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This comprehensive book explores the techniques, materials, and real-world applications of thermal spray coatings across various industries, including power generation, aerospace, medical, and automotive sectors.
Readers will learn about the basic science and engineering aspects of thermal spray technology, its historical developments, and the diverse range of materials used, from metallic to ceramic materials, and nano-crystallization materials. Distinct thermal spray techniques are explained (flame spray, detonation-gun spray, high-velocity oxy-fuel spray, electric arc spray, plasma spray and cold spray). Chapters on advanced topics also give an understanding of crucial material properties such as high temperature corrosion, oxidation, erosion or wear resistance, and biocompatibility.
Key features
- Contributions from materials science experts with references for each topic
- Gives a comprehensive overview of materials and distinct spray techniques used in thermal coatings
- Dedicated chapters for applications of thermal coatings in different industries
- Covers recent trends and new advances such as surface modification techniques to improve functionality and performance
This book is intended as a resource for an in-depth understanding of the fundamentals and applications of thermal spray coatings for students, professionals and researchers in materials science and chemical engineering disciplines.
Readership
Students, professionals and researchers of materials science and engineering disciplines.
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Seitenzahl: 485
Veröffentlichungsjahr: 2024
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Thermal Spray Coatings: Materials, Techniques, and Applications is a comprehensive book that discusses the development, application, and future scope of thermal spray coatings. In this book, the authors try to explore the techniques, materials, and applications of thermal spray coatings in various industries, including power generation, aerospace, medical, and automotive.
The first chapter of the book is dedicated to the historical development of thermal spray coatings, providing a detailed account of the evolution of this technology. It also outlines the various types of thermal spray coatings and the materials used in their production. The second chapter discusses the latest trends in coatings, including the use of new materials and techniques to enhance the performance and durability of coatings. The third chapter focuses on reliable surface modification techniques and how they can be used to improve the functionality and performance of various materials. We explore the different types of surface modification techniques, including thermal spray coatings, and how they can be used to enhance the properties of materials. In the fourth chapter, we examine the high-temperature corrosion of coal-based thermal power plants, gas turbines, and steam turbines, and how thermal spray coatings can be used to mitigate these issues. We also discuss the challenges associated with using thermal spray coatings in high-temperature applications. Chapter five explores how thermal spray coatings can be used to combat corrosion, wear, erosion, and abrasion in hydropower plants. This chapter outlines the different types of thermal spray coatings used in hydropower plants and how they can improve the efficiency and performance of the equipment. The sixth chapter provides a comprehensive review of the thermal spray coating technique, its applications, future scope, and challenges. It covers the basics of thermal spray coating, including the various techniques used in the process, the materials used, and the properties of the coatings produced. Chapter seven is dedicated to cold spray coating of nano-crystallization material, providing a critical review of the method, properties, and challenges associated with this technique. We examine the benefits and limitations of cold spray coating and the potential applications of this technology. In chapter eight, we provide an overview of orthopedic implant materials, the problems associated with these materials, and the different coating materials that can be used to enhance the biocompatibility and corrosion resistance of these implants. Chapter nine examines the issue of corrosion in medical devices and the different detection methods used to identify and prevent corrosion. This chapter provides an in-depth analysis of the different types of corrosion that can occur in medical devices and the impact they can have on patient safety. Finally, chapter ten explores how thermal spray coatings can be used to enhance the corrosion resistance and biocompatibility of implants. We examine the different types of coatings used in this application, their properties, and their potential impact on patient outcomes.
Overall, this book provides a comprehensive overview of thermal spray coatings, including their historical development, recent trends, and future scope. It explores the different techniques, materials, and applications of thermal spray coatings in various industries and provides critical insights into the challenges associated with this technology. It is hoped that this book will serve as a valuable resource for researchers, engineers, and professionals working in the field of thermal spray coatings.
