Effects of Graphene Nanoplatelets and Nanosized Al₄C₃ Formation on the Wear Properties of Hot Extruded Al-Based Nanocomposites

Abstract

This study investigates the influence of graphene nanoplatelets (GNPs) and the formation of nanosized Al₄C₃ on the tribological performance of hot extruded aluminum-based nanocomposites. Al/GNP nanocomposites with varying GNP contents (0, 0.1, 0.5, and 1.1 wt.%) were fabricated through powder metallurgy, including ball milling, compaction, and hot extrusion at 500 °C, which was designed to facilitate the formation of nanosized carbides during the extrusion process. The effect of GNPs and nanosized carbides on the tribological properties of the composites was evaluated using dry friction pin-on-disk tests to assess wear resistance and the coefficient of friction (COF). Microstructural analyses using scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed the uniform distribution of GNPs and the formation of nanosized Al₄C₃ in the samples. Incorporating 0.1 wt.% GNPs resulted in the lowest wear mass loss (1.40 mg) while maintaining a stable COF (0.52), attributed to enhanced lubrication and load transfer. Although a higher GNP content (1.1 wt.%) resulted in increased wear due to agglomeration, the nanocomposite still demonstrated superior wear resistance compared to the unreinforced aluminum matrix. These findings underscore the potential of combining nanotechnology with precise processing techniques to enhance the wear and friction properties of aluminum-based composites.

1. Introduction

Aluminum Metal Matrix Composites (AMMCs) are integral to industries such as automotive, aerospace, and electronics, owing to their advantageous properties: high strength-to-weight ratio, low density, and outstanding thermal and electrical conductivity [1]. These characteristics make aluminum a preferred material for applications where weight reduction without compromising strength is crucial [2,3].

To further enhance aluminum’s mechanical and tribological properties, reinforcing it with various materials has emerged as a promising approach. Reinforcements such as Al₂O₃ [4], known for its hardness and wear resistance; SiC [5,6], which offers high thermal stability and strength; and B₄C, recognized for its exceptional hardness and low density, are frequently employed in AMMCs. Additionally, composite coatings on aluminum and its alloys, such as nanocomposite coatings [7,8], ceramic coatings [9,10], and ceramic coatings [11], are widely used to enhance surface hardness and wear resistance. Graphite is another common reinforcement due to its self-lubricating properties and ability to reduce the COF effectively [12,13]. Graphene nanoplatelets (GNPs) are renowned for their exceptional mechanical strength, high surface area, and excellent lubricating properties [14,15]. These characteristics make GNPs an attractive reinforcement material in composite fabrication. Incorporating GNPs into aluminum matrices can significantly enhance load transfer capabilities, improve wear resistance, and reduce the COF [16]. Studies have shown that aluminum composites reinforced with GNPs exhibit superior mechanical and tribological behaviors, making them suitable for high-performance engineering applications.

Fathy et al. demonstrated the superior mechanical and tribological properties of hybrid Al-Al₂O₃/GNP nanocomposites synthesized using the electroless deposition of nickel-coated reinforcements [17]. Their work achieved remarkable improvements in compressive strength, hardness, and wear resistance, attributing these enhancements to the mechanical robustness and self-lubricating properties of GNPs, along with the prevention of detrimental Al₃C₄ phase formation. Similarly, Ghazaly et al. showed that AA2124-GNP composites, fabricated via powder metallurgy, exhibited an optimal tribological performance at 3 wt.% graphene reinforcement, transitioning from a severe to a mild wear regime compared to unreinforced aluminum [18]. Expanding on these findings, Mishra et al. analyzed the wear behavior of Al-GNP composites and reported that 1 wt.% GNPs maximized hardness and wear resistance, with homogeneous microstructures contributing to the composite’s stability [19]. Kumar and Xavior further explored the fatigue and wear properties of Al6061-GNP composites, revealing improved surface hardness and reduced mass loss under varying loads, owing to the dispersion of graphene flakes through ultrasonication and ball milling [20]. Innovative manufacturing methods also play a key role in enhancing AMMC properties. Bao et al. utilized ultrasonic-assisted casting followed by T6 heat treatment to fabricate ADC12-GNP composites, achieving a 63.3% reduction in wear rate and significant improvements in surface roughness and coefficient of friction due to self-lubrication and nanoscale precipitate interactions [21]. Similarly, Wu et al. utilized selective laser melting (SLM) to produce AlSi10Mg-GNP composites, reporting a 42.9% reduction in wear rate and substantial hardness improvements, demonstrating the versatility of GNP reinforcements in AMMCs [22]. Additionally, Cygan et al. incorporated multilayered graphene and graphene oxide into alumina matrix composites using spark plasma sintering (SPS). Their results highlighted a 53.3% reduction in wear rate for 1 wt.% GNP composites, emphasizing the synergy between alumina and graphene-family materials for tribological performance [23].

