An overview of cold spray coating in additive manufacturing, component repairing and other engineering applications

5.1 High spray deposition efficiency (DE)

For most metals, alloys and composites, very high DE values have been attained using the CS technique. DE values of over 95% have been attained for aluminium, copper and their alloys, for example. Investigations showed that by (a) reducing O₂ content of feedstock; (b) stress relieving powder; (c) optimising the size of particle distribution; and (d) optimising the spray parameters, very high DE values can be obtained for most materials, including refractory metals (such as tantalum, niobium and others) and high strength materials (superalloys and Ti–6Al–4V alloys) [29].

5.2 High deposition rate

The CS beam’s spray footprint is small and clearly defined. A typical spray beam has a diameter of around 10 mm with sharp edges. A narrow spray beam combined with a high powder input rate results in extremely high deposition rates. In most cases, a 1–2-mm-thick coating may be applied in a single pass to most materials. CS, unlike other coating methods, creates ultra-thick layers with exceptional binding strength. This property has been taken advantage of in the development of CS as a technique for fabricating near-net-shape objects as well as additive manufacture of features [30].

5.3 No grit blasts

A triplex method (grit blast–spray coat–shot peen) may be seen as the CSP. The velocity distribution over the spray beam follows a Gaussian pattern, as one would anticipate. When the system’s parameters are optimised, particles in the core have a velocity greater than the V _cr, while particles in the rim have a velocity less than the V _cr. When such a spraying beam is scanned, the particle at the leading-edge strike on the substrate surface at a velocity less than the critical deposition velocity (V _c) which causes solid particle erosion of the surface and in situ micro-grit blasting. Particles in the core collide with a velocity greater than V _c on the substrate surface, deform plastically across the recently cleaned rough surface, and produce a coating. Particles in trailing edge, which collide with a lower V _c, not only sputter loosely attached particles but also shot peen newly created coatings. Experiments have demonstrated that several substrate materials, such as aluminium, copper, and titanium, do not require traditional grit blasting [6].

5.4 High density

Only particles having impact velocity greater than V _c plastically deform and deposit in CS procedure. High strain rate deformation adds extra thermal energy to the conversion of kinetic energy to thermal energy on impact throughout deformation. These tiny processes cause hydrodynamic instabilities, which result in the creation of a metal vapour jet. This jet creates vapour deposition of material at the inter-particulate contacts, filling any gaps or fissures that may present. As a result, CS may be thought of as a hybrid of particle and tiny vapour deposition techniques. Furthermore, any weakly bonded particles are flicked away, and spray beam’s trailing edge particles micro-peen the deposited splats. Finally, the underlying layer is peened with each succeeding pass of spray beam shot, increasing its density. CS creates near-theoretical density coating due to the combination of these events [31].

5.5 No or little masking

The particle beam in the CS method is narrow and highly defined. Despite the fact that a normal spray beam has a width of roughly 10 mm, customised rectangular nozzles may be designed to produce spray beams as tiny as 1–2-mm wide with sharp edges. As a result, in many applications, masking of areas where overspray is prohibited is unnecessary [32].

5.6 Flexibility in substrate-coating selection

Many applications need substrate-coating pairs with different materials, which are challenging to achieve using traditional methods. Even common materials like aluminium over copper or copper over aluminium can result in undesirable interfaces. When a molten copper droplet strikes an aluminium substrate, a brittle Cu–Al intermetallic phase forms, which can cause the coated structure to break during service. As a result, it is critical to carefully examine phase diagrams and pick substrate-coating couples to guarantee that no intermetallic phases emerge during high-temperature processing, which might cause the coated system to fail prematurely. The creation of weak interfaces is prevented since CS does not heat and melt the coating material. As a result, the engineer has freedom to choose materials based on design needs [33].

5.7 High bond strength

Cold-sprayed coatings have a high binding strength to a variety of substrate materials, including metals (aluminium, copper, titanium, nickel and so on), alloys (Inconel, steels, etc.) and composites (aluminium, copper, titanium, nickel, etc.) (metal matrix composites, carbon composites, etc.). Some materials, such as aluminium, can be used to coat glass. Even ultra-thick coatings have appropriate binding strength values, as can be observed [34].

