Metallization of polymers and composites: State-of-the-art approaches

: Polymers and their composites are widely used for designing structures in aerospace, automotive, electronic, sport industries due to their lightweight, cost, and processing advantages. However, the surface of polymeric materials typically exhibits intrinsic deficiencies, limiting their durability and functionalities, e.g., low wear resistance, low thermal and electrical conductivity, low adhesion, low bioactivity, low reflectiveness, and weak photochemical resistance. Polymer metallization is an emerging concept that addresses these deficiencies by forming a metallic skin on polymeric surfaces. Herein, the working principles, recent advances, challenges, functional capabilities, and applications of the state-of-the-art polymer metallization methods in the fields of additive manufacturing, coating technologies, and material science are reviewed on nano-, micro-, and macroscales. The polymer metallization methods applied to polymeric and polymer composite substrates are physical vapor deposition, electrochemical plating, a family of thermal spray methods (such as flame spaying, arc spraying, plasma spraying, and cold spraying), and a series of polymer–metal direct bonding methods (such as adhesive bonding, injection overmolding, and fusion joining techniques, including ultrasonic joining, friction spot joining, electromagnetic induction joining, and laser joining). Understanding the key aspects within these approaches would guide scientist and engineers for optimizing the design and durability of structural materials made of polymers/composites.


Introduction
Polymeric-based materials have been conceptualized and explosively developed in the past five decades. Almost all sectors of the global economy-construction, transportation, manufacturing, mining, and education-have embraced and benefited from these materials [1]. The global polymer market reached US$650 billion in 2020 and is expected to exceed $1 trillion bar in this decade [1]. The editorial board of Materials Today acknowledged that polymer composites are among the top 10 advances made in material science over the past five decades [2]. Developments in polymer materials over the years have influenced our lifestyles and shaped the dynamic field of modern material science.
Polymers have burgeoned owing to their specific properties and economic factors. Despite their outstanding physical and mechanical characteristics (e.g. strength-to-weight ratio, friction performance, corrosion resistance, transparency, etc. [3]) as well as their cost and processing advantages, polymers exhibit multiple shortcomings under certain conditions, such as low wear resistance, poor thermal and electrical conductivity, weak adhesiveness and bioactivity, low light reflectiveness, and sensitivity to sunlight [3]. Sensitivity to sunlight-seemingly a trifling issue-induced one of the most epic automobile recalls in the US history (8.8 million vehicles) because the seatbelt-release buttons were composed of acrylonitrile butadiene styrene (ABS), a plastic that degrades in sunlight [4]. Bio inertness of polymers results in biofouling of man-made marine structures, such as offshore oil and gas and wind facilities, and vessels [5]. The US Navy alone spends nearly $1 billion annually in maintaining its vessels free of barnacles, oysters, and other organisms [6]. Due to the low electrical conductivity of composites, aircraft fuselage are vulnerable to lightning strikes, burdening aircraft industrialists and operators for $2 billion annually [7]. Various solutions were developed for either issue, e.g. paining for UV and fouling protection, wire mesh and metal foils for lightning strike protection, but often this solutions are temporary, costly or labor-intensive [8]. Overall, polymers offer numerous advantages; however, their deficiencies present a major barrier to the further spread of these materials.
Polymer metallization (PM) is an emerging additive-manufacturing concept that masks the deficiencies of polymers under a metallic layer that serves as a functional skin. The skin can considerably enhance the target performance of the polymer surface or instill new capabilities inherent to neither the polymer nor the bulk metal. As it will be shown further in this review, the functional skin can improve the electrical, thermal, and mechanical properties as well as chemical and photochemical wear resistances of the surface, thereby improving the strength and stability of polymeric parts. The new capabilities of metallized polymers-including phenomenological, chemical, mechanical, and electrical responses-have inspired innovations in various industries (such as automotive, aircraft, airspace, marine, energy, electronics, packaging, sport, design, and furniture) and the development of numerous small commodities in daily life. Many of these commodities, e.g., a shiny pen on a reader's table, exhibit glossy metallized surfaces that hide their polymer base. PM has been unnoticed and unheralded but may define the future of polymericbased materials.
The best PM method in a particular case and the justification for its brilliance are pertinent points. Some of the most promising PM methods are physical vapor deposition (PVD) [9,10], electroless/chemical plating [11,12], thermal spray methods [13,14], and direct polymer-metal bonding methods [15][16][17]. The technological advantages and disadvantages, which are specific to each method, define the range of functionalities conferred by the metallic skin [18]. Unfortunately, the technological capabilities of the aforementioned methods are poorly defined when applied to polymeric substrates. Metal deposition on polymers is a nonconventional and delicate approach with numerous considerations: the selection of the deposited metal [19][20][21], thermal stability of the polymeric substrate [22][23][24][25][26], size and shape of the substrate [27], process productivity and costs [28], compliance with the existing manufacturing infrastructure, and environmental hazards [29]. For example, polymer electroplating poses an environmental hazard [30] that is noncompliant with green regulations, costing manufacturers penalties worth millions of dollars [31][32][33][34]. Often, paying these fines is less onerous than modifying the existing manufacturing infrastructure to ecofriendly the electroplating process. Moreover, as it is shown further, each method can metallize only a small number of polymers with few metals of limited thicknesses. Several metallization methods, often titled in different way, have been reviewed in relation to a specific application, e.g. of sputtering on polymer membranes [9] and optical devices [10], electroless plating on semiconductors [11], while the methods for macro-scale metallization are not visible in this topic due to been renown as techniques for joining dissimilar materials and hybrid structures. Reviews on PM via cold spray appeared recently in a great number [22][23][24][25][26], several reviews available for electroless plating [35,36], while other PM methods were not reviewed from the technological point of view due to a low number of articles available. In addition, the reviews dedicated to each method cannot provide a comprehensive picture of PM. Finally, PM is an emerging additive-manufacturing concept, where most of the progress appeared in the past few years. To sum up, the PM subject is fractured across different research fields and a comprehensive summary of the capabilities of each PM method is needed. This is the wide scope review assembling the methods developed in the fields of additive manufacturing, coating technologies, and material science for metallization of polymer-based parts in the nano-, micro-, and macroscales. We review the working principles, recent advances, challenges, and applications of PVD, electroless plating, a family of thermal spray methods (including flame spraying, arc spraying, plasma spraying, and cold spraying), and a series of polymer-metal direct bonding methods (such as adhesive bonding, injection overmolding, and fusion bonding techniques, including ultrasonic joining, friction-based joining, electromagnetic induction joining, and laser-based joining). The methods preferable for different purposes are identified, and a range of functional capabilities and promising applications of PM is described. Understanding the key aspects within these techniques would guide scientist and engineers for optimizing the design and durability of structural materials made of polymers/composites. In summary, we define PM as the process of connecting a metallic layer to a polymeric part using chemical and/or mechanical adhesion between their surfaces. This review concludes with visions for the further development of polymers and polymer composites using PM techniques.

Physical vapor deposition (PVD) on polymeric-based materials 2.1. General characteristics
PVD is a gaseous-state deposition process that vaporizes a material from a solid or liquid source and deposits the vapor on a substrate [37]. This process is performed in a vacuum environment and can be used to deposit films of one or more elements. PVD is usually performed using evaporation (vacuum evaporation) or sputtering. Figure 1 presents the sputtering process. The evaporation setup is very similar to the sputtering one but replaces the magnetron and target with a heated filament, which is always placed below the substrate (i.e., in the bottom-up configuration). The deposition mechanisms of both sputtering and evaporation PVD are discussed below.
The metallic coatings deposited using the PVD techniques are thin (with thicknesses of several nanometers to several micrometers). The coatings closely follow the contours of the coated parts and are decently adhered to the substrates using appropriate pretreatments. Notably, PVD processes are line-of-sight processes, implying that the coating is formed on the surface facing the source material. Therefore, the coating uniformity (e.g., thickness) may vary on surfaces not directly facing the source material, e.g. free-formed parts with a complex geometry. In modern PVD equipment, this issue is usually alleviated by rotating the substrates in front of the source material (e.g., using a planetary substrate holder). Overall, PVD allows the thin-film metallization of polymeric parts with no hidden surface.