Thermal spray coatings are a method of surface modification in which various metallic and non-metallic materials are sprayed in molten, semi-molten, or even solid state on a prepared substrate. The coating material is present in two forms: wire or powder. The most common thermal spraying techniques include cold spray, electric arc spray, plasma spray, detonation gun spray, flame spray, and high-velocity oxy-fuel spray. The coating's thickness, which is calculated in millimeters or microns and has distinguishing features from the base material's surface, is acceptable in many industrial sectors and is ideal for on-site industrial applications. These processes also offer affordable solutions in many industrial sectors and are capable of providing surface modification approaches with enhanced surface properties comprising better texture and high mechanical strength in terms of hardness, scratch resistance, and porosity. This chapter presents the evolution of coatings developed during the last few decades using various coating processes and materials for the protection of service components. Coating measures are developed for use in thermal power plants, gas steam, and the automotive industry for the treatment of components, able to work in harsh environments of flue gases and chemicals.
Thermal spraying (TS) is a coating process that develops coatings by depositing finely divided metallic or non-metallic surface components in a melted or semi-molten state on a prepared substrate [1]. The foundations of the schematic thermal spray method are depicted in Fig. (1). Surface modification needs the employment of surfacing material in powder, rod, or wire form depending on the mode of heat
generation: combustion or electrical [2]. Plasma jets (non-transferred direct current arcs or induction discharges), wire arcs, and transmitted arcs are examples of electrically driven thermal sprayed coatings. Plasma jets, as opposed to wire arcs, which are formed across two consumable, automatically extending wires, require powdered or liquid precursors [3]. For both techniques, the substrates made of metal, glass, ceramic, composite, and plastic are held much below their melting point. In contrast, the transferred arc technique welds the finished coating to the metallic substrate while the surface material is still in powder form [4].
Fig. (1)) Schematic of fundamental thermal spraying technique.Thermal spray coating is a versatile technique used to apply protective or functional coatings to a wide range of surfaces. It involves heating coating materials to a molten or semi-molten state and then propelling them onto a substrate to form a coating. Hybrid approaches combine thermal spray with other deposition methods, such as physical vapour deposition (PVD) or chemical vapour deposition (CVD), to create coatings with unique properties. These techniques leverage the advantages of both methods to achieve improved adhesion, thickness control, and coating quality. Some thermal spray methods allow in-situ alloying of the feedstock materials, enabling the creation of tailored compositions and properties. Additionally, composite coatings can be produced by mixing different materials in the spray process, resulting in coatings with customized combinations of properties. Novel thermal spray coating methods also focus on reducing environmental impact. This includes exploring greener propellant gases and alternative feedstock materials and improving the efficiency of the process to minimize waste. The development of coatings for medical implants, free-standing shapes, thermal insulation or barriers, oxidation protection, corrosion or oxidation resistance, abradable seals, electrically conductive or insulating materials, dimensional restoration or manufacture, and wear protection is now mainly owing to these techniques. Application, cost (related to the cost of process investment), and economic viability of the method all influence the coatings that should be sprayed [5]. The parts utilized in various applications function under varying load circumstances and come across a wide range of problems. Surface modification by these thermally sprayed techniques allows the deposition of almost every class of material, such as metals, alloys, ceramics, and composites, with better thickness and potential coating characteristics to protect these components operating in hostile conditions [6]. A coating is formed by laying distinct compacted particles, one on top of the other. Compacted particles, known as lamellae or splats, develop preceding surface structures nearby that are cool and hardened. A precise morphology and metastable phases produced by the rapid solidification rate of 1 million °C/sec exhibit properties that are not attainable with conventional material production procedures. The deformation of the particle affects the flattening of the particles or droplets. The flattening rate (disc diameter vs. initial particle size) for molten particles can reach a maximum of 5. Less flattening is produced by operations at lower temperatures. The material being sprayed and the deposition method both affect how close together the granules or drips are. When heated, a class of substances called self-bonding materials demonstrates an exothermic reaction that provides the heat needed to create an interfacial connection. Most materials lack the ability to form a chemical connection combined with the base, so the surface must be toughened using method, such as grit blasting, to establish a foundation for mechanical bonding. Plastics, glass, ceramics, metals, paper, and composites are the surfaces that can be coated. The compact ability of the coating's structural components and the coating's bonding strength to the supporting base material are what determine the coating's toughness. Compact ability is determined by material qualities as well as porosity inside the coating. Its porosity comprises voids between flattened particles as well as micro fractures in ceramics. As porosity correlates to a loss in strength, it has been the subject of more sophisticated coating techniques that can, if necessary, reduce porosity.
