Nanocoatings for ballistic applications: A review

3.1 Energy absorption mechanism in the nanocomposites under a ballistic impact

The mechanical performance of polymer composites tends to increase with the reinforcement of nanoparticles. This nano-reinforcement is generally recognised as the third phase. However, the mechanism involved in the performance of nanocomposites can be completely different compared to composites having reinforcements at the micron level [63]. This situation is due to the higher surface area of the nanoparticles. For example, upon ballistic impact, polymer resins without nanoparticles will undergo failure mechanisms like radial cracks, shear yielding, and shear plugging. However, if nanoparticles are incorporated into polymer resins, the failure mechanism takes place through crack bridging, crack deflection, debonding of nanoparticles, and plastic void growth [17]. In this direction, it is essential to understand the mechanism involved in the absorption of impact energy for nanocomposites. Eq. (1) helps in understanding how the total energy of the projectile is absorbed by the nanocomposites. It provides the energy lost (E _L) by the projectile in the process of perforating the target:

where m denotes the projectile’s mass, V denotes the projectile’s incident velocity, and V _r denotes the projectile’s residual velocity. Eq. (2) describes the energy the laminate has absorbed overall (E _Total):

where E Fra is the absorption of energy due to fibre failing in tensile mode, E Cone is the energy carried by the travelling projectile, E Def is the absorption of energy due to the deformation of secondary yarns, E Dela is the absorption of energy due to delamination, and E Mat Crack is the absorption of energy due to the generation of cracks in the matrix. Eq. (3) can be used to calculate the nanocomposite laminates ballistic limit velocity ( V Bl ):

3.2 Pinning of cracks

The incorporation of nanoparticles in the composites helps in the stoppage of crack propagation. It is commonly recognised as a mechanism of crack pinning. From Figure 4, it can be seen that the addition of nano-silica resulted in the pinning of cracks [64]. This is one of the dominant mechanisms when it comes to energy absorption [65].

Figure 4

SEM images of crack pinning in epoxy resin (a) without nano-silica, (b) with 2.5 vol% nano-silica, and (c) with 13.4 vol% nano-silica [64].

4.1 Nano-silica-based shear thickening fluid (STF)

STF is a smart anti-ballistic material that tends to change from liquid to solid phase upon ballistic impact to resist the impact caused by high-speed projectiles [42]. Upon the disappearance of the impact, the solid phase returns to its initial state (liquid phase) reversibly. STF is a non-Newtonian fluid in which shear stress increases non-linearly with an increase in the shear rate. The viscosity of the STF increases suddenly as the shear rate crosses the critical value called the critical shear rate. It is generally known as shear thickening behaviour, and this process is reversible, i.e. upon release of impact force, STF tends to come back to its initial flow state. Figure 5 shows the shear thickening phenomenon. STF is generally made up of two phases: the dispersed phase and the dispersion medium. The dispersed phase is made up of sub-micron or nanoscale particles of silica, poly-methyl methacrylate (PMMA) or calcium carbonate, etc. On the other hand, the dispersion medium consists of water, polyethylene glycol (PEG), silicon oils, and polypropylene glycol (PPG) [72,73]. The key to attaining the shear thickening behaviour is the proper interaction between the dispersed phase and the dispersion medium. For dispersed phase, fumed silica or colloidal silica nanoparticles, and for dispersion medium, PEG is extensively used by most researchers. The main reason behind choosing PEG as a dispersion medium is due to its non-toxic nature, thermal stability, and ease of handling [74,75,76]. To achieve the optimal distribution of particles throughout the STF, breaking of intermolecular interaction among nanoparticles is essential [77]; this can be accomplished by the use of techniques like sonochemical, magnetic stirrer, high-speed homogenisation, ultrasound sonication, etc. [78,79].

Figure 5

Mechanism of shear thickening [25].