Working principles of PVD methods
In vacuum evaporation, the material to be deposited is thermally evaporated in a vacuum environment (10 −3 -10 −7 Pa), transported in vacuum, and condensed on the substrates [38]. Thermal evaporation is performed by the resistive heating of tungsten wires, firing using electron beams, and inductive heating as well as by applying an electric arc or laser beam [38]. The substrates are mounted on the holder at a considerable distance from the source material (up to 30 cm [38]) to avoid their heating, which is extremely important when coating polymers [39]. The material being evaporated can be attached to a heated wire or placed in a boat or crucible.
In sputtering, the source material (called the target) is vaporized using a sputtering process, in which the target is bombarded with particles (typically Ar+ ions). Atoms and/or ions are then ejected from the source material ( Figure 1). Note that sputtering is not a thermal vaporization process and that the source material is ejected by the transfer of kinetic energy from the bombarding ions [40]. The sputtered material is transported in vacuum and condensed on the substrates. A critical review on the numerical simulation of process fundamentals of sputtering process can be found in [41]. A major development in the sputtering technique is magnetron sputtering, in which magnetic fields are used to enhance the density of the plasma near the target and thus increase the sputtering rate. The magnetron-sputtering configuration can sustain the discharge at lower pressures and requires lower voltages than conventional sputtering, which adds a flexibility advantage [27]. The target materials can receive power from direct-current (DC), radiofrequency (RF), or pulsed-DC current (p-DC) suppliers. The latter two suppliers are used in the deposition of nonconductive materials. One of the latest developments in magnetron sputtering is high-power impulse magnetron sputtering (HiPIMS) [42], which achieves a high ionization degree of the sputtered species and affords films with a high density and enhanced properties.
The evaporation and sputtering techniques were pioneered nearly two centuries ago and now deliver a wide technological variety of PM. The sputtering and evaporation processes can be traced back to the publications of Grove [43] and Faraday [44], respectively, in the 1850s. Vacuum thermal evaporation was first applied to PM in the 1940s [45], and sputtering PM was available in the 1970s [46]. Research on PM based on magnetron sputtering date back to the 1980s [47]. Today, all PVD-based deposition techniques can be identified in the literature as methods to metallize polymers. Recent advancements and challenges of PVD Progress in PM using PVD has been motivated by the development of novel deposition techniques that upscale the metallization process, improve the adhesion of metallic films to polymer substrates, and achieve specific coating functionalities. Table 1 shows the classification of research on PM using PVD based on different substrate and coating materials, deposition methods, applications, and coating functionalities. In the following section, the progress achieved in these major categories is overviewed.
Owing to various modifications of the PVD process, coatings with particular qualities can be achieved. Zhang et al. [48] used the High Power Impulse Magnetron Sputtering (HiPIMS) deposition method to improve the properties of CrN films deposited on ABS substrates. This variation of magnetron sputtering deposition can afford the high ionization of sputtered species, which benefits the structural and overall physicochemical properties of the deposited films. HiPIMS provides a denser coating morphology, higher corrosion resistance, and better mechanical properties (hardness, wear resistance) than conventional DC magnetron sputtering. Another PVD technique, pulsed electron ablation (PEA), has recently been used for depositing Ag films on polyurethane (PU) [49]. In the PEA process, the target material is evaporated using a pulsed electron beam. The feasibility of PEA for coating central venous PU catheters was also reported. Later, Heinß and Fietzke [50] deposited Al on various polymers (such as polycarbonate (PC), ABS, and polylactic acid) using PEA. TiN was deposited on a polymethyl methacrylate (PMMA) substrate using pulsed laser deposition [51], and Ti/TiN and Zr/ZrN coatings were deposited on polysulfone substrates using cathodic arc deposition [52]. Overall, many PVD processes have shown the ability to metallize polymers.
The poor adhesion of metallic coatings on polymer substrates is the critical problems of most PM methods including PVD. The interfacial polymer-metal adhesion has been improved using various strategies, such as the application of a base coat before metallizing. Base coat is the layer of material which demonstrates higher interfacial adhesion to polymeric substrate and serve as an intermediate layer between the substrate and final metallic coating. Vergason [53] deposited Cr films on an ABS substrate using magnetron sputtering rather than electroless plating. After selecting the proper base coat and optimizing the deposition process, the coating performance was similar to that of the well-established Cr-electroplated components regularly used in the decorative sector of the automotive industry. The adhesion can also be improved via the plasma pretreatment of polymer substrates before deposition. Recently, Pedrosa et al. [54] employed a plasma activation process in an Ar environment before depositing CrN films with various N contents on ABS substrates. During the activation process, the bombardment of Ar ions caused various topographical (e.g. roughening) and chemical changes (e.g. formation of functional groups on the surface and dangling bonds) that benefited the polymer-metal adhesion. The power level and Ar-plasma pretreatment duration were adjusted to enhance the hydrophilicity of the polymer surface and hence the adhesion of the deposited Ni films. Qian et al. [50] also employed plasma pretreatment before depositing Al on PC, ABS, and PLA polymers using the electron beam evaporation process. Interestingly, they found that the substrate adhesion of the Al coating was insufficient in solo Ar plasma pretreatment but improved after incorporating oxygen with Ar during substrate pretreatment. PM can also be preceded by ultraviolet (UV) and ozone (UVO) treatment [55]. The authors reported the formation of polar OH and C=O groups, which were later found to improve the properties of aluminum films deposited using vacuum evaporation. After optimizing the deposition process, Frutos et al. [56] improved the mechanical properties of TiSi coatings deposited on polyetheretherketone (PEEK) substrates. By varying the bias voltage during deposition, they varied the intensity of the bombardment of the growing film. Using nanomechanical testing procedures, the fracture toughness of the films was improved after deposition under the optimal substrate bias. In summary, the adhesion at the metal\polymer interface is one of the critical factors of PM. Pretreating the polymer and application of intermediate coatings can enhance the quality of the deposited metallic skin.
Improving the mechanical properties and wear resistance of polymeric surfaces was the primary motivation of many PM studies. Mauer and Shulz [57] deposited Ti, Al, Cr and Ti/TiN multilayers on carbon fiber-reinforced polymers (CFRPs) using magnetron sputtering to improve their erosion resistance. They conducted an extensive solid-particle erosion study of CFRPs coated with multilayers up to 30-μm thick. The PVD-deposited coatings improved the erosion behavior of the CFRPs, showing their potential for improving the erosion performance of polymer composites. A similar study was conducted on CFRPs coated with different thicknesses of Ti/TiN multilayers [58]. Solid-particle and rain erosion tests were performed, and the erosion resistance were reported as functions of the coating thicknesses under various test conditions. Baptista et al. [59] reported the wear resistance of Cr-metallized PC. In their study, thin Cr layers (total thickness: 150-250 nm) were deposited using magnetron sputtering. The wear behavior and adhesion of the coatings were assessed in abrasion and scratch tests. The wear performance of the coatings increased with the thickness. Carvalho et al. [60] deposited metal-containing thin films on polymers to improve their tribological properties. They deposited Ag-doped amorphous carbon coatings (thickness: ~200 nm) using magnetron sputtering on PU medical stent substrates. When sliding against porcine liver counterbodies, the coated stents exhibited a lower coefficient of friction than the uncoated PU stents. Note that Ag imbued these coatings with antimicrobial properties. The toughness of PVD-deposited coatings on polymers can be improved using multilayered structures. For example, Lackner et al. [39] deposited Cr/CrN multilayers on epoxy polymer composites followed by a top layer of a-C to further improve the tribomechanical properties. The multilayered Cr/CrN coating achieved higher toughness than single-phase CrN. Evidently, PVD can elevate the erosion, abrasion, friction, and scratch resistances of polymer parts.
Imparting antimicrobial properties to metallized polymers was motivated by biomedical research. Liu et al. [61] deposited nanometer-thick Ag films on PEEK substrates and assessed the antibacterial potential and cytotoxicity of the coated material. The coating exhibited high antimicrobial activities but no cytotoxicity. Using the HiPIMS technique, Chen et al. [41] deposited Ag with different thicknesses (30-170 nm) on a polyethylene terephthalate (PET) fabric. In this case, antimicrobial activities were reported even for the thinnest coating. Moreover, the antimicrobial activities were sustained even after 20 washing cycles. PM has also been employed for improving the biocompatibility of vascular grafts. Bolbasov et al. [62] deposited Ti coatings on electrospun scaffolds composed of polycaprolactone (PCL) and poly 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) using magnetron sputtering. The coated scaffolds were more biocompatible and showed higher proangiogenic activities than their uncoated counterparts.
Reflective aluminum and silver coatings are often deposited on polymer substrates to improve their optical performance in various scenarios. Polyvinyl chloride (PVC) was aluminized using vacuum evaporation, achieving 100% reflectivity in the visible range [63]. Moreover, the aluminized PVC part showed good flexibility. After depositing aluminum using vacuum evaporation on PC, a reflectivity ~90% was achieved, attributed to the optimized pretreatment of the PC substrates before deposition [55].
The PVD deposition of Al, Cu and Ag coatings can also enable the electromagnetic shielding of polymers. For example, Mihut et al. [64] deposited copper, silver, or aluminum coatings on high-density polyethylene (HDPE) and styrene butadiene copolymer (SBC) substrates reinforced with carbon nanofillers. The depositions were performed using vacuum evaporation by employing tungsten coils or boats, and the resulting coatings showed thicknesses of 100-300 nm. All coatings-even the thinnest (100 nm)-showed a shielding effect. The electromagnetic shielding of polymers using PVD-deposited coatings was also demonstrated by Heinß and Fietzke [50], who deposited 5-μm-thick Al coatings on different polymeric substrates using electron beam evaporation. Relatively to the conventional PVD process, high deposition rates (100 nm/s) and the intended electromagneticshielding functionality were achieved.
Among disadvantages of the sputtering methods are the sensitivity of the process to the environmental conditions, uniformity of coating thickness along with increasing the thickness, inadequate coating of the hidden surface on free form parts with complex geometries, and, as finally poor interfacial adhesion. First, to achieve the preside thickness of the coating film the sputtering conditions in the chamber, such as vacuum, temperature, substrate-to-source distance and current must be accurately controlled [56]. Second, due to the different deposition efficiency in the center and periphery of the plasma cloud, the coating thickness grows non-uniformly [49], which might be an issue when designing function materials (e.g. optics and electronics). Third, PVD is most suitable for metalizing flat parts, the plasma cloud does not penetrate the obstructed surface, in results of what we often observed that even sidewalls of micro-textured surface remain uncoated. Finally, atom-by-atom deposition involve only molecular forces (Electron Volts) of adhesion at the metal-polymer interface, which are negligible in the mechanical engineering applications (Newtons) [65]. In other coating methods the improved adhesion is typically achieved with grit blasting, aimed to roughen the surface and introduce an interlocking effect [66]. However, the minimum roughness achieved by this method is in the micro-scale (even when using a micro-blasting system with the smallest abrasives [67,68]), while the coating thickness by PVD is in nanoscale. Thus, chemical surface pretreatment methods must be considered instead of mechanical roughening.

Applications of PVD
Metallization of plastics has emerged from the need to overcome various drawbacks related to their properties. The main applications of metallized plastics via PVD are listed below:  Decorative coatings (glassware, trophies, and medallions) [53,65]  Optical coatings (lenses and headlamps) [73]  Electrical devices (capacitors, solar cells, electromagnetic interference shielding, and data storage) [50,70]  Materials with improved mechanical and wear properties [59,74]  Biomedical applications (biocompatibility and antibacterial activities) [56,61].
Metallic decorations to improve the appearances of plastic parts are often performed using a PVD technique. Polymers such as ABS are typically metalized with a Cr coating via eco-unfriendly electro-chemical plating [53], thus PVD is a promising alternative approach for preparing a thin conductive layer of Cr or Al [46]. When a reactive gas (such as an N-or C-containing gas) is added to the PVD process, PVD can also achieve compound films (e.g., TiN, TiCN, or CrN) with various colors, which is a distinct advantage in decorative applications. In the other example, TiN coatings are widelly employed as protective layers on cutting tools to increase their durability; the characteristic golden appearance of TiN coatings has attracted attention in decorative applications [75]. For instance, jewelry coated with TiN has the visual appearance of gold but does not require frequent repolishing such as genuine gold products.
One of the most large-scale applications of PVD is optical coating. Most headlamp reflectors in the automotive industry are coated with a reflective Al coating, selected for its good reflectivity, silvery brilliance, high adherence, and low price [76]. The headlamp reflectors are usually composed of PC or ABS. Reflective Al coatings on PC also provide flexible mirrors in solarconcentrated technologies [55], and reflective Al films are the default coating choice for PC-based compact disks [77].
Electric and microelectronic applications have considerably boosted the use of PVD-based PM. For example, Al coatings deposited on PC and ABS plastics using electron beam evaporation are employed as electromagnetic shielding materials [50]. Furthermore, Al coatings are used in the fabrication of flexible solar cells, which have garnered considerable interest in the renewable energy sector. Here, solar cells are formed using PVD techniques by depositing the conjugated metal electrodes (e.g., Ag) on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) substrates [70]. Solar cells can be similarly formed using the vacuum evaporation method [70]. The electrodes formed using these methods can realize next-generation ultrathin and ultralight flexible solar cells. The back contact of a solar cell is typically a Mo layer deposited using DC magnetron sputtering [69]. Copper-indium-gallium-selenide absorber layers can also be deposited on the solar cell substrate using vacuum evaporation. The aforementioned numerous discoveries related to solar applications have facilitated the global shift toward renewable energy. Sputtering and evaporation enable the fabrication of soft and stretchable electronic devices that can be adapted to the human body and other free-form surfaces. Such materials are deployable in broad applications, from wearable sensors (such as artificial skin) to flexible displays and soft robotics. Future electronic systems are difficult to imagine without PM via PVD.
PM using PVD has enabled improved wear resistance and overall durability of frequently used polymers. The typically used main coating materials are Cr [59], TiN [32], and CrN [48], and the commonly employed main coated polymers are PC and ABS. Wear-resistant coatings on polymers have been discussed in detail in the previous section and are summarized in Table 1.
In the biomedical field, PEEK has emerged as an alternative to metallic implants [61]. The disadvantages of PEEK-low biological activities resulting reduced bone adhesion-are surface-related problems that can be alleviated using PVD techniques. Many studies have reported improved biological activities of PEEK after metallization. Han et al. [78] deposited Ti on PEEK using electron beam evaporation, and Frutos et al. [56] deposited TiSi thin films on PEEK using magnetron sputtering. PVD metallization can endow polymers with antibacterial activities. An example of this application is the magnetron sputtering deposition of Ag films on PEEK substrates [61].  [69], (e) Conductive Ag coating formed by magnetron-sputtered on PMMA, and (f) antibacterial Ag coating deposited on a PET fabric using magnetron sputtering [71] 3. Electroless plating/chemical plating of polymeric-based materials 3.1.

