Beyond traditional coatings, a review on thermal sprayed functional and smart coatings

Thermal spraying has been present for over a century, being greatly refined and optimised during this time, becoming nowadays a reliable and cost-efficient method to deposit thick coatings with a wide variety of feedstock materials and substrates. Thermal sprayed coatings have been successfully applied in fields such as aerospace or electricity production, becoming an essential component of today's industry. To overpass the traditional capabilities of those coatings, new functionalities and coherent responses are being integrated, opening the field of functional and smart coatings. The aim of this paper is to present a comprehensive review of the current state of functional and smart coatings produced using thermal spraying deposition. It will first describe the different thermal spraying technologies, with a focus on how different techniques achieve the thermal and kinetic energy required to form a coating, as well as the environment to which feedstock particles are exposed in terms of temperature and velocity. It will then deal with the state-of-the-art functional and smart coatings applied using thermal spraying techniques, with a discussion on the fundamentals on which the coatings are designed, the efficiency of its performance and the industrial applications, both current and potential. The inherent designing flexibility of thermal sprayed functional and smart coatings has been exploited to explore exciting new possibilities on many different fields. Applications such as anti-bacterial and anti-fouling coatings, superhydrophobic surfaces, electrical and heating devices for functional coatings and self-healing, self-lubricating and sensors for smart coatings are here presented and discussed. All these exciting developments pave the way for the numerous applications that are to come in the next decade, making the field of thermal sprayed coatings a unique opportunity for research.

current state of functional and smart coatings produced using thermal spraying deposition. It will first describe the different thermal spraying technologies, with a focus on how different techniques achieve the thermal and kinetic energy required to form a coating. It will as well focus on the environment to which feedstock particles are exposed in terms of temperature and velocity. It will first deal with the state-of-the-art functional and smart coatings applied using thermal spraying techniques; a discussion will follow on the fundamentals on which the coatings are designed and the efficiency of its performance; finally, the successful applications, both current and potential will be described. The inherent designing flexibility of thermal sprayed functional and smart coatings has been exploited to explore exciting new possibilities on many different fields. Some applications include, but not limited to, prevention of bacteria contamination and infection on hygienic environments. Here, thermal spray has been used to efficiently deposit anti-microbial compounds on medical furniture and appliances and to develop biocidal and biocompatible coatings for prosthetic implants. The attachment of hard and soft foulers such as algae or molluscs, which represents a considerable issue for any marine or freshwater installation, can be prevented on components where the use of traditional antifouling strategies such as paints is not optimal such as polymers. Another interesting approach pursued is the development of superhydrophobic surfaces, with contact angles as high as 160° and slide angles below 5°, leading to high droplet mobility. This adds capabilities as selfcleaning or corrosion resistance in addition to the characteristic robustness of thermal sprayed coatings. The electric and magnetic properties of the feedstock materials have also led to the application of thermal spraying techniques in the creation of patterned structures with desired electromagnetic properties for their use on microelectronics. The possibility to intercalate layers of thermal sprayed materials doped with optical-reactive elements has led to the development of online and offline temperature sensors which can be readily integrated in current thermal barrier coatings. To finalise the examples of the many applications of thermal sprayed functional and smart coatings, autonomous self-healing or self-lubricant coatings have been developed.

