Constructing Ti3C2Tx/Carbon Fiber hybrids for enhancing the interfacial strength and erosion wear resistance of EP based composite coating

doi:10.21203/rs.3.rs-1820069/v1

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Constructing Ti3C2Tx/Carbon Fiber hybrids for enhancing the interfacial strength and erosion wear resistance of EP based composite coating

https://doi.org/10.21203/rs.3.rs-1820069/v1

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The composite coatings are indispensable for protecting the offshore steel structures serviced in the splash zone. But the mechanical properties of conventional epoxy resin (EP) coatings are poor, which would lead to the short-term lifespan. The carbon fiber (CF) with excellent mechanical properties is an ideal filler to reinforce the EP coating, thus we chemically grafted Ti₃C₂T_x nanosheets onto CF via dopamine to enhance the compatibility between CF and EP. While improving the interface adhesion between CF and EP, the mechanical strength and erosion wear resistance of the CF/Ti₃C₂T_x@EP coating were also significantly enhanced. The flexural properties of CF/Ti₃C₂T_x@EP composite coating was evaluated by three-point bending test, and the flexural strength of CF/Ti₃C₂T_x@EP was increased by 41.95% compared with pure EP coating. The tribological behaviors and erosion wear resistance of those composite coatings were carried out by UMT-3 tribometer and erosion test rig. Comparing with the pure EP coating, the wear rate of CF/Ti₃C₂T_x@EP was decreased by 43.67%, and the erosion mass and volume of CF/Ti₃C₂T_x@EP were respectively reduced by 21.31% and 44.50%, this were ascribed to the increased interfacial combination strength between the Ti₃C₂T_x/CF hybrids and EP matrix. We investigated the failure behavior and revealed the interfacial strengthening mechanism of the CF/Ti₃C₂T_x@EP composite coating, which would evoke widespread interest in developing high performance and long-term protective composite coatings used in the splash zone of marine environment.

Carbon fiber

Ti3C2Tx nanosheets

hybrids

Interfacial strengthening

Erosion wear resistance

Tribological properties

As the basic construction materials of wind power engineering, steel structures are widely used for the offshore resource development¹. However, considering the impact of the complex harsh service environment in the splash zone, such as drying and wetting, oxygen-saturated water, strong ultraviolet, erosion wear by waves, sediment, grits and sand, the service life of steel structures is greatly reduced. Therefore, it is essential to prevent or postpone the failure of steel structure in the splash zone^2,3.

Organic coating is one of the most promising methods of metal protection because of its low cost, simple operation, and high protection performance, it can not only prevent corrosion species from reaching the metal interface, but also could provide protection against external impact damage^4–6. As an extensively implemented organic coating, epoxy resin (EP) coating has excellent adhesion with metal matrix and stability under acidic or alkaline conditions. However, EP is a class of brittle materials with low energy storage modulus, most importantly, there are inherent defects such as cracks and pinholes inside the coating during the curing process^7,8. Moreover, during the transport and the pile driving process, the protective coating on the steel pipe may be damaged, even peeled off from the metal substrate, caused by the low mechanical properties. Hence, the mechanical properties of epoxy resin coating are requested urgently to be improved. For eliminating the inherent defects in organic coatings, many researchers used inorganic fillers such as nanoparticles (Anatase-TiO₂, silica), two-dimensional lamellar materials (graphene oxide, carbon nitride and hexagonal boron nitride), fibers (carbon fibers, aramid fibers) to reinforce the EP resin^9–12.

Carbon fiber reinforced composite materials (CFRP) have many unique characteristics, such as low density¹³, high specific strength and erosion wear resistance^14,15. However, the surface of original carbon fiber is low surface energy and chemically inert. Due to the poor surface wettability of CFs, it is almost impossible to fabricate chemically bond with most polymer matrices, thus CFRP failures generally originated from the fiber/resin interface^16–18. Therefore, the surface modification strategy is used to improve the performance of the interfacial compatibility and combination strength between CF and resin, thereby enhancing the comprehensive performance of the composite material. At present, many surface treatment methods have been proposed, mainly including surface oxidation (wet oxidation¹⁹, electrochemical oxidation^20,21), plasma treatment²², surface sizing^23,24, chemical vapor deposition²⁵ and the enhancement based on nanomaterials^26,27. Most of the above-mentioned functionalized methods increase the surface energy and reactive functional groups of CFs, the interfacial strength between carbon fiber and epoxy resin is enhanced further. Xiong et al.²⁸ introduced CF loaded with carbon nanotubes into the EP coating, and investigated the effect of fiber modification on improving the mechanical properties of composite coating. The results suggested the interfacial shear strength (IFSS) and inter-layer shear strength (ILSS) increased by 86.81% and 11.13% respectively, which was ascribed to the increase of the surface roughness on CF and the stronger interfacial combination with resin. Zhang et al.²² introduced anhydride groups onto CFs through plasma treatment to obtain the more hydrophilic and rough surface, and enhanced the adhesion between fiber and resin. The IFSS of carbon fiber/epoxy resin after plasma treatment grafted with maleic anhydride groups (MAH) was higher than that without plasma treatment grafted with MAH. This was attributed to the increase of surface-activated groups, in which the plasma-grafting polymer improved the interface strength between the CF and EP. However, the majority of surface treatment have many deficiencies, for example, complex preparation process and high energy consumption. These methods also reduced the mechanical properties of CFs, although the stronger interfacial strength between carbon fiber and EP was obtained.

