Tribological Behavior of SPS-Prepared Al-Matrix–ZrO₂-Nanofiber Composites with Graphene Nanoplatelets Solid-Lubricating Surface Films

Abstract

In this study, the tribological compatibility of ZrO₂-nanofiber-strengthened Al-matrix composites with graphene nanoplatelets (GNPs)-derived surface film acting as a solid lubricant was investigated. The substrate materials prepared by Spark Plasma Sintering (SPS) included the pure aluminum monolith (reference material) and two Al–ZrO₂ nanocomposites with either 1 or 3 wt.% of ZrO₂ nanofibers. The GNPs-derived solid lubricant films were dry mechanically burnished into the metallographically polished surfaces. The durability of these burnished films was evaluated by performing tribological friction experiments using a ball-on-disk method. Thus, a friction load capacity of GNP-derived tribofilms on the substrate materials and its effect on the coefficient of friction (COF) were evaluated. The results showed that the films burnished on the surfaces of Al–ZrO₂ nanofiber composites were more resistant to much higher loads than films burnished on monolithic aluminum. The obtained findings indicated that ZrO₂ nanofiber protrusions likely stabilize a GNP-derived carbon tribolayer on the polished composite surfaces. As a result, the reinforcement of aluminum with ceramic nanofibers led also to a significant reduction in COF. The highest improvement of tribological performance was observed for the Al–ZrO₂ nanofiber composite with 1 wt.% ZrO₂ nanofibers. The increase of ZrO₂ nanofibers up to 3 wt.% was no more efficient due to nanofiber clustering leading to lower stability of the carbon friction film. Our objective was to isolate the role of the aluminum substrate, specifically, ZrO₂ nanofiber protrusions in the formation and retention of a GNP-derived carbon tribofilm under room-temperature, ambient-air dry sliding.

1. Introduction

Aluminum (Al) alloys are indispensable in various industrial sectors, including automotive, aerospace, and mechanical engineering, due to their attractive properties such as low density, high strength-to-weight ratio, and good ductility [1]. However, their application in high-wear environments is often limited by their relatively low hardness and inadequate wear resistance [2]. Enhancing their tribological properties is therefore crucial for broadening their application range. An effective strategy to achieve this is the development of metal matrix composites (MMCs) reinforced with ceramic particles [3,4]. Zirconia (ZrO₂) has emerged as a promising ceramic reinforcement material owing to its high hardness, excellent fracture toughness, and chemical stability [5]. Zirconia (ZrO₂) has emerged as a promising ceramic reinforcement material owing to its high hardness, excellent fracture toughness, and chemical stability [5]. Beyond its role as a reinforcement in metal matrices, zirconia has recently attracted significant attention in advanced functional ceramics and extreme environments. Recent work demonstrates that irradiation temperature governs defect evolution and radiation-induced polymorphic transformations (t-ZrO₂ → c-ZrO₂), highlighting zirconia’s phase stability and structural adaptability under harsh conditions [6]. Such properties, combined with its chemical inertness and mechanical robustness, make ZrO₂ an attractive candidate for tribological reinforcement in lightweight metallic matrices. In particular, short ZrO₂ nanofibers can significantly enhance mechanical properties, including hardness and wear resistance, through mechanisms such as dispersion strengthening and crack bridging [7]. Recent studies confirm that zirconia-based reinforcements not only improve hardness but also contribute to enhanced fracture toughness and thermal stability, making them highly suitable for demanding tribological conditions [8,9]. Spark Plasma Sintering (SPS) is particularly advantageous for the fabrication of such composites. This technique enables rapid densification of powder materials at relatively lower temperatures and shorter sintering times, minimizing grain growth and preserving a fine microstructure, which results in optimal mechanical properties of the composites [10,11]. SPS has been successfully applied for manufacturing various aluminum-based composites with ceramic and carbon reinforcements, achieving superior strength and wear resistance compared to conventional sintering techniques [12,13,14]. In addition to the development of the composite itself, surface modification plays a critical role in minimizing friction and wear. Over the past decade, graphene nanoplatelets (GNPs) have gained recognition as exceptional solid lubricants. Their outstanding strength, low shear resistance, and high thermal conductivity make them ideal for tribological applications [15]. Recent reviews consolidate the mechanistic picture of graphene-enabled (super)lubricity—ranging from incommensurate sliding between basal planes to the evolution of transfer films during sliding—providing guidance on how to stabilize low-shear tribofilms across length scales [16,17]. Synergy between a hard ceramic skeleton and a lamellar carbon film is increasingly evidenced in aluminum-based systems. Hybrid Al–graphene–ceramic composites (e.g., Al–GNS–SiC produced by friction-stir processing) show concurrent strengthening and friction reduction, consistent with a load-bearing/anchoring role of the ceramic phase that stabilizes graphene-rich tribofilms [18]. Al–GNP composites maintain markedly low coefficients of friction even at elevated temperatures when a robust transfer film forms on the counterface [19]. Meta-analytical and data-driven studies further indicate that graphene content, contact load, and matrix hardness dominate friction/wear responses in Al–GNP MMCs, with relatively low graphene additions often sufficient for substantial tribological gains [20]. Likewise, in oxide-based coatings, the inclusion of graphite or ZrO₂ promotes the in-situ formation of protective tribofilms and reduces wear, underscoring the generality of this load-bearing + solid-lubricant paradigm [21]. Furthermore, the lamellar structure of graphene enables easy shear between layers, leading to a significant reduction in the coefficient of friction (COF) [22]. The application of a thin GNP-derived tribofilm on the surface of tribological components has been shown to significantly enhance their tribological performance [23]. The combination of mechanical polishing followed by GNPs deposition on the sample surface prior to tribological testing can form an effective lubricating film with low friction, further reducing the COF and wear. Research also indicates that GNPs can provide oxidation resistance and enhance thermal/electrical conductivity, which is beneficial for tribological systems operating under severe conditions [24,25]. Recent investigations highlight the synergistic role of graphene with ceramic reinforcements, demonstrating improved load-bearing capacity and stability of lubricating films [16,26].