Despite these advancements, incorporating GNPs into aluminum matrices introduces challenges. One critical issue is the formation of Al₄C₃ carbides at the interface between GNPs and aluminum during composite production. Al₄C₃ is a brittle and thermodynamically unstable compound that weakens interfacial bonding and causes localized stress concentrations. Studies have confirmed interfacial reactions in C/Al composites, leading to Al₄C₃ phase formation [24,25]. This interfacial reaction has recently drawn significant attention as a key factor influencing both interfacial properties and mechanical performance [26,27]. Interestingly, Al₄C₃ has been shown to enhance load transfer efficiency due to its anchoring effect and the transformation of interfacial bonding from mechanical to strong chemical bonding [28,29,30]. Understanding the role of Al₄C₃ in shaping the tribological properties of aluminum–GNP composites is essential. Research demonstrates that adding 3 wt.% graphene to aluminum composites reduces the wear rate by 34% and lowers the COF by 25%. However, higher graphene content, such as 5 wt.%, can lead to agglomeration, resulting in increased wear rates [31].

A notable research gap is present in the investigation of AMMCs reinforced with GNPs. While existing studies primarily focus on broader tribological characteristics or utilize different fabrication techniques, the combined influence of controlled GNP content (0, 0.1, 0.5, and 1.1 wt.%) and nanosized Al₄C₃ formation, achieved through a specific powder metallurgy process involving ball milling, compaction, and hot extrusion at 500 °C, has not been investigated.

The originality of this study lies in its investigation of the combined effects of varying GNP content and the in situ formation of nanosized Al₄C₃ carbides on the tribological performance of aluminum-based nanocomposites. Previous research, including the works of Ghazaly et al. [18] and Mishra et al. [19], has predominantly focused on broader tribological properties or specific GNP concentrations without addressing the simultaneous effects of nanosized Al₄C₃ formation during processing. Unlike earlier studies that utilized alternative fabrication techniques such as selective laser melting (Wu et al. [22]) or ultrasonic-assisted casting (Bao et al. [21]), this research employs powder metallurgy coupled with hot extrusion at 500 °C to facilitate the controlled formation of nanosized Al₄C₃ carbides. This method allows for precise evaluation of the interplay between reinforcement distribution, carbide morphology, and tribological performance. This study investigates the influence of GNPs and the formation of nanosized Al₄C₃ on the tribological performance of hot-extruded Al-based nanocomposites. Al/GNP nanocomposites with varying GNP content (0, 0.1, 0.5, and 1.1 wt.%) were fabricated through powder metallurgy techniques, including ball milling, compaction, and hot extrusion at 500 °C, which facilitated the in situ formation of nanosized carbides during the extrusion process. Microstructural analyses conducted using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) confirmed the uniform distribution of GNPs within the Al matrix and the formation of nanosized Al₄C₃. The wear characteristics were compared to those of a prior study [32], which investigated the effects of GNPs and predominantly micron-sized Al₄C₃ on the tribological performance of aluminum-based nanocomposites. These composites were fabricated through extrusion at 400 °C, followed by annealing at 610 °C for 3 h to promote Al₄C₃ formation. This comparison highlights the impact of processing conditions and nanoscale carbide formation on the COF and wear rate of the composites, providing new insights into optimizing the tribological properties of Al/GNP nanocomposites.

