Surface Coating Technologies: A Concise Review

1. Introduction

Surface engineering has become an essential strategy for tailoring the interfacial properties of materials to meet the increasingly stringent requirements of advanced engineering applications. Since many forms of material degradation initiate at the surface, coatings provide an effective means of improving surface-dependent properties such as wear resistance, corrosion protection, oxidation resistance, thermal stability, electrical performance, and biocompatibility. As a result, coating technologies are widely used to extend component lifetime and enhance functionality in sectors including aerospace, energy, electronics, automotive engineering, and biomedicine [1].

The development of modern deposition technologies has enabled increasingly precise control over coating thickness, composition, morphology, adhesion, and microstructure. Among the available techniques, physical vapour deposition (PVD), chemical vapour deposition (CVD), atomic layer deposition (ALD), laser cladding, and electrodeposition have attracted significant attention because of their versatility and broad industrial relevance [1,2,3]. Figure 1 presents the timeline of the development of various coating technologies. While PVD and CVD are widely used to produce dense, adherent, and functional thin films, ALD offers exceptional conformality and atomic-scale thickness control for high-aspect-ratio structures, making it particularly valuable for nanostructured coatings, microelectronics, energy devices, and biomedical surfaces. Laser cladding, in contrast, enables the formation of thick, metallurgically bonded coatings for surface repair and wear- or corrosion-resistant applications. Electrodeposition remains one of the most practical and scalable methods for depositing metallic, alloy, composite, and functional coatings on complex geometries.

In parallel with advances in processing methods, the range of coating materials has expanded considerably. Carbon-based coatings, including graphene-based films and diamond-like carbon, nanostructured ceramics, metallic alloys, composite coatings, and bioactive polymers have opened new possibilities for multifunctional surface design. These materials allow coatings to move beyond conventional passive protection and provide additional functions such as antimicrobial activity, self-cleaning behavior, controlled drug release, thermal insulation, electrical conductivity, and catalytic activity. Consequently, modern coating systems are increasingly designed not only to protect substrates but also to actively enhance their performance in specific service environments.

A major direction in contemporary surface engineering is the development of multifunctional, hybrid, and smart coatings. Such systems can combine several properties within a single coating architecture, for example, simultaneous wear and corrosion resistance, or mechanical durability combined with biological activity. Smart coatings that respond to external stimuli such as temperature, pH, light, or mechanical damage are also gaining importance. Self-healing coatings, for instance, can repair micro-cracks and extend the operational life of structural components, while drug-eluting and bioactive coatings have significantly improved the performance of biomedical implants. These developments are particularly relevant for addressing challenges related to energy efficiency, healthcare, infrastructure durability, and sustainable manufacturing.

Despite these advances, several limitations continue to restrict the wider implementation of advanced coating technologies. Many deposition processes involve high equipment costs, complex processing conditions, limited scalability, high energy consumption, or the use of hazardous precursors and electrolytes. In addition, the relationship between processing parameters, coating microstructure, and final functional performance is not always systematically established, making it difficult to optimize coatings for specific applications. Environmental considerations, including material utilization, emissions, waste generation, and life cycle impact, are also becoming increasingly important in the selection and development of coating processes.

This review provides a concise overview of major coating technologies, their classification, and their applications, with particular emphasis on CVD, PVD, ALD, laser cladding, and electrodeposition. The discussion focuses on a short description and classification of each technique and their advantages and limitations. It also emphasizes the relevance of these technologies across major application domains, including aerospace components, biomedical implants, energy systems, electronic materials, corrosion-resistant structures, and wear-resistant engineering parts. By bringing these major coating approaches together in a single discussion, this review seeks to explain their individual contributions to surface engineering while identifying how future improvements in process control, material design, and scalability can support more durable, economical, and environmentally responsible coating systems.

2. Deposition Techniques

Modern deposition techniques are the cornerstone of advanced coating technologies, enabling precise control over material properties, thickness, and functionality. These methods, including Physical Vapour Deposition (PVD), laser cladding, Chemical Vapour Deposition (CVD), Atomic Layer Deposition (ALD), and electrodeposition, enable precise control over coating properties and therefore cater to diverse industrial demands, from aerospace to flexible electronics. ALD excels in creating ultra-thin films for electronics, while laser cladding produces wear-resistant coatings for aerospace components. Electrodeposition has evolved to fabricate nanostructured coatings with enhanced corrosion resistance and catalytic activity, crucial for energy applications. Hybrid techniques combining electrochemical synthesis with other methods are emerging, offering multifunctional layers for sensors and smart coatings.

The following section discusses the different deposition techniques, their applications, along with recent advancements, challenges and outlook.

2.1. Physical Vapour Deposition (PVD)

Physical vapour deposition (PVD) comprises a group of vacuum-based coating techniques in which material is physically removed from a solid source and transported in the vapour phase to a substrate, where it condenses to form a thin film [4]. Unlike chemical vapour deposition, PVD relies on physical processes such as evaporation or momentum transfer rather than chemical reactions, enabling relatively lower processing temperatures and high-purity coatings.

PVD techniques are widely used for depositing metals, alloys, and compound coatings such as nitrides, carbides, and oxides. These coatings typically exhibit high hardness, good adhesion, and controlled microstructure, making PVD particularly suitable for applications in tribology, optics, microelectronics, and decorative coatings.

2.1.1. Classification of PVD Techniques

PVD processes can be broadly categorized based on the mechanism used to generate vapour species.

(a) Evaporation-based techniques

In evaporation processes, the coating material is heated to its vaporisation temperature in a vacuum environment and subsequently condenses on the substrate (Figure 2a). Evaporation techniques are commonly used in optical coatings and microelectronics but are inherently limited by their line-of-sight nature, which restricts coating uniformity on complex geometries.

Thermal Evaporation: Material is heated using resistive heating or crucibles until evaporation occurs. This method is relatively simple and provides high deposition rates, but it offers limited control over film density and adhesion.

Electron Beam Evaporation: A focused electron beam is used to heat and evaporate the target material. This allows the deposition of high-melting-point materials and provides improved control over film purity and deposition rate compared to thermal evaporation.

(b) Sputtering-based techniques

Sputtering involves the ejection of atoms from a target material due to bombardment by energetic ions (typically from plasma). These ejected atoms then deposit onto the substrate (Figure 2b). Sputtering techniques are favored for their excellent film uniformity, strong adhesion, and ability to deposit a wide range of materials, including hard coatings such as TiN, CrN, and AlTiN.

Direct Current (DC) Sputtering: Suitable for conductive targets, offering stable deposition and good film uniformity.

Radio Frequency (RF) Sputtering: Enables deposition of insulating materials by alternating the electric field, preventing charge buildup on the target.

Magnetron Sputtering: Utilizes magnetic fields to confine electrons near the target surface, increasing plasma density and improving deposition efficiency. This is one of the most widely used industrial PVD techniques.

High-Power Impulse Magnetron Sputtering (HiPIMS): An advanced variant that employs high-power pulses to generate a highly ionized plasma. HiPIMS produces dense, defect-free coatings with superior adhesion and mechanical properties, making it particularly suitable for demanding applications such as aerospace and high-performance tooling.

(c) Arc-based deposition

Cathodic Arc Deposition: In this technique, a high-current, low-voltage arc is used to vaporize material from the cathode target. The resulting plasma contains highly ionized species, which contribute to dense coatings with excellent adhesion. However, the presence of macroparticles (droplets) can negatively affect surface quality and may require additional filtering techniques.

2.1.2. Applications of PVD Coatings

PVD coatings are extensively used across a range of industrial applications. Here, we will focus on a few specific sectors.

Cutting tools and tribological components

Single layer and multilayer PVD coatings are a promising alternative to CVD coatings, and are widely used to make cutting tools like drills and inserts harder and longer-lasting. They are also vital for molds and dies, where they provide better wear and corrosion resistance while helping parts release more easily. Though arc evaporation-PVD has significantly higher deposition rate, magnetron sputtering is favored for wear-resistant coatings in cutting tools and automotive components due to its excellent adhesion, uniformity, surface smoothness and precision [5]. Moreover, it allows coating of steel tools without exposing the substrate to excessive heat, which maintains the tool’s structural integrity. Sputtered coatings, particularly when optimized with techniques like ion-beam assistance or bias, produce dense microstructures that improve wear and corrosion resistance. However, the induction of residual stress is inevitable in PVD process [6].