The aim of this study was to investigate the effect of incorporating short ZrO₂ nanofibers into an aluminum matrix via SPS processing on the resulting microstructural, mechanical, and tribological properties of produced MMCs. Particular attention was paid to how the presence of nanofibers influences hardness, wear resistance, and frictional coefficient. Moreover, the impact of applying a thin GNP-derived tribofilm onto the polished sample surface on resulting tribological behavior of the composites was examined. It was hypothesized that the synergistic effect of ZrO₂ nanofiber reinforcement and GNPs surface modification should lead to a significant improvement in the overall tribological performance of Al matrix composites, paving the way for their application in demanding engineering environments. We selected 1 and 3 wt.% of ZrO₂ nanofibers, with aim where SPS densification is still practical for Al while avoiding excessive porosity/agglomeration, and to test whether the formation and retention of a carbon tribofilm saturates at low volume fractions. Accordingly, this study was designed to isolate the substrate’s contribution to tribofilm stability. We tested the hypothesis that ZrO₂ nanofiber protrusions act as pinning/load-sharing features that promote the formation and retention of a GNP-derived carbon tribofilm on aluminum during dry sliding at room temperature in ambient air, a regime relevant to lightweight guide rails, small linear stages, bushings, and actuators

2. Materials and Methods

Aluminum powder (~325 mesh powder with purity 99.5%, Thermo Scientific Chemicals, Kandel, Germany) with the addition of either 1 or 3 wt.% of zirconium dioxide milled nanofibers (D × L, 200–800 nm ± 100 nm × 2–10 μm, Sigma-Aldrich, St. Louis, MO, USA) have been used as the base materials. The above powders were homogenized by mixing in 3D Turbula mixer with a rotation of 55 rpm for 2 h in ethanol with 15 steel balls in 125 mL polymer jar. Finally, the mixtures were sintered using SPS (HP D 10SD, FCT Systeme, Frankenblick, Germany) at 500 °C for 7 min. with uniaxial pressure of 50.9 MPa (16 kN) under vacuum. Heating and cooling rate was 100 °C/min. The temperature was measured with an optical pyrometer focused through a 10 mm hole on the bottom of the graphite punch and automatically regulated from 400 °C to the final sintering temperature. Applied pulse on/off ratio was 15:3 ms. The sintered compacts were disc-shaped, measuring 20 mm in diameter and approximately 4 mm in height. The sample surfaces were then ground and mechanically polished. The densities of the sintered samples were measured by the Archimedes hydrostatic weighing method. The hardness was determined by the Vickers method. The measurements were performed using the hardness tester WILSON-HARDNESS Tukon 1102 (Illinois Tool Works Test & Measurement, Shanghai, China) on metalographically ground and polished samples. The hardness testing conditions per measurement were 9.8 N loading force with 10 s loading time and 0.98 N loading force with 15 s loading time corresponding to the HV1 and HV0.1 unit scale, respectively. Prior to burnishing, the metallographically polished surfaces were etched using Keller’s etchant (95 mL H₂O, 2.5 mL HNO₃, 1.5 mL HCl, 1.0 mL HF) for ~15 s, followed by immediate rinsing with deionized water and ethanol and air-drying. Metallographically etched samples were then subjected to dry mechanical burnishing with GNPs powder (GNPs aggregates with a specific surface area 500 m²/g, sub-micron particles powder, Alfa Aesar) using a polishing microfiber cloth. Samples were first degreased in acetone and then rinsed in isopropanol. About 1 g of GNPs powder was evenly spread on a polishing microfiber cloth and the samples were rubbed for 10 min. on