2. Materials and Methods

2.1. Materials and Fabrication

The nanocomposite materials consisted of Al powder with a purity of 99.5% and an average particle size of 37 µm (Figure 1a), and GNPs with a thickness of 6–8 nm and purity of 99.5% (Figure 1b). The Al powder and graphene nanoplatelets were mixed using a planetary agate ball mill equipped with 7 balls, each weighing 11.6 g. The milling parameters were as follows: speed of 700 revolutions per minute, milling time of 30 min, at room temperature, in an argon atmosphere. Four different compositions were prepared with GNP content at 0, 0.1, 0.5, and 1.1 wt.%, each mixture weighing 300 g. This study builds upon previous research [33,34] that investigated aluminum–graphene composites fabricated via powder metallurgy. Prior findings [34] demonstrated that Al₄C₃–graphene interfaces are coherent, providing a precondition for carbide nucleation on GNPs when the crystal orientation relationship between them is Al₄C₃(008)//C(002). The coherent boundary between the two phases also serves as a prerequisite for the strength of their connection. Li et al. [35] synthesized in situ nano-Al₄C₃ in CNT/Al composites, achieving high strength and exceptional ductility via friction stir processing. The improvement in ductility and strength is attributed to the strong Al₄C₃–Al interface and nanoscale size. However, the focus of this work was to develop nanocomposites featuring nanosized carbides as a result of the in situ reactions during the extrusion process. To achieve this, the compacts were heated to 370 °C for 20 min, followed by preheating the mold to 500 ± 10 °C, where the compacts remained for 4.5 min before being extruded under a pressure of 457 MPa for 60 s per piece. The final products were aluminum-based nanocomposite bars reinforced with GNPs, 12 mm in diameter, cooled in air. The formation of nanosized carbides during extrusion at 500 °C, resulting from the reaction between graphene and aluminum, was successfully achieved, eliminating the need for additional annealing of the extrudates at higher temperatures. Figure 2 illustrates the production process scheme for the Al/GNP nanocomposites.

2.2. Characterization Methods

The extruded rods were characterized using SEM with EDS and subsequently subjected to dry sliding wear testing. The samples were wet ground in longitudinal sections on grinding paper with grit sizes ranging from 320 to 3000, followed by a two-step polishing process using an OPS suspension from Struers. The microstructure was examined by etching with a 0.5% HF water solution, and observations were made using scanning electron microscopy. SEM and EDS analyses were conducted using a HIROX SH-5500P SEM (Hirox Japan Co., Ltd., Tokyo, Japan) equipped with a ‘QUANTAX 100 Advanced’ EDS system (Bruker, Billerica, MA, USA). The operating conditions included an accelerating voltage of 20 kV, a working distance of 3–7 mm, and a secondary electron detector.

The Vickers microhardness of the composite cross-sections was quantified utilizing a Polyvar Met optical microscope (Reichert Jung, Vienna, Austria) integrated with a semi-automated micro-Vickers hardness measurement system (Micro-Duromat 5000, Reichert Jung, Vienna, Austria). Measurements were obtained using a calibrated Vickers indenter, characterized by a diamond pyramid with an apex angle of 136°. A standardized load of 0.05 kgf was applied for a dwell time of 10 s, followed by an additional 10 s of sustained pressure to ensure uniform indentation depth and minimize viscoelastic effects. Tribological properties were evaluated under dry sliding conditions using a Ducom TR-20 rotary (pin/ball-on-disk) tribometer (Ducom Instruments Pvt. Ltd., Bangalore, India). Test specimens were machined from the extruded rods on a lathe to achieve a height of 20 mm and a diameter of 12 mm, minimizing structural flaws that could affect wear behavior. The test parameters for the dry sliding wear tests were selected based on a prior study that evaluated annealed Al/GNP nanocomposites [32]. Specifically, a pin-on-disk system was utilized with a load of 30 N, a linear velocity of 0.9 m∙s⁻¹, and a sliding distance of 540 m, and a counter disk made of EN-31 steel with a hardness of 62 HRC.

3. Results and Discussion

3.1. Microstructural Characterization

The microstructural characterization of non-annealed Al/GNP nanocomposites, as illustrated in Figure 3 and analyzed using EDS data from

Table 1. EDS analysis results (in mass norm. %) of Al/GNPs (0–1.1 wt.%) prior to wear testing, focusing on regions 1–2 as highlighted in Figure 3.

