Wear- and Corrosion-Resistant Coatings for Extreme Environments: Advances, Challenges, and Future Perspectives

Abstract

Tribological processes in extreme environments pose serious material challenges, requiring coatings that resist both wear and corrosion. This review summarizes recent advances in protective coatings engineered for extreme environments such as high temperatures, chemically aggressive media, and high-pressure and abrasive domains, as well as cryogenic and space applications. A comprehensive overview of promising coating materials is provided, including ceramic-based coatings, metallic and alloy coatings, and polymer and composite systems, as well as nanostructured and multilayered architectures. These materials are deployed using advanced coating technologies such as thermal spraying (plasma spray, high-velocity oxygen fuel (HVOF), and cold spray), chemical and physical vapor deposition (CVD and PVD), electrochemical methods (electrodeposition), additive manufacturing, and in situ coating approaches. Key degradation mechanisms such as adhesive and abrasive wear, oxidation, hot corrosion, stress corrosion cracking, and tribocorrosion are examined with coating performance. The review also explores application-specific needs in aerospace, marine, energy, biomedical, and mining sectors operating in aggressive physiological environments. Emerging trends in the field are highlighted, including self-healing and smart coatings, environmentally friendly coating technologies, functionally graded and nanostructured coatings, and the integration of machine learning in coating design and optimization. Finally, the review addresses broader considerations such as scalability, cost-effectiveness, long-term durability, maintenance requirements, and environmental regulations. This comprehensive analysis aims to synthesize current knowledge while identifying future directions for innovation in protective coatings for extreme environments.

1. Introduction

1.1. Overview of Extreme Environments

Extreme environments are characterized by physical or chemical conditions that significantly exceed ambient or normal service conditions, thereby accelerating material degradation through wear, corrosion, or their synergy. These environments span a wide range of harsh conditions. High-salinity coastal regions, high-temperature industrial zones, and chemically aggressive surroundings are common examples. Other challenging domains include high-pressure and abrasive settings, such as those found in deep-sea operations. Additionally, extreme environments exist in mining areas, cryogenic temperatures, and high-vacuum conditions. Ionizing radiation in nuclear energy systems and plasma exposure in aerospace and fusion reactors also present significant challenges [1,2].

The definition of an extreme environment can vary depending on the field of application, with each sector presenting unique challenges. In such settings, materials are routinely subjected to elevated mechanical, chemical, and thermal stresses that challenge their structural integrity and operational reliability. Corrosion in these environments is driven by exposure to water, salts, chemicals, and atmospheric pollutants, factors that are intensified under extreme temperature and pH conditions [3]. For example, steel railway structures exposed to seasonal and daily climatic fluctuations face corrosion due to condensation, frost, rain, and snow. Lazorenko et al. [4] highlight the vulnerability of such structures and the critical role of coatings in mitigating environmental impact. The global economic burden of corrosion is immense. Bender et al. [5] estimate the worldwide cost of corrosion damage at USD 1.8 trillion as of 2001. The authors postulate that effective corrosion protection strategies, particularly coatings, can enhance public safety, prolong asset life, ensure reliable performance, and reduce maintenance costs, making corrosion control a cornerstone of sustainable engineering.

The pursuit of wear- and corrosion-resistant materials dates back to the early 20th century, when protective paints and metallic claddings were first applied to ships and industrial equipment [6]. By the 1950s, thermal spray coatings emerged for aerospace and turbine applications, enabling high-temperature surface protection [7]. In the 1980s, the miniaturization of coating technologies evolved, with chemical and physical vapor deposition (CVD/PVD) enabling micrometer-scale coatings for electronics and cutting tools [8]. In the early 2000s, the integration of nanotechnology led to nanocomposite and self-assembled coatings with tailored properties such as enhanced hardness, barrier performance, and self-healing capabilities [9]. This shows the shift from bulk material reliance to surface engineering as a key strategy for extending component life in extreme service environments.

Beyond natural environmental exposures, many industrial systems operate in artificially extreme conditions, such as those found in gas turbines and thermal and nuclear power plants. Mondal et al. [10] emphasize the significance of thermal barrier coatings (TBCs) in such systems. TBCs provide critical insulation that extends component life and boosts operational safety. With the intensifying effects of climate change, especially in coastal zones experiencing higher humidity and temperature, there is a growing need for ultra-durable coatings. Wang et al. [11] highlights the urgency of developing coatings that can withstand the evolving harshness of these environments, especially in the context of rising global climate instability.

1.2. Importance of Wear and Corrosion Resistance in Critical Applications

In high-stakes industries such those of aerospace, automotive, marine, energy, and biomedical engineering, the application of wear- and corrosion-resistant coatings is essential to ensure the reliability, safety, and performance of critical components. These systems often operate in environments where mechanical stress, extreme temperatures, and corrosive agents act simultaneously. This necessitates advanced protective strategies.

Cui et al. [12] demonstrated that smart coatings with self-healing properties that adapt to environmental stimuli delayed corrosion initiation by over 72 h in salt spray testing, and restored up to 90% of barrier performance after localized damage. These smart systems not only delay corrosion initiation but also repair localized damage, offering a more sustainable solution to long-term material degradation. Another notable example is the work of Feng et al. [13], which demonstrated that alloying Inconel 625 with 2.5 wt.% Al enhanced its wear resistance by 45% and corrosion resistance by 58% in acidic environments, due to the formation of a more stable and adherent passive film. The treated coatings showed improved resistance to plastic deformation and abrasive wear. Figure 1 shows the SEM morphology of Inconel 625 with different Al contents. At 2.5% Al, the coating formed fine pits and a stable oxide film, enhancing corrosion resistance. However, higher Al levels reduced Cr content, which resulted in weakening the protective Cr oxide layer and increasing corrosion. This highlights the impact of strategic alloying in hostile service conditions.

Huang et al. [14] highlighted the superior performance of Fe-based amorphous coatings. They found an improvement in wear rate by an order of magnitude lower than that of crystalline steel coatings, along with superior corrosion resistance in both acidic and saline media. Their results underlined the potential of advanced material formulations in improving the durability of coatings exposed to extreme environmental stressors. These findings collectively illustrate that wear- and corrosion-resistant coatings are not mere auxiliary features but critical design elements in ensuring the longevity, efficiency, and safety of modern engineered systems.

1.3. Scope and Objectives of the Review

The growing demand for high-performance materials in industries such as aerospace, energy, marine, and biomedical sectors has fueled the development of advanced coating technologies. Components operating in these environments face severe challenges from high temperatures, corrosive chemicals, abrasion, and cyclic mechanical stresses. Without adequate protection, these components rapidly degrade, resulting in premature failure, increased downtime, and elevated maintenance costs. This review provides a comprehensive examination of wear- and corrosion-resistant coatings designed for extreme environments. It focuses on both traditional and emerging coating materials, including ceramic-based, metallic, polymeric, and nanostructured systems, and explores their advantages and limitations across various operational conditions. Application techniques such as thermal spraying (e.g., plasma spray, HVOF, cold spray), vapor deposition (CVD and PVD), electrodeposition, additive manufacturing, and in situ coatings are discussed in detail. Additionally, the review addresses key degradation mechanisms, such as adhesive and abrasive wear, oxidation, hot corrosion, pitting, stress corrosion cracking, and tribocorrosion, which limit the service life of coatings. A critical evaluation of coating performance in sectors such as aerospace, oil and gas, power generation, marine infrastructure, and biomedical implants is also provided.

