Tribological and Thermo-Mechanical Properties of TiO2 Nanodot-Decorated Ti3C2/Epoxy Nanocomposites

The micromorphology of fillers plays an important role in tribological and mechanical properties of polymer matrices. In this work, a TiO2-decorated Ti2C3 (TiO2/Ti3C2) composite particle with unique micro-nano morphology was engineered to improve the tribological and thermo-mechanical properties of epoxy resin. The TiO2/Ti3C2 were synthesized by hydrothermal growth of TiO2 nanodots onto the surface of accordion-like Ti3C2 microparticles, and three different decoration degrees (low, medium, high density) of TiO2/Ti3C2 were prepared by regulating the concentration of TiO2 precursor solution. Tribological test results indicated that the incorporation of TiO2/Ti3C2 can effectively improve the wear rate of epoxy resin. Among them, the medium density TiO2/Ti3C2/epoxy nanocomposites gained a minimum wear rate. This may be ascribed by the moderate TiO2 nanodot protuberances on the Ti3C2 surface induced a strong mechanical interlock effect between medium-density TiO2/Ti3C2 and the epoxy matrix, which can bear a higher normal shear stress during sliding friction. The morphologies of worn surfaces and wear debris revealed that the wear form was gradually transformed from fatigue wear in neat epoxy to abrasive wear in TiO2/Ti3C2/epoxy nanocomposites. Moreover, the results of thermo-mechanical property indicated that incorporation of TiO2/Ti3C2 also effectively improved the storage modulus and glass transition temperature of epoxy resin.


Introduction
Epoxy resin is a kind of typical thermosetting polymer that possesses high mechanical strength, low shrinkage rate, excellent chemical stability, high adhesive strength, etc., and has been widely used in aerospace, petrochemical engineering, automobile components and other fields [1][2][3][4]. However, the intrinsic high brittleness, low resistance to thermal shock, and poor wear resistance of epoxy resin, originating from its highly crosslinked network structure, seriously restrict its practical application and development in many industrial fields, especially in the antifriction and high-frequency vibrating components and parts of machinery [5][6][7]. Therefore, it is quite necessary to improve wear resistance and dynamic mechanical properties of epoxy resin.
To date, the physical blending with fillers is considered as one of the most commonly used ways to improve the multifunctional performance of polymer, especially wear resistance [8][9][10]. In this manner, the high anti-friction feature of fillers can be integrated into polymer matrix composites, thereby improving their wear resisting property. Previous studies have demonstrated that many kinds of wear resistant fillers, such as ZnO [11], PTFE [12], graphite [13], graphene oxide [14], and WS 2 [15], can effectively improve the wear resistance of epoxy resin, and the particle size of fillers could influence significantly the wear rate of epoxy resin [16]. Shi et al. [17] further found that the nano-sized TiO 2 particles imparted a better wear resisting property to the epoxy resin when compared to the micro sized TiO 2 fillers, mainly ascribing to the stronger interfacial interaction between nano-TiO 2 and epoxy matrix. Therefore, it can be speculated that the interfacial interaction of filler-polymer matrix plays an important role in the wear resisting property of polymer composites, as well as other properties. The micromorphology of micro/nano particles is one of the key factors that influence the interfacial interaction of particle-polymer. Ramezanzadeh et al. [18] reported that the different micromorphology of iron oxide particles could induce the distinguishing interfacial interaction between iron oxide and epoxy, thereby resulting in the different mechanical properties. In recent years, some publications have indicated that the micromorphology of fillers could influence the tribological and/or other properties of polymer [19,20]. Even, the composite particles with unique micromorphology had been synthesized for improving the tribological property of polymer [21,22]. However, how to design the micromorphology of fillers for imparting the optimal tribological and mechanical property to polymer matrix still remains a challenge.
Inspired by the micro/nano composite structure from previous reports [23,24], we plan to design and synthesize a composite particle with unique micro/nano structure by integrating nano-structured particles into micro-structured particles. Herein, Ti 3 C 2 MXene, a new kind of 2D transition metal carbides firstly discovered by Gogotsi et al. [25] in 2011, was selected as the micro-structure particles, and the TiO 2 was chosen as the nano-structured particles. It has been widely reported that both Ti 3 C 2 and TiO 2 particles can improve the tribological and/or mechanical properties of epoxy and other polymers [26][27][28][29][30]. In this work, we attempted to synthesize the TiO 2 decorated Ti 3 C 2 (TiO 2 /Ti 3 C 2 ) composite particles that possessed the unique micro/nano structure through hydrothermally growth of TiO 2 nanodots on Ti 3 C 2 microparticles, to improve the tribological and thermo-mechanical properties of epoxy resin. After removal of Al atom layers of Ti 3 AlC 2 by HF, the accordion-like Ti 3 C 2 microparticles were prepared. Subsequently, TiO 2 nanodots were hydrothermally grown on the Ti 3 C 2 surface to construct the micro/nano structured TiO 2 /Ti 3 C 2 , and the TiO 2 /Ti 3 C 2 /epoxy nanocomposites were fabricated by ultrasound-assisted blending of epoxy with TiO 2 /Ti 3 C 2 (Scheme 1). The micromorphology of different decoration degrees of TiO 2 /Ti 3 C 2 composite particles and the fracture surface of TiO 2 /Ti 3 C 2 /epoxy nanocomposites were investigated, and the tribological and thermo-mechanical properties of TiO 2 /Ti 3 C 2 /epoxy nanocomposites were tested. Moreover, the underlying wear mechanism of TiO 2 /Ti 3 C 2 /epoxy nanocomposites was analyzed. s 2021, 14, x FOR PEER REVIEW 3 of 15 Scheme 1. Illustration for preparation of TiO2/Ti3C2/epoxy nanocomposites.

