Nitrogen-doped graphene sheets as e�cient nano�llers at ultra-low content for reinforcing mechanical and wear-resistant properties of acrylic polyurethane coatings

The enhancement to the mechanical and wear-resistant properties of polymer coatings plays a vital role for their application in hostile serving environment and nano�ller is effective for this destination. Herein, we systematically investigate a new nano�ller, nitrogen-doped graphene sheets (NGS), which possess a multilayer sheet-like morphology and share a good compatibility with water. After the incorporation of NGS into a two-component waterborne polyurethane (WPU), the mechanical and wear-resistant properties of NGS/WPU composite coatings signi�cantly improve and wear resistance behaves best at an ultra-low content, reaching up to 0.05 wt ‰ . Wherein, Young’s modulus is elevated by 52.67% and tensile strength is appreciably boosted by 58.87%. Simultaneously, apparent reduction of weight loss of 78.74% is observed in the abrasion testing, and the ductility of NGS/WPU composite �lms is reduced by 48.38%. These make it possible that an ultra-low content of nano�ller e�ciently reinforces polymer-based composites to achieve a trade-off between mechanical properties. Moreover, the wear-resistance mechanism is investigated, and the interaction between NGS and WPU segments is explored to �nd the reason that the mechanical and wear-resistant properties of NGS/WPU composite coatings are improved at an ultra-low content.

Compared to such conventional llers, graphene has drawn great attention recently owing to its exceptional mechanical, electrical, thermal, structural properties and has already shown promising results in tribology as a lubricant additive [14,15]. However, as the pristine graphene without modi cation is introduced into the polymer matrix, it usually results in agglomeration inside the composite material [16]. Therefore, various studies of graphene modi cation have been done to simultaneously improve its dispersion in composites, mechanical properties and wear resistance. And graphene decorated with uorine, phosphorus, sulfur has been applied on lubrication inside composites. For instance, Ye et al. claimed that the wear rate of polyimide composite coatings decreased by about 37.14% with the concentration of uorinated graphene at 0.5% under dried sliding condition [17]. Mu et al. reported that the wear loss and average friction coe cient were reduced by 87% and 9% respectively after adding 1% GO-Tr in [CH][P] [18]. And Feng et al. showed that the tensile strength of the composite lm increased by 46.53% and the wear-resistance of composite lms improved signi cantly after introducing hydrophilic sulfonic groups into the SGO/WPU composites as the added amount of SGO was 0.8 wt% of WPU [19]. Also, as an important part of graphene derivatives, nitrogen-doped graphene sheets (NGS) can be easily prepared without doing harm to the environment [20]. In addition, NGS possessing good lubricating properties due to the introduction of nitrogen atoms can form a rm, effective and protective lm on the interface of the friction pair to prevent direct contact between the counterparts [21].
And the introduction of nitrogen atoms can reinforce the interaction between the graphene sheets and the polymer matrix to reduce the damage of the polymer matrix in the friction stage. For instance, Shi et al. found that the wear rate of UHMWPE was reduced by about 59.46%, and the microhardness increased to its 115%, when the content of nitrogen-doped graphene is 1% [22]. To the best of our knowledge, it is di cult to achieve the large-scale utilization of NGS inside the wear-resistant polymer coatings due to the high cost. Therefore, the concentration of NGS must be limited to an ultra-low loading but still promote the signi cant progress on mechanical and wear-resistant properties of composite coatings.
In the present study, a novel NGS prepared via arc discharge on a graphite rod are incorporated into WPU to obviously achieve signi cant reinforcement on mechanical properties and wear resistance. Wherein, NGS as an e cient nanoparticle have an excellent dispersion in WPU matrix especially at an ultra-low content reaching up to 0.05 wt‰, which shows a possibility that NGS, compared to others, possess an effective function in polymer-based nanocomposites as the content is ultra-low. Simultaneously, the enhancement mechanism of mechanical properties and wear resistance of NGS/WPU composites is investigated basing on the interaction between NGS and PU segments and the produced protective lm in wearing process.

Materials
Hydroxy acrylate (HAR) (solid content 45%), hexamethylene diisocyanate (HDI) were provided by Hubei Double Bond Fine Chemical Co., Ltd. The defoamer was purchased from BYK Additives Co., Ltd. Propylene glycol methyl ether acetate (PMA) was obtained from Shanghai Macklin Biochemical Co., Ltd. Graphite rods were provided by Foshan Nanhai Jusheng Graphite Products Co., Ltd.