Introduction
The selection of the materials to be used on industrial applications is dictated by their intrinsic properties, which must satisfy the specified needs for the component being designed and manufactured. A clear example would be structural components, where strength or fracture resistance is of the upmost importance. Nevertheless, any designed component will face a determined environment during service operation. This interaction can drastically limit its lifetime or even change its properties to an extent where it will no longer satisfy the expected As the examples above illustrate, the interaction between the environment and the material plays a critical role, and at the heart of that interaction, lies the surface. The surface of any component represents the interface between the environment and the material, and any interaction that is due to take place will happen at the surface. In order to protect the surface, one successful strategy has been to deposit a relatively thin layer (when compared to the dimensions of the bulk material) of a different material, with the properties required to face the expected environment. One of the aspects that has made coatings such a popular solution is the fact that a wide range of deposition techniques are available. To cover the whole catalogue of deposition techniques available is outside the scope of this review, which will focus on thermal spray technologies.
Thermal spray comprises those deposition processes in which an energy source is used to heat the initial feedstock particles (which could be presented in the form of suspension, powder, wire or rod), being then accelerated and propelled towards the substrate using a gas stream (Ref 4).
The combined thermal and kinetic energies of the particles allows the bonding with the surface of the substrate upon impact, effectively building up the coating as the particles reach the surface. Another aspect that has contributed to the wide-spread use of thermal spraying is the flexibility in the choice of materials that can be deposited with these techniques. As a general definition, any material with the capability of melting without experiencing decomposition is suitable for thermal spraying (Ref 5).
Due to the unique combination of a wide range of deposition techniques and materials available for coatings, thermal spraying has been successfully applied in numerous fields, such as One of the most successful routes in the development of thermal sprayed coatings has been the combination of a proven material system which, and due to the flexibility of allowed sprayed materials, is combined with an added component responsible for the novel functionality. The presence of a solid base of thoroughly investigated and field-tested thermal sprayed coatings has provided an unparalleled starting point for the development of more capable and tailor designed functional coatings. On its simplest definition, a functional coating can be described as a coating with an added functionality beyond the traditional protective capabilities ( Ref 13). The classical protective case would be the already mentioned corrosion or wear protection.
However, although these new functionalities provide functional coatings with a wide range of applications and possibilities, their behaviour is still passive on its interaction with the environment. A smart coating, on the other hand, aims to provide coatings with an active response to certain stimuli, generated either by intrinsic or extrinsic events ( Ref 14,15). In summary, all smart coatings can be considered functional coatings due to the presence of a functionality beyond simple protection, but not all functional coatings can be categorised as smart due to the lack of an active response to external stimuli. It should be noted that the categorisation used in this review regarding the functional and smart coatings is not intended to be definitive-different definitions are present in existing literature. The distinction was chosen to provide a more structured approach to the review.
Several decades of investigation on the science behind thermal sprayed coatings and the relatively new addition of functional and smart coatings has provided an invaluable opportunity for numerous industrial applications. However further research is still required to provide costefficient methods with proven added value to the companies. With the unparalleled success of deposition techniques such as plasma spraying or high velocity oxy-fuel (HVOF) thermal spraying as an example, other thermal spray technologies still need to reach that level of market penetration. This will only be attainable through strong beneficial cases with a clear understanding of the processes involved. The addition of new capabilities through the introduction of functional and smart coatings represents an added opportunity for exciting, ground-breaking research. It is essential that the industry becomes involved too, setting the requirements and needs of a market more demanding than ever, in which these technologies can present a benefit.
In this work, and due to the importance that the different deposition techniques have on the produced functional and smart coatings, an overview of the main thermal spray technologies available is first presented. Then, attending to the division previously defined between functional and smart coatings, an extensive and comprehensive study of the current developments in the field is undertaken. With the use of thermal spraying techniques as common factor, the current state-of-the-art for functional and smart coatings is presented attending to the different functionalities achieved.

Thermal Spraying Technologies
Thermal spraying processes incorporate those technologies on which metallic or non-metallic coatings are deposited through the same principle. A heat source melts the feedstock material and a jet is used to impart kinetic energy to the molten particles. They then impinge the substrate surface and rapidly cool down to form a solid splat, continuously building up the desired thickness ( Ref 16). A basic schematic of the thermal spray process can be seen in Figure   1. The flexibility on thermal sources and jet configurations give rise to a plethora of different deposition technologies, as presented in Figure 2, each one producing coatings with different microstructures and physical properties.  The development of thermal spraying processes has been central to the evolution of functional and smart coatings, allowing new materials to be deposited and new substrates to be coated, broadening the range of accessible possibilities. This section presents an overview of the most common thermal spraying processes used in the fabrication of functional and smart coatings, with a brief description of their working principles.