Preparation of Ti3C2 Microparticles
Ti3AlC2 powders were added into HF solution (40%) at a ratio of 0.1 g per milliliter.
After stirring vigorously for 12 h at room temperature (RT), the sediments were centrifugally collected at a speed of 4000 rpm, followed by washed with deionized water until the pH value was approximate to neutrality. With the wet powders were dried for 12 h at 60 °C in a vacuum drying oven, the Ti3C2 microparticles were obtained.

Preparation of TiO2 Decorated Ti3C2 (TiO2/Ti3C2) Composite Particles
Ti3C2 microparticles (1 g) were added into a mixed solution of deionized water and absolute ethyl alcohol (30 mL, volume = 1:1) to form a Ti3C2 dispersion solution. Simultaneously, the desired molar mass of TBOT (0.25, 0.5, and 1.0 mol) were added into 45 mL of absolute ethyl alcohol to prepare the TiO2 precursor solution. Afterwards, the TiO2 precursor solution was added dropwise into the Ti3C2 dispersion solution with stirring vigorously, followed by transferred into a 100 mL of Teflon-lined autoclave and kept at 180 °C for 24 h. After cooled to RT, the sediments were centrifugally collected at a speed of 4000 rpm, and dried at 60 °C for 12 h in a vacuum drying oven, the TiO2/Ti3C2 composite particles were obtained. According to the molar mass of TBOT, the obtained TiO2/Ti3C2 were named as 0.25 M, 0.5 M, and 1.0 M TiO2/Ti3C2. Scheme 1. Illustration for preparation of TiO 2 /Ti 3 C 2 /epoxy nanocomposites.

Preparation of Ti 3 C 2 Microparticles
Ti 3 AlC 2 powders were added into HF solution (40%) at a ratio of 0.1 g per milliliter.
After stirring vigorously for 12 h at room temperature (RT), the sediments were centrifugally collected at a speed of 4000 rpm, followed by washed with deionized water until the pH value was approximate to neutrality. With the wet powders were dried for 12 h at 60 • C in a vacuum drying oven, the Ti 3 C 2 microparticles were obtained.

Preparation of TiO 2 Decorated Ti 3 C 2 (TiO 2 /Ti 3 C 2 ) Composite Particles
Ti 3 C 2 microparticles (1 g) were added into a mixed solution of deionized water and absolute ethyl alcohol (30 mL, volume = 1:1) to form a Ti 3 C 2 dispersion solution. Simultaneously, the desired molar mass of TBOT (0.25, 0.5, and 1.0 mol) were added into 45 mL of absolute ethyl alcohol to prepare the TiO 2 precursor solution. Afterwards, the TiO 2 precursor solution was added dropwise into the Ti 3 C 2 dispersion solution with stirring vigorously, followed by transferred into a 100 mL of Teflon-lined autoclave and kept at 180 • C for 24 h. After cooled to RT, the sediments were centrifugally collected at a speed of 4000 rpm, and dried at 60 • C for 12 h in a vacuum drying oven, the TiO 2 /Ti 3 C 2 composite particles were obtained. According to the molar mass of TBOT, the obtained TiO 2 /Ti 3 C 2 were named as 0.25 M, 0.5 M, and 1.0 M TiO 2 /Ti 3 C 2 .

Preparation of TiO 2 /Ti 3 C 2 /Epoxy Nanocomposites
Firstly, the desired mass fraction of TiO 2 /Ti 3 C 2 were ultrasonically dispersed into absolute ethyl alcohol, followed by epoxy resin was mixed with TiO 2/ Ti 3 C 2 dispersion solution for 1 h under ultrasound. With stirring vigorously, the mixed solution of TiO 2 /Ti 3 C 2 and epoxy was reduced pressure distilled at 60 • C until the absolute ethyl alcohol was fully evaporated. The MHHPA containing TEAB was then added into the mixture of TiO 2 /Ti 3 C 2 and epoxy, followed by stirring for 1 h at RT in a vacuum. The TiO 2 /Ti 3 C 2 /epoxy mixture was subsequently poured into stainless steel mold and cured for 2 h at 120 • C, the TiO 2 /Ti 3 C 2 /epoxy nanocomposites were finally obtained. Herein, the mass ratio of epoxy resin, MHHPA and TEAB was set to 100:80:0.3, and the mass fraction of TiO 2 /Ti 3 C 2 (relative to nanocomposite) was set to 0.25%, 0.5%, 0.75%, and 1%.