Recently, carbide, nitride and boride composite coating materials have been the most extensively employed ceramic/ceramic nano-coating materials in different sectors. They are mixed into a nano-composite-based coating like B4C/HfC, B4C/SiC, TiC/TiB2, TiC/TiN, TiN/TiB2,etc. Several nano-coatings of HfC/SiC, B4C/SiC, and HfC/B4C are frequently utilized in knives to keep the surface from oxidating at high temperatures. Typical metal nano-oxide coatings, such as ZrO2, Fe2O3, TiO2, CdO, and ZnO, make up the 2nd ceramic coating. Nano ZrO2/ceramic structural coating technique is one of the techniques that has gained the interest of researchers. Different ceramics are coated with nano zirconia coating powder, improving performance as compared to conventional coatings. Heat shielding, high-temperature resistance, corrosion resistance, and abrasion resistance are the principal uses of ZrO2 nano-coatings. Compared to conventional zirconia coatings, they have a higher thermal expansion coefficient and lower thermal conductivity, and their mechanical qualities are also excellent [6].
Although it appears that production demands are increasing, the phrase “better, quicker, cheaper” is ubiquitous and applicable in all the industries today. High demands and aggressive service conditions frequently result in component or system failure. An entirely eroded Pelton turbine nozzle needle, after actual use over millions of hours, was damaged in the case of turbines. Assuming this service life is regarded improper, the whole component or the worn out area must be protected. Typically, the ultimate alternative is chosen for economic reasons. This calls for the application of surface coatings. Depending on the needs, the whole component or simply the attack-prone area is coated. A component's surface varies substantially based on the environment it serves. External necessities comprise appropriate wear protection, corrosion resistance, thermal insulation, electrical insulation, and a more aesthetically attractive appearance. The parts are rarely unprotected from a single provision situation in practice. Generally, an arrangement of abrasive wear and significant heat stress is present. The most frequent conditions that the surface layer must withstand are wear and corrosion.
Historical developments and advancements in the area of thermal spraying methods can be categorized into different classes based on their characteristics and specifications. Thermal spraying was first documented in patents issued by Swiss scientist Max Ulrich Schoop between 1882 and 1889 [7]. Wear and corrosion are the two situations that the coat must survive most frequently. Subsequently, the torches were altered to accommodate powdered feedstock materials. The feedstock powder particles became entangled in a heated expanding jet stream, where they received enough heat while being driven towards the substrate's surface. On impacting the surface, these molten and semi-molten particles formed splats and solidified rapidly. The outcome was the coating that developed incrementally from these impacting droplets [8].
Coatings can be applied using a variety of methods and materials. Specialist knowledge is required to pick the appropriate combination for the respective application. Certain techniques are not appropriate for specific coating materials, and not all methods can achieve the required coating thicknesses. In addition, the equipment required for some operations might be expensive and sophisticated. Cost analysis can be used to decide whether a coating is a workable solution. Since not all coating processes are created equal in terms of their environmental impact, today's regulations demand that ecological factors be looked at as well. Thermal spray is the coating method that offers the widest variety of coating materials, coating thicknesses, and potential coating features.
In 1908, electric arc spraying was developed by Schoop, which was capable of spraying some more metals like steel, stainless steel (SS) and Zn, with some improvements such as better process and equipment control [9]. Schoop established the spraying methods throughout his whole life, and some of them have patents. However, all these early processes were not capable of combating corrosion issues when rapid industrialization occurred after World War II, as most of them were designed with the bottom-up approach. During this period, believers in thermal spraying technology realized the importance of surface engineering in the industries, and they firmly pushed the approach upstream by developing TS technology [10]. Countries like the USA, Germany, and Russia were the main contributors to this technology for such uses over time. The main principle approaches of thermal spraying during 1920–1950 were powder flame and wire spraying [11]. Primary applications were corrosion control and reclamation of parts in the paper industry.