this cloth in a circular motion by applying a slight amount of pressure (~10 N), i.e., a procedure similar to the metallographic polishing technique. GNPs powder was replenished after 5 min of rubbing. Excess non-adherent GNPs powder was removed from the surface using an air spray. The local microstructures were analyzed using a Scanning Electron Microscope (SEM EVO MA15, Carl Zeiss, Jena, Germany) in backscattered electron (BSE) mode. Elemental chemical mapping of studied materials was performed by using energy dispersive spectrometer (EDS OXFORD INSTRUMENTS X-Max⁵⁰, High Wycombe, UK). Tribological tests of burnished samples were conducted on an automatic tribometer (HTT CSM Instruments, Peseux, Switzerland) under rotational dry sliding conditions, employing a ball-on-disk configuration at room temperature and atmospheric pressure. A 6 mm diameter ZrO₂ ceramic ball was used as the friction counterpart. Tribological tests were performed at a sliding velocity of 0.1 m/s and four different normal loads (1, 2, 3 and 4 N) on a total sliding distance of 50 m. Bare Al versus ZrO₂ nanofiber-reinforced samples without GNP-derived tribofilm are expected to show uniformly higher and unstable COF and severe wear. After tribological tests the wear tracks were not etched, loose debris was removed by gentle compressed air and directly examined by SEM/BSE.

3. Results and Discussion

3.1. Starting Powders

Figure 1 shows the micrographs of the starting powder (Al and ZrO₂ nanofibers) and the mixed powder mixture of Al-1 and 3 wt.% ZrO₂ nanofibers. It is seen that the powders are uniformly mixed and many small particles of ZrO₂ nanofibers are dispersed on the surface of Al.

3.2. Sintering Regime

Figure 2a shows a typical sintering profiles for the Al–1 wt.% ZrO₂ nanofiber composite. The temperature cycle consists of a preheating stage at ~400 °C (0–6 min), a ramp to 500 °C with a small overshoot to ~520 °C while the force increases to ~16 kN, an isothermal hold at 500 °C and ~16 kN (dwell of 7 min), followed by force release and cooling from 500 °C to 450 °C proceeded at 50 °C min⁻¹ then between 450 °C and 400 °C, controlled cooling of 100 °C/min as recorded by the SPS pyrometer. Figure 2b shows punch displacement versus time, highlighting densification kinetics. A rapid densification stage starts at ~6.2 min and yields ~5–6 mm displacement by ~7.0 min, followed by a slower creep-controlled stage up to a final displacement of ~7 mm by ~11.5 min. After the end of the final stage of densification, no noticeable displacement of the punches was observed although force and temperature were kept for several additional minutes.

3.3. Density

The apparent and relative density values were determined using the Archimedes’ method for aluminum-based composite materials with the addition of 0, 1, and 3 wt.% of ZrO₂ nanofibers prepared by the SPS method. The pure aluminum sample reached a relative density of 96.37% after sintering. With an increasing content of ZrO₂ nanofibers, a slight increase in apparent density from 2.602 g/cm³ to 2.633 g/cm³ was observed, which is a consequence of the higher density of zirconia compared to aluminum. However, the addition of the ceramic phase led to a decrease in relative density from 96.01% for the sample with 1 wt.% of ZrO₂ nanofibers down to 94.39% for the sample with 3 wt.% of ZrO₂ nanofibers. This observation indicated an increase in microstructural porosity of the composites with increasing amounts of ZrO₂ nanofibers, likely due to the agglomeration of the nanofibers, thus preventing complete densification of the composite during the sintering process [27]. Moderate porosity can aid in tribofilm/lubricant retention and damping of local stresses, which is desirable in small, dry-friction mechanisms.