Different theories have been suggested by researchers to describe the behaviour of shear thickenings, such as order–disorder theory, dilation theory, hydrocluster theory, and contact-rheology theory [77,80,81]. The behaviour of STF depends upon crucial parameters such as the presence of solid content, particle shape, particle aspect ratio, size of the particles and their distribution, interaction among particles, particle hardness and temperature, pH, and nanofillers [4]. Generally, to prepare STF, first nano-silica particles are dispersed in ethanol with the help of ultrasonication; then, a known amount of PEG is added to this solution with subsequent ultrasonication, and finally, the prepared STF is dried in an oven to evaporate excess ethanol. To perform the impregnation of fabric, STF is again diluted with ethanol and ultrasonicated; then, the fabric is soaked in the STF solution and dried in an oven to evaporate the excess ethanol, as depicted in Figure 6.

Figure 6

The preparation process of STF-impregnated fabrics: (a) SEM images of silica particles, (b and c) impregnation process, and (d) drying treatment [82].

In the case of neat panels, when a fabric is hit by a bullet, angular orientations act as a facilitator in the propagation of stress waves through primary yarns in different directions. This beneficial effect (facilitated by angular orientation) is insignificant due to the smaller number of primary yarns and relatively insufficient contribution from secondary yarns in energy absorption. The involvement of secondary yarns can be enhanced by impregnating the fabric in STF. Thus, the use of STF further enhances the advantage of angular orientation by increasing the dome formation area (Figure 7) [83].

Figure 7

Propagation of stress waves in neat and STF-treated fabrics [83].

SEM images of the Kevlar fabric impregnated in STF (35 wt%) have indicated good dispersion of silica nanoparticles on the surface of Kevlar fabric(Figure 8) at different magnifications. Deposition of STF in the inter-yarn gaps at the yarn crossover region was also observed. The specific energy absorption (SEA) capacity of the Kevlar/STF composite was 2.3 times higher compared to the neat Kevlar fabric.

Figure 8

SEM images of Kevlar fabric impregnated with STF (35 wt%) [84].

A rheometer is generally used to understand the rheological response of the developed STF. Figure 9(a) and (b) depicts the influence of nano-silica wt% on the rheological response of the developed STF, and Figure 9(c) and (d) presents the rheological response of the developed STF at different operating temperatures. From the figure, it is evident that the viscosity of all the developed STFs varies non-linearly with respect to the applied shear rate. Upon application of the shear rate, STF experienced both shear thinning as well as shear thickening. At lower shear rates, a shear-thinning response was observed and at higher shear rates, a shear-thickening response was observed [42,79,85,86]. From Figure 9(a) and (b), it can be observed that with the increase in the nano-silica wt%, critical shear rate values have decreased, and once the shear rate exceeds its critical value, a sudden jump in the viscosity of the STF can be observed. When the effect of the temperature was investigated on the rheological response of the STF, it was found that with the increase in the temperature, the shear rate required for attaining the highest viscosity and critical value of the shear rate also increases and decreases in peak viscosity values, as depicted in Figure 9(c) and (d). Also, results of the rheological study have revealed that, with the increase in the particle size (from 100 to 500 nm), the critical shear rate values decrease. Along with the many advantages in ballistic impact applications, STF exhibits certain limitations like temperature sensitivity, limited durability, and complex manufacturing processes [87].

Figure 9

Rheological behaviour of STF: (a) [84] and (b) [42] for different wt% of nanosilica and (c) [88] and (d) [89] at different temperatures.

4.2 Graphene nanoparticle coating

As graphene is a rigid and high-strength material, the potential future of graphene in ballistic applications has been investigated [90,91,92]. Previously, graphene layers were employed in manufacturing body armours to reduce blunt trauma and dissipate the projectile’s kinetic energy away from the strike zone. However, the manufacturing of graphene-layered body armour is quite expensive. With the development of nanotechnology, graphene nanoparticles have been used in the manufacturing of body armours.