Transition-metal nitride (such as Titanium Nitride (TiN), Titanium carbonitride (TiCN), Titanium carbide (TiC), Titanium Aluminum Nitride (TiAlN), Aluminum Titanium Nitride (AlTiN), Chromium Nitride (CrN)), alumina (Al₂O₃) coatings, and combination of these materials are widely used in engineering applications due to their superior physical and chemical properties. Hard coatings of these materials significantly enhance wear and corrosion resistance, reduce friction, and improve tool lifespan in machining and forming operations [7,8,9,10,11,12,13,14]. Diamond-like carbon coatings are also hard but suffer lower friction than the hard nitride coatings [5]. Among the several PVD techniques available for deposition of hard coatings, magnetron sputtering and cathodic arc vapour deposition methods are widely used to deposit wear resistant Ti-Al based coatings [15,16,17,18,19,20,21]. Shakib et al. [12] assessed the tribological behavior of TiN/TiCN multi-layer hard coatings on steel substrates. The results showed that integrating Ti interlayer with subsequent TiN/TiCN layers yielded hard Ti-based coating with enhanced mechanical and tribological behavior. In another study, Panjan et al. [22] performed a comparative study of microstructural, mechanical and tribological behavior of TiN hard coatings deposited by different PVD deposition techniques, including low-voltage electron beam evapouration, magnetron sputtering and cathodic arc deposition. A comparison of SEM images is shown in Figure 3 illustrate that the TiN coatings deposited using different PVD techniques, differ not only in the details of columnar microstructure but also in their coating porosity, degree of preferred orientation, and surface roughness [22]. Figure 4 shows the comparison of the wear rate measurements for BAI (low-voltage electron beam evaporation system), CC7 (magnetron sputter deposition system) and AIP (cathodic arc deposition system) TiN coatings performed in different atmospheres (nitrogen, oxygen and ambient air). It was concluded that the wear coefficient increased with the surface roughness of the coating and the most for the AIP coating.

Meysami et al. [23] enhanced the wear behavior and hardness of cold work D5 tool steel through TiCrN multilayer nanocoating using cathodic arc-PVD. In their work, Guimaraes et al. [24] compared the CrN coatings prepared using HiPIMS and DC magnetron sputtering (DCMS). The morphologies of the CrN coating deposited using the two methods show significant differences when influenced by the bias voltage. These disparities arise from the higher ionization rate inherent to HiPIMS and the applied bias voltage. Hardness results showed higher values for HiPIMS coatings. In another similar work [25], a preferential growth of as-deposited CrN film on (200) facet was realized by HiPIMS when compared with DCMS on (111) facet. Consequently, the HiPIMS-deposited coatings had a very compact microstructure with high hardness and corrosion current with an order of magnitude smaller than that of CrN films deposited by DCMS. The friction coefficient and wear resistance studies showed that CrN coatings deposited by HiPIMS exhibited lower friction coefficients compared to DCMS (Figure 5).

Hybrid CVD–PVD multilayer coating strategy has been reported to significantly enhance cutting tool performance and longevity in demanding machining applications. Machining tests on advanced ceramic multilayer coatings on AISI 316 stainless steel by Codau et al. [26] revealed that the hybrid CVD and HiPIMS-PVD-coated tools exhibited significantly better tool life compared to the CVD-coated tools.

In automotive sector, coatings are extensively used for engine components (piston rings, valves), bearings, and fuel cells to reduce friction and improve durability [27,28,29,30]. Among the several PVD techniques, magnetron sputtering is most favored for coating the piston ring of internal combustion engine [28,31]. Tribological test have confirmed that carbon-, nitride and carbide-based coatings (such as CrN, MoN, CrAlN, TiSiN, TiC and DLC) effectively reduces the wear rate compared with uncoated piston rings [29,31,32,33,34,35,36,37].

Aerospace and high-temperature coatings

Thermal barrier coatings are widely utilized protective coatings in aerospace industry to improve to improve oxidation resistance, thermal stability, functional performance and life span of components such as gas turbine engines, turbine blades, aircraft landing gear and engine blocks [38,39,40,41]. Several PVD based thermal barrier coating methods have been studied. HIPIMS enhance coating density and reduce defects, critical for aerospace turbine blades exposed to extreme temperatures, while EB-PVD coating method has also been used in aircraft engine applications of gas turbines [38,42,43]. For instance, Shen et al. [38] studied LaNdZrO and YSZ ceramic coatings deposited by EB-PVD, and NiCoCrAlYHf metallic coating using AIP-PVD. Protective Cr/CrN coatings deposited using magnetron sputtering-PVD have been utilized for space application [44]. Still further, hybrid CVD-PVD has also been used for barrier coatings of aircraft engines [39].

Microelectronics and thin-film devices

PVD is employed for the deposition of metal interconnects, barrier layers, and functional thin films in semiconductor devices [45,46,47,48,49]. TiN, due to its relatively high thermal, structural stability and low electrical resistivity, is employed in microelectronic and photovoltaic devices [45,49]. High carrier mobility of 800 cm³/Vs and a resistance of 5e-4 Ohm-cm has been reported in TiN films [49]. For instance, Mühlbacher et al. [45] sputter deposited single-crystalline TiN/Cu bilayers on MgO (001) substrates and demonstrated excellent diffusion barrier properties of single-crystalline TiN.

Decorative and optical coatings

PVD enables the fabrication of reflective, anti-reflective, and aesthetically appealing coatings with precise thickness control [50,51,52,53]. Hard transition-metal nitrides exhibit different colours such as gold-yellow, bronze, brown, violet, dark-blue and almost black by small variations of their stoichiometric composition. In particular, the characteristic golden colour, abrasion resistance, and chemical inertness of TiN coatings is attractive for decorative purposes, including watch cases, sanitary hardware, household appliances, door handles and jewellery items and architectural glass. PVD is also used to develop optical coatings on display screens enabling anti-reflective or reflective properties.

Biomedical coatings

Similar to CVD, the PVD coatings are excellent for wear protection of orthopaedic implants (hips, knees and other prosthetic joints) and dental implants (screws, abutments) [54,55]. Besides implants, PVD coatings are used to provide corrosion protection, antimicrobial properties, sharpness and cutting edge retention of the surgical tools (e.g., scalpels, blades, drills, reamers, orthopaedic bone saw blades).

Owing to its intrinsic biocompatibility, antibacterial properties, chemical inertness and good tribological properties, coatings of Ti and its alloys such as TiN is extensively used for wear protection of medical implants [56]. Moreover, the haemocompatibility of TiN encouraged TiN-coatings in cardiology for ventricular assist devices and for pacemaker leads [54,57,58].

2.1.3. Advantages, Challenges and Outlook

PVD offers several advantages, including relatively low deposition temperatures compared to CVD, high coating purity due to vacuum processing, and good control over film thickness and composition. Recent developments in PVD technologies have focused on enhancing plasma characteristics and ionization efficiency to improve coating density, adhesion, and microstructural control. High-power impulse magnetron sputtering has emerged as a significant advancement, generating a highly ionized metal flux that promotes dense film growth, reduced defect density, and improved interfacial bonding compared to conventional sputtering techniques. In addition, strategies such as ion-assisted deposition, substrate biasing, and reactive sputtering have enabled improved control over residual stress, phase composition, and functional properties of coatings.

However, PVD techniques are generally limited by their line-of-sight deposition nature, which restricts their ability to uniformly coat substrates with complex geometries. Also, achieving strong consistent adhesion on certain substrates often requires additional surface treatments or interlayers. Furthermore, issues related to scalability, process cost, and energy consumption can restrict large-scale industrial implementation. To overcome the intrinsic line-of-sight limitation of conventional PVD processes, hybrid approaches combining PVD with techniques such as atomic layer deposition have been developed. These systems enable the formation of conformal multilayer and nanolaminate coatings, integrating the high density of PVD films with the excellent conformality of ALD.

Looking forward, future developments in PVD are expected to focus on improving coating conformality, enhancing process scalability, and reducing energy consumption. The integration of advanced plasma diagnostics, real-time monitoring, and data-driven process optimization is likely to enable greater control over the coating quality and reproducibility. In addition, continued development of hybrid and multifunctional coating systems is expected to expand the applicability of PVD in emerging fields such as flexible electronics, energy systems, and environmentally sustainable surface engineering.

2.2. Laser Cladding

Laser cladding is an advanced surface modification and additive manufacturing technique in which a high-energy laser beam is used to melt and deposit a coating material onto a substrate, forming a metallurgically bonded layer (Figure 6). The laser cladding process involves complex interactions between laser energy, material feedstock, and substrate. Key parameters include laser power, scanning speed, powder feed rate, and shielding environment, all of which influence melt pool dynamics, microstructure evolution, and final coating properties. The feedstock material, typically in the form of powder or wire, is delivered into the laser-induced melt pool and rapidly solidifies to produce a dense coating with refined microstructure.

As a directed energy deposition process, laser cladding enables precise control over heat input, dilution, and coating geometry. The localized nature of the process minimizes the heat-affected zone (HAZ), thereby reducing thermal distortion and preserving substrate properties. Rapid solidification during processing promotes fine microstructures, which are often associated with improved mechanical and tribological performance.

Laser cladding using titanium and high-entropy alloys (HEAs) offers significant surface modification advantages, primarily by creating high-performance, metallurgically bonded coatings on materials like steel, aluminum, and titanium alloys [59,60,61,62]. Particularly, HEAs, composed of five or more metallic elements are promising for oil drilling, aerospace, and marine engineering applications owing to their high strength, exceptional hardness, corrosion resistance, and high-temperature stability [63,64,65,66,67,68,69,70]. Advances in real-time monitoring and multi-material cladding (e.g., metal-ceramic composites) enhance process reliability and enable functionally graded coatings by dynamically adjusting feedstock composition during deposition.

2.2.1. Variants of Laser Cladding

Laser cladding processes can be categorized based on feedstock delivery, processing speed, and application configuration.

(a) Powder-Fed Laser Cladding: In this widely used configuration, powder is delivered through a nozzle into the laser beam. It offers high flexibility in material selection and is suitable for both coating applications and additive manufacturing.