3.4. Microstructures

Figure 3 shows the microstructures of pure aluminum and composite materials Al with of 1 wt.% ZrO₂ nanofibers, and Al with of 3 wt.% ZrO₂ nanofibers, all fabricated via the SPS technique. The images were acquired using SEM in BSE mode, where contrast depends on the atomic number of the elements.

The reference sample of pure aluminum shown in Figure 3a exhibits a near-fully dense with isolated porosity and indicates grain coarsening. The addition of 1 and 3 wt.% ZrO₂ nanofibers has an observable effect on the aluminum matrix microstructure (Figure 3b,c). The average grain size of the aluminum matrix is slightly reduced compared with the pure Al sample. Based on Z-contrast, i.e., the phases containing heavier elements appear brighter in BSE images, the bright particles distributed throughout the microstructures in Figure 3b,c are identified as ZrO₂ clusters. These particles are predominantly located at the grain boundaries of the aluminum matrix. The observed grain refinement in both composite samples can be supported by Zener pinning mechanism, in which minor secondary phase particles (in this case ZrO₂) effectively anchor grain boundaries during sintering, thus inhibiting the grain growth [28]. The microstructure of the sample with a higher nanofiber content (3 wt.% ZrO₂) exhibits a visually higher concentration and more pronounced clustering of the bright ZrO₂ phase corresponding to its increased mass fraction. Due to the higher content, ZrO₂ particles form a denser and partially interconnected network along the grain boundaries. The observed agglomerations of ZrO₂ particles are typical features thermodynamically driven by diffusion and surface energy minimization during the powder mixture preparation and subsequent processing at elevated temperatures. The comparison of microstructures in Figure 2 clearly demonstrates that the ZrO₂ particles act as an effective reinforcing phase in the aluminum matrix, fulfilling two key roles: (i) they induce grain refinement via the Zener pinning mechanism, which provides a theoretical potential to enhance mechanical properties like hardness and strength (in accordance with the Hall–Petch relationship [29]), although, as will be shown, this effect is counteracted by an increase in porosity; and (ii) they form an intergranular network of hard ceramic phase that reinforces grain boundaries, increasing the material’s resistance to plastic deformation and wear. The refined microstructures with homogeneously distributed hard-phase particles provide a theoretical basis for improvement of tribological properties observed in the composite materials.

Figure 4a shows the Raman spectra of GNP-derived tribofilm burnished on the Al-3 wt.% ZrO₂ nanofiber composite from three different places (black, red, green) recorded in the 150–2600 cm⁻¹ range. The spectra display the D band at ~1350 cm⁻¹ (defect/edge-activated) and the G band at ~1580 cm⁻¹ (sp²-bonded carbon). The I D/I G ratios decrease from ~0.7 (black) to ~0.45 (red) and ~0.25 (green), indicating progressively lower defect density (highest structural order for the green sample). The 2D band (~2680–2720 cm⁻¹), which would inform about the number of graphene layers, is not captured in this range. An typical light micrograph of GNP-derived tribofilm burnished on the Al-3 wt.% ZrO₂ nanofiber composite is shown in Figure 4b.

Elemental chemical maps for the Al-1 wt.% ZrO₂ nanofiber composite obtained by EDS analyses on the polished metallographic cross-section are shown in Figure 5. The aluminum map analyzed at the Al-Kα energy line reveals a homogeneous Al distribution in matrix without evidence of localized concentration or segregation. Zirconium map analyzed at the Zr-Lα energy line indicates finely dispersed Zr-rich regions, confirming a uniform distribution of ZrO₂ phase in the matrix. The oxygen map analyzed at the O-Kα energy line suggests the presence of oxide phases within the matrix, likely associated with the ZrO₂ nanofibers. However, it may also indicate the presence of Al₂O₃ oxide films on Al grains as a result of high chemical affinity of aluminum to oxygen. The carbon map analyzed at the C-Kα energy line shows a homogeneous carbon distribution, indicating successful mechanical deposition of the GNP-derived tribofilm on the polished surface. The obtained EDS results indicate the effective incorporation of ZrO₂ reinforcement phase and GNPs solid lubricating phase within prepared Al-matrix nanocomposite materials.