Preparation of NGS
NGS were prepared by using the arc discharge on graphite rod. Firstly, a graphite rod was put into a sealed container with the air exhausted. And then nitrogen and hydrogen were introduced in the sealed container to 0.19 bar pressure. After that, arc discharge was produced between the anode (graphite rod) and the cathode for 15 minutes, which succeeded in peeling graphite into NGS. When the reaction nished, the power was turned off, and the hydrogen gas was replaced by introducing nitrogen. The NGS obtained in the container was collected for further use.

Preparation of acrylic WPU composites with NGS
NGS were ultrasonically dispersed in ultrapure water for 0.5 h. The prepared dispersion was mixed with HAR by ball milling at the speed of 200 r min − 1 for 2 h. Then HDI diluted with 20 wt% of PMA was added dropwise to the HAR emulsion with NGS and the mixture were mechanically stirred. Finally, the obtained solution was carefully casted on plexiglass plates and aluminum plates until the solvent was fully removed at room temperature for 168 hours to obtain dried coatings for testing. The whole process is illustrated in Fig. 1. The composite coatings were gained and labeled as 0.01NG, 0.03NG, 0.05NG, 0.07NG, 0.1NG and 0.3NG, which contain 0.01 wt‰, 0.03 wt‰, 0.05 wt‰, 0.07 wt‰, 0.1 wt‰ and 0.3 wt‰ NGS respectively. The composite lms on the plexiglass plates after drying were soaked in 80 ℃ water for two hours to obtain peeled composite lms, which were then prepared into strips of 80 mm × 10 mm × 0.03 mm for stress and strain testing. The composite coatings on the aluminum plates (60 µm) were adopted for abrasion test.

Characterization
Morphologies of the samples were observed using Hitachi SU 8220 scanning electron microscopy (SEM).
Smartlab Rigaku X-ray powder diffraction (XRD) with Cu Kα radiation (λ = 1.5405 Å) at 40 kV and 30 mA was adopted to explore the transformation of degree of crystallinity of the materials. Transmission electron microscopy (TEM) images recorded by the TITAN G2 ETEM microscope (FEI, American) at 300 kV were employed to nd the dispersion of NGS in the HAR and the micro morphology of NGS. The surface chemical states of NGS were studied through X-ray photoelectron spectroscopy (XPS) with the ESCALAB 250 XI (Thermo Fisher Scienti c, USA) by a monochromatic Al X-ray source. Fourier-transform infrared (FTIR) spectra of all samples were operated on the Bruker VERTEX 70 infrared spectrometer employing transmission mode in the range 4000 cm − 1 -400 cm − 1 . The roughness of the surface of all samples was measured by Atomic Force Microscope (AFM), Bruker Dimension Icon. Thermogravimetry (TGA) was carried out with an Instrument NETZSCH STA 449 F5 under the atmosphere of nitrogen at a ow rate of 50 mL min − 1 , heating and cooling process were performed at a rate of 10 ℃ min − 1 . Tensile testing was performed on rectangularshaped specimens using universal tensile testing machine (DZ-106, Zonhow Test Equiment Co., Ltd, China) and the stretching speed is 5 mm min − 1 , at least 5 samples were tested in each group. The loss factor (tan δ) is measured by dynamic shear rheometer (DSR), Anton Paar Smart Pave 102. The Contact angles were carried out on the Attention Theta Lite instrument with deionized water at room temperature.
According to ISO 7784.2, the wear resistance of the coatings was determined via using the rotating friction wheel on a Taber abrasion testing machine, where the composite coatings were subjected to two paralleled CS-10 Rubber wheels rotating for 500 hundred cycles in the opposite directions. The disk on which the samples xed rotated at 60 r min − 1 . The pressure loaded on the composite lm via the rotating rubber wheels was 750 g. Wear weight loss was employed to characterize the abrasion resistance of composite coatings and calculated according to the formula as follows: where M W refers to the wear weight loss, M 1 refers to the mass of composite coatings before the Taber abraser test and M 2 refers to the mass of composite coatings after the test. Reported results were the mean values of at least ve measurements.

