18.2.1 Physical vapour deposition

PVD first involves the generation of a vapour of the target substance, e.g. a gas mixture plasma, via a high-energy source. Next, the vaporized material is condensed onto the surface of the substrate in a very low-pressure chamber (0.1–10 Pa) to gradually form a thin film on the substrate. The creation of a high-density plasma using a low-pressure gas requires a static magnetic field of ~0.03 T. The field is formed by a configuration of equivalent (or balanced) magnets within the region in front of the sputtering target and traps electrons close to the surface. The magnetic field lines are predominantly concentrated behind the target, while the plasma density near the substrate is about η_e (~10¹⁰ cm⁻³). Magnetron sputtering or direct current magnetron sputtering involves the application of a direct potential to the substrate to induce and sustain the plasma. A related technique, unbalanced magnetron sputtering (UMS), was developed to deposit alloys and their nitrides and carbides. However, this type of coating technology still relies on relatively high energies and is rarely utilized for nanofilled polymers due to the hazardous conditions.

18.2.2 Chemical vapour deposition

CVD is based on the production of a vapour from the solid target material via a heating process and chemical reactions in the vapour phase. Hence, this deposition involves a homogeneous gas phase or heterogeneous chemical reactions, which occur on or in the vicinity of the heated surface, leading to the formation of powders or films, respectively.²⁰ CVD is typically performed at very high temperatures, up to 1000 °C, in order to activate the chemical reactions in the vapour phase. These extremely high temperatures lead to a breakdown of the chemical structure of the solid targets, resulting in different properties compared with the original material. Therefore, low-temperature CVD at 350–700 °C has been carried out using inorganic and metallo-organic precursors. Even lower activation temperatures of 200–400 °C have been achieved using a plasma, as described below. The two most widely used CVD devices for providing a high-temperature stage are hot-wall CVD (HWCVD) and hot-filament CVD (HFCVD) reactors, due to their low cost compared with other coating techniques. However, HWCVD is more appropriate for the deposition of polymer nanocomposites because it does not require the excessively high filament temperatures of HFCVD. Even so, these coating technologies are rarely used for polymer nanocomposites, except for very special cases like the deposition of a nanodiamond.

Plasma-assisted CVD is an efficient way to reduce the temperature in CVD. It uses a microwave plasma at an extremely low pressure (10–100 Pa), instead of at high temperature, to activate the chemical reactions. The method requires only a low environmental temperature and uses a 2.45-GHz microwave plasma exciting source. This deposition method has been used to deposit silicon dioxide, carbon nitride and cubic boron nitride. Polymers containing fluorine, such as polyfluorohydrocarbon and polyperfluorocarbon,^{21, 22} have also been deposited on specific surfaces.²³

18.2.3 Chemical and electrochemical deposition

Chemical and electrochemical deposition is an area of considerable interest, both from a fundamental and an applied standpoint; these methods can be used for electroplating and solution analysis. The low ionizing temperature (in most cases, the temperature can be ambient) should have a negligible hazardous effect on the coating targets. The key objective of chemical and electrochemical deposition is to reduce the precursors to active species through either reducing agents or an input voltage applied to the polyelectrolyte matrix. Based on this prerequisite, conductive polymers such as polyaniline (PANI), polythiophene (PTh), polypyrrole and poly(3,4-ethylenedioxythiophene) (PEDOT) have been the most commonly used for the conductive polymer matrix in nanocomposites. The mechanism for this coating technology is rather simple and normally includes two steps: (1) the target precursors are driven into becoming active monomers, including cathodic or free radicals, and (2) the active monomers diffuse to the cathode (the working electron source and target substrate) and gradually accumulate on the surface of the substrate. However, in the electrochemical method, reduction requires an external current and the sites for anodic and cathodic reactions are separate. For chemical deposition, reduction is induced by a reducing agent and the anodic and cathodic reactions occur together on the workpiece.²⁴ In addition, these reactions only proceed on catalytically active surfaces, i.e. newly coated metallic surfaces should be catalytically active enough to promote redox reactions. Electrochemical deposition has been widely used with conductive polymer nanocomposites to immobilize nanofillers such as heavy metal colloids or specific enzymes, which are inside the conducting polymer matrix. In addition, bioactive thin films may be able to act as biotransistors to convert analogue biosignals into electronic signals.