Inspired by the adhesive proteins secreted by mussel, dopamine (DA) has drawn great interests with its superior adhesive ability. Dopamine is a bio-inspired compound rich in catechol and amine functional groups, and can self-polymerize on the surface of almost all organic and inorganic materials under the mild conditions. Moreover, the residual amino and catechol groups of PDA can form covalent bonds or hydrogen bonds with the specific functional groups via Michael addition or Schiff base reactions, making the PDA layer serve as a versatile platform for further functionalization⁹. Zeng et al.²⁹ grafted GO on the surface of CFs with the assistance of PDA thin layer, aiming to increase interfacial compatibility of fiber/resin, with nearly 70% higher interfacial shear strength (IFSS) compared to untreated CFs, which demonstrated the great improvement of mechanical interlocking and interfacial bonding strength of CF/resin.

Ti₃C₂T_x (MXene), a class of two-dimensional inorganic compound, has large specific surface area, excellent mechanical properties, where T corresponds to surface-terminating functional groups (e.g., F⁻, OH⁻, and O²⁻)³⁰. Generally, Ti₃C₂T_x is widely used in the coating as a filler to play its role of corrosion barrier, lubrication, and wear reduction. Early research of our group confirmed that Ti₃C₂T_x (MXene) can remarkably improve the corrosion resistance of pure EP coating³¹. It is believed that the grafting of Ti₃C₂T_x nanosheets onto CF surface could improve the surface roughness of CFs and provide strong interfacial bonding at the fiber/resin interface.

In order to improve the mechanical strength between fiber and epoxy, we put forward a functional and non-damaging route to modify carbon fiber in this work by using polydopamine (PDA) with Ti₃C₂T_x (MXene) grafted on the fiber. The Ti₃C₂T_x nanosheets were grafted on CF using PDA, which could enhance the mechanical strength and corrosion resistance of the CFRP. The improvement of the interface mechanical interlocking between CF and EP enhanced the mechanical properties of CFRP, and the two-dimensional Ti₃C₂T_x nanosheets prevented the permeating of corrosive species into the composite coating. The Ti₃C₂T_x/CF hybrids were efficiently dispersed into epoxy coatings, the mechanical strength of the composite coating and the erosion wear resistance properties of carbon fiber reinforced composite coating were investigated in simulated splash zone of marine environments. Finally, the erosion and wear resistance mechanism of the composite coatings was proposed.

2.1 Materials

CF powder was provided by Toray Group of Japan, and the single-layer Ti₃C₂T_x colloidal aqueous solution was provided by Nanjing MKNANO Tec Co., Ltd. China. The epoxy resin E-51 was purchased from Shandong Yousuo Chemical Engineering Technology Co. Ltd. China, and the curing agent 1618 was obtained from Nanjing Haozhuo Materials Technology Co. Ltd. China. Tris-(hydroxymethyl) aminomethane (Tris) and dopamine hydrochloride (DA) were obtained from the Aladdin Group. Sodium hydroxide (NaOH, AR), acetone (AR) and absolute ethanol (AR), concentrated nitric acid (HNO₃, AR), and concentrated hydrochloric acid (HCl, AR) were all obtained from Sinopharm Chemical Reagent. All chemical reagents were used directly without further purification. In addition, the tribological behaviors and erosion wear resistance were evaluated respectively using Q235 steel blocks (1×1×1 cm³) and Q235 steel plates (5×5×0.5 cm³) as substrates. Noted that prior to the experiment, all Q235 steel should be polished with abrasive paper (400 #, 800 #, and 1500 #) and then ultrasonically cleaned with anhydrous ethanol.