General characteristics
In electroless plating, metal atoms are deposited on a nonconductive surface via an electrochemical reaction. A polymeric workpiece is immersed in a reducing agent to form a continuous metallic layer of nearly 1-μm thickness. The electroless plating process enables additional electroplating of a thick metallic coating once the surface has become electrically conductive [55]. This method easily allows the preparation of micron-scale metallic coatings on a polymeric part with complex thin-walled geometries using simple equipment. The electroplating process was originally developed for metallic parts in the 19 th century [79] and was first applied to polymeric parts in the North American and European automotive industries in the 1960s [80]. Today, electroplated plastics are widely employed in houses, cars, offices, and furniture. Many of these commodities are not even acknowledged to be made of polymer, due to the shiny metallic coatings, such as vehicle brakes and water taps. The typical metals of choice for electroless plating and electroplating processes are copper, nickel, gold, silver, and chrome [35]. The electroless plating process was the first industrially commercialized PM method and remains the primary and benchmarking process in PM research.
The plating of metallic films on polymers involves two main stages: electroless plating and electroplating. The first stage (electroless plating) involves various chemical processes that modify the surface properties of polymers, mainly to increase its surface conductivity. The second stage (electroplating) provides a thicker coating layer with glossy surface finish. Kuzmik [81] proposed 14 foundational electroplating steps in the classic electroless plating process of nonconductive materials such as ABS, polyethylene, and polypropylene (Figure 3 a). The process proceeds in a series of stages: (i) pretreatment stage that involves cleaning or degreasing (Step 1) and chromic acid etching, followed by polymer surface modification via acid etching, neutralization, and water rinsing (Steps 2-5); (ii) catalytic stage that involves surface sensitization and activation by immersion in different chemical baths (Steps 6-10); (iii) electroless plating stage that involves chemical plating without an external current source (Steps 11-13); and (iv) final electroplating stage that involves the deposition of a thin metal layer on the electroless-plated substrate (Step 14). Subsequently, Kuzmik [81] combined two or more of the steps involving degreasing, Chromic acid etching, sensitization, and activation to prepare a plastic substrate for the electroless plating of metals. More recently, Charbonnier [82] confirmed a different route for the conventional method [81] with combined sensitization, activation, and acceleration steps. Figure 3 b presents schemas of two commonplace electroless plating processes. The first process involves sensitization (dipping in a SnCl 2 solution) and activation (dipping in a PdCl 2 /HCl solution) steps. The second process, which is more practical today, involves activation (dipping in a PdCl 2 /SnCl 2 /HCl colloidal solution) and consequent acceleration (dipping in a HCl or NaOH solution) steps. In summary, polymer electroplating is a multistep procedure wherein a workpiece is placed in more than one dozen chemical baths to form conductive and then coated metallic layers. Only some of these electro-chemical plating routes have been discovered to date.

Reactions and mechanisms of major electroless steps
Before discussing recent advances and enhancements in conventional electroless plating, the most critical steps and mechanisms underlying the coating formation and adhesion strength must be clarified. Herein, the current understanding of the reactions in the electroless steps and their effects on the integrity of polymer surfaces are described. The discussion is based on recent studies and our electroplating experience.
Cleaning or degreasing is performed to remove greasy substances from the polymer surface after the injection molding process and eliminate remnant particles and dust from handling and packaging. During the cleaning step, the polymer plate is immersed in a bath containing an alkaline surfactant (NaOH). This step can increase the wettability of the surface by reducing the surface tension. Appropriate degreasing functions as a pre-etching process that preliminarily modifies the initial surface morphology and chemical properties [83].
The etching step is conducted to promote the formation of useful functional or chemical groups (chemical modification) and roughen the surface (morphological modification). Both modifications improve the chemical adhesion and mechanical interlocking at the polymer-metal interface. Etching is typically performed by immersing the polymer substrate in a bath containing a mixed solution of chromium trioxide (CrO 3 ) and sulfuric acid (H 2 SO 4 ) (called sulfochromic acid) at 65°C-75°C . Here, we describe the chemical and morphological modifications of a commonly electroplated polymer-ABS-as an example. The hexachrome group in the sulfochromic etching solution removes or dissolves butadiene rubber particles via a redox reaction involving -CH double-bond opening to create a -COOH functional base and other polar groups, such as -SO 3 H or -CONH. The oxidation process consumes large amounts of acid and Cr 6+ and produces large quantities of Cr 3+ and H 2 O. The full oxidation of a 1:1:1 acrylonitrile:butadiene:styrene copolymer occurs as follows [84]: . During etching, the butadiene rubber particles are removed and the remaining micropores enlarge the total surface area of ABS. As butadiene particles are generally etched at a considerably faster rate than the styrene acrylonitrile (SAN) phase, optimizing the etching duration and concentration of the etching mixture is recommended [85]. Excessive exposure of the polymer surface to acid-called overetching-can reduce the interlocking potential of the polymer topography and weaken the overall ABS. Over time, it causes the discoloration and severe degradation of the ABS surface. Overetching occurs when a high concentration of etching mixture is applied over a long duration. After performing experiments using various grades of ABS polymers, the optimal etching duration was found to vary between 3 and 10 min (Figure 4). At this etching duration, the level of interlocking guarantees the cohesive failure of the coated film during peeling. Exposure durations exceeding this range do not increase the peeling forces. We will soon present these findings in a separate report. Kato et al. [63] investigated the effect of the sulfochromic etching duration on the morphology of two commercial grades of ABS along the thickness direction ( Figure 5). Long etching durations severely damaged the polymer material. The effects of different etching stages (underetching, optimum etching, and overetching) on an ABS surface were schematically presented by Bucknall [86] (Figure 6 (a)). The synergy between the etching effects on the surface porosity and functional groups transforms the nature of the polymer surface from hydrophobic to hydrophilic. Hydrophilicity ensures the pore penetration of the liquid solutions in the subsequent sensitization and activation steps and the deposition of a palladium seed layer for consequent chemical plating. After etching, the micropores and their interconnected cavities provide numerous sites for the mechanical interlocking (or anchoring) of the ABS surface and chemically plated metallic layer [87] (Figure 6 (b)). Therefore, the removal of butadiene rubber particles provides the desired roughness and mechanical interlocking for strong adhesion to the deposited metallic layers ( Figure 6 (c)). Such an appropriately triggered mechanical interlocking ensures coating adhesion strength because the fracture process is confined to the ABS substrate and is prevented at the metal-ABS interface [88]. Additionally, the etched surface has well developed surface area which facilitates the chemical adhesion. Thus, it is difficult to claim which of this adhesion factors is the governing one. We recently offered two strategies for decoupling the chemical and mechanical aspects of adhesion: surface passivation [89] and manipulation of the interfacial area [90,91]. The surface morphology appeared a significant factor in overall adhesion whereas, surface wettability is crucial for subsequent sensitization and chemical plating steps. For a detailed review of this topic as well as quantitative estimation of chemical and mechanical adhesion aspects refer to our experimental reports [90,92,93] and review [94]). In summary, the etching process provides both wettability and porous topography on the ABS surface, which are essential for coating formation and to achieve robust adhesion between metal-plastic. Peeling of the metal layer from an ABS substrate with strong mechanical interlocking features [86] The sensitization step is performed in stannous chloride/hydrochloric (SnCl 2 /HCl), which provides Sn 2+ sites for palladium deposition. The basic reaction during the sensitization is thought to proceed as follows [96] (where the subscripts sol and ads denote solution and adsorption, respectively): . The activation step forms an atomic catalytic layer on the polymer surface. The activation agent (catalyst) is a precious metal such as palladium, which is deposited (seeded) on the porous surface (bonding sites) to nucleate the following deposition of a thin metallic layer during the electroless plating step: . In this reaction, the divalent tin (Sn 2+ ) ions reduce the palladium ions. Although metallic palladium nuclei (Pd 0 ads ) are produced on the substrate, the adsorbed Sn 2+ ions (Sn 2+ ads ) are oxidized to Sn 4+ and then desorbed from the substrate [96]. This reduction occurs at a tin:palladium ratio of 2:1 [84]. The catalyzed (sensitization + activation) part is then rinsed with water and chemically accelerated to remove the nonreactive parts and eliminate the excess stannous hydroxide surrounding the palladium metal (noncatalytic ionic components). The intact palladium remains for electroless plating. This step also eliminates excess colloids, thereby avoiding unwanted chemical deposition.
The electroless (also called "chemical") plating step enables a continuous thin metal layer (typically, a 0.5-0.7-μm-thick copper layer [83]) that facilitates the subsequent electroplating processes. When the palladium-activated part is immersed in a metal:formaldehyde bath, the metal is reduced at the palladium sites while the autocatalytic reaction proceeds till the substrate is in the solution. The pH level plays a critical role in this reaction process. The basic reaction of electroless copper plating is [35] . Based on the immersion duration, the electroless plating step in a cupric sulfate:formaldehyde bath (Cupric sulfate CuSO4.5H2O, potassium sodium tartrate tetrahydrate NaKC4H4O6.4H2O, and Formaldehyde HCHO mixed in proportion of 15 g/L, 30 g/L, and 100 ml/L, respectively) generally increases the surface conductivity of ABS (P400). The electrical resistance of ABS can reach 90-110 kΩ after immersion for 24-48 h.
In the final electroplating step, the desired metal coating is transferred from the immersed anode (a bulk metal piece, e.g. copper plate) to the cathode (the polymeric part to be plated) via controlled electrolysis. Chemically plated polymers are electroplated using the same process as metallic parts [79,80,83,84,97], and their discussion is omitted for brevity.

3.3.
Recent advances in ecofriendly electroless plating The standard electroless procedure, in which plastic surfaces are etched in sulfochromic acid, triggers health and environmental concerns. Sulfochromic acid contains toxic Cr 6+ , which are carcinogenic, mutagenic, allergenic, and potentially harmful to reproduction when inhaled [98][99][100]. Moreover, the sulfochromic acid solution discharged as waste pollutes the environment. Chromium enters the air, water, and soil in the form of Cr 3+ and Cr 6+ . Chromium accumulation in plants retards their development and induces morphological changes. In soil and water, chromium accumulation reduces the crop yield and grain quality [98]. To avoid these hazards, the concentration and volume of chromium in etching baths are highly controlled under different directives (e.g., the regulations of the Environmental Protection Agency under section 112 and Seveso II (96/82/CE)). The mishandling of the etching process and noncompliance with regulations have cost the electroplating industry millions of dollars in fines (see the EPA achieve [31][32][33][34]). In the next five years, the electroplaters in Europe and China will need to replace sulfochromic etching with more environmentally friendly alternatives in electroless plating.
The environmental factor has driven active searches for ecofriendly alternatives for preparing polymer surfaces for chemical plating. Researchers in academia, the electroplating industry, and polymer production have explored alternatives for hexavalent chromium in the polymer etching process with various degrees of success [35]. The challenge is not only to remove the need for hazardous chromic-acid etching processes but also to provide a drop-in technology that requires minimal changes to current electroplating infrastructures and production facilities. Recent studies and developments of some ecofriendly solutions for the electroplating of ABS are summarized in Table 2 and discussed in-depth in the subsequent section. Laser ablation [112] Optophysic treatment [87] Plasma etching [85,113] Catalytic etching Photocatalytic [114,115] 3.3.1.