18.2.4 Roll-to-roll processing deposition

R2R (Fig. 18.1) processing deposition is a high production capacity and low-cost industrial coating technology, which has been employed in the manufacture of flat devices such as organic light-emitting diodes (OLEDs), photovoltaics (PV) and electrophoresis displays (EPDs). A simple roll coating is defined by Coyle et al. as follows: the fluid flows into the space between a pair of rotating rollers that control both the thickness and the uniformity of the coated film. There are some variations of this definition, like reverse-roll coating, ²⁵ gravure coating, ²⁶ knife-over-roll coating (gap coating) and slot die coating, which have been used industrially for specific purposes. R2R is a 'non-stop' coating process and can be combined with most other deposition techniques. Many practical techniques, for instance R2R casting, R2R sputtering and R2R plasma-assisted CVD, have been used in the manufacture of nanocomposites for photoconversion thin films.

18.3.1 Anticorrosion properties

Organic coatings have long been used to protect metals against corrosive environments in both academic and industrial settings. The primary objective of organic coatings is to act as a physical barrier against aggressive species, such as O₂ and H₂O. In the past decade, scientists have focused on a new type of coating: hybrid organic–inorganic coatings. These coatings combine the flexibility and ease of processing of polymers with the hardness of inorganic materials and have been successfully applied to various substrates. For example, Yeh and co-workers demonstrated that the incorporation of organophilic clay platelets into a polymer matrix, in the form of organic-based coatings, may effectively enhance the corrosion protection of polymers on metallic surfaces.^27–38 This is because of the good dispersion (intercalation and exfoliation, Fig. 18.2) of the plate-like clay platelets in the polymer matrix, which effectively increase the length of the diffusion pathways for oxygen and water. In addition, the dispersion may also decrease the permeability of the coatings.

Apart from clay, another important material with this advantage is SiO₂. The incorporation of low-cost SiO₂ nanoparticles into polymers can significantly improve thermal properties,^{39, 40} mechanical properties,^{41, 42} anticorrosion,^{43, 44} wear resistance,^{45, 46} barrier properties^{46, 47} and the customization of electronic packaging properties.⁴⁸ Nanoparticles can significantly alter the mechanical properties of the polymer close to the particle surface due to changes in polymer chain mobility.⁴⁶

Graphene, a monolayer of sp²-hybridized carbon atoms arranged in a two-dimensional lattice, has also attracted tremendous attention as a nanofiller in recent years due to its exceptional thermal,^49–52 mechanical^53–61 and barrier properties.^62–66 The incorporation of graphene fillers can significantly reduce gas permeation through a polymer composite relative to the neat polymer matrix. A percolating network of platelets can provide a 'tortuous path', which inhibits molecular diffusion through the matrix, thus resulting in significantly reduced permeability (Fig. 18.3). Nguyen et al. demonstrated that the incorporation of graphene nanosheets into a polymer matrix improves the barrier capability. At low concentrations (below 0.05 vol%), crumpled graphene sheets are as effective as clay-based nanofillers with loadings that are approximately 25–130 times higher.⁶³

Nanofillers can also be loaded into conductive polymers (in addition to non-conductive polymers) to form coating materials, while retaining their barrier capabilities. Conductive polymers such as PANI, polypyrrole, and PTh have also been applied as anticorrosion coatings. PANI in particular is one of the most promising electrode materials due to its simple synthesis and environmental stability.⁶⁷ PANI and its derivatives have been extensively studied as anticorrosive coatings for various metals.^68–70 For instance, Ahmad and MacDiarmid⁷¹ and Wei et al.⁷² evaluated the corrosion protection of PANI by performing a series of electrochemical measurements. Wessling and co-workers reported that conducting polymers form a complex with Fe (passivation oxide layers, Fig 18.4), which improved corrosion protection.^73–75 A possible passivation oxide layer mechanism is shown in Fig. 18.5. The chemical composition of the passivation oxide layers was determined by X-ray photoelectron spectroscopy (XPS). Binding energy plots versus intensity for iron oxide layers on the exterior of the sample and for the oxides right up against the iron surface are shown in Fig. 18.6. These plots indicate that the passive oxide layer is predominately composed of Fe₂O₃ on the outside with an Fe₃O₄ layer sandwiched between this layer and the nascent steel.