2.2 Pretreatment and oxidation of carbon fibers

We used chemical treatment to remove sizing agent and impurities on the surface of carbon fiber: the original CF was added into NaOH solution (2 M) and stirred under 400 r/min at 80°C for 2 h. Then, CF was refluxed with acetone and stirred under 400 r/min at 60°C for 4 h, and the powders were collected after centrifugation and DI washing. Subsequently, in order to generate carboxyl active groups on the surface of CFs, CFs were dispersed and oxidized in concentrated nitric acid (HNO_3, 68wt%) at 80°C for 4 h. Then they were taken out and washed with deionized water for several times to neural, this obtained fibers were named as CF-COOH.

2.3 Grafting PDA on the CF surface

Buffer solution was prepared with 10 mM Tris and pH was control at 8.5 with 1 M HCl solution. The CF-COOH were immersed in the Tris-HCl dopamine solution and stirred under 400 r/min at 25 ℃ for 24 h. The CF/PDA were collected via multi-centrifugation and cleaning with deionized water. Finally, the CF/PDA powders were dried overnight at 60 ℃ in a vacuum oven.

2.4 Preparation of Ti₃C₂T_x/CF hybrids

The schematic preparation process of Ti₃C₂T_x/CF hybrids was shown in Fig. 1. CF/PDA powder (10 mg) was added into Ti₃C₂T_x solution (0.1 mM), then stirred at 25°C for 12 h. Then, Ti₃C₂T_x/CF hybrids could be successfully prepared, repeatedly washed to neutral with alcohol and deionized water, and dried in a vacuum oven at 40 ℃ for 12 h.

2.5 Fabrication of CF/Ti₃C₂T_x@EP composite coating

The preparation process of the composite coating (CF/Ti₃C₂T_x@EP) was as follows Fig. 1. 0.225 g Ti₃C₂T_x/CF hybrids were added to 18.610 g epoxy resin, and mixed in turn at 600 r/min for 30 s, 1500 r/min for 1 min, 3000 r/min for 5 min, to form a uniform dispersion of CF/Ti₃C₂T_x@EP composite. Afterwards, 11.165 g curing agent was added to the above dispersion and stirred for 5 min. The CF/Ti₃C₂T_x@EP composite was poured on the mold (80 mm×12 mm×3 mm) and Q235 steel plate (the average thickness of coating was 1700 ± 50 µm), and then we analyzed the mechanical test and erosion wear behaviors, respectively. All kinds of samples were cured at 20 ℃ for 2 h, 40 ℃ for 2 h, and 60 ℃ for 2 h. According to the above process, pure EP, CF@EP and CF/PDA@EP coatings were prepared.

2.6 Characterization techniques

The surface morphologies of CF, CF/PDA, CF/Ti₃C₂T_x were observed using a scanning electron microscopy (S-4800). In addition, the chemical composition of carbon fiber before and after modification was characterized using Fourier Infrared spectrometer (FTIR, NICOLET 6700, USA) and Raman analyzer (Renishaw, UK), and X-ray photoelectric spectroscopy (XPS, Axis Supra Ultra DLD, UK) equipped with a monochromatic Al Kα source were used to illustrate the chemical states and elemental combinations of carbon fiber, calibrated with C 1s as a reference peak of 284.6 eV. The mechanical properties of composites were evaluated by the testing machines (Landmark 370, MTS, USA) with a loading speed of 5 mm/min. Moreover, the morphologies of the tensile-fractured surfaces were observed by SEM. The weight loss of CF, CF/PDA, CF/Ti₃C₂T_x was analyzed by the thermal weight loss instrument (TGA2019F1) under nitrogen atmosphere.

2.7 Tribological test

The tribological behaviors of the coatings were characterized by a UMT-3 tribometer (Bruker-CETR, USA) with a reciprocating ball-on-plate mode. The experimental parameters of dry sliding friction were as follows: the Al₃O₂ ball with 3 mm diameter was used as friction pair, the wear track distance was 5 mm, load was 10 N, frequency was 5 Hz, sliding time was 30 min. Each tribological test was repeated three times to obtain an average value. The morphologies of wear scar surface were observed by SEM (S-4800) and 3D laser scanning confocal microscopy (UP-Lambda).