5.8 Minimum thermal input to the substrate

The substrate is heated to different degrees by the flame in traditional thermal spray processing. As a result, temperature-sensitive materials, such as magnesium and polymers, are difficult to treat. Furthermore, temperature-induced substrate warping is an issue, especially when the specimen thickness is tiny. There is no high-temperature jet to heat the substrate in CSP; therefore, substrate only gets striking particles’ stagnation enthalpy. As a result, CS can be utilised to repair temperature-sensitive parts and components [35].

5.9 Compressive residual stresses

The majority of coating processes result in tensile residual stresses in the deposits. These strains cause micro-cracking on the coating’s top surface in addition to bond failure. As a result, the coating’s fatigue qualities deteriorate, and the coating’s performance attributes in the real application environment deteriorate. Because the coating is created by plastic deformation in the solid state in the CSP, the residual tension over the whole coating thickness is compressive. Coatings with compressive residual stresses are required in many high-tech sectors, such as gas turbines, since such a stress condition results in higher fatigue performance. Post-spray shot peening is used in some key applications to improve fatigue performance. Because the CS procedure generates compressively stressed coatings, they have higher fatigue qualities and do not require the extra shot peening phase [36].

5.10 Ultra-thick coatings

Most methods generate tensile stresses during coating process. These stresses build up at substrate-coating contact when coating thickness is raised, resulting in a loss in bond strength. The coating spontaneously spalls off the substrate when the coating thickness reaches a threshold amount. Because a cold-sprayed coating is compressively strained, ultra-thick layers may be created without bond failure over a wide range of substrates. In some applications, such as replacement of thick electroplated layers, this has proved advantageous [37].

5.11 No oxidation

Thermal spray treatment of oxygen-sensitive metals including aluminium, copper, magnesium, titanium and their alloys is challenging since raising the temperature dramatically promotes oxidation. The material is not heated to a high degree by CS. Furthermore, the particles are propelled by an inert gas, which effectively protects the splats that develop on the substrate. As a result, in the CS procedure, oxidation of the particle is virtually totally avoided. In fact, cold-sprayed coatings have been shown to have a somewhat lower oxygen concentration than the starting material. This deoxygenation takes place in the following way. A thin oxide layer coats each tiny particle in the feedstock material. The brittle oxide skin shatters and is carried away by the gas jet as the particle collides with the substrate surface, while the nascent metal surface attaches to the underlying surface [38].

5.12 No grain growth

CS is a solid-state consolidation method that works with temperature-sensitive materials including nanomaterials and amorphous materials at low temperatures. CS generates nanostructured coatings with no discernible grain formation, unlike other powder consolidation processes like as pressing and sintering, thermal spray and so on. Experiments have demonstrated that CS consolidation of micro-WC powders can attain unique features, such as extremely high toughness. Amorphicity retention has also been shown when amorphous alloys are CS treated [37,39].

5.13 No phase changes

During any high-temperature operation, oxidation, breakdown, development of metastable phases, preferential loss of some ingredient and so on are to be expected. Because the particles in the CS technique remain close to ambient, no phase shifts occur [38].

5.14 High strength and hardness

According to the mechanical property evaluation, the tensile strength of cold-sprayed coatings will always be greater than the bulk values. Furthermore, coatings have a higher hardness than bulk values due to its high degree of plastic deformation of the particles. Finally, there are various advantages to using the CS method. Oxidation, breakdown, phase change, grain growth and other detrimental high-temperature processes are all avoided. Phase-pure coatings with a wrought-like microstructure and relatively close density, as well as high electrical and thermal conductivities and improved corrosion resistance, are created using compressive rather than tensile stresses. Spray coatings can be used to create protective layer or to repair/refurbish oxygen- and temperature-sensitive surfaces. Ultra-thick (several cm thick) coats with high binding strength to a variety of substrates are feasible. Cold spraying eradicates necessity grit blasting on many substrate materials. The CS beam’s imprint is much narrow and well defined, allowing for a faster rate of coating layer growth and greater control of the coating’s shape without the use of masking. That can be used to create free-form and feature pieces. Because CS generates almost minimal grain growth, it may be utilised to combine nanomaterials into coating and structures [34].

5.15 High conductivity

Cold spraying produces thick coatings with excellent inter-particle bonding. In addition, coatings have a high degree of phase purity, with holes, oxides and other contaminants being negligible or non-existent. As a result, cold-sprayed coatings have a high thermal and electrical conductivity. Cold-sprayed coatings exhibit bulk conductivity values from over 92%, opposed to 40–63% for thermal-sprayed coatings, according to tests [39].

