Thermal Spray Coatings: Materials, Techniques & Applications

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.

Surface modification of metal substrates by coatings has remained a challenging research topic because of the conflicting demands for various properties. Functionally graded coatings (FGCs) have recently attracted the attention of researchers all over the globe owing to their mechanical, electrical, thermal and tribological characteristics in a variety of advanced engineering applications. These coatings are usually characterized by low porosity, good adhesion and base material compatibility, which includes temperature and geometry. However, coatings often experience some problems like variable thermal expansion coefficient (TEC) as compared to their base metals. Thus, to overcome this issue, the functionally graded material (FGM) layers may be employed. Hence, the purpose of this chapter is to describe a general idea of FGM coatings, including classifications of production methods and their diverse applications

Loss of material due to corrosion, erosion, wear and oxidation is a major problem in various industries. Recently, various surface modification methods have been employed to improve the service life of distinct engineering parts by improving their resistance to corrosion, wear and erosion. These methods boost thermal and biocompatibility in addition to the mechanical and physical qualities. To offer a thorough review of surface modification techniques, including mechanical, chemical, and thermal procedures, this chapter has three main objectives. Overall, this chapter provides a detailed study on working principles, merits, demerits, and applications of various surface modification techniques.

With the demand for high-fidelity coating involving metallic, multilayer, protective, and ceramic-based coatings, deposition methods have been introduced to achieve quality coatings. High temperatures cause erosion-corrosion wear, oxidation, and hot corrosion in materials operating in corrosive conditions. Among the various methods used to protect surfaces from deterioration, the method of applying coatings by high-velocity oxy-fuel spraying deserves special mention because it creates coatings with increased hardness and resilience, low (less than 1%) porosity, and high erosioncorrosion and wear resistances. Deposition of these coatings using a typical thermal spray process finds applications in the protection of base material in automobile, aerospace, orthopaedic, thermal power plant, and gas pipeline sectors. We present the potentials of the coatings and their respective protective properties. This chapter provides the optimization and overviews of the use of various recently used coating materials developed for the application in automotive, power plant, defence, gas and steam, and orthopaedic sectors.

Hot corrosion is a critical challenge in the designing and functioning of coal-based power plants, gas turbines and steam turbines. The economic loss due to hot corrosion is very high. Many researchers are working to combat hot corrosion, but only a few researchers have reduced hot corrosion to some extent by various surface modification techniques. However, coatings deposited by the thermal spray process offer better results in combating hot corrosion. Thermal spray techniques are a promising way to apply dense, defect-free adherent coatings to components, increasing both their performance and lifetime. Thus, the core objective of this chapter is to provide a review of different thermal spray coating methods, coating materials, advantages, and disadvantages. Finally, the most recent industrial advances in thermal spray technologies to combat corrosion in high-temperature applications are provided.

Hydropower plants, thermal power plants, offshore, chemical, food processing, oil sectors, etc., all have difficulties with erosion, abrasion, and corrosion regularly. These issues impact a variety of hydraulic equipment and pipeline circuit components (pipelines, elbows, reducers, separators, tees, and seals). One application where these three issues consistently arise is a hydropower plant. However, one of the main issues with Indian hydropower facilities is silt erosion in the hydro-turbines and their parts. Hard particles like quartz, feldspar, and other minerals may be found in Indian rivers. More than 50% of the quartz in the silt contributes to several issues with hydro-turbines, including sediment erosion, leaky flow, disruptions in secondary flow, etc. As a result, these issues have an impact on the hydro-power plant's overall performance. The numerous failures of the components placed in hydropower facilities' impulse and response turbines are discussed in this chapter. Additionally, this chapter provides information on different turbine materials and their characteristics. Based on silt characteristics, material properties, and flow phenomena in various hydro-turbines, several numerical models of erosion abrasion are addressed. Different thermal spraying methods for turbine materials are compared and contrasted. To regulate wear and safeguard hydro-turbines, this chapter reviews the literature on wear mechanisms, models, pilot plant loops or rigs/testers, and protective strategies.