Plasma spraying
Atmospheric plasma spraying (APS) uses thermal plasmas produced through direct current (DC) arc or radio frequency (RF) discharge as the heat source of the deposition process. This allows flame temperature over 8000 K (reaching as high as 14000 K in the jet core ( Ref 16,18)) and particle velocities ranging from ~20 m s -1 up to ~500 m s -1 depending on the particle size distribution ( Ref 19). The elevated temperatures produce a high proportion of particles melted, which in addition to the relatively high velocities, give rise to excellent deposition densities, bond strengths and low porosity coatings when compared to most thermal spraying processes (Ref 16). The high cost-efficiency and good quality of the coatings obtained by using APS has led to a successful implementation in numerous industries.

Suspension/solution precursor plasma spraying
Due to the need of adequate flowability for the feedstock powder, APS is limited to the deposition of particles with an approximate lower limit size of 10 -100 µm ( Ref 20,21). In order to allow the use of nano-scaled powders, different solutions have been developed as an alternative to the traditional injection of powder. The main representatives of these alternatives are suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS) ( . The differentiation factor between the two methods is shown in Figure 3, with the precipitation of the deposited particles in-flight in the case of SPPS as opposed to the direct deposition (apart from the physical changes related to the exposure to the high temperature in the flame) in SPS. These techniques increase the flexibility of the plasma deposition technologies, already widely used in the industry, accessing smaller particle size for the feedstock materials, allowing deposited coatings with different microstructures. A field where SPS and SPPS have found great application is the fabrication of thermal barrier coatings (TBC) for high temperature applications.

Wire arc spraying
Wire arc spraying, also known as twin wire arc spray or electric arc spray, is based on the feeding of two consumable conductive wires (or a core of a non-conductive material on a conductive wire) between which a direct current electric arc is stablished. Once the material is molten, the molten layer is accelerated towards the substrate surface by a stream of atomizing gas. This promotes a further in-flight atomization of the molten particles before their deposition and posterior solidification at the substrate surface ( Ref 16,43). The advantages of this deposition method are several. They include the reduced cost of the process, both in terms of equipment and operational costs, being a very cost-efficient deposition technique. It also shows absence of unmelted or semi-molten particles, a high deposition rate when compared to other thermal spray processes and a low thermal transfer to the substrate. All these factors make wire arc spraying one of the less expensive techniques, nevertheless, the particular characteristics of the produced coatings such as high porosity or low bonding strength make its use somewhat limited.

Flame spraying
Flame spray was the first of the thermal spray techniques to be devised, being developed by Schoop around 1909 (Ref 44). The basic principles are still applied to today's modern conventional flame spray guns. The combustion of fuel gases is used to impart heat to the feedstock particles. At the same time, it produces an expanding gas flow which in combination of additional gases creates the required jet applied to accelerate the material towards the surface to be coated. Typical temperatures for this technique are around 3000 K and particle velocities of up to 100 m s -1 are usually applied (Ref 16); however, in order to improve the initial design on which flame spraying is based, several variations have been developed with a focus on different flame temperatures and particles velocities.

High velocity oxy-fuel spray
Kinetic spray

Functional Coatings
The definition of functional coating varies slightly depending on the context on which it is used or the different points of view present in among the experts in the field of thermal spraying; however, in this work a functional coating has been defined as a deposited coating which has a passive, integrated, new functionality beyond the traditional protective capabilities. As such, in this section, the aforementioned definition is applied to classify the functional coatings developed attending to its functionality. For instance, its capability to kill pathogens or prevent infections on orthopaedic implants, the hindering of adhesion and growth of algae and hard shell organisms on water submerged equipment, creation of water-and ice-repellent surfaces or the deposition of coatings with electromagnetic or electrochemical properties. In the case of anti-adherent surfaces, the creation of a broad spectrum morphology is complicated due to the non-specificity of the method. A surface with low attachment of a specific bacterial strain might not present the same behaviour with other pathogens, limiting its general application. In addition to non-specificity, anti-adherent surfaces suffer a great functionality loss when wear is present, due to the alteration of the designed morphologies.
Despite these deficiencies, the absence of antibiotics or similar agents as the active principle presents a promising approach to prevent the appearance of antibiotic resistant bacteria. As it stands now, this method is mainly applied as a secondary approach in combination with other principles, rather than the main solution against pathogen proliferation (Ref 67).
The loading of an antibiotic or antibacterial agent into the coating has been a popular approach to achieve anti-microbial surfaces, although the method presents one main drawback. The finite nature of an embedded reservoir within the coating implies a time constraint in the duration of the effect. After said time, the reservoir will be depleted and the coating will fail to prevent