5.16 High corrosion resistance

Because of its high density, phase purity and homogeneous microstructure, cold-sprayed coatings offer outstanding corrosion characteristics. In fact, cold-sprayed aluminium and other coatings have shown to outperform bulk materials in terms of corrosion resistance [30].

7.1 CS additive manufacturing (CSAM) process

A wide variety of industrial parts have cylindrical structures, CSAM has the ability to manufacture those parts. The cylindrical structures are developed via CS technique using an exterior spindle to support the mandrel substrate. The CSAM Al pipe and flange after machining are shown in Figure 5. Al flange was developed for an Irish corporation. Initially, the Al powder coated on the Al pipe was controlled and turned through the spindle; after the development of the components, it is passed to the machining process to get the successful flange.

Figure 5

CSAM pipe after machining: (a) Al alloy cylindrical pipe and (b) Al alloy flange.

Figure 6 displays the photograph of CSAM Cu flange with the thickness of 50–60 mm thickness [48]. It is important to remember that CSAM can only generate an irregular structure, not a net-shape structure. As a result, to achieve the desired form, CSAM coatings usually need a post-machining (deductive manufacturing) operation. Any type of internal and external walls of the rotational structure (cylindrical shape) can also be fabricated through the CSAM [49,50,51]. The production technique is same to flange manufacturing, especially for external wall. Figure 7a shows 1–10 scale of canister manufactured with CSAM for the removal of CANDU spent fuels. A copper coating of 10 mm is deposited on the cast iron cylinder with pours 0.3% and density about 8,900 kg/m³, the deposit fabricated with high tensile, thermal and mechanical characteristics. For a few parts, the mandrel substrate is not required and has to be extracted at the time of deductive manufacturing. Figure 7b depicts a CSAM 10Ta-W alloy tube ultilised for the manufacturing of gun barrel liners; then 10Ta-W alloy powder was coated on the cylinder shaped Al mandrel substrate, and then internal mandrel was extracted. The extracted component is freely dipped into NaOH solution.

Figure 6

Photographic view of CSAM copper flange [48].

Figure 7

Photograph of CSAM cylindrical structure: (a) scaled canister for the disposal of CANDU spent fuels [49] and (b) 10Ta-W alloy donor tube utilised for the gun barrel liners [50].

The fabrication strategy for internal wall manufacture varies depending on the internal diameter. If the internal diameter is huge enough to accommodate a typical CS nozzle and its mounting attachments, then the manufacturing procudure was similar as developing the external wall. However, if the component is too small to fit a CS nozzle, the nozzle must be mildly tilted to accommodate the tiny area; therefore, due to the deposition angle, the deposit content is fewer. Figure 8a illustrates the dense copper coating in the cylindrical structural component of the food-processing equipment through CSAM [52]. For some instant, the gun (nozzle) must be angled to the aimed substratum at the time of manufacturing owing to the restriction of the inner diameter of the product. In this specially engineered nozzle need to be used having very small internal diameter. [53]. Figure 8b shows a specially designed small CS nozzle to the minimum diameter interior walls and Figure 8c reveals the interior Cu coating wall with the thickness of 80 mm on the metal tube with specially designed small nozzle. CSAM is also able to generate other unique cylindrical structures as shown in Figure 8. The CSAM parts of intricate structures, such as cone and gear, are presented in Figure 9. The cone and gear component is produced with higher hardness with low porosity. To develop such components, a detailed design and scan pathway should be created prior fabrication. Nozzle moving speed and scan phase with pathway must be connected with the spindle axis [54].

Figure 8

CSAM cylinder: (a) internal wall of pressure ring for food processing machine; (b) small-size CS nozzle; and (c) internal wall of small-space cylindrical pipe [51,52].

Figure 9

CSAM parts of intricate structures: (a) cone and (b) gear [51].