3.5. Mechanical Properties

Vickers hardness measurements at loads of 1 kg (HV1) and 0.1 kg (HV0.1) revealed the effect of ZrO₂ nanofiber addition on the mechanical properties of the composites. The pure aluminum matrix exhibited an average hardness of 36.5 ± 0.5 HV1 and 39.0 ± 1.6 HV0.1. Contrary to the expected strengthening effect of the ceramic phase, the addition of 1 wt.% and 3 wt.% of ZrO₂ did not lead to an increase in hardness. On the contrary, a slight decrease was recorded for the sample with 1 wt.% of ZrO₂ nanofibers (36.1 ± 0.4 HV1 and 38.8 ± 1.3 HV0.1), with the lowest values being measured for the sample with 3 wt.% of ZrO₂ nanofibers (36.2 ± 0.8 HV1 and 36.8 ± 1.4 HV0.1). This decrease in hardness directly correlates with the previously observed reduction in relative density, indicating that the increased porosity in composites with a higher ZrO₂ content has a negative impact on mechanical properties, which offsets the potential strengthening contribution of the nanofibers [30].

This hardness behavior observed in both composites can be attributed to several microstructural factors. The addition of ZrO₂ in the form of short nanofibers changes the mechanical response of the aluminum matrix; however, at relatively low reinforcement levels (1–3 wt.%), a significant increase in hardness is not achieved. In the case of the Al-1 wt.% ZrO₂ nanofiber composite, the slight decrease in hardness compared to pure aluminum is likely caused by inadequate particle dispersion or poor interfacial bonding between the matrix and the reinforcement phase, which can lead to localized mechanical weakening.

At the higher ZrO₂ content (3 wt.%), the further decrease in hardness suggests that under the current SPS processing conditions, the reinforcing particles caused an increase in structural porosity and did not become the predominant factor in strengthening the matrix [31].

3.6. Coefficient of Friction

Figure 6 illustrates the evolution of the coefficient of friction as a function of sliding distance for pure aluminum and composite materials under normal loads ranging from 1 to 4 N. All samples were fabricated by SPS, followed by polishing, etching, and surface modification through dry mechanical deposition of GNPs. Tribological tests were conducted using a ball-on-disk configuration at a sliding speed of 0.1 m/s over a total distance of 50 m, using a 6 mm diameter ZrO₂ ball as the counterbody.

The pure Al sample with a GNP-derived tribofilm (denoted as Al-0ZrO_2nf-GNPs) exhibits an typical initial run-in period (Figure 6a) lasting up to approximately 13 m, during which the coefficient of friction reaches values around 0.3. After this stage, the coefficient of friction decreases to approximately 0.25. However, beyond 39 m, a sudden rise in coefficient of friction to 0.65 is observed, followed by pronounced fluctuations between approximately 0.52 and 0.6 for the remainder of the test. This instability indicates a sudden failure of the GNP-derived tribofilm, leading to repeated breakdown and reformation of the tribofilm on the soft Al substrate. The abrupt COF rise is consistent with tribofilm exhaustion/delamination on the soft Al substrate. Once the carbon film locally loses continuity, adhesive–abrasive wear dominates and COF spikes with stick–slip. This contrasts with the composites where ZrO₂ nanofiber protrusions enhancing the stability of a carbonaceous tribofilm.

In contrast, the composite materials with 1 wt.% (denoted as Al-1ZrO_2nf-GNPs) and 3 wt.% (denoted as Al-3ZrO_2nf-GNPs) ZrO₂ nanofibre reinforcement display immediate and significant improvement in tribological behavior. For both composites, the COF stabilizes from the beginning of the test at a low and steady value of approximately 0.2 (Figure 6a). The frictional behavior remains highly stable throughout the test duration, without the fluctuations seen in pure Al. This improved performance is attributed to a synergistic effect in which the hard ZrO₂ nanofibers enhance the matrix resistance to local plastic deformation, thus providing a stable support for the GNP-derived tribofilm. The graphene then functions effectively as a solid lubricant, ensuring low friction. The results clearly demonstrate that even a 1 wt.% addition of ZrO₂ nanofibers is sufficient to significantly enhance the tribological behavior of the system under the given test conditions.