Characterization and Measurement
The surface morphologies of the samples were investigated by field-emission scanning electron microscope (FE-SEM, Hitachi S4800, Tokyo, Japan). The phase composition of the samples was analyzed by X-ray diffractometer (XRD, Bruker D8, Karlsruhe, Germany). Fourier transform infrared (FTIR) spectra of Ti 3 AlC 2 , Ti 3 C 2 and TiO 2 /Ti 3 C 2 were recorded at room temperature by an infrared spectrometer (Nicolet 5700, Madison, WI, USA) with the range from 400 cm −1 to 4000 cm −1 . The densities of TiO 2 /Ti 3 C 2 /epoxy nanocomposites were measured with a density tester (GP-300E, Guanwei Instrument, Shanghai, China), and the hardness of TiO 2 /Ti 3 C 2 /epoxy nanocomposites were examined by Vickers microhardness tester at a load of 0.5 N and loading time of 15 s (HV-1000, Shangcai Instrument, Shanghai, China). For the testing of tribological property, the TiO 2 /Ti 3 C 2 /epoxy nanocomposites were cut into strip-shaped specimens (16 mm × 4 mm × 4 mm), and specimen surfaces were polished with 1200 mesh SiC abrasive paper. After alcohol washed and dried at 60 • C, the tribological testing of specimens was performed by a vertical universal friction and wear testing machine (MM-W18, Shijin Co. Ltd., Jinan, China) at a rotating speed of 780 rpm with the load of 4 N and 8 N for 1 h duration under dry friction condition. Six replications were carried out for each nanocomposite. The friction coefficients were evaluated as the computer-calculated average value. The average wear rates were calculated according to the formula: W = V/(F × L), where the W, V, F, and L are the wear rate, wear volume, applied load, and sliding distance, respectively [31]. Thermo-mechanical properties were measured using DMA 850, (TA Instruments, Newcastle, DE, USA). The TiO 2 /Ti 3 C 2 /epoxy nanocomposites were cut into strip-shaped specimens (30 mm × 10 mm × 4 mm) to match with machine. A temperature scan was conducted from 40 to 190 • C with a heating rate of 5 • C/min at a frequency of 1 Hz and a strain of 0.05% in a single cantilever mode. The storage modulus and tan δ (the ratio of loss modulus to storage modulus) were obtained from dynamic mechanical analysis, and the glass transition temperature (Tg) was gained from the peak value of tan δ.

Surface Morphology of TiO 2 /Ti 3 C 2 Microparticles
The Ti 3 C 2 microparticles were prepared by HF etching, and the TiO 2 /Ti 3 C 2 composite particles were synthesized via hydrothermal method. As shown in Figure 1A, the original Ti 3 AlC 2 microparticles exhibit irregular morphology, with an approximate size of 10 µm, and the selected area magnified image ( Figure 1B) clearly displays the tightly stacked layer structure of Ti 3 AlC 2 microparticles. After etched with HF solution, lots of interlayered gaps are produced in etched Ti 3 AlC 2 microparticles ( Figure 1C), implying the removal of Al atom layers in Ti 3 AlC 2 and the possible formation of Ti 3 C 2 microparticles [32]. After hydrothermally reacted with TiO 2 precursor solution, the surface of Ti 3 C 2 is covered with large amounts of nanodots ( Figure 1D-F), which maybe suggests that the TiO 2 are successfully grown on the surface of Ti 3 C 2 . It can be seen that the density of nanodots on Ti 3 C 2 surface is gradually increase with increasing the molar mass of TOBT, especially, the obvious aggregated nanodots are found on the surface of Ti 3 C 2 when the molar mass of TOBT reaches 1.0 mol.

XRD and FTIR Analysis of TiO2/Ti3C2 Composite Particles
The phase composition of as-prepared samples was analyzed by XRD pattern. It can be seen from Figure 2A that three strong diffraction peaks are found in the pattern of Ti3AlC2, with the peaks at 9.6°, 39.1° and 41.9° are well indexed to the (002), (104), and (105) lattice plane of hexagonal Ti3AlC2 (JCPDS No. 52-0875), respectively [33]. After HF etching, the characteristic (104) diffraction peak of Ti3AlC2 has thoroughly disappeared. Additionally, the (002) diffraction peak is broadened, and shift to the lower angles compared to their original location, which indicate the conversion of Ti3AlC2 to Ti3C2 [34]. After hydrothermal reaction, three new diffraction peaks are observed in the TiO2/Ti3C2 composite particles, with peaks at 25.3°, 37.9°, and 48.0° assigned to (101), (004), and (200) lattice plane of anatase (JCPDS no. 21-1272), respectively [35]. More importantly, the intensity of (101) diffraction peak at 25.3° markedly enhances with the increase of molar mass of TBOT ( Figure 2B), which indicates that the density of TiO2 nanodots can be regulated with TiO2 precursor solution. FTIR spectra show that four obvious adsorption bands are observed in Ti3C2 and TiO2/Ti3C2, with 3411 cm −1 , 3180 cm −1 , 1635 cm −1 , and 1322 cm −1 corresponding to the stretching vibration of hydroxyl group (-OH) of H2O, stretching vibration of Ti-OH, blending vibration of -OH of H2O and stretching vibration of covalent C-F bond of Ti3C2, respectively [27,36]. It is worth noting that, compared with Ti3C2, a strong adsorption band at 542 cm −1 is found in the spectra of TiO2/Ti3C2, which is assigned to the stretching vibration of Ti-O bond of TiO2 [37].