(b) Wire-Fed Laser Cladding: This method employs a continuous metallic wire as feedstock. Compared to powder-fed systems, it provides higher material utilization efficiency and reduced material waste, making it attractive for large-scale deposition.

(c) Extreme High-Speed Laser Application (EHLA): EHLA is an advanced variant characterized by very high processing speeds and the ability to produce thin coatings (typically < 0.1 mm). The process significantly reduces heat input, resulting in minimal dilution and a very narrow heat-affected zone.

(d) Internal Bore Laser Cladding: This specialized configuration is designed for coating the internal surfaces of cylindrical components such as pipes and tubes, enabling in situ repair and protection of otherwise inaccessible regions.

(e) 3D Freeform / Direct Energy Deposition (DED): This approach utilizes multi-axis systems to enable additive manufacturing of complex three-dimensional geometries. It extends laser cladding beyond surface coating to full component fabrication.

(f) Conventional (low-speed) Laser Cladding: The traditional form of laser cladding operates at relatively lower speeds and higher heat input. It is commonly used for industrial repair, hardfacing, and deposition of thick coatings.

2.2.2. Applications

Laser cladding is an economical and effective process for a wide range of coating applications. It is extensively applied in industries requiring high-performance and repairable coatings.

Aerospace industry

Laser cladding is used for the repair and protection of turbine blades and engine components, where wear- and oxidation-resistant alloys such as nickel- and cobalt-based high-entropy alloy systems provide improved durability under high temperature and oxidative environments [71,72,73,74,75,76,77,78,79,80]. Wang et al. [78] fabricated a new refractory high-entropy alloy Ti₃₅Ni₃₄Cu₁₀Zr₁₀Nb₈Ta₃ coating on copper substrate, significantly improving hardness and wear resistance (Figure 9). Morais et al. [75] exhibited that laser surface treatment enhanced metallurgical bonding between the coating and the substrate, preventing delamination and peel-off in Al-cladded AA2024-T3, an aluminum alloy widely used in the aerospace industry.

Recently, Liu et al. [81] prepared AlCoFeCuMox high-entropy alloy coatings with excellent mechanical and corrosion properties, rendering them suitable for aerospace applications, including unmanned aerial systems. In another work, Guan et al. [82] investigated the mechanical property of 30CrMnSiNi2A steel repaired by laser cladding with AerMet100 Steel, providing an effective solution for critical aerospace components such as landing gear. Zhao et al. [83] improved wear and corrosion resistance in laser cladding fabricated TC11-xMo coatings. The worn surface of the TC11–10Mo coating showed highest degree of smoothness and minimum wear depth, wherein the coating was rubbed against GCr15 steel balls measuring 6 mm in diameter.

Energy sector

In the energy sector, laser cladding is employed to protect critical components and boost the efficiency and durability against corrosion, erosion, and high-temperature degradation of the power generation equipment such as parts of oil and gas systems, including turbines, boilers, heat exchangers, deep-earth drilling tools, pipelines, and valves. These coatings are essential for maintaining structural integrity and operational efficiency in aggressive service conditions. Compared with conventional alloys, high entropy alloys, have garnered much attention for surface protection of engineering components operating under complex mechanical and corrosive environments as in deep earth engineering applications such as ultra deep drilling and geothermal exploitation [70,84,85,86,87,88]. Wu et al. [70] studied the tribological behaviour of 1 wt% hBN-reinforced CoCrFeNiMo_0.5Ti_x (x = 0, 0.2, 0.4, 0.6) high-entropy alloy composite coatings on 42CrMo material used for oil drill pipe joints. Results indicated that the CoCrFeNiMo_0.5Ti_0.4-hBN exhibited the best resistance to dry wear and tribocorrosion. Wang et al. [85] reported on the Ni-WC laser-clad coating in mineralizing solution. Cui et al. [87] designed laser cladding of FeCoNiCrMox (x = 0, 0.5, 1.0, 1.5) coatings for strengthening Ti6Al4V components for drilling equipment in deep complex environment. The potentiodynamic polarization studies of Ti6Al4V and FeCoNiCrMo_x coatings in 3.5 % NaCl solution, revealed that the Mo-0.5 and Mo-1.0 coatings significantly enhanced the corrosion resistance.

Automotive industry

Laser cladding in the automotive industry is a high-precision, eco-friendly surface engineering process used to enhance wear and corrosion resistance of components, such as brake discs, engine valves, camshafts and crankshafts. The automotive industry extensively uses laser surface treatments for repair/remanufacturing and aesthetic appeal of automotive parts, allowing restoration of worn components with minimal distortion and reduced material waste, enabling cost-effective repair and remanufacturing strategies. The process is particularly beneficial for automotive components made of aluminium alloys [73,89,90,91,92].

Laser Cladding, a directed energy deposition process is extremely favorable to improve durability of brake discs and reduce brake dust emissions that can cause health issue. With the introduction of the Euro 7 emissions standard which limits brake dust emissions (brake

particle emission < 7 mg · km^-1), new coating solutions such as metal matrix composites (MMC) are gaining importance. Laser cladding provides protective, defect-free, metallurgically bonded layers with high adhesion, creating strong and durable coatings for brake discs that improves brake disc lifetime and performance while helping to meet environmental regulations [90,93,94,95]. Wang et al. [90] utilized rotatable inner-surface laser cladding to generate stainless steel cladding layer to enhance wear resistance of inner surface of car engine cylinders. They explored the process parameters for laser cladding an Fe-based layer onto an AlSi9Mg surface employing low-speed (LSLC) and high-speed laser cladding (HSLC) layers. In comparison to AlSi9Mg substrate, the microhardness was significantly greater in cladded layers, while the wear depth was nearly ten times shallower. Zhang et al. [96] successfully deposited Co-Cr-Ti composite coating on Ti6Al4V (TC4) alloy that are extensively used in various automotive components, such as connecting rods, intake valves, and turbochargers, to achieve higher fuel efficiency and reduced energy consumption targets. The Co-Cr-Ti composite coating on TC4 alloy substrate by laser cladding enhanced microhardness, wear and corrosion resistance. In particular, the coating demonstrated a notable enhancement in friction and wear resistance at both room temperature and 500 °C. Altuncu et al. [94] systematically investigated the efficiency of HSLC for enhancing the wear and corrosion resistance of automotive brake discs, meeting the Euro-7. Three distinct MMC coatings were fabricated on GG25 grey cast iron substrate. Consistent coating quality and high performance under challenging conditions such as wear, corrosion and thermal-cycle loads was reported. Moreover, the coated brake discs complied with the Euro-7 regulations and with ECR90 standards.

Functionally graded coatings

Beyond conventional protective applications, laser cladding enables the fabrication of advanced surface layers such as functionally graded coatings in which the composition and properties change gradually across the coating thickness [97,98,99,100]. Instead of a single uniform layer, the material is tailored step by step, for example, transitioning from a tough, compatible base layer to a hard, wear-resistant top layer. This gradual variation in composition and microstructure improves stress distribution and thermal compatibility, creating a strong metallurgical bond between the coating and the substrate. A gradual composition shift creates a steady gradient in the coefficient of thermal expansion (CTE), preventing sharp stress concentrations at the interface. By spreading thermal and mechanical stresses over a transitional layer, FGCs minimize the destructive residual stresses and eliminate sharp interfaces, preventing delamination and warping, particularly in dissimilar material systems like ceramics on metals. Particularly, key advantages of functionally graded coatings include low dilution and minimal heat-affected zone, high density and purity, and the ability to smoothly transition from hard surface layers to more ductile substrates.

These coatings are widely used for wear and corrosion protection (e.g., Ni-WC or TiC on steel or magnesium alloys), high-temperature applications such as thermal barrier coatings, and repair or additive manufacturing of high-value components. Common material systems include Ni-WC/Al-Ni, AlCuTiVCr/Cu-Al high-entropy alloys, and TiC/Ni composites for high-strength applications [100,101,102,103,104,105,106,107,108]. For instance, Wan et al. [101] reported on a novel laser-cladded AlCuTiVCr/Cu-Al lightweight, high-melting-point HEA graded coating on the surface of low-melting-point Mg-Li substrate. The design of gradient coating comprised of AlCuTiVCr protective layer, Cu-Al transition layer and Mg-Li alloy substrate, that formed excellent metallurgical bonding, and the protective layer achieved superior corrosion resistance. Müller et al. [109] applied functionally graded coating by laser cladding to improve the lifetime of die-casting dies. They studied the hardness distribution of the gradient structure before and after heat treatment (for two hours at 580 °C) [109]. The heat treatment did not affect the hardness of the base material and the three top layers, which is essential to maintain its mechanical properties. In another work, Wang et al. [107] formed H13/Ni60 functional gradient coating on the surface of H13 steel by using the laser cladding method, improving significantly the hardness and wear resistance of the substrate.

2.2.3. Advantages, Challenges and Outlook

Laser cladding offers significant advantages in producing high-performance coatings with strong metallurgical bonding and minimal dilution, ensuring that the deposited layer retains its designed composition and properties. The localized heat input reduces thermal damage to the substrate and enables precise control over coating geometry, making it particularly suitable for repair and remanufacturing of high-value components. Additionally, the ability to process a wide range of materials, including metal alloys and composite systems, and to fabricate functionally graded structures enhances its versatility. Recent developments in laser cladding have focused on improving process reliability, material performance, and functional versatility. The integration of real-time monitoring and closed-loop control systems has significantly enhanced process stability and reproducibility. In addition, multi-material and composite cladding approaches have enabled the fabrication of metal–ceramic coatings with superior wear resistance and thermal stability. The convergence of laser cladding with additive manufacturing technologies, particularly directed energy deposition, is further expanding its role in the fabrication and repair of complex, high-value components.