When analyzing the influence of normal loads from 1 to 4 N (Figure 6a–d) on the tribological performance of the prepared materials, distinct differences in behavior between the pure Al and the ZrO₂ nanofibers reinforced composites were observed. The reference pure Al sample with GNPs coating exhibited unsatisfactory tribological performance at all loading levels. Even at the lowest load of 1 N (Figure 6a), the coefficient of friction was high and unstable. As the load increased, this behavior worsened, with the average COF increasing and fluctuations becoming more pronounced. At loads of 3 N and 4 N (Figure 6c,d), catastrophic failure occurred, manifested by extremely high coefficient of friction values (approaching 1.0 at 4 N) and intense stick-slip oscillations. This indicates failure of the protective film and the onset of severe adhesive-abrasive wear, characteristic of contact between soft Al and the ZrO₂ ball counterbody.

In contrast, both composite materials showed dramatic improvements over pure aluminum at all applied loads. At low loads of 1 and 2 N (Figure 6a,b), both 1 wt.% and 3 wt.% ZrO₂ nanofiber composites exhibited very low and highly stable coefficient of friction values in the range of ~0.20–0.25. Their performance under these conditions was nearly identical, demonstrating the excellent lubricating capability of the formed tribofilm. At 3 N (Figure 6c), both composites maintained a low and stable coefficient of friction, although a slight increase in the average value (~0.25–0.30) was noted. Nevertheless, the stability remained very high. At the highest load of 4 N (Figure 6d), a key difference between the two composites emerged. The Al-1 wt.% ZrO₂ nanofiber composite sample exhibited the lowest and most stable coefficient of friction (~0.2–0.25) throughout the test, while both pure Al and Al-3 wt.% ZrO₂ nanofiber composite showed a sudden increase in COF after ~35 m, indicating tribofilm breakdown and unstable frictional behavior.

3.7. Wear Analysis

Figure 7 shows the evolution of penetration depth as a function of sliding distance, serving as an indicator of wear and plastic deformation in the tested materials. Measurements were carried out under normal loads ranging from 1 to 4 N over a total sliding distance of 50 m. The reference pure aluminum sample exhibits low resistance to wear and plastic deformation. The penetration depth increases continuously from the beginning of the test, reaching a final value of approximately 30 µm at the end of the 50-m path under a 1 N load. This trend reflects significant plastic deformation of the soft aluminum matrix and substantial material removal due to both adhesive and abrasive wear mechanisms, which could not be effectively mitigated even by the applied GNP-derived tribofilm.

In contrast, the composite materials show an opposite behavior. The incorporation of ZrO₂ nanofibers into the aluminum matrix significantly enhances the material’s resistance to wear. For both composite samples (with 1 wt.% and 3 wt.% ZrO₂ nanofibers), the penetration depth remains minimal and stable throughout the entire test. The final penetration depth for these samples reaches only about 5–10 µm, which is more than 30% lower than that of the pure aluminum. This improvement is a direct result of the strengthening effect provided by the ceramic nanofibers, which effectively bear the applied load, provide stable load-bearing points and restrict the plastic flow of the soft matrix. It is evident that even at a reinforcement level of 1 wt.%, excellent wear resistance is achieved under a 1 N load, while a further increase to 3 wt.% does not result in any significant improvement under these specific conditions.

The analysis of penetration depth, which directly indicates the extent of wear and plastic deformation, revealed a clear influence of normal load and material composition on the service life of the tested materials. The reference sample of pure aluminum failed under all tested loading conditions. The wear rate was directly proportional to the applied load. While a final penetration depth of approximately 30 µm was recorded at a 1 N load, this value increased dramatically with increasing load. At 4 N, catastrophic failure occurred, with the penetration depth exceeding 1300 µm. This result confirms that the soft aluminum matrix is incapable of withstanding high contact pressures, leading to severe ploughing and adhesive wear mechanisms [32]. In contrast, both composites demonstrated significantly higher wear resistance—by orders of magnitude—compared to pure aluminum. At loads of 1 N, 2 N, and 3 N, both composite materials exhibited similar and excellent performance. The penetration depth remained low and increased only slightly and in a controlled manner with increasing load. This behavior confirms that the reinforcing ceramic phase effectively hinders plastic deformation and material removal under mechanical stress [33,34].

