coating that delivers superior surface characteristics and performance using coating modification technologies. This method is very helpful in challenging circumstances, such as corrosive environments, conditions requiring for wear protection, thermal insulation, or high levels of stress. Yttria-stabilized zirconia is employed as a protective overlay coating in thermal barrier coatings (TBCs) to safeguard nickel or cobalt super-alloys from exceptionally high temperatures[
7,
8].
Experiments have been conducted to optimize the process parameters of flame-spraying to produce high-quality coatings. The effectiveness of the coating is determined by the cohesiveness and adhesives strengths of the coating, which are regulated by the spraying process parameters used during the coating deposition process [
9,
10]. In industrial settings, flame-sprayed molybdenum coatings are commonly used to improve the performance of engineering components such as shafts, pistons, and piston rings. By adjusting the spraying settings, the Coating efficacy of metallic glass based on Fe was improved, revealing that the amorphous phase initially increased but decreased as power was increased [
11]. It was believed that the oxidation and melting phases of the powders, as well as the heat radiation from the flame, contributed to this phenomenon. The correlation between the micro-hardness and spraying power was positive.
Zirconium diboride is a top-tier material in the ultra-high-temperature ceramics (UHTCs) group, valued for its outstanding combination of chemical stability, high thermal and electrical conductivity, and resistance to erosion and corrosion. It is suitable as a thermal protection system. Enhancing the coating's strength, hardness, and wear resistance can be achieved by optimizing the spray parameters. Spraying settings are vital for enhancing the properties of coatings. [
12].
Modern materials known as metal matrix composites combine a robust metallic matrix with a rigid ceramic or soft reinforcing. These materials can be adapted for specific applications and have higher quality characteristics than monolithic materials[
13]. Due to their high specific strength and stiffness, wear resistance, and dimensional stability, metal matrix composites have advantages in various applications, including those involving machines, cars, and aeroplanes.
Several processing parameters can impact the quality of composite materials, with one or two being particularly significant. To fully maximize the economic and technical potential of any manufacturing process, it's crucial to operate it with the most optimal parameters. The Taguchi method is an important optimization process that can help achieve this. [
14,
15]. The Taguchi method of parameter design offers a reliable technique for developing high-quality systems through a systematic, efficient, and simple approach that aims to improve design performance, quality, and cost-effectiveness. This approach is especially advantageous in cases where the design parameters are subjective and limited in scope. By adjusting the design parameters, the Taguchi parameter design boosts the performance characteristics and minimizes the impact of fluctuations in the system's performance [
16]. The Taguchi method facilitates a thorough comprehension of the separate and combined impacts from a minimal amount of simulation attempts. This method consists of several steps that adhere to a specific sequence of experiments, which eventually lead to an enhanced understanding of the performance of a product or process.
The thermal spray (TS) technique involves various Spraying distances, gas pressure, coat-face temperature, number of passes, and flow rate are process parameters, which significantly affect the coating properties of the treated materials. Research indicates that a longer spraying distance can produce a smoother and more uniform coating, whereas shorter distances tend to yield coatings with higher hardness values [
17]. By using thermal spray (TS) technology, the thickness of the coating on the treated substrate can be increased by increasing the number of passes during the spraying process [
18], and porosity decreases with higher particle temperature [
19], The microstructure of the coating becomes denser when exposed to higher rates of oxygen and acetylene fuel, which leads to a decrease in the coating's porosity. Additionally, when the particle velocity increases due to elevated oxygen and acetylene fuel rates, the pressure within the combustion chamber rises, and particle spread improves, enhancing contact and extending the particle's kinetic energy [
20]. Hence, controlling process parameters is crucial to avoid coating defects.
The increased need for materials that can endure challenging industrial environments and the limitations of conventional materials has spurred the creation of new alloys for thermal coating. Flame-sprayed molybdenum coatings are a prominent illustration of such coatings that are extensively used in multiple industrial settings to enhance the functionality of engineering components, such as shafts, pistons, and piston rings. These coatings exhibit superior thermal stability, corrosion resistance, and wear resistance. The coating's effectiveness is dependent on its cohesive and adhesive strengths, which are influenced by the coating deposition process's spraying parameters. This study focuses on optimizing the flame-spraying process parameters to create top-notch coatings, with previous research successfully using Taguchi methods for optimization. [
10,
21,
22].
Zirconium diboride is a high-quality ultra-high-temperature ceramic material that has outstanding physical, structural, transport, and thermodynamic properties. As a result, it can be used in a variety of applications, including refractory linings [
23,
24], electrodes [
25], microelectronics [
26], and cutting tools [
27]. ZrB2 is an excellent material for thermal protection systems due to its high melting temperature, chemical stability, and resistance to erosion/corrosion, as well as its exceptional electrical and thermal conductivity. Additionally, adjusting the spraying settings is crucial for enhancing the coating's characteristics. [
12]. The performance of the Fe-based metallic glass coating was improved by modifying the spraying parameters, leading to an increase in the amorphous phase that first rose and then decreased with increasing power [
28]. The proposal was made that the phenomenon was influenced by the oxidation and melting levels of the powders, as well as the heat radiation from the flame. An association was observed between spraying power and microhardness. In general, improving the spray parameters can increase the strength, hardness, and resistance to wear of the coating.
The quantity of times the process is carried out: The size and arrangement of particles present in the input material can affect the coherence and permeability of the coating. Particles ranging from 1 to 50 µm undergo partial melting and are swiftly propelled by either an electric arc or flame before being scattered onto a surface to produce a coating [
10].
Spray distance : The thickness, density, and mechanical properties of a coating can be influenced by the distance and angle at which it is sprayed. Generally, the spraying distance falls in the range of 100-300 mm, and the spray angle is between 45 and 90 degrees [
29].
Gas pressure: The melting and speed of the feed-stock material can be affected by both the spray temperature and pressure, ultimately affecting the microstructure and mechanical characteristics of the coating. The temperature at which the spray is released can vary from 3000 to 11000 °C, while the pressure can range from 1 to 10 MPa[
2].
Measuring the density of cut stainless steel samples accurately is challenging, and optimizing thermal spray parameters to achieve dense coating surfaces is crucial. To address these issues, the primary objective of this study was to create a Mo/ZrB2 composite coating using the thermal spray (TS) process. The Taguchi approach was employed to optimize torch parameters.
To achieve a Mo/ZrB2 composite coating with high wear resistance, an L9 orthogonal array was utilized in the Taguchi method to determine the main effect of various parameters. The coating's composition was discovered to be primarily influenced by the operational parameters, including spraying distance, gas pressure, and number of passes, among others. Nevertheless, the chemical composition of the coating was indirectly taken into account as it is modified by varying the operational parameters, rather than being treated as an independent parameter.. The use of L9 helped to remove the interaction between the parameters. Statistical analysis software MINITAB 17 was employed to design and analyze the experiments in this study.