In 1937, the first patent on the commercial use of TS coatings for high-temperature corrosion safeguards for the boiler was introduced by Quinlan and Grobel. Consequently, for applications related to ceramic oxides, the period 1950s noticed the development of DC atmospheric plasma spraying (APS), which uses Ar as the primary gas for plasma stream generation. With further improvements in TS technology, it became a candidate for protecting parts/components and enhancing performance in the aircraft engine industry between 1970 and 1980 [12]. In the early 1980s, by employing rocket engine technology, two scientists, Browning and Witfield, developed high-velocity oxy-fuel (HVOF) technology for metal powder spraying [13]. The powder particles in molten or semi-molten states are deposited onto the substrate at supersonic velocities of approximately 1000ms−1. This process has applications in various fields where the engineering components are exposed to corrosion and wear-related conditions. Furthermore, incorporating the concept of “Hypervelocity” Impact, Browning was the first to develop high-velocity air-fuel (HVAF) [14]. The HVAF technique presents flexibility in the fuels used, like propane, hydrogen, natural gas, or propylene. As the system uses a significant air volume for combustion, the jet temperatures are restricted between 1900 °C and 1950 °C. Coatings developed by HVAF exhibit reduced oxidation in comparison with HVOF-processed counterparts [15]. Thermal spraying processes vary based on in-flight particle velocity and temperature, as shown in Fig. (2).
Fig. (2)) Different thermal spraying techniques, particle temperature and particle velocity.In recent decades, thermal spraying technology has continued to evolve with a focus on improving coating quality, efficiency, and environmental impact. Advancements include the development of cold spraying, where particles are accelerated to high velocities without melting, resulting in minimal thermal damage to the coating material. Cold spraying is suitable for temperature-sensitive materials. Another modern development involves the application of nanomaterials through thermal spraying techniques. Nanocoatings offer enhanced properties such as improved hardness, wear resistance, and reduced friction. They find applications in fields like electronics, medical devices, and renewable energy. Automation and robotics have been integrated into thermal spraying processes to improve consistency, precision, and efficiency. This has led to greater adoption of thermal spraying for large-scale industrial applications. As environmental considerations have become more important, researchers and industries have worked on developing thermal spraying processes with reduced emissions and waste generation. Efforts have been made to optimize coating materials and deposition methods to minimize environmental impact. With advancements in materials science and process control, thermal spraying can now produce coatings with tailored properties to meet specific requirements. This customization has opened up new possibilities in areas like aerospace, energy, and medical applications.
The structure of the deposited coating depends on the spray conditions of the process and the parameters used, especially the coating material selected for spraying. The most commonly noticed feature in any spray thermal coating is its lamellar grain structure. This type of structure is formed by the flattening of molten and semi-molten in-fight particles upon striking the substrate's cold surface, followed by rapid setting [16]. The flattened particles are known as splats, which have different surface textures. The common coating formation structure is shown in Fig. (3). Depending upon the spray process and material selected, thermally sprayed coatings have micro-cracks, porosity, and heterogeneous and anisotropic structures. The heterogeneous structure is formed due to the different conditions of the in-flight particles, as it is practically impossible to achieve the same temperature and velocity conditions for all the particles because of different shapes and sizes [17]. Low porosity coatings are necessary for preventing wear and corrosion since they also have better adherence. Thus, it is essential to manage and anticipate porosity during coating deposition. Porosity can be produced by several different causes, including splat curling brought on by thermal tensions, gas entrapment beneath hitting particles, and incompletely filled voids when molten particles touch a rough surface. Pores created by gas entrapment in thermally sprayed coatings are usually microscopic and located at the interface between the splats [18, 19].
The HVOF spray method results in denser coatings. Porosity is barely discernible. The regions are made up of WC hard phase that is encased in a ductile cobalt and chrome matrix. Here, the usual thickness of the coating is between 0.2 and 0.3 mm (0.008 to 0.012 inches). Because of the cooling and solidification processes, thermal sprayed coatings typically have substantial internal tensions. Internal coating stress is caused by the heated particle's contraction during cooling. These stresses can be resisted by creating compressive stresses if such a relationship between the thermal expansion coefficients for said base material and the coating material is taken into account. It is crucial to maintain temperature control through coating, using a process to know if the base has to be chilled or reheated. On rare occasions, the bond strength requirements are not met by the adhesion of a ceramic coating to a substrate. Between the substrate and the ceramic coating, a bond coat is frequently added to strengthen the bond. This layer is usually formed of a NiAl or NiCr alloy. In addition to providing further corrosion protection, these intermediate coatings can also be used for other purposes. The most typical splat curling at edges may be a significant cause of porosity. The top surface of the splat can contract, but the bottom surface, which is attached to the substrate, cannot. Splat curling or splat cracking are two methods for alleviating stress. Metallic splats are more ductile and, therefore, less likely to crack than ceramic splats. Hence, compared to ceramic coatings, curling up is a more significant factor in the development of porosity in metallic coatings [15]. The following reactions, which result from the heat input provided to the spray particles and the amount of time these in-flight particles spend in the environment, have an impact on the structure and final composition of the coatings:
Fig. (3)) Photographs of splats formed after coating.1. The reaction of metal complexes (e.g., hard material decomposes when there is oxygen).