3.8. Wear Mechanisms

From a macroscopic perspective, the wear track on the pure Al sample (Figure 8a) exhibits severe plastic deformation, with visible signs of plastic flow directed laterally and a wavy surface morphology. On the microscale, a dense network of both discontinuous and continuous cracks is observed, characteristic of delamination wear. Locally, the presence of torn-off flakes and plastically stretched bands is evident, indicating a predominant adhesive–abrasive wear mechanism. The highest concentration of bright cracks correlates with substantial surface degradation.

The wear track on the Al-1 wt.% ZrO₂ nanofiber composite sample exhibits a more compact structure, with less pronounced plastic flow and sharper track edges compared to pure Al. Microscopically, the cracks form a finer and more regular network, with many terminating in blunt ends without complete delamination of surface lamellae. Protruding ceramic particles locally absorb the applied load and help to disperse plastic deformation. The reduced presence of cracks indicates partial suppression of delamination and stabilization of the surface tribofilm. The wear track appears highly homogeneous, with minimal material extrusion and a seemingly continuous tribofilm. Microscopically, the presence of shorter, branched cracks without delamination features is noted.

At higher ZrO₂ content, a rigid ceramic skeleton is formed, which effectively suppresses continuous plastic flow. The overall crack density is further reduced, correlating with the highest surface integrity and the lowest expected wear depth. The ZrO₂ nanofibers (hardness > 11 GPa) increase local load-bearing capacity and restrict dislocation mobility, thereby progressively reducing the surface plastic flow typically observed in pure Al. While adhesive–delamination wear dominates in the 0 wt.% ZrO₂ nanofiber sample, the addition of 1 wt.% ZrO₂ nanofiber shifts the wear regime toward a mixed adhesive–abrasive mechanism. With 3 wt.% ZrO₂ nanofiber, the system transitions to a predominantly abrasive micro-grooving mechanism, with minimal ductile tearing of the matrix.

As the ZrO₂ nanofiber content in the Al matrix increases, the tribofilm becomes more continuous and less topographically complex. Although microcracks become more numerous (due to the increased brittleness of the ceramic phase), they remain shallow and weakly connected, thereby preventing the propagation of macro-delamination. Practically, this translates into a reduced coefficient of friction and lower material loss after 50 m of sliding, in accordance with the observed trend of decreasing relative crack area.

The addition of milled ZrO₂ nanofibers to the Al matrix—even at 1 wt.%—significantly enhances the wear resistance of the sintered aluminum by altering the tribofilm morphology and reducing delamination cracks. Increasing the nanofiber content to 3 wt.% further homogenizes the surface, minimizes plastic flow, and results in the lowest observed crack density, synergistically improving the tribological performance. This trend is consistent with findings reported by Ghahari and Shabani [35]. The higher ZrO₂ nanofiber content (3 wt.%) forms a denser network of rigid micro-islands that redistribute contact pressure and mitigate localized plastic shearing. As a result, the tribofilm evolves into a finer, more continuous structure with a lower probability of delamination events.

SEM microstructural analysis clearly shows that increasing the ZrO₂ nanofiber content from 1 wt.% to 3 wt.% leads to a finer, more cohesive tribofilm with a mosaic-like texture and denser, more uniform distribution of the ceramic phase. This effectively supports a load-bearing effect while suppressing and refining delamination cracks, thereby enhancing surface integrity. This microscopic evidence supports the earlier macroscopic observations and reinforces the conclusion that the addition of 3 wt.% ZrO₂ nanofiber to the Al matrix represents an optimal compromise between matrix strengthening and suppression of brittle tribofilm failure.

Figure 9 shows the SEM images of wear tracks after tribological testing of the Al-ZrO₂ nanofiber composites reinforced with 1 and 3 wt.% of short ZrO₂ nanofibers with GNPs applied to the composite surfaces. The tribological tests were carried out against a zirconia ball with a sliding distance of 50 m and under a load of 4 N.

At lower loads (1–3 N), both Al-1 wt.% ZrO₂ nanofiber composite and Al-3 wt.% ZrO₂ nanofiber composite develop compact wear tracks with suppressed delamination compared to pure Al. The 3 wt.% material often shows a more rigid ceramic skeleton and reduced plastic flow in the matrix, yielding slightly cleaner groove morphology. However, these morphological advantages at low load do not translate into superior high-load performance.

