Preparation of Aluminum Matrix Composites Reinforced with Hybrid MAX–MXene Particles for Enhancing Mechanical Properties and Tribological Performance

Abstract

This study presents a novel methodology for the fabrication of aluminum matrix composites (AMCs) reinforced with a hybrid of MAX phase (Ti₃AlC₂) and MXene (Ti₃C₂T_x) particles via vacuum hot-pressing sintering, aiming to enhance the mechanical properties and tribological performance of aluminum matrix composites. The hybrid-reinforced aluminum matrix composites were fabricated with Ti₃AlC₂/Ti₃C₂T_x reinforcements at a 1:1 mass ratio, incorporating reinforcement contents of 5 wt.%, 15 wt.%, and 25 wt.%, respectively. The optimized vacuum hot press sintering process was as follows: firstly, a cold press pressure of 20 MPa was applied to the composite powder, and then hot press sintering was carried out by means of segmental pressurization with a sintering pressure of 20 MPa, a temperature of 500 °C, and a heat preservation of 1 h before cooling in the furnace. It was found by micro-morphological characterization and mechanical property testing that with the increase of Ti₃AlC₂/Ti₃C₂T_x reinforcement content (5 wt.%→15 wt.%), the micro-hardness of the composites (31.9→76.1 HV_0.2), compressive strength (41.7→151.9 MPa), and tribological properties (friction coefficient 0.68→0.50) were significantly improved; however, when the content of reinforcement exceeded 15 wt.%, the deterioration of properties triggered by the increase in pore defects and particle agglomeration leads instead to a decrease in compressive strength (by 12.3%), apparent modulus of elasticity (specimen’s compressive specific stiffness, by 9.8%) and frictional stability (coefficient of friction recovered to 0.62). The 15 wt.% hybrid reinforcement composites demonstrated optimal strength-toughness synergies, exhibiting a 361.6% increase in yield strength and a 597.1% increase in apparent modulus of elasticity compared to pure aluminum. Furthermore, the friction coefficient exhibited a 26.47% reduction in comparison to pure aluminum, thereby substantiating enhanced tribological performance. The observed enhancements are attributed to the synergistic effects of the MAX and MXene phases, where MXene improves interfacial wettability and densification, while MAX particles enhance overall strength through diffusion reinforcement.

1. Introduction

Aluminum alloys, recognized for their low density, high specific strength, excellent thermal and electrical conductivity, and superior formability, have become indispensable structural and functional materials across modern industrial sectors, including aerospace, automotive manufacturing, electronics, and high-end equipment [1]. In the 1960s, Professor Novotny first systematically reported MAX-phase materials (chemical general formula M_n+1AX_n, where M is a transition metal, A is a dominant element, and X is C or N) [2]. These layered ternary compounds consist of alternating stacks of metal layers (M layers) and covalently bond-dominated ceramic layers (AX layers), combining the high thermal conductivity, electrical conductivity, and machinability of metals with the high strength, high temperature resistance, and oxidation resistance of ceramics [3,4]. Typical MAX-phase materials, exemplified by Ti₃AlC₂, Ti₃SiC₂, and Ti₂AlC, exhibit considerable promise for utilization in domains such as energy storage, high-temperature structural applications, and aerospace engineering, attributable to their distinctive property profile [3]. In recent years, the study of MAX phase as a reinforcement for aluminum matrix composites has gradually emerged. Research has demonstrated that composites such as Ti₃AlC₂/Al exhibit substantial enhancements in properties such as hardness, compressive yield strength, and wear resistance when compared to pure aluminum and aluminum alloys [5,6,7,8]. However, the inadequate wettability between the MAX phase and the aluminum matrix results in insufficient interfacial bonding strength, which is susceptible to triggering interfacial debonding failure [9], thereby severely restricting the optimization of their properties. Furthermore, the MAX phase is susceptible to interfacial reaction with Al, resulting in the formation of brittle intermetallic compounds (e.g., Al₃Ti) during conventional preparation processes (e.g., the fusion casting method). This phenomenon further compromises the performance of the composites [9,10,11,12].

In 2011, Barsoum’s team at Drexel University successfully prepared novel two-dimensional transition metal carbides or nitrides (MXene, e.g., Ti₃C₂T_x) by selectively etching the A-layer in the MAX phase [13,14]. As a class of transition group metal carbides or nitrides, MXene is derived from its precursor MAX phase—a layered ceramic particle that exhibits both ceramic and metal properties, including high hardness, excellent electrical conductivity, and good processability—which has already been proven to be an effective reinforcing phase for aluminum matrix composites. MXene not only inherits the high toughness and conductivity of the MAX phase but also, due to its abundant surface functional groups (e.g., –O²⁻, –OH⁻, –F⁻), exhibits significantly enhanced wettability with the aluminum matrix compared to the parent MAX phase [15,16,17]. This improved interfacial compatibility, coupled with MXene’s unique open accordion-like structure, endows it with a strong “anchoring effect” that can significantly enhance the densification of composites. Additionally, its interlayer slip property has been demonstrated to improve the plastic toughness of the materials [18,19,20,21]. However, the loosely stacked, accordion-like structure of MXene also results in weak interlayer bonding, making it susceptible to crack initiation under mechanical loading and leading to premature failure [22,23]. Notably, the prevailing focus of contemporary research endeavors has been on the functional applications of MXene, with scant attention devoted to addressing its structural defects. Against this backdrop, developing new aluminum matrix composites by compositing MAX and MXene particles as reinforcements holds great research significance and application value.