Structural characterization of NGS
To con rm the compatibility of NGS with water after the defects formed in the structure of it, the solubility of NGS in water is tested and is shown in Fig. 2a. The mixtures are the same concentration (1 mg mL − 1 ) and are sonicated for 10 min. And the NGS solution is stable for a month without precipitate.
In addition, the contact angle ( Fig. 2b) tested between NGS and water is about 30 degrees, revealing that NGS share good compatibility with water. Based on the excellent dispersion of NGS in the water ambient, which brought its great compatibility with waterborne polymer, it's necessary to carry out the characterization of the NGS and analyze. The morphology of NGS is examined via using SEM and TEM ( Fig. 2c-2e), it is found that NGS has a sheet-like morphology, the lattice fringes of which is 0.36 nm after the introduction of nitrogen. And the chemical components evaluated by XPS are summarized in Table 1. After introducing N to the structure of graphene, the N content of corresponding NGS increased to 9.49%, which was signi cantly higher than that of the pristine arc-discharged graphene sheets (AGS). eV, corresponding to sp 2 C − sp 2 C, N − sp 2 C, N − sp 3 C, C = O and O − C = O bonds, respectively [23]. Additionally, the detailed high resolution XPS N1s spectra of NGS are further adopted to distinguish whether the C-N bond is formed. As shown in Fig. 2g, the NGS N1s spectra can be tted into three different peaks of pyridinic nitrogen (397.7 eV), pyrrolic nitrogen (398.8 eV) and quaternary nitrogen (400.7 eV), which directly claims that N successfully formed linkage with the pristine graphene structure [24].

Morphology of composites
The compatibility of nano ller with polymer matrix usually determines whether their reinforcing effect is good, which is di cult to quantify owing to many factors [25]. Before introducing NGS into WPU matrix, the compatibility of NGS with water is explored via contact angle exhibited in Fig. 2b. It is noticeable that the NGS share good compatibility with water due to the 30 degrees contact angle. However, when NGS, as nano llers, are introduced to WPU, it has always been a problem for their dispersion [26]. Therefore, TEM images of WPU/NGS are adopted to observe the dispersion of the NGS in WPU matrix.
As shown in the TEM images ( Fig. 3a-3b), it is noted that the morphology of pristine WPU, which indicates that it stacks layer by layer like steps. And the high-resolution morphology shows that virgin WPU is amorphous. Moreover, as plotted in Fig. 3c-3h, the lattice fringes of NGS can only be observed in the eld of edge of the sample as the NGS content is lower than 0.07 wt‰. In other words, there is a relationship between the matrix and the ller NGS, where NGS are embraced by matrix or the NGS insert into the space between the WPU layers. When the NGS content is as high as 0.07 wt‰ (see Fig. 3i-3j), the state observed is different from the former, where the lattice fringes of NGS can be clearly seen in a wide range. And NGS, existing on the surface, surround around the WPU matrix owing to the increasing of NGS content. However, with the continuous increasing of NGS content (see Fig. 3k-3n), the NGS agglomerate and claim a separated relationship with the matrix as evidenced by the obvious lattice fringes. As a result of the relationship between the NGS and the matrix, the composites behave robustly in the following tests, as the addition of NGS is no more than 0.07 wt‰. While the content is more than 0.07 wt‰, the samples exhibit more inferior mechanical properties than the former ones.