However, the process also presents several limitations. Rapid thermal cycles can produce steep temperature gradients, causing residual stresses, distortion, and in some cases cracking. Maintaining tight control of the process window to achieve the desired microstructure while minimizing defects (e.g., porosity, cracking, incomplete fusion/overlap) remains challenging and typically requires specialized expertise. Surface finish can be relatively rough and may necessitate post-processing. In addition, high capital and operating costs for laser systems (and, where applicable, consumables such as feedstock) can hinder adoption in cost-sensitive sectors. Process parameter complexity, scalability to consistent high-throughput production, integration into existing lines, and material/absorptivity constraints are additional barriers. Finally, applicability can be limited by material-laser interactions such as low absorptivity or narrow processing windows for certain substrates.

Looking forward, research in laser cladding is expected to focus on physics-based process modeling (including thermal–fluid flow and dilution), in situ diagnostics, and closed-loop control to improve reproducibility and part-to-part quality. Data-driven optimization, including machine learning for parameter selection, defect detection, and predictive maintenance, is likely to accelerate process qualification. On the hardware side, wider adoption of efficient fiber/diode laser sources, improved beam delivery, and higher-throughput systems, combined with automation and robotics, should enhance scalability for industrial production. Continued development of cost-effective laser platforms and optimized feedstock powders/wires will be essential for broader deployment. In parallel, functionally graded and multi-material coatings will expand performance tailoring for aerospace, energy, and biomedical components, and hybrid additive–subtractive repair and remanufacturing workflows are expected to grow. Futuristic applications are emerging, with powder-based laser cladding being investigated for in-space metal repair and on-demand manufacturing in microgravity; however, robust powder handling and process stability remain key challenges [110,111].

2.3. Chemical Vapour Deposition (CVD)

Chemical vapour deposition (CVD) is a widely employed thin-film deposition technique in which volatile precursor species undergo chemical reactions at or near a heated substrate surface to form a solid coating. The process typically involves the transport of gaseous precursors into a reaction chamber, followed by their decomposition and/or reaction under controlled temperature and pressure conditions. The resulting solid material is deposited on the substrate, while gaseous by-products are continuously removed [112,113,114]. Figure 7 shows the schematic of the main steps involved during the CVD process [115].

CVD can produce dense, uniform, and well-adhered coatings with controlled composition and microstructure. Due to its non-line-of-sight nature, it is particularly suitable for coating substrates with complex geometries. The technique supports a broad range of material systems, including metals, oxides, nitrides, carbides, borides, and carbon-based materials such as diamond and diamond-like carbon (DLC). Consequently, CVD is extensively used in applications requiring high-performance coatings, including wear-resistant layers, corrosion protection, semiconductor devices, optical coatings, and thermal barrier systems.

2.3.1. Classification of CVD Techniques

(a) Activation method

Thermal CVD: In thermal CVD, chemical reactions are driven by elevated temperatures, typically in the range of 500–1400 °C. This method often utilizes inorganic precursors such as halides or hydrides and is known for producing coatings with high crystallinity and purity. However, the high processing temperatures may limit its applicability to thermally sensitive substrates [112,116].

Plasma-Enhanced CVD (PECVD): PECVD employs plasma to generate reactive species, thereby reducing the required substrate temperature (typically 200–300 °C). Plasma activation enhances reaction kinetics and enables deposition on temperature-sensitive materials. Additionally, PECVD allows improved control over film properties such as density, stress, and composition. It is widely used for depositing dielectric layers, silicon-based films, and DLC coatings [112,113,117]. For example, Zhang et al. [118] deposited silicon nitride films at 80 ºC under plasma-enhanced conditions.

Photo-Assisted CVD: In this approach, ultraviolet or laser radiation is used to initiate or enhance chemical reactions. This enables lower processing temperatures and selective area deposition, although its industrial application remains comparatively limited [112].

Microwave CVD (MWCVD): In this deposition technique, microwave energy is utilized to generate a plasma, which activates gaseous precursor molecules and enables the growth of high-quality materials on a substrate. Unlike conventional thermal CVD, Microwave CVD relies on plasma rather than high temperatures to drive chemical reactions, allowing deposition at lower substrate temperatures while maintaining excellent film quality.

MWCVD is widely used for the synthesis of diamond films, graphene, silicon-based coatings, and other advanced materials, and is valuable for applications in electronics, optics, energy devices, and protective coatings.

(b) Precursor type

Metal-Organic CVD (MOCVD): MOCVD utilizes metal–organic precursors that can be vapourized at relatively low temperatures. This technique provides excellent control over film composition and uniformity, making it particularly suitable for the deposition of compound semiconductors, oxides, and nitrides [112,113,119]. Compared to conventional CVD, MOCVD enables lower-temperature processing and is widely used in optoelectronic and microelectronic applications.

Other specialized variants include initiated CVD (iCVD) and oxidative CVD (oCVD) [120,121,122,123]. The iCVD is an all-dry method for designing organic and hybrid polymers, enabling uniform, high-purity, and pinhole-free films with thicknesses ranging from >15 µm to <5 nm via free-radical mechanisms (Figure 8) [122,124]. While, oxidative CVD (oCVD) is a liquid-free technique used for depositing electrically conductive conformal polymer coatings directly on the substrate as both monomer and oxidant in the vapour phase are introduced into the reactor simultaneously. For example, Kaviani et al. [121] deposited oCVD PEDOT polymer coatings on carbon cloth, imparting stable all-organic electrodes for oxygen electroreduction. Baek et al. [125] fabricated highly sensitive and fast-response pressure sensors through conformal oCVD polymers.

(c) Operating pressure

Atmospheric Pressure CVD (APCVD): APCVD operates at atmospheric pressure and is characterized by high deposition rates and relatively simple equipment [126]. However, film uniformity and thickness control may be limited compared to low-pressure systems, making this method more suitable for thick deposition.

Low-Pressure CVD (LPCVD): LPCVD is conducted under reduced pressure, which enhances film uniformity and step coverage while minimizing gas-phase reactions [126]. In contrast to the APCVD, the low-pressure CVD allows thin-film deposition and is therefore used for depositing high-quality thin films such as polysilicon and silicon-based dielectrics.

Ultra-High Vacuum CVD: This variant operates under ultra-high vacuum conditions to achieve superior purity and precise control over film growth, particularly in advanced semiconductor applications [126].

(d) Other CVD variants: Additional CVD-based processes include hot-filament CVD, chemical vapour infiltration (CVI), fluidized-bed CVD, and electrochemical vapour deposition (EVD) [112,127]. Chemical vapour infiltration is particularly useful for producing ceramic matrix composites by infiltrating porous structures, while fluidized-bed CVD is employed for coating powders. Although atomic layer deposition (ALD) is often considered a specialized form of CVD due to its reliance on sequential surface reactions, it is treated separately in this review due to its distinct self-limiting growth mechanism.

2.3.2. Applications of CVD Coatings

CVD is extensively applied across multiple industries. Here, only a selected few will be discussed.

Cutting tools and wear-resistant components

Cutting performance and wear resistance can be significantly improved by coatings [26,128,129,130]. Components such as piston rings, cylinder bodies for injection molding, screws for extrusion, and bearings are prone to wear during sliding [131]. Using CVD-coated cutting tools and components offer excellent wear resistance and good adhesion to the substrate. Hard coatings of advanced materials such as titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN) and aluminum oxide (Al₂O₃₎ significantly improve wear resistance, reduce friction, and extend tool life [26,132]. For instance, Zhu et al. [132] studied the mechanical and tribologicial properties of TiN/Ti(C,N)/Al₂O₃ multilayer coatings deposited using thermal CVD system and investigated the effects of operational conditions on tool wear. Tool-life tests (Figure 9) revealed good adhesion between κ-Al₂O₃ and Ti(C,N)-based layers and exhibited the longest tool life, double of that of a commercial CVD Al₂O3 multilayer coating [132].

Figure 9. Tool-life plots for cutting tests of 24CrMoV5-1 steel. Reprinted from ref. [132].

Figure 9. Tool-life plots for cutting tests of 24CrMoV5-1 steel. Reprinted from ref. [132].

In another study, Chen et al. [133] successfully enhanced the wear resistance of the CVD-coated inserts employing a composite process for CVD post-treatment. In their work, TiN/MT-TiCN/TiN/α-Al2O3 composite coating was deposited on a cemented carbide substrate through CVD and assessed to evaluate the influence of different post-treatment processes. The longest tool life was achieved with the inserts that underwent the combined process.