2.8 Erosion wear test

The erosion wear resistance test was carried out by means of an erosion rig. In order to simulate the erosion condition in the marine environment, the liquid-solid two-phase flow was used to test the erosion wear resistance of the as-prepared coatings. Silicon carbide used for testing was lumpy particle with sharp edge, and the size distribution was obtained by laser particle size meter. The cumulative distribution of SiC particle size below 90 µm accounted for 27.2%, 62.8% in the range of 90 to 180 µm, and 10% for the particle size was greater than 180 µm. The mixture was sprayed onto the surface of the composite plate at a jet velocity of 3.13 m/s, the impact angle was 90°, and the test time was 5 min per sample. Before and after testing, samples were washed with ethanol to remove surface contaminants, and weighed with an analytical balance (0.1 mg accuracy). Each data was obtained from the average of the three measurements.

3.1 Characterization of Ti₃C₂T_x/CF hybrids

Surface morphologies and structure of CF, CF/PDA and CF/Ti₃C₂T_x were observed by SEM. As shown in the Fig. 2, the diameter of CFs prepared in this work was approximately 7 µm. The pristine CF exhibited a smooth and clean surface, with a small amount of sizing agent residue (Fig. 2a and b). In addition, after self-polymerized of dopamine under the alkaline condition, the CF surface formed a thin layer of PDA particles (Fig. 2c and d), and the surface of CF-PDA was rougher. Later, after grafting Ti₃C₂T_x nanosheets on the surface of CF/PDA, the surface of CF/Ti₃C₂T_x was rougher than that of CF/PDA. Under the higher magnification, it could be seen that scaly wrinkles were covered on the CF/PDA surface, which confirmed that the successful grafting of Ti₃C₂T_x nanosheets (Fig. 2e and f). Moreover, Ti₃C₂T_x as an anchor point could increase the surface roughness and specific surface area of CFs, and hopefully strengthen the mechanical interlocking between carbon fiber and epoxy resin.

3.2 Structural characterization

The FTIR spectra of CF, CF/PDA, and CF/Ti₃C₂T_x were shown in Fig. 3a. The typical peaks of CF respectively appeared at 3439 cm^-1 and 2923 cm^-1 were ascribed to the telescopic vibration peaks of O-H and C-H bond, while the peak at 1630 cm^-1 was attributed to the telescopic vibration of C = C bond³². In addition, after surface polymerization of dopamine, typical peaks of amide bond characteristics appeared at 1512 cm^-1 and 1384 cm^-1 in CF/PDA, corresponding to the telescopic vibrations of N-H and C-N bond³³. This phenomenon suggested that dopamine successfully grafted on the surface of carbon fiber. For CF/Ti₃C₂T_x, and the new peaks at 1262 cm^-1 and 578 cm^-1 were attributed to the telescopic vibrations of C-F and Ti-O bond³⁴, which indicated that the carbon fiber surface had been successfully grafted with the Ti₃C₂T_x two-dimensional sheets.

The structure of carbon fibers was further analyzed using Raman spectroscopy. As shown in Fig. 3b, carbon fiber showed two inherent characteristic peaks (D peak and G peak), which were important indicators to describe the structural defects of carbon materials. The D-band peak at 1358 cm^− 1 was associated with amorphous carbon atoms, while the G-peak at 1580 cm^− 1 was caused by the ordered graphite structure. At the same time, the area ratio (I_D/I_G) of D peak to G peak intensity was usually used to evaluate the degree of graphitization of the sample. The higher the I_D/I_G, the higher the degree of surface disorder³⁵. As mentioned above, the I_D/I_G value increased from 1.73 for CF to 2.07 for CF/PDA and 2.68 for CF/Ti₃C₂T_x. As could be observed from the Raman curve, there was little change in the peak positions of D and G band, which implied that the solid structure of CF was not destroyed, and introducing PDA and Ti₃C₂T_x caused more disorder structure on the CF surface.

XPS was used to investigate the chemical composition and binding pattern of carbon fiber surfaces, C 1s spectra of CF, CF/PDA and CF/Ti₃C₂T_x were shown in Fig. 3c, respectively. Apparently, the prominent C 1s, N 1s, and O 1s spectra were centered on ~ 283 eV, ~ 397 eV, and ~ 535 eV, respectively, corresponding to the basic components of CF, CF/PDA, and CF/Ti₃C₂T_x (Fig. 3c)³⁶. The N atomic content was low of the pristine CF, possibly attributing to incomplete removal of the sizing agent on the CF surface. Correspondingly, the N atom content on the CF/PDA surface increased, indicating the self-polymerization of the PDA on the CF surface. In addition, after the Ti₃C₂T_x incorpration, there were a significant Ti 2p and F 1s binding energy peaks at ~ 458 eV and ~ 684 eV. In Fig. 3d-f, C 1s spectra of CF, CF/PDA and CF/Ti₃C₂T_x surfaces were shown respectively. In Fig. 3d, the C 1s peak of the CF was resolved into three component peaks, C = C/C-C (284.6 eV), C-O (285.9 eV), and O-C = O (288.4 eV) peaks³⁷. These three identical characteristic peaks also appeared in the fine spectrum of C 1s of CF/PDA and CF/Ti₃C₂T_x (in Fig. 3e and f), and a new C-N peak (286.5 eV) appeared in the fine spectrum of CF/PDA, indicating that the self-polymerization of dopamine on the surface of the carbon fiber. In addition to the above four characteristic peaks, there was also a Ti-C peak of 281.4 eV on the CF/Ti₃C₂T_x surface³⁸, which demonstrating the successful attachment of Ti₃C₂T_x nanosheets on the surface of CF/PDA.