The application of graphene nanoplates on the high-performance fabric surface results in the increase of tensile strength, tenacity, and Young’s modulus of the nanocomposites [93]. Reinforcement of Aramid fabric (Twaron^®) was performed with graphene oxide (GO) solution; subsequently, heat treatment was carried out at 120°C. The ballistic behaviour of the fabric was improved by increasing the coefficient of friction of the fibres. A comparison between neat and GO-coated fabrics with one and two filtrations was carried out; GO-coated fabric with two filtrations has shown better resistive force (50% increase) in comparison with neat fabrics [31]. All the samples were compared using optical microscopy (OM) and scanning electron microscopy (SEM). As shown in Figure 10, smooth surfaces were observed on OM (a) and SEM (b) for neat aramid fabric. Figure 10(c) and (d) shows the partial covering of GO flakes from the first filtration. Moreover, Figures 10(e) and 26(f) show almost complete covering of small GO flakes on aramid from the second filtration [31]. The uniform distribution of GO flakes during coating helps in obtaining a superior ballistic performance of the aramid [90].

Figure 10

(a) OM of neat aramid, (b) SEM of neat aramid, (c) OM of aramid+ GO (1), (d) SEM of aramid+ GO(1), (e) OM of aramid + GO(2), and (f) SEM of aramid + GO(2) [31].

A molecular dynamic test was carried out on graphene-coated polyethylene samples, and it was found that graphene has the potential to transform polyethylene into a BIR high-performance material. Over an eight-fold increase in the ballistic impact, the resistant capacity of the polyethylene can be achieved by coating a layer of graphene on it. The impact of graphene was more pronounced when graphene was added in the form of a coating on the surface rather than as reinforcement. In the multi-layered body armour system, graphene at the topmost layers only contributed to the absorption of ballistic impact energy, as depicted in Figure 11 [94].

Figure 11

Velocity profile of the projectile: (a) coated sample and (b) multi-layered reinforced sample [94].

To understand the significance of the incorporation of graphene nanopowder in its ballistic properties, experiments were conducted on a glass fibre-based composite. Two types of composites with 2 and 3 mm thicknesses were developed using the vacuum-assisted resin transfer moulding technique. Upon the addition of 0.1 wt% of graphene nanoplatelet to the composite, the energy absorption capacities of the 2 and 3 mm thick samples improved by 9.2 and 8.2%, respectively. An increase in the energy absorption capacity was attributed to the enhanced ductile properties of the composite [95].

To evaluate ballistic performance, an investigation was carried out on curaua fibres (CFs) coated with graphene oxide. The coating was done by immersing the CF in the graphene oxide solution (0.1 wt% with respect to the fibre), and then the coated fibres were dried in an oven for 24 h at 80°C. The presence of graphene oxide was confirmed by the Raman and Fourier transform infrared spectroscopy. From the FTIR analysis, it was evident that graphene oxide exhibited changes in the characteristic bands of the CFs [96]. Changes in the surface morphologies of the CFs after the deposition of graphene oxide can be observed in SEM images, as presented in Figure 12. At higher magnifications, cracks have opened up on the CF surface due to the intensity of the electron beams, as presented in Figure 12(a), with a white arrow indicating the crack location. However, graphene oxide-coated CF was more stable, and the surface was more intact without any visible cracks, as shown in Figure 12(b).

Figure 12

SEM images of the surface of the fibres: (a) CFs and (b) graphene oxide-coated CFs [96].

4.3 CNT coating

CNTs have been extensively employed in various applications owing to their excellent chemical, electrical, magnetic, and mechanical properties. CNTs are used in the making of useful articles like bulletproof vests, artificial muscles, expansion-proof blankets, and reinforced materials [97]. To explore the capability of CNTs in ballistic behaviour, it is essential to understand the mechanism of bullet impact involving how the velocity, momentum, force, and energy vary in the time horizon when the bullet strikes. CNTs with a larger area can withstand projectiles with higher velocity. Body armours from CNTs exhibit sustained resistance, even when multiple bullets are shot at the same location [98]. The performance of polymer composites at cryogenic temperatures can be enhanced with the inclusion of an appropriate amount of CNTs. CNTs are categorised into two types: single-walled and multi-walled CNTs. From an economical perspective, MWCNTs are comparatively less expensive than SWCNTs [99].