Although PANI has outstanding anticorrosion properties, the adhesion, mechanical properties and barrier effect of the PANI coating can still be improved to enhance the efficiency of corrosion protection. For example, different nanofillers, such as inorganic nanoparticles,⁷⁶ graphite,⁷⁷ carbon nanotubes^{78, 79} and nanoclays (layered silicates),^{36, 37} have been used as additives to strengthen the mechanical and barrier performance of polymers.^{80, 81} Shabani-Nooshabadi et al.⁸² synthesized PANI/montmorillonite nanocomposites and coated them on aluminium substrates. The corrosion rate was found to be 190 times lower than that observed for uncoated Al. The redox catalytic and barrier properties affected corrosion performance. Yeh and co-workers³⁶ synthesized a PANI/clay nanocomposite and coated it onto cold-rolled steel (CRS). The results indicated that a PANI coating containing 1 wt% clay enhanced the corrosion protection of the CRS electrode.

Nanoparticles have been extensively used as stabilizers in the preparation of PANI dispersions and colloids,^{83, 84} and various hybrid systems consisting of PANI and silica particles have been reported.^85–87 Currently, nanoparticle/PANI hybrid systems have mostly been constructed through the chemical polymerization of aniline in the presence of nanoparticles, or by modifying the nanoparticles with PANI prepared in advance. Radhakrishnan et al. used nano-TiO₂ as a metal oxide additive in the composite, which gave better dispersion of the formulation as well as better barrier properties in the coatings.⁸⁸ Luo et al. prepared a SiO₂ particle/PANI hybrid system (the SiO₂ particles were covered with a PANI nanofilm) on a glassy carbon (GC) electrode surface through a simple electrochemical method for use in biosensor applications.⁸⁹

Nanoparticles (SiO₂, TiO₂) and nanoclays used as fillers to synthesize PANI nanocomposites can usually improve thermal, mechanical and anticorrosion properties. However, the peak current of the PANI nanocomposite is reduced as a result.⁸⁹ This phenomenon may be due to the presence of the non-conductive nanofiller, which partially blocks the electrode surface (Fig. 18.7). A reduction of the electrical conductivity of the PANI/clay nanocomposite was observed in comparison to neat PANI. This is expected because the clay component is not electrically conductive and because incorporating the clay into the PANI matrix lowers the molecular weight and decreases electrical conductivity (Fig. 18.8).

18.3.2 Electrical properties of polymer/carbon nanotube(CNT) materials

Another widely used nanofiller, namely, the carbon nanotube (CNT), has clearly demonstrated its capability as a filler in diverse multifunctional nanocomposites. An enhancement of electrical conductivity by several orders of magnitude at very low percolation thresholds (<0.1 wt%) has been observed for CNTs in polymer matrices. Electrically conducting composites with a volume conductivity higher than 10^–10 S/cm are considered to be an important group of relatively inexpensive materials for many engineering applications (Fig. 18.9),^{19, 90, 91} such as electrically conducting adhesives, antistatic coatings⁹² and electromagnetic interference (EMI) shielding for electronic devices.

Ajayan and co-workers⁹³ first reported the electrochemical oxidation of aniline and the behaviour of PANI films formed on multiwalled CNT (MWNT) electrodes. The results suggest that PANI films on nanotubes are oxidized more readily and produce a higher current density during anodic oxidation compared with conventional electrodes such as Pt. This is attributed to the unusual surface topology and relatively large surface area of the nanotube electrodes. The morphology of PANI films formed on nanotube surfaces is granular and nanostructural.

Zengin et al.⁹⁴ reported that PANI could be doped with CNTs to construct a donor-acceptor (DA) nanofilled thin film. CNTs are relatively good electron acceptors, whereas PANI is a good electron donor. CNTs dispersed well in PANI may serve as electron acceptors and conducting bridges. Conductivity increases from 3.36 S cm^–1 for HCl-doped PANI to 33.374 S cm^–1 for doped PANI/CNTs (10 wt% CNT). The Fourier transform infrared (FTIR) spectrum of PANI/CNT composites illustrates several differences from the spectrum of neat PANI (Fig. 18.10). The composite spectrum exhibits an increased quinoid (Q) to benzenoid (B) band (1600–1500 cm^–1) intensity ratio compared with neat PANI. These data reveal that PANI/CNT is richer in quinoid units than pure PANI. This fact may suggest that PANI/CNT interactions promote or stabilize the quinoid ring structure. The π-bonded surface of the CNTs might interact strongly with the conjugated structure of PANI, especially through the quinoid ring. The band at 1150 cm^–1 was described by Macdiarmid and co-workers⁹⁵ as an electronic-like band, and it is considered to be a measure of the degree of delocalization of the electrons, and, thus, it is a characteristic peak of the conductivity of PANI. In PANI/CNT composites, the intensity of the signal at 1143 cm⁻¹ increased and shifted to 1123 cm⁻¹. This increase of the electronic-like absorption peak indicates increased electron delocalization in the composite compared with pure PANI and agrees well with the increased conductivity measurements.