Photocatalytic effect for anti-microbial applications
Parting from the already presented approach of release-based anti-microbial coatings, the photocatalytic effect provides an alternative method for the development of biocidal surfaces.
This method does not rely on embedded agents to provide the biocidal effect, but it also differs from the traditional contact-killing solutions. The photocatalytic effect is based on the illumination of a material, which decomposes compounds by oxidation. As seen in Figure 5, the illumination of the TiO2 coating with photons carrying energy equal or greater than the band gap results in the creation of electron-hole pairs in the titania conductance and valence band, respectively. There is a probability that these charge carriers will transfer or diffuse to the coating surface, where they can interact with adsorbed water and molecular oxygen. The produced electrons usually take part in photoreduction reactions, such as the production of O2radicals, whereas the correspondent holes produce the photooxidation of water molecules, forming free hydroxyl radicals (OH -). These present already  In conclusion, thermal spray methods might not be optimal for the development of contactkilling coatings due to the limitations on temperatures experienced by the feedstock during the process, but they have been proven an excellent solution for others. The possibility to efficiently deposit a robust coating loaded with anti-microbial components represents an excellent opportunity for the medical and dental field. The localised delivery (both in location and time) of biocidal agents is a desired characteristic rather than a drawback for such applications, preventing surgical site infections and extended release of potentially noxious substances. These characteristics, combined with the quick deposition of biocidal components such as copper over large components, i.e. hospital furniture, and the possibilities provided by the photocatalytic effect shown by well-studied components such as TiO2, makes the area of thermal sprayed antimicrobial coatings a thriving one.

Membranes for water filtration
Although not a direct anti-microbial application in the sense covered in the previous sections, thermal spray has been applied for the development of membranes for water purification. The ability to deposit a thick film with controlled porosity allows for the design of membranes with tailored mean pore size, effective in the removal of particulates from water. Despite the efforts made by some authors to create ceramic membranes using technologies such as wire-arc (Ref

Anti-fouling
The attachment of different aquatic species, classified as soft foulers (primarily algae) and hard foulers (comprehending hard shelled molluscs such as barnacles and mussels) (Ref 133) to surfaces exposed to submersion in water represents a critical factor when considering the    substrates applied in the marine industry, such as PU on seismic streamer skins, would be beneficial to clarify the potential side-effects and extend the use of thermal spray for anti-fouling applications, being currently limited to niche applications