Though fibres like UHMWPE are used significantly in ballistic and aerospace applications, it possesses poor adhesion properties with the polymers due to their non-polarity and chemical inertness [100,101,102,103]. To overcome this limitation, two approaches are generally employed: (i) the use of surface treatment of the fibre to enhance the surface energy and (ii) the inclusion of nano-reinforcements in the matrix to decrease the surface energy [104,105,106]. The introduction of polar functional groups onto the polymer matrix can be achieved by incorporating various nano-reinforcements, like multi-walled CNTs, carbon nanofibres (CNFs), nano-clay, and graphene nanoplatelets (GnPs) [107,108,109]. CNTs are known for possessing outstanding Young’s modulus (greater than 1 TPa) and tensile strength (100 times greater than that of the strongest steel). An investigation was carried out on the UHMWPE/epoxy composite by incorporating MWCNT as a secondary reinforcement. To enhance the compatibility with the epoxy matrix, chemical treatment is carried out on UHMWPE and MWCNTs by using glycidyl methacrylate and amino-thiol. Results showed that chemical treatment increases the wettability, adhesion, tensile properties, and inter-laminar shear stress [110]. The study conducted on carbon fibre/MWCNT/epoxy nanocomposites found that the inclusion of MWCNT results in an increase in conductivity. thermogravimetric analysis has shown that with the increase in MWCNT content, the thermal stability of the nanocomposites is also enhanced [111]. As the army bases are located in various climate conditions, soft body armours must possess good thermal stability.

An investigation was carried out by growing CNT on the E-glass fabric using the CVD method. The introduction of CNT in the composite resulted in the reduction of ILSS and bending properties. However, when the ballistic impact test (V50) was carried out, the composite consisting of CNT exhibited an inflated V50 value (up by 11.1%) due to the enhanced energy absorption properties [15]. An evaluation of the BIR capacity of the newly developed composite was carried out. The composite was made up of CNT/polystyrene ethyl acrylate (PSt-EA)/C-STF/Kevlar. The rheological behaviour of the STF developed from PSt-EA was found to increase significantly upon the addition of CNT. The study also highlighted the importance of adding an optimum amount of CNT (1.0%) into the STF. In the current work, when the amount of CNT addition exceeded 1%, it reduced its significance. Figure 13(a–d) depicts the results of the yarn pull-out tests. From the results, it can be observed that for the neat Kevlar fabric, highest pull-out force recorded was around 2.5 N, and it increased to 9 N for the Kevlar/STF sample. Upon the addition of CNT, the pull-out force reached nearly 15 N, which was the highest among all the samples. An increase in the pull-out force was attributed to the enhanced inter-yarn friction (from 0.50 to 0.80) upon the addition of CNTs. Figure 14(a–c) depicts the SEM images of neat Kevlar, Kevlar/STF, and Kevlar/STF/CNT. Figure 14(d) presents the deposition pattern of PSt-EA and CNT nanoparticles [22].

Figure 13

Yarn pull-out test: (a) neat Kevlar, (b) Kevlar/STF, (c) Kevlar/STF/CNT, and (d) pull-out forces at 20 mm/min for all the samples [22].

Figure 14

SEM images of (a) neat Kevlar, (b) Kevlar/STF, (c) Kevlar/STF/CNT, and (d) deposition patterns of PSt-EA and CNT nanoparticles [22].