XRD and FTIR Analysis of TiO 2 /Ti 3 C 2 Composite Particles
The phase composition of as-prepared samples was analyzed by XRD pattern. It can be seen from Figure 2A that three strong diffraction peaks are found in the pattern of Ti 3 AlC 2 , with the peaks at 9.6 • , 39.1 • and 41.9 • are well indexed to the (002), (104), and (105) lattice plane of hexagonal Ti 3 AlC 2 (JCPDS No. 52-0875), respectively [33]. After HF etching, the characteristic (104) diffraction peak of Ti 3 AlC 2 has thoroughly disappeared. Additionally, the (002) diffraction peak is broadened, and shift to the lower angles compared to their original location, which indicate the conversion of Ti 3 AlC 2 to Ti 3 C 2 [34]. After hydrothermal reaction, three new diffraction peaks are observed in the TiO 2 /Ti 3 C 2 composite particles, with peaks at 25.3 • , 37.9 • , and 48.0 • assigned to (101), (004), and (200) lattice plane of anatase (JCPDS no. 21-1272), respectively [35]. More importantly, the intensity of (101) diffraction peak at 25.3 • markedly enhances with the increase of molar mass of TBOT ( Figure 2B), which indicates that the density of TiO 2 nanodots can be regulated with TiO 2 precursor solution. FTIR spectra show that four obvious adsorption bands are observed in Ti 3 C 2 and TiO 2 /Ti 3 C 2 , with 3411 cm −1 , 3180 cm −1 , 1635 cm −1 , and 1322 cm −1 corresponding to the stretching vibration of hydroxyl group (-OH) of H 2 O, stretching vibration of Ti-OH, blending vibration of -OH of H 2 O and stretching vibration of covalent C-F bond of Ti 3 C 2 , respectively [27,36]. It is worth noting that, compared with Ti 3 C 2 , a strong adsorption band at 542 cm −1 is found in the spectra of TiO 2 /Ti 3 C 2 , which is assigned to the stretching vibration of Ti-O bond of TiO 2 [37]. To determine the optimal mass fraction of TiO2/Ti3C2 composite particles, the tribo-

Determining the Optimal Mass Fraction of TiO2/Ti3C2 through Testing the Tribological Properties of 0.25 M TiO2/Ti3C2/Epoxy Nanocomposite
To determine the optimal mass fraction of TiO2/Ti3C2 composite particles, the tribological property of TiO2/Ti3C2/epoxy nanocomposite with different content was investigated. As shown in Figure 3, both the friction coefficient and wear rate of all the TiO2/Ti3C2/epoxy nanocomposites show a trend of first decreasing and then increasing when the mass fraction of TiO2/Ti3C2 is increased from 0% to 1%. Among them, the TiO2/Ti3C2/epoxy nanocomposite with 0.5% mass fraction presents the lowest friction coefficient and minimum wear rate. Herein, the friction coefficient of TiO2/Ti3C2/epoxy nanocomposites at 0.5% mass fraction are determined to 0.60 for Ti3C2/epoxy, 0.66 for 0.

Fracture Surface Analysis of TiO2/Ti3C2/Epoxy Nanocomposites with Different TiO2/Ti3C2
To understand the internal microstructure of 0.5% TiO2/Ti3C2/epoxy nanocomposites, the morphology of fracture surfaces of TiO2/Ti3C2/epoxy nanocomposites incorporated To understand the internal microstructure of 0.5% TiO 2 /Ti 3 C 2 /epoxy nanocomposites, the morphology of fracture surfaces of TiO 2 /Ti 3 C 2 /epoxy nanocomposites incorporated with different TiO 2 /Ti 3 C 2 was investigated. As shown in Figure 4A, large numbers of riverlike stripes are clearly observed on the fracture surface of neat epoxy, which are typical characteristics of brittle thermosetting polymer [38]. For Ti 3 C 2 /epoxy nanocomposite ( Figure 4B), the stripes on the fracture surface decrease, and simultaneously some loose particles are emerged on the fracture surface. After incorporation of 0.25 M TiO 2 /Ti 3 C 2 composite particles into epoxy matrix, the stripes on the fracture surface of composite greatly decrease, but the loose particles are still found on the fracture surface ( Figure 4C), which maybe implies the weak interfacial interaction between 0.25 M TiO 2 /Ti 3 C 2 and the epoxy matrix. When incorporated with 0.5 M TiO 2 /Ti 3 C 2 , it seems that some particles are embedded in the epoxy matrix, and the loose particles can hardly be found on the fracture surface ( Figure 4D), maybe ascribing to the strong interfacial interaction between 0.5 M TiO 2 /Ti 3 C 2 and epoxy matrix. However, a large number of particles are observed on the fracture surface of 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite ( Figure 4E), suggesting the poor interfacial interaction between TiO 2 /Ti 3 C 2 and matrix. In addition, the density and hardness of TiO 2 /Ti 3 C 2 /epoxy nanocomposites (0.5% mass fraction) are gradually enhanced with the increase of decorated TiO 2 ( Figure 4F), with the average density of 1.24, 1.21, 1.23, 1.29, 1.33 g·cm -3 and average hardness of 135, 139, 144, 153, 156 Hv0.5 for neat epoxy, Ti 3 C 2 /epoxy, 0.25 M TiO 2 /Ti 3 C 2 /epoxy, 0.5 M TiO 2 /Ti 3 C 2 /epoxy, and 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposites.
decrease, but the loose particles are still found on the fracture surface ( Figure 4C), which maybe implies the weak interfacial interaction between 0.25 M TiO2/Ti3C2 and the epoxy matrix. When incorporated with 0.5 M TiO2/Ti3C2, it seems that some particles are embedded in the epoxy matrix, and the loose particles can hardly be found on the fracture surface ( Figure 4D), maybe ascribing to the strong interfacial interaction between 0.5 M TiO2/Ti3C2 and epoxy matrix. However, a large number of particles are observed on the fracture surface of 1.0 M TiO2/Ti3C2/epoxy nanocomposite ( Figure 4E), suggesting the poor interfacial interaction between TiO2/Ti3C2 and matrix. In addition, the density and hardness of TiO2/Ti3C2/epoxy nanocomposites (0.5% mass fraction) are gradually enhanced with the increase of decorated TiO2 ( Figure 4F