2. Creation of non-volatile metal compounds, primarily from highly reactive metals in the presence of oxygen, nitrogen, and hydrogen, as well as oxides, hydrides, and nitrides [20].
TS coatings display a positive level of process-dependent porosity. Flame and electric arc spray exhibit the highest porosity values. HVOF coatings, however, result in extremely dense layers with porosity < 0.5%. The porosity of typical plasma coatings ranges from 1 to 2%. In a regulated environment, plasma spray can be exceedingly dense. As the spray cannon passes through the exterior repeatedly, the material is deposited incrementally in layers, with a usual film thickness of 10 to 20 m (400 to 800 in). Compounds may incur on the outer surface of the layer between layers. Spraying in a vacuum or inert atmosphere helps in the reduction of oxidation. Fine dust and defrosted substances from overspray can be drawn into the covering. Coating substance, when sprayed, does not stick to the surface, resulting in dust. These particles are pushed towards the coating surface by sequential spray passes, where they are entangled in the surface coating and retained.
The detonation process is similar to combustion, although it operates differently. While combustion produce a steady flame in which particles are continuously moved, detonation is a pulsing process [21]. As seen in Fig. (4), powder is pumped into a chamber with oxygen and fuel before being ignited by a spark. As a gunshot, the combustion carries the mixture along a lengthy barrel to the substrate. This method achieves higher velocities than HVOF and results in high-density, low-porosity coatings with good bond strength. The disadvantage of the explosion process is the need of a big barrel. Substrates in tight spaces are difficult to be coated correctly with the detonation gun. Between each shot, the barrel is purged with nitrogen. The frequency of purging, injection, and igniting cycle ranges from 3 to more than 10 times per second [22]. A nozzle or a torch/gun creates a thermo-kinetic state for melting and conveying the co-products and serves as the material delivery device in thermal spray. Schoop made the initial discovery early in the 20th century. When zinc particles were exposed to a flame, the particles stuck to the surface. This technology started a chain reaction of advances that led to numerous coating techniques. High-velocity flame spraying, two-wire arc spraying, plasma spraying, detonation gun spraying (HVOF), and cold spraying are all included in the list of thermal spraying procedures. The types of feedstock used in spraying processes (grains, wire, ceramic rod, and fresh drops that have atomized), material formation, and either medium temperature or medium speed transportation can all be classified.
In this process, a combination of fuel and O2 is ignited in a long barrel using a spark plug. The detonation-pressure wave arises from the explosion of the mixture, which heats up and propels the powder particles with a high velocity toward the surface of the substrate. It is a must that proper cooling arrangements should be provided for both the coating and the substrate during the operation [23]. The process is continuously pulsed in a frequency range of 6–100 Hz. The resulting coating deposit is well-bonded, highly dense, and extremely hard. The high noise levels of 145+ dBA restrict its usage in an open environment and make it mandatory to perform inside the acoustical enclosures. Coating porosity is <2%, and its oxide content is in the range of 0.1 and 0.5 wt%. The powder flow rates are limited to 1–2 kgh−1, and the process deposition efficiency is approximately 90%. Sprayed materials are mainly in the powder form of metals, cermets, and alloys [24-26]. This process also sprays some oxides, but the feedstock grain size is limited to 20 µm. The major application areas include the coatings for abrasion, adhesion, and corrosion resistance. The splat formation at different temperatures by flame spray processes is shown in Fig. (5).