Nonetching approaches
Several nonetching approaches for polymer electroplating have recently been developed. In most of these approaches, the polymer is premixed with an additive element; alternatively, a type of glue is employed. Despite some positive outcomes, these approaches can affect the mechanical properties of polymeric parts and deliver lower coating adhesion strength than that achieved using conventional procedures. In one example, ABS, which is originally hydrophobic and nonconductive, was blended with conductive polyether-ester-amide (PEEA) using the injection molding process [101]. After blending, the copolymerized ABS was infused with Pd(II) hexafluoroacetylacetonate (Pd(hfa) 2 ) assisted by supercritical CO 2 (scCO 2 ). Here, scCO 2 improves the boundary and clarity of the butadiene and PEEA phases. The Pd(hfa) 2 infusion into copolymerized ABS was performed for 60 min in an autoclave set to 10 MPa and 80°C. To remove the CO 2 content, the Pd-infused ABS was placed in a vacuum oven for 24 h. In the electroless plating process, the substrate was first immersed in a mixture of ethanol and Ni-P plating solution at 65°C. Ethanol improves the diffusivity of the substrate, enabling easier coating. The Ni-P metal was then grown around the Pd nanoparticles, agglomerating with the other metal particles until a metallic layer is formed on the substrate. This layer anchored to the ABS substrate, generating a strong polymer-metal adhesion by up to 0.91 N/mm in the coating peeling test. However, the tensile strength further decreased to 30.3±0.7 MPa when 20% PEEA was copolymerized, which could be critical to strength-sensitive applications. Bazzaoui et al. [102,103] deposited polypyrrole (PPy) on ABS and rendered it conductive via oxidation. In this process, ABS was first immersed in a 0.3 M pyrrole solution (75 mL) for 1 h. The solution was then stirred, and a 0.9 M FeCl 3 solution (25 mL) was added dropwise to the stirred monomer solution. The polymerization was completed after 2 h of constant magnetic stirring. The synthesized PPy/ABS was rinsed with a mixture of distilled water and methanol for several times. After drying, the ABS was fully coated with PPy. The electrolysis was performed in a 200-mL beaker using PPy/ABS as the cathode and copper or nickel as the anode. The use of PPy in polymerization is quite promising because no hazardous chromic-acid etching and electroless steps are required. However, the peeling strength and surface characteristics (roughness and contact angle) obtained using this process have not been reported. Grafting is another technique that forms an activated and rough interphase between the plastic surface and coated metal layer [75]. Garcia et al. [75] dissolved 0.1 M 1-4-phenlyenediammonium dihydrochloride in a 0.5 M HCl solution and then added 0.1 M NaNO 2 (15 mL) stepwise, yielding an aryldiazonium salt. Next, acrylic acid (15 mL), an ABS substrates, and iron powder (2.25 g) were successively added to the solution. After immersion at 38°C for 2 h, the ABS substrate was rinsed with 1 M NaOH and deionized water (DI). Finally, as a copper activation step, the PAA-modified ABS substrate was immersed in a solution containing 0.6 M NH 3 and 0.1 M CuSO 4 .5H 2 O for 10 min at room temperature. The adhesion abilities of the electroless-plated substrates were evaluated using a simple Scotch tape test. In particular, a regular tape was bonded on the ABS surface with a grid made/notched using a sharp knife. The test is considered passed if no (or minimal) metal coating could be detached from the ABS grid. Electroless-plated copper and nickel on metallized ABS passed the Scotch tape test with PAA grafting but failed without PAA grafting [75]. However, this semiqualitative test lacks a competitive claim of the actual peeling strength. In summary, nonetching approaches are effective to some extent; however, their ecofriendliness trades off the mechanical properties of the polymers. Furthermore, as mechanical interlocking by the nonetched polymer topography is absent or reduced, nonetching approaches cannot exploit the cohesive failure mode of the polymers during coating delamination and thus cannot guarantee competitive coatingadhesion strength.

3.3.2.
Wet etching using mild solutions The replacement of sulfochromic acid with nonhazardous, mild solutions for the wet etching of ABS has recently received considerable attention [105,[108][109][110]116,117] [117], diluted nitric acid and a combination of oxidants [107], or chemical reagents and low-concentration sulfochromic acid [111]. Tang et al. [108][109][110] etched ABS in a mixture of H 2 SO 4 and H 2 O 2 at room temperature for 5 min. The sensitizing and activation processes were performed using chitosan at room temperature for 5 min and using a nickel sulfate solution, respectively. During the activation process, the bonding between chitosan and ABS was improved by the formation of chemically stable hydrophilic functional groups. This etching technique afforded a rough surface with some observable cavities on ABS, but its performance was not validated in the coating adhesion test. Teixeira and Santini [116] incorporated H 2 NO 3 into the H 2 O 2 -H 2 SO 4 mixture reported by Tang et al [108][109][110] as an etching solution for ABS. The etching was performed at 60°C-65°C for 5-15 min. Similar to the finding reported by Tang et al., Teixeira and Santini [116] found that the etching process could roughen the ABS surface. The specimens etched in their proposed solution passed a simple Scotch tape adhesion test. However, this semiqualitative test does not enable competitive claims. If the mixed solution is too mild to entirely remove the butadiene from the ABS surface, the cavities may provide a reduced extent of mechanical interlocking compared with that obtained using conventional chrome etching. In another promising approach, Li et al. [105] mixed H 2 SO 4 with MnO 2 colloids as an ABS etchant at 70°C. They investigated the influence of the etching duration and MnO 2 content on the surface morphology. When the MnO 2 content and etching duration were 30 g/L and 20 min, respectively, the surface roughness and adhesion strength reached 320 nm and 1.19 N/mm, respectively. Etching durations of 10 and 30 min reduced the average surface roughness (Ra) to 72 and 150 nm, respectively, and the adhesion strength to 0.12 and 1.1 N/mm, respectively. After etching ABS in 50 g/L MnO 2 for 20 min, the Ra roughness of 388 nm was achieved. Although this procedure seems to be feasible for an ecofriendly approach, the adhesion strength was not measured; hence, the influence of the MnO 2 content could not be ascertained. This colloid mixture introduced cavities and reduced the contact angle to 25.7°. The etched surface was enriched with -COOH and -OH groups, improving the adhesion. To facilitate the polymer etching process, Zhao and Wang [117] added Na 4 P 2 O 7 and H 2 O to a mixture of H 2 SO 4 and MnO 2 . Etching was performed at 60°C for 10 min. This approach enabled numerous cavities on the ABS surface and reduced the contact angle to 31.2°. The adhesion strength was maximized at 1.3 N/mm when the Na 4 P 2 O 7 content was 58 g/L and the H 2 SO 4 concentration was 12.9 M. In summary, several alternative ecofriendly routes for wet etching have been proposed. Most, if not all, of these alternatives combine sulfuric acid with less toxic acids and other chemicals. Although they achieve reasonable adhesion strengths, the etching durations are one order of magnitude longer than those of conventional sulfochromium etching methods.

3.3.3.
Dry etching using energetic treatments Dry etching has been performed using energetic treatments, such as laser treatments [118], optophysical method [87], or atmospheric pressure plasma treatment (APPT) [85]. These ABS surface treatments were developed as replacements of toxic sulfochromic etching. The aim is to specifically remove butadiene particles from the ABS surface and subsurface or to simply form patterns on the ABS surface. Zhang et al. [118] first mixed ABS with a metal oxide (e.g., CuO, Cr 2 O 3 , or CuO-Cr 2 O 3 ) and targeted a 1064-nm 10-W fiber laser on the ABS surface. The treated ABS surface was cleaned with alcohol and DI water and then subjected to electroless copper plating. This strategy was successful only in the case of a mixture of ABS and CuO-Cr 2 O 3 , where a B pass (good adhesion ability) was achieved in a Scotch tape test after electroless Cu plating. However, the laser treatment apparently reduced the wettability; the contact angle of ABS and CuO-Cr 2 O 3 after the treatment was 134°, while it was 92.5° on neat ABS, suggesting that the laser-induced microstructure did not improve the surface wetting and chemical work of adhesion. Cacho et al. [87] introduced the optophysical method, in which ABS was treated with a combination of corona discharge and UV light exposure. The specimen was set on a table and moved back and forth under a corona discharge (20 kV and 1.6 A) or UV light (365 nm and 125 W). Under both irradiations, the source-specimen distance was 5 mm. Using the optophysical method, the functional groups on the ABS surface could be modified; in particular, carbonyl groups (C=O) and hydroperoxide (-COOH) were formed and enhanced and polybutadiene particles were removed. The corona discharge step particularly increases the oxygen content on the ABS surface, and its high voltage (20 kV) selectively ablates (or removes) the polybutadiene particles. Such a high voltage decreases the threshold height of electron capture than that of UV treatment; the energy and charges are converted to thermal agitation that causes the dielectric and mechanical breakdown of the polybutadiene particles (the insulating material in ABS). The resulting microholes on the ABS surface increase the overall ABS roughness, reducing the contact angle to 36°. The peel strength between ABS and the electroplated metallic coating reached 0.81 N/mm. Song et al. [85] proposed an APPT prior to electroless plating, observing that APPT selectively removed butadiene particles from the surface. The APPT effectively improved the surface heterogeneity and reduced the water contact angle to 15°. The adhesion strength gradually improved as the treatment duration was increased from 30 to 120 s. The peel force measured on the specimen reached 1.8 N/mm after the APPT for 120 s. Unfortunately, the mentioned studies did test the same grade of ABS polymer and the obtained adhesion forces should be compared directly. In summary, dry etching methods affect the ABS surface in two ways: they render the surface more hydrophilic and/or selectively etch the butadiene particles. Both ways facilitate adhesion, either by enhancing the chemical interactions or by mechanically interlocking the substrate-coating interface. The drawback of dry etching methods is the added pretreatment step. The existing infrastructures of electroplating facilities disallow a simple switch from wet to dry etching. Moreover, the uniform dry etching of free-form parts is technologically challenging. Upscaling the dry etching process to the production volumes of wet etching presents another manufacturing challenge. An alternative route to greener electroplating may consist in combining the in-mold texturing (e.g. hot embossing [119]) with quick acid etching. We recently tried this approach and achieved two times higher adhesion strength while reducing the etching time threefold as compared to the conventional treatment in sulfo-chromium acid [120,121]. Further research on this approach is of high interest for electroplaters and automated equipment manufacturers.

Applications
The electroless plating process and subsequent electroplating are perhaps the most widely applied PM techniques. Electrochemical plating endows plastic materials with several additional functions such as abrasion resistance, electrical conductivity, and (mostly) decorative effects [75]. Polymer electroplating is performed in many industries [35,79,80] such as microelectronics, construction and civil engineering, toy production, housing utilities, jewelries, and automotive and aerospace industries (Figure 7). The use of polymers (thermoplastics and thermosets) in these industries is increasing, particularly in the manufacturing of secondary parts by the automotive and aerospace industries. Electroplated polymer parts offer three main advantages over conventional metal parts: (i) higher fuel efficiency of vehicles owing to a decrease in the total weight, (ii) cost reduction because plastics are cheaper than metals, and (iii) safety enhancement because polymeric parts are nonconductive and thereby prevent human electrocution. Plastic electroplating imparts an aesthetic aspect by providing a metallic look or decorative luster. Metallization complements the high flexibility of polymers in processing, forming/shaping, and coloring.
Among the electroplated polymers are ABS, polysulfone, polytetrafluoroethylene (PTFE), polyoxymethylene (polyacetal), PPy, PC, and phenolic acid. The vast majority (90%) of electroplating applications worldwide involve ABS, which is favored for its high rigidity, excellent abrasion and impact resistances, good electrical insulation properties, high weldability and dimensional stability, high melting point, low cost, remarkable surface quality and recyclability. ABS is occasionally blended with PC to achieve specific properties [107]. Polymers are usually electroplated with copper (for low cost and high electrical conductivity), nickel (for corrosion or wear protection), gold or silver (for aesthetic appeal), or chrome (for a smooth surface finish, good corrosion resistance, and aesthetic appeal). General characteristics Thermal spraying has evolved over the last three decades, becoming a prospective approach for depositing almost any material on any substrate. The term thermal spraying embraces a family of methods, in which metallic powder is sprayed as a coating layer on a solid surface [122]. The layer is formed by the melting and deformation of the spray particles during flight or on impact. The heat for melting the sprayed powder is provided by electrical, chemical, or kinetic energy sources, depending on the method employed ( Figure 8). All methods use a gun in which the feedstock material for deposition (the coating precursor) is either melted or accelerated using an electric arc, flame, plasma, or preheated gas and propelled toward the target. The coating precursor is a metal, plastic, or ceramic in the form of powder, wires, or rods that are separated into particles within the jet and stick to the target on impact. When the target is fully covered by the impacting particles, the later incoming particles stick to other particles and the coating layer is achieved. Various thermal spraying methods and their thermal spray characteristics are mapped in Figure  9 [24,[123][124][125].
The primary concern in PM based on thermal spraying is the survivability of the workpiece at the working temperature of the coating process [126]. The steady-state temperature of the workpiece can remain low; however, the local temperature increases because of single-particle impact, threatening the underlying polymers. The average substrate temperature can be controlled to below 150°C, which is compatible with most polymers. However, the temperature generated at each individual impact of a supersonic particle can reach thousands of degrees [127]. This thermal effect facilitates the formation of metallurgical bonds between the particles and the impacted metallic substrate but is detrimental to the polymeric substrates. The sprayed powder, particularly when melted, can degrade the mechanical properties of the polymeric parts [126]. From this perspective, cold spraying and other spray methods with reduced thermal intensity are more suitable for PM than plasma spraying. Apparently, each combination of coating-substrate methods can construct a coating without thermal or erosive damage only within a narrow window of processing parameters. More details on the difference between spraying on metal and polymeric substrate were elaborated in [14].
The process parameters of thermal spraying can be categorized into three main categories: jet energy, feedstock materials, and spraying parameters. The jet energy is controlled by the electrical settings forming the arc or plasma and the type, pressure, and temperature of the gas. The jet energy largely affects the temperature, velocity, impact energy, and deformability of the sprayed powder and thus the overall deposition efficiency, which can be as high as 70% [128]. The rest of the powder is simply renounce back from the target. The feedstock material is determined based on the type of metal (typically, a soft and low-melting-point metal), powder size (10-100 µm), particle shape (such as spherical, angular, and dendritic) and flow rate (up to 1 g/s [129]). The spraying parameters enable the tuning of the powder impact energy, workpiece heating, and coating area. These parameters include the nozzle diameter (1-3 mm), nozzle standoff distance (10-200 mm), gun travel speed, and number of passes. The large variability of deposition parameters enables diverse coating characteristics.
Thermally sprayed coatings can be several millimeters thick and can be deposited over a larger area of free-form parts at higher deposition rates than other coating processes. Sprayed coatings on polymers are characterized by high porosity (up to 20% for flame spraying [130]) or low porosity (below 1% for cold spraying [131,132]), high adhesion strength (up to 20 MPa [128,133]), high electrical conductivity (up to 50% of the bulk [132]), and high hardness. Most thermal spray applications are passive protective coatings; however, electronic [122] and biomedical device [134] applications are emerging.