Surface Morphology of TiO2/Ti3C2/Epoxy Nanocomposites with Different TiO2/Ti3C2 before Friction Tests
The differences in surface irregularity of materials have an important influence on the results of friction tests. So, the surface morphology of TiO2/Ti3C2/epoxy samples were investigated before friction tests were carried out. As shown in Figure 5, the surface morphologies of all the samples seem to be similar, ascribing to the same treatment conditions. Additionally, the neat epoxy and TiO2/Ti3C2/epoxy nanocomposites display a homogeneous and regular surface morphology, without apparent protuberant or sunk part, which

Surface Morphology of TiO 2 /Ti 3 C 2 /Epoxy Nanocomposites with Different TiO 2 /Ti 3 C 2 before Friction Tests
The differences in surface irregularity of materials have an important influence on the results of friction tests. So, the surface morphology of TiO 2 /Ti 3 C 2 /epoxy samples were investigated before friction tests were carried out. As shown in Figure 5, the surface morphologies of all the samples seem to be similar, ascribing to the same treatment conditions. Additionally, the neat epoxy and TiO 2 /Ti 3 C 2 /epoxy nanocomposites display a homogeneous and regular surface morphology, without apparent protuberant or sunk part, which indicates that the effect of surface irregularity of TiO 2 /Ti 3 C 2 /epoxy nanocomposites on results of friction tests is negligible.

Tribological Properties of TiO 2 /Ti 3 C 2 /Epoxy Nanocomposites with Different TiO 2 /Ti 3 C 2
The tribological property of TiO 2 /Ti 3 C 2 /epoxy nanocomposites was evaluated by measuring their friction coefficient and wear rate. As shown in Figure 6A, the neat epoxy exhibits the highest friction coefficient of 1.02 under a normal load of 4 N, and the friction coefficient dramatically decreases to 0.67 after incorporation of 0.5% Ti 3 C 2 into epoxy matrix. When the TiO 2 /Ti 3 C 2 composite particles were used as the fillers, the friction coefficient of TiO 2 /Ti 3 C 2 /epoxy nanocomposites increases gradually with increasing the decorated TiO 2 , and the friction coefficient of 0.25 M TiO 2 /Ti 3 C 2 /epoxy, 0.5 M TiO 2 /Ti 3 C 2 /epoxy, and 1.0 M TiO 2 /Ti 3 C 2 /epoxy are determined to 0.74, 0.76, and 0.88, respectively. Furthermore, the friction coefficient of neat epoxy, Ti 3 C 2 /epoxy, 0.25 M TiO 2 /Ti 3 C 2 /epoxy, 0.5 M TiO 2 /Ti 3 C 2 /epoxy, and 1.0 M TiO 2 /Ti 3 C 2 /epoxy under the normal load of 8 N are calculated to 0.93, 0.60, 0.68, 0.72, and 0.81, respectively, which is lower than that under the normal load of 4 N. This can be ascribed to the negative exponential relation between applied load and friction coefficient that can be expressed as µ = kN n − 1 [39], where the µ, k, N are the friction coefficient, constant, and applied load, respectively, and n is a constant between 2/3 and 1, depending on the amount of interaction between elastic and plastic deformation. The wear rate of neat epoxy and TiO 2 /Ti 3 C 2 /epoxy nanocomposites is shown in Figure 6B. It can be seen that the wear rate of neat epoxy is 15.02 × /epoxy nanocomposite gains the minimum wear rate, which may be attributed that the medium density TiO 2 nanodots grown on Ti 3 C 2 provide abundant anchors for the strong interaction between TiO 2 /Ti 3 C 2 composite particles and epoxy matrix [40]. Besides, the wear rate of all the samples under the normal load of 8 N is always higher than that under the normal load of 4 N, mainly ascribing that the enhanced normal load produced a higher shear stress, which resulted in the detachment of more epoxy particles or fillers from the surface of epoxy nanocomposites. indicates that the effect of surface irregularity of TiO2/Ti3C2/epoxy nanocomposites on results of friction tests is negligible.