Table 1. Current status of MAX/MXene-reinforced composite materials research.

Researchers	Reinforcement Type (MAX/MXene)	Preparation Method	Key Findings
Gonzalez-Julian J et al. [24]	Cr2AlC (MAX phase)	Pressureless sintering (Ar atmosphere)	High-purity Cr2AlC with low density; secondary heat treatment needed for densification.
Alhabeb M et al. [25]	Ti3SiC2 (MAX phase)	Not specified	TiCxNi3(Al, Ti)-Ni composite prepared; hardness increased by nearly 50%, and bending strength, fracture toughness improved significantly vs. traditional Ni and Ni alloys.
Zhang et al. [26]	Ti3C2Tx (MXene)	Pressureless sintering (650 °C) + hot extrusion	3 wt.% Ti3C2Tx/Al composite: hardness (0.27→0.52 GPa) and tensile strength (98→148 MPa) improved vs. pure Al.
Liu et al. [27]	Ti3C2Tx (MXene)	High-shear mixing + SPS	Partial MXene oxidized to TiO2 in situ, forming nail-plate reinforcement (NSR); Cu matrix yield strength increased by 70.3%.
Zhou et al. [28]	Ti3C2Tx (MXene)	Powder metallurgy	Ti3C2Tx tightly bonded to Al matrix; interfacial load transfer enhanced by Al penetration anchoring effect; 0.26 vol.% Ti3C2Tx increased composite strength by ~66% vs. pure Al.

summarizes the current state of research on MAX/MXene-reinforced composites.

The predominant method for producing aluminum-based composite materials is powder metallurgy. Among these, vacuum hot-press sintering technology has been widely adopted due to its ability to effectively suppress interfacial reactions and enhance density. For instance, Wang et al. successfully achieved a uniform distribution of Ti₃AlC₂ particles in an aluminum matrix using this process, thereby significantly enhancing the mechanical properties of the composite material [7]. From the perspective of strengthening mechanisms, single MAX phases primarily enhance composite strength through dislocation strengthening and dispersion strengthening, while MXene enhances interfacial bonding through the “anchoring effect” of its two-dimensional structure. However, extant studies have not systematically verified the synergistic effects of these two mechanisms [14,23]. Furthermore, in the context of performance regulation, numerous studies have examined the impact of single reinforcing phase content on composite hardness and strength. However, there is a paucity of in-depth exploration into the balance between tribological and mechanical properties in composite reinforcement systems [10].

The present study proposes a novel hybrid reinforcement strategy involving the co-doping of Ti₃AlC₂ (MAX phase) and Ti₃C₂T_x (MXene phase) into an aluminum matrix. The objective of this approach is to achieve a synergistic effect that overcomes the limitations of single-phase reinforcement materials. Specifically, MXene, with its abundant surface functional groups (–O²⁻, –OH⁻, –F⁻), has been shown to significantly improve the interfacial wettability between the reinforcement material and the aluminum matrix. Its accordion-like structure is conducive to densification through strong anchoring effects. Concurrently, the rigid Ti₃AlC₂ particles impede crack propagation and augment overall strength through diffusion strengthening. This effect effectively compensates for the deficient interlayer bonding strength of MXene.

This paper systematically investigated the effects of mixed reinforcement content (5 wt.%, 15 wt.%, 25 wt.%) on microstructure, mechanical properties, and tribological properties. The employment of vacuum hot-press sintering technology (a powder metallurgy process) entails the optimization of etching time (2 h) and sintering parameters (500 °C, 20 MPa). This approach effectively circumvents the destruction of the MXene structure by molten aluminum in conventional melting casting, thereby facilitating the attainment of a uniform distribution of the two reinforcing phases and interfacial strengthening. This study pioneers a Ti₃AlC₂–Ti₃C₂T_x dual-phase reinforced aluminum matrix composite, systematically examining the synergistic reinforcement mechanisms at varying phase ratios. Multi-technique characterization elucidates the MAX–MXene interplay, correlating their content with the material’s mechanical and tribological properties. This work addresses the existing gap in systematic research on MAX–MXene synergy and establishes an experimental basis for optimizing aluminum composite formulations.

2. Experimental Materials and Methods

In this study, Ti₃C₂T_x was prepared by subjecting Ti₃AlC₂ to high-frequency (HF) acid etching and subsequently compositing it with Ti₃AlC₂ as a reinforcement within an aluminum matrix. The specific process entails the preparation of the MXene phase, ball milling, and the amalgamation of the reinforcement with aluminum powder. Subsequent steps involve vacuum hot pressing and sintering molding. The subsequent sections delineate the process parameters and optimization process of each preparation step with great particularity.