Specimen	Analysis №	Al	C
Al (Figure 3a)	1	100.00	–
Al + 0.1 wt.% GNPs (Figure 3c)	1	13.55	86.45
	2	49.14	50.86
Al + 0.5 wt.% GNPs (Figure 3e)	1	31.59	68.41
	2	90.44	9.56
Al + 1.1 wt.% GNPs (Figure 3g)	1	18.20	81.80
	2	67.06	32.94

, provides valuable insights into the influence of GNP content on wear performance and material properties. Each SEM image highlights specific regions of interest, particularly the distribution of GNPs, the formation of Al₄C₃ carbides, and the changes in surface morphology before and after wear tests. The corresponding EDS results further elucidate the elemental composition within these zones, revealing the complex interplay between the aluminum matrix and the reinforcing GNPs.

For pure Al (Figure 3a,b), the SEM image before the wear test (Figure 3a) shows a smooth and homogeneous surface typical of unreinforced Al, with no visible secondary phases or particles. The EDS analysis confirms the composition is 100% aluminum, as expected. However, after the wear test (Figure 3b), the surface exhibits severe plastic deformation with pronounced grooves and roughness, indicating substantial material loss. This behavior aligns with the wear test results, where pure Al showed the highest mass wear (3.95 mg) and a relatively high COF of 0.58.

With the addition of 0.1 wt.% GNPs (Figure 3c,d), the microstructure undergoes noticeable changes. Before the wear test (Figure 3c), the SEM image reveals the presence of Al₄C₃ carbides and dispersed GNPs. EDS analysis for this region shows two distinct zones: Zone 1 contains 86.45% carbon and 13.55% Al, indicating a GNP-rich area, while Zone 2 shows a more balanced composition of 50.86% carbon and 49.14% Al, indicative of Al₄C₃ carbide formation due to aluminum–carbon interactions. After the wear test (Figure 3d), the surface displays finer grooves compared to pure aluminum, indicating improved wear resistance. However, the COF increased to 0.62 compared to 0.58 for pure Al. This increase can be attributed primarily to the agglomeration of Al₄C₃ carbides, which likely altered the surface characteristics and influenced the wear behavior. These carbide clusters would have increased interfacial shearing during sliding, contributing to a higher COF. Despite this, the addition of 0.1 wt.% GNPs significantly reduced mass wear to 1.40 mg—a 64.6% reduction compared to pure aluminum. The relatively fine dispersion of GNPs and the formation of a lubricating tribolayer during wear mitigated wear damage but could not fully offset the friction-increasing effect of the carbide.

At 0.5 wt.% GNPs (Figure 3e,f), the microstructure before the wear test (Figure 3e) shows better GNP dispersion but still features some agglomeration and Al₄C₃ carbides. The EDS analysis highlights Zone 1 with 68.41% carbon and 31.59% aluminum, indicating the presence of GNP-rich areas, while Zone 2 is predominantly aluminum (90.44%) with minimal carbon (9.56%), revealing the presence of Al₄C₃ carbides and dispersed GNPs. Notably, the carbides appear visibly larger in the SEM image compared to the 0.1 wt.% GNP sample, suggesting more significant clustering of Al₄C₃ particles. After the wear test (Figure 3f), the wear surface displays more defined grooves than the 0.1 wt.% GNP sample, but the wear resistance remains superior to pure Al. The wear results show a mass wear of 1.85 mg, which is significantly higher than the 1.40 mg observed for the 0.1 wt.% sample. This increase can be attributed to the larger Al₄C₃ carbides, which may contribute to higher wear rates under dry sliding wear conditions. The diminishing returns with increased GNP content highlight the adverse effects of carbide agglomeration on the material’s wear resistance. Nevertheless, the COF remains relatively low at 0.61, demonstrating that the reinforcing effects of GNPs are still beneficial, albeit less pronounced than at lower GNP concentrations. While the GNPs contribute to reducing friction, the larger carbides overshadow these effects, resulting in the higher wear observed at 0.5 wt.% GNPs.