Several methods to modify CNTs have been reported in the literature;^96–101 for instance, physical treatments such as ultrasonication and harsh chemical treatments in strong acids such as sulphuric acid, nitric acid or their mixtures. These treatments work well to improve dispersibility. Unfortunately, these approaches often result in significant damage to the CNT framework, which involves sidewall opening, breaking, and conversion into amorphous carbon. ^{102, 103} The damage to the CNTs definitely diminishes their original outstanding electrical, thermal and physical properties. Recently, the direct Friedel–Crafts acylation of nanofibres and nanotubes has been developed by Baek and co-workers. ^104–109 The reaction condition in this approach is known to be a less destructive chemical modification. The CNTs were functionalized with 4-aminobenzoic acid via direct Friedel-Crafts acylation in a mild polyphosphoric acid (PPA)/phosphorous pentoxide (P₂O₅) medium. The 4-aminobenzoyl-functionalized CNTs should improve the electrochemical properties of PANI/CNT nanocomposites. Cyclic voltammetry (CV) is generally used to investigate the electrochemical properties of conductive materials. The output current of PANI/CNT nanocomposites was observed to be much larger than that of pure PANI. The results suggest that ion inclusion and exclusion in PANI/CNT nanocomposites were much more effective during the redox process (Fig. 18.11). The results are promising for the possible deposition of high-quality PANI films on nanotubes for future applications in, for example, devices, sensors¹¹⁰ (in which high currents would improve performance) and nanocomposites for electromagnetic shielding.^{64, 111}

EMI, also called radio-frequency interference, is a serious issue due to the rapid proliferation of electronics and wireless systems in fields such as navigation and space technology.¹¹² EMI not only affects the performance of electronic devices but may also be harmful to life forms, including humans. Therefore, some kind of shielding material must be employed to prevent electromagnetic noise or pollution. Various types of materials have been used for EMI shielding, including metals, carbon materials and conducting polymers. Recently, the EMI shielding and microwave absorption properties of conducting polymers have attracted increased attention due to their good electrical conductivity and processability.¹¹³ The EMI shielding efficiency (SE) of a composite material depends on many factors, including the intrinsic conductivity of the filler, dielectric constant and aspect ratio.^{114, 115} The small diameter, high aspect ratio, high conductivity and mechanical strength of CNTs, including single-walled CNTs (SWNTs) and MWNTs, make them an excellent option for creating conductive composites for high-performance EMI shielding materials at low filling rates. For example, MWNTs have been added to polymer matrices for EMI shielding materials and tested in the frequency range 8.2–12.4 GHz (X band) with 20 dB for 7% MWNT in polystyrene (PS).^{116, 117} Joo and co-workers studied the electrical conductivity and EMI properties of MWNTs in poly(methyl methacrylate) (PMMA) containing Fe. They achieved 27 dB for a 40% MWNT loading. ¹¹⁸ The microwave absorption properties of MWNT composites with encapsulated Fe, with different phases and shapes of the included Fe, have also been studied.¹¹⁹ Recently, Xiang et al. synthesized MWNT composites with silica and studied their microwave attenuation in the X band.¹²⁰ Grimes et al.¹²¹ reported that polymer/SWNT composites possess a high real permittivity component (polarisation, ε') as well as an imaginary permittivity component (absorption or electric loss, ε") in the 0.5–2 GHz range. They also found that the permittivity decreased rapidly with increasing frequency.

Eklund and co-workers¹²² demonstrated that epoxy/SWNT nanocomposites can be used as effective lightweight EMI shielding materials. The highest EMI shielding effectiveness for epoxy/SWNT composites was found for materials with 15 wt% SWNT. The SE was found to be ~49 dB at 10 MHz, and the shielding effectiveness is around 15–20 dB in the 500 MHz to 1.5 GHz range.