Flame spraying
Flame spraying (FS) is a thermal spray process in which feedstock materials-such as powder, rods, or wire droplets-are melted and accelerated in a flame of combustion gas, such as acetylene or oxygen [14]. The melted powder is then propelled toward the target via a nozzle in a flowing inert carrier gas such as argon. The particle velocity in FS is usually below 100 m/s [135]. The energy for powder deformation during deposition is mostly sourced from the flame jet; hence, the flame temperature should be sufficiently high to melt the metallic powder [14]. The flame temperature range for most metals is 2000°C-3000°C [135]. The FS process is controlled based on the fuel:oxygen ratio, carrier gas pressure, nozzle geometry, and standoff distance.
The FS metallization of polymeric-based substrates has mostly been studied for Al powders because aluminum quickly melts in the jet and exerts the minimal thermal impact on the polymeric surface. Electrically conductive coatings of Al-12Si have been sprayed on PU elastomers [136]. Both powder and wire feed materials have been used to successfully form electrically conductive Al coatings on polyester textiles, causing no thermal or chemical degradation to the fabric [137,138]. Increasing the thickness of the Al layer on the polymer fabric proportionally increases the electrical conductivity while retaining the flexibility of the textile. Spraying glass fiber-reinforced plastic (GFRP) with a thin Al-12Si layer improves the heat transfer through the entire coating-polymer ensemble, increasing the potential applicability of FS to novel deicing systems in the aerospace sector [139]. A 100-μm-thick NiCrAlY layer deposited on FRP plates using the FS process was demonstrated to prevent ice accumulation on polymer structures exposed to frost conditions [140]. A 90-μm-thick Zn-Al alloy layer with a porosity of <1% and an electrical response of 2682 S/m was deposited on ABS, with no evidence of cracks or rust in the coating [141]. The typical porosity of flamesprayed aluminum coatings reaches 20% [130], higher than those achieved using most thermal spray methods. To achieve a low porosity, a high jet temperature is required; however, the upper limit of the jet temperature is set based on the polymeric substrate. Overall, only low-melting-point metals can be deposited on polymers via FS.
Despite the appropriate selection of powder, thermal degradation can occur. For example, FRP structures can be damaged after spraying with aluminum coatings [142]. Thus the nozzle is typically kept at 100 -200 mm distance from the substrate, which is for an order of magnitude larger that in cold spray. To prevent the FRP from thermal damage and erosion, Therrien et al. [112] employed a garnet-sand interlayer that thermally insulated the composite fibers and enhanced the mechanical interlocking of the Al-12Si coating on the FRP workpiece [143]. The thermal impact on the substrate was reduced by injecting air into the FS torch during deposition. Under a sufficiently large air supply, the flame temperature and thermal load on the PU substrate were reduced and the mechanical properties of the substrate were preserved [136]. Damage to a coated substrate can be evaluated by correlating the intentional tensile deformation of the substrate with the change in the electrical resistance of the coatings [144]. Using an analytical model [136], the dependency of the thermal field in polymeric substrates on the air pressure, standoff distance, and other FS process parameters can be determined. Apparently, the melting of particles and the thermal field in the substrate are the key controlling factors of the FS coating process, which can be explored in future research.

4.3.
Wire arc spraying In wire arc spraying, the deposition material is melted using an electric arc and accelerated using a gas stream. The electric arc acts between the tips of two continuously feeding wires. The wires melt into droplets at their tips. These molten droplets are blown away using a high-velocity gas stream. The gas stream performs three functions: 1) forms large primary droplets by removing the molten metal from the wire tips; 2) atomizes these droplets into secondary microscale particles; and 3) accelerates the particles toward the substrate [145]. Increasing the gas velocity reduces the sizes of the primary and secondary droplets and facilitates their acceleration but also amplifies the oxidation of particles and finally elevates the oxide content in the coating. The oxidation problem can be avoided using a shroud and a nonoxidizing gas such as nitrogen or carbon dioxide [145]. Moreover, increasing the droplet velocity improves the deposition efficiency and most of the coating properties. Because its production cost is nearly three times lower than that of FS [146], wire arc spraying is potentially usable in industrial PM.
Several researchers have attempted to deposit metals on polymers via arc spraying; however, most of these efforts have encountered a tradeoff between poor adhesion strength and substrate damage. Fortunately, this tradeoff can be mitigated using specific surface conditioning. Liu et al. [126] arc-sprayed a soft metal on a CFRP substrate as an interlayer for the subsequent deposition of a wear-resistant WC-Co coating. The interlayers formed from materials with high melting points-Ni or Cu-were easily delaminated from the substrate, presumably because of thermal damage. Al and Zn are more suitable materials for deposition. With melting points above the upper critical temperature endured by the CFRP but lower than those of Cu or Ni, these materials enable a lower extent of thermal damage. However, the bond strength of the arc-sprayed Al coating was only half that of the plasmasprayed Al coating (7.5 vs. 14.2 MPa). Sandblasting prior to arc spraying enhanced the coating bond strength by nearly 20% (9.4 MPa) [147]. A uniform Zn coating was achieved on an CRFP substrate; however, the mechanical properties of the substrate decreased by nearly 15% [148]. To avoid substrate damage, researchers later covered the CFRP surface with epoxy overflow, copper powder, or aluminum mesh before spraying Zn [148]. A powder layer protects the carbon fibers but deteriorates the adherence of the coating layer to the substrate, as evidenced by the easy delamination of the powder during a bending test. Alternatively, the aluminum mesh increased the adhesion strength by ~50% compared with that of the coated substrate with no protective layer [148]. Devaraj et al. [149] sprayed (260 ± 20)-μm-thick Al and Zn coatings on two thermoplastic substrates, namely, ultrahigh-molecular-weight polyethylene (UHMWPE) and PTFE. Increasing the surface roughness enhanced the mechanical interlocking of the coating bond that nearly doubled the overall pull-off force. Similarly, increasing the initial substrate temperature from 20°C to 100°C almost doubled the adhesion strength because the softened polymer allowed the deep penetration and anchoring of the incoming Zn particles. However, low-density Al particles lacked sufficient kinetic impact energy to deeply embed in the substrate, particularly in hard UHMWPE. Therefore, the arc-sprayed Al layer adhered to PTFE to some extent but could not adhere to UHMWPE. Clearly, the metal-coating bonding to thermoplastics can be elevated by preheating the part before arc spraying.
Owing to its low cost and processing simplicity, the wire arc spraying of polymers is eminently suitable for producing erosion-and corrosion-free parts in the civil and transportation industries. Low resistance to solid-particle erosion has limited the use of polymer composites in aerospace applications. The strength, stiffness, fatigue, and corrosion resistance of PC can meet the requirements of turbine engine parts; however, its poor resistance to heat and erosion by dust particles are major obstacles. Arc spraying has an impressive record of preventing erosion and corrosion damage to metal parts, including bridges, pipelines, and offshore equipment [146]. Accordingly, this approach has been employed as a protective-coating technique in PM [126,147]. A 100-µm-thick Al erosion-resistant coating formed via arc spraying improved the thermal fatigue and erosion resistance of PMC [147]. In a thermal fatigue test of the coated sample, no cracks were found after 50 cycles of heating to 370°C and cooling in air. Only minor cracks were detected after a further 43 cycles quenched in water. In an erosion test, the mass and thickness losses of the coated PMC were one-half and only one-tenth those of the uncoated PMC, respectively. Furthermore, the annual corrosion rate of arc-sprayed Al coatings at an elevated temperature (70°C) is five times slower than that of steel [146]. Evidently, arc-sprayed coatings can improve the thermal fatigue and erosion resistance of the polymeric parts.
In summary, low-melting-point metals-such as Zn and Al-can be deposited on both thermoset and thermoplastic polymers. Arc-sprayed coatings can effectively protect large-scale polymeric structures from erosion, corrosion, and thermal cycling damage. Future research should focus on upscaling the arc spray technologies for application to numerous and massive structures. The formation of erosion/corrosion coatings via wire arc spraying for protecting polymeric parts installed in tidal-and water-immersion zones will show significant prospects.