Pattison et al. [55] fabricated the three sheathed thermocouples and solid hemisphere mould tool through titanium particle, coating produced with high mechanical strength as shown in Figure 10. They also investigated the CSAM of Ti–6Al–4V/Al and Ti/Cu substratum; it produces the deposition with excellent adhesive and cohesive properties to the component as shown in Figure 11. Recently, a synthesis of CSAM and traditional deductive manufacturing has shown promising results in manufacturing intricate structural components. The researchers fabricated the intricate – structured cooling channel made of copper elements which meets the real time application as shown in Figure 12. Similarly, Figure 13 depicts a CSAM-fabricated part with a structure of intricately. To develop these parts, the additive components were first coated onto a uniquely engineered substratum. After deposition, the substratum was extracted and, then, the CSAM components get retained. From this, an intricate part was developed using a hybrid of CSAM and deductive manufacturing. In this technique, substrate must be constructed in such a way that it is simple to remove and post-process machining is minimal. Thus, CSAM can also be used to change the current part. Figure 14 depicts the modification technique of attaching a new component onto a bearing lid through CSAM. After CSAM, an entirely new part was acquired with no clear distinction between the boundary of new element and original section [56]. Manufacturing of such complicated mechanisms is essential for CSAM technique, as it greatly extends the range of applications.

Figure 10

(a) Three sheathed thermocouples embedded within a titanium part and (b) titanium hemisphere formed by mould tool [55].

Figure 11

Fragments of bimetal Ti–6Al–4V/Al and Ti/Cu plates manufactured through CS process and machining is made by milling after deposition [55].

Figure 12

AM with copper and tool steel with cooling channel-drawing [1].

Figure 13

CS interpreted concept: (a) mandrel after deposition and (b) mandrel removed and post-machined [55].

Figure 14

Modification technique of adding a new component to a bearing cap through CSAM [56].

CSAM can build array structural components, as illustrated in Figure 15, the production method requries a uniquely engineered mask to avoid the deposit of superfluous materials and enable to deposition on the specified template to fabricate the high densities pyramidal fin arrays [57,58,59,60]. By changing the SOD and model structure of the wire mask, the fin array structure and density may be precisely managed [61]. The thermal and mechanical characteristis are better comparision with the conventional rectangle shaped fins; this is owing to the greater convection heat dissipation coefficients [62]. Cormier et al. [63] observed that the heat transfer efficiency can be enhanced by raising fin altitude and fin density at the expense of a greater pressure drop. Mask-attached CSAM may also be utilised to develop chip heat sinks, metal markings and various components (unique structure).

Figure 15

CSAM pyramidal fin arrays heat sink: (a) schematic view of the manufacturing technique and (b) photograph of a CSAM fin array heat sink [57].

7.2 CS repair and restoration of components

The mechanical component gets degraded after certain periods of time owing to electrochemical or tribological actions or other factors. The defected parts are often unable to be restored and must be changed owing to a shortage of appropriate restoration techniques [64]. CSAM is a economical method that has the tremendous ability to restore the defected parts; this is owing to the ability to neglect the thermal defect of the fundamental substratum and able to sustain the same characteristic of the coating material. CSAM had effectively used to recover a numerous of detorated and defected parts in different disciplines [65]. From this process the feed stock material does not directly deposited on the defected area due to its intricate surface geometry, and the ability for the degraded surface of the defected region will affect bonding behaviour. So, prior machining is needed above the affected area to rebuild the degraded region. Surface healing needs to be carried out on the restored area with various machining operations to get the better surface finish that is acceptable to the CSAM [66]. Even for the CSAM, the deposited products must be carried out for the machining process to attain the stanadard geometries. The typical restore procedure of a component is shown in Figure 16.

Figure 16

Repairing steps for CSAM [66].

In the aviation sector, the restoration and maintenance of aerospace components is the more challenge owing to the poor behaviour of electrochemical and tribological properties. This happens on the component owing to high speed impact and rotation [67]. The Mg and Mg alloy posses the excellent properties (greater strength-to-weight ratio) compared with other alloys. So, it is widely used in the manufacturing of aircraft transmission gearboxes. The application of Mg alloy in the gear boxes is challenging because the Mg alloy gets oxidized due to the anodic reaction with other metal. So, the periodic service is required to improve the life span of the gear boxes to avoid the failure risk of the aircraft [68]. To overcome this corrosion and wear problem, CSAM is used to repair or restore the components by depositing Al or Al alloy materials on the defected area as shown in Figure 17. Schell [69] reveals enhanced adhesion, cohesion properties of the component with improved wear and corrosion behaviour in restored components. The repaired components meet real-time application in aircraft.

Figure 17

Comparing the deteriorated and restored parts by CSAM: (a) S-92 helicopter gearbox sump; (b) oil tube bores in CH47 helicopter accessory cover; (c) UH-60 helicopter gearbox sump; and (d) UH-60 rotor transmission housing [68].