Tribological Properties of TiO2/Ti3C2/Epoxy Nanocomposites with Different TiO2/Ti3C2
The tribological property of TiO2/Ti3C2/epoxy nanocomposites was evaluated by measuring their friction coefficient and wear rate. As shown in Figure 6A, the neat epoxy exhibits the highest friction coefficient of 1.02 under a normal load of 4 N, and the friction coefficient dramatically decreases to 0.67 after incorporation of 0.5% Ti3C2 into epoxy matrix. When the TiO2/Ti3C2 composite particles were used as the fillers, the friction coefficient of TiO2/Ti3C2/epoxy nanocomposites increases gradually with increasing the decorated TiO2, and the friction coefficient of 0.25 M TiO2/Ti3C2/epoxy, 0.5 M TiO2/Ti3C2/epoxy, and 1.0 M TiO2/Ti3C2/epoxy are determined to 0.74, 0.76, and 0.88, respectively. Furthermore, the friction coefficient of neat epoxy, Ti3C2/epoxy, 0.25 M TiO2/Ti3C2/epoxy, 0.5 M 8 N, 0.5 M TiO2/Ti3C2/epoxy nanocomposite gains the minimum wear rate, which may be attributed that the medium density TiO2 nanodots grown on Ti3C2 provide abundant anchors for the strong interaction between TiO2/Ti3C2 composite particles and epoxy matrix [40]. Besides, the wear rate of all the samples under the normal load of 8 N is always higher than that under the normal load of 4 N, mainly ascribing that the enhanced normal load produced a higher shear stress, which resulted in the detachment of more epoxy particles or fillers from the surface of epoxy nanocomposites.

Worn Surface Analysis of TiO2/Ti3C2/Epoxy Nanocomposites with Different TiO2/Ti3C2
To understand the wear mechanism, the worn surfaces along with wear debris of neat epoxy and TiO2/Ti3C2/epoxy nanocomposites under the normal load of 8 N were investigated. As shown in Figure 7A,B, the neat epoxy displays a relatively rough worn surface with lots of ploughed furrows, suggesting the fatigue wear of neat epoxy, which is the typical wear form of brittle polymer [41]. During the sliding friction, the ploughed furrows can easily induce the detachment of small pieces of epoxy from the bulk epoxy under the shear stress, so large numbers of small-sized wear debris are observed on the steel counterpart surface ( Figure 7C), which results in the serious wear behavior of neat epoxy. After incorporation of 0.5% Ti3C2, the amounts of surface ploughed furrows of Ti3C2/epoxy nanocomposite remarkably decrease ( Figure 7D,E), and the worn surface becomes smooth. In addition, the size of wear debris of Ti3C2/epoxy nanocomposite is obviously bigger than that of neat epoxy ( Figure 7F), implying the improved fatigue wear in Ti3C2/epoxy nanocomposite. For 0.25 M TiO2/Ti3C2/epoxy nanocomposite, the smoothness of its worn surface is further enhanced, and noticeable ploughed traces have scarcely been found ( Figure 7G,H). Simultaneously, the large amounts of flake-like wear debris are collected from counterpart surface of 0.25 M TiO2/Ti3C2/epoxy nanocomposite ( Figure 7I), which implies the improvement of wear resisting property of epoxy resin. The improved wear resisting property mainly attributed that the original brittle "epoxy to steel" contact wear is partly substituted with tough "TiO2/Ti3C2 to steel" contact wear [42]. When the 0.5 M TiO2/Ti3C2 composite particles are incorporated into epoxy matrix, the worn surface of nanocomposite becomes quite smooth ( Figure 7J,K), indicating the excellent wear resisting property of 0.5 M TiO2/Ti3C2/epoxy nanocomposite. Additionally, some tiny particles are presented on the worn surface, which suggests that the transformation of wear form of 0.5 M TiO2/Ti3C2/epoxy nanocomposite from fatigue wear to abrasive wear may be made. Moreover, the flake-like wear debris with large size are collected from the counterpart surface ( Figure 7L). This can be explained that, on one hand, the moderate TiO2 nanodots grown on TiO2/Ti3C2 act as the anchors to form strong mechanical interlock between

Worn Surface Analysis of TiO 2 /Ti 3 C 2 /Epoxy Nanocomposites with Different TiO 2 /Ti 3 C 2
To understand the wear mechanism, the worn surfaces along with wear debris of neat epoxy and TiO 2 /Ti 3 C 2 /epoxy nanocomposites under the normal load of 8 N were investigated. As shown in Figure 7A,B, the neat epoxy displays a relatively rough worn surface with lots of ploughed furrows, suggesting the fatigue wear of neat epoxy, which is the typical wear form of brittle polymer [41]. During the sliding friction, the ploughed furrows can easily induce the detachment of small pieces of epoxy from the bulk epoxy under the shear stress, so large numbers of small-sized wear debris are observed on the steel counterpart surface ( Figure 7C), which results in the serious wear behavior of neat epoxy. After incorporation of 0.5% Ti 3 C 2 , the amounts of surface ploughed furrows of Ti 3 C 2 /epoxy nanocomposite remarkably decrease ( Figure 7D,E), and the worn surface becomes smooth. In addition, the size of wear debris of Ti 3 C 2 /epoxy nanocomposite is obviously bigger than that of neat epoxy ( Figure 7F), implying the improved fatigue wear in Ti 3 C 2 /epoxy nanocomposite. For 0.25 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite, the smoothness of its worn surface is further enhanced, and noticeable ploughed traces have scarcely been found ( Figure 7G,H). Simultaneously, the large amounts of flake-like wear debris are collected from counterpart surface of 0.25 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite ( Figure 7I), which implies the improvement of wear resisting property of epoxy resin. The improved wear resisting property mainly attributed that the original brittle "epoxy to steel" contact wear is partly substituted with tough "TiO 2 /Ti 3 C 2 to steel" contact wear [42]. When the 0.5 M TiO 2 /Ti 3 C 2 composite particles are incorporated into epoxy matrix, the worn surface of nanocomposite becomes quite smooth ( Figure 7J,K), indicating the excellent wear resisting property of 0.5 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite. Additionally, some tiny particles are presented on the worn surface, which suggests that the transformation of wear form of 0.5 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite from fatigue wear to abrasive wear may be made. Moreover, the flake-like wear debris with large size are collected from the counterpart surface ( Figure 7L). This can be explained that, on one hand, the moderate TiO 2 nanodots grown on TiO 2 /Ti 3 C 2 act as the anchors to form strong mechanical interlock between 0.5 M TiO 2 /Ti 3 C 2 and epoxy matrix, which can bear a higher normal shear stress during sliding friction; on the other hand, due to the formation of large size of flake-like wear debris, the "composite to flake-like wear debris of composite" contact wear may be established, which can reduce the wear rate of 0.5 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite greatly. For the 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite, a large number of tiny particles are presented on the worn surface ( Figure 7M,N), and the size of wear debris of 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite are much smaller than that of 0.5 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite ( Figure 7O), which reveals that the dominant wear form of 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite is abrasive wear, instead of fatigue wear. These results demonstrated that the micromorphology of TiO 2 /Ti 3 C 2 composite particles can strongly influence the wear behavior of epoxy nanocomposites. 0.5 M TiO2/Ti3C2 and epoxy matrix, which can bear a higher normal shear stress during sliding friction; on the other hand, due to the formation of large size of flake-like wear debris, the "composite to flake-like wear debris of composite" contact wear may be established, which can reduce the wear rate of 0.5 M TiO2/Ti3C2/epoxy nanocomposite greatly. For the 1.0 M TiO2/Ti3C2/epoxy nanocomposite, a large number of tiny particles are presented on the worn surface ( Figure 7M,N), and the size of wear debris of 1.0 M TiO2/Ti3C2/epoxy nanocomposite are much smaller than that of 0.5 M TiO2/Ti3C2/epoxy nanocomposite ( Figure 7O), which reveals that the dominant wear form of 1.0 M TiO2/Ti3C2/epoxy nanocomposite is abrasive wear, instead of fatigue wear. These results demonstrated that the micromorphology of TiO2/Ti3C2 composite particles can strongly influence the wear behavior of epoxy nanocomposites.

Dynamic Mechanical Analysis of Ti 3 C 2 /Epoxy Nanocomposites with Different TiO 2 /Ti 3 C 2
To further analyze the thermo-mechanical properties of neat epoxy and TiO 2 /Ti 3 C 2 / epoxy nanocomposites, dynamic mechanical analysis was performed. It can be seen from Figure 8A that the storage modulus of TiO 2 /Ti 3 C 2 /epoxy nanocomposites in the glassy state is gradually enhanced with the increase of decorated TiO 2 nanodots of TiO 2 /Ti 3 C 2 composite particles, followed by reach to the maximum value at 0.5 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite and decrease in 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposite. Herein, the storage modulus of Ti 3 C 2 , 0.25 M TiO 2 /Ti 3 C 2 /epoxy, 0.5 M TiO 2 /Ti 3 C 2 /epoxy, and 1.0 M TiO 2 /Ti 3 C 2 /epoxy at 40 • C are determined to 2481.3 MPa, 2543.4 MPa, 2645.9 MPa, 2388.5 MPa, which is increased by 11.0%, 13.8%, 18.4%, and 6.9%, respectively, compared with neat epoxy (2235.2 MPa at 40 • C). As shown in Figure 8B, the Tg of all the samples show a similar trend of raising first and then falling, as observed in their storage modulus, with the Tg of neat epoxy, Ti 3 C 2 /epoxy, 0.25 M TiO 2 /Ti 3 C 2 /epoxy, 0.5 M TiO 2 /Ti 3 C 2 /epoxy, and 1.0 M TiO 2 /Ti 3 C 2 /epoxy are determined to 146.3 • C, 152.5 • C, 153.9 • C, 156.3 • C, and 143.3 • C, respectively. The increased storage modulus and Tg can be attributed that, on one hand, the TiO 2 nanodot protuberances maybe provided the anchors to form the mechanical interlock between TiO 2 /Ti 3 C 2 and epoxy matrix, which physically hindered the molecular mobility of epoxy matrix [43]; on the other hand, the enhanced contact area between TiO 2 /Ti 3 C 2 and epoxy resulting from TiO 2 nanodots, as well as abundant functional groups (-OH, -F) on TiO 2 and Ti 3 C 2 surface induced a strong interaction between TiO 2 /Ti 3 C 2 and epoxy matrix, which restricted the molecular motion of the local epoxy around TiO 2 /Ti 3 C 2 [44]. The minimum Tg in 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposites maybe ascribed to the poor interaction between 1.0 M TiO 2 /Ti 3 C 2 and epoxy matrix, as well as the reduced degree of cross-linking reaction of epoxy resin resulting from the high-density TiO 2 nanodots of 1M TiO 2 /Ti 3 C 2 [45].
3.9. A Proposed Mechanism for the Improved Tribological and Thermo-Mechanical Properties of TiO 2 /Ti 3 C 2 /Epoxy Nanocomposites On the basis of the abovementioned results, a plausible mechanism is proposed for shedding light on the improved tribological and thermo-mechanical properties of TiO 2 /Ti 3 C 2 / epoxy nanocomposite, as illustrated in Scheme 2. For the low density TiO 2 /Ti 3 C 2 incorporated epoxy nanocomposite, the sparse TiO 2 nanodots grown on Ti 3 C 2 surface only provide a small amount of TiO 2 nanodot anchors, which induces a weak interfacial interaction and mechanical interlock between TiO 2 /Ti 3 C 2 and epoxy matrix. As a result, the tribological and thermo-mechanical properties of TiO 2 /Ti 3 C 2 /epoxy nanocomposite are enhanced to some extent. When the medium density TiO 2 /Ti 3 C 2 are used as fillers, the moderate TiO 2 nanodots provide lots of effective anchors (suitable gap between adjacent anchors), supporting the strong interfacial interaction and mechanical interlock between TiO 2 /Ti 3 C 2 and epoxy matrix. Therefore, the tribological and thermo-mechanical properties of TiO 2 /Ti 3 C 2 /epoxy nanocomposite are greatly enhanced. However, the dense TiO 2 nanodots hardly provide effective anchors for the TiO 2 /Ti 3 C 2 -epoxy interfacial interaction, leads to a poor mechanical interlock between TiO 2 /Ti 3 C 2 and epoxy matrix. Thus, the worse tribological and thermo-mechanical properties are gained in the high density TiO 2 /Ti 3 C 2 incorporated epoxy nanocomposite.

A Proposed Mechanism for the Improved Tribological and Thermo-Mechanical Properties of TiO2/Ti3C2/Epoxy Nanocomposites
On the basis of the abovementioned results, a plausible mechanism is proposed for shedding light on the improved tribological and thermo-mechanical properties of TiO2/Ti3C2/epoxy nanocomposite, as illustrated in Scheme 2. For the low density TiO2/Ti3C2 incorporated epoxy nanocomposite, the sparse TiO2 nanodots grown on Ti3C2 surface only provide a small amount of TiO2 nanodot anchors, which induces a weak interfacial interaction and mechanical interlock between TiO2/Ti3C2 and epoxy matrix. As a result, the tribological and thermo-mechanical properties of TiO2/Ti3C2/epoxy nanocomposite are enhanced to some extent. When the medium density TiO2/Ti3C2 are used as fillers, the moderate TiO2 nanodots provide lots of effective anchors (suitable gap between adjacent anchors), supporting the strong interfacial interaction and mechanical interlock between TiO2/Ti3C2 and epoxy matrix. Therefore, the tribological and thermo-mechanical properties of TiO2/Ti3C2/epoxy nanocomposite are greatly enhanced. However, the dense TiO2 nanodots hardly provide effective anchors for the TiO2/Ti3C2-epoxy interfacial interaction, leads to a poor mechanical interlock between TiO2/Ti3C2 and epoxy matrix. Thus, the worse tribological and thermo-mechanical properties are gained in the high density TiO2/Ti3C2 incorporated epoxy nanocomposite. Scheme 2. Illustration for the interaction between TiO2/Ti3C2 and epoxy matrix in TiO2/Ti3C2/epoxy nanocomposites.

Conclusions
In summary, the TiO2 decorated Ti3C2 were synthesized by hydrothermal growth of TiO2 nanodots onto the surface of accordion-like Ti3C2, and the different decoration degree (low, medium, high density) of TiO2/Ti3C2 composite particles were prepared by regulat-Scheme 2. Illustration for the interaction between TiO 2 /Ti 3 C 2 and epoxy matrix in TiO 2 /Ti 3 C 2 / epoxy nanocomposites.

Conclusions
In summary, the TiO 2 decorated Ti 3 C 2 were synthesized by hydrothermal growth of TiO 2 nanodots onto the surface of accordion-like Ti 3 C 2 , and the different decoration degree (low, medium, high density) of TiO 2 /Ti 3 C 2 composite particles were prepared by regulating the concentration of TiO 2 precursor solution. Tribological test results indicated that the incorporation of TiO 2 /Ti 3 C 2 can effectively improve the wear rate of epoxy resin, and the improvement effect was dependent on the density of TiO 2 nanodots of TiO 2 /Ti 3 C 2 . Among, the medium density TiO 2 /Ti 3 C 2 could improve the wear rate of epoxy resin at the greatest extent, with a wear rate of only 2.67 × 10 −14 m 3 /(N·m) was obtained under the normal load of 8 N. This mainly ascribed that the moderate TiO 2 nanodot protuberances on the Ti 3 C 2 surface induced a strong mechanical interlocking effect between medium density TiO 2 /Ti 3 C 2 and epoxy matrix, which can bear a higher normal shear stress during sliding friction. The morphologies of worn surfaces and wear debris revealed that the wear form was gradually transformed from abrasive wear in neat epoxy to adhesive wear in 1.0 M TiO 2 /Ti 3 C 2 /epoxy nanocomposites. Moreover, the results of thermo-mechanical property indicated that incorporation of TiO 2 /Ti 3 C 2 also effectively improved the storage modulus and glass transition temperature of epoxy resin. Our results demonstrated that the tribological and thermo-mechanical properties of TiO 2 /Ti 3 C 2 /epoxy nanocomposites can be strongly influenced by the micromorphology of TiO 2 /Ti 3 C 2 , which provides a promising strategy for improving the tribological and thermo-mechanical properties of epoxy nanocomposites.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.