At the highest GNP concentration (1.1 wt.%, Figure 3g,h), the microstructure reveals the significant agglomeration of GNPs and Al₄C₃ carbides before the wear test (Figure 3g). EDS analysis confirms the presence of highly carbon-rich regions (Zone 1: 81.80% carbon, 18.20% aluminum) alongside Al₄C₃-dominant zones (Zone 2: 67.06% aluminum, 32.94% carbon). The Al₄C₃ particles are visibly larger compared to those in the 0.1 and 0.5 wt.% nanocomposites and appear to form distinct agglomerates. These Al₄C₃ agglomerates act as stress concentrators, significantly reducing the effectiveness of the reinforcement. After the wear test (Figure 3h), the surface shows increased roughness with clear evidence of particle pull-out, leading to slightly higher mass wear (2.70 mg) and a COF of 0.56. Despite the slightly lower COF compared to the 0.1 wt.% and 0.5 wt.% composites, the COF value remains within the deviation range observed for the other nanocomposites, indicating no significant improvement in friction behavior at this GNP concentration. The contact surface is visibly the worst among the nanocomposites but still better than that of the unreinforced aluminum material. This performance indicates that excessive GNP content promotes the formation of larger Al₄C₃ carbides and their agglomeration, which compromises both wear resistance and overall mechanical properties.

The microstructural characterization of non-annealed Al/GNP nanocomposites highlights the influence of GNP content on wear performance and material properties. Pure Al shows a smooth surface before wear, with significant plastic deformation and roughness after wear, resulting in the highest mass wear (3.95 mg) and a COF of 0.58. The addition of 0.1 wt.% GNPs introduces Al₄C₃ carbides and GNP dispersions, reducing the mass wear to 1.40 mg—a 64.6% reduction—despite a slight COF increase to 0.62 due to carbide agglomeration. At 0.5 wt.% GNPs, better GNP dispersion is observed, but larger Al₄C₃ carbides lead to increased mass wear (1.85 mg). At 1.1 wt.% GNPs, excessive GNP content results in significant Al₄C₃ agglomeration, acting as stress concentrators, leading to the highest roughness and mass wear among the nanocomposites (2.70 mg) and no significant improvement in COF (0.56). While GNP addition improves wear resistance compared to pure Al, excessive concentrations compromise mechanical properties due to larger carbide formations and their agglomeration. These findings align with studies such as that conducted by Wu et al. [22], who identified an optimal GNP content of around 0.3 wt.% for Al–GNP composites, beyond which agglomeration and reduced performance were observed. Similarly, Ghazaly et al. [31] reported that incorporating a high fraction (5 wt.%) of graphene caused significant agglomeration and higher wear rates. Zhang et al. [36] further demonstrated that 1 vol.% GNSs reinforcement significantly refined Al grains and improved the reinforcement distribution in the matrix, highlighting the importance of balancing reinforcement content to optimize mechanical and tribological properties. While GNP addition improves wear resistance compared to pure Al, excessive concentrations compromise mechanical properties due to larger carbide formations and their agglomeration.

3.2. Wear Behavior and Microhardness

The tribological performance of the non-annealed Al–GNP nanocomposites was evaluated under dry sliding wear conditions using a pin-on-disk tribometer. The test specimens were machined with a height of 20 mm and a diameter of 12 mm to minimize structural flaws. Testing was conducted at room temperature with a normal load of 30 N, a sliding speed of 0.9 m∙s⁻¹, and a total sliding distance of 540 m. An EN-31 steel disk with a hardness of 62 HRC served as the counter surface. The wear performance was assessed by measuring mass loss using a high-precision analytical balance, and the COF was recorded during testing. The results were compared to previously reported annealed Al/GNP samples [32], comparing the effects of annealing on the COF and mass wear across varying GNP concentrations.

Figure 4 presents the COF vs. sliding distance test results for Al with GNPs (0–1.1 wt.%) under dry sliding friction conditions at room temperature, conducted at a 30 N load, 0.9 m∙s⁻¹ sliding speed, and a 540 m sliding distance, including both annealed and non-annealed materials.

Table 2. Descriptive statistics of the COF for annealed and non-annealed Al–GNP nanocomposites at varying GNP contents (0–1.1 wt.%).