Conducting polymers can be combined with other nanocomponents to enhance EMI shielding. The microwave absorption properties of CNT/conducting-polymer nanocomposites with a core-sheath nanostructure prepared by in situ polymerization have been measured.¹²³ The conductivity of the PANI/CNT nanocomposites was higher than pure PANI and also pure CNTs -an example of the synergistic effect of the two components. PANI/CNT nanocomposites can be used for shielding in the K_u band (12.4–18.0 GHz) as their total shielding effectiveness is in the range of −27.5 to 39.2 dB. The microwave absorption properties of one-dimensional PANI/hexanoic acid (HA)/TiOi nanocomposites have also been studied.¹²⁴ The results showed that the conductivity of the composite significantly decreased after addition of TiO₂ and that the PANI/HA/TiO₂ nanocomposites synthesized at 0 °C had a maximum reflection loss (RL) of 31 dB (>99.9% power absorption) at 10 GHz. To increase the conductivity and microwave absorption of the above system, SWNTs were also incorporated.¹²⁵

When the SWNT content was 20%, the PANI/HA/TiO-/CNT nanocomposite exhibited an RL of less than −15 dB with a broad bandwidth (4 GHz), while an RL less than −20 dB with a narrow bandwidth (1 GHz) was observed when the SWNT content reached 60%. Although only a few reports on the use of one-dimensional nanostructured conducting polymers for microwave absorption and EMI shielding are available, it is believed that one-dimensional nanocomposites could be important in this field.⁶⁴

18.3.3 Electroactive polymer coatings

Although PANI nanocomposites have excellent anticorrosion properties, the poor solubility of PANI in common organic solvents has limited its practical application in many fields. Recently, electroactive polymers incorporating an aniline oligomer have attracted immense research interest because of their advanced properties such as good solubility, mechanical strength and film-forming ability.¹²⁶

A novel route for oxidative coupling polymerization has been developed by Wei, Zhang and co-workers.^127–131 The fabricated copolymers not only contained well-defined conjugated segments, but also provided a better understanding of the structure–property relations and the conducting mechanism of conjugated polymers; knowledge which is limited by the complexity of their molecular structure and their poor solubility in organic solvents. Zagorska and co-workers^{132, 133} prepared new electroactive polymers combining the electrochromic properties of PTh and oligoaniline. Both the oligoaniline side chains and the PTh main chain could be doped in different ways. Wang and co-workers^134–136 prepared electroactive polymers by including the aniline oligomer in the main and side chains, which exhibited electrochemical properties, a high dielectric constant and electrochromic behaviour.

Albertsson and co-workers^137–140 incorporated the electroactive aniline oligomer into a polymer to obtain biodegradable and electroactive copolymers. The aniline oligomer has a well-defined electroactive structure, good processing properties and high degradability. Wei and co-workers ^{141, 142} incorporated aniline pentamer (AP) into polylactide and obtained copolymers that showed good electroactivity, biodegradability and processing properties. Electroactive polyimide (EPI) and electroactive epoxy (EE) have been extensively studied as electrochemical sensors,⁴ functional membranes,⁶ electrochromic materials ¹⁴³ and anticorrosion coatings.^{144, 145}

EPIs derived from bis-amino-terminated aniline trimer units were prepared by Wei and co-workers.¹⁴⁶ Yeh and co-workers presented the first evaluations of the effect of an amine-capped aniline trimer (AT) on the corrosion protection efficiency of as-prepared EPI.¹⁴⁵ A possible mechanism for the enhanced corrosion protection of EPI coatings on CRS electrodes is: (1) the EPI coatings may act as a physical barrier and (2) the redox catalytic properties of the aniline pentamer units in EPI may induce the formation of a passive metal oxide layer on the CRS electrode, as evidenced by scanning electron microscopy (SEM) and electron spectroscopy for chemical analysis (ESCA) studies (Fig. 18.12). The electrochromic performance of EPI was investigated by studying electrochromic photographs and measuring UV absorption spectra. Across different voltages, the colour of the EPI thin film was found to change from grey (at 0.0 V), to green (at 0.4 V), to blue (at 0.6 V) and to metallic blue (at 1.0 V), as shown in Fig. 18.13.¹⁴³