2.1. Preparation of MXene Phase (Ti₃C₂T_x)

The Ti₃AlC₂ precursor was prepared by etching the Ti₃C₂T_x compound using a HF solution with a concentration of 40%, as previously described in the literature [23,29]. First, weigh 2 g of Ti₃AlC₂ powder into a PTFE beaker. Add a stir bar and 50 mL of 40% HF solution. Cover the beaker opening with vented plastic wrap, place it on a magnetic stirrer in a water bath, and heat to 50 °C. Perform etching for varying durations: 1 h, 2 h, and 3 h. Stirring should be maintained throughout both the heating and etching processes to evaluate the effect of etching time on the product morphology. To optimize the etching effect, after etching, centrifuge the solution 9–10 times with deionized water at 4500 rpm for 3 min per cycle until the pH of the supernatant reaches approximately 6. Filter off the water, then dry the sample in a vacuum oven at 80 °C for 24 h. Store the sample in a vacuum desiccator at approximately 0.01 MPa vacuum for subsequent use [30]. Following the completion of the scanning electron microscopy characterization, it was ascertained that the Ti₃C₂T_x lamellae exhibited optimal structural integrity after undergoing etching for a duration of two hours. Consequently, the preferred etching process, as determined by the experimental findings, is as follows: The high-frequency (HF) concentration was set at 40%, the etching temperature was maintained at 50 °C, and the etching time was set at 2 h.

Figure 1a,b show the XRD diffraction patterns of Ti₃C₂T_x and Al powder derived from Ti₃AlC₂ after different etching durations, respectively. All diffraction peaks of Ti₃AlC₂ match the standard reference pattern almost exactly, with no additional impurity peaks observed, indicating the high purity of Ti₃AlC₂. The XRD patterns reveal changes in the crystal structure of Ti₃C₂T_x. Compared to Ti₃AlC₂, the etched Ti₃C₂T_x exhibits distinct differences, forming characteristic diffraction peaks unique to MXene. A continuous hump is observed in the 30–45° 2θ range, and the characteristic peak corresponding to the (002) plane shifts from approximately 9.5° in Ti₃AlC₂ to around 7.3° in the etched sample. This shift indicates a reduction in lattice parameters, suggesting that Al atoms are removed during etching, leading to an expansion of the interplanar spacing. The XRD results for MXene obtained after different etching durations also show variations. After 2 h of etching, the Ti₃C₂T_x diffraction peaks remain relatively intact, with a moderate shift in the (002) plane. Other characteristic peaks are also clearly visible, indicating that the interlayer spacing of the resulting Ti₃C₂T_x is nearly fully opened without structural damage.

2.2. Reinforcement/Aluminum Powder Ball Mill Mixing

A planetary ball mill was utilized for the ball milling and mixing of Ti₃AlC₂, Ti₃C₂T_x and pure aluminum particles. A 1:1 mass ratio of Ti₃AlC₂ to Ti₃C₂T_x was selected as an enhancer, and mixed enhancement powders with mass fractions of 5%, 15%, and 25%, respectively, were added to the aluminum matrix powders for vacuum ball milling. The following procedure was employed in the synthesis of the hybrid powder by ball milling was:

(1)

The procedure entails the weighing of 100 g of Ti₃AlC₂, Ti₃C₂T_x, and Al powders in a ball-milling jar. Subsequently, 1 kg of grinding balls is added at a ball-to-material ratio of 10:1. The mixture is then supplemented with approximately 33 g of anhydrous ethanol, constituting 3% of the total mass of the ball material.

(2)

In order to prevent the powder from oxidizing, it is necessary to vacuum to approximately 0.05 MPa after covering the lid of the ball milling tank.

(3)

The speed of the ball mill was set to 150 revolutions per minute (rpm), alternating between clockwise and counterclockwise rotations. Each cycle lasted 30 min, with 10 min intervals allocated to allow the machine to cool down. The experiment spanned six cycles, with the cumulative effective ball milling time amounting to 360 min. Following the ball milling process, the aluminum powder particles underwent a transformation in shape, exiting the ball mill as irregularly shaped particles that deviated from their original spherical form. Concurrently, the surface energy of the powder experienced a substantial increase, a property that would prove advantageous in subsequent sintering tests.

(4)

Subsequently, the ball milling product should be subjected to vacuum drying in order to ensure its preservation for future use.

2.3. Vacuum Hot-Pressing Sintering

The experiment is a single-phase system sintering, which principally relies on the mass flow transfer of aluminum particles. The sintering process is carried out in the solid state, without generating new phases or compounds. Consequently, the pure aluminum sintering experiment can be utilized to establish a process window for the selection of sintering parameters for composites. In accordance with the fundamental tenets of powder metallurgy [31,32], the sintering temperature is typically elevated in comparison to the recrystallization temperature of the metal. This elevated temperature is a critical factor in accelerating the self-diffusion rate of the metal atoms, thereby facilitating the desired phase transformation. The sintering process comprises three distinct stages: low-temperature pre-sintering, medium-temperature sintering, and high-temperature holding. The temperature of the latter stage is calculated to be 0.70–0.85 times the melting point of the metal. The melting point of the hybrid powder has been determined to be 663 °C based on the DSC experiments, indicating a range of 382 °C to 523 °C. In accordance with the findings of preceding research, temperatures of 460 °C, 480 °C, 500 °C, and 520 °C were selected for the sintering experiments. However, the sample was completely melted at 520 °C during practical operation, so the maximum sintering temperature was set at 500 °C.