4.4 Nano-clay

Nano-clays are made up of layered silicates having octahedrally shared Al(OH)₃/Mg(OH)₃ edge and Si atom bound tetrahedrally as structural elements. Nano-clays are obtained from organic sources [112]. In comparison with other nanoparticles, nano-clays are less expensive from an economical perspective and are pro-environment. These advantages have encouraged researchers to further explore the utilisation of nano-clays in nanocomposites for various applications. An investigation was carried out by Pol et al. [41] to identify the influence of nano-clays on the ballistic behaviour of the E-glass/epoxy composite panel. Nano-clays are loaded into the epoxy system by varying their weight percentage (0–7 wt%) with respect to the resin. With the incorporation of 3 wt% of nano-clays, 16% enhancement in the Young’s modulus was achieved. On the other hand, for 5 wt% of nano-clays, enhancement of Young’s modulus was restricted to 8%. Upon the addition of 5 wt% of nano-clays, energy absorption and damaged area of the panel increased significantly at 142 m/s. Another experimental work was carried out on Kevlar fibre, and nano-clay-reinforced epoxy composite. Graphene and nano-clay (up to 10 wt%) were added to the Kevlar/epoxy (70/30 vol%) composite as nanofillers. Results from the flow velocity and drop-down tests indicated that the addition of nano-clay had a more significant effect in terms of absorption of energy and impact resistance capability in comparison with graphene. In the penetration behaviour study, nano-clays outperformed graphene [38].

Assessment of ballistic performance of E-glass/nano-clays/polyester composite was carried out at various pre-load conditions like without preload, and preload in both uniaxial and biaxial directions for tension and compression. Results indicated that the application of preload would result in a decrease in energy absorption and ballistic behaviour of the composites. For comparison, two types of samples were developed, one without nano-clays and another one with nano-clays (1.5 wt%). Upon the addition of nano-clays, the surface morphology of the composite changed significantly, as shown in Figure 15 [113]. Ballistic test results revealed that the incorporation of nano-clays in the composite results in an increase in energy absorption for both no-preload and preload conditions [113].

Figure 15

SEM images of the composite (a) without nano-clay and (b) with nano-clays (1.5 wt%) [113].

4.5 Other nanoparticles

Apart from nano-silica, graphene, CNTS, and nano-clays, other nanoparticles like silica carbide (SiC), titanium boride (TiB₂), boron carbide (B₄C), graphite, ZnO nanowires, and aluminium oxide (Al₂O₃) were also employed as reinforcement in the development of BIR composites. An examination was conducted to understand the effect of silane-treated alumina nanoparticles on the ballistic performance of the UHMWPE/epoxy composites. The addition of silane-treated alumina nanofillers was done at different weight percentages like 0, 1, 3, and 5 wt%. Among all the samples, energy absorbed by the composite consisting of 5 wt% of alumina was the highest (183.2 J), and the minimum number of plies required for stopping the bullet was also the least. When the ballistics test was carried out at an initial projectile velocity of 373 m/s, the residual velocity recorded for the sample having 5 wt% of alumina was also the least among all [114].

To improve the inter-yarn fraction of aramid fabrics, ZnO nanowires were grown on the surface of aramid fabric [115]. The increase in the inter-yarn friction was attributed to the interlocking mechanism between the grown ZnO nanowires and fibres. In addition to the inter-yarn friction, ZnO nanowire growth improved the tensile strength (13%) and Young’s modulus (10%) of the fabric. In another work, to improve the fracture toughness of fibre-reinforced composites (glass/carbon/epoxy), alumina nanofillers (Al₂O₃) and graphene nanoplatelets (GNPs) were added as hybrid nano-reinforcements [116]. Upon the addition of nanoparticles, nanobubbles observed for the neat samples are filled with nanoparticles (Figure 16), which resulted in the improvement of fracture toughness of the composite.

Figure 16

(a) Glass fibres, (b) carbon fibres, (c) hybrid glass and carbon fibres, (d) hybrid glass and carbon fibres at 1.5 wt% GNPs and1.5 wt% Al₂O₃. (e) Hybrid glass and carbon fibres at 3 wt% Al₂O₃. (f) Higher magnification of hybrid glass and carbon fibres at 1.5 wt% GNPs and 1.5 wt% Al₂O₃ [116].