Al alloys are widely used in aviation sector owing to its good mechanical, electrochemical and tribological properties. Several components of the helicopter are fabricated by using the Al alloy. Electrochemical properties are poor in snap ring groove region. It is a common type of defect which affects the life span of chopper mast support. When the pitting corrosion occurs on the mast supports, it needs not to be replaced with a new one [70]. It can be restored to real time application using CSAM as shown in Figure 18a. The thorough restoration process includes removing of pitting corrosion from the defected area and refilling the removed element with CSAM deposition of Al alloy, and to get the required geometry post-machining is carried out. Kilchenstein [68] restored the pitting corroded F18-AMAD gearbox made of Al alloy using CSAM. This increased the life of the gear box and reduced the cost of component replacement owing to the excellent coating properties which helped to meet the real time application as shown in Figure 18b. The same result was obtained by Howe [71] who restored the Al alloy T-700 engine frontal frame by CSAM with excellent corrosion and wear resistance as shown in Figure 18c.

Figure 18

Comparing the deteriorated and restored parts by CSAM: (a) AH-64 helicopter mast support, (b) F18-AMAD gearbox, and (c) front frame of T-700 engine [70].

CSAM is effectively implemented to restore the corroded surface of inner bore of Al alloy valve actuator. It does not cause thermal damage to the underlying substrate and offers enhanced capabilities compared to conventional repair techniques as shown in Figure 19a. The CSAM actuator is assembled in the engine section to service in the actual time application after passing through all property tests [72]. Furthermore, in the automobile sector, CSAM restored the Al alloy Caterpillar-3116 and 3126 engines’ oil pump housings affected by corrosion as shown in Figure 19b. Lyalyakin et al. [73] reported that nearly 30 number of oil pump housings are restored, and these repaired products are passed to real-time service; still no failure reports are reported.

Figure 19

Comparing the deteriorated and restored parts by CSAM: (a) inner bore surface of a navy valve actuator and (b) oil pump housing of Caterpillar engine [72].

In the aeroplane, the front landing gear steering actuator barrels are fabricated by nickel super alloy because of its better corrosion and wear resistance, but at the time of flight, the steering actuator barrels are affected by corrosion and wear due to humid air and impact of foreign particles while landing. To overcome this problem, the CSAM is introduced to repair the pitting corrosion and cracks in the B737 front landing gear steering actuator barrels [69]. Figure 20 shows the photograph of repaired B737 nose wheel steering actuator barrel by CSAM. It helps to increase the life span of the component by periodic servicing. In the aviation sector, another type of damage that components can suffer is mechanical defects due to fatigue, unexpected impact, repair or improper service. These kinds of mechanical defects are restored by CSAM. Figure 21 illustrates the CSAM repair technique of a mechanical process via damaging the flap transmission tee box assembly of an aeroplane component. The defected parts were restored to the original state after post-machining process. CSAM is even utilised to restore the mechanically defected huge cast automobile components as shown in Figure 22a [74]. It also used to restore the defected mould via CSAM as shown in Figure 22b. Then, in a cutting force experiment using bulk material, the restored mould showed the same output and reveals good tribological behaviour than the original parts [75].

Figure 20

Repaired B737 nose wheel steering actuator barrel by CSAM [69].

Figure 21

Repairing steps of a mechanical means deteriorated flap transmission tee box housing of an aero-plane by CSAM [69].

Figure 22

Comparing the deteriorated and restored parts by CSAM: (a) larger cast automotive components [74] and (b) mould [75].

The skin surfaces of boeing and defence aircraft are mainly made of Al alloy material, with Al cladding to avoid electrochemical behaviour. When the aircraft flies continuously, the aeroplanne surface has a greater possibility of affecting erosion as well as wear defect because of high-velocity impact of foreign particles in the atmosphere. The destruction region behaves as a weak spot, promoting to corrosion, and passes into the defensive Al clad film, bottom to the base Al alloy. When the Al alloy is heavily affected by corrosion, the refurbishment and failure expense were huge. As a result, the weakened aluminium cladding must be repaired prior to corrosion spreads to the base substrate [76]. To overcome this problem, various types of thermal spray process are often used to restore the defected aircraft skin when comparing the CSAM with other convectional thermal spray process [77]. The conventional thermal spray process produce coatings with high temperature it affect the thin Al alloy skin surface of the aircraft. CSAM has the ability to restore this destruction without risking damage to the corresponding base material because it works at very low temperature [78]. Figure 23 illustrates the restoration of an Al cladding on an Al alloy plate through CSAM. The affected area tends to be absolutely covered by the Al coating, with no discernible difference from the Al cladding. The result reveals that the restored component with higher hardness and fatigue strength, as well as the corrosion resistance of the component, is enhanced [79].