Mechanical and wear-resistant properties
As observed in Fig. 4a, compared with the pristine WPU coating, the weight loss of the NGS/WPU composite coatings suffered abrasion obviously decreases and obtains the minimum with the addition of 0.05 wt‰. Based on the improvement of wear resistance, the mechanical properties of the coatings need to be investigated. It can be seen from Fig. 4b that the ultimate tensile strengths of the composite lms increase rst and then decrease with the increasing of the NGS content. When the content of NGS reaches up to 0.03 wt‰, the strength of the material comes to the peak value, which is about 88.75% stronger than the pristine sample. When the content of NGS further increases, the strengths decrease. However, the strength will have a peak value again while the content of NGS is 0.1 wt‰. That can be explained that the property of composites usually has two or more peak values as the content of ller increases, which is caused by the reduction of average ller size and the broadening of the size distribution due to the increased shear stresses during the processing [2].
As indicated in Fig. 4c, after incorporating NGS, it is evident that the elongation at break has a noticeable decline compared to the original sample. With the NGS content at 0.03 wt‰ and 0.05 wt‰, the elongation at break reduces by 66.69% and 48.39% respectively compared with the original sample. However, compared to the samples with 0.01 wt‰, 0.03 wt‰ and 0.05 wt‰ NGS, the elongation of break increases signi cantly with the content of NGS at 0.07 wt‰ or more. This can be explained as follows: rstly, when the content of NGS is low, the NGS evenly disperse and do not agglomerate, which can be also con rmed from the morphology in Fig. 3c-3d and Fig. 3e-3f respectively. The NGS exist like nails rmly riveting in the matrix of the material, thereby inhibiting the thermodynamic movement of polymer segments. Secondly, although previous studies have demonstrated that llers will agglomerate in polymers to form a network structure if they are up to high content [27]. But for this work, we guess that the NGS agglomerates to enlarge the interface area among the matrix and NGS, and the composite materials rst deform at the larger interface area once the materials suffered pulling or damage. Although the strengths of the composite materials have a certain increase compared with the original material, the dominant role for resisting deformation is not the NGS but the matrix. However, it is interesting to see that, among all the samples, the strength and Young's modulus of 0.03NG samples are the best (Fig. 3b and Fig. 3d), but the weight loss of 0.05NG is the least in the actual situations, the reason for which is that the performance of the material is based on its own comprehensive performances in the actual serving environment, rather than relying on just one of its performances [3]. Although the tensile strength and Young's modulus of 0.05NG are not the best among all the samples, the good toughness of it can avoid being quickly peeled off from the surface of the aluminum plate in the process of contacting with the rubber grinding wheels and moving relatively. In addition, the representative stress-strain curves (Fig. 4e) also simultaneously indicate obvious difference on mechanical properties of WPU composite lms with different NGS content. And as shown in Fig. 4f, the cross-section of 0.05NG composite lm born with a rough morphology symbolizes that there is a strong interface interaction between the NGS and WPU matrix, which remains to be investigated.

Microstructure of composite coatings surface and wear resistance
Besides, to more directly verify the enhancement of abrasion resistance, the SEM pictures ( Fig. 5a-5n) of the abrasion surface of the samples are adopted, it can be clearly seen that the wear surface of 0.05NG was smoother and completer than that of others. For the pristine sample ( Fig. 5a-5b), the surface of the coating is torn into slightly larger pieces compared with the coatings containing NGS. With the content of NGS gradually increasing, some parts of the surface of the coatings are reinforced, which brings about a smoother morphology, but others have not been enough enhanced by NGS are inclined to be not strong enough to resist ploughing and shearing from the process of abrasion. Thereby, the cracks induced by the shearing of harder particles easily propagate, which results in the exfoliation of the coating and rough locality in the weaker area. However, at the time the addition of NGS is over 0.05 wt‰ (Fig. 5i-5n), the inhomogeneity of composite coatings is exposed again because of the uneven dispersion of NGS, which can be noted from the deep grooves due to the ploughing of hard particles in the rubber grinding wheels [28]. Also, the uneven dispersion of NGS is re ected in Fig. 3i-3n). But, the whole process of abrasion still waits for us to investigate.
Obviously, with the increasing of the wearing cycles, it's not di cult to speculate that the wear type is changeable basing on the mass loss of the coatings in different cycles, as observed in Fig. 6a. In the early stage of wearing, the harder rubber grinding wheels contact the surface of the softer composites and move relatively to form a friction pair, the wear of which was referred to as abrasive wear, which is certi cated from the surface change of rubber grinding wheels, as shown in Fig. S1. This stage was also the most severe stage of the whole process, which can be seen from the slope in the rst 300 cycles in Fig. 6a. In this stage, the Young's modulus and the strength of the material play the main role. Under the action of hard grinding wheel particles, the ability of the material surface to resist deformation is depended on the Young's modulus of the composite, and whether it is easy or not to be peeled off from the surface of the aluminum plate relies on the strength of the composite. With the increasing of wear cycles, parts of the surface of the composites are peeled off by the harder rubber grinding wheels. The friction pair is no longer formed by the direct contact between the grinding wheels and the surface of the composites. Then, the peeled material fragments and the wear debris of the rubber grinding wheels adhere to the surface of the grinding wheels due to periodic wearing. Therefore, adhesive wear in this stage is dominant [29]. In this process, there is a certain shear force due to relative sliding between the composites and the rubber grinding wheels. On the one hand, the toughness of the composite is challenged in this stage, but it is not only limited to the toughness but also the strength and the Young's modulus [28]. This is the reason that although the toughness of 0.1NG and 0.3NG is higher, the weight loss of them in the adhesion wear stage is still large. On the other hand, with the wear cycles increasing, NGS participate in the formation of friction pairs, which can be veri ed via comparing the roughness (as shown in Fig. 6b-6c) of the composite coatings bearing abrasion and the weight loss. Apparently, it's facile to observe that the higher the roughness, the larger the weight loss of composite coatings is. The mechanism can be concluded that the lubrication of NGS and a protective lm forming on the surface of the composite greatly reduce the weight loss [1,29,30]. For pristine sample, the surface of the sample easily becomes rough due to no participation of NGS, and the rougher the sample surface, the larger the friction is, which gives rise to the drastic abrasion. Once the NGS is incorporated in the matrix, the roughness decreases owing to the lubrication and the protective lm in the process of wearing, which weakens the effect of friction on the samples. In addition, the enhanced ultimate tensile strength of composite coatings is strong enough to resist the shearing force from the hard particle of rubber grinding wheels. But when the content of NGS is more than 0.05 wt‰, the agglomerated NGS in the matrix results in the inhomogeneity of the composites, which is also observed from the TEM pictures, as shown in Fig. 3i-3n. Once NGS agglomerate into bulk, the enhancement of NGS on the mechanical properties is weakened and coatings are easily inclined to be stripped off owing to stress concentration occuring on the interface between NGS and PU matrix. On account of that, there is a little bounce-back of weight loss for sample 0.1NG and 0.3NG. Also, the AFM surface height more directly indicates that the improvement of NGS on the surface roughness of coatings, as shown in Fig. 6d-6e.