Apart from ceramic coatings, CVD diamond and related hard carbon allotrope films exhibit extremely high hardness, wear resistance and low coefficient of friction, and are therefore widely used in machining industry for various wear and antiwear tools [134,135]. Synthetic diamond coatings deposited via microwave plasma CVD, exhibit unmatched hardness (90 – 100 GPa) and thermal conductivity (2000 W/m·K). They are used in cutting tools for machining high-strength alloys, extending tool life by 10 to 20 times compared to tungsten carbide [136]. Zhang et al. [137] studied the tribology and cutting performance of boron doped diamond composite coated tool using hot filament CVD technique. Cutting experiments show that boron doped microcrystalline diamond-nanocrystalline diamond film showed excellent wear resistance and the best cutting performance [137]. Zhao et al. [138] demonstrated superior performance of CVD diamond tool when compared with coated cemented carbide tool. In another work, Parray et al. [139] showed that, in contrast to ordinary cemented tungsten carbide (WC-Co) cutting tools, the CVD diamond coatings on WC-Co inserts enhanced machining performance of EN24 steel.

Microelectronics

Among the different deposition techniques for fabricating functional coating systems, CVD has been employed industrially in microelectronics and a large number of other sectors [140,141,142] . CVD is fundamental to semiconductor fabrication, enabling deposition of polysilicon, dielectric films, and metal interconnects with high precision and uniformity. CVD is used for achieving thin films of polycrystalline silicon (poly-Si) or doped poly-Si for use as carrier-selective passivating contacts which are one of the key enabling technologies for the tremendous efficiency improvements in crystalline silicon solar cells [142,143,144,145]. Particularly, APCVD technique enables a low-cost, simple manufacturing process to achieve poly-Si passivating contacts [146]. For integration of transistors in semiconductor industry, metal lines and low-dielectric constant films are used as intermetal dielectrics (IMDs) to reduce RC delays and cross-talk in multilevel metallization process. In this regard, silicon carbon-nitride (SiCN) films or porous low-k SiCOH deposited by PECVD are widely used as a metal diffusion barriers [147,148,149,150].

Superconducting materials

CVD is used to produce high quality coatings of superconductor materials such as MgB₂, Nb₃Sn with controlled thickness for superior RF performance levels in Superconducting Radio-Frequency (SRF) cavities [151,152,153,154,155].

Superhydrophobic coatings

Transparent, wear-resistant, superhydrophobic inorganic coating on glass substrates are desirable for applications in high building glass curtain, wind shield glass, and solar cell cover glass. Zheng et al. [156] prepared transparent and wear-resistant superhydrophobic film on glass via two-step phase separation and CVD process. Transparent, superhydrophobic polymer nanocone array coatings with excellent mechanical durability and anti-icing performance were fabricated via a facile one-step iCVD process [157].

Besides, ceramic-based superhydrophobic coatings on metal surface provides a protective layer against corrosion [158,159]. For instance, Cao et al. [158] in their study showed better anti-icing performance of the F-SiO₂-coated surface as compared to the uncoated surface.

Biomedical coatings

Biomedical coatings are revolutionizing healthcare by enhancing the integration, safety, and functionality of medical devices. These coatings address critical challenges such as infection, biocompatibility, and therapeutic delivery, enabling advancements in implants, prosthetics, and drug delivery systems. Chemically inert passivation layer or antimicrobial coatings on implantable devices not only isolates adjacent channels electrically but also protects the implanted device from the implant-associated infections [160,161,162,163,164,165]. CVD-based coating techniques particularly show distinct advantages for antibacterial application since it allows precise control over coating chemistry and thickness regardless of the substrate characteristics [163,164,165,166]. CVD transforms medical implants by not only adding a high-performance layer that boosts biocompatibility and durability, but also by making surfaces harder and more resistant to wear or corrosion, thereby extending their lifespan [164]. Beyond protection, these coatings can even be engineered to assist with controlled drug delivery directly at the implant site.

Amorphous silicon carbide (a-SiC) coating is widely considered for the biocompatible coating of implantable devices [167]. Su et al. [166] fabricated self-regenerating antibacterial coating surfaces via vapour deposition (iCVD) of layered polymer nanocoatings. The trilayered coating consisted of top and bottom antibacterial PDE layers and an intermediate biodegradable PMAH-based layer, as depicted in Figure 10. Rico et al. [163] deposited graphene on a medical-grade cobalt-chromium alloy surface by RF-PECVD that exhibited high biocompatibility, antibacterial and antifouling properties.

Carbon-based and tantalum (Ta) coatings by CVD on metal alloy substrates such as austentic stainless steel, SiC, Co-Cr, and Ti alloys are promising for orthopaedic, vascular stent and dental implants [168,169,170,171,172,173,174].

2.3.3. Advantages Challenges and Outlook

CVD offers several key advantages, including excellent coating conformity, high density, strong adhesion, and broad material compatibility. The ability to tailor microstructure through process parameters makes it highly versatile for advanced applications. Recent developments in CVD have focused on reducing processing temperatures, improving precursor efficiency, and enabling environmentally sustainable processes. Plasma-enhanced techniques have significantly expanded the applicability of CVD to temperature-sensitive substrates by lowering reaction temperatures while maintaining film quality. In parallel, advances in precursor chemistry have enabled the controlled growth of nanostructured and functional materials, including graphene, diamond-like carbon (DLC), and other two-dimensional materials, with tailored microstructures and properties.

However, several challenges remain. High processing temperatures in conventional CVD can restrict substrate selection and induce thermal stresses. Additionally, the use of toxic, corrosive, or hazardous precursor gases and the relatively high energy consumption present environmental and safety concerns, while also increasing operational costs posing economic challenges. Achieving uniform deposition over large areas and complex geometries, particularly in industrial-scale systems, remains another critical challenge.

Looking forward, future developments in CVD are expected to focus on low-temperature and energy-efficient processing routes, including further refinement of plasma-assisted and catalytic CVD techniques. The design of safer, more sustainable precursor chemistry will be essential to reduce environmental impact and improve process safety. In addition, integration with hybrid deposition approaches and advanced process control strategies is anticipated to enhance scalability, enabling broader adoption of CVD in next-generation coating technologies across electronics, energy, and structural applications.

2.4. Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD), one of the most sophisticated coating methods, is a vapour-phase thin-film deposition technique based on sequential, self-limiting surface reactions, enabling atomic-scale control over film thickness and composition. In a typical ALD cycle, precursor gases are introduced alternately into the reaction chamber, where each precursor reacts with the substrate surface in a controlled manner, forming a single atomic layer per cycle, offering unprecedented control over film growth at the atomic scale (Figure 11). Excess precursors and reaction by-products are removed through purging steps, ensuring high film uniformity. Due to its unique growth mechanism, ALD is particularly well suited for depositing conformal coatings on complex and high-aspect-ratio structures, making it indispensable in nanoscale applications.

2.4.1. Classification of ALD Techniques

ALD processes can be categorized based on activation method and reactor configuration.

(a) Thermal ALD: It is the most conventional form of ALD and relies purely on thermal energy rather than plasma to activate precursors. It typically operates within a defined ALD temperature window (150-350 ºC), providing excellent conformality over high-aspect-ratio structures and precise control over film thickness, however, is limited to precursors that are sufficiently reactive.

(b) Plasma-enhanced ALD (PEALD): Also known as plasma-assisted ALD, it is a thin-film deposition technique that uses plasma-generated reactive species (such as radicals) rather than thermal energy to grow high-quality, ultra-thin coatings at low temperatures. It is used in semiconductor manufacturing to deposit materials like titanium nitride (TiN), silicon nitride (SiN) and aluminum oxide (Al₂O₃) on temperature-sensitive substrates, offering improved material density, reduced impurities, and faster process times [176,177,178].

(c) Spatial ALD (SALD): It is an advanced, high-throughput thin-film coating technique where precursor gases are separated physically in space rather than time, allowing for continuous, rapid deposition at atmospheric pressure. Unlike traditional ALD, which is slow due to sequential gas purging, Spatial ALD continuously injects precursors, with inert gas barriers keeping them separate. The process is easily scaled up and compatible with roll-to-roll (R2R) and sheet-to-sheet (S2S) processing, while maintaining the high-quality, pinhole-free, and conformal, atomic-level thickness control of traditional ALD. Spatial ALD is a “game-changer” for industrializing ALD technology, reducing costs and energy consumption while enhancing production efficiency. SALD is used for developing flexible electronics and display technology; coating electrodes, membranes, and separators for lithium-ion batteries and fuel cells (e.g., Al₂O₃, TiO₂, ZnO); applying passivation layers to high-efficiency solar cells and as barrier layers for creating thin films to protect against moisture and oxygen [179].

(d) Area-Selective ALD (AS-ALD): is a bottom-up, thin-film deposition technique that selectively grows material only on designated surface areas rather than the entire substrate. By using inhibitor molecules or inherent surface differences to block deposition elsewhere, it achieves nanoscale thickness control while eliminating complex masking steps, reducing cost and edge placement errors. AS-ALD is crucial in advanced semiconductor manufacturing and nanotechnology, including next-generation nanoelectronics [180] and patterning 2D materials (e.g., selective growth on 2D materials like MoS₂ or graphene) [181].

(e) Molecular Layer Deposition (MLD): It is a variant of ALD that uses sequential, self-limiting surface reactions to deposit organic or organic-inorganic hybrid materials at the molecular level, producing polymer-like thin films and hybrid coatings with subnanoscale precision. MLD is often used for flexible coatings, battery electrodes, biomedical devices, and barrier layers [182,183].

Among these variants, thermal ALD remains dominant for high-quality films, while plasma-enhanced and spatial ALD are increasingly adopted to overcome limitations related to temperature and throughput, respectively.