Given the established pressure limit of 25 MPa within the sintering mold, a triad of pressures—namely, 10 MPa, 15 MPa, and 20 MPa—was designated as the sintering pressures. A total of three sintering temperatures were then selected for the sintering test, with each combination representing an individual sintering sample. The resulting densities of the prepared samples are presented in

Table 2. Densification of sintered pure aluminum after 20 MPa cold pressing.

Number of Samples	Sintering Temperature (℃)	Sintering Pressure (MPa)	Densification (%)
1	460	10	83.07%
2	460	15	88.61%
3	460	20	90.14%
4	480	10	93.40%
5	480	15	94.35%
6	480	20	96.18%
7	500	10	94.60%
8	500	15	96.95%
9	500	20	99.38%

. As illustrated in Table 2, increasing the sintering temperature and pressure resulted in a progressive increase in the density of the sintered samples. Notably, at a sintering temperature of 500 °C and a sintering pressure of 20 MPa, the density of the pure aluminum samples sintered attained 99.38%, exhibiting near-complete density. Consequently, the sintering temperature of 500 °C and the sintering pressure of 20 MPa were ultimately identified as the optimal sintering temperature and pressure parameters for the fabrication of the composites, respectively. The preferred sintering process parameters are enumerated in

Table 3. Preferred sintering process parameters.

Sinter Point	Sintering Pressure	Sintering Vacuum	Heating Rate	Cooling Method
500 °C	20 MPa	5 × 10−3 Pa	10 °C/min	Furnace cooling

.

2.4. Experiment and Methods

For microstructural characterization, 2 mm thick slices were sectioned from the central region of the specimen gauge. Phase identification was performed using X-ray diffraction (XRD, Bruker D8 DISCOVER A25(Bruker, Karlsruhe, Germany)) with Co-Kα1 radiation, operating at 40 kV and 30 mA. XRD scans were collected over a 2θ range of 5° to 90° at a scan speed of 3°/min. Microstructural observations were primarily conducted using a scanning electron microscope (SEM, FEI Helios NanoLab G3 UC(FEI, Brno, Czech Republic)). Prior to SEM imaging, the sample surfaces were mechanically polished to a mirror-like finish and then vibrationally polished for 8 h to eliminate residual stress.

Vickers microhardness was measured using a LECO LM248AT (LECO Corporation, St. Joseph, MI, USA) tester, with applied loads ranging from 5 to 2000 gf and a stepper resolution of 1 µm. Quasi-static compressive tests were performed using a GNT100 (GNT, Houston, TX, USA) electronic universal testing machine, which has a maximum load capacity of 100 kN, force resolution of 0.2 N, and displacement resolution of 0.017 µm. The compression specimens were cylindrical, with dimensions of Φ8 mm × 12 mm. Before testing, both ends of the specimens were polished to a smooth finish to minimize the effects of surface impurities or friction on the compression results.

Tribological properties were evaluated using a UMT Tribo(Bruker, Karlsruhe, Germany) multifunctional friction and wear tester. The tests were conducted in linear reciprocating mode at room temperature, with applied loads ranging from 20 mN to 200 N, a stroke length of 1 mm, and a frequency of 0.1 to 5 Hz.

3. Results and Discussion

3.1. Characterization of MXene Enhancers

Figure 2 presents the SEM morphology of Ti₃C₂T_x powders fabricated under varying etching times. It can be observed that the Al atoms between layers in the original close-packed particle structure of Ti₃AlC₂ have been selectively etched, resulting in the expansion of the layer spacing. Consequently, the particle structure has undergone a transformation from close-packed to open, accordion-like. Figure 2 illustrates the Ti₃C₂T_x powder morphology under varying etching times:

(1)

The etching process was conducted for a duration of one hour. As a result, the layer spacing was observed to undergo a substantial widening. However, it was noted that a number of layers remained partially open.

(2)

The etching process was conducted for a duration of two hours. This resulted in a moderate degree of interlayer opening, with almost no unopened layers and a uniform morphology. The interlayer openings were found to be uniformly and moderately open. This was performed to maintain the hardness of the original MAX particles. Additionally, a larger number of surface functional groups were observed. These groups can maximize the provision of interfacial wettability with the aluminum substrate. This, in turn, allows for the formation of a strong interfacial bond.

(3)

The etching process was conducted for a duration of three hours. This resulted in an expansion of the layer spacing to a greater extent than before, accompanied by the occurrence of over-etching. This, in turn, led to the fracturing of certain lamellar structures.