The key objectives of this paper are (a) to understand the current advancements in coating technologies for extreme environments; (b) to assess the effectiveness of smart, self-healing, environmentally benign, and nanostructured coatings; (c) to highlight the integration of machine learning in the optimization of coating design; and (d) to evaluate the sustainability, scalability, cost-efficiency, and long-term reliability of modern coating systems. Lastly, this review identifies current research gaps and proposes future directions, including the development of coatings for emerging applications such as hydrogen storage systems and space exploration. By addressing the multifaceted challenges of extreme environments, this work aims to serve as a valuable resource for researchers, engineers, and industry professionals seeking to enhance the resilience and longevity of materials through advanced coating technologies.

2. Types of Extreme Environments and Their Challenges

Engineering materials and components often face harsh conditions that push their limits. These extreme environments, ranging from high temperatures and corrosive chemicals to high pressure, abrasive wear, cryogenic temperatures, and the vacuum of space, pose unique challenges that demand specialized design and protection strategies. Figure 2 provides a schematic representation of various extreme environments encountered in engineering applications. Together, these environments emphasize the critical need for advanced materials and protective coatings to ensure durability and performance.

2.1. High-Temperature Environments

Extreme temperatures pose a serious challenge to structural integrity and surface durability in sectors such as aerospace, power generation, and advanced manufacturing [15]. High-temperature environments promote degradation through oxidation, sulfidation, thermal fatigue, and tribological wear mechanisms, including abrasion, sliding, and erosion. These factors reduce component life and compromise performance [16]. To combat such deterioration, advanced coatings with high thermal stability and oxidation resistance are employed. Oxide ceramics like Al₂O₃, Cr₂O₃, and TiO₂ form protective barriers that prevent further oxidation [17]. Metallic and intermetallic coatings, particularly those based on nickel aluminides or cobalt superalloys, exhibit stable oxide formation and thermal fatigue resistance. Carbide- and boride-based coatings (e.g., WC, TiC, CrB, TiB₂) provide excellent hardness and protection in abrasive, high-temperature settings [10,18,19]. Mechanically mixed layers (MMLs), incorporating oxides and metal particulates, act as self-lubricating, wear-resistant barriers [20]. Surface engineering methods, such as thermal spraying, plasma nitriding, water jet peening (WJP), and multifunctional cavitation (MFC), further enhance coating adhesion and mechanical stability [21]. For example, MFC-treated Cr-Mo steel shows improved corrosion resistance at 500 °C due to the formation of dense oxide films and beneficial residual stresses. Nickel aluminide coatings are particularly effective around 650 °C, maintaining surface protection through slow-growing oxide scales [22]. Ultimately, the selection of coating materials and deposition techniques must consider thermal exposure levels, loading conditions, and chemical interactions. Ongoing advances in carbide-based coatings, surface modification, and multilayer systems continue to expand the usability and longevity of components in aggressive thermal environments.

2.2. Corrosive Chemical Environments

In industries such as oil and gas, chemical processing, and marine operations, materials are routinely exposed to highly corrosive chemicals that accelerate degradation. These environments involve not only chemical attack but also mechanical wear, leading to a complex failure mode known as tribocorrosion, where friction and corrosion act synergistically to erode material surfaces [23,24]. Common chemical agents, such as CO₂, H₂S, acids, alkalis, and chloride ions, induce localized corrosion phenomena, including pitting, crevice corrosion, and stress corrosion cracking [25,26]. These effects compromise the structural integrity of pipelines, valves, reactors, and drilling components. Fusion-bonded epoxy (FBE) and thermal spray aluminum (TSA) coatings are widely adopted in oil and gas operations for their excellent corrosion resistance and mechanical durability [27,28]. In chemical plants, exposure to strong acids and reactive compounds necessitates the use of specialized coatings such as fluoropolymers, ceramic linings, and nanocomposite barriers. These coatings prevent material erosion and minimize chemical reactivity, even under high flow velocities or turbulent mixing [29,30].

The grain boundary size and density in coating microstructures can significantly influence corrosion behavior. Nanostructured or ultrafine-grained coatings, which have a higher density of grain boundaries, potentially exhibit accelerated corrosion under certain conditions. This is due to increased diffusivity and higher energy sites that promote electrochemical activity [31,32]. However, if engineered properly, these coatings can also form more stable oxide films and enhance corrosion resistance. One of the effective strategies to mitigate the negative impact of excessive grain boundary effect on corrosion is heat treatment. Heat treatment can lead to grain growth, stress relief, and improved boundary stability. This enables the reduction in localized corrosion susceptibility of coatings [33,34].

Materials like stainless steels and nickel-based alloys offer inherent resistance but benefit significantly from additional protective coatings. Deposition methods like plasma electrolytic oxidation (PEO) or carbide-based thermal spray coatings create dense, adherent layers that extend component life [35]. Regular maintenance and inspection remain essential to ensure long-term performance in these chemically aggressive environments.

2.3. High-Pressure and Abrasive Conditions

High-pressure and highly abrasive environments, such as those encountered in deep-sea mining, subterranean drilling, and mineral processing, pose serious threats to material integrity [36]. These environments are characterized by extreme hydrostatic pressure, mechanical abrasion from particulates, and fluctuating loads, all of which can induce rapid wear and corrosion [37]. Steel remains a material of choice for such operations, yet its limited erosion resistance makes it susceptible to premature failure, especially in areas like the “splash zone”, where alternating wet and dry exposure accelerates degradation. In deep-sea environments, corrosion is further exacerbated by factors such as high salinity, low oxygen levels, low temperatures, and microbial activity [38]. Protective strategies include the use of sacrificial anodes in cathodic protection systems and the application of organic and thermal spray coatings. Organic coatings can significantly reduce moisture ingress and chemical attack, while hard facing or ceramic coatings provide resistance against erosion and impact wear. Structural reliability in high-pressure operations depends on preemptive surface treatment and material selection. For example, studies on Q235 mild steel exposed to deep-sea conditions have demonstrated the critical impact of galvanic corrosion, underscoring the need for enhanced coating technologies to mitigate failure risks [39]. Given the potential for catastrophic equipment failure, reliable coatings are not only a performance solution but also a safety imperative.

2.4. Space and Cryogenic Applications

Space presents a uniquely harsh environment where components are subjected to rapid thermal cycling, extreme vacuum, intense radiation, and cryogenic temperatures [40]. Materials used in spacecraft and satellite systems must endure these conditions without degradation, making advanced surface coatings critical for mission success. Thermal cycling, from as high as 200 °C in sunlight to below −200 °C in planetary shadow, causes expansion and contraction that can lead to microcracking, delamination, and fatigue in protective coatings [41,42]. Additionally, sustained thermal loads can surpass material melting points, requiring coatings with exceptional thermal conductivity and mechanical stability [43].

Coating materials currently in development for space applications include hexagonal boron nitride (hBN), diamond-like carbon (DLC), MoS₂, and ultra-thin nanocomposites [44,45,46]. These materials maintain functional integrity across extreme temperatures, prevent cold welding in vacuum, and reduce wear under sliding contact. Cryogenic applications pose further challenges due to the difficulty of simulating space conditions in laboratory settings [47]. Testing tribological performance at very low temperatures often yields limited data, making it difficult to validate new coating systems. For instance, lunar regolith, a highly abrasive and electrostatically charged dust, is difficult to replicate, yet poses a substantial wear threat to lunar infrastructure and robotics. Despite the challenges, advances in low-friction coatings, adaptive materials, and self-healing films are promising to enhance components’ durability and functionality in extraterrestrial environments. As interest grows in lunar bases, deep-space missions, and Mars exploration, the development of resilient coatings remains a key enabler of sustainable space operations.