An electroactive epoxy thermoset (EET) coating, an electroactive polymer, has been studied.¹⁴⁴ A higher concentration of conjugated amino-capped aniline trimer (ACAT) units existed in the as-prepared EET samples, and a higher redox current was observed in the CV studies. Electrochemical corrosion experiments showed that a higher concentration of the ACAT segment in the as-prepared EET led to better corrosion protection of the coated CRS electrode. The mechanism for enhanced corrosion protection is similar to that for the EPI material. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) showed that incorporating a higher percentage of ACAT with a rigid aromatic structure in as-prepared EET led to a significant increase in thermal and mechanical properties (Fig 18.14).

A superhydrophobic electroactive epoxy (SEE) from the surface structure of fresh Xanthosoma sagittifolium leaves was prepared and coated on the surface of a CRS electrode using a nanocasting technique for corrosion protection. ¹⁴⁷ The electrode was found to have a water contact angle (CA) of 153°, which was significantly higher than smooth EE coated on CRS by spin coating (CA = 97°), as shown in Fig. 18.15.

Nanofillers such as clay, SiO₂ and TiO₂ also can be included in EPI and EE to capitalize on the properties of these nanoparticles.^148–150 The good dispersion of these nanofillers in the polymer matrix leads to an effective increase in the length of the diffusion pathways for oxygen gas and water vapour and to a decrease in the gas permeability of the coatings, as shown in Fig. 18.16.

18.3.4 Friction and wear properties

Both friction and wear belong to the discipline of tribology and surface engineering; friction is the force between two surfaces in contact (the force resists the relative motion of the two objects), and wear is the erosion of material from a solid surface by the action of another solid. Scientists and engineers consider that surface engineering is one of the most crucial branches of technology in present society. Surface engineering is defined as the design of a surface/substrate composite system to achieve performance that could not be achieved by either the surface composition or the substrate alone, through engineering the substrate surface to improve the appearance, to provide protection from environmental damage or to enhance the mechanical or physical performance of the surface.⁴⁵ Damage, such as corrosion, friction, wear, heat, radiation, weathering and the like, occurs on the surface of a component or is transferred via the surface into the component. Therefore, surface protection is of considerable significance to modern materials technology, and a wide variety of coatings can improve the wear resistance of a substrate. The addition of a second phase is one of the methods used to improve the tribological properties (such as the coefficient of friction and wear rate) of polymer materials. Factors that have an influence on the friction and wear characteristics of polymer composites are particle size, morphology and the concentration of the filler.

An epoxy/SiO₂ composite was prepared by blending and the subsequent addition of a curing agent, and the wear properties were studied. The mixtures obtained were then gently transferred onto a clean, flat, smooth poly(tetrafluoroethylene) (PTFE) sheet, and an aluminium disc was placed on top of the epoxy composite. The PTFE/composite/aluminium sandwich was cured at 23 °C for 168 h and then the thickness of the cured composite was measured. The results showed that spherical silica particles improved the wear resistance of the epoxy matrix even though the filler concentration was relatively low (0.5–4.0 wt%), as shown in Fig. 18.17.¹⁵¹

The wear mechanisms of epoxy and particulate-filled epoxy matrix composites under the pin-on-disc condition were studied by Durand et al.¹⁵² During wear testing, the unfilled epoxy exhibited brittle behaviour and cracks formed perpendicular to the sliding direction. Therefore, material waves were created and wear debris was produced (Fig. 18.18(a)). With the incorporation of uniformly sized submicron spherical silica particles in the epoxy matrix, the propagation of cracks into the epoxy matrix was hindered by particles on or near the surface layer. This resulted in the formation of finer material waves and, consequently, less debris (Fig. 18.18(b)). Furthermore, hard filler particles in a polymer matrix can reduce the wear rate when the applied stress is less than the critical value.¹⁵³ Both of these factors could contribute to a reduction in the wear rate of an epoxy matrix.

Wang et al.^154–157 incorporated nano-Si₃N₄, nano-SiO₂, nano-SiC and nano-ZrO₂ into poly(ether ether ketone) (PEEK) and the testing results indicated that these nanometre-sized particles could effectively reduce the wear rate of PEEK, which was attributed to a thin, uniform and tenacious transfer film that formed on the surface of the counterpart. Bahadur and co-workers ^158,159 investigated the tribological properties of poly(phenylene sulphide) (PPS) filled with nanometre TiO₂ i ZnO, CuO or SiC, as shown in Table 18.1. An increase in wear resistance and a decrease in the coefficient of friction were observed under specific conditions.