Interface interaction between NGS and WPU segments
Based on the distinct improvement of mechanical properties and wear resistant by comparing the WPU and its composites, it is indicated that there may be interactions between NGS and WPU matrix. According to previous studies, it has been shown that the introduction of nano ller into the polyurethane can lead to microphase separation, due to the interaction between nano ller and HS (urea and urethane) or SS of polyurethane [31,32]. The common method is to capitalize Fourier-transform infrared (FTIR) spectroscopy to investigate the level of microphase separation of WPU and its composites with the NGS. As the plot exhibited in Fig. 7a-7b, the C = O region in the range of 1600 cm − 1 − 1720 cm − 1 corresponds to the H-boned urea, free urea, H-bonded urethane and free urethane respectively. It is shown that addition of NGS, on the one hand, weakens the C = O stretching of H-bonded urea and the peaks of corresponding composites have a right shift; on the other hand, the peak intensity of C = O stretching of H-bonded urethane also recedes or disappears with the participation in the WPU. These comprehensively indicate that the NGS are likely to capture the electron belonging to the H-bonded urea and the H-bonded urethane owing to the high electronegativity of nitrogen [33]. Hence, it is credible that there is an interaction between the WPU matrix and NGS.
In order to further investigate the interaction between WPU matrix and NGS. TGA is captured to explore the thermal stability of these composites, as illustrated in Fig. 7c-7e, which is also an indicator for the level of microphase separation of these composites [31]. Interestingly, compared with the pristine sample, the thermal stabilities of the composites slightly decrease while being heated from 150 ℃ to 370 ℃ and then increase from 370 ℃ to 500 ℃. The reason for the different tendency of weight loss of the composites may be due to microphase separation. The decomposition of WPU has two stages: the HS decomposes in the rst stage and the SS decomposes in the second stage [31]. When these two segments are blended, the SS will effectively inhibit the decomposition of the HS. And in this study, the presence of NGS contributes to microphase separation, which isolate the segments and weaken the inhibiting effect, thus the thermal degradation increases as the microphase separation increases in the early temperature range. In the second temperature range, the thermal stabilities increase at higher microphase separation level due to the lack of residual HS.
To further demonstrate the effect of NGS on the microphase separation, the XRD patterns are adopted. As the XRD patterns of amorphous diffraction peaks of all samples shown in Figure. 7f, compared with the virgin sample, the diffraction peaks of the samples with NGS are shifted to smaller angles, which indicates domain spacing of the composites are related to the increase in the number of hard segments phases [34]. In other words, the NGS can interact with hard segments of WPU and the nucleation of NGS will induce more fraction of noncrystalline part of HS to form a crystalline phase along with the NGS, which coincides with the TEM morphology plotted in Fig. 3c-3n [25]. As a direct evidence, this consequence further supports our argument on the results of FTIR spectra and TG analysis. To detect whether there is an effect for NGS on the soft segments or not, the differential scanning calorimetry (DSC) of the WPU and its composites is employed to see the effect of NGS on the thermal properties such as the melting point of SS (T m−s ) and enthalpy change at Tm-s (ΔHm-s) [35]. As given in Fig. 7g and Table 2, the Tm-s of WPU composites clearly increases with the addition of NGS and the ΔH m−s of them also has a signi cant increase. These results suggest that there is a disturbance for SS in the melting process as the participation of NGS in the WPU matrix. Additionally, we capitalize the DSR of the corresponding WPU composites to further investigate changes in glass-transition temperature (T g ). As the curves depicted in Fig.   7h, Tan δ peak decreases as more NGS are incorporated, which means that the existence of NGS within the WPU matrix lowers damping capacity. Besides, T g (see Table 2) of pristine WPU is located at 68.7 ℃, while the T g of corresponding WPU composites has a distinct right shift after the introduction of NGS. Till the NGS content reaches up to 0.3 wt‰, the T g comes to its peak value, 85.5 ℃. According to the previous study, the presence of nano ller in PU not only has a hindering effect on the thermodynamic motion of soft segments but also change the level of microphase separation [36]. And microphase separation of PU causes fewer hard segments to be alongside soft segments. If the hindering effect of hard segments plays a less important role than that of nano ller in determining T g shift, the T g will shift to higher temperature [31]. And the synergistic effect brings the enhancement to the mechanical properties of composite polymer material. Herein, we can ensure that the presence of NGS really has a signi cant suppression on the motion of soft segments.