The EDS results of Ti₃C₂T_x powders obtained at varying etching times are presented in Figure 3. The uniform composition of Ti₃C₂T_x powder is evident in the distribution of Ti and C elements.

3.2. Effect of Reinforcement Content on Microstructure and Densification of Aluminum Matrix Composites

The composite samples containing 5 wt.%, 15 wt.%, and 25 wt.% mixed reinforcements were prepared using the preferred vacuum hot press sintering process, and their microstructural evolution is shown in Figure 4 and Figure 5. As demonstrated in Figure 4 and Figure 5, the content of reinforcement has a substantial impact on the uniformity of its distribution and the density of the composites, as evidenced by characterization techniques such as optical microscopy and scanning electron microscopy (SEM).

3.2.1. Microstructural Evolution

The enhancer content of 5 wt.% is as follows: As demonstrated in Figure 3(a1–c1) and Figure 4(a1–c1), the enhancer (gray-black particles) is uniformly dispersed at the grain boundaries of the Al substrate, with no apparent agglomerates or holes. This indicates that the enhancer’s vacuum hot-pressing sintering microstructure and morphology are uniform, despite its low content.

As illustrated in Figure 3(a2–c2) and Figure 4(a2–c2), at an enhancer content of 15 wt.%, there is a tendency for enhancers to cluster in the local area, accompanied by a modest number of submicron-sized pores (indicated by the red circle in Figure 4(a2)). Concurrently, there is a decrease in the degree of densification. The distribution of titanium (Ti) elements, as determined by energy-dispersive spectroscopy (EDS) surface scanning, aligns closely with the morphology observed through scanning electron microscopy (SEM). This finding serves to substantiate the hypothesis that the clustered area is indeed an enhancer-enriched region.

As the content of the reinforcing body increases, the tendency for agglomeration and the formation of a continuous network structure become more pronounced. This phenomenon, depicted in Figure 3(a3–c3) and Figure 4(a3–c3), results in the generation of a substantial number of strip micropores and point-like pores at the interface. Consequently, the densification process is further diminished.

3.2.2. Interface Bonding and Anchoring Effect Mechanisms

Figure 6 presents the high-magnification SEM analysis of the reinforcement particles in the 15 wt.% reinforcement composites, unveiling the distinctive interfacial interactions between the reinforcement and the Al matrix.

Ti₃C₂T_x reinforcement: The initial layered structure, reminiscent of an accordion, becomes partially closed during the sintering process due to the penetration of the flowing mass transfer of Al into the interlayer gap. This results in a morphology that resembles that of MAX. This phenomenon is not a reversible transformation of the etched structure, but rather a mechanical anchoring effect caused by the capillary force-driven mass transfer of molten Al, which enhances the interfacial load transfer efficiency.

Ti₃AlC₂ reinforcement: Ti₃AlC₂ exhibits a metallurgical bond with the aluminum substrate interface characterized by no obvious gaps. High-magnification SEM observations reveal the presence of a diffusion layer approximately 50–100 nm thick at the interface (Figure 6b). The formation of this interface structure is attributed to the diffusion of Ti elements into the aluminum substrate during hot-press sintering, resulting in the formation of an Al–Ti solid solution zone, which effectively enhances the interface load transfer efficiency. Additionally, the layered structure of Ti₃AlC₂ can alleviate stress concentration through interlayer slip when subjected to force, while synergistically interacting with the anchoring effect of MXene to further suppress interface cracking.

3.3. Effect of Reinforcement Content on the Properties of Aluminum Matrix Composites

To guarantee statistical reliability, all mechanical properties (including compressive and hardness measurements) and tribological tests were evaluated using five parallel specimens for each condition. The data were statistically processed, and the results are presented as the mean value with the corresponding standard deviation (SD). Accordingly, error bars derived from these standard deviations are incorporated in all relevant performance graphs (e.g., density, hardness).

3.3.1. Densification Test

A total of three density tests were conducted on each of the prepared composites, employing Archimedes’ drainage method. The results of these tests were then averaged, and the findings are presented in

Table 4. Density of composites with different reinforcement contents.

Reinforcement Content (wt.%)	Density (g/cm3)
0	2.68 ± 0.006
5	2.71 ± 0.006
15	2.72 ± 0.006
25	2.71 ± 0.006

.

Theoretically, the density of pure aluminum is 2.7 g/cm³, while that of Ti₃AlC₂ is 4.25 g/cm³, and that of Ti₃C₂T_x is 3.7 g/cm³. The theoretical density of the 5 wt.% reinforced body composites was calculated to be 2.74 g/cm³, the theoretical density of the 15 wt.% reinforced body composites was 2.835 g/cm³, and the theoretical density of the 25 wt.% reinforced body composite was 2.93 g/cm³.

The densities of the composites were calculated and are presented in

Table 5. Densification of composites with varying reinforcement content.