3. Coating Materials for Extreme Environments

3.1. Ceramic-Based Coatings

The growing demands in clean energy, aerospace, and nuclear sectors have intensified the need for coatings capable of withstanding extreme environments. Among the most promising candidates are ceramic-based coatings due to their inherent resistance to corrosion, wear, and high temperatures. A comprehensive review by Wyatt et al. [48] highlights the potential of Ultra-High-Temperature Ceramics (UHTCs), refractory materials containing early transition metals with melting points between 3000 °C and 4200 °C. These ceramics exhibit excellent oxidation resistance and structural integrity, making them ideal for applications in hypersonic, nuclear energy, and space travel.

Another innovative class is High-Entropy Ceramics (HECs), discussed by Ward et al. [49], which differ from high-entropy alloys through their use of multi-element compositions involving ionic and covalent bonding. HECs demonstrate superior oxidation and corrosion resistance, particularly fluorite, silicate, and disilicate-based variants. These materials are especially suitable for high-temperature electronics, thermal protection systems, and components in aerospace propulsion and power electronics, due to their thermal stability and chemical compatibility with silicon carbide-based systems. Grilli et al. [50] further explore thermal barrier coatings (TBCs) and oxidation-resistant ceramics designed to reduce dependency on critical raw materials. These coatings can withstand aerospace engine temperatures above 1500 °C and provide essential protection against corrosive and oxidative conditions.

3.2. Metallic and Alloy Coatings

Metallic and alloy coatings offer another viable solution for extreme environments, combining high-temperature tolerance with mechanical durability. Superalloy coatings deposited using air plasma spraying, such as Co-based L605 coatings, have shown enhanced oxidation and hot corrosion resistance in extreme conditions [51]. Dixit et al. [52] examine Refractory High-Entropy Alloy (RHEA) coatings, including HfNbTaZr and MoNbTaVW systems, which exhibit exceptional thermal stability with yield strength maintained above 1200 °C. Owing to their high-temperature strength, these RHEAs are particularly suited for aerospace turbine components and thermal protection systems. Additionally, the MoNbTaVW system offers superior radiation resistance due to its non-neutron-absorbing composition. This makes it a promising candidate for nuclear reactor environments. Kumar et al. [53] investigate Ni-based coatings (Ni-5Al and Ni-20Cr), applied via wire arc spraying. These coatings demonstrate strong adhesion and corrosion resistance at high temperatures, making them suitable for boiler tubes and other power generation components.

Table 1. Summary of selected metallic/ceramic coatings for extreme conditions.

Coating Type	Temperature Rating	Corrosion Resistance/Key Benefit	Reference
Ni-20Cr	900 °C	High resistance in molten salt environments	[53]
RHEAs	>1200 °C	Strong mechanical properties; variants for radiation or corrosive use	[52]
TBCs	~1300 °C	Prevent oxygen ingress; ideal for aerospace engine applications	[50]
Self-Healing Coatings	~1200 °C	Maintain integrity and corrosion resistance over long service durations	[54]

provides selected metallic/ceramic coatings used for extreme conditions.

The following table (

Table 2. Summary of typical properties of ceramic-based and metallic/alloy coatings used in extreme environments (property ranges are approximate and may vary depending on processing, composition, and conditions.).

Coating Type	Hardness (GPa)	Thermal Stability (°C)	Corrosion/Oxidation Resistance	Advantages	References
Ultra-High-Temperature Ceramics (UHTCs)	20–30	3000–4200	Excellent	Extremely high melting points, oxidation resistance at ultra-high temps	[55]
High-Entropy Ceramics (HECs)	15–25	>1600	Excellent	Tunable properties, compatibility with SiC electronics	[56]
Thermal Barrier Coatings (TBCs)	10–15	~1500	Good	Thermal insulation, reduced thermal conductivity	[57,58]
Superalloy Coatings	5–10	~1100–1200	Very Good	High creep strength, good aerospace performance	[59,60]
Refractory High-Entropy Alloys (RHEAs)	12–18	>1200	Very Good to Excellent	High strength at elevated temperatures, radiation resistance	[61]
Ni-Based Alloy Coatings	6–10	~1000–1100	Good	Strong adhesion, cost-effective, suitable for boiler tubes	[62,63]

) summarizes the key properties of ceramic-based and metallic/alloy coatings. The table highlights differences in hardness, thermal stability, and corrosion/oxidation resistance, offering insights into each coating class’s advantages and ideal application domains.

3.3. Polymer and Composite Coatings

Polymer coatings offer UV resistance due to their transparency to wavelengths above 296 nm, making them ideal for outdoor applications [64]. Variants include conductive, porous, adsorptive, and reactive polymer coatings. Adsorptive polymers (e.g., polyethylene, PET, nylon) are used in packaging, pharmaceuticals, and cosmetics to improve the shelf life and stability of sensitive products. For more demanding conditions, composite coatings, comprising polymer, metal, or ceramic matrices reinforced with particles, offer superior wear resistance and mechanical strength. As material restrictions tighten, composites become increasingly vital. Sun et al. [65]. demonstrated that combining anticorrosive fillers with waterborne epoxy resins significantly enhances corrosion protection, offering a viable and eco-friendly alternative. For aerospace and space applications, high-performance polymers like polyimides (such as Kapton), PTFE (Teflon), and PEEK are commonly used because they can handle extreme temperatures, resist radiation, and remain stable in vacuum conditions [66,67].

Despite the advantages, polymer and composite coatings have considerable limitations that can restrict their long-term performance in extreme conditions. Most polymer matrices are thermally sensitive and tend to degrade at elevated temperatures (typically above 200–300 °C) [68]. While some polymers exhibit initial UV resistance, prolonged exposure to UV radiation can cause surface cracking and degradation [69].

3.4. Nanostructured and Multilayered Coatings

Nanostructured coatings, with grain sizes in the 1–100 nm range, are used in aerospace, electronics, and industrial sectors [70]. Their high surface-area-to-volume ratios confer enhanced hardness, thermal stability, and even self-healing capabilities [71]. Common nanostructured materials include nanocrystalline Ni, having high hardness and corrosion resistance [72]; TiO₂ and Al₂O₃ nanocoatings, having excellent wear and oxidation resistance [73]; and ZnO and SiO₂, having optical transparency and UV shielding property [74]. These coatings bridge the gap between atomic-level design and macroscale applications, enabling tunable optical and thermal properties.

Recent studies show that nanostructured coatings also exhibit improved erosion–corrosion resistance due to their refined grain structure, which promotes uniform passive film formation and impedes localized attack [75,76]. For example, in marine environments, nanocrystalline aluminum coatings provide strong resistance to pitting and cavitation damage. This is mainly due to their ability to form a dense, protective oxide layer that can partially heal itself, helping maintain the coating’s integrity over time [77]. Additionally, surface patterning techniques such as riblet or sawtooth-shaped nano-patterns have shown promise in mitigating erosion and influencing nanobubble behavior. Molecular dynamics simulations show that these structures can modify local fluid flow and bubble collapse dynamics. This can, in turn, effectively reduce surface impact energy during the cavitation phenomenon. This may also limit the microstructural damage and delay the initiation of localized corrosion [78,79,80].

Multilayered coatings, composed of stacked layers with distinct functionalities, offer tailored protection by combining the benefits of different materials. Examples include TiN/CrN, TiAlN/CrAlN, and AlTiN/Si₃N₄ multilayers [81,82]. Liu et al. [83] reported that TiAlN/CrAlN nano-multilayers exhibit exceptional wear and thermal resistance, making them ideal for cutting tools and aerospace engine components. ZrN/ZrCN, NbN/CrN, and MoN/NbN multilayers are also being explored for applications requiring high-temperature oxidation resistance and diffusion barrier performance.

Table 3. Comparison of nano-structured vs. multilayered coatings [70,83,84,85,86].