3.3 Mechanical properties

By means of the three-point bending test, the comprehensive interface performance between carbon fibers and resin matrix was evaluated. The stress-strain curve and the averaged flexural strength were shown in Fig. 4a. The flexural strength of specimens showed an increasing trend in turn, compared with EP, the flexural strength of CF@EP, CF/PDA@EP and CF/Ti₃C₂T_x@EP were also significantly increased (Fig. 4b), and the amine group of dopamine could participate in the EP curing process, effectively enhancing the interfacial binding of the composite material. More importantly, compared with pure EP, the flexural strength of CF/Ti₃C₂T_x@EP increased by 27% (from 59.74 MPa to 75.89 MPa). This significant reinforcement can be attributed to the fact that Ti₃C₂T_x nanosheets grafting on the carbon fiber greatly enhanced the interfacial strength of carbon fiber/resin and restricted the delamination and failure of CF composites.

TGA was used to analyze the CF after PDA and Ti₃C₂T_x grafting on the surface of carbon fibers. As shown in Fig. 4c, the original carbon fiber showed a mass loss of 2.02% at 344.2°C, and the fiber itself had excellent thermal stability, so its mass loss should be attributed to the residual sizing agent on the surface. For CF/PDA, the degradation temperature was at 316.8°C, which corresponded to the decomposition of the PDA backbone, then the mass loss before this temperature could be considered as the elimination of moisture and other residues, as well as the fracture decomposition of some catechol and amine groups at about 180°C, it turned out that the PDA was chemically grafted on the CF, and the mass loss was about 3.40 wt%. At the same time, Ti₃C₂T_x was relatively stable under nitrogen atmosphere, so the residual mass of CF/Ti₃C₂T_x at 800 ℃ was proportional to the loading amount of Ti₃C₂T_x nanosheets (9.90%). Therefore, TGA also demonstrated the successful construction of a multi-dimensional reinforcement structure on the surface of carbon fiber using a simple solution method.

The adhesion strength between coatings and steel matrix were characterized, as shown in the Fig. 4d. The adhesion strength of pure EP coating on the steel matrix was 2.92 MPa, it could be seen from the surface morphology of the steel substrate was smooth due to the weak adhesion strength between the steel substrate and the pure EP coating. By contrast, CF@EP, CF/PDA@EP and CF/Ti₃C₂T_x@EP coatings on the steel substrate were increased by 20.22%, 29.81% and 41.95%, respectively. Moreover, there were more residual coatings on the surface of the steel substrate, suggested that the CF/Ti₃C₂T_x@EP composite coating had larger adhesion strength on the surface of the steel substrate.

3.4 Interface performance

As shown in Fig. 5, the cross-section morphologies of the composite materials were observed by SEM, and the damage type and interfacial binding mechanism were also analyzed. Clearly, in Fig. 5a, river and stair patterned morphology appeared on the fracture surface of pure EP, which was due to the stress concentration³⁹. The internal structure of the pure EP coating was defective, these defects would promote propagation of cracks and reduce the mechanical strength of the whole coating.

When CFs were filled in the composite material, the cracks of the epoxy resin were significantly reduced (Fig. 5b), which indicated that the presence of carbon fibers improved the mechanical strength of the composite materials. And at the same time, a significantly pull-out failure mechanism was also observed on the fiber/resin interface, this was due to the poor interfacial combination between pristine CF and epoxy resin.