2.4.2. Applications of ALD Coatings

ALD is widely utilized in applications requiring ultrathin, highly uniform coatings, such as:

Microelectronics

ALD has become indispensable in semiconductor device fabrication, enabling deposition of high-ĸ dielectric (such as ZrO₂, Al₂O₃, and HfO₂) layers, channel materials, metal gate electrodes, diffusion barriers, and gate oxides [184,185,186,187]. Intel was the first to implement ALD in CMOS circuits with HfO₂-based high-к films, replacing traditional thermally grown SiO₂ gate dielectrics. Figure 12 shows a SEM image of ALD-deposited SiO₂ film having a conformality of >95% [188].

Recently, Liu et al. [189] successfully fabricated high-performance ZnO thin-film transistors at few μm-resolution using low temperature-ALD (at 150 ºC), patterned by high-resolution reverse-offset printing (Figure 13). Highly conductive ultrathin niobium carbide thin films (2.6 nm) as diffusion barriers for Cu and Ru interconnects were grown by PE-ALD on SiO2 substrates at 100–400 °C [190]. Figure 14 (a-b) shows the four-point bending test performance of the PEALD-NbCx coatings as adhesion promoting layer for Cu and Ru interconnects, showing high critical loads and scratch resistance in the mechanical tests [190]. Figure 14c shows the uniform and conformal coating of NbCx film onto the complex topography of the 3D trench, with a step coverage of ~100% achieved using PE-ALD [190]. Development of flexible wearable electronics has also benefited through advances in ALD nano-coating [191,192,193]. Li et al. [191] fabricated high-performance flexible field-effect transistors based on ferroelectric Zr-doped HfO₂ (HZO) and ultrathin ITO channels on a flexible MICA substrate (Figure 15), where 100 nm Al₂O₃ and 11.4 nm HZO was grown by ALD at 250 °C.

Energy conversion and storage applications

ALD coatings improve the stability and performance of lithium-ion batteries, supercapacitors, and perovskite solar cells by enhancing interfacial stability and preventing degradation. In solar cells (including silicon, organic, thin film, perovskite, and quantum dot cells), ALD coatings are used as a surface passivation layer, buffer layer, window layer, absorber layer, electron/hole contact or transparent conductive oxide [143,194,195,196,197,198,199,200,201]. For example, research has shown that ALD facilitates controllable ultra-thin and dense surface coating of quantum dots (QDs) at the atomic scale, enabling stability and longevity of their exceptional optoelectronic performance, indispensable for QD-based devices such as QLEDs, solar cells, microLED displays [202,203,204,205]. Cheng et al. [203] reported on photo-stability enhancement of Al₂O₃-passivated colloidal CdSe/ZnS QDs. Selective surface passivation via ALD protects QDs from photo-oxidation and enhances photo-stability, critical for LED applications. Xie et al. [202] coated TiO2 nanorod arrays/CdS quantum dots via ALD, enhancing their photoelectrochemical and photocatalytic performance.

Leveraging conformal coatings and pin-hole-free films with atomic-level thickness, ALD has shown great potential in deposition and surface modification of advanced electrode materials in electrochemical energy storage devices [202,206,207,208,209,210,211,212,213,214]. For example, Lopa et al. [214] fabricated ALD-deposited tungsten trioxide/molybdenum trioxide (WO₃/MoO₃) nanohybrid heterostructure electrodes, achieving high energy storage capacity and stability.

Protective and barrier coatings

Thin ALD layers serve as membranes/barrier films that are used for filtration of water and air, and also act as effective moisture and gas barriers in flexible electronics and packaging applications [215,216,217,218,219,220]. For instance, Yang et al. [220] prepared protein-activated ALD-TiO₂-coated membrane, resulting in superhydrophilic nanoarmour on the membrane that demonstrated robust wettability and crude-oil-repellent stability.

Catalyst Surface Engineering

ALD enables atomic-scale surface engineering of catalysts and nanostructured materials with exceptional control over thickness, composition, and conformality. For designing efficient and durable catalysts, ALD has shown unique merits by allowing precise tuning of active sites and surface chemistry through the deposition of highly dispersed nanoparticles or even single atoms, and ultrathin overcoats, thereby tailoring the support-catalyst interface for enhancing electrochemical performance [221,222,223,224,225,226]. Currently, the molecular- and plasma enhanced-ALD techniques are widely used for the development of advanced catalysts [226]. Song et al. [227] presented a pioneering work on creating a sub-nano WO₃ bridge layer between Pt nanoparticles and carbon substrate (nitrogen-doped carbon nanotubes, NCNT) through ALD, resulting in Pt-WO₃-NCNT catalyst exhibiting remarkable stability and catalytic activity. Chen et al. [223] prepared highly efficient and stable Pt-MoO₃/CNT electrocatalyst via ALD for methanol oxidation and oxygen reduction reaction.

2.4.3. Advantages Challenges and Outlook

ALD offers several distinct advantages over conventional deposition techniques, including

atomic-scale thickness control (film thickness can be precisely tuned by controlling the number of deposition cycles), excellent conformality (uniform coatings can be achieved on complex geometries, porous materials, and high-aspect-ratio structures), high film quality (dense, pinhole-free films with excellent uniformity and reproducibility) and low-temperature processing (compared to conventional CVD, ALD can operate at relatively lower temperatures, enabling deposition on temperature-sensitive substrates). Recent developments in ALD have focused on improving scalability and expanding its applicability. Spatial ALD has emerged as a promising approach to increase deposition rates by eliminating purge steps, thereby enabling continuous processing. In addition, roll-to-roll ALD systems have been developed for large-area and flexible substrates, facilitating applications in flexible electronics and advanced coatings. Another significant advancement is area-selective ALD, which enables deposition on predefined surface regions without the need for conventional lithography, making it highly attractive for next-generation nanofabrication and device integration.

Despite its advantages, several challenges limit the widespread industrial adoption of ALD. The inherently slow growth rate associated with cyclic, self-limiting reactions restricts throughput, particularly for thick coatings or large-scale production. Furthermore, the availability of suitable precursors remains constrained by requirements such as volatility, thermal stability, and reactivity, which can limit material versatility and increase process cost.

Moreover, the precise timing and control of precursor exposure and purging steps increase system complexity of the ALD process.

Looking forward, ongoing research is directed toward overcoming these limitations through the development of high-throughput ALD techniques, such as spatial and plasma-enhanced variants, as well as the design of novel precursor chemistries. In addition, hybrid deposition strategies that combine ALD with techniques such as PVD or CVD are expected to play a crucial role in balancing precision, conformality, and scalability. These advancements are anticipated to expand the application of ALD in emerging fields including energy storage, nanoelectronics, and multifunctional coatings.

ALD-based monolithic three-dimensional architecture provides a promising platform for next-generation electronic systems, integrating enhanced functional diversity (preserving the performance of two-dimensional devices) and higher integration density (along the vertical dimension). Application fields include neuromorphic hardware, integrated photonic circuits, and flexible electronic devices [175].

2.5. Electrodeposition

Electrodeposition is a versatile electrochemical technique used to deposit metallic, alloy, and composite coatings from electrolyte solutions onto conductive substrates [228]. The process is typically based on the reduction of metal ions at the cathode under an applied potential (or current), enabling control over coating thickness, composition, and microstructure [229]. According to IUPAC, electrodeposition is the deposition of dissolved or suspended species onto an electrode under an applied electric field, a broad definition that encompasses processes such as electrophoretic deposition, electropainting, and both anodic and cathodic electrodeposition. In many coating studies, however, “electrodeposition” is used more narrowly to mean cathodic metal deposition (“electroplating”), where a metallic film forms on a substrate through electrochemical reduction of metal ions from the electrolyte; the deposited amount is governed by the charge passed [230].

One of the key advantages of electrodeposition is its ability to produce nanostructured and composite coatings through controlled manipulation of deposition parameters such as current density, electrolyte composition, and bath temperature. Techniques such as pulse and pulse-reverse electrodeposition have been extensively developed to refine grain size, enhance coating density, and improve functional performance. The method’s low energy consumption and near-ambient processing conditions make it environmentally favorable; however, achieving uniform coatings on complex geometries often requires careful control of electrodeposition parameters including bath chemistry and agitation/mass transport conditions [231,232,233,234,235,236,237,238,239].

2.5.1. Classification of Electrodeposition Techniques

Electrodeposition techniques can be broadly classified based on the nature of the charged species involved, the deposition mechanism, and modifications to the electrochemical process used to tailor coating properties.

a) Cationic Electrodeposition (CED) and Anionic Electrodeposition (AED): Based on the charge of the depositing species, electrodeposition is commonly divided into cationic electrodeposition (CED) and anionic electrodeposition (AED). In CED, positively charged species migrate toward the cathode and are deposited as a coating, whereas in AED, negatively charged species move toward the anode [240,241]. Cathodic electrodeposition (CED) is an important advance in coating technology, developed to overcome key limitations of conventional methods. It enables rapid formation of highly uniform films with strong corrosion resistance, broad and consistent coverage (including complex geometries), and low material waste. The use of waterborne resins further improves the process by reducing or eliminating VOC emissions, lowering cost, and improving environmental performance.