Feature	Nanostructured Coatings	Multilayered Coatings
Structure	Nano-scale grains/particles	Stacked layers with distinct functions
Thickness	1–100 nm	Nanometers to micrometers
Material Type	Ceramics, metals, hybrid composites	Ceramics, polymers, metals, composites
Properties	High hardness, toughness, stability	Combined mechanical, thermal, and chemical benefits
Applications	Biomedical, aerospace, electronics	Optical barriers, wear and corrosion resistance
Deposition methods	Physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition	Magnetron sputtering, pulsed laser deposition (PLD), atomic layer deposition (ALD), sol–gel

presents a comparison of nanostructured and multilayered coatings

An overview of the comparative summary of the four major coating materials discussed in this section is provided in

Table 4. Comparative summary of the coating materials highlighting strengths, limitations, and suitable extreme environments.

Material type	Advantages	Limitations	Suitable extreme environments/Applications	References
Ceramic-Based Coatings	Excellent thermal and oxidation resistance, high hardness, corrosion protection	Brittle, low fracture toughness, often require high-temperature processing	High-temperature environments, chemically aggressive media, and thermal barrier applications	[87,88,89]
Metallic and Alloy Coatings	Good ductility, thermal conductivity, electrical conductivity, oxidation resistance (in some alloys)	Lower hardness than ceramics, susceptible to corrosion if unprotected	High-pressure and thermal cycling environments, marine and structural applications	[90,91]
Polymer and Composite Coatings	Lightweight, corrosion-resistant, chemically versatile, easy to apply	Low thermal stability, prone to UV degradation over time	Chemical plants, biomedical devices, electronics, and moderate corrosion and abrasion environments	[92,93,94]
Nanostructured and Multilayered Coatings	Enhanced hardness, wear, and corrosion resistance, tunable properties	Complex fabrication, scalability issues, and cost	Wear-prone components, aerospace and microelectronics, dynamic or multifunctional environments	[70,95,96]

, highlighting their advantages, limitations, and ideal service environments in extreme conditions.

4. Coating Techniques and Processes

4.1. Thermal Spray Coatings: Plasma Spray and High-Velocity Oxygen Fuel (HVOF)

Thermal spray technologies, including plasma spray and high-velocity oxygen fuel (HVOF), are widely used to deposit thick, dense coatings on substrates of various geometries. These methods operate by heating feedstock powders and propelling the molten or semi-molten particles at high velocities onto a surface, where they solidify to form protective layers [97,98]. In plasma spray, an electric arc is used to ionize gas, creating a high-temperature plasma jet. Coating materials introduced into the jet are melted and sprayed onto the substrate. HVOF, in contrast, operates at lower flame temperatures but higher particle velocities. It uses the combustion of oxygen and fuel gases in a high-pressure nozzle to generate a supersonic jet that propels partially molten particles onto the surface. A schematic illustration and comparison of both plasma spray and HVOF are given in Figure 3.

A major advantage of HVOF is its lower oxidation tendency. Because of the shorter residence time of particles in the high-temperature zone, HVOF coatings exhibit reduced phase transformations and decomposition. These coatings typically achieve porosity levels below 1%, resulting in superior wear resistance and long-term durability [100,101,102]. Furthermore, the rapid cooling of the deposited particles can lead to the formation of nanocrystalline microstructures, which can be tailored further through post-deposition heat treatments [103]. However, it is to be noted that HVOF systems generally involve higher operational and equipment costs compared to conventional plasma spraying. This is due to more complex combustion and gas handling systems associated with HVOF. Both plasma spray and HVOF are regarded as excellent methods for producing dense, adherent, corrosion- and wear-resistant coatings across various industrial components.

4.2. Cold Spray Technology

Cold spray (CS) technology is a solid-state coating technique where fine metallic particles are accelerated to supersonic speeds in a high-velocity gas stream and plastically deform upon impact, adhering to the substrate without melting. Developed in the 1980s by the Institute of Theoretical and Applied Mechanics in Russia, this technique is especially useful for heat-sensitive materials. Unlike thermal spray methods, CS avoids thermal degradation and phase changes, preserving the original material properties [31,104]. This makes it ideal for coating temperature-sensitive substrates such as aluminum alloys and for applications requiring electromagnetic interference or radiation shielding [105]. A schematic diagram of the CS process is shown in Figure 4.

CS coatings exhibit low porosity, excellent mechanical strength, and high electrical and thermal conductivity. Applications range from aircraft component repair (e.g., turbine blades) to large-scale structural parts such as fuel tanks and rocket nozzles. Titanium and tantalum alloys can be deposited without compromising their structural properties [106]. However, CS has limitations: (a) it cannot effectively deposit hard or brittle materials that lack sufficient ductility; (b) the process often requires helium gas, which is costly; (c) achieving high coating efficiency depends on precise control of parameters such as gas pressure and temperature [107]. Despite these constraints, CS is increasingly indispensable for critical applications where precision and material integrity are essential. Recent advances like nitrogen–helium gas mixing and pre-heating strategies help to reduce helium consumption while maintaining performance [108]. Progress made in hybrid cold spray systems helps combine mechanical milling, laser assistance, or friction-based pre-treatments to improve the deposition of harder materials [109,110].

4.3. Chemical and Physical Vapor Deposition (CVD and PVD)

Vapor deposition techniques, specifically chemical vapor deposition (CVD) and physical vapor deposition (PVD), are thin-film deposition methods used to create dense, hard coatings with atomic-level precision. In PVD, solid materials are vaporized via sputtering or thermal evaporation and then condensed onto the substrate in a vacuum. Common PVD techniques include electron beam evaporation, ion plating, and magnetron sputtering, with the latter providing high-density, uniform coatings [111,112]. CVD, on the other hand, involves the introduction of volatile precursor gases into a reaction chamber, where chemical reactions occur at the substrate surface, forming a film. This method allows precise control over composition and thickness, and is often used for advanced materials like graphene [113]. A schematic comparison of PVD and CVD processes is given in Figure 5.

Table 5. Comparison of PVD and CVD methods [111,112,113].

Attribute	Physical Vapor Deposition (PVD)	Chemical Vapor Deposition (CVD)
Starting Material	Solid (target material)	Gaseous precursors
Deposition Mechanism	Physical (evaporation/sputtering)	Chemical reaction at the substrate surface
Coating Properties	High hardness, wear and temperature resistance	Precise thickness and composition
Typical Applications	Cutting tools, optics, and electronics	Semiconductors, protective coatings

compares the PVD and CVD methods. CVD is often preferred for complex geometries and when stoichiometric control is needed, while PVD is advantageous for producing hard coatings in high-vacuum environments.

4.4. Electrochemical and Electrodeposition Methods

Electrochemical methods involve the application of electrical energy to drive chemical changes and deposit coatings. Electrodeposition, commonly known as electroplating, is the most widely used electrochemical technique and involves the reduction in metal cations from an electrolyte onto a conductive substrate. Localized Pulsed Electrodeposition (L-PED) represents an advanced, high-resolution version of electrodeposition. By applying short, localized pulses of current through a micro-nozzle, it enables the 3D printing of nanoscale metallic structures, allowing engineers to fine-tune grain structure and material properties [115]. These methods are critical in the fabrication of integrated circuits and micro-electromechanical systems (MEMS), where control over composition and geometry at micro- and nano-scales is essential.

Table 6. Electrochemical coating methods and applications [116,117].

Method	Description	Applications	Advantages	Limitations
Electrochemical Techniques	Electron transfer-based coating processes	Batteries, sensors, corrosion testing	Broad material range, scalable	Requires conductive substrates
Electrodeposition	Ion reduction and metal deposition	Electronics, corrosion protection	Uniform coating, cost-effective	Limited to conductive surfaces
Localized Pulsed Electrodeposition (L-PED)	Pulsed micro-deposition using electrolyte meniscus	3D microelectronics, nano-sensors	Tailored microstructure, high precision	Complex, requires precise control

presents electrochemical coating methods and applications.