For CF/PDA@EP, under the action of tensile stress, the damage spreaded from the surface to both sides of the carbon fiber, and the PDA enhanced substantial interface compatibility between carbon fiber with the resin matrix (Fig. 5c). Importantly, for CF/Ti₃C₂T_x@EP composite (Fig. 5d), the morphological characteristics were rough the same as those of CF/PDA@EP. But the large number of residual resins on the fiber surface illustrated that better interfacial compatibility and interfacial strength between CF/Ti₃C₂T_x and EP resin. The results showed that although the presence of carbon fibers increased the mechanical strength of the reinforced composite, the interfacial combination strength of pristine carbon fiber/epoxy resins was not strong enough, and the grafting of PDA and Ti₃C₂T_x on the CF surface increased the mechanical meshing effect of the interface.

3.5 Tribological properties

As shown in Fig. 6, in order to describe the wear resistance properties of EP, CF@EP, CF/PDA@EP and CF/Ti₃C₂T_x@EP coatings, their friction coefficients, wear rates and wear scar were characterized, respectively. The friction coefficients (COF) of EP, CF@EP, CF/PDA@EP and CF/Ti₃C₂T_x@EP coatings were 0.66, 0.54, 0.52 and 0.47, respectively (Fig. 6e), the COF of CF/Ti₃C₂T_x@EP was reduced by 40.4% compared with EP. In addition, the trend of wear rate of these coatings was similar to that of COF, in the order of CF/Ti₃C₂T_x@EP < CF/PDA@EP < CF@EP < EP. The wear rate of EP (2.29×10^− 4 mm³/N·m) was higher than other composite coatings, and the wear rate of CF/Ti₃C₂T_x@EP was the lowest, which was 43.67% much lower than that of the EP coating. It was concluded that the Ti₃C₂T_x/CF hybrids possessed better wear resistance, which was well consistent with its flexural strength.

In Fig. 6f, the cross-sectional profiles of the wear scar were characterized in order to further character the anti-wear properties of coatings. Clearly, CF/Ti₃C₂T_x@EP displayed the minimum depth of wear scars (38.9 µm) and pure EP composites with the maximum depth of 91.3 µm, and the surface area of the wear scar decreased with the increase of fiber/epoxy’s interfacial strength. The improvement of tribological properties of the CF/Ti₃C₂T_x@EP composites was attributed to the increase of surface roughness on the modified CF. As shown in Fig. 6a-d, the surface features of wear scars were visualized by three-dimensional morphology. We took the lowest point of wear scars as the zero point, and the color bar represented the depth of the wear scars. Apparently, the results of the 3D morphology and histogram of the depth of the wear scars were consistent, and the wear resistance of CF/Ti₃C₂T_x@EP was the best.

3.6 Analysis of wear scar morphology and anti-wear mechanism.

The failure modes of epoxy resin during the wear process were mainly plastic deformation and fatigue deformation⁴⁰. In order to analysis the anti-wear mechanism, we observed the wear scars of EP and its composites by SEM (as shown in Fig. 7). Regarding to the pure EP coating, there was no doubt that the dominative wear mechanism was the fatigue wear, which was induced by the repeated local stresses during friction sliding. Prolongating cracks and resin fragments were observed from the wear scar, which showed the pure EP coating could not resist the crack initiation and crack growth because of poor wear resistance properties.

For pristine CF/EP in the Fig. 7b, the wear surface was smoother than that of the pure EP coating, the width of wear track was reduced to 1.04 mm, and we observed the failure of interface between original CF and resin under higher magnification. Due to the high mechanical properties of CF, the introduction of CF can indeed improve the anti-wear properties of the coating. However, due to the weak interface combination performance of fiber/resin, the anti-wear capability of the CF@EP coating was limited. For CF/PDA@EP, the number of surface cracks was significantly reduced (Fig. 7c), which meant that the interfacial strength of CF/PDA@EP was stronger than that of CF@EP. In the case of grafting Ti₃C₂T_x on the surface, less groove and fiber peeling-off on the wear surface of the CF/Ti₃C₂T_x coating (Fig. 7d₁) could be detected, comparing to that of the pristine CF/EP coating, the width and length of cracks was significantly reduced, and the fiber/resin interface was in favorable conditions. The higher magnification morphologies of Fig. 7d₂ illustrated that the grafting of Ti₃C₂T_x improved the fiber-resin interface property significantly. Moreover, the higher surface roughness of CF/Ti₃C₂T_x enhanced the mechanical meshing effect at the interface between the fiber and resin, thus the tribological properties of CF/Ti₃C₂T_x@EP were greatly improved. We can attribute it to that the modification of the fiber surface improved the wear resistance of the composite coating by enhancing the interfacial combination strength with the resin⁴¹.