Corrosion is a major issue that can cause implant failure, leading to adverse reactions in the surrounding tissue and sometimes causing systemic complications. Several researchers are currently exploring ways to enhance the corrosion resistance of orthopedic implants, which is essential to improve their performance and longevity. The most common strategies used to enhance the corrosion resistance of orthopedic implants are selecting corrosion-resistant materials, surface treatments, coatings, and improved implant design. Surface treatments, such as passivation, anodization, and micro-arc oxidation, can also create a thin oxide layer on the surface of implants to act as a barrier against corrosion. Coatings (hydroxyapatite, diamond-like carbon, metal oxide coatings) and good implant design can also be used to provide a protective barrier and alter the surface chemistry. Further research can be focused on developing new materials and surface treatments that are more corrosion-resistant, as well as advanced implant designs that can minimize stress concentrations and enhance load distribution. By implementing these strategies, orthopedic implants can provide better treatment for patients with a higher level of safety and efficacy. This chapter mainly focuses on corrosion types, causes, merits, demerits, corrosion detection methods and remedial actions.

In the recent era, distinct metallic materials such as titanium, stainless steel, titanium alloys, and Co-Cr alloy are widely used for implant manufacturing. But for successful implantation, these biomaterials require good biocompatibility, corrosion resistance, low elastic modulus, which is required closer to actual human bone, high strength, and non-cytotoxic. These biomaterials have primarily been used in specific applications such as orthopaedic fixation devices, dental implants, and cardiovascular stents. The corrosion of metal implants, on the other hand, determines the service period of implantation due to the release of incompatible metal ions into the human body, which may cause allergic reactions. As a result, the focus of this chapter is initially on metal biomaterials and their properties. The causes of implant failure are then highlighted, with a focus on corrosion mechanism details. Finally, various surface modification techniques, such as thermal-based surface modification techniques, are discussed in detail, as are their applications in improving corrosion resistance, biocompatibility, and osseointegration of various biomaterials. 

Orthopedic Implant is a high-risk medical device. Its main function is stabilization and fixation of bone but some are functional devices like hip arthroscopy, knee joint replacement implants, spinal cages, etc. Some common materials used to manufacture implants are Titanium, Titanium alloy (Ti6Al4V) as per ISO 5832-3, Stainless Steel-316 as per ISO 5832-1, tantalum, bioabsorbable material like PLLA, PGA, PLDLA, etc. The implant should have some fundamental properties such as being biocompatible, corrosion resistant, and having good mechanical properties. Though the implants have these properties, some complications like bacterial adhesion cause infection, poor osseointegration, and loosening of the implant. To overcome these complications, one of the effective and simple solutions is coating. The coating can enhance osseointegration, reduce infection, increase bone ingrowth and mechanical strength, etc. The coating of a material with desirable properties over the implant is a tough and complex process. The antibacterial coating materials are chitosan, gentamicin, Rifampicin, Titanium oxide, etc. Similarly, the coating material for osseointegration is hydroxyapatite (HA), extracellular matrix (ECM), magnesium coating, etc. There are different technique for coating materials like the Dip-Coating method, magnetron sputtering, sol-gel technique, electrophoretic deposition, etc. Although coating is the most effective way to overcome some above-mentioned complications, most of the implants are sold on the market without coating. Coating is a complicated and costly process. It is still in its niche in research and development, however, it has a lot of potential for the future. Hence, in this chapter, the author mainly focuses on orthopedics implant materials, associated problems, and distinct coating materials techniques, which are discussed in detail.

In the 1980s, a deposition technique known as cold spray solid-state coating was created. Cold spray technology, unlike conventional thermal spray techniques, can maintain the natural properties of the feedstock, prevent damage to the constituent elements of the substrate and create extremely solid coatings. Nanostructured coatings have the potential to significantly enhance their properties compared to conventional, non-nanostructured coatings. Furthermore, surface coating on metal substrates is a very difficult challenge for the researcher due to the contradictory requirements for various properties. The ability of cold spray to form coatings with nanostructures has also been demonstrated to a great extent. This work aims to provide an in-depth analysis of nanostructured cold-sprayed metal coatings. First, a description of the cold spray technique is given. Next, the issue of Nano crystallization in standard metal coatings is discussed. Then, microstructures and properties of nanomaterial-reinforced metal matrix composite (MMC) coatings and cold-sprayed nanocrystalline metal coatings are discussed. In conclusion, a summary and future prospects for cold spray technology are given. To conclude, the process of developing nanostructured metal coatings has been completed.