Material_Type	GNPs, wt._%	Mean	Std	Min	Max
Annealed	0.0	0.46	0.09	0.39	0.52
Non-Annealed	0.0	0.58	0.01	0.57	0.59
Annealed	0.1	0.54	0.01	0.53	0.55
Non-Annealed	0.1	0.62	0.05	0.59	0.66
Annealed	0.5	0.56	0.03	0.54	0.59
Non-Annealed	0.5	0.61	0.07	0.56	0.65
Annealed	1.1	0.51	0.01	0.50	0.52
Non-Annealed	1.1	0.56	0.01	0.54	0.57

presents the descriptive statistics of the COF values for annealed and non-annealed Al–GNP nanocomposites. The annealing process consistently reduced the COF across all GNP concentrations, with the most pronounced effect observed in pure aluminum (0 wt.% GNPs). Non-annealed samples exhibited a COF of 0.58, which was 26.1% higher than the annealed samples (0.46). This indicates that annealing enhances the surface interaction by refining the matrix structure, potentially leading to improved load distribution during dry sliding wear tests. At 0.1 wt.% GNP, the COF for non-annealed samples was 0.62, 14.8% higher than the annealed samples (0.54). This difference diminished further with increasing GNP content; at 0.5 wt.% GNPs, the non-annealed samples showed a COF of 0.61, only 8.9% higher than annealed samples (0.56). At the highest GNP concentration (1.1 wt.%), the COF difference narrowed further to 9.8% (0.56 for non-annealed vs. 0.51 for annealed samples). These results suggest that annealing enhances frictional performance, particularly in pure aluminum and composites with low GNP content, possibly by promoting better GNP–matrix bonding and a more uniform microstructure.

Figure 5 shows the mass wear results for Al with GNPs (0–1.1 wt.%) under dry sliding friction conditions at room temperature, conducted at a 30 N load, 0.9 m∙s⁻¹ sliding speed, and a 540 m sliding distance, including both annealed and non-annealed materials.

Table 3. Descriptive statistics of the mass wear (mg) for annealed and non-annealed Al–GNP nanocomposites at varying GNP contents (0–1.1 wt.%).

Material_Type	GNPs, wt._%	Mean	Std	Min	Max
Annealed	0.0	4.60	0.42	4.30	4.90
Non-Annealed	0.0	3.95	1.06	3.20	4.70
Annealed	0.1	2.25	0.64	1.80	2.70
Non-Annealed	0.1	1.40	0.28	1.20	1.60
Annealed	0.5	1.75	0.92	1.10	2.40
Non-Annealed	0.5	1.85	0.07	1.80	1.90
Annealed	1.1	3.00	0.28	2.80	3.20
Non-Annealed	1.1	2.70	0.14	2.60	2.80

presents the descriptive statistics for the mass wear (mg) of annealed and non-annealed Al–GNP nanocomposites. Mass wear trends revealed a more complex relationship between annealing, GNP content, and tribological performance. For pure aluminum (0 wt.% GNPs), annealed samples exhibited 16.5% higher mass wear than non-annealed samples (4.60 mg vs. 3.95 mg), likely due to microstructural changes such as grain refinement induced by annealing, which may reduce hardness in the absence of reinforcements. At 0.1 wt.% GNPs, this trend became even more pronounced, with annealed samples showing 60.7% higher mass wear (2.25 mg vs. 1.40 mg). The higher wear in annealed samples could be attributed to the thermal treatment altering the matrix-reinforcement interaction, reducing the effectiveness of GNPs in mitigating wear. Interestingly, the trend reversed at 0.5 wt.% GNPs, where annealed samples exhibited 5.7% lower mass wear compared to non-annealed samples (1.75 mg vs. 1.85 mg). This reversal suggests that at intermediate GNP concentrations, annealing enhances the stress transfer capabilities of the GNPs and improves their dispersion within the matrix, resulting in better wear resistance. At 1.1 wt.% GNP, the trend shifted again, with annealed samples showing 11.1% higher mass wear than non-annealed samples (3.00 mg vs. 2.70 mg). The excessive GNP content likely led to agglomeration and the formation of larger Al₄C₃ particles in both cases, but the annealing process may have exacerbated this effect. These results are consistent with prior research [32] on the tribological performance of GNP-reinforced composites.