3.7 Erosion wear properties

In order to evaluate the erosion wear resistance of the coatings, an erosion test was carried out with liquid-solid two-phase flow (SiC particles and deionized water) as an erosion medium. Figure 8a-d visualized the 3D morphologies of the coatings’ erosion wear area. After the continuous impact of SiC particles, the surface of the coating formed pits with different size, and the surface of the erosion area was rougher than the uneroded area. In addition, the erosion resistance of the coatings can be analyzed by the mass loss, volume loss, erosion area and maximum depth of the erosion pits.

To assess the integrity of the coating under the attack of SiC, Fig. 8e showed the mass loss of the coatings. The mass loss of the CF/Ti₃C₂T_x@EP was 148.07 mg, compared with CF/PDA@EP (156.67 mg), CF@EP (164.67 mg), EP (188.17 mg), which was reduced by 5.11%, 10.12% and 21.31%, respectively. However, SiC will be embedded in coatings under high-speed impact, thus we used the volume loss to characterize the erosion wear resistance of the composite coating. It could be seen that the volume loss of CF/Ti₃C₂T_x@EP was the lowest, 44.50% lower than that of pure EP coating. Through the histogram of the surface area and the maximum depth of the erosion pits (Fig. 8f), the CF/Ti₃C₂T_x@EP coating behaved the best erosion resistance, and the maximum depth and surface area were reduced by 15.91% and 7.70% when compared with pure EP, respectively. Therefore, by introducing CF in the EP coating, the erosion resistance of the composite coating in the marine environment improved significantly, and combined with the above test results, it was believed that the CF/Ti₃C₂T_x@EP coating displayed the best erosion wear resistance.

3.8 Analysis of erosion morphology and erosion wear mechanism

Figure 9a-d showed the erosion wear morphology of the composite coatings, and an energy dispersive spectrometer (EDS) was used to investigate the distribution of Si and Ti elements on the erosion surface. The surface of the pure EP coating was severely damaged, and due to the impact of silicon carbide particles, the epoxy resin matrix was crushed, and cracks and cavity structures appeared on the eroded surface (Fig. 9a). The enrichment areas of SiC on the pure EP coating were scattered around cracks and holes. Large SiC particles were deeply embedded in the resin matrix, which was due to the poor ability of the pure EP coating resisting the impact of SiC particles.

Subsequently, for the carbon fiber reinforced epoxy resin coating, from Fig. 9b, the erosion wear area of the CF@EP coating was obviously reduced, and the number of cracks and cavity structures were much lower than those on the pure EP coating. Unfortunately, cracks propagated under continuously impact, weak interface combination led the resin around the carbon fibers peeled off, which caused the carbon fibers to be exposed to the erosion environment. However, in the energy spectrum of the CF@EP coatings, the Si element was enriched around the carbon fibers, big sized SiC particles were broken when continuously impacting to the carbon fiber, which indicated that carbon fibers resisted the severe impact and the resin matrix under fiber was free of impacting.

For the erosion wear zone of CF/PDA@EP coating, CF/PDA was not obviously damaged, and the interface combination between CF/PDA and resin was excellent (Fig. 9c). Under the same condition, the enrichment area of Si showed that SiC was broken into smaller particles, indicating that PDA modified carbon fibers can resist the serious impact of silicon carbide. For CF/Ti₃C₂T_x@EP coating (Fig. 9d), the continuous impact of SiC particles resulted in a small part of the epoxy matrix was peeled off compared to other three type coatings, and the CF/Ti₃C₂T_x was not completely exposed to erosion wear environment. The energy spectrum of Ti and Si elements indicated the interface combination between the CF/Ti₃C₂T_x and epoxy was still strong, but the SiC particles were broken into very small size. We can infer Ti₃C₂T_x nanosheets bonded on the surface of CF increased their surface roughness, and the interfacial strength between the carbon fiber and the resin matrix improved. The grafting of Ti₃C₂T_x nanosheets on the surface of fiber greatly enhanced the erosion resistance of the carbon fiber reinforced composite coating.

In Fig. 9e, the erosion resistance of the coatings was further evaluated by characterizing the particle size distribution and morphology of SiC particles before and after erosion wear test. Before the erosion wear test, SiC particles showed the characteristics of rough surface and sharp edge, particle size was less than 305 µm. In terms of differential distribution, the peak value showed that the particle distribution of SiC was about 114.5 µm-164.3 µm. After the erosion test, from the distribution curves, the SiC particle size was about 68.47 µm-97.12 µm (Fig. 9f), which was due to that the SiC particles were destroyed due to the high-speed impact on the coating surface.