4.5. Electrophoretic Deposition and Aerosol Deposition

In addition to conventional techniques such as thermal spraying, PVD, and CVD, several other methods are widely used in the ceramics industry for depositing functional coatings. Electrophoretic deposition (EPD) is a versatile and low-cost technique that involves the movement of charged ceramic particles in a liquid suspension under an electric field. This allows for a uniform coating on substrates that have complex shapes. A schematic illustration of the EPD process is given in Figure 6. Key factors affecting coating morphology in this process include particle size, suspension properties (like viscosity, conductivity, and stability), deposition time, applied voltage, and substrate conductivity. EPD is effective for producing dense, crack-free ceramic layers and has been applied in areas such as thermal barriers, bioactive coatings, and corrosion protection [118,119].

Another promising method is aerosol deposition (AD), a room-temperature process in which submicron ceramic particles are accelerated in a gas stream, impact, and become consolidated on the substrate without melting. This enables the formation of dense, nanostructured ceramic coatings with strong adhesion and minimal thermal damage to the substrate [120]. While AD shares some similarities with the CS process as a solid-state coating process, it differs in using finer ceramic particles and operating at lower temperatures and gas pressures, making it especially suitable for ceramic coatings on temperature-sensitive substrates [121].

4.6. Additive Manufacturing and In Situ Coating Techniques

Additive manufacturing (AM), or 3D printing, constructs components layer-by-layer based on digital designs. It offers advantages over traditional subtractive methods by reducing material waste, enabling complex geometries, and accelerating production cycles [122]. In situ coating techniques, applied during the AM process, enhance surface functionality and reduce post-processing steps. One such technique is Laser Metal Deposition (LMD), where metal powder is injected into a laser-induced melt pool and solidifies to form a strong metallurgical bond [123,124]. LMD is effective for surface enhancement, repair, and part reconstruction. Another promising method is Cold Spray Additive Manufacturing (CSAM)—an adaptation of CS technology for AM. CSAM builds components by depositing layers of cold-sprayed material, preserving original material properties while enhancing hardness, corrosion resistance, and thermal protection. These combined approaches allow for simultaneous structure and function customization, offering unprecedented design flexibility. They are especially valuable in aerospace, biomedical, and energy sectors, where high-performance materials are required in tailored geometries.

4.7. Coating Techniques for Polymers

Unlike metallic and ceramic coatings, which often rely on high-temperature or vacuum-based processes, polymer coatings are typically applied using low-temperature, solution-based, or dry methods that preserve the chemical integrity of the polymers [125]. Common techniques include dip coating, where the substrate is immersed in a polymer solution and slowly taken out, allowing a uniform film to form and dry on the surface [126]. Spray coating or polymer metallization is another widely used technique for large or geometrically complex parts, utilizing atomized droplets to deposit coatings efficiently [127]. For precise, thin-film applications, as in the case for microelectronics, spin coating is preferred, where a polymer solution is deposited on a rotating substrate to ensure uniform thickness and smooth surfaces [128]. Electrospinning is an advanced method that enables the fabrication of nanofibrous polymer coatings with high surface area, controllable fiber diameter, and porous structures, which are particularly useful in filtration, tissue engineering, and responsive surfaces [129].

5. Wear and Corrosion Mechanisms in Coatings

5.1. Adhesive and Abrasive Wear

Wear mechanisms critically influence the durability and performance of materials, particularly in extreme service environments. Among these, adhesive and abrasive wear are two dominant modes that highlight the response of materials to mechanical stress. A clear understanding of these phenomena, especially in systems such as polyimide composites and carbon fiber-reinforced polymers (CFRPs), is essential for optimizing materials to withstand harsh operating conditions.

Adhesive wear occurs due to intimate contact between surfaces, resulting in material transfer or loss. Studies have shown that the chemical structure of polyimide composites greatly influences their adhesive wear resistance. Both thermosetting and thermoplastic polyimides demonstrate excellent performance against smooth metallic counterparts. However, incorporating fillers like graphite and PTFE (polytetrafluoroethylene) yields mixed results, enhancing performance in some formulations while reducing it in others [130,131].

Abrasive wear, conversely, is induced by hard particles sliding across the material surface, leading to material removal. Fillers and reinforcing fibers can negatively impact abrasive wear resistance. The wear rate tends to increase with applied load and the size of abrasive particles, transitioning from surface deformation to cutting-dominated wear modes. Importantly, materials with good adhesive wear resistance often perform poorly in abrasive environments. Composites that showed very good performance in adhesive wear studies were observed to be unsuitable in abrasive wear performance [130,132].

Additional influences include environmental effects, particularly for CFRPs. UV-A exposure, thermal shocks, and moisture can accelerate wear. UV radiation leads to surface degradation, while the presence of moisture during abrasion increases wear rates. Interestingly, thermal shocks can sometimes enhance resistance by altering the microstructure beneficially [133]. In summary, the interplay between adhesive and abrasive wear is dictated by the composite formulation, environmental conditions, and operating stress. Strategic material selection and design are thus essential for achieving a balanced wear resistance profile in demanding applications.

5.2. Oxidation and Hot Corrosion

High-temperature applications in energy, aerospace, and marine sectors require materials that can withstand oxidation and hot corrosion, two of the most severe degradation mechanisms under such conditions. Oxidation involves the reaction of metal surfaces with oxygen to form oxide scales. While some oxides, such as Cr₂O₃, offer temporary protection, their volatility in the presence of water vapor, forming chromium oxy-hydroxides, leads to rapid degradation via chromium depletion. In contrast, alumina-forming alloys (Al₂O₃) exhibit more stable oxide layers and superior high-temperature oxidation resistance. However, thermal cycling can induce cracking and spallation of these protective layers, thereby compromising protection [134].

Hot corrosion occurs in environments with molten salts, such as Na₂SO₄ and V₂O₅, which actively degrade oxide layers through fluxing reactions. Impurities such as sulfur and chlorine exacerbate this process by destabilizing oxide scales and enhancing corrosive attack. Alloys rich in chromium and aluminum offer better protection due to their ability to form stable, adherent oxide films that are less susceptible to fluxing [96]. Mitigation strategies include (a) optimizing alloy composition by increasing Cr and Al content; (b) applying protective coatings such as aluminides and MCrAlY, which form stable oxide barriers; and (c) tailoring designs to account for specific environmental stressors like moisture and thermal shock. A deep understanding of oxidation and hot corrosion mechanisms is crucial for developing next-generation materials for high-temperature applications. Advances in alloy development and coating technologies are essential to ensuring long-term performance and reliability under extreme service conditions.

5.3. Stress Corrosion Cracking and Pitting

Pitting corrosion and stress corrosion cracking (SCC) are localized damage mechanisms that significantly compromise the structural integrity of metallic systems, particularly in marine and humid environments. Pitting corrosion originates from localized breakdowns in passive films or protective coatings, often induced by chloride ions. These pits act as stress concentrators and frequently serve as initiation sites for SCC. Under insulation and in marine settings, fluctuations in temperature and humidity exacerbate pit formation and growth [135]. The progression of pitting generally follows three stages: nucleation, propagation, and repassivation [136]. During propagation, autocatalytic reactions lower the local pH, promoting continued metal dissolution. While some pits stabilize, others deepen and expand, compromising structural integrity and potentially triggering SCC under applied stress. SCC arises from the synergistic action of mechanical stress and a corrosive environment. Often nucleated at pitting sites, SCC propagates due to localized stress intensification and aggressive chemistry. Long-term studies underscore the necessity for accurate field data and predictive models to assess SCC risks effectively [137]. Mitigation techniques include (a) non-destructive testing (NDT) methods such as ultrasonic and radiographic inspections for early pit detection [135]; (b) material selection, with a preference for highly resistant stainless-steel alloys; and (c) predictive modeling, based on long-term datasets, to enhance inspection planning and life-cycle management [137]. Together, pitting and SCC highlight the need for proactive corrosion management, particularly in environments exposed to saltwater, humidity, or insulation-induced moisture ingress.