The Model Schematic And Schematic Of Abrasion Of Composite Coatings
Since the level of microphase separation of WPU is di cult to observe by adopting the existing characterizing method owing to its nanometer in size, we plot a schematic of the interfacial interaction of WPU matrix with NGS to reveal the relationship between the NGS and the WPU segments, as shown in Fig. 8. Based on the above argument, it is indicated that the NGS not only accompanies along with the hard segments due to interfacial interaction, but also tangle with the SS, which agrees well with the rough morphology of crosssection of 0.05NG composite lm, as shown in Fig. 4f. Therefore, for one thing, the HS of WPU prefers to unite with NGS and regulates itself along with NGS to form a paralleling structure. For another, once the SS forms a crosslinking knot, the mobility of the SS will be heavily limited. All the relationships among the NGS, HS and SS meanwhile lead to the improvement in the mechanical properties of the corresponding NGS/WPU composites. In addition, as depicted in Figure 9, the NGS together with wearing debris transforming into rm, effective protective lm adheres to the surface of friction pair consisting of coatings and rubber grinding wheels, slowing the rate of wear down. Thus, the interface interaction and the transforming protective lm produced together lead to a rise of mechanical and wear-resistant properties of NGS/WPU composite coatings when the content of NGS, compared to others (Table 3), is at an ultra low level.

Conclusions
In this study, we added NGS to the polymer matrix to improve the wear-resistant and mechanical properties of WPU and explored the enhancement mechanism. The wear-resistant tests exhibited that the weight loss of NGS/WPU composite coating with an ultra-low content of 0.05 wt‰ NGS under abrasion was reduced by 78.74% comparing with the pristine WPU. In addition, the ultimate tensile strength and Young's modulus of the lm with addition of 0.05 wt‰ are enhanced by 58.87% and 55% respectively. However, as the content of NGS increases to over 0.05 wt‰, there is an increasing trend for weight loss, and the ultimate tensile strength and Young's modulus is also reduced comparing with 0.05 wt‰, which can be attributed to the NGS agglomeration weakening the interaction between NGS and WPU. But as the addition of NGS is no more than 0.05 wt‰, the wear-resistant and mechanical properties signi cantly improve, which is bene ted from the even dispersion and the interaction among the NGS, the hard segments and soft segments of WPU, on the other hand, the rm, effective and protective lm produced between the friction pair in the wearing process.
Well-dispersed NGS not only combines with hard segments and promote the crystallization of hard segments, but also forms a crosslinking point with the soft segments, which weakens the ductility, while greatly improves the mechanical strength of NGS/WPU composite coatings. And then the improvement of mechanical strength accompanying with the protective lm bring about the rise of wear resistance of NGS/WPU composite coatings. According to the excellent dispersion of NGS in the WPU matrix, the interaction of NGS with WPU segments at an ultra-low content and the produced effective protective lm due to the existence of NGS in wearing process, we believe that the speci ed NGS or its new derivatives can be designed to achieve tailored properties of composites and be brought into real application in the future. Figure 1 Schematic diagram about the preparation route for NGS/WPU composite coatings.