The influence of various nanofillers like GNPs, CNTs, and hexagonal boron nitride sheets (BNNS) on the ballistic performance of the glass/epoxy composite was studied. Five variants of nanocomposites, namely (i) glass/epoxy, (ii) glass/epoxy + GNPs, (iii) glass/epoxy + CNTs, (iv) glass/epoxy + BNNS + GNPs, and (v) glass/epoxy + BNNS + CNTs, were developed for carrying out a comparative study. Ballistic impact test results show that an increase in impact resistance was observed for all the samples consisting of nanofillers compared to samples without nanofillers (glass/epoxy). Among all the samples, glass/epoxy + BNNS + GNPs recorded the lowest exit velocity when tested at the impact velocity of 134 m/s, as depicted in Figure 17(a). Across samples, SEA was highest for glass/epoxy + BNNS + CNTs (16.3% higher compared to neat samples as shown in Figure 17(b)). SEA of all the nanocomposites was higher than that of the neat sample [47].

Figure 17

Influence of nanomaterials on the (a) exit velocity of the projectile and (b) upon ballistic impact (134 m/s) SEA by all the samples [47].

4.6 Dispersion of nanoparticles

The incorporation of nanoparticles in the development of soft body armour systems is considered one of the prominent methods to enhance its ballistic impact performance without compromising on the weight aspects. However, the dispersion of nanoparticles in the composites in a homogeneous manner is a very challenging task. Non-homogenous dispersion of nanoparticles can restrict the exploitation of their advantages [117]. Agglomeration of nanoparticles results in the reduction of their overall effectiveness [118]. During the synthesis of nanoparticles, two types of clusters are generally formed, known as aggregates and agglomerates. Aggregate clusters are formed by the covalent bond between the primary particles, whereas agglomerate clusters are formed by weak bonding forces, which can be separated into primary particles [119,120]. To obtain proper homogenous dispersion of nanofillers in a suitable liquid medium, researchers have used mechanical stirring and ultrasonication with high intensity [121,122]. Similarly, GnPs were dispersed in epoxy resin with acetone, ultrasonication, and magnetic stirring (hot plate), as depicted in Figure 18 [19].

Figure 18

Steps involved in the dispersion of GnPs in the epoxy resins [19].

To improve the dispersion of CNTs and break the agglomerates, plasma functionalisation was carried out on the surface of CNTs [113]. After functionalisation, CNTs were able to gel with the matrix material and form a good homogenous solution. Plasma functionalisation brought changes to the surface morphology by developing small defects and functional groups [123]. The hydrothermal reaction is another effective method for the dispersion of nanoparticles, in which a solution consisting of nanoparticles and solvent undergoes a heterogeneous reaction at high temperature and pressure. In an earlier study, MWCNTs were dispersed in various organic media like ethanol, dichloromethane, hexane, and isopropyl alcohol by employing a hydrothermal reaction. Raman spectroscopy characterisation revealed that, among all the dispersion media, the dispersion and debundling ability of MWCNTs increased significantly for dichloromethane. Results obtained from other characterisation techniques like UV-Vis spectroscopy and transmission electron microscopy were also in agreement with the Raman spectroscopy results [124].

In another study, the dispersion of MWCNs was assessed in different media like sodium hydroxide, sodium silicate, and combined sodium hydroxide and sodium silicate. Among them, the dispersion of MWCNTs was found to be excellent in the sodium silicate medium. The dispersion process was carried out with naphthalene sulfonate and ultrasonication, as shown in Figure 19. The effect of ultrasonication in the dispersion of MWCNT is evident in Figure 19. The MWCNT bundle had the smallest size, and its behaviour was most stable in sodium silicate in comparison with other media. Control over crack propagation and spatial distribution is best for MWCNTs dispersed in sodium silicate [125].

Figure 19

Effect of ultrasonication in the dispersion of MWCNTs [125].