Compared with traditional solvent-based coating approaches, CED relies primarily on water-based resin systems such as acrylic, epoxy, epoxy–acrylate, and polyurethane. The process requires specially engineered resins that can be rendered electrophoretically mobile and deposited onto cathodic substrates. As a result, CED coatings typically deliver improved corrosion protection, strong adhesion to metals, and effective coverage of irregular surfaces and intricate designs. Despite these advantages, relatively few reviews comprehensively address CED process parameters, recent developments, and strategies for tailoring resins for CED applications.

These techniques are widely used in industrial coating processes, particularly for corrosion protection in automotive and structural applications due to their ability to provide uniform and adherent films.

b) Electrophoretic Deposition (EPD): A closely related but distinct technique is electrophoretic deposition (EPD), which involves the movement and deposition of charged particles, often non-conductive materials such as ceramics or polymers, under an applied electric field [242]. Unlike conventional electrodeposition, EPD enables the fabrication of coatings from suspensions of colloidal particles and is particularly suitable for producing uniform coatings on complex geometries.Because particles generally retain their charge after deposition, layer control depends strongly on using compatible, similarly charged powders and appropriate solvent–binder–dispersant systems. EPD is increasingly applied beyond traditional wear-resistant and anti-oxidant coatings, including functional films for microelectronics and solid oxide fuel cells, bioactive coatings for implants, nanoscale assembly, complex compounds, and ceramic laminates [243,244,245,246,247,248]. Its main intrinsic limitation is that water is typically unsuitable due to gas evolution at the electrodes, but this is mitigated by the availability of many non-aqueous solvents.

c) Specialized Electrodeposition Variants

In addition to these fundamental classifications, several advanced and modified electrodeposition techniques have been developed to enhance coating performance and functionality. Some of them are outlined below.

Pulse Electrodeposition (PED): employs pulsed or pulse-reverse current instead of continuous direct current, enabling improved control over nucleation and growth processes, resulting in refined grain structures and enhanced mechanical and corrosion-resistant properties [249,250].

Composite Electrodeposition: Involves the co-deposition of solid particles, such as Al₂O₃, SiC, or carbon-based nanomaterials, within a metallic matrix to produce coatings with superior hardness, wear resistance, or self-lubricating characteristics.

Electrophoretic co-deposition: Extends the EPD approach by enabling the simultaneous incorporation of multiple material phases, such as ceramic nanoparticles and carbon nanotubes, to form multifunctional nanostructured coatings, frequently used in functional applications like fuel cell components.

Brush Electrodeposition: A localized variant used for repair and selective coating, where the electrolyte is applied via a moving electrode brush, allowing precise material deposition on targeted regions. Brush electrodeposition methods allow localized repair or enhancement of worn or damaged surfaces without the need for full-scale re-coating.

Autodeposition: A chemically driven coating process that does not require an external power supply. Instead, film formation occurs through a self-limiting redox reaction at the substrate surface. Although distinct from conventional electrodeposition, it is often discussed within this broader category due to its similar applications in corrosion protection and surface finishing.

Ultrasound-assisted electrodeposition (UAED): UAED has gained attention as a practical route to improve the quality and performance of electrodeposited metallic coatings compared with conventional depositions. Recent work shows that applying ultrasound can influence deposit morphology, crystal growth, and the resulting mechanical properties. These effects are commonly attributed to enhanced mass transport arising from cavitation, acoustic streaming (microflow), and acoustic pressure fluctuations, which reduce concentration polarization and promote more uniform ion supply at the electrode surface. As a result, UAED often produces finer-grained deposits with smoother surfaces, higher density, and fewer defects, translating into improved hardness and, in many cases, better wear and corrosion resistance.

Despite these benefits, several barriers still limit broad industrial uptake, including scale-up constraints, added energy demand, and potential electrolyte instability under prolonged sonication. Future studies should therefore prioritize optimization of ultrasound conditions (e.g., power, frequency, duty cycle, and transducer configuration), integration with advanced waveforms or hybrid deposition routes, and system-specific process design tailored to the coating material and application. Overall, UAED remains promising for energy, environmental, and high-performance coating applications by addressing deposit non-uniformity issues associated with concentration polarization in traditional electrodeposition.

2.5.2. Applications of Electrodeposition

Electrodeposition is one of the most frequently used methods for preparing thin metal films and applying metal or polymer coatings to conductive surfaces and is widely employed in applications ranging from corrosion protection and decorative coatings to energy systems and microelectronics. Common applications include:

Corrosion protection and wear-resistance

Electrodeposited coatings such as nickel, zinc, and chromium provide uniform, adherent layers that protect metal substrates from oxidation and corrosion in marine, automotive and tribological applications. Nanocomposite coatings (e.g., Ni-P-SiC, Ni-B-TiN, Ni-CeO₂, Ni-Co-Al₂O₃) enhance hardness and reduce friction, extending the service life of mechanical components, gears, and cutting tools [251]. Besides a broad spectrum of coating materials, including chromium, nickel, cobalt, zinc and copper, alloying elements to a single metal coating can result in better in-service performance, achieving enhanced mechanical, tribological and corrosion protection. Extensive work has been done to investigate the influence of co-deposition of nanoparticles (e.g., SiC, CNTs, Al2O3), electrodeposition conditions and parameters on mechanical, tribological, wear and corrosion performance [252,253,254,255,256,257,258,259,260,261,262,263,264,265].

Decorative finishing

Electrodeposition enables highly uniform and aesthetically appealing finishes for consumer products, jewellery, and architectural metals, while simultaneously providing corrosion protection and mechanical durability [266,267,268,269,270,271]. Electrodeposition for decorative applications provides a lustrous, durable finish on metal and plastic substrates, enhancing aesthetic appeal and corrosion resistance for jewellery, automotive trim, and household fixtures. Common materials include gold, silver, nickel, copper, brass, and chromium alternatives (tin-cobalt).

For copper, industry has traditionally used alkaline cyanide baths, valued for good coverage and controllability, but these are increasingly replaced because of toxicity, disposal burden, and relatively low efficiency and brightness [272]. Lower-impact alternatives such as pyrophosphate electrolytes exist, though they are less stable and more condition-sensitive [273]. Acid copper sulfate–sulfuric baths are now dominant in many applications because they are simple, inexpensive, highly conductive, and can produce bright, levelling deposits at high current efficiency [274]. Nickel plating is still common for bright finishes and as a functional intermediate layer, but its use, especially in items that contact skin, has been constrained by health regulations, driving “nickel-free” and low-release (“hypoallergenic”) approaches and accelerating interest in substitutes. In that context, bronze and palladium coatings have grown in importance as barrier and decorative layers, though some established bath families rely on strongly alkaline, cyanide-containing chemistries that require tight control. Gold plating remains central where high corrosion resistance, conductivity, and appearance are needed. Beyond these, silver, rhodium, platinum, ruthenium, and chromium are used for specific combinations of reflectivity, hardness, inertness, or wear resistance, with a broader trend away from the most hazardous electrolytes (notably cyanides and hexavalent chromium) toward more compliant alternatives.

Modern techniques, such as pulsed current electrodeposition, improve brightness, uniformity, gloss, and deposition efficiency. For instance, Mariani et al. [275] demonstrated that Au-Ni deposits obtained by pulsed currents had finer and compact grains, leading to smoother, glossier, brighter finish and markedly improved corrosion resistance than the deposits obtained using direct current. In another work, Mariani et al. [276] presented a systematic study of pulse and pulse reverse plating on acid copper bath for decorative and functional applications, and concluded that pulsed current show improved characteristics compared to those obtained with direct current, while reverse pulsed currents are preferable for applications where homogeneity of metal thickness is favored over aesthetics. Verrucchi et al. [269] reported on electrodeposition of Sn-Ru Alloys by employing direct, pulsed, and pulsed reverse current for decorative applications. The uniformity of the electrodeposited layer was analyzed by XRF (Figure 16). The sample deposited using pulsed reverse current (t_C = 2 ms) showed a remarkably homogeneous thickness with a standard deviation of 2 nm.

Functional surfaces

Electrodeposited catalysts, including cobalt-phosphide or nickel-iron alloys, are used for hydrogen and oxygen evolution reactions in electrocatalysis, as well as for battery electrodes. Additionally, electrodeposited electrodes are employed in water purification, fuel cells, and energy storage devices. In the field of energy, electrodeposited materials such as cobalt phosphide on conductive substrates have demonstrated excellent electrocatalytic activity for hydrogen evolution reactions, highlighting the technique’s relevance in sustainable energy technologies. Jia et al. [240] successfully deposited two-dimensional conductive metal-organic framework on nickel foam for use as electrodes for supercapacitors that demonstrated outstanding performance.

Besides corrosion and wear resistance, the electrodeposited Ni-Fe coating is largely employed for magnetic (such as pole pieces in magnetic valves, magnetic heads for tape recorders, flux guides in thin film write heads and shields for magnetic recording or the cores of on-chip inductors etc) and electronic applications (e.g., memory, recording and storage devices in computers, Null-balance transformer, pulse transformers, relay parts etc.) [277,278,279,280,281,282]. Rasmussen et al. [278] developed a new bath formulation for pulse reverse electroplating of CoNiFe for their magnetic microsystems’ applications. Sun et al. [281] electrodeposited Ni₈₀Fe₂₀ Permalloy for use as the magnetic core to increase the inductance and quality factor of integrated flexible parylene-based MEMS inductors. In another work, a giant magnetoresistance ratio of 5.4% was achieved for electrodeposited [Ni₈₀Fe₂₀/Cu/Co/Cu] spin-valve multilayers on NiFe buffer layers [280].