Hydrophobicity
A hydrophobic surface is defined as having superior water repellent properties, which present several advantages such as reduced contact time with corrosion agents or self-cleaning capabilities, produced by the rolling droplets of water, which carries away dirt. In order to quantitatively evaluate a surface, two parameters are commonly used, namely water contact angle (CA) and slide angle (SA). Contact angle is defined by the tangent to the liquid-vapour interface where it meets the surface, as shown in Figure 6 by the angle θ. The slide angle is the tilt angle required for a static droplet deposited on a surface to start rolling down. As a general definition, any surface with CA > 90° can be considered hydrophobic, while values of CA > 150° and SA < 10° are generally required for a surface to be considered as superhydrophobic. Hydrophobicity has two contributions that explain the particular behaviour presented. First of all, a low surface energy which ensures that the attraction between the water droplets and the surface is minimized. Secondly, a structured surface morphology. As seen on Figure 6, the different levels on the morphology have a direct impact on the hydrophobicity of the surface.
Two main models are used to describe different hydrophobic states present on surfaces. The  superhydrophobic surface and those involved in the development of icephobic (or pagophobic as they prefer to called them, from the Greek word "pagos" for ice). Figure 7 gives an overview of the main mechanisms considered when designing an icephobic surface. Here, three parameters play the key role. First, topography, which determines the presence of a Wenzel or Cassie-Baxter state, as explained in the previous section. Secondly, elasticity, being a common example the use of silicon for ice cube trays accounting for its flexibility and low surface energy.
Thirdly, the liquid extent, where a micro/nanoporous material is infused with lubricant liquid with a low freezing point, providing a smooth liquid interface that reduces droplet retention and ice adhesion strength.   Table 3.   Traditional fabrication methods for those ceramic components present several disadvantages that thermal spray could help to overcome. The high temperature required for the firing process, for instance, presents a bottleneck on the mass production of SOFCs, greatly increasing their cost. It also implies problems such as thermal mismatch stresses on large components, side reactions between adjacent layers and the difficulty of using low-melting materials in the multi- The production of cheaper SOFCs for electricity generation would represent a major breakthrough for the industry, leading to a greater industrial uptake. Nevertheless, to achieve this, the above-mentioned design requirements should be met, such as porous electrodes or thin, defect-free electrolytes.  that for the cases in which the substrate is electrically conductive, an intermediary ceramic layer, which possesses dielectric properties, is required to prevent short circuiting and leakage current in the heating system. Therefore, it is required to develop a multi-layered coating system that consists of the two main elements, namely the heating element that is usually a metallic alloy and an electrically insulating ceramic layer. VPS, and HVOF, to produce resistance heating elements that can generate uniform and easily controlled flux. It was found that HVOF and VPS processes compared to the APS could manufacture consistent coatings with physical properties that are close to the bulk materials. In this study, a metallic layer (nickel-chromium alloy Ni80-Cr20), which served as the heating element, was deposited onto a ceramic layer (alumina) that served as the electrically insulating layer. The layers had a thickness in the range of 75 to 300 µm. The configuration of the studied resistive heating system is shown in Figure 9. Michels et al. (Ref 216) found that the thickness of alumina that was required to isolate the heating element from the substrate was around 100 µm for HVOF, but 200 µm for APS.

Process
Furthermore, the highest fluxes that could be generated from the fabricated heating system before failure was as high as 10.6 MW/m 2 and 17.2 MW/m 2 for HVOF and VPS films, respectively.
The heaters failed at very high level electrical currents. It was found that the failure mode of the thermal-sprayed heating system was delamination of the Ni-Cr layer from the insulating ceramic layer. Therefore, the evolved thermal stresses between the deposited ceramic and metallic films due to the mismatch between the thermal expansion coefficients were suggested as the root    The other main issue associated with direct deposition of metallic coatings on glass or glass ceramic substrates is evolution of critical residual stresses within the coating system due to the noticeable difference between the CTE of the glass substrates and that of the metal coatings that can lead to cracking and delamination at the coating-substrate interface. It was found that the areas that were coated with alumina and NiCrAlY materials had a profound impact on the stability of the bilayer coating system. For some of the samples in which alumina coating was deposited over a wider region, delamination was not observed (see Figure 12(a)). However, for the case in which both layers were deposited onto an accurate geometrical path, delamination of the top coating was observed, which was due to the accumulation of stresses at the edges (see Figure 12  was found that the best adhesion and stability was achieved for the case in which titania (TiO2) was deposited on the glass ceramic. Furthermore, it was found that addition of NiCrAlY to TiO2 feeding powder can enhance the electrical conductivity of the coating layer. However, delamination of the coating is more probable for cases in which higher content of NiCrAlY is added in the TiO2/NiCrAlY mixed phase. The micrographs that were taken from the cross section of the developed coating systems are shown in Figure 13.      It is due to the above-mentioned detrimental consequences that utilization of an effective solution that is capable of mitigating the problems associated with accretion of ice seems to be of great importance. In this regard, coatings with anti-icing properties such as ice phobic coatings have been studied extensively; however, it has been proven that the passive anti-icing coatings are inefficient to prevent formation of ice on the blades alone, and their efficiency has been demonstrated only in less severe sites ( Ref 242). Therefore, an efficient de-icing method is still needed to remove the ice layer from the blades. substrate successfully without any damage to the substrate. In order to prevent damage to the fibres of the composite substrate due to the deposition of high-temperature molten powder particles, a layer of garnet sand was used as an intermediary layer between the FRPC and the metal alloy coating. Furthermore, the garnet sand layer that was formed by sparkling garnet on top of the last layer of epoxy prior to the curing process was also used as the roughening agent.
It was found that the use of the garnet sand layer was beneficial for both deposition of the flamesprayed coatings and also protecting the substrate against thermal damages during the flame spraying deposition process. The NiCrAlY and Ni-20Cr coatings that were fabricated in this research were as thin as 80 ± 15 µm (n = 20) and 100 ± 15 µm (n = 20), respectively. The SEM images that were taken from the cross-section of the coated FRPC sample are shown in Figure   19(a) and Figure 19(b).