Reinforcement Content (wt.%)	Densification (%)
0	99.38 ± 0.21
5	98.79 ± 0.21
15	96.06 ± 0.21
25	92.38 ± 0.20

. The variation in density with reinforcement content is illustrated in Figure 7. The decrease in the densification of the composites with the increase in the reinforcement content from 99.38% (pure aluminum) to 92.38% (25 wt.%) can be attributed to the increase in reinforcement, which resulted in an increase in agglomerates. This phenomenon occurred due to the inability of the reinforcing particles to form a completely tight bond with each other, resulting in a greater number of holes. The results of the densification test of the composites, which contained three proportions of 5 wt.%, 15 wt.%, and 25 wt.% of reinforcers were consistent with the metallographic organization and SEM test results. These results indicated that as the content of the reinforcer particles increased, the composites became more porous, and the densification gradually decreased. However, the densification of the three types of aluminum matrix composites with three types of reinforcements exceeds 92%, with the highest density approaching 99%, suggesting that the prepared composites exhibit enhanced density.

3.3.2. Microhardness Test

As illustrated in

Table 6. Vickers microhardness of composites with varying reinforcement contents.

Reinforcement Content (wt.%)	Hardness (HV0.2)
0	31.90 ± 0.10
5	45.27 ± 1.63
15	76.13 ± 3.08
25	86.74 ± 7.95

and Figure 8, the nonlinear strengthening pattern of the reinforcement content on the hardness of the composites is evident. The hardness of sintered pure aluminum was measured at 31.9 HV_0.2, while the hardness of composites containing 5 wt.%, 15 wt.%, and 25 wt.% of reinforcement were enhanced to 45.27, 76.13, and 86.84 HV_0.2, respectively. However, the strengthening effect exhibits a substantial decay after exceeding 15 wt.%. The increase in hardness is 139% at the 0→15 wt.% stage, and the subsequent increase in hardness is only 14% at the 15→25 wt.% stage. This phenomenon is the result of a dynamic competition between dislocation pinning reinforcement and hole-damaging effects. As the reinforcement content increases, the Orowan dislocation pinning mechanism becomes the predominant mechanism. The holes created by the subsequent increase in reinforcement also increase, leading to a decrease in the densification. This decrease partially offsets the increase in hardness, resulting in a net decrease in the hardness value of the composites when the reinforcement content is increased from 15 wt.% to 25 wt.%.

3.3.3. Uniaxial Compression Testing

Uniaxial compression tests were used to characterize the strength and apparent modulus (specimen’s compressive specific stiffness) of aluminum matrix composites reinforced with particles. As illustrated in Figure 9 and substantiated in

Table 7. Compression properties of composites.

Reinforcement Content (wt.%)	Compressive Yield Strength (MPa)	Apparent Elastic Modulus (GPa)	Stress at Cracking (MPa)	Strain at Cracking
0	42.00	1.31	uncracked	uncracked
5	112.60	7.78	uncracked	uncracked
15	151.87	7.80	261.04	30.61%
25	116.21	6.38	272.41	33.12%

, the compressive failure mode of the specimens with increasing reinforcement content undergoes a transition from ductile bulging (5 wt.%) to brittle splitting (25 wt.%). The mechanical response of these specimens exhibits nonlinear characteristics. As demonstrated in Figure 9, an increase in reinforcement content resulted in the manifestation of a bulging belly phenomenon, concurrently accompanied by the formation of cracks, particularly at a reinforcement content of 25 wt.%. It is evident that the specimen exhibited a compression direction of the crack at an angle of 45°. Numerous fine cracks also appear perpendicular to the main crack. This phenomenon can be attributed to the strengthening effect of high particle content, which enhances the brittleness of the composite material. The presence of a high concentration of reinforcing particles inherently makes the material more brittle. As the material deforms under external forces, cracks initiate. With further deformation, stress concentrations around pores or agglomerates lead to the formation of secondary cracks.

Figure 10 shows the compression stress–strain curves of composites with different filler contents: (a) Ti₃AlC₂ and Ti₃C₂T_x, and (b) Ti₃AlC₂ [5]. As shown in Figure 10a, the compression curves of the Ti₃AlC₂ and Ti₃C₂T_x two-phase reinforcements are depicted; the strength corresponding to a plastic strain of 0.2% is designated as the compressive yield strength of the material. The slope of the initial straight-line segment of the engineering stress–strain curve is identified as the apparent modulus of elasticity of the material. The results are documented in Table 7. A close examination of the data presented in Table 7 reveals a consistent upward trend in the yield strength and apparent modulus of elasticity of the composites, ranging from 0 wt.% to 15 wt.% of reinforcement content. However, this trend undergoes a reversal at 15 wt.%, exhibiting a decrease rather than an increase, up to 25 wt.%. A comprehensive analysis was conducted, incorporating the densities of the composites and the findings from metallographic and SEM image analysis. This analysis led to the conclusion that the densities of the 25 wt.% reinforced composites exhibited a decrease despite an increase in the reinforcements. The investigation revealed that the agglomeration of Ti₃AlC₂ and Ti₃C₂T_x particles was more pronounced, resulting in the formation of additional pores. The material that was loaded with these agglomerates and pores was found to experience stress concentration, which led to premature material failure. The premature yielding of the material, as well as its effect on the strength and apparent modulus of elasticity of the composites, has been demonstrated to exceed the enhancement of these properties by the reinforcing particles. This phenomenon ultimately results in a decrease in the strength and apparent modulus of elasticity of the 25 wt.% reinforcing composites. As shown in Figure 10b, the compressive stress–strain curve of the Ti₃AlC₂ single-phase reinforced material is presented [5]. A comparative analysis reveals that the dual-phase (Ti₃AlC₂–Ti₃C₂T_x) reinforced composite achieves a significant improvement in plasticity without a loss of compressive strength. This synergistic enhancement is attributed to the effective interplay between the two reinforcing phases.