The summary of the influence of various nanofillers on the ballistic behaviour of the composites in terms of absorption of energy and the ballistic limit is presented in Table 1.

5.1 Sol–gel

It is an economical chemical process in which sol is a suspension of solid particles in a phase consisting of liquid and gel is solid particles submerged in the liquid solvent [138]. In this process, a chemical solution acts as a precursor to form an integrated system from discrete particles. Sol–gel coating can be done in various forms like spraying, dip coating, and spinning. As the sol–gel coating process is considerably slow, it is not considered a cost-effective option for industrial applications where a high rate of production is required [142]. However, sol–gel coating is significant when substrate has to be protected against decreasing ion release and corrosion. Inter-yarn friction of a given fabric material plays a significant role in determining its BIR capacity. The sol–gel method can be employed to introduce nanoparticles on the surface of high-performance fabrics (Figure 22), which improves inter-yarn friction and eventually helps in absorbing the projectile energy during the ballistic impacts [138].

Figure 22

Schematic representation of the sol–gel method [7].

5.2 PVD coating

PVD-coated materials are employed in a wide range of applications, from industrial parts to decorative objects where corrosion and wear-resistant slim films are required [140]. The main benefit of this method is that customised corrosion, mechanical, and aesthetic properties can be obtained. Generally, the PVD process is carried out in a high-vacuum chamber where solid or liquid materials get vaporised, and subsequently, vapour condensation takes place, which in turn produces a dense and solid film [143]. Evaporation and spluttering are the two commonly employed PVD types. Coating layers obtained from the PVD process are quite thin; hence multi-layered coating and careful selection of material become even more important. To summarise, PVD coating has the potential to be employed in various applications like aerospace, automotive, biomedical, and defence [144]. The PVD process offers flexibility in the usage of organic as well as an inorganic coating material for obtaining the hard and corrosive resistant layer. The utilisation of the PVD process for the deposition of polymer material is considered a challenging option due to polymer degradation. Owing to its higher coating initiation temperatures of PVD, it is not preferred to develop polymer-based nanocomposites for ballistic impact applications. In this direction, further research is required to overcome these challenges.

5.3 CVD coating

CVD is another vapour deposition method to produce a coating on the substrate. Like the PVD process, this process is carried out in a high-vacuum chamber. However, the coating material in CVD is in the form of gas [145]. In this process, chemical gases are fed into the high-vacuum chamber, and chemical reactions among these gases produce a layer of coating [46]. A wide range of materials in different forms and compositions can be used to coat the substrate using the CVD process, including nitrides, oxynitrides, carbides, a composition consisting of Si with Ge and O, carbon in different forms like a diamond, fluorocarbons, graphene, polymers, nanotubes/nanofibres/fibres W and Ti. The CVD process can be performed in various categories like an ultra-high vacuum, low pressure, and atmospheric pressure. Ultra-high vacuum and low-pressure processes are frequently used. Further classification of the CVD process is done based on material properties, substrate heating, and the kind of plasma employed in the vaporisation of the materials. In the case of the CVD process, heating of the substrate at 900° is required, which means it cannot be employed for the materials sensitive to the temperatures. On the other hand, PVD is considered a potential option for temperature-sensitive materials.

The growth of CNTs occurs on top of the full 30 cm × 30 cm region on the de-sized GF2 sample. The density of the grown CNT particles was slightly non-uniform across the substrate area. The de-sizing process was responsible for the slightly uneven growth of CNTs [146]. The SEM images of the grown CNTs at different magnifications are shown in Figure 23(b–g). The CNT forest was composed of MWCNTs and the growth of these MWCNTs around the exposed cylindrical surfaces of the glass fibres (Figure 23(c–g)). The presence of CNT forest on the upper and lower surfaces of the fibre was visible, as shown in Figure 23(f). It indicates the good coverage of MWCNTs around the fibre surface. Penetration of the grown MWCNTs in the thickness direction of the fabric was also observed.