Figure 19: a) Low-and b) high-magnification SEM images taken from the cross-section of the coated FRPC (Ref 243).
It was observed that with a supplied power as low as 2.5 W over 3 V the temperature of the FRPC sample with dimensions of 20 mm × 120 mm was 15C above the ambient temperature when no air was flowing over the sample (Ref 243). In addition, application of the efficient coating-based heating element was successful in increasing temperature of the FRPC sample under forced convective conditions. In another study by the same authors (Ref 244), it was shown that the embedded coating-based de-icing element was also able to melt the ice that was formed on top of the coated polymer-based composite while the sample was exposed to forced convection conditions.

Heat tracing of metallic pipelines used for water conveyance and drainage
It is well-known that the solidification of water inside the pipes that are exposed to temperatures below the freezing point of water for an extended period of time is of great concern. Given the volume expansion of the enclosed freezing liquid upon transformation into It is due to the aforementioned serious consequences that developing an efficient heating system that can mitigate the solidification of entrapped water inside pipes is of great importance in industrial sector because installation of the insulation, as the most cost-effective and available solution to reduce the heat loss rate and prolong the duration of freezing (Ref 250), is only sufficient when the stagnant water inside the pipe is exposed to cold environment for a short period of time. In this regard, conventional heat tracing system is used in the industry to overcome this challenge.
Thermal-sprayed coating systems, as electrical resistive heating systems, can be used as an efficient alternative to mitigate the problems associated with frozen pipes. The other advantage that application of the coating-based heating systems offers is the possibility of using them as strain and temperature sensors so that the heating system can act as a closed-loop control system that operates independently. In this respect, any changes in the electrical resistance of the heating element could be monitored. Then, the changes can be attributed to the fluctuations in temperature or potential deformation/damage of the metal alloy coating.
The possibility of deposition of a thermal-sprayed multi-layered coating on a low carbon steel substrate and its functional performance were studied ( Ref 251). In this study, nickel-50 wt.% chromium (Ni-50Cr) was selected as the heating element owing to its high electrical resistivity.
Due to the electrical conductivity of the steel pipe, an intermediary layer, as an electrical insulation, was deposited between the conductive substrate and the heating element to ensure that the short circuit does not occur and the free electrons do not move through the pipe that has less electrical resistance compared to the coating-based heating element.
Because of the unique electrical and thermal properties of alumina, this material was selected to act as the electrically insulating layer to prevent the short circuit and malfunction of the heating system. While the flow of free electrons from the coating to the steel pipe was obstructed by utilization of alumina due to its dielectric properties, the heat was freely transferred to the pipe and the ice inside the pipe due to the high thermal conductivity of the alumina coating, which is relatively higher than other ceramics. It has been reported by Cold-sprayed dense copper coating was also deposited onto the NiCr in this study to make an ideal electrical connection between the metal alloy coating and the power supply. The coatingbased heating element was heated by way of Joule heating in proportion to the supplied power. Figure 20 shows the multi-layered coating system that was deposited onto the pipe assembly.