Plasma spraying
Plasma is formed by the electrical discharge of electrons between a tungsten electrode and a copper-based nozzle inside a deposition gun. The arc is stabilized using a gas, such as argon, helium, nitrogen, or hydrogen, or a mixture of these gases. Furthermore, the gas is heated, accelerated, and partially ionized using the arc, affording a hot plasma jet (up to 12,000°C in the core region). A separate stream of carrier gas delivers metallic powder into the plasma jet, where the powder is quickly melted and accelerated up to 600 m/s. The melt impacts the substrate, spreading and solidifying in the form of splats and eventually forming a lamellar-structured coating. The process can proceed under various conditions, such as vacuum, low-pressure, inert-gas, and underwater conditions [155]. Atmospheric plasma spraying is the most cost-effective and widely used plasma-spraying process and is discussed further.
Plasma spraying is not commonly used for metallizing polymers and composites, mainly because polymers are considerably less thermally stable than metallic or ceramic substrates. As plasma generates considerably higher temperatures than other thermal spray methods (Figure 9), it requires a more accurate regulation of the process parameters that affect substrate heating, such as the distance of the plasma torch, gas and plasma temperatures, and cooling system. Beydon et al. [156] applied an intermediate thermoplastic layer on a CFRP as a bond coating to enhance the adhesion of the subsequently sprayed copper powder. The bond coating captured and anchored the molten copper powder even after aging; thus, it could be deposited on CFRP parts long before spraying. Guanhong et al. [157] sequentially plasma-sprayed aluminum and alumina powders on CFRP, affording a well-adhered multilayered wear-resistant coating. The shear adhesion strengths of the bond (aluminum) and top (alumina) layers were 5.2 and 1.1 MPa, respectively. To achieve top coatings with higher adhesion strength than the bottom coating, polymers with an increased thermal resistance and cooling of the substrate during spraying have been recommended. In another study [151], the aluminum and alumina coating layers were deposited using a combination of plasma spray and microarc oxidation. Affi et al. [158] deposited an aluminum interlayer on a CFRP substrate via plasma spraying and topped it with another layer via cold spraying. The cold-sprayed aluminum layer showed a lower volume resistivity than the plasma-sprayed aluminum coating because the high operational temperature in the case of the latter facilitated the oxidation of particles. Therefore, highly conductive coatings should be fabricated at decreased temperatures. Huang et al. [159] sequentially coated quartz fiber-reinforced polyimide with aluminum and zirconia powders via plasma spraying, forming a thermal barrier coating. In a thermal ablation test above 800°C, the weight loss of the sample was three-fold lower than that of the uncoated polyimide. Later, CoNiCrAlY and Zn powders were successfully sprayed as a bond coating on a wear-resistant zirconium top layer [160]. Coating failure was attributed to residual-stress degeneration during bonding, thermal stress induced by the mismatched coefficients of thermal expansion of the bonded materials, and substrate oxidation owing to high processing temperatures. Wypych et al. [161] deposited a low-porosity titanium layer on polyethylene and glass fiber-reinforced polyamide via plasma spraying. Being composed of Ti and its oxides, the coatings showed a very high hardness of nearly 400 HV and a Young's modulus of 200 GPa. Ganesan et al. [150] demonstrated that surface pretreatment notably affects the coating bond strength. A chemically etched and brit-blasted CFRP achieved higher adhesion strength than a sample thermally treated using a plasma plume. The enhanced adhesion strength was most likely associated with the improved surface roughness; however, the chemical affinity between the polymer and copper splats may also have contributed. The initial substrate roughness, spraying distance, number of layers, and cooling time between the layer depositions must be appropriately selected to achieve coating interlocking and avoid the thermal deformation and degradation of the polymer substrate [162]. The various latent physical, chemical, and mechanical interactions between the striking metal particles and polymeric substrate during plasma spray require further quantitative analyses.
Atmospheric plasma spraying is the most widely used technology for applying bioactive coatings on metallic and ceramic orthopedic prostheses. Moreover, over the past three decades, it has been continuously applied to polymeric prostheses ( Figure 11). PEEK and carbon fiber-reinforced (CRF) PEEK are the most promising replacements of metallic materials for multiple biomedical devices intended for compromised tissue substitution. The plasma-spray deposition of bioactive hydroxyapatite (HA) and titanium on PEEK was first reported in the 1990s; however, the sprayed coatings were very weakly bonded (bonding strength < 3 MPa) [163]. Later, Auclair-Daigle et al. [164] reported a specific polymer-surface preparation that boosted the bonding strength between the HA coating and CFR-polyamide to an outstanding 23 MPa. Further, Beauvais and Decaux [165] reported a chemically compliant HA coating on PEEK that preserved the tensile, flexural, and impact strengths of PEEK; unfortunately, the adhesion strength between PEEK and HA was quite weak (7.5 MPa, half that of the ISO requirements). Bureau et al. [166] achieved an HA adhesion strength of 21 MPa on a CFR-PEEK by overmolding an HA interlayer on the PEEK substrate before plasma spraying. Comparable strengths of HA and bioactive titanium oxide coatings were achieved via vacuum plasma spraying (refer to the literature [155] for review). One disadvantage of plasma spraying is the high degree of crystallinity in plasma-sprayed HA coatings [155]. The chemical degradation of PEEK caused by plasma-emitted UV radiation is another concern. Even in the absence of oxygen, direct UV absorption causes polymer-chain scissions with the formation of hydroxyl and ester groups, which degrade the mechanical properties and discolor the surface [167]. However, a very recent study [152] reported a PEEK substrate with a defectless microstructure and no chemical change after the deposition of combinations of Ti, TiO 2 , and HA powders using atmospheric plasma spraying. Since it is one of the most promising and commercialized applications, future research can be focused on achieving higher crystallinity of HA coating during plasma spray.  Figure 11. Thermal spray applications to polymer metallization (clockwise starting from 12:00): PEEK-based knee joint [168], titanium sprayed on PEEK [169], a coated knee joint (Marle Orthopaedics Manufacturing Ltd), electrical circuit written on a polyethylene film [122], a conductive flexible fabric [137], copper particles embedded in HDPE [170], a fouled CFRP [171], a fouled polymer-hulled boat and underwater column (open source), lightning strike [8], an aircraft part (bond jumper) after a lightning strike [172], and the cold spraying of a wing [173] 4.5.

Cold spraying
Cold spraying is a powder coating technology in which metallic particles are bonded to the substrate and to each other by increasing the local temperature on impact. Since its application to polymers only a decade ago, this additive manufacturing method has garnered tremendous momentum. Half the studies on PM using cold spraying have been published in the last three years (2018-2021) [22]. Moreover, the first three literature reviews of cold spraying [22,25,26] have been released in the past year (2021). The subject has emerged quickly, and we summarize the key findings reported in the literature [22]. The simulation of the process fundamentals can be found in [25,174].
Whether cold spraying belongs to the family of thermal spraying is arguable because the supplied thermal energy accelerates the powder particles rather than melting them. In cold spraying, a preheated gas-such as helium, nitrogen, or air-is supplied to a converging-diverging nozzle, in which the heat energy increases the gas velocity. Before exiting the nozzle, the gas stream is mixed with a metallic powder. The momentum of the gas motion is transferred to the powder, accelerating it to a supersonic speed before impacting the target. Upon impact, the kinetic energy of the metallic particles is converted to the deformation energy and finally reverts to heat, melting and adhering the particles to the target [175]. The cold spraying process can be comprehended as sandblasting but using metallic particles and a hot gas. With such small modifications, sandblasting becomes a material deposition process rather than a material erosion process. Remarkably, cold spraying was discovered during the investigation of high-speed solid-particle erosion in the 1990s [176]. In the past three decades, this approach has also been variously described as cold-gas dynamic spraying, kinetic spraying, supersonic particle deposition, and dynamic or kinetic metallization [14,24,[177][178][179]. Among thermal spray methods, cold spraying employs the least temperature of preheated gas and thus delivers solid metallic powder to the target. This solid-state delivery process reduces powder oxidation and thermal damage to the target [23,174,[180][181][182][183]. Owing to its reduced operating temperatures, cold spraying has become an attractive coating option for thermally sensitive substrates [174]. Perhaps the simplest and cheapest realization of cold spraying is the low-pressure cold spray system [24]. Cold spraying is considered to satisfy the PM requirements of different applications [14]. The first successful report on PM based on cold spraying was published by a team from Cambridge University in 2006 [184]. Nearly 50 research articles inspired by this work are available today. Clearly, cold spraying is the most studied of the thermal spray methods for polymeric substrates. Table 3 lists the polymers and composites that have been successfully metallized via cold spraying, which are mostly limited to PEEK and CFRP targets. In thermoplastics, the impact energy is mainly channeled into plastic deformation within the substrate, leading to particle embedment [185]. Alternatively, thermosets undergo microfracturing instead of plastic deformation, which generally lowers the deposition efficiency [186]. To ameliorate this situation, thermosets can be coated with a thermoplastic-such as PLA-before cold spraying [187] [188]. In both thermoplastic and thermosetting cases, the powder material for deposition should be soft and easily meltable; suitable materials are Sn, Cu, Zn, Al, and mixtures of these. Harder particlessuch as Fe and Cu-commonly erode the substrate surface and remain embedded without completely forming the coating layer [170,189,190]. In most of the published data, the particle size ranges between 20 and 40 µm (Table 3). A heavier powder induces severe stress at the coating-substrate interface, facilitating its delamination from the substrate [191]. Liberati et al. [21] experimented with 12 different powders and concluded that the particle size, morphology and density individually do not notably influence the deposition efficiency, but the impact energy does. The difficulties of depositing hard metals can be mitigated by first spraying an interlayer of a light material such as Sn [192][193][194] and Al [158]. A hard coating layer of Fe, Ti, or Cu powder is then deposited. The type of gas influences the particle velocity, amount of oxygen in the deposition, and structural integrity of the composite. Helium best accelerates the metallic powder [24,133], facilitating the deposition efficiency, hardness, and density of the coating [184]. However, helium is expensive and increases the coating fabrication cost substantially. The flow velocity of air might be insufficient for forming low-porosity, low-oxide soft metallic layers and depositing several hard metals [195]. The most popular choice is relatively cheap inert nitrogen (Table 3). An advanced solution could include two guns with different gases, one for forming the first layer and the other for increasing the thickness of the coating. The gas temperature in the converging section of the nozzle is typically set at 200°C-500°C. The gas temperature decreases rapidly along the nozzle axis in the diverging section and beyond the nozzle edge, enabling a short nozzle standoff distance (10-100 mm) and an accurate spraying spot. Furthermore, the standoff distance can be adjusted for the preheating and proper melting of the substrate, allowing deep particle embedment [170]. The main difficulty of cold-sprayed PM is determining the appropriate set of process parameters, particularly when the system uses dense metallic powders such as Cu and Fe. Nearly one-third of the studies listed in Table 3 achieved only particle embedment. Forming the coating above the embedded layer remains a major challenge owing to erosion, particularly on thermosetbased composites. However, coatings formed using particle embedment show unique properties that can widen the applications of polymers in various sectors, as discussed in the subsequent section.
Some cold spray applications to polymeric parts are shown in Figure 11. Many studies on PM based on cold spraying were inspired by the vulnerability of polymeric composites to lightning strikes [19,131,158,193,[196][197][198][199][200]. The electrical conductivity of polymer composites is typically below 5 MS/m [192]. In lightning tests, cold-sprayed coatings adequately shielded CFRP parts from a 200-A current C-waveform strike [196]. An electroconductive Sn layer sprayed on various polymers [186,191,194] could be integrated into the electroplating process as a greener alternative to chemical plating, shortening the 14-step process chain [201] to a single-step process. The sprayed coatings are usually rough and matte in appearance; however, a decorative glossy appearance can be achieved by electropolishing the sprayed layer. Further research is needed to deliver various cold spraying-electroplating hybrid finishing methods in the technological market. Another application of cold spraying is the biofunctionalization of polymericbased orthopedic prostheses. Polymeric PEEK bioimplants are thermally degraded using high-temperature coating techniques [202]. Moreover, the widely used plasma-sprayed bioactive HA coatings become amorphous after remelting [203,204]. The low operational gas temperature of cold spraying preserves the high crystallinity of HA, thus promoting osseointegration [134]. This implies that cold spraying can deliver a biocompatible surface treatment in various orthopedic applications. The cold-spray embedment of metallic particles into polymers was suggested as an effective antifouling technology [170,189,190]. In a previous study, embedding copper powder in HDPE and PU parts prevented the fouling of the parts during eight months in the Coral Sea [189,190]. In addition, a cold-sprayed copper coating reduced the biofouling of marine structures by 85% [205]. Thus, cold spraying is a promising solution to address the biofouling of seawater piles and inlets, underwater cables, ropes and marine buoys, polymerhulled vessels, and fish farm equipment. Cold spraying can be directly applied on site, for instance, to oceanographic and oil platforms, which is a distinct technological advantage. Note that embedded metallic powders (unlike continuous coatings) do not affect other properties of the bulk polymer part. The discontinuous coating preserves the flexibility and some level of transparency of the substrate, prevents delamination of coatings, and participates in the chemical reactions and tribological performance of polymeric parts [170,206]. Using a nanoparticle deposition system based on cold spraying [207,208], nanoscale titanium oxide powder was obtained on PET and PMMA; the resulting materials are suitable for flexible electronics and photocatalysis. Similar to direct microetching via powder blasting [209], cold spraying can achieve microdeposition through a mask with microopenings [209]. Finally, direct three-dimensional (3D) printing via cold spraying has recently been demonstrated [210]. Upgrading the process using a micronozzle would enable the direct writing of conductive electrodes in solar cells, flexible polymer sensors, and microelectromechanical systems.
Compering the cold spaing parameters on metallic and polymeric substrates [22], the last one has a significatly narrower window of process prameters, ensuring the ebsences of erosion or thermal damage to the substate, while providing the particles bonding. There are a number of simulations and theoretical work on temperature field during the single particles impact [180,211], due to many assumptions, the error may exceed the narrow window of process prameters for polymeric substrates. Thus, the future studies would need to focus on the experimental identification of threshould particle velocities, mass, etc for bonding/rebouncing during the cold spray process. Compared with other coating technologies, thermal spray methods offer certain advantages as well as unique coating characteristics and process capabilities. Some advantages are listed below:  High coating-deposition rate (up to 100 µm/s)  Cost-effectiveness: cheap consumables, simple maintenance, and personal training  Unlimited coating thickness (from micro-to millimeter scale)  Superior coating adhesion strength (up to 20 MPa)  Tailored coating qualities: porosity, hardness, conductivity, bioactivity, and transparency  High process stability: linear deposition rate, no vibrations, and no wear  Good process flexibility: over 10 process parameters can be tuned for depositing various metals on various polymers  Process scalability: workpiece of any size and shape can be processed provided that the spray gun can approach the target.
Multiple gun assembles and particle-acceleration systems with enhanced scalability can be adopted  One-step process: eliminates the need for cleaning, grease removal, surface roughening, heat pretreatment, masking, and other pretreatments  Field application: mobile equipment that can apply coatings on site (e.g., offshore oil and gas production felicities, oceanographic platforms, remote stations, and military parts)  Ecofriendliness: no requirement for rare materials no hazardous waste production  3D printing capabilities and direct writing of conductive patterns on polymers  Hybridization: thermal spraying is easily combined with other processing methods, e.g., cold spraying with selective laser melting or an abrasive jet for additive manufacturing or material replacement, respectively. Premixed powders or multiple nozzles with different powders can be used to realize microstructural composite coatings with tailored properties. Thermal spray methods also show various disadvantages that have limited their competitiveness:  Directly sprayed coatings are limited to soft metals. Hard metals should be sprayed on soft interlayers.  The spatial resolution of the deposition is of the millimeter scale.  The structural, mechanical, and electrical properties of the coating are inferior to those of the bulk material. The coatings are rough and porous.  The process is dusty and noisy and requires a cabinet.  PM based on thermal spraying remains in its infancy, and empirical knowledge is lacking. No reference books, guidelines by equipment manufacturers, or extensive lists of publications are available.  The lack of the fundamental understanding of the process-material-property relation necessitates pilot experiments and the optimization of the polymer-spraying process in each case. 5. Direct bonding methods between polymeric-based materials and metals (macroscopic "metallization") PM can be performed at the structural level, where the metallic components are directly joined to their polymeric counterparts. At this level, the aim of polymer-metal direct bonding is minimizing the total structural weight or enhancing the mechanical properties (bending, compression, torsion, and impact resistance) of the polymeric parts. Although PM is intuitively associated with thin films, its concept is far broader. PM occurs whenever a metallic material and a polymeric material are joined into a single structure along the polymer-metal interface. Direct bonding methods include adhesive bonding, injection overmolding, and a family of fusion bonding techniques. Direct bonding methods enable PM at the largest scale. Heavy-duty coatings with a structural geometry of several millimeters in thickness can be realized for durable hybrid structures.