Fig. (4)) Representation of detonation gun spray. Fig. (5)) Flame formation temperature subjected to D-Gun techniques.Depending on the substance being coated, the powder particles' physical condition at the point of contact with the substrate that range from metals like Al, Ni, to Cu, can be completely molten, while in ceramics like Al2O3 and Cr2O3, low-melting adhesive sections with partial melting only occur in the case of a variety of powders like Cr3C2-NiCr and WC-Co. The DSC technique uses cyclic combustion, compared to alternative thermal spray procedures that are ongoing [25, 27]. Some of the studies subjected to D-gun sprayed coatings are listed in Table 1.
Another kind of combustion spray is HVOF (High-velocity oxy-fuel). The only difference between HVOF and flame spray is the much higher spray stream velocity. The temperature and heating technique are the same. HVOF cannons come in a variety of designs to achieve the necessary high velocities. In Fig. (6), one type is shown. This depicts the feeding of oxygen and fuel into a combustion compartment. The burning of this mixture generates a high-pressured flame stream that travels through an extended jet at a high speed. Compared to flame spray, the velocity of the flame stream is increased when powder is added. High particle impact velocity still results in high-density coatings despite the low particle temperature [27]. The tremendous impact energy distorts unmolten particles, therefore, they do not need to be melted to generate a high-quality coating. Dense coating creation is also caused by particle heating, which occurs when moving energy in the particles is transformed into thermal energy during impact [28].
Hard cermet coatings of excellent quality, including WCCo, are well known to be produced via the HVOF technique. During deposition, a minor degree of decarburization and carbide dissolution occurs as a result of the high temperatures that are exposed to the WC-Co coating. As a result, brittle W2C and CoW-C are formed, which has an impact on the coating's mechanical properties. Given that plasma spraying occurs at significantly lower temperatures, there is still a significant difference between the amount of decarburization and carbide dissolution [29-30]. The HVOF process typically uses hydrogen, propylene, kerosene, propane, or acetylene as the combustion fuel gas. HVOF spray devices fall into two distinct categories based on the type of combustion fuel and chamber pressure. Chamber pressures in the first class, high velocity, surpass 241 kPa. The second class, known as hypervelocity, operates with kerosene as its normal fuel and has a chamber pressure in the range of 620 to 827 kPa. High combustion chamber pressures often result in coatings with compressive stresses, which are often advantageous for the coating's function. But, compared to lower-pressure guns, the deposition efficiency of the hyper-velocity kerosene guns suffers. Kerosene guns have a deposition efficiency of 35–50%, compared to conventional HVOF weapons with efficiency ranging from 50% to 70%. [31-33].
Fig. (6)) Schematic of HVOF spraying.HVOF & HVAF spraying processes produce gas streams of very high velocity, which are achieved using the de Laval nozzle. Using kerosene as liquid fuel, high velocity and low flame temperatures can be achieved in the HVOF spray system [34]. Some of the studies on HVOF-sprayed coatings are listed in Table 2. Consequently, unwanted reactions such as oxidation and phase dissolution can be avoided in composite coatings. Thus, low porosity and better mechanical characteristics such as fracture hardness and impact strength are achieved with the liquid-fueled HVOF method. The observation is that the medium temperature range of the liquid fuel HVOF system can still influence the properties of some materials and composites that are heat sensitive. In such conditions, the HVAF spraying is an effective solution in which the compressed air replaces pure oxygen. The flame temperature is further reduced as excess nitrogen is present in the compressed air [35, 36].
A coating layer is created by the intense deformation of feedstock powder particles during the process of cold gas spray (CGS), a developing technique [38, 39]. Powder particles are carried and accelerated by CGS utilising a convergent-divergent nozzle and a highly pressurised hot gas (N2 or He). These velocities range from 300 to 1200 m/s, as shown in Fig. (7). This hot carrier gas also heats the powder particles below their melting point, which assists in the deformation of powder particles upon impact.