The considerations of the results highlight the critical role of GNP content in influencing the wear resistance and COF of Al–GNP nanocomposites. Optimal performance is achieved through a balance between adequate GNP dispersion and the minimization of agglomeration and Al₄C₃ carbide clustering, which can act as stress concentrators and reduce reinforcement efficiency. The interplay between microstructural features and experimental conditions underscores the importance of precise control over GNP content and processing techniques to optimize tribological properties. These findings emphasize the need to tailor reinforcement levels to avoid diminishing returns at higher concentrations, ensuring practical applicability in engineering contexts.

The microhardness of the Al–GNP nanocomposites obtained through hot extrusion, as shown in Figure 6, reveals significant variations with changing GNP content. The pure aluminum sample exhibits the lowest average microhardness (37 HV), indicative of its unreinforced structure. The addition of 0.1 wt.% GNP results in the highest microhardness (51 HV), which can be attributed to the uniform dispersion of GNPs and the formation of nanosized Al₄C₃ carbides. At 0.5 wt.% GNPs, the microhardness decreases slightly to 44 HV, likely due to the partial agglomeration of GNPs. This trend continues at 1.1 wt.% GNPs, where excessive GNP content further increases agglomeration and clustering, resulting in a microhardness of 40 HV. These findings suggest that while the addition of GNPs improves the hardness of the composite, maintaining an optimal GNP content is crucial to avoid diminishing returns caused by agglomeration and clustering.

4. Conclusions

This study investigated the tribological properties under dry sliding wear conditions of hot-extruded Al/GNP nanocomposites fabricated at 500 °C using powder metallurgy techniques and evaluated the effects of GNP content and in situ nanosized Al₄C₃ formation during extrusion. The non-annealed composites’ wear results were compared with those obtained under the same testing conditions for annealed Al/GNP nanocomposites. The results reveal several key findings:

Microstructural Insights:

• The incorporation of GNPs into the aluminum matrix and the in situ formation of nanosized Al₄C₃ carbides were successfully achieved through extrusion at 500 °C.
• SEM and EDS analyses confirmed the uniform distribution of GNPs and the formation of the Al₄C₃ particles. However, with increasing GNP concentrations, the Al₄C₃ particles became visibly larger, and at higher concentrations (1.1 wt.%), agglomeration and the formation of larger Al₄C₃ clusters were observed, which acted as stress concentrators.
• Microhardness measurements showed that 0.1 wt.% GNPs led to the highest hardness (51 HV) due to uniform dispersion and nanosized Al₄C₃ formation, while higher GNP contents (0.5 and 1.1 wt.%) resulted in decreased hardness due to agglomeration and clustering effects.

Tribological Performance:

• Non-annealed nanocomposites exhibited improved wear resistance compared to pure aluminum. The addition of 0.1 wt.% GNPs resulted in a significant reduction in mass wear (64.6%) compared to pure Al, demonstrating the reinforcing effects of GNPs and the formation of a lubricating tribolayer.
• However, excessive GNP content (1.1 wt.%) resulted in increased wear and compromised performance due to agglomeration and carbide clustering.
• Annealing consistently reduced the COF across all GNP concentrations, with the most pronounced effect observed in pure aluminum. For instance, the COF of annealed pure aluminum (0.46) was 26.1% lower than that of non-annealed pure aluminum (0.58). Among the nanocomposites, the best COF results were observed at 1.1 wt.% GNPs for both annealed (0.51) and non-annealed (0.56) samples.
• Mass wear trends were more complex. Non-annealed composites outperformed annealed counterparts at lower GNP contents, particularly at 0.1 wt.% GNPs, where the non-annealed sample exhibited significantly lower mass wear of 1.40 mg compared to 2.25 mg for the annealed sample. This highlights the superior wear resistance of the non-annealed composite at this concentration. At 0.5 wt.% GNPs, the trend reversed slightly, with the annealed sample showing marginally better wear resistance (1.75 mg) compared to the non-annealed sample (1.85 mg).