5.4. Tribocorrosion: Synergistic Wear and Corrosion

Tribocorrosion refers to the synergistic interaction between mechanical wear and electrochemical corrosion, leading to accelerated degradation of materials. It is particularly prevalent in passive metals such as aluminum, stainless steels, and titanium, which form oxide films that are mechanically and electrochemically unstable under simultaneous wear and corrosion [138]. The tribocorrosion process begins with plastic deformation from sliding contact, disrupting the oxide layer and increasing surface friction. This promotes further wear, exposing fresh material to corrosion. In many cases, the process becomes cyclical; corrosion promotes wear, and wear enhances corrosion. In advanced stages, broken wear debris may embed into mating surfaces, increasing roughness and friction, which can lead to severe material degradation [139].

A study by Masahashi et al. [140] shows how titanium-based hip implants undergo tribocorrosion in chloride-rich body fluids during joint motion. This leads to repeated breakdown of the passive TiO₂ layer, and eventually leads to metal ion release, inflammation, and implant failure. Another study by Zhang et al. [141] demonstrated how high-strength aluminum alloys such as AA7075-T6, when subjected to fretting contact in 3.5 wt.% NaCl (simulated seawater), exhibit pronounced tribocorrosion. Increased sliding speeds and load accelerated the synergistic degradation.

Industries affected by tribocorrosion include (a) petrochemical and chemical processing, (b) aerospace and automotive, (c) marine and mining operations, and (d) food processing and biomedical applications. Due to its widespread impact, developing coatings that can withstand both wear and corrosion is a critical area of research. These coatings must maintain mechanical integrity while providing corrosion protection under dynamic conditions. In summary, tribocorrosion presents a complex degradation mechanism requiring integrated solutions that combine material selection, surface engineering, and environmental control. Continued research is essential to mitigate its effects and improve the longevity of components in aggressive service conditions.

6. Applications of Wear- and Corrosion-Resistant Coatings

6.1. Aerospace and Aviation Components

Wear- and corrosion-resistant coatings are critically important in aerospace and aviation, where components endure extreme mechanical, thermal, and chemical stresses. One widely used material in these sectors is the AA7075-T6 aluminum alloy, valued for its high strength-to-weight ratio. However, this alloy is susceptible to corrosion, particularly in aerospace environments [142]. To mitigate degradation, protective coatings have been applied to aerospace-grade components to enhance oxidation and corrosion resistance. The evolution of coating technologies for aerospace applications can be categorized into three main generations. (1) First-generation coatings consisted of monolithic nitride or carbide layers deposited via PVD. While effective under moderate conditions, their performance declined under high temperatures and velocities [143]. (2) Second-generation coatings introduced improved toughness and thermal stability, offering better resistance under harsher conditions. However, their performance remained dependent on factors such as coating thickness and interfacial quality. (3) Third-generation coatings, currently under development, integrate self-healing mechanisms and multi-layered nanostructures to deliver superior corrosion resistance and durability in high-stress aerospace conditions [144]. These advancements highlight the continuous need for multifunctional coatings that provide high-performance protection in aerospace operations.

6.2. Oil and Gas Drilling Equipment

The oil and gas industry rely heavily on wear- and corrosion-resistant coatings to safeguard drilling equipment exposed to highly abrasive and chemically aggressive environments. Drills often penetrate through hard rock formations such as granite and shale, which contain high-density particulates that accelerate mechanical wear [145]. Additionally, extreme subsurface temperatures and corrosive drilling fluids exacerbate material degradation. Aluminum-based drill pipes, preferred for their lightweight nature, are especially vulnerable to corrosion. Coatings such as tungsten carbide provide substantial protection, extending service life and ensuring equipment reliability [146]. Emerging solutions include smart coatings with self-healing capabilities, which autonomously repair micro-damage. Recent research focuses on hybrid nanostructured coatings that combine mechanical durability with chemical resistance, providing cost-effective protection under extreme operational conditions [1].

6.3. Marine and Offshore Structures

Marine and offshore environments present uniquely aggressive corrosion challenges due to saltwater exposure, microbial activity, and variable climatic conditions. Corrosion in these settings can lead to structural failure and pose significant safety risks [147]. Advanced coatings have been developed to mitigate these effects. (a) Laser-clad coatings involve melting metal powders onto surfaces to form metallurgically bonded layers, enhancing mechanical integrity and corrosion resistance. (b) Graphene-based polymer nanocomposites act as barriers to moisture and ions, effectively preventing the initiation of corrosion [148]. Given the environmental sensitivity of marine ecosystems, recent innovations emphasize low-VOC (volatile organic compound) or solvent-free coatings, such as laser-clad systems, which reduce the risk of environmental contamination compared to traditional solvent-based coatings [149].

6.4. Nuclear and Power Generation Systems

The advancement of nuclear power technologies, including the transition to Generation IV reactors, necessitates coatings that can withstand elevated temperatures, radiation, and chemically aggressive coolants. Traditional chromium-based electroplating processes using Cr (VI) compounds pose corrosion risks. Alternatives such as Cr₂O₃ and Cr₃C₂–NiCr coatings are being explored to improve corrosion resistance while reducing environmental toxicity [150]. New reactor designs require coatings compatible with materials such as oxide dispersion-strengthened alloys (ODS), refractory metals, carbides, nitrides, ceramics, and high-entropy alloys [151]. In these high-demand applications, nanocomposite metallic coatings have shown promise in enhancing thermal stability, hardness, wear resistance, and corrosion protection, thereby increasing system longevity and efficiency [152]. Ongoing innovations in coating technologies are pivotal for the safe, efficient, and extended operation of next-generation power systems.

6.5. Biomedical Implants in Aggressive Body Environments

Biomedical implants operate in complex physiological environments that demand exceptional biocompatibility, corrosion resistance, and mechanical integrity. Coatings must withstand bodily fluids, variable pH, and mechanical loading while minimizing adverse responses such as toxicity, inflammation, and foreign body rejection. Materials such as polyether ether ketone (PEEK), titanium dioxide, transition metal nitrides, and carbon-based coatings are widely employed to enhance the performance of implants. These coatings prevent the release of toxic metal ions, resist microbial colonization, and preserve surface functionality [153]. Compared to alloying or heat treatment, surface coatings offer a cost-effective method to improve performance without altering the bulk material. Their application in the micrometer range allows for precision tailoring of surface properties [154]. Modern medical coatings are engineered to complement the implant substrate, preserving the desired mechanical properties while enhancing its biological interface. Wear-resistant coatings on bone-mimetic implants, for example, have the potential to significantly extend implant lifespan and improve patient mobility [155].

7. Advances and Innovations in Coating Technologies

7.1. Self-Healing and Smart Coatings

Self-healing and smart coatings represent a transformative advancement in surface protection technologies. These coatings are designed to autonomously respond to external stimuli, such as mechanical stress, heat, or chemical exposure, by reinforcing or even repairing themselves. There are three primary mechanisms by which self-healing coatings function: (1) intrinsic healing, where the polymer matrix itself reflows to close microscopic cracks; (2) microcapsule-based healing, where embedded capsules release monomers that polymerize upon damage; (3) corrosion-triggered healing, where embedded capsules release corrosion inhibitors in the presence of corrosive agents [156]. These smart coatings are typically stimulus-responsive, requiring triggers like light, heat, or mechanical force to activate their healing functions [157]. However, sub-threshold stimuli may result in delayed or absent healing, and in some cases, poor compatibility between the active agents and the host matrix can lead to premature or incomplete release of the healing compounds. Moreover, the finite reservoir of active agents limits the number of times a coating can self-heal. Once the embedded capsules are depleted or the coating becomes too thin due to wear compensation, performance may degrade [158]. While self-healing and smart coatings present a novel strategy to extend the service life of materials, their incorporation introduces new challenges, such as balancing mechanical strength, environmental stability, and compatibility with host matrices. Figure 7 presents a schematic of a self-healing coating utilizing microcapsules that release monomer to repair a crack in response to mechanical damage.

7.2. Environmentally Benign Coating Process

Environmentally benign coating processes aim to reduce or eliminate the use of hazardous substances such as volatile organic compounds (VOCs), heavy metals, and toxic solvents. These efforts are increasingly vital due to regulatory pressures and environmental concerns. Key approaches include the following. (1) Waterborne coatings, which significantly lower VOC emissions, are now widely used in the automotive and architectural sectors [159]. (2) Powder coatings, a solvent-free alternative, minimize waste through recoverable overspray, offering high material efficiency [160]. (3) Bio-based coatings, derived from renewable resources, are biodegradable and reduce dependency on petrochemical feedstocks [161]. Additionally, nanotechnology enables the creation of coatings with enhanced performance and durability, reducing the frequency of recoating and associated environmental impacts [162]. The adoption of these sustainable coating technologies reflects a broader industrial shift toward eco-friendly manufacturing, reducing both ecological footprints and long-term costs.

7.3. Nanostructured and Functionally Graded Coatings

Nanostructured and functionally graded coatings leverage precise control at the micro- and nanoscale to deliver superior mechanical and chemical properties. Nanostructured coatings, composed of grains smaller than 100 nm, exhibit enhanced hardness, wear resistance, and corrosion protection due to their high grain boundary density and surface area [163]. These coatings are critical in applications such as aerospace, biomedical devices, and energy systems where failure is not an option. Functionally graded coatings (FGCs), in contrast, feature compositional or structural gradients tailored to substrate properties. This gradient minimizes thermal and mechanical mismatches, improving adhesion, toughness, and thermal shock resistance [164]. For instance, jet engine components often employ FGCs to act as thermal barriers, protecting against extreme heat and cyclic mechanical loading. Nanotechnology further enhances these coatings by enabling adaptive functionalities such as self-healing, anti-fouling, and real-time sensing, thus merging smart behavior with structural performance [165].

7.4. Role of Machine Learning in Coating Design and Optimization

Machine learning (ML) is revolutionizing coating development by shifting the paradigm from trial-and-error experimentation to data-driven optimization. ML models can rapidly analyze vast experimental and simulation datasets to (a) predict coating performance based on composition, deposition methods, and process parameters; (b) optimize formulations for target properties such as hardness, corrosion resistance, or thermal stability [166]; and (c) improve process control by adjusting variables like curing time, spray rate, or substrate temperature to ensure coating uniformity and quality [167]. Beyond predictive modeling, ML enables the development of smart, adaptive coatings integrated with sensors and real-time feedback systems. These coatings can dynamically respond to changing environmental conditions, such as humidity, mechanical stress, or temperature, enhancing performance and durability. As the availability of high-throughput material data grows, ML will become integral in the design of next-generation sustainable coatings, accelerating innovation and reducing development costs.

Despite the promise that ML offers, the integration of ML into coating design faces several significant challenges. A major limitation is the availability and quality of materials data. Also, coating systems are complex, which makes it hard to identify the key features that affect their performance. This is because important properties often result from complicated interactions between the material’s microstructure, surface chemistry, and the environment it is exposed to. Using ML in real-world settings can be difficult, as it requires fast data collection, good digital systems, and automation. Even with these difficulties, progress is being made. New tools, smarter ML models, and better databases help to overcome these problems. As these technologies continue to improve, ML will become a powerful tool that can withstand extreme environments while also reducing costs and improving long-term performance.

8. Challenges and Limitations

8.1. Scalability and Cost of Coating Techniques

Scalability and cost are critical factors in selecting coating techniques for industrial applications. While technical performance is paramount, a coating method’s ability to scale efficiently and economically is essential for practical deployment. This section reviews the scalability and cost implications of PVD, CVD, and thermal spray coatings.

Among PVD techniques, magnetron sputtering and high-power pulsed magnetron sputtering (HPPMS) are notable for their ability to produce dense, uniform coatings with high adhesion. These processes are suitable for large surface areas and offer excellent control over coating properties. However, scalability is constrained when applied to large, complex components (e.g., aerospace parts), primarily due to the need for vacuum chambers, substrate heating, and precise deposition environments. Cost remains a significant limitation. The high capital investment required for vacuum systems, along with energy-intensive operations, makes PVD economically viable primarily for high-value, low-volume applications. Operational costs are compounded by material waste and complex maintenance requirements [168].

CVD processes, including low-pressure CVD (LPCVD) and plasma-enhanced CVD (PECVD), offer better scalability than PVD for large-scale production. These techniques are extensively used in the semiconductor, aerospace, and automotive industries for their ability to produce high-purity, conformal coatings over large or intricate geometries. However, CVD systems involve high-temperature reactions, complex precursor handling, and often high-vacuum environments, all of which increase system complexity. While more scalable than PVD, CVD systems can be cost-prohibitive, especially for HV-CVD, due to expensive precursor chemicals and energy consumption. Nevertheless, for applications demanding exceptional coating quality and uniformity, these costs are often justifiable [169].

Thermal spray techniques such as air plasma spraying (APS) and solution precursor plasma spraying (SPPS) are highly scalable and ideal for high-throughput applications. Widely used in turbines, engine components, and wear-resistant surfaces, these methods accommodate various substrates and geometries. Initial capital costs for thermal spray equipment can be high, but the lower energy requirements, ambient operating conditions, and use of inexpensive precursor materials (e.g., powders or solutions) result in lower operational costs. SPPS, in particular, allows the creation of nanostructured coatings with high performance at moderate cost, making thermal spraying a cost-effective solution for large-scale applications in industries like automotive, power generation, and infrastructure [170]. Figure 8 presents an example of scaling up thin-film batch-coating design to roll-to-roll processing for high-volume manufacturing.

8.2. Long-Term Durability and Maintenance

In addition to cost and scalability, long-term durability and ease of maintenance are essential to ensure the extended performance of coated components, especially in harsh environments. This section addresses key considerations related to high-temperature wear, thermal spray coatings, cavitation resistance, and modern maintenance strategies.

High-Temperature Wear Resistance: CrN and Cr (NO) coatings are often applied to materials such as M50 tool steel and 304 stainless steel to withstand extreme temperatures and mechanical loads. These coatings offer excellent oxidation resistance, low friction coefficients, and stable mechanical properties at elevated temperatures. Their resistance to wear transitions, from mild to severe wear, makes them suitable for tools, bearings, and high-speed moving parts [172].

Thermal Spray Coatings for Wear Resistance: Cermet- and ceramic-based coatings applied via thermal spray techniques are frequently used for components subjected to abrasive and erosive wear (e.g., turbine blades, valves, boiler tubes). Coatings that incorporate WC, Cr₃C₂, Al₂O₃, or ZrO₂ exhibit excellent thermal stability and resistance to mechanical degradation. Post-deposition treatments, such as laser sealing or densification, enhance durability by reducing porosity and minimizing crack initiation [173].

Cavitation and Erosion Resistance: Fe-based amorphous and nanocrystalline coatings produced by arc spraying offer strong protection against cavitation erosion in hydraulic systems, pumps, and impellers. Their high hardness and elasticity help absorb cavitation shock waves, while fine nanostructures increase resistance to surface fatigue without causing brittleness. Optimizing surface preparation (e.g., roughness control) further enhances performance [71].

Maintenance Strategies: Several approaches improve the maintainability of wear-resistant coatings. (1) Recoatable systems: Coatings such as HVOF or arc-sprayed layers can be stripped and reapplied in the field, reducing downtime [174]. (2) Sacrificial wear layers: Outer layers in multilayer coatings are designed to wear out first, preserving the core functional layers and extending overall service life [175]. (3) Adaptive wear strategies: Emerging smart coatings are being developed to harden under stress or release lubricants upon wear initiation. Though still in research phases, these self-regulating systems could revolutionize long-term maintenance.

8.3. Environmental and Regulatory Considerations

Environmental sustainability and regulatory compliance are increasingly shaping the future of coating technologies. Factors such as humidity, air pollution (SOx, NOx), saltwater exposure, and temperature fluctuations accelerate corrosion, further exacerbated by climate change. Global corrosion-related losses are estimated to exceed USD 1.8 trillion annually, but effective mitigation strategies could reduce these losses by 25%–30% [176].

The regulatory landscape includes the following elements. (a) REACH Legislation and Cr (VI) Phase-Out: The European Union’s REACH directive promotes the elimination of hexavalent chromium (Cr (VI)), a potent but hazardous corrosion inhibitor. Alternatives such as Cr (III) coatings, Zr-based treatments, tartaric/sulfuric anodizing, and silane-based primers are under development. While environmentally safer, these substitutes require further testing for long-term durability [177,178]. (b) Green Chemistry in Oil and Gas: There is growing emphasis on biodegradable, non-toxic inhibitors, polymer-based liners, and fiber-reinforced composites as replacements for traditional steel systems that rely on hazardous chemicals [179]. (c) Material Recycling: Increased use of post-consumer aluminum scrap introduces trace elements that may compromise corrosion resistance, necessitating new alloy designs and compatible coating solutions [180]. (d) International Standards: Organizations such as ISO TC 265 support the development of standards for CO₂ transport and corrosion control, defining safe impurity thresholds and acceptable materials to mitigate corrosion in carbon capture pipelines [181].

Figure 9 presents key components contributing to environmentally friendly anticorrosive polymeric coatings. The figure highlights the integration of advanced strategies, including (a) smart additives for responsive behavior under environmental stimuli, (b) hyperbranched and hybrid polymers to improve adhesion and durability, (c) bio-based corrosion inhibitors to reduce reliance on toxic chemicals, (d) superhydrophobic surfaces to prevent moisture ingress, and (d) renewable raw materials that align with circular economy principles. Together, these innovations underscore the shift toward sustainable, high-performance coating systems that meet both environmental and regulatory demands.

9. Future Directions

9.1. Development of Sustainable Coating Materials

As global demand rises for environmentally responsible and high-performance protective coatings, research is increasingly focused on developing materials that are both effective and sustainable. Industries such as paper manufacturing are exploring advanced coating technologies to enhance physical properties and introduce new functionalities. One such innovation is the use of core–shell pigments, which improve dispersion and mechanical performance. These pigments typically consist of a titanium dioxide shell deposited on talc particles, offering superior optical, physical, and antibacterial properties compared to conventional TiO₂ and talc-based coatings [183].

In parallel, under current trajectories, global reliance on synthetic polymers is projected to nearly quadruple by 2100, starting from a baseline of ~460 Mt in 2019, with a tripling to ~1.2 billion tons by 2060 due to population and economic growth [184]. This has accelerated the pursuit of biopolymer-based coatings as a sustainable alternative. Derived from renewable biomass sources, biopolymers such as cellulose, chitosan, and polylactic acid offer multiple benefits: reduced dependence on petroleum, inherent biodegradability, and lower toxicity. These materials not only reduce environmental impact but also align with circular economy principles, making them attractive for future coating systems. Among these, chitosan, which is a naturally occurring polysaccharide derived from chitin, offers excellent film-forming ability, antimicrobial properties, and good barrier performance against gases [185]. Similarly, cellulose-based coatings (including nanocellulose and cellulose acetate) provide renewable, biodegradable, and mechanically robust solutions [186]. However, several challenges limit their broader use in industrial settings. Most of the bio-based polymers suffer from water sensitivity, which leads to swelling or degradation in humid environments [187]. In addition, their adhesion to hydrophobic or metallic surfaces remains poor, necessitating surface treatments or chemical modifications to improve bonding and durability [188]. In a circular economy approach, bio-based coatings are made from natural or waste materials like plant fibers or shells from seafood, which would otherwise be discarded. These coatings are designed to break down naturally or be reused. This helps reduce waste and brings down our reliance on fossil-based resources.

Figure 10 shows a conceptual map of renewable sources for biopolymer-based coating materials.

9.2. Exploration of Coatings for Emerging Applications

As new technologies evolve, especially in extreme and specialized environments, the need for advanced coatings becomes increasingly critical. For instance, space-based applications demand coatings capable of withstanding conditions such as high vacuum, extreme temperature fluctuations, and intense radiation (e.g., gamma rays, protons) [71]. The development of durable coatings for space laser optics is essential for ensuring performance stability and protecting sensitive components during long-term missions. Beyond sensitive electronics, the structural and optical components of spacecraft also benefit from protective coatings. These coatings enhance resistance to impacts from micrometeoroids and anthropogenic space debris, which can compromise critical surfaces like windows and solar panels [189]. Emerging areas like hydrogen storage and transport also require advanced barrier coatings to prevent hydrogen permeation and embrittlement. Ceramic and multilayer metallic coatings are being explored for their gas impermeability, thermal stability, and corrosion resistance under demanding conditions [190]. Protective coatings in these scenarios extend operational life, improve reliability, and reduce mission risk. AM enables new approaches to coating design and application. Techniques such as CS and directed energy deposition allow for the localized deposition of functional or repair coatings with reduced material waste. AM also provides gradient and multifunctional coatings for flexible electronics, biomedical devices, and smart infrastructure [191]. Continued innovation in coating materials and deposition methods is expected to unlock broader functionality in emerging sectors, including flexible electronics, biomedical implants, hydrogen storage systems, and smart infrastructure.

10. Summary

Corrosion and mechanical wear remain persistent challenges across industrial, structural, and aerospace sectors. Protective coatings serve as a first line of defense, enhancing the durability, functionality, and service life of critical components. This review highlights the diverse types of protective coatings, each tailored to meet specific operational demands from ambient conditions to extreme environments like space or high-temperature reactors. Looking ahead, the global push for sustainability and resource efficiency will drive innovation toward eco-friendly coatings that deliver advanced performance while minimizing environmental footprint. Biopolymers, smart additives, core–shell structures, and green processing technologies are just the beginning of this transition. As human activity expands into harsher environments and demands for material performance grow, protective coatings will play an integral role in enabling future technologies. Whether safeguarding everyday consumer products or enabling complex aerospace systems, the continued advancement of coating science is poised to significantly affect performance, sustainability, and quality of life. Despite current challenges such as scalability, cost, and regulatory constraints, emerging research directions and industry leadership signal a promising future. The next generation of coatings will likely be smarter, more sustainable, and more capable, evolving in lockstep with the complex demands of modern engineering and global sustainability goals.