Under the highest load (4 N), the Al-1 wt.% ZrO₂ nanofiber composite retains a continuous, low-shear tribofilm and maintains a low, stable COF throughout the 50 m sliding distance. By contrast, the Al-3 wt.% ZrO₂ nanofiber composite exhibits a late-stage tribofilm breakdown (≈35 m) manifested by a sudden COF rise and increased instability. We attribute this to nanofiber clustering, which concentrates stresses and interrupts the GNP-derived tribofilm, undermining film continuity under sustained load. Thus, across all loads, the 1 wt.% composite delivers the most robust tribological response, while the 3 wt.% composite provides comparable performance at 1–3 N but loses stability at 4 N.

Thanks to the synergistic effect of ZrO₂ nanofibers and GNPs, a more homogeneous tribofilm was formed, which reduced the intensity of metallic contacts and thus the wear rate. This effect corresponds with the findings of other authors, who report that the combination of ceramic reinforcing particles with carbon nanomaterials (e.g., GNPs) leads to the formation of a stable tribofilm and improved wear resistance [36,37].

During sliding, the GNPs deposited on the composite surface undergo mechanical exfoliation and lateral transfer, forming a thin, continuous tribofilm that covers both the Al matrix and protruding ZrO₂ nanofibers. This tribofilm evolves dynamically through repeated shear and compaction, leading to a lamellar carbon-rich layer with low shear resistance, consistent with mechanisms described for graphene superlubricity [38]. Although direct imaging of a GNP-derived tribofilm was not obtained, the data suggest that ZrO₂ nanofiber protrusions act as pinning/load-sharing sites that support tribofilm stability and may reduce local plastic flow within the ductile Al matrix. This synergistic interface provides a stable support that prevents premature delamination of the tribofilm and maintains a low and steady coefficient of friction.

The tribofilm therefore consists of a hybrid structure: a carbon-dominated top film incorporating fine wear debris and oxide fragments, underlain by a mechanically mixed subsurface zone enriched in Al–O and Zr–O compounds. Such hybridization enhances adhesion of the carbon tribofilm and improves thermal dissipation at the sliding interface. According to the model proposed by Liu et al. [38], friction reduction arises from incommensurate sliding between graphene basal planes and from the weak van der Waals bonding between transferred graphene sheets, which minimizes shear stress. In the present composites, these effects are reinforced by the nanoscale roughness and hardness contrast introduced by ZrO₂ nanofibers, leading to the formation of a self-sustaining graphene/oxide tribofilm capable of maintaining lubrication even under increased load.

4. Conclusions

(1)

Al-1 wt.% ZrO₂ nanofiber and Al-3 wt.% ZrO₂ nanofiber composite materials were successfully prepared using SPS. Microstructural analysis showed that the addition of 1–3 wt.% ZrO₂ nanofibers to the aluminum matrix led to visible grain refinement and the formation of a homogeneously distributed ZrO₂ phase. Performed EDS analyses confirmed uniform distribution of the reinforcement ZrO₂ phase. However, the both produced composites showed visible clustering of ZrO₂ particles.

(2)

The application of a GNP-derived tribofilm to pure aluminum is insufficient to ensure favorable tribological performance. The soft aluminum matrix does not provide adequate mechanical support for the tribofilm, which fails under applied load, resulting in a high and unstable coefficient of friction.

(3)

The incorporation of short ZrO₂ nanofibers into the aluminum matrix significantly enhances the load-bearing capacity and resistance to plastic deformation. The hard ceramic fibers act as a structural phase that stabilizes the surface and facilitates the formation of a stable and effective GNP-derived tribofilm, leading to a dramatic reduction in the coefficient of friction.

(4)

Under increased load, reinforcement content becomes critical. While both composites perform similarly at 1–3 N, at 4 N the Al-1 wt.% ZrO₂ nanofiber composite maintains a low and stable coefficient of friction for the entire 50 m, whereas the Al-3 wt.% ZrO₂ nanofiber composite shows tribofilm failure and a sudden coefficient of friction rise after ~35 m, consistent with nanofiber clustering that disrupts tribofilm continuity.

(5)

Overall, the Al-1 wt.% ZrO₂ nanofiber composite exhibits the best performance across the tested conditions, combining the most stable low coefficient of friction with the highest wear resistance. The 3 wt.% composite ranks second, matching 1 wt.% at 1–3 N but losing stability at 4 N, while pure sintered aluminum performs worst.