Figure 23

Restoration of Al cladding layer on Al alloy panel with self-machining damages by CSAM: (a) schematic representation of self-machined damage and (b) restoration technique [79].

Fatigue analyses of CSAM-restored Al alloy 2099 plates were performed. Figure 24a reveals that the photograph view of the notch was developed during the machining process on the Al alloy 2099 plate area. Then, the notch was restored through CSAM using the Al alloys 2198 and 7075. On the coated frames, crack growth studies were conducted [80]. Figure 24b indicates the crack behaviour based on the number of cycles for the defected and restored Al alloy frame. The crack size of the restored frame was reduced owing to the higher adhesive behavior of CSAM coating with the base material. This investigation reveals that the CSAM has the potential for enhancing the fatigue behaviour of defected plate. In the aviation sector, the aeroplane fuselages were affected by the multi-site damage (MSD) problem. Almost every type of aeroplane fuselage is manufactured by Al alloy lap joint skin frame. As a result, flaws and damages related to lap joints can develop after long periods of time. The corrosion arises between the interface of the fastener bore and skin. Furthermore, various fastened strip repairs serve as a weak point, allowing cracks to form more frequently. These affect the MSD on the surface of the aeroplane.

Figure 24

(a) Photograph of machining process made notch on Al alloy 2,099 panel surface and (b) crack length vs number of loading cycles [80].

Using CSAM with quality sealant, the sealant humidity inside the joint produces the interface among the mating frames and fasteners and aeroplane upper surface can be avoided [73]. Figure 25 illustrates the photograph of CSAM coatings on the fuselage fasteners. As a result, the layers of the fasteners are totally closed through the CSAM coatings. As a result, the fatigue strength of the deposit is improved; it leads to improve the fuselage structural integrity of the aircraft. The aeroplane propellers are affected by severe erosion owing to high-velocity stick by foreign object and moisture air in the environment. However, a high amount of blade damage is developed at the time of aeroplane taking off and landing. The current technique used to restore this defect involves removing of the eroded blade up to the point of defect by machining process. To overcome this failure, CSAM is used to deposit the material in the eroded region. Then, the restored blade is fitted to the aircraft which meets the real time application [81]. The restoration technique of Al alloy blades by CSAM is shown in Figure 26.

Figure 25

CS coating on simulated fuselage fasteners [76].

Figure 26

Restoration technique of Al alloy blades by CSA [81].

7.3 CS coatings on biomedical application

Cold gas dynamic sprayed hydroxyapatite (HAP) deposit is commonly used due to its excellent biological properties. Then, the osteoconduction element will produce skelet-developing cells with adjacent skelet tissue [82]. Choudhuri et al. [83] investigated the bio-ceramic deposit (HAP and Ti) for various purposes of medical implants and dental to improve bone amalgamation. Several investigations were carried out on HAP deposit (Ca₁₀(PO₄)₆ (OH)₂ for various medical surgical applications since HAP deposit delivers same crystallised and chemicalised properties as that of skelet mineral [84,85,86]. Cinca et al. [87] investigated the coating properties of bioactive HAP, CS deposit on Ti–6Al–4V and Al7075T6 base material. As a result that coated specimen is less expensive for biomedical purpose, yet it is ductile in nature. Al-Mangour et al. [88] revealed the effect of heat treatment on hardness properties and microstructure of CS 316L-deposited stainless steel by helium and nitrogen as a process gas. They concluded that heat treatment leads to reduce porosity, enhanced interparticle bonding and ductility.

Dikici et al. [89] investigated the electrochemical properties of kinetic-sprayed 316L steel powder on the Al5052 alloy base substrate. Based on the investigation, coated specimen has excellent adhesive properties. To further enhance the hardness and electrochemical characteristics, heat treatment techniques are adopted. Gardon et al. examined the Ti coatings on polyetheretherketone base material. According to the results, no polymer deterioration was found at the time of coating. It even discovered that titanium with biocompatible polymer implants may be effectively coated. It results in homogenous, dense and high bonding deposit [90]. El-Eskandrany and Al-Azmi [91] investigated the Cu50Ti20Ni30 coatings on SUS 304 base material to assess its antibacterial characteristics for food and medical industries. The findings of this study revealed that bacterial prevention is efficient. Biofilm formation developed after coatings offers viable options for managing microbial growth.