Figure 23

Growth of MWCNTs on the de-sized GF2 samples: (a) optical microscopic image of GF1, GF2, and CNT-GF2 fabrics and (b–g) SEM images presenting the growth of MWCNTs at various magnifications [15].

Although the use of the CVD technique for coating the nanoparticles on the fibre surfaces has been done in the past, they are very less in number, particularly in the development of BIR nanocomposites. The primary reason for limited usage is due to the higher coating initiation temperature associated with CVD, which restricts its application on low melting point materials like polymers. In this direction, further research is required.

5.4 Thermal and other spray coatings

The thermal spray coating process is known to use the heat source generated from the combustion of chemical, electric, and plasma to melt the assigned material and eventually spray the molten material on the substrate to obtain the protective coating. The coatings obtained from this type are reliable from a wear and corrosion resistance point of view. The thickness obtained from this technique varies from 20 µm to some millimetres, which is quite higher in comparison with the thickness obtained from PVD, CVD, or electroplating. The categorisation of thermal spray coating into different types is done based on their process specification and characteristics. Frequently used categories are detonation, plasma, high-velocity oxyfuel, high-velocity air fuel, and warm/cold and wire arc spraying. Although thermal spray coatings are carried out on fibres for different applications [57,147], it is yet to be explored for ballistic impact applications. In addition to thermal spray coating, other spray coating techniques like ultrasonic spray and hand spray coating techniques have been employed [135,148]. To enhance the surface properties of carbon fibres, CNTs were spray coated on them (Figure 24) and then the coated carbons fibres were reinforced with high-density polyethylene to produce composites. A significant change in the morphology of the CNT-coated carbon fibres was observed compared to uncoated carbon fibres as depicted in Figure 25. The composites prepared from CNT-coated fibres exhibited enhanced mechanical properties [135]. Similarly, this method has the potential to be employed for developing BIR materials.

Figure 24

Ultrasonic spray coating of MWCNTs on carbon fibres [135].

Figure 25

SEM images of CNTs spray-coated carbon fibres [135]. (a) raw CFs, (b) CNTs, (c) and (d) CFs with CNT spray coatings.

5.5 Electroless coating

The electroless coating, also called auto-catalytic coating, is a non-galvanic kind of coating technique in which several reactions take place simultaneously in a water-based solution without the aid of electricity [149,150]. The electroless coating process is carried out in four steps, namely, surface treatment, surface sensitisation, surface activation, and deposition of material on the substrate, as depicted in Figure 26 [149,151]. Generally, electroless coating bath consists of a metal ion source, complexing agents, reducing agents, buffering agents, wetting agents, and a stabiliser [151]. Temperature and pH are the two controllable parameters in the electroless coating [152]. Electroless coating on different fibres was performed in the past to improve the conductivity, strength, and EMI shielding capability [149,153]. However, the potential of electroless coating on fibres for ballistic impact applications is yet to be explored. The electroless NiP-coated fibres exhibit the island-shaped protuberances (below 70°) and tree-shaped protuberances (80°), as depicted in Figure 27.

Figure 26

Electroless-coating process [149,151,156].

Figure 27

SEM images of electroless NiP-coated carbon fibres [149]. (a) Island-shaped protuberance and (b) Tree-shaped protuberance.

Electroless coating technique is considered one of the effective options, owing to its cost-effectiveness, higher throwing power, higher potential to coat conductive as well as non-conductive surfaces, and potential to coat the substrate continuously and uniformly [154,155].

The selection of techniques in the development of nanocomposites depends on the type of fibres, nano-reinforcements, the thickness of the deposition, and operating conditions. To date, most of the researchers have developed nanocomposites for ballistic impact applications by modifying the matrix with nano-reinforcements and by dip-coating methods. Significant scope lies ahead in exploring the potential of sol–gel, CVD, PVD, spray, and electroless-coating techniques in the development of BIR nanocomposites.