Adhesive bonding
Polymeric structures can be metallized by bonding the metallic and polymeric substrates (similar or dissimilar) using an adhesive. This technique achieves integrated structures without considerably modifying the geometry or properties of either substrate [16]. Hybrid systems in various industries, such as aerospace (fuselage, scarf repair, propellant tank, and fiber metal laminate), marine (ship hulls), automotive (car side rails, front-end carriers, and drive shafts), and robotics [16,229], have been produced using this method. Figure 12 a shows the adhesive bonding of metals and polymers during the fabrication of automotive structures [17]. The metal-adhesive and plastic-adhesive interfaces obtained using adhesive bonding are shown in Figures 12 b and c, respectively [230].
The adhesives used in this technique should exhibit strengths above 5 MPa and should be resistant to high and low temperatures and moisture. The recommended adhesives are epoxy (one-or two-part systems; for high strength and high temperature resistance), cyanoacrylates (for fast bonding to rubber and plastics), anaerobics (for the bonding of cylindrical shapes), acrylics (for fast curing with no surface treatment), polyurethanes (for flexibility at low temperatures and fatigue resistance; for example, BETAMATE TM from Dupont), phenolics, bismaleimide, and polyimides (for high temperature resistance) [16,229,231]. Before bonding, the polymer and metal substrates must be treated to remove contaminants (dirt, particles, and surface oxide layers) and improve the mechanical interlocking and adhesion strength between the adhesive and substrates [230]. Pretreatments include chemical cleaning, acid etching, grit blasting, sand polishing and brushing, plasma treatment, UVO treatment, and laser treatment [232].
Recently, adhesives as the bonding agents of 3D-printed metals (e.g., titanium alloy), thermoplastics (e.g., ABS, polyamide, and PLA), and composites (e.g., carbon/polyamide) have attracted increasing interest in the additive manufacturing (3D printing) community. The surfaces of 3D-printed materials are typically rough and resemble the treated surface, which benefits their adhesive bonding to other materials. For example, a Ti-6Al-4V coating (3D printed using electron-beam melting technology) has been directly bonded to a carbon/PPS composite using adhesive epoxy without a pretreatment process [233]. However, metals must generally be treated before bonding them to 3D-printed thermoplastics. For example, Zhou et al. [211] sandwiched a PLA lattice core between two magnesium alloy face plates that was pretreated using grit blasting and etching to improve their adhesion to PLA [234].
The adhesive bonding process can be time-consuming because the adhesive must be cured appropriately between the metal and polymer [235]. Moreover, the adhesion quality must be assessed using advanced nondestructive evaluation techniques (X-ray, acoustic emission, and ultrasonic scanning). Finally, adhesive bonding typically fails catastrophically, indicating a low fracture toughness and low long-term safety of the entire structure [15].

Injection overmolding
Polymeric substrates can be metallized by injecting molten thermoplastics through several holes and directly overmolding them onto preheated metallic structures [237]. The metallic structures are typically preheated to above the melting temperature of the thermoplastic (e.g., 300°C for 30 min) [238]. The surface of the metallic structures should be treated to generate microroughness, which enables microscale mechanical interlocking between the metal and thermoplastic [17]. This mechanical interlocking occurs when the polymer can penetrate the cavities of the metallic surface, forming strong polymer-metal bonds. This penetration depends on the viscosity of the polymer and the surface temperature of the metal. In polycarbonate-steel or polycarbonate-aluminum bonding, increasing the mold temperature decreases the viscosity of the plastic; the polycarbonate then penetrates the metal surface to a microroughness of <1 μm. This injection overmolding process is adopted by the automotive industry because the processing time is quick (<1 min) and complex parts are easily joined [238]. Steel-polycarbonate and aluminum-polycarbonate [238] composites for automotive parts (such as bumper cross beams, door modules, and tailgates; Figures  13 a and b) [17] have been formed using this technique. In these structures, the polymeric supports prevent the buckling or crushing of the metallic parts while minimizing the overall weight of the metallic parts.
Polymer-metal bonding typically fails because of low adhesion between the two parts. The polymer-metal adhesion can be improved by enhancing the metallic surface. For instance, electrogalvanizing a steel surface enables the formation of hierarchical microstructures that enhance the normal and shear strengths of polymer-metal joints [239]. Using a nanosecond fiber laser, Rodríguez-Vidal et al. [240] formed textured micropatterns on low-alloy steel to improve its bonding to GFRPs. Other surface texturing methods, e.g. grit-blasting [66], micro-abrasive jet [241], or controlled solid particles impact [242] could be considered for more productive tailoring of the metallic substrate topography. However, injection tooling is considerably more expensive than adhesive bonding and the cohesion between the metal and injection-molded polymer depends on the location of injection nozzle gate. In particular, the plastic exhibits void-free interfaces when located near the injection gate (Figure 13 c), but is prone to voids (a ) (b ) (c ) (trapped air pockets) when located at a farther distance from the injection gate (Figure 13 d). If the polymer contains short fibers, the metal-plastic bonding can be inhibited based on the fiber geometry (length and diameter). In particular, if the fibers are too large to penetrate the microscale cavities, voids and subsequent premature polymer-metal debonding will occur [17]. Injection overmolding can induce shape distortion (warping) or microstructural changes during preheating. To resolve this issue, Jun et al. [230] proposed the use of organic etching on the metal part in a 2.0 M HCl solution at room temperature, followed by direct adhesion between the metal and plastic. However, the appropriate bonding of the polymer-metal adhesion requires 24 h. Decreasing the polymer-metal bonding time without temperature assistance remains a challenge.

Fusion bonding
The fusion bonding of metal and polymer substrates can be achieved using various joining processes: ultrasonic joining, friction-based joining, electromagnetic induction joining, and laser-based joining. In fusion bonding process, the polymer adjacent to the metal is melted while the metal is heated. Once the plastic has solidified, the metal and polymer are strongly bonded through direct mechanical interlocking and/or chemical bonds [235]. These techniques employ different heat generators to realize the polymer-metal adhesion.

5.3.1.
Ultrasonic joining In ultrasonic joining, the metal and polymer parts are clamped and subjected to high-frequency vibrations (e.g., 20 kHz) (Figure 14 a) [15,235]. The clamping pressure and vibration amplitude must be optimized to ensure proper adhesion between the metal and polymer. An aluminum alloy requires a pressure and vibration amplitude of 0.2 MPa and 80 μm, respectively, when bonding to PPy [231] and 0.1 MPa and 30 μm, respectively, when bonding to ABS [235]. A CFRP bonded to an aluminum alloy requires a vibration amplitude of 40 μm [15]. Under this setting, the thin subsurface layer of the polymer is rapidly melted (within 0.1-0.5 s) because the thermal conductivity of the polymer is 100-1000 times lower than that of the metal. The molten polymer occupies microvoids on the metal surface, creating a direct bonding between the metal and polymer (Figure 14 b). Ultrasonicallyassisted pins can possibly be inserted between two materials to improve joint strength and toughness as have been demonstrated by z-pinning technique for composites [244]. For approximately two decades, ultrasonic joining has been used to bond metals to thermoplastic parts and thermoplastic composites with synthetic (carbon and glass) or natural fibers [245]. Although ultrasonic joining is generally applied to thermoplastic-thermoplastic, thermoplastic-metal, and thermoplastic-composite bonding, it can also assist the cocuring of thermoplastic films on a thermoset adhesive [246]. However, an ultrasonically-joined plastic-metal interface is very sensitive to thermal cycles. Thermal cycling wear is enhanced by the large CTE difference between the metals and polymers (e.g., CTE = 78 × 10 −6 K −1 for ABS and 22 × 10 −6 K −1 for AA502, a nearly fourfold difference). Such a large CTE mismatch causes the accumulation of residual stress and subsequent failure at the interface [235]. In the current practice, ultrasonic joining is limited to relatively small joints composed of metals and thermoplastic polymers. Large components require powerful equipment that produces loud audible noise [28].