Fig. (7)) Schematic of cold spraying.The particles severely distort the impact, a result of the substrate, due to the high kinetic impact on the powdered feedstock, which results in high bonding and densification. When particles stretch a critical speed, further known as the minimum velocity, in which particular thermo-mechanical circumstances are realized, particles connect to the substrate surface and the feedstock particles securely cling to the substrate. For several materials, critical velocities have been calculated and determined as a function of material characteristics and particle sizes. Critical velocity ranging from 150 to 900 ms-1 has been recorded for several materials that CS has deposited thus far [40]. The extension of the gas flow at the nozzle's deviation zone prevents the feedstock particles from disintegrating before they reach the substrate, which lowers the temperature and decreases oxidation as well as phase transitions and unwanted microstructural changes. Owing to its effective deposition and ability to create extremely dense deposits that are more dense and thicker, CGS has garnered interest on a global scale [41]. The thermal power plant is where all of these coatings processes are most frequently used [42-44].
The plasma spraying gun is comprised of a copper anode and a tungsten cathode. An electric arc is discharged over the two electrodes to heat a working gas, often argon or a mixture of argon + helium, argon + hydrogen, or argon + nitrogen. With velocities of about 800 ms-1 or higher, the heated gaseous mixture leaves the nozzle as a high-temperature plasma of roughly 15,000 K (ionised gas) [45]. As depicted in Fig. (8), a powdered feedstock material for coating is released mostly radially into the high-temperature plasma jet.
Fig. (8)) Schematic of plasma spraying.Nonetheless, the plasma spraying methods can be divided into groups according to the setting in which they are used. They comprise:
• Atmospheric Plasma Spraying (APS)
The APS method generates heat using a direct current (DC) arc source and a non-carburizing or non-oxidizing flow rate. According to the different spraying needs, many spray gun designs are available, with the power output for regularly applied plasma spray sources varying between 20 and 250 kW [19, 25]. The type of feedstock, gun type, injection of powder mechanism, plasma gas, etc., may affect the spray rates. APS coatings, however, exhibit strong abrasion resistance, adhesive, and sliding wear [46-48]. A common application of APS is the restoration and repair of worn surfaces. It also creates thermally resistant or conductive surfaces.
• Controlled Atmosphere Plasma Spraying
When the oxidation of in-flight particles is the issue, this technique can be used, and the process is carried out in an inert atmosphere (generally in the presence of argon gas). Antechambers (connecting zones) are required so that air does not enter the main chamber. The atmospheric chamber must be cooled with water if the spray chamber volume is less than 10m3. The higher cost of inert-APS is the main factor compared to APS, as the equipment required to recycle argon gas is expensive [49, 50]. This method can deposit the materials whose melting point is high, approximately 4000 °C, such as TaC.
• Low-Pressure or Vacuum Plasma Spraying
Comparatively, this method is similar to the inert atmospheric spray method in that it is performed in a closed chamber but with a pressure that is slightly less than atmospheric pressure (varying from 10–50 kPa). As compared to inert atmospheric spraying, the equipment is more expensive. Before coating deposition, a layer of oxide is removed from the substrate exterior using transferred arc etching, and the coating method is carried out with inverted divergence. The object is treated to an extremely high temperature to create a solid permeation interaction seen between the coating material and the component during the deposition operation. This technique is mostly employed for coating the turbine blades. Increased coating density, increased control over coating thickness, enhanced bonding (even with a surface of an uneven form), and high deposition efficacy are a few benefits of this technology [51, 52]. Moreover, the plasma's diameter and length become relevant at low pressures, and by using nozzles (convergent/divergent), a pressurized plasma jet can be created. Low oxide content and thick and adhesive coatings are produced by operating in the absence of oxygen and at high temperatures [53-55].
• Induction Plasma Spraying
The induction plasma spraying procedure is performed in a regulated environment but at atmospheric pressure or in a light vacuum. The procedure is quite similar to VPS, however, the component is either rotated or moved linearly while the spraying torch is fixed. It allows for the spraying of larger particles (about 200–250 m) than those made possible by DCAPS torches. Installation plasma power levels fall between 50 and 400 kW, with powder process parameters of 2 to 10 kg/h. It is mostly used for spraying ceramics and metals. Powder spheroidization creates powders with good flowability that are required for numerous applications unrelated to spray techniques [56, 57]. Plasma spheroidization improves the quality of metal powders for additive manufacturing. Plasma technology develops metal powders that are more spherical and deliver better properties for additive manufacturing.
• Plasma Transferred Arcs (PTA) Process
It results from the combination of a welding method that uses electrically conductive materials primarily as the anode and a thermal spraying procedure. A secondary current is applied through the workpiece and plasma that regulates the melting of the surface and penetration depth. Also, it requires less electrical power in comparison with the non-transferred type arc processes. Wires or powders with particle sizes up to 100 m are acceptable materials for use as feedstock. The PTA method makes it simple to spray cermets, metals, and alloys. The maximum current, which varies between 200 and 600 A, might be used to classify the various firearms. In comparison to alternative spray methods, thicker coatings (10 mm or even more) can be produced, and then a strong metallic bond with the component is created. The component has to be positioned as horizontally as feasible while spraying [58]. The reported minimum porosity and procedure deposition efficiency are both above 90%. For coatings deposited using the PTA technique, the wear resistance and high-temperature corrosion resistance are excellent. It is used to deposit the coatings over heavy components employed in oil fields and the mining industry [59].
Thermal spraying coatings, a widely utilized technique in various industries, involve the deposition of protective or functional layers onto surfaces through the application of heat and momentum to feedstock materials. These feedstock materials, often in the form of powders or wires, play a crucial role in determining the properties and characteristics of the resulting coatings. Typically, feedstock materials encompass a diverse range of substances, such as metals, ceramics, polymers, and composite materials. These materials are chosen based on their compatibility with the substrate material and the desired coating properties, including hardness, corrosion resistance, thermal insulation, and wear resistance. The selection of appropriate feedstock materials is a critical step in the thermal spraying process, as it directly influences the performance and longevity of the coatings in a wide array of applications spanning aerospace, automotive, energy, and manufacturing sectors. The types of materials commonly deposited by thermal spraying are shown in Fig. (9). Essential fields that will determine the growth in TS coatings in the future include:
Fig. (9)) Commonly used feedstock material. Continuous advancements in process control equipment (such as robotics, control of motion, and real-time sensors, etc.) for better coatings.Better methods for nondestructive testing and coating evaluation.Improved CS method optimisation.Thermal spraying of the components in a near-net shape.Spray formation of ceramics made up of superconducting oxide at high critical temperatures.Stereolithography and other rapid prototyping techniques.An improved process for developing feedstocks and for quality assurance.Novel disciplines and materials, including composites, nanophase materials, and coatings for biological applications [60].Even though there are several studies on materials made conventionally, additive manufacturing—which makes use of thermal spraying technologies, notably the CS process—is currently the focus of research [61-63]. In comparison to the traditional production method, CS additive manufacturing (CSAM) enables the layout of products with complicated symmetrical geometries built with substantially reduced lead time. CSAM technology is probably going to be incorporated in the industries in the coming years [64]. Overall, thermal spray coatings can be effective against hot corrosion, erosion, and oxidation in various fields [65-75].
For customizing the surface qualities of engineering components, the thermal spraying technique provides a potential and affordable way to deposit coatings. As-sprayed coatings can be employed in a variety of applications, including those involving aviation engines, power generation machinery, coal-fired boiler components, ships, orthopaedics, and dental care. However, industries like aerospace and automotive are the primary consumers in the market associated with thermal spraying coatings. An extensive range of operating parameters in the thermal spraying process influences the final properties of the deposited coatings. For this cause, extensive research has been carried out to clarify the relationships between the process handling parameters and the final coating properties. The other growing sector which is in current demand is biomedical implants. Osteoconductive scaffolds can be made using a quick and easy thermal spraying process that mimics the biological deposition of a composite coating on polymeric tissue engineering scaffolds.
Furthermore, additional functionality for the tissue engineering scaffolds can be added or enhanced using biomimetic techniques. It is evident that new, cutting-edge, and useful technologies, particularly in the nanotechnology field, will emerge and be applied to surface-modify biomaterials and scaffolds for tissue engineering. Thermal spraying coatings can be applied to parts by two techniques, based on combustion flame principles and based on electrical energy. Combustion flame-based is a commercially and widely used technique to get hard, well-bonded, and dense coatings for corrosion and wear resistance applications. Ceramics, cermets, metals and alloys, composites, and polymers are the widely used materials in thermally sprayed coating applications. Among these, ceramics alone account for the largest share in the market of thermally deposited coatings.