Furthermore, the installed thermowells and thermocouples that were used for measuring the transient temperature of the ice/water during the heating tests can be seen in Figure 20. Given the probable damage to the plastic pipes due to the high temperature of the flame and the molten feedstock particles, deposition of the flame-sprayed coatings has been tried only for the case of metallic pipes so far. It was concluded that the combination of the powder materials, namely Ni-50Cr, alumina, and copper, was ideal for the heating task. While the pipe assembly that was filled with solid ice was exposed directly to the circulation of ambient cold air at a temperature of -25C without any thermal insulation, even a power as low as 20 W was enough to heat and melt the ice within the pipe ( Ref 251). For the case in which a proper thermal insulation was wrapped around the pipe, even 10 W was sufficient to heat and melt the ice inside the pipe easily. Furthermore, no electrical or mechanical damages were detected during the microstructural evaluation of the multi-layered coating system by using a scanning electron microscope (SEM). These promising results speak to the high efficiency and operational advantage of the coating-based heating element for being used in industrial applications. Figure   21 shows the SEM images from the coating system after conducting quite a few heating tests with various supplied powers.
It was observed that the measured average thickness of alumina as low as 175 µm (n = 10) was enough to protect this layer from dielectric breakdown and the whole coating system from short circuit and the resulting malfunction. Furthermore, the average thickness of the deposited NiCr coating was measured to be as thin as 110 µm (n = 10). This shows that only little amount of NiCr powder is required for developing this heating system, and therefore, fabrication of this coating system is feasible from financial viewpoint. Due to the lamellar structure of the coating and the presence of the unmolten particles that is common in flame spraying process, the obtained microstructure of the NiCr coating was porous. The average porosity of the coating was measured to be 7. Although during the early stages of pressurization of the entrapped freezing liquid the pipe undergoes elastic deformation, it is likely that the ceramic coating fails due to the fact that its failure strain is less than the yield strain of the steel pipe. Therefore, better understanding the freezing behaviour of the enclosed water and the potential damage to the coating layers and the pipe for understanding the critical time for initiation of the heating and decreasing the energy consumption of the heating system can also be the subject of future studies.
Furthermore, potential application and feasibility of using the metal alloy coatings deposited onto steel pipes as both temperature and strain sensors and the reliability of the obtained measurements by reading and monitoring the slight changes in the electrical resistance of the coating-based heating element could be another subject for further study and research in the future.

Smart Coatings
Smart coatings are considered the next step in the development of more capable coatings, accounting for their active response to intrinsic or extrinsic events, in contrast to the passive behaviour presented by traditional and functional coatings. In this section, an overview of the most significant developments in the field of smart coatings applied through the use of thermal spraying deposition techniques will be presented.

Sensors
The unique capability of smart coatings to produce an active and coherent response to a wide   Another potential application, the so-called "rainbow" sensor, with both erosion and temperature profiling capabilities, is presented on Figure 24. The concept is based on a TBC with several layers doped with different luminescence elements, each one of them producing a signal with a distinguishable and characteristic frequency under laser illumination. As the top layers disappear due to the effect of erosion, their corresponding peaks cease to show in the corresponding spectra, marking the progression of erosion.  Embedded sensors present several advantages over intrinsic smart sensing coatings, since sensor and coating can be designed separately and then integrated. The only requirement is that the embedded sensor does not chemically interact with the coating, or excessively modify its physical and mechanical properties. The concept was first described by Fasching et al. (Ref 272) in 1995, including the description of two such embedded sensors, a sprayed thermocouple and a humidity sensor. Although the reported sensors presented faults and needed further optimisation, they represented the proof-of-concept for embedded sensors using thermal spray techniques that promote further research. machining methods has also been reported, although specific information of materials deposited and geometry of the patterns has not been disclosed. This device showed promising results in terms of sensibility and accuracy, although further research into the impact of real service conditions into the sensing capabilities of the system would be required to assess its applicability into the industry.