Friction-based joining
Here, we discussed a couple of widely-used friction-based joining techniques, namely friction lap joining and friction spot joining. Friction lap joining (FLJ) is a friction-based technique used for joining dissimilar materials (e.g., metal-polymer, metalcomposite) that was proposed by Joining and Welding Research Institute (JWRI), Japan [223]. FLJ requires the metal and plastic parts to be initially overlapped. The tool with a downward force is pressed upon the metal surface, and travels along the overlapping region. The frictional effect heats up the metal, resulting in a melting of plastic base underneath. The re-solidification of the plastic upon the removal of rotating tool makes up the metal-to-plastic joint. The mechanical performance of metal-plastic joint under fatigue loading can be improved using this joining technique, in which increasing welding duration could enhance the fatigue life [248]. Nonetheless, FLJ is still limited to flat or slightly curved parts, and at present cannot be used to join free-form or geometrically-complex parts. Another friction-based joining technique is called friction spot joining (FSpJ), which was proposed by Helmholtz-Zentrum Geesthacht, Germany. FSpJ is one variant of the friction spot welding of metals, polymers, or composites, and have been used in the production of aircraft and automotive structures due to their rapid process. In FSpJ, metal and polymer parts must be clamped at their top and back sides, respectively. The pneumatic piston subsequently moves against the metal part in either the rotational mode (sleeve plunge type), linear mode (pin plunge type), or combined modes. The sleeve and pin plunge types differ considerably in their initial steps. In the sleeve plunge type, the sleeve touches the metal part and is rotated while the pin is retracted. In the pin plunge type, the pin touches the metal part while the sleeve is being retracted. The subsequent phenomena for both types are the same: (i) the friction generated between the rotating sleeve and metal increases the local temperature, plasticizes the metal, and creates a keyhole; (ii) the plasticized metal then flows into the reservoir created by the retracted pin (the flowing process); (iii) the pin is then pushed back against the metal, forcing the plasticized metal into the keyhole (the refill process), and (iv) the tool is retracted and the metal and polymer are bonded under pressure and cooling [16]. Important parameters in FSpJ is pressure, rotational speed, and duration.

Electromagnetic induction joining
In EIW, the polymer-metal interface (e.g., polypropylene-aluminum) is heated using a high-frequency induction coil [231]. The alternating electromagnetic field of EIW typically generates frequencies in the kilohertz-megahertz range, inducing heat-producing eddy currents and magnetic polarization [250]. Figure 16 a shows the feeding of metal and plastic sheets into a consolidation roller at the specified speed. Here, the induction coil is placed near the joints, immediately after the rolls. The coils can be a single turn (Figure 16 b), solenoid, or pancake-shaped coil. The coil is often accompanied by an air jet nozzle that cools the welded interface (Figure 16 c) [251]. Mechanical interlocking between the metal and polymer is formed using transcrystalline spherulitic microstructures between the fusion and heat-affected zones (Figure 16 d). EIW is used in the production of automotive parts and consumer wearables, such as safety shoes [252]. The drawback of the methods is that the precise position of the EIW inductor and its effective heat distribution, along with the penetration depth into the joining parts, must be established by performing extensive parametric studies leading to an optimized metric, particularly when they the parts exhibit complex geometries. Thus, part-fitting equipment and a number of trials are needed to determine the process parameters. Future work should focus on a generalized window for the process parameters and shape-adaptive EIW tools. Figure 16. Electromagnetic induction welding: (a) schematic of the process [250], (b) allocation of coil [251], (c) finite element method (FEM) simulation of the thermal effect [251], (d) transmission electron microscopic (TEM) image of the welded aluminum-polymer interface [202].

Laser-based joining
Katayama and Kawahito [253] proposed LW or laser transmission bonding for the adhesion of metals to transparent polymers. By applying a 807-nm 170-W diode laser beam, they joined transparent PET to 304 stainless steel (Figure 17 a) [253]. This technique was adopted for the joining of polyimide and titanium [254], PET and titanium [255], PET and magnesium [256], and PET and aluminum [257,258]. As the aforementioned polymers are transparent (for instance, PET shows a transparency of 90% [253]), the metal sheet can act as an absorbent. Before LW, the metal sheet is placed beneath the polymer sheet and both materials are clamped using thick glass sheets at 1 MPa. The laser is then precisely directed through the transparent polymer sheet to the polymer-metal interface. When the tip of the laser beam hits the interface, the metal is heated and the polymer is melted [255]. During the melting process, tight chemical bonds could be formed at the interface between metal and carbon chains of the polymer at the molecular level, which is hypothetically located at the metal-plastic boundary shown in Figure 17 b. Determining the optimum laser power is essential for tight bonding. The LW technique shows great promise in the biomedical industry. Examples of its potential use are polymeric and metallic parts for cochlea implants, neural stimulation, and implantable drug delivery systems [255]. This technique is also suitable for the joining of CFRP composites with metals [16]. The current challenge is eliminating the trapped gas (bubbles) caused by the thermal decomposition and outgassing of the polymer during heating. These bubbles may concentrate stress, leading to the premature failure of the joint [16]. Such a challenge, i.e., trapped gas (bubbles), may also be encountered by other methods that are generating heat on the polymer, e.g., welding, friction-based joining. Figure 17. Laser polymer-metal welding: (a) schematic of the joining process using a diode laser [214]; (b) SEM image of the metal-plastic interface [257] 6. Summary and future directions Polymer metallization (PM) is generally associated with thin films, however, we have shown that its concept is much broader. Our review discovered that PM is associated with methods ranging from the atomic-scale deposition of thin metallic films on polymers up to the welding of heavy-duty metallic and polymeric parts. These methods are applicable for connecting polymers with metal films of different thicknesses, ranging from nanoscale to macroscale. Thus, we define PM as the process of connecting a metallic layer to a polymeric part using chemical and/or mechanical adhesion between their surfaces. This section briefly summarizes the capabilities and prospects of the reviewed PM methods.
The PDV process, which is used to coat a polymer with a thin metal film, shows a plethora of applications. The improvements achieved by PDV range from mechanical properties to biocompatibility. Metallization using PVD is usually performed by employing two methods: magnetron sputtering and vacuum evaporation under heated filaments. These processes are well established in the semiconductor industry and are easily adaptable to polymer coating. More recently, novel PVD processessuch as HiPIMS as well as electron-beam and pulsed electron-beam evaporation-have emerged. PM studies exploiting these novel methods are expected to increase in the future. Stringent health and environmental regulations have driven the search for ecofriendly alternatives (e.g., [259]) to conventional electroless and electroplating processes, which release toxic acid. PVD processes are a promising alternative to conventional chemical plating for various polymers. Future PVD research can be focused on improving the adhesion strength at the interface of polymer and deposited metallic thin film, scaling up the sputtering process to accommodate a larger number of free-formed parts, and integrating the PVD method as the metal seeding step into the electroplating procedure.
The most widely employed PM technique is the electroless plating process and the subsequent electroplating. This technique deposits 1-10-μm-thick films of copper, nickel, chrome, silver, gold, or another metal on polymeric substrates. Although the plastic material can vary, 90% of electroplated polymers worldwide are ABS polymers. The electrochemically plated coating protects the polymer from prolonged harsh environmental conditions and serves a decorative purpose. Electroless plating is widely used in electronics, automotive industries, and commodity goods. Present research is aimed at greening the classic electroless process, which has been established for the past five decades. In particular, the hazardous chromium in the wet etching step will be replaced with more ecofriendly solutions. This review outlined nonetching, mild wet etching, and dry etching, which aim to improve the surface wettability and/or interlocking potential. Nonetching methods affect the material properties and may create insufficient interlocking. Mild acids complete the polymer etching nearly 10 times more slowly than harsh acids. Dry etching performs well, but its commercial viability is questionable. Meanwhile, the Environmental Protection Agency pressurizes electroplaters in Asia, Europe, and China with various regulations for the chemical waste coming from the electroplating process. Overall, chemical plating leads the PM industry but must be greenerized. Future research can be focused on clarifying the relative contributions of different adhesion mechanisms of the plated coatings, developing eco-friendly polymer pretreatment methods which would rely on the dominant adhesion mechanism, and modifying the chemistry of polymer and/or plating solutions.
Thermal spray methods are dominantly used for the thick-layer deposition of soft metals. Thermal spraying is perhaps the simplest, most productive, and cost-effective approach for depositing thick (submillimeter scale) metallic layers on polymers. Polymers can be coated using several members of the thermal spray method family: flame spraying, electric arc spraying, plasma spraying, and cold spraying. All of these methods are feasible for PM but risk thermal damage to the workpiece. Most of them are limited only to metals with a low melting temperature or require protective precoatings. Cold spraying is the most suitable technique for polymeric substrates and has received more research attention than the other thermal spray methods. This technique can metallize any polymer (thermosets, thermoplastics, and composites, including fiber-reinforced composites). Because the deposition is applied in a solid state, the obtained coatings undergo negligible oxidation and exhibit highly distorted microstructures that improve the hardness of the part. In addition to hardness and wear-resistant applications, thermally sprayed coatings are considered for the lightning strike protection of composite aircraft fuselage, protection of underwater structures from biofouling, preparation (a) (b) of biocompatible polymeric implants, and formation of conductive patterns and discontinuous coatings of various materials on polymeric substrates. The widespread use of thermal spraying as a PM method is mainly inhibited by the cost of equipment and the need to independently determine the window of process parameters that ensures deposition without damaging the substrate. Overall, cold spraying on polymers shows the most promise among the spraying methods but remains in the research and development stage and is expected to progress in the future. Further research can be focused on identification of the window of deposition parameters of harder metallic powders on a wider range of polymeric substrate, without causing surface erosion, thermal damage or jeopardizing the coating adhesion strength. Direct polymer-metal bonding realizes PM layers with millimeter-scale thicknesses and can afford macroscale hybrid structures. Direct bonding techniques include adhesive bonding, injection overmolding, and a family of fusion bonding techniques, such ultrasonic welding, friction spot welding, electromagnetic induction welding and laser welding. Strong adhesive bonding requires surface preparation and the selection of an appropriate glue. Overmolding the molten polymer over a hot metal is a rapid PM method but is costly and may be unsuitable for fiber-reinforced composites. The fusion bonding techniques employ different methods for heating the polymer-metal interface. In the ultrasonic welding, the heat is supplied by high-frequency vibrations, which is effective for many polymer-metal combinations but may require a large power input for vibrating large parts. The friction spot welding, which generates heat via friction, is a good alternative to mechanical fastening but is limited to thermoplastic parts with a simple geometry. Electromagnetic induction welding is suitable only for standardized parts in large production series because its process parameters and equipment should be tuned to each part. Laser welding transmits the ablation heat of a metal substrate through a transparent polymer. Steel and titanium have been laser-welded only to transparent polymeric substrates. These techniques have been employed to fabricate lightweight automotive parts and polymeric parts with increased carrying capacities. They are also used for the structural reinforcement of implants in the biomedical industry. Future attention can be given to the effective surface-roughening methods, rapidly curing adhesives, possibility to overmold polymer composites, simple inspection methods for bonding quality, and other methods of suppling the heat energy at the metal/polymer interface for fusion bonding.

Conclusions
The PM methods can be divided into three categories based on their physical principle: the atomic deposition of metals in a plasma or electrolyte, metallic powder spraying, and direct thermal fusion. These different categories can complement each other in one toolset for any PM application to enable the decoration of metallic layers with thicknesses ranging from nanometers to millimeters. By understanding the advantages and disadvantages of the methods in each category, we can develop multiple hybrid methods.
In all these techniques, a common critical problem is encountered: how to ensure the needed level of adhesion at the polymer-metal interface so the integrity of the application is non compromised at long-term? We found that chemical bonding can maintain a nanoscale metallic film but it is deemed insufficient for the use in micro-and macroscale thick metallic layers. A welldeveloped mechanically interlocking interface is mandatory for the thick PM. More efficient ecofriendly, cost-effective, and scalable polymer-surface preparations are needed for most PM methods.
Application of PM ranges from flexible electronics to heavy-duty polymer-metal hybrid structures. Thin film deposition is necessary for most types of sensors and will further evolve to enable the Internet of Things. Thick PM will soon become integrated in the manufacturing chains of high-speed transportation, zero-emission vehicles, hydrogen infrastructures, and other emerging features in the near future. Considering the importance of these applications, the development of PM methods is expected to reap rewards in the material sciences, manufacturing industry, and global sustainability.

Highlights
 Recent methods for depositing metals on polymers and polymer composites.  Surface preparation strategies for enhancing adhesion strength at the metal/polymer interface.  Coating qualities, properties, and performance of metallized polymeric parts.  Design and applications of metallized polymers and polymer composites

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: