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Article

High-Temperature Tribological Behavior of Polyimide Composites with Dual-Phase MoS2/MXene Lubricants: A Synergistic Effect Analysis

by
Xingtian Ji
1,2,
Pengwei Ren
1,
Hao Liu
1,
Yanhua Shi
2,
Yunfeng Yan
1,* and
Jianzhang Wang
1,*
1
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
2
School of Mechanical Engineering, Liaoning Petrochemical University, Fushun 113001, China
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(7), 373; https://doi.org/10.3390/jcs9070373
Submission received: 9 June 2025 / Revised: 12 July 2025 / Accepted: 15 July 2025 / Published: 17 July 2025
(This article belongs to the Special Issue Polymer Composites: Fiber Architecture, Interfacial Engineering, and Processing)
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Abstract

Polyimide (PI), owing to its high heat resistance and low density, is often employed as a substitute for metallic materials in high-temperature environments, such as aircraft engines, bearings, and gears. However, the relatively high friction coefficient of pure PI limits its application under harsh conditions. Therefore, this study synthesized a composite lubricant with binary fillers to improve this performance. This study employed the hydrothermal method to synthesize MoS2/MXene composite lubricating fillers and systematically investigated the high-temperature tribological properties of PI composites reinforced with these fillers. The results demonstrated that the optimal PI composite containing 5% MoS2/MXene exhibited a 14 °C increase in initial decomposition temperature compared to pure PI. Additionally, its thermal conductivity was enhanced by 36%, while the hardness (0.398 GPa) and elastic modulus (6.294 GPa) were elevated by 12.4% and 18.6%, respectively, relative to the pure PI. In terms of tribological behavior, all composite formulations displayed typical temperature-dependent friction characteristics. It is worth noting that MXene’s high hardness and thermal conductivity inhibited the occurrence of abrasive wear. At the same time, the substrate was strengthened, and thermal resistance was enhanced, thereby delaying the plastic deformation of the material at high temperatures.

1. Introduction

Polyimide (PI), a class of high-performance polymer materials, plays an indispensable role in aerospace, microelectronic packaging, and high-temperature lubrication owing to its excellent thermal stability, mechanical strength, chemical resistance, and dielectric properties [1,2,3]. These characteristics are particularly critical in extreme operating environments in which temperatures exceed 300 °C. The molecular architecture of polyimides, which is characterized by alternating rigid aromatic rings and flexible imide linkages in the polymer backbone, confers remarkable physicochemical stability, rendering them ideal matrix materials for demanding engineering applications [4,5,6]. For example, polyimides demonstrate exceptional performance as high-temperature bearing bushings in aero-engine sealing components and as insulating films for electronic devices [7]. Despite these advantages, conventional polyimides exhibit significant limitations under increasingly severe operational conditions, particularly in tribological applications [3,8]. On the one hand, their inherent coefficient of friction (COF) ranges from 0.3 to 0.5, which promotes frictional heating and potential material degradation during high-speed sliding contact [9,10]. On the other hand, their inadequate thermal conductivity (~0.1–0.3 W/(m·K)) impedes effective heat dissipation, thereby accelerating wear mechanisms [11]. These material constraints motivate extensive research into advanced modification strategies to enhance performance characteristics.
Recent investigations have demonstrated the efficacy of nanomaterial reinforcement in addressing these limitations. Song et al. [12] functionalized carbon fiber (CF) by depositing polydopamine (PDA) and hydrothermally growing MoS2 on its surface before compositing it with PI. The results demonstrated that the PI/CF-MoS2 composite exhibited significantly enhanced thermal stability, tensile strength, and tribological properties compared to pure PI. Specifically, the tensile strength increased by 43%, while the COF and wear rate decreased by 57% and 77%, respectively. Similarly, Lu et al. [13] investigated the tribological properties of PI composites reinforced with functionalized graphene under dry sliding conditions. At an optimal graphene content of 4%, the COF and wear rate were reduced by 38.2% and 25%, respectively, while the tensile strength and modulus increased by 51.9% and 56.5% compared to pure PI. In another study, Wang et al. [14] incorporated Mo2CTx nanoflakes into the PI matrix. The addition of these nanoflakes markedly improved the composites’ thermal stability, hardness, and tribological performance relative to unmodified PI.
In recent years, nanofiller-reinforced polyimide composites have attracted considerable research interest due to their tailorable functional properties [15,16]. Within this domain, two-dimensional (2D) nanomaterials have emerged as particularly promising reinforcement candidates, owing to their distinctive layered architectures and surface characteristics. Notably, MoS2 and MXene have become focal points of investigation—the former for its exceptional solid lubrication properties and the latter for its multifunctional interfacial characteristics. The strategic incorporation of these materials as hybrid fillers in polyimide matrices can induce synergistic effects, leading to simultaneous enhancements in tribological performance, thermal management capacity, and mechanical strength. This composite design approach shows significant potential for meeting the demanding operational requirements of extreme environments characterized by elevated temperatures, heavy mechanical loads, and high-speed conditions.
MoS2 is a representative layered transition metal sulfide characterized by a crystal structure of S-Mo-S sandwich units held together by weak van der Waals forces. This unique structure facilitates interlayer sliding under shear stress, endowing MoS2 with exceptional solid lubrication properties [17,18]. Studies indicate that MoS2 can achieve an ultra-low COF of ~0.02 in vacuum or inert atmospheres [19], making it particularly valuable for space machinery and vacuum bearing applications. However, under humid or oxidizing conditions, the surface of MoS2 readily reacts with water or oxygen to form MoO3, leading to substantial degradation of its lubricating performance [20]. To address this limitation, researchers have improved the environmental stability of MoS2 through strategies such as nanosizing [21,22] (fabrication of few-layer MoS2 nanosheets) and surface functionalization (silane coupling agent modification). When incorporated into polyimide matrices, MoS2 nanosheets reduce friction by physically shielding friction pairs and forming protective transfer films at the interface [23]. However, MoS2 alone has limitations: it decreases the composite’s thermal conductivity, and excessive loading promotes particle agglomeration, restricting its practical application.
MXenes, a novel class of 2D transition metal carbides/nitrides, exhibit exceptional properties, such as metallic conductivity and a high specific surface area. These characteristics make them highly advantageous for applications in thermal conductivity, electromagnetic shielding, and energy storage [24]. Notably, Ti3C2Tx MXene exhibits a remarkable in-plane thermal conductivity of 40–50 W/(m·K) at room temperature (RT), surpassing that of graphene oxide (about 2–5 W/(m·K)) by an order of magnitude. This superior thermal performance originates from its highly ordered lattice structure and efficient phonon transport mechanisms. Moreover, the abundant surface functional groups (-OH, -O, -F) on MXene nanosheets can significantly improve interfacial adhesion with polymer matrix, while their inherent high hardness and layered structure contribute to outstanding wear resistance in the resulting composites [25]. Experimental studies have confirmed that MXenes significantly enhance the wear resistance of polymers [26]. Notably, MXene’s lubrication mechanism differs fundamentally from that of MoS2. While both materials exhibit solid lubricant properties, MXene’s tribological performance relies more heavily on the chemical inertness of its surface functional groups and interlayer shear behavior. Furthermore, MXenes demonstrate superior environmental stability, particularly in humid or wet conditions. However, their lubricating performance alone is generally inferior to that of conventional solid lubricants, such as MoS2 or graphite. Additionally, their high electrical conductivity may compromise the intrinsic insulating properties of polyimide matrices.
The engineered construction of MoS2/MXene heterostructured hybrid fillers synergistically integrates their complementary functional advantages [27,28]. MXene’s high thermal conductivity improves heat dissipation, while MoS2 provides lubrication. The surface functional groups of MXene prevent MoS2 agglomeration, and the incorporation of MoS2 mitigates adverse effects on the matrix’s insulation properties. This synergy operates through multiple mechanisms: MXene’s 3D thermal network facilitates efficient heat dissipation, MoS2 forms a transfer film to reduce friction, and the hybrid filler’s rigid structure resists deformation.
PI is widely employed as a substitute for metal materials in high-temperature environments that exceed 300 °C, owing to its exceptional thermal stability and mechanical strength and low density. These properties make it an ideal candidate for critical applications, such as aircraft engine components, bearings, and gears, where weight reduction and resistance to thermal degradation are paramount. However, the inherent high friction coefficient of pure PI materials remains a significant drawback, leading to increased wear, energy loss, and reduced service life under harsh operating conditions. This limitation restricts its broader adoption in demanding tribological applications in which both thermal and frictional performance are crucial. To overcome these limitations, we employed a precisely controlled synthesis approach to fabricate MoS2/MXene heterostructures, enabling the systematic investigation of the effects of filler morphology, composition ratio, and dispersion state on the performance of PI composites. Through a comprehensive analysis of high-temperature interfacial tribological behavior, we successfully established clear structure–property correlations between the nanofillers and polymer matrix, elucidated the temperature-dependent wear transition mechanisms, and developed effective synergistic reinforcement strategies. This study provided new ideas and technical reserves for the design modification of PI composite materials to improve their self-lubricating properties at normal and high temperatures.

2. Materials and Methods

2.1. Materials

Ti3AlC2 powder (200 mesh) and lithium fluoride (LiF) were purchased from Shanghai McLean Biochemical Co., Ltd. (Shanghai, China). Hydrochloric acid (HCl, 36–38%), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), and thiourea (CH4N2S) were obtained from Chengdu Colony Chemicals Co., Ltd. (Chengdu, China), while polyimide powder (YS-20) was supplied by the Shanghai Research Institute of Synthesis Resin (Shanghai, China). All reagents were of analytical grade.

2.2. Preparation of MXene and MoS2/MXene Hybrids

MXene was synthesized through HF acid etching following a previously reported method [27], as illustrated in Figure 1a. The preparation process involved three main steps. First, 30 mL of HCl and 1 g of LiF were precisely measured and transferred into a reaction kettle. After thorough mixing for 10 min, 1 g of Ti3AlC2 was gradually added in three equal portions. The etching reaction was then conducted in a 60 °C oil bath for 48 h. The resulting product was carefully washed with deionized water until the solution reached neutral pH. Subsequently, the prepared MXene was ultrasonically dispersed for one hour in a nitrogen (N2) environment to minimize oxidation. Finally, the suspension was centrifuged at 300 rpm to collect the supernatant, which was then freeze-dried at −72 °C for 36 h to obtain black MXene powder. The layered MXene suspension shown in Figure 1b was dark green in color and exhibited a typical Tyndall effect [29,30].
The MoS2/MXene hybrid material was synthesized via a hydrothermal method [27,31]. In a typical synthesis, 0.201 g of (NH4)6Mo7O24·4H2O and 0.173 g of CH4N2S were first dissolved in 60 mL of deionized water under continuous magnetic stirring for 10 min. After the complete dissolution of the precursors, 1.181 g of pre-synthesized MXene powder was added to the homogeneous solution. The resulting mixture was then stirred for an additional 30 min to ensure uniform dispersion. The reaction mixture was then transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 220 °C for 36 h to allow hydrothermal growth. Upon completion of the reaction, the autoclave was naturally cooled to room temperature. The product was collected via centrifugation at 6000 rpm for 5 min, followed by three successive washing cycles using deionized water and anhydrous ethanol alternately to remove any unreacted species. Finally, the purified product was freeze-dried at −72 °C for 36 h to obtain the MoS2/MXene hybrid powder.

2.3. Preparation of PI Matrix Composites

The PI matrix composites were fabricated through a hot press sintering process. First, the constituent materials were mechanically blended according to the predetermined ratios (Table 1) and then oven-dried at 120 °C for 2 h to remove residual moisture. The dried mixture was then loaded into a mold and subjected to pressure sintering at 30 MPa. The thermal treatment protocol involved heating the mold to 370 °C (holding for 120 min) under controlled temperature conditions, followed by natural cooling to below 200 °C. The sintering procedure is shown in Figure S2. Finally, the pressure was released, and the composite samples were demolded for subsequent testing.

2.4. Characterization

The crystal structure of the materials was analyzed using high-resolution X-ray diffraction (D8 Discover25, Bruker, Bielefeld, Germany) and a micro-confocal Raman spectrometer (LabRAM HR Evolution, HORIBA Jobin Yvon S.A.S., Paris, France). The material surface and abrasive components were analyzed using X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). The thermal properties of the materials were analyzed using a simultaneous thermal analyzer (STA 449 F3, Netzsch, Selb, Germany) under an N2 atmosphere, with heating from room temperature to 800 °C at a rate of 10 °C/min. The thermal diffusion coefficients of the composites were measured from room temperature to 300 °C using a laser thermal conductivity meter (STA449F3, Netzsch, Selb, Germany). Material microstructures and wear patterns were observed using scanning electron microscopy (JSM-6701F, JEOL, Musashino, Japan) and environmental scanning electron microscopy (Quanta 650 FEG, FEI, Hillsboro, OR, USA). The morphology of the hybrid filler was characterized using transmission electron microscopy (JEM-F200, JEOL, Musashino, Japan). The wear rate was measured using a non-contact 3D surface profiler (MicroXAM-800, KLA-Tencor, Milpitas, CA, USA). The hardness and elastic modulus were measured using a nanoindentation tester (STEP500, Anton Paar, Graz, Austria) with an applied force of 4 mN and a holding time of 10 s. The glass transition temperatures (Tg) and loss factors of the composites were measured using a dynamic mechanical analyzer (DMA 242 E, Netzsch, Selb, Germany) with a heating rate of 5 °C/min to 280 °C.

2.5. Tribological Tests

High-temperature tribological tests were conducted using a THT 07-135 (CSEM, Neuchatel, Switzerland) tribometer (Figure 2) in a ball-on-disk configuration. The upper specimen consisted of a GCr15 steel ball (φ3 mm), while the lower specimens were PI composite disks (φ25 × 8 mm). The test parameters included a 10 N applied load, a 50 mm/s sliding speed, and a 2000 m total sliding distance (66 min duration), with temperatures of RT, 100 °C, 225 °C (Tg), and 300 °C. The friction coefficient was recorded automatically, while the wear rate was calculated as the product of the wear scar cross-sectional area and its circumference, measured using a 3D profilometer.

3. Results and Discussion

3.1. Characterization of Structures and Properties

The structural morphologies of MXene and MoS2/MXene were characterized using scanning electron microscopy (SEM). Figure 3a revealed that the MXene exhibited distinct wrinkled surfaces with corrugated undulations, which are characteristic of interlayer stress relaxation during the etching process. The layers displayed irregularly jagged edges, forming a partially stacked accordion-like architecture [32,33]. This unique morphology provided both abundant active sites and a high specific surface area, facilitating effective composite formation with other materials [34,35,36]. Figure 3b,c present the SEM of the MoS2/MXene hybrid, which demonstrates the uniform coverage of MXene surfaces by hydrothermally synthesized MoS2 nanosheets. The MoS2 growth occurred both on MXene surfaces and within interlayer spaces, with localized nanoflower-like agglomerations evident in specific regions. Furthermore, the HR-TEM images of MoS2/MXene (Figure 3d) confirmed that the MXene substrate retained its characteristic 2D nanosheet layered structure. Measured lattice spacings of 0.25 nm and 0.68 nm corresponded to the interlayer distances of MXene and MoS2, respectively [37]. Energy-dispersive X-ray spectroscopy (EDS) mapping in Figure 3g–k confirmed the homogeneous surface distribution of constituent elements (C, Ti, O, Mo, and S) throughout the MoS2/MXene hybrid structure.
Figure 4a presents the XRD patterns of Ti3AlC2, MXene, and MoS2/MXene. The disappearance of the (104) peak (characteristic of the Al layer) confirmed the successful HF etching of Al atomic layers, transforming the densely packed MAX phase into layered MXene [31,38,39]. Following hydrothermal treatment, the XRD pattern revealed distinct diffraction peaks at 14.51°, 34.11°, 38.42°, 48.31°, and 59.79°, corresponding to the (002), (101), (103), (105), and (110) crystallographic planes of MoS2, respectively [40,41]. Figure 4b presents the Raman spectra of Ti3AlC2, MXene, and MoS2/MXene. The Raman peak at 401 cm−1 in Ti3AlC2 corresponded to in-plane Al-Ti vibrational modes. The complete disappearance of this peak in MXene confirmed the successful etching of the Al atomic layers [42]. The characteristic peaks of MoS2/MXene at 362 cm−1 and 391 cm−1, which correspond to the E12g and A1g of MoS2, respectively, represented the in-plane inverse vibrations of Mo and S atoms [31,37]. These results confirmed both the presence of MoS2 in the composites and its characteristic structural features.
The surface chemistry of MXene and MoS2/MXene was investigated using X-ray photoelectron spectroscopy (XPS). The broad-scan XPS spectrum of MXene (Figure 5a) revealed the presence of O 1s and F 1s peaks, confirming abundant surface functional groups (-OH and -F) due to HF acid etching [41]. In the MoS2/MXene sample, the full spectrum showed characteristic Mo 3d and S 2s peaks (Figure 5a). Additionally, the C 1s spectra (Figure 4b) displayed bonds related to C-O-Mo (285.91 eV) and C-S-Mo (288.8 eV), indicating the formation of new chemical bonds between MXene’s polar functional groups and MoS2 [37]. The Ti 2p spectrum of the MoS2/MXene composite (Figure 5c) was deconvoluted into five peaks at 455.56, 457.23, 459.44, 461.4, and 463.04 eV, which were assigned to Ti-C 2p3/2, Ti(II) 2p3/2, Ti-O 2p3/2, Ti-C 2p1/2, and Ti-O 2p1/2, respectively. In addition, the Mo 3d spectra (Figure 5d) exhibited two characteristic peaks at 229.04 eV (Mo 3d5/2) and 232.17 eV (Mo 3d3/2), corresponding to Mo4+, which confirmed the successful incorporation of MoS2 without oxidation. Similarly, the S 2p spectrum (Figure 5e) showed two peaks at 161.63 eV (S 2p3/2) and 162.51 eV (S 2p1/2), which is consistent with the reported binding energies of S2− in MoS2 [31].
The thermal stability of the four composites was evaluated under an N2 atmosphere using thermogravimetric analysis (TGA). Figure 6a,b present the TGA and differential thermal analysis (DTA) curves for PI, M-1, M-2, and M-3, with detailed data shown in Table 2. During the initial heating stage (before 540 °C), all composites exhibited minimal decomposition, which was primarily attributed to moisture desorption and the volatilization of small molecules. The onset decomposition temperatures for the four composites ranged from 548 °C to 562 °C. Pure PI exhibited an onset decomposition temperature of 548 °C. As the temperature increased, thermal degradation occurred through the scission of the PI main chain, followed by the decomposition of both imide and aryl ring structures [43]. This process released gaseous products, including CO2, CO, and nitrogen oxides, yielding a residual mass of 61% at 800 °C. The TGA curves of the remaining three composites indicated that incorporating these fillers enhanced the thermal stability of the PI matrix to varying degrees. Among these, the hybrid MoS2/MXene composite (M-3) exhibited the highest thermal stability. This improvement was primarily attributed to the strong Mo-S bonding energy within the MoS2 layers. Even under thermal expansion conditions, interlayer slippage did not disrupt the intralayer chemical bonds, thereby delaying the overall decomposition. Additionally, MoS2 acted as a physical barrier [38,44]. The enhanced thermal decomposition temperature achieved by MXene could be attributed to several key factors. First, MXene’s high specific surface area and interlayer spacing effectively inhibited the diffusion of small-molecule gases (CO2, CO) generated from PI thermal decomposition. This gas confinement effect reduced the decomposition reaction rate, thereby delaying significant material degradation until higher temperatures were reached [45]. Second, MXene’s high thermal conductivity [46] facilitated uniform heat distribution throughout the PI matrix, preventing localized thermal degradation and thereby delaying the onset of decomposition. This thermal regulation mechanism (combining high conductivity with heat homogenization) minimized localized chain scission in PI caused by thermal hotspots, thereby enhancing the composite’s bulk thermal stability [47]. Figure 6c,d present the thermal conductivity and Tg of the four composites, respectively. A lower tan δ peak intensity indicated stronger matrix–filler interfacial bonding [48]. Notably, the incorporation of MoS2 significantly reduced the tan δ peak height, confirming enhanced interfacial adhesion within the composites.
A nanoindentation analysis revealed the superior mechanical properties in the M-3 composite, which exhibited peak values for both hardness (0.398 GPa) and elastic modulus (6.294 GPa) among all tested formulations (Figure 7b). These enhanced mechanical characteristics directly contributed to improved load-bearing capacity during sliding friction applications.

3.2. Tribological Performance

The high-temperature tribological performance of pure PI and its composites (M-1, M-2, and M-3) was evaluated using a THT 07-135 high-temperature tribometer. Figure 8a presents the temperature-dependent friction coefficients for all four materials. In particular, 225 °C was selected for friction testing, as it approached the composite’s Tg, effectively evaluating tribological properties during material softening. The coefficients of friction for all materials decreased initially and then increased with temperature. At room temperature, the coefficient of friction of M-1 was 0.113 lower than that of pure PI, representing a 39% reduction. In comparison, the addition of MXene reduced the coefficient of friction by only 0.042 (a 14% decrease). This phenomenon was attributed to the 2D layered structure of MoS2, in which the layers were bonded by weak van der Waals forces. During friction, these layers slipped easily, forming a low-shear strength transfer film that significantly reduced the coefficient of friction [17]. Additionally, the 5% MoS2 filler was uniformly dispersed in the PI matrix, effectively carrying the load and reducing the direct wear of the PI substrate [26]. When the temperature increased to 100 °C, the friction coefficients of all four materials decreased to varying degrees compared to their values at room temperature. This phenomenon was primarily attributed to the temperature increase, which enhanced the mobility of the PI molecular chains, improved material toughness, reduced surface roughness, and altered molecular interactions between contact surfaces. These changes collectively lowered frictional resistance, resulting in a decreased coefficient of friction. When the temperature approached the glass transition point, the coefficient of friction increased sharply. This occurred because, at elevated temperatures, the PI matrix softened and began to undergo thermal degradation. As the material’s mechanical properties deteriorated significantly, it could no longer effectively withstand applied loads. Furthermore, the contact surface became prone to adhesion, abrasion, and other phenomena, leading to a rapid increase in the coefficient of friction [23]. Notably, the M-1 sample exhibited the most pronounced slope increase in the temperature range between the Tg and 300 °C (Figure 8a). This accelerated wear rate resulted from two concurrent mechanisms: the softening of the polymer matrix and the rapid oxidation of MoS2 to MoO3 occurred under combined thermal (ambient temperature) and tribological (frictional heating) effects. This phase transformation disrupted the layered structure, ultimately leading to lubrication failure. In contrast, the M-3 composite exhibited a slower rise in the friction coefficient due to MXene’s superior mechanical properties, which counteracted temperature-induced stiffness degradation [49]. Figure S3 shows the coefficient of friction curves for all composites at 200 °C and 300 °C.
Meanwhile, the wear rates of the four composites remained nearly constant at increasing temperatures up to the Tg but increased rapidly beyond the Tg (Figure 8b). The primary cause of this behavior was the reduction in material rigidity at elevated temperatures, which led to surface softening, diminished load-carrying capacity, and consequently, an increased wear rate [14]. However, the material’s surface roughness decreased simultaneously, resulting in more uniform contact between the composites and the friction pair. This reduced local stress concentration and lowered the wear rate. These two factors had opposing effects on the wear rate and canceled each other out before the Tg was reached, thereby keeping the wear rate relatively constant. However, when the temperature exceeded the Tg, the combined effects of ambient heat and frictional heating caused the material to rapidly lose strength. During grinding with the metal counterpart, it became highly susceptible to plowing and scraping, leading to severe wear and inefficiency. As a result, the wear rate increased sharply. Among the four materials, M-2 exhibited the best wear resistance. This was primarily attributed to MXene’s inherent rigidity and thermal conductivity, which enhanced the composite’s structural stability. Additionally, MXene facilitated heat dissipation, delaying material softening at elevated temperatures. These combined effects contributed to improved wear resistance [50,51].
To more comprehensively analyze the friction behavior of the composites at different temperatures, we examined their wear morphology using SEM (Figure 9). As shown in Figure 9a1–a4, the pure PI wear track was smooth and exhibited no obvious crack defects. However, in Figure 9a4, plastic deformation occurred at high temperatures, and surface cracking developed under the combined influence of pressure and temperature. For M-1 at the Tg (Figure 9b3), extensive delamination occurred on the wear surface due to the combined effects of high ambient temperature and frictional heating. This rapid oxidation formed MoO3, altering the laminar structure and exacerbating lubrication failure. Consequently, the friction coefficient and wear rate increased significantly compared to those at 100 °C. When 5% MXene was added to the PI matrix, the wear surface became smoother, and the shedding of MXene particles helped reduce severe wear, providing some anti-wear effect. However, micro-pit traces resembling corrosion were observed on the wear tracks of the M-2 composites at 300 °C (Figure 9c4), likely due to MXene oxidation under high-temperature sliding conditions [52]. In contrast, the 5% MoS2/MXene composite exhibited a more irregular wear surface with numerous small debris particles (Figure 9d1–d4).
The elemental composition and chemical state evolution of the abrasive generated during high-temperature friction were characterized by XPS, as shown in Figure 10. Deconvolution of the Mo 3d spectrum for the M-1 composite (Figure 10a) revealed binding energies at 228.14 eV (Mo4+ 3d5/2), 231.74 eV (Mo4+ 3d3/2), 233.92 eV (Mo5+ 3d3/2), and 236.7 eV (Mo6+ 3d3/2), confirming the partial oxidation of MoS2 to MoO3− and MoO3 during friction [53]. Furthermore, the S 2p and O 1s spectra in Figure 10b,c confirmed the presence of S2− and O2− in the wear debris, respectively. Similarly, deconvolution of the Mo 3d spectrum for the M-3 composite (Figure 10d) revealed binding energies at 229.42 eV (Mo4+ 3d5/2), 232.48 eV (Mo4+ 3d3/2), and 235.48 eV (Mo6+ 3d3/2), demonstrating that frictional heating induced the oxidation of MoS2. Deconvolution of the Ti 2p spectrum revealed the presence of oxidized Ti, with a characteristic peak at 464.69 eV.
Figure 11 illustrates the schematic tribological behavior of all four composites in the ball-on-disk test, and Figure S4 shows the electron microscopy images of the steel ball’s surface morphology after friction. The pure PI sample exhibited a smooth wear track without apparent surface damage. The incorporation of MoS2 and MXene enhanced the tribological performance of PI composites, attributed to their distinctive 2D lamellar structures that facilitated interlayer sliding during friction. For the MoS2 composite, the weakly bonded van der Waals interfaces between the MoS2 layers enabled easy interlayer sliding, promoting the formation of a low-shear strength transfer film on the contact surface and leading to significant friction reduction. For the MXene composite, the PI matrix exhibited enhanced hardness and thermal stability due to MXene’s inherent mechanical rigidity and high thermal conductivity, thereby reducing wear. Simultaneously, the exposed MXene nanosheets functioned as effective solid lubricants, yielding moderate reductions in both the coefficient of friction and wear rate [41]. When the filler was MoS2/MXene, MoS2 protected the MXene, mitigated oxidative damage, and maintained good lubricity. Additionally, more MoS2/MXene nanosheets were exposed on the surface, which helped to prevent damage to the PI substrate caused by frictional heat generation [27].
The MoS2/MXene hybrid materials (M-3) exhibited excellent comprehensive tribological properties, but their friction coefficient and wear rate were not optimal among all tested formulations. This phenomenon revealed the complex balance of functional synergy in composite material design. From the perspective of the mechanism of action, although the high hardness of MXene improved the load-bearing capacity, its interlayer shear strength was significantly higher than that of MoS2, which made the pure MoS2 formulation (M-1) more advantageous in terms of lubricity at room temperature. In the high-temperature range (>Tg), the stability of MXene’s Ti-O bonds resulted in a lower wear rate compared to the pure MoS2 system (where oxidation caused a sharp increase in the wear rate of M-1). The hybrid material achieved compromise performance through C-O-Mo bonding, but it was possible that part of the MXene surface was not effectively covered by MoS2. The stress concentration points formed in the uncovered areas explained why its wear rate remained higher than that of the pure MXene formulation (M-2).

4. Conclusions

In this work, MoS2/MXene hybrids were successfully synthesized via a hydrothermal method. These hybrids were then used as fillers to prepare a series of PI-based composites, including pure PI, 5% MoS2-PI, 5% MXene-PI, and 5% MoS2/MXene-PI. The influence of different fillers on the thermal properties, mechanical properties, and high-temperature tribological behavior of the composites was systematically investigated. In conclusion, the following findings were obtained:
1.
In terms of thermal stability, the MoS2/MXene hybrid filler demonstrated the most significant enhancement effect, increasing thermal conductivity by 36% compared to pure PI. This improvement was primarily attributed to the high bonding energy of Mo-S covalent bonds within the MoS2 layer.
2.
Regarding mechanical properties, all three fillers substantially enhanced the hardness of the PI matrix, with this improvement being achieved through effective filler–matrix interfacial interactions that concurrently suppressed wear behavior.
3.
In tribological performance studies, all composites exhibited characteristic temperature-dependent friction behavior. The coefficient of friction followed a consistent ‘decrease-then-increase’ trend with rising temperature, reaching a minimum value at 100 °C. The 5% MoS2-PI composite demonstrated a 39% reduction in the coefficient of friction at room temperature compared to pure PI. Meanwhile, the 5% MXene–PI composite exhibited superior wear resistance, achieving a 64% decrease in room temperature wear rate relative to pure PI. MXene’s inherent high hardness and laminated structure provided dual benefits by suppressing abrasive wear while delaying plastic deformation onset through matrix stiffening.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs9070373/s1, Figure S1: Schematic diagram of the THT 07-135 high-temperature friction and wear test device; Figure S2: Sintering temperature program and pressure for the PI composite materials; Figure S3: Friction coefficient curves of four composite materials at different temperatures (a) 200 °C (b) 300 °C; Figure S4: SEM micrographs and EDS elemental mappings of four friction steel balls at Tg: (a) PI; (b) M-1; (c) M-2; (d) M-3; Figure S5: Surface of M-3 material after friction at 320 °C; Table S1: Thermal conductivity of four composite materials at different temperatures (W/(m·K)); Table S2: Friction coefficients of four composite materials at different temperatures; Table S3: Wear rates of four composite materials at different temperatures (×10−5 mm3/Nm).

Author Contributions

Conceptualization, X.J., Y.Y. and J.W.; Methodology, X.J.; Formal analysis, X.J. and P.R.; Investigation, X.J. and Y.S.; Resources, P.R.; Data curation, X.J. and P.R.; Writing—original draft, X.J.; Writing—review & editing, H.L., Y.Y. and J.W.; Supervision, Y.S.; Funding acquisition, Y.Y. and J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China [grant number 52475228] and Technical foundation projects of Taihang National Laboratory [grant number: F2024-4-010].