Electronics and Microfabrication

The technique is widely used to fabricate metallic interconnects, conductive traces, and microstructures for printed circuit boards (PCBs), microelectronic devices, and MEMS components, benefiting from precise thickness control and high reproducibility [283,284,285,286,287]. In this context, electrodeposited nano-twinned metals have been studied extensively for applications in interconnects in nanoelectronic devices and nanoelectromechanical systems owing to their excellent strength, ductility, and electrical conductivity [285]. Nanoscale twin metal formation has been achieved by both direct current and pulsed current deposition [288,289,290,291,292,293,294,295,296,297,298]. For example, Dong et al. [297] reported nanotwinned Cu micro-cone arrays fabricated by pulse electrodeposition without any template. It was observed that the morphologies of the micro-cones were controllable by Cl^- concentration in the electrolyte. Also, it was realized that the nanotwinned micro-cone array enabled the low-temperature bonding a 50% enhancement, rendering it promising for advanced 3D electronic packaging applications.

Metal-recovery

Electrodeposition is widely used in hydrometallurgy, particularly in the fields of metal recovery. Examples include the extraction of precious metals, such as gold, silver, and copper, from solution by means of electrodeposition or electrochemistry-based metal recovery [299,300,301,302,303]. Li et al. [300] recovered nickel from real electroplating wastewater using a cost-effective manner by integrating electrodeposition with adsorption pretreatment technique. Wang et al. [303] performed stepwise recovery of gold and silver from E-waste via stepwise electrodeposition. In another work, Li et al. [302] employed flotation-assisted electrodeposition process to recover copper from waste printed circuit boards.

Biomedical Coatings

Electrodeposition of biocompatible materials such as hydroxyapatite (HAp) or calcium phosphate on implants improves osseointegration, corrosion resistance, and overall biocompatibility in physiological environments [304,305]. Chitosan-based coatings modified with gold or zinc nanoparticles are considered effective strategy to prevent bacterial infections in implants [306,307]. Among the wet methods, the various electrodeposition approaches for chitosan, HAp and reinforced HAp nanocomposite (such as HAp-CNTs, HAp-GO, HAp-Chitosan, HAp-TiO₂) coatings include direct/pulse electrodeposition, electroless and electrophoretic deposition [304,305,308,309,310,311,312,313,314].

Coatings on titanium and titanium alloy (such as Ti6Al4V, NiTi) are favored for implants due to their outstanding biocompatibility and osseointegration, low density, resistance to corrosion, and high mechanical strength [315,316,317,318,319,320,321,322,323,324,325]. Besides, Zn-, Mg-based alloys, Co-Cr alloys and 316 L stainless steels are also used in load-bearing implant applications [326,327,328,329,330,331,332,333].

For example, electrodeposition of HAp-TiO₂ composite coating improved the corrosion resistance of NiTi by almost 50% [316], while, pulsed electrodeposition of HAp-Ta₂O₅ composite coatings on NiTi improved the corrosion resistance and decreased the Ni ion release of the samples up to 205 % and 91 %, respectively [324]. In a recent work, Souri et al. [334] reported on electrochemical deposition of homogeneous HAp-Ag nanostructured coating which enhanced the antibacterial and corrosion resistance of Ti6Al4V alloy.

2.5.3. Advantages, Challenges and Outlook

Electrodeposition offers several compelling advantages that make it a versatile and widely used surface engineering technique. Its electrochemical nature allows precise control over coating thickness, composition, and microstructure by adjusting parameters such as current density, bath composition, temperature, and deposition time. This enables the fabrication of uniform and adherent coatings even on substrates with moderate geometric complexity. In comparison to other deposition techniques, such as PVD or CVD, electrodepositions offer some advantages in terms of simplicity of the process and the low cost of the equipment. While CVD achieves uniform deposition over a large-area, it often requires high temperature and high pressure, consuming high energy, and may also produce harmful byproducts. Likewise, even though PVD prepares high-quality films, the complexity of the process, slow deposition rate and expensive equipment are its downside.

The process is also inherently energy-efficient, as it operates at ambient or mild temperatures, and is scalable for both small components and large industrial surfaces. Moreover, advanced variants such as pulse electrodeposition and composite co-deposition allow the creation of nanostructured or multifunctional coatings, enhancing properties like hardness, corrosion resistance, wear resistance, and catalytic activity. Electrodeposition also accommodates a wide range of metals, alloys, and particle-reinforced composites, including environmentally friendly coatings, further broadening its applicability across industries such as automotive, aerospace, marine, electronics, and energy storage. Deep eutectic solvents have gained attention as non-aqueous electrolyte media for metal electrodeposition. Their appeal stems from a combination of properties often aligned with greener processing, including low volatility, good thermal stability, relatively low toxicity, biodegradability, and low cost [335].

Despite these advantages, electrodeposition presents several limitations that must be addressed for optimal performance. Achieving uniform deposition on highly complex or recessed geometries remains challenging due to variations in local current density and mass transport limitations. Coatings may develop internal stresses or defects such as porosity, cracks, or nodules, which can compromise mechanical integrity and long-term durability. The choice of electrolyte, additives, and particle dispersions critically affects coating quality. Furthermore, the use of certain electrolytes and additives pose environmental or safety concerns, necessitating the development of greener and more sustainable bath chemistries.

Additionally, while pulse and composite techniques improve performance, they often involve more complex setups, longer processing times, or increased operational costs, limiting high-throughput industrial adoption in some cases. Careful optimization of process parameters and bath formulations is therefore essential to balance performance, reproducibility, and sustainability in electrodeposition-based coatings.

Recent advances in electrodeposition have focused on enhancing coating performance, uniformity, and functional versatility through improved process control and electrolyte engineering. Techniques such as pulse and pulse-reverse electrodeposition have enabled the fabrication of fine-grained and nanostructured coatings with superior mechanical properties, corrosion resistance, and adhesion. The incorporation of nanoparticles into the plating bath has further facilitated the development of metal–matrix composite coatings with tailored functionalities, including enhanced wear resistance, catalytic activity, and multifunctionality. Emerging strategies, such as electrodeposition under magnetic or ultrasonic fields, have improved mass transport and coating uniformity, addressing limitations associated with complex geometries and substrate accessibility.

Looking forward, research is expected to focus on the development of environmentally benign electrolytes, optimized bath chemistries, and advanced real-time monitoring techniques to improve deposition control and reproducibility. The integration of electrodeposition with complementary surface engineering methods and additive manufacturing approaches is likely to enable multifunctional and hybrid coatings with enhanced mechanical, chemical, and functional performance. These developments position electrodeposition as a key technology for emerging applications in energy storage, catalysis, advanced electronics, and biomedical devices, highlighting its continued relevance in next-generation surface engineering.

3. Conclusions and Future Perspectives

Surface coating technologies have become indispensable in modern manufacturing and materials engineering, offering effective routes to enhance durability, functionality, surface protection, and overall component performance. This review has highlighted the major coating approaches, including physical techniques such as PVD, CVD, as well as chemical methods such as ALD, and electrodeposition. Together, these technologies provide control over the coating thickness, composition, structure, adhesion, and surface functionality, enabling their use across a wide range of industrial sectors.

The evolution of coatings is marked by the convergence of advanced deposition techniques, novel materials, and intelligent design, enabling multifunctional systems that address sustainability and performance. Combining PVD, CVD, and ALD allows manufacturers to engineer materials from the atomic scale up to the macro scale. This hybrid approach, often called sequential or synergistic deposition, allows engineers to exploit the unique strengths of each method while mitigating their individual limitations. For instance, by using ALD for precision interfaces, CVD for high-quality bulk materials, and PVD for dense metallic finishes, industries can create faster microchips, longer-lasting tools, and next-generation energy devices that would not be possible using any single method alone. Recent progress has been driven by advances in materials design, process optimization, and digital tools. The integration of artificial intelligence is particularly important, as it can support parameter optimization, defect prediction, process adjustments in real-time monitoring, and faster development of coating systems with improved reliability and reproducibility. At the same time, emerging concepts such as self-healing coatings, biomimetic surfaces, smart responsive coatings, and hybrid deposition processes are expanding the functional scope of coated materials.

Sustainability will be a decisive factor in the future development of coating technologies. Many conventional processes still involve high energy consumption, hazardous chemicals, solvent use, or volatile organic compound emissions. Consequently, greener approaches such as low-temperature deposition, water-based sol-gel processing, solvent-free formulations, recyclable materials, and bio-based coatings should receive greater attention. Life cycle assessment can guide the selection of materials and processes with lower environmental impact, reduced waste, and improved resource efficiency. Aligning coating technologies with circular economy principles will be essential for achieving both high performance and environmental responsibility.

Overall, surface coating technologies are expected to remain a key driver of innovation in materials science and industrial engineering. Their future development will depend on the integration of advanced materials, precise processing, AI-driven optimization, multifunctional design, and sustainable manufacturing. By addressing current challenges related to scalability, durability, cost, and environmental impact, surface coatings can continue to support safer, cleaner, and more efficient technologies across diverse application areas.