7.4 CS coatings on antimicrobial application

In various food preservation sectors and hospitals, the growth of bacteria on the surface happens owing to the possibility of increased bacterial infection [92]. In the hospital floors, comprising nurse station, patient rooms and kitchens were contaminated by bacteria [93,94,95]. Champagne and Helfritch [96] examined the antimicrobial characteristis of Cu and its alloys’ opposition to variety of microorganisms that cause health issue in health-care and food sectors. Even so, a preferred technique of copper surface coatings in these applications has not yet been found. Champagne and Helfritch examined the antimicrobial effectiveness of various Cu surfaces. The various methods of thermal spray technique were used to coat Cu on the substrate. Compared with other spray techniques, CS was found to be more efficient. It is resulted in 99% microbial degradation after a 2 h. Da Silva et al. [97] studied the antibacterial and electrochemical behaviours of cold gas dynamic-sprayed copper deposit on carbon steel alloy. The investigation reveals that no electrochemical behaviour was detected between the base material and deposit at the completion of 1,100 h of immersion in Cl solution.

7.5 CS coatings on electrical application

The CS coatings have a superior electrical and thermal conductivities; they are widely used to improve the electrical properties of any alloy metal and are best equipped for electrical application. CS deposit is primarily used in electrical industries since ceramic coatings are commonly used as insulating element. Tazegul et al. [98] examined the copper and Al₂Cu coatings on copper substrate for electric tonics applications. The amount of Al₂Cu particle varies from 0 to 15% by volume. According to this study, the composite deposit with the presence of 5 and 10% volume of Al₂Cu particle reveals the better tribological behaviour and electrical conductivity to the coatings due to the high dense coating. Wang et al. [99] investigated the impact of heat-treated Ag-SnO₂ coatings on copper substrate for voltage switching devices. The experiment reveals that eroded region of Ag-SnO₂ deposit is steady and stable at 850°C. Furthermore, the contact resistance of the deposit is reduced and stabilised in 3.4 mΩ. They revealed that heat treatment is the most important factor that impacts on contact resistance. Li et al. [45] examined the copper coatings on copper substrate with the thickness of 7 mm to improve the electrical resistivity of the deposit. The experiment reveals that coating posses the excellent cohesive and adhesive behaviours, resulting in the enhancement in electrical resistivity contract to uncoated copper substrate.

8.1 New design techniques

New design and production options are opened up by combining CS with conventional subtractive manufacturing techniques:

• Finishing the inside surface of channels that have been machined. These coatings might be added to offer anti-corrosion or other protective characteristics, as well as lower drag coefficients.

• Adding minute, intricate elements to substantial machined components.

• Making a component out of a lightweight material and applying desirable or conductive surface coatings.

• Manufacturing the same kind of lightweight component but adding materials to improve structural integrity while preserving weight-savings.

8.2 Post-processing of CS coatings

The completion of conventionally made components frequently involves heat treatment and other post-fabrication procedures. There is growing investigation into post-deposition heat treatments, even though CS coatings are normally left as-deposited. With the use of brief induction heating periods, which restrict the heat input and the depth of heating, for instance, there have been favourable test results lowering the porosity of the coatings [100].

8.3 Increased use for repairs

As manufacturers explore for methods to minimise costs, specialised repairs are an increasingly significant service. To swiftly restore damaged components to their original dimensional tolerances, CS coating can be used. These same coatings may also have improved performance qualities like anti-corrosion defence that will further increase the lifespan of the restored component. For enhanced wear resistance without sacrificing corrosion resistance, hard, galvanically inert phases like ceramic particles can be used [101]. The choices to repair massive components already in situ will also become more attractive as progress is made in the development of portable CS systems. By choosing this method, there will be more options for fixing major things rather than having to wait for them to be shipped off-site or spending more money to replace them entirely. A rising number of businesses will think about incorporating CS technology into their manufacturing and repair capabilities as awareness of its adaptability, rapid setup and improved economic costs rises [102].