The enhancement of yield strength and apparent modulus of elasticity is attributed to the addition of reinforcement particles, specifically Ti₃AlC₂ and Ti₃C₂T_x. These particles exhibit a synergistic effect in conjunction with pressure and temperature during the hot pressing and sintering process, resulting in enhanced interfacial bonding of the composites. This, in turn, leads to an improvement in the effective transfer of loads from the aluminum matrix to the reinforcing particles when subjected to external forces. Consequently, this process enhances the strength and hardness of the materials. Concurrently, the micron-sized hard particles of the reinforcing body can readily form Orowan rings around them when subjected to loads, thereby instigating an Orowan strengthening mechanism. This hinders some dislocation movements and consequently enhances the deformation resistance. In conclusion, the 15 wt.% reinforcement composites demonstrated optimal strength, exhibiting an increase of 361.60% in yield strength and 597.10% in apparent modulus of elasticity compared to pure aluminum.

3.3.4. Friction Performance Test

As illustrated in Figure 11a, the measured variation curves of the friction coefficients of composites with varying reinforcer contents are presented. Pure aluminum exhibits the highest friction coefficient, which exceeds 0.75, while the lowest friction coefficient, recorded at 15 wt.% reinforcer composites, is approximately 0.33. The friction coefficients of the 5 wt.% and 25 wt.% reinforcer composites are intermediate, while the friction coefficients of the 25 wt.% reinforcing composites are greater than those of the 5 wt.% reinforcing composites.

The variation in the average friction coefficient of composites with different reinforcement contents is shown in Figure 11b. The average friction coefficient decreases and then increases with the increase in reinforcement content (pure aluminum: 0.68→15 wt.%: 0.50→25 wt.%: 0.62). The average coefficients of friction of the composites were all lower than that of pure aluminum, and the friction resistance was improved. The lowest average coefficient of friction was reduced by 26.47% compared to pure aluminum. This phenomenon can be attributed to the self-lubrication process initiated by the rubbing of the reinforcer particles against the steel ball. As this process progresses, the laminar structure of the reinforcer particles undergoes a gradual disintegration, leading to a reduction in the overall friction resistance.

However, an increase in the reinforcement content from 15 wt.% to 25 wt.% resulted in an increase in the average friction coefficient of the aluminum matrix composites from 0.50 to 0.62. Notably, the friction coefficient of the 25 wt.% composites exceeded that of the 5 wt.% composites. The underlying mechanism for this phenomenon is attributed to the agglomeration of reinforcement particles at high contents, particularly at 25 wt.%. This agglomeration introduces both surface and internal voids, which reduce the composite’s overall density. As a result, the interfacial bonding between the aluminum matrix and the Ti₃AlC₂/Ti₃C₂T_x particles is weakened. During sustained friction against hard steel balls, the weakened interface promotes the dislodgement of agglomerated particles. This particle loss compromises the material’s self-lubricating properties, leading to an increased coefficient of friction.

Figure 12 presents the SEM images of the friction wear surface morphology of composites with varying reinforcer contents, and the aforementioned changes in friction coefficient are corroborated by the SEM images. Regardless of the filler content, plow-like scratches aligned with the friction direction formed on the composite surface. This represents the classic abrasive wear morphology left by hard steel balls after high-speed dry sliding wear on the composite surface. As illustrated in Figure 12a, the 5 wt.% reinforcer composite demonstrates a distinct composition, wherein the aluminum matrix predominates due to the minimal reinforcer content. The aluminum matrix undergoes incessant plastic deformation, which cumulatively generates and propagates subsurface cracks. These cracks, in turn, initiate the process of peeling wear. This phenomenon results in a reduction in the coefficient of friction due to the self-lubrication of the reinforcement particles, although the magnitude of this reduction is negligible. As the friction time increases, a small amount of Ti₃AlC₂ and Ti₃C₂T_x particles will be shed. These particles can be seen in Figure 12a in the red arrows, specifically in the circle part. Figure 12b shows the result for 15 wt.%. The composition of reinforcer composites is modified to include elevated levels of hard particles, thereby enhancing the composite’s hardness and reducing its vulnerability to plastic deformation. This is accompanied by a reduction in the friction contact area. Despite these modifications, the material undergoes densification, resulting in a minimal number of holes. This facilitates the intimate integration of the reinforcing body with the matrix, thereby maximizing the self-lubricating properties of the Ti₃AlC₂ and Ti₃C₂T_x particles. Consequently, the composite material exhibits a further decrease in its friction coefficient (Figure 12c). The percentage is expressed as a percentage of the whole. The presence of a high content of reinforcing body particles has been shown to result in a number of undesirable outcomes. These include the intensification of agglomeration, an increase in holes and defects, a weakening of the bonding force between the reinforcement and the matrix, an increase in particle shedding during friction, and a decrease in the self-lubrication of the composite material. These phenomena ultimately led to an increase in the friction coefficient of the 25 wt.% reinforcer composite material rather than a decrease.