Li et al.¹⁶⁰ reported that PTFE filled with nano-ZnO could greatly reduce the wear rate of this polymer. The nano-ZnO antiwear mechanism works by preventing the destruction of the PTFE's banded structure during the friction process. Sawyer et al.¹⁶¹ explored the friction and wear behaviour of PTFE composites filled with 40-nm Al₂O₃, and they found that the friction coefficient of the composites increased slightly compared with an unfilled sample and that the wear resistance increased monotonically with increasing filler concentration. Lai et al.^162–164 used nanometre attapulgite and ultrafine diamond to improve the tribological performance of PTFE and analysed the mechanism of the filler action in reducing the wear rate of the PTFE polymer through DSC and SEM. Yu et al.¹⁶⁵ compared the friction and wear behaviour of poly(oxymethylene) (POM) filled with micrometre and submicrometre copper particles. The submicrometre copper particles were more effective in lowering the wear and coefficient of friction of the composites. Ng et al.¹⁶⁶ dispersed TiO₂ nanoparticles in epoxy, and the resultant composites not only appeared to be tougher than the traditional microparticle-filled epoxy but also had a higher scratch resistance. Cai et al.^{167, 168} researched the tribological properties of polyimide (PI) filled with Al₂O₃ and CNTs, and they reported that the fillers could effectively enhance the friction resistance and antiwear capacity of the composites. Lai et al.¹⁶⁹ showed also that nano-attapulgite particles could improve the tribological behaviour of PI.

Li and co-workers ¹⁷⁰ studied the effect of SiO₂ particle size on the friction and wear behaviour of PI/SiO₂ hybrids synthesized through a sol–gel method. Nanosilica could simultaneously provide PI with strength, toughness, friction and wear resistance. The friction coefficient of the hybrid with 100-nm SiO₂ particles was the lowest and approximately 20% lower than that of pure PI. However, the lowest wear rate was recorded for the hybrid with 300-nm SiO₂ particles and was approximately 20% lower than that of neat PI. The strength and toughness of PI/SiO₂ hybrids are enhanced by approximately 50 and 200%, respectively, compared with pure PI when the size of the silica is less than 300 nm. These results are shown in Fig 18.19.

Gu and co-workers¹⁷¹ studied the indentation and scratch behaviour of a SiO₂/polycarbonate (PC) composite coating at the microscale and nanoscale. The experimental results showed that hardness and stiffness increased after the addition of nano-SiO₂. The scratch tests indicated that the nano-SiO₂/PC coating exhibits a smaller scratch depth (Fig. 18.20) and a lower frictional coefficient. The addition of nano-SiO₂ particles was reported as increasing the hardness and the modulus of the PC film. The elastic recovery of the nanocomposite was also improved. It was suggested that the improvement of these mechanical properties could be ascribed to the excellent performance of the nanoparticles and the increased crystallinity of the nanocomposites.^172–174

The friction and wear properties of polyester (PE)/clay nanocomposites prepared by a mechanical stirrer was studied by Balasubramanian and co-workers.¹⁷⁵ PE/clay nanocomposites have a slightly higher hardness than pristine PE. The lowest specific wear rate and friction coefficient were obtained for PE containing 3 wt% clay, as shown in Fig. 18.21.

The tribological properties of poly(vinylidene fluoride) (PVDF)/clay nanocomposites were studied by Li and co-workers.¹⁷⁶ Table 18.2 and Fig 18.22 show the crystallinity and mechanical properties of the nanocomposites. The data shows that adding 1–2 wt% clay to PVDF was effective in improving the mechanical and tribological properties of neat PVDF. The nanoclay acts as a reinforcing element for the load bore and thus decreased plastic deformation, which in turn reduced the friction coefficient, while the increased polarity of the material caused by crystal transformation may be the main reason for the lower wear. The PVDF/clay nanocomposite containing 5 wt% nanoclay had the highest wear rate. Weak compatibility between the nanoclay and PVDF, and also decreased crystallinity, may be responsible for the higher wear rate.

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