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a) Preparation of MXene from Ti3AlC2 via HF acid etching and MoS2/MXene using the hydrothermal method; (b) Tyndall effect in MXene dispersions.
Figure 1. (a) Preparation of MXene from Ti3AlC2 via HF acid etching and MoS2/MXene using the hydrothermal method; (b) Tyndall effect in MXene dispersions.
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Figure 2. Schematic diagram of the THT 07-135 high-temperature friction and wear test device.
Figure 2. Schematic diagram of the THT 07-135 high-temperature friction and wear test device.
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Figure 3. SEM images of (a) MXene and (b,c) MoS2/MXene; (d–f) TEM images of MoS2/MXene; (g–k) EDS mapping.
Figure 3. SEM images of (a) MXene and (b,c) MoS2/MXene; (d–f) TEM images of MoS2/MXene; (g–k) EDS mapping.
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Figure 4. Ti3AlC2, MXene, and MoS2/MXene: (a) X-ray diffraction pattern; (b) Raman spectrum.
Figure 4. Ti3AlC2, MXene, and MoS2/MXene: (a) X-ray diffraction pattern; (b) Raman spectrum.
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Figure 5. XPS of MXene and MoS2/Mxene: (a) full spectra; (b–c) C 1s and Ti 2p; (d–e) Mo 3d and S 2p spectra of MoS2/MXene.
Figure 5. XPS of MXene and MoS2/Mxene: (a) full spectra; (b–c) C 1s and Ti 2p; (d–e) Mo 3d and S 2p spectra of MoS2/MXene.
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Figure 6. (a) TGA curves; (b) DTG curves; (c) thermal conductivity at different temperatures; (d) loss factor for four composites.
Figure 6. (a) TGA curves; (b) DTG curves; (c) thermal conductivity at different temperatures; (d) loss factor for four composites.
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Figure 7. (a) Load displacement curves; (b) hardness and elasticity modulus of the four composites.
Figure 7. (a) Load displacement curves; (b) hardness and elasticity modulus of the four composites.
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Figure 8. (a) Coefficient of friction; (b) wear rate curves of four composites at different temperatures.
Figure 8. (a) Coefficient of friction; (b) wear rate curves of four composites at different temperatures.
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Figure 9. Wear patterns of four composites at different temperatures: (a1–a4) PI; (b1–b4) M-1; (c1–c4) M-2; (d1–d4) M-3.
Figure 9. Wear patterns of four composites at different temperatures: (a1–a4) PI; (b1–b4) M-1; (c1–c4) M-2; (d1–d4) M-3.
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Figure 10. XPS of composites’ abrasive chips: (a–c) Mo 3d, S 2p, and O 1s for M-1; (d–e) Mo 3d and Ti 2p spectra for M-3.
Figure 10. XPS of composites’ abrasive chips: (a–c) Mo 3d, S 2p, and O 1s for M-1; (d–e) Mo 3d and Ti 2p spectra for M-3.
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Figure 11. Schematic wear mechanisms of different composites: (a) sliding schematic; (b) PI; (c) M-1; (d) M-2; (e) M-3.
Figure 11. Schematic wear mechanisms of different composites: (a) sliding schematic; (b) PI; (c) M-1; (d) M-2; (e) M-3.
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Table 1. PI matrix composites with different ratios (wt.%).
Table 1. PI matrix composites with different ratios (wt.%).
MaterialsPIMoS2MXeneMoS2/MXene
PI100%000
M-195%5%00
M-295%05%0
M-395%005%
Table 2. TGA data of four composites under N2 atmosphere.
Table 2. TGA data of four composites under N2 atmosphere.
MaterialsTi (°C)T5 (°C)Tp (°C)Rw (%)Tg
PI54855357961221
M-155156058160223
M-255956658458229
M-356256958660230
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Ji, X.; Ren, P.; Liu, H.; Shi, Y.; Yan, Y.; Wang, J. High-Temperature Tribological Behavior of Polyimide Composites with Dual-Phase MoS2/MXene Lubricants: A Synergistic Effect Analysis. J. Compos. Sci. 2025, 9, 373. https://doi.org/10.3390/jcs9070373

AMA Style

Ji X, Ren P, Liu H, Shi Y, Yan Y, Wang J. High-Temperature Tribological Behavior of Polyimide Composites with Dual-Phase MoS2/MXene Lubricants: A Synergistic Effect Analysis. Journal of Composites Science. 2025; 9(7):373. https://doi.org/10.3390/jcs9070373

Chicago/Turabian Style

Ji, Xingtian, Pengwei Ren, Hao Liu, Yanhua Shi, Yunfeng Yan, and Jianzhang Wang. 2025. "High-Temperature Tribological Behavior of Polyimide Composites with Dual-Phase MoS2/MXene Lubricants: A Synergistic Effect Analysis" Journal of Composites Science 9, no. 7: 373. https://doi.org/10.3390/jcs9070373

APA Style

Ji, X., Ren, P., Liu, H., Shi, Y., Yan, Y., & Wang, J. (2025). High-Temperature Tribological Behavior of Polyimide Composites with Dual-Phase MoS2/MXene Lubricants: A Synergistic Effect Analysis. Journal of Composites Science, 9(7), 373. https://doi.org/10.3390/jcs9070373

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