Self-healing
The appearance of cracks and other defects within coatings, sometimes being too small or buried deep within the material to be detected, is a recurring problem that affects the coating mechanical properties and its correct functionality. The motivation for coatings that can repair themselves without any external intervention is clear, although its realisation has not been possible until recently. The concept of self-healing within a coating was first demonstrated by  surfaces of open porosity, exposed to higher oxygen partial pressures as seen in Figure 27   show that arc spray produced the coatings with higher porosity, capable of better accommodating the microcapsules, which showed the best self-lubrication performance, without any noticeable difference regarding microcapsule size. The lower temperature and particle velocity of the technique also plays a fundamental role, ensuring that the microcapsules do not suffer damages during deposition.
The development of additional ceramic and metallic matrixes with tailored microcapsules to the thermal spray deposition techniques chosen, and filled with liquid for different applications, such as corrosion inhibitors or self-healing components, would represent the next step in smart thermal sprayed coatings, with a vast potential in several industries were these coatings are already widely applied.

Conclusion
The introduction of functional coatings expanded the capabilities of thermal sprayed coatings by adding a novel functionality on top of the passive protective characteristics already present.
These new capabilities present unique opportunities, such as anti-microbial coatings on equipment and appliances on sterile environments, limiting the surface contamination and cross-transfer of pathogens. On the other hand, well-known thermal sprayed coatings, with a proven record of success on surgical implants can be improved by adding anti-microbial agents that reduce rejection after surgery. The new functionalities would benefit other areas such as the protection of ships hulls and marine equipment against the attachment and growth of foulers, with a severe impact on efficiency and economic turnover.
With the precise aim to provide an increased level of protection, functional coatings that effectively reduce the impact of harmful phenomena caused by environmental conditions have been developed. A clear example would be the introduction of hydrophobic capabilities, presenting a dual purpose. Firstly, water droplets rolling over a hydrophobic surface drag dirt, providing a self-cleaning effect. Secondly, a reduced contact time between the liquid environment and the surface reduces the impact of corroding elements present in the environment. Finally, an important development has been the use of the conducting capabilities of deposited materials. Once more, this characteristic has a dual application, as the deposition of electrical or magnetic materials on a tailored pattern represents a huge opportunity in the field of micro-electronics. Nevertheless, another implication is the creation of resistive heating elements, which provides an invaluable tool for the de-icing of components exposed to harsh environments, such as wind turbine blades, or the prevention of the solidification of water on pipelines.
Despite the great advancements that functional coatings have represented, their applications are limited by an inherent passive behaviour on their interaction with their environment.
Therefore, the next step in the development of more capable coatings relied on the introduction of active capabilities with a coherent respond to stimuli, or smart coatings. From the definition itself, the first natural application of smart coatings would be as sensors. Particularly successful has been the application of the photoluminescence effect on rare earth-doped thermal sprayed coatings on high temperature environments, which provides a remote, live or historic In conclusion, functional and smart coatings represent the crystallisation of the wide range of capabilities that thermal spray offers. The successful track record presented in the past, along with the new opportunities provided by the latest developments, points out to an exciting future. Further research will allow to comprehend the deposition and bonding mechanism on multi-component coatings, key to the addition of functional and smart abilities. In addition, the impact that these modified compositions have on the performance of the coatings needs to be thoroughly understood to ensure that the same level, if not improved, of performance is achieved. With these goals in mind, the next steps required for functional and smart coatings would be, first, to translate the current knowledge acquired into readily available feedstock materials tailored to the chosen thermal deposition technique and desired functionality. The great strength of thermal spraying, its flexibility, represents a disadvantage when trying to accommodate all the requirement of the different deposition techniques. The new developments will have to leave the "proof-of-concept stage" and present a sound production process compatible with the standards of the industry, if possible, with minimal deviation from