Based on the above analysis, the erosion wear resistance of the coatings depended on how carbon fibers acted in the erosion wear zone and how the erosion stress was propagated within the coating⁴². In Fig. 10a, SiC particles caused surface scratches under continuous impact, and the stress concentration in the resin matrix (EP) caused severe damage and large erosion wear pits on the coating. Therefore, by introducing carbon fibers into the coating (Fig. 10b), the germination and propagation of cracks could be restricted, providing discontinuous paths for the impact delivery of the resin matrix, endowing the composite coating better erosion wear resistance. However, the weak interface adhesion between carbon fiber/resin cannot defend against the severe impact of the SiC particles, which can cause the carbon fibers to be peeled off or broken from the resin matrix. As shown in Fig. 10c, the adhesion strength of carbon fiber/resin interface was improved by surface modification, so the surface roughness and specific surface area of the modified fiber increased, thus improving its interfacial strength with the resin matrix, making the composite coating more durable in severe erosion environments.

In summary, we fabricated Ti₃C₂T_x/CF hybrids by using a simple and environmental-friendly two-step method, which was intended to enhance the mechanical properties, wear resistance and erosion resistance of carbon fiber reinforced composites by improving the interfacial adhension strength of fiber/resin. It was found that CF/Ti₃C₂T_x@EP showed excellent mechanical properties (flexural strength of 75.89 MPa), and the adhesion strength with the steel substrate was 41.95% higher than that of pure EP. The COF and wear rate of CF/Ti₃C₂T_x@EP composite coating was reduced by 40.4% and 43.67%, respectively, compared with pure EP coating. The mass loss and volume loss of CF/Ti₃C₂T_x@EP were reduced by 21.31% and 44.5% compared to pure EP coating, which was ascribed to that CF/Ti₃C₂T_x can fasten the resin matrix and prevent the origination and extension of cracks. The impressive interfacial combination of CF/Ti₃C₂T_x and epoxy resin rendered the coating showing enhanced erosion wear resistance. In a nutshell, CF/Ti₃C₂T_x can enhance the mechanical properties, wear resistance and erosion resistance of EP coating. The enhancement strategy in this work can provide a protective measure for marine steel structures, which ensure that the composite coating showing excellent performance in high-impact environments.

Funding

The authors gratefully appreciate the financial support provided by National Natural Science Foundation of China (51775540 and 52105230), Zhejiang Province Key Technology Project. (2022C01183), Zhoushan Municipal Science and Technology Research Program (2020C12007) and the Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (Grant No.2021R01005).

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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No competing interests reported.

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Constructing Ti3C2Tx/Carbon Fiber hybrids for enhancing the interfacial strength and erosion wear resistance of EP based composite coating

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental Section

2.1 Materials

2.2 Pretreatment and oxidation of carbon fibers

2.3 Grafting PDA on the CF surface

2.4 Preparation of Ti₃C₂T_x/CF hybrids

2.5 Fabrication of CF/Ti₃C₂T_x@EP composite coating

2.6 Characterization techniques

2.7 Tribological test

2.8 Erosion wear test

3 Results And Discussion

3.1 Characterization of Ti₃C₂T_x/CF hybrids

3.2 Structural characterization

3.3 Mechanical properties

3.4 Interface performance

3.5 Tribological properties

3.6 Analysis of wear scar morphology and anti-wear mechanism.

3.7 Erosion wear properties

3.8 Analysis of erosion morphology and erosion wear mechanism

4 Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

Constructing Ti3C2Tx/Carbon Fiber hybrids for enhancing the interfacial strength and erosion wear resistance of EP based composite coating

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental Section

2.1 Materials

2.2 Pretreatment and oxidation of carbon fibers

2.3 Grafting PDA on the CF surface

2.4 Preparation of Ti3C2Tx/CF hybrids

2.5 Fabrication of CF/Ti3C2Tx@EP composite coating

2.6 Characterization techniques

2.7 Tribological test

2.8 Erosion wear test

3 Results And Discussion

3.1 Characterization of Ti3C2Tx/CF hybrids

3.2 Structural characterization

3.3 Mechanical properties

3.4 Interface performance

3.5 Tribological properties

3.6 Analysis of wear scar morphology and anti-wear mechanism.

3.7 Erosion wear properties

3.8 Analysis of erosion morphology and erosion wear mechanism

4 Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

2.4 Preparation of Ti₃C₂T_x/CF hybrids

2.5 Fabrication of CF/Ti₃C₂T_x@EP composite coating

3.1 Characterization of Ti₃C₂T_x/CF hybrids