3.4. Mechanism Analysis of Performance Evolution

3.4.1. Synergistic Mechanism of MAX–MXene Composite Reinforcement

The mechanical properties of the 15 wt.% composite-reinforced composite material are significantly improved, attributed to the synergistic effect between Ti₃AlC₂ and Ti₃C₂T_x. The abundant functional groups on the MXene surface (–O²⁻, –OH⁻, etc.) enhance the interfacial wettability between the reinforcing phase and the aluminum matrix, promoting the formation of strong metallurgical bonding (as shown in Figure 5b). During sintering, the “accordion structure” of Ti₃C₂T_x partially closes due to Al penetration, creating a mechanical “anchoring effect” that significantly enhances interfacial load transfer efficiency. Concurrently, rigid Ti₃AlC₂ particles hinder dislocation movement via the Orowan dislocation pinning mechanism and form a transition layer with the aluminum matrix through diffusion, further reinforcing overall strength. This dual effect of “MXene interface enhancement + MAX phase strength reinforcement” enables the 15 wt.% composite material to achieve optimal synergistic strength and toughness.

3.4.2. Microstructural Causes of Performance Degradation at High Reinforcing Phase Content

When the reinforcing phase content exceeds 15 wt.%, the performance degradation (e.g., reduced compressive strength) is primarily attributed to two factors:

(1)

Particle agglomeration: Ti₃AlC₂/Ti₃C₂T_x forms a continuous network structure (Figure 4(a3)), which acts as a stress concentration point under external loads, triggering crack initiation and reducing the effective bearing area.

(2)

Pore defects: An increase in strip-like and punctate pores (Figure 3(a3–c3)) weakens the matrix continuity, with density decreasing from 96.06% to 92.38%, leading to premature failure of the material under compression.

3.4.3. Mechanism of Enhancement Phase Content Regulation on Tribological Properties

The non-monotonic variation in the coefficient of friction stems from the balance between self-lubrication and interfacial bonding force:

At 15 wt.%, the material has high density, and the reinforcing phase is firmly bonded to the matrix. During friction, the layered structure of Ti₃AlC₂/Ti₃C₂T_x gradually peels off, forming a lubricating film on the worn surface, reducing the friction coefficient by 26.47% compared to pure aluminum.

At 25 wt.%, severe agglomeration and porosity weaken the interfacial bonding, causing the reinforcing phase to easily detach from the matrix during prolonged friction, disrupting the lubricating film and exacerbating abrasive wear, resulting in a friction coefficient increase to 0.62.

4. Conclusions

By employing vacuum hot-pressing sintering, this study successfully fabricated aluminum matrix composites reinforced with a hybrid of MAX phase (Ti₃AlC₂) and MXene (Ti₃C₂T_x) particles. The main conclusions obtained are as follows:

(1)

The mechanical and tribological properties evolve with reinforcement content, influenced by a balance between strengthening and defect degradation. Microhardness increases nonlinearly (31.9→86.84 HV₀.₂), but the strengthening effect sharply diminishes beyond 15 wt.% (with the increment dropping from 139% to 14%) due to the competing effects of dislocation pinning and pore/agglomeration damage. The 15 wt.% composite achieves the optimal strength-toughness synergy—yield strength (151.87 MPa) and compressive specific modulus (7.80 GPa) are 361.6% and 597.1% higher than pure Al. This enhancement is driven by the Al-penetration anchoring effect of Ti₃C₂T_x and the diffusion strengthening of Ti₃AlC₂. Tribologically, the friction coefficient drops to 0.50 (26.47% lower than pure Al) at 15 wt.% (coincident with 96.06% densification); excess reinforcement content weakens bonding, disrupting lubrication and raising friction.

(2)

These findings confirm that 15 wt.% is the optimal reinforcement content, while highlighting a key synergistic mechanism: MXene improves interfacial wettability and load transfer via anchoring, while Ti₃AlC₂ enhances strength and crack resistance through rigid particles and diffusion layers. This “interface enhancement + structural reinforcement” strategy addresses single-phase shortcomings (e.g., poor MAX phase wettability and MXene interlayer weakness), providing a design principle for high-performance composites.

(3)

The 15 wt.% AMC holds significant promise for aerospace and automotive lightweight, wear-resistant components, with sintering facilitating industrial scalability. Future research will utilize gradient-content samples and advanced characterization techniques to further elucidate the microscale synergistic effects, which will guide the development of tailored composites.