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Article

Investigating the Effects of Graphene Nanoplatelets and Al4C3 on the Tribological Performance of Aluminum-Based Nanocomposites

by
Mihail Kolev
1,*,
Rumyana Lazarova
1,
Veselin Petkov
1,
Yana Mourdjeva
1 and
Diana Nihtianova
2
1
Institute of Metal Science, Bulgarian Academy of Sciences, Equipment and Technologies with Center for Hydro-and Aerodynamics “Acad. A. Balevski”, Boulevard “Shipchenski Prohod” 67, 1574 Sofia, Bulgaria
2
Institute of Mineralogy and Crystallography “Acad. I. Kostov”, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Metals 2023, 13(5), 943; https://doi.org/10.3390/met13050943
Submission received: 1 April 2023 / Revised: 24 April 2023 / Accepted: 9 May 2023 / Published: 13 May 2023
(This article belongs to the Section Structural Integrity of Metals)
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Abstract

:
The study investigates the effects of graphene nanoplatelets (GNPs) on the tribological properties of aluminum-based nanocomposites, both annealed after extrusion and non-annealed. It also examines the role of nanosized Al4C3 (aluminum carbide), which forms in the annealed Al/GNPs nanocomposite, on the tribological performance of the nanocomposites. The nanocomposites were fabricated using the powder metallurgy method. The microstructure of the composite materials was characterized using SEM, EDS, XRD and TEM techniques. The coefficient of friction (CF) and mass wear of the composites were measured using a pin-on-disk test under dry sliding friction conditions. The results showed that adding GNPs increased the coefficient of friction (CF) of the nanocomposites by up to 44% at 0.1 wt.% GNP, but the CF decreased by 15% at 1.1 wt.% GNP. The optimal concentration of GNPs for minimizing the CF and mass wear of Al-based nanocomposites was 0.1 wt.%. Additionally, the presence of Al4C3 in the annealed Al/GNP nanocomposite had a positive effect on the CF at low GNP concentrations, with a 38% increases at 0.1 wt.% GNP, but this effect diminished as the GNP concentration increased. The study also found that the mass wear of the nanocomposites increased with the GNP concentration, with a 46% increase in the mass wear from 0.1 wt.% GNP to 0.5 wt.% GNP and a 202% increase from 0.1 wt.% GNP to 1.1 wt.% GNP. The presence of Al4C3 also affected the mass wear, with the effect diminishing as the GNP concentration increased. The study observed an increase in the mass wear with the increase in the GNP concentrations, but the mass wear of the annealed Al/GNPs with 1.1 wt.% GNP and Al4C3 was 52% lower than the Al composite with 1.1 wt.%. Overall, this study provides insights into the role of GNPs and Al4C3 on the tribological performance of aluminum-based nanocomposites.

1. Introduction

Various engineering applications rely on aluminum-based composites due to their desirable properties: low density, high strength-to-weight ratio, good corrosion resistance and excellent thermal conductivity. However, these composites have poor tribological properties, tend to wear out quickly and have high friction. To enhance their performance and durability for demanding applications, researchers have added different reinforcements to the aluminum matrix [1,2,3,4].
Advanced fabrication techniques such as friction stir processing [5], liquid method [6], ball milling with sintering [7], hot extrusion [8,9,10] and spark plasma sintering [11,12] have been employed to produce metal–matrix composites reinforced with graphene. Despite its drawbacks, powder metallurgy remains a favorable and less expensive alternative for the fabrication of these composites, particularly Al–G composite materials, making it a highly promising prospect for industrial applications [13,14].
Graphene nanoplatelets (GNPs) are one of the most promising reinforcements for aluminum composites due to their exceptional mechanical, electrical, thermal and chemical properties [15]. GNPs consist of a few layers of graphene sheets with a thickness of less than 10 nm and a lateral size of up to several micrometers [16]. GNPs can enhance the mechanical properties of aluminum composites by increasing their stiffness, strength, hardness and ductility. Moreover, GNPs can reduce the friction and wear of aluminum composites by acting as solid lubricants or protective layers at the sliding interface [17,18,19].
In recent years, many researchers have studied aluminum-based composites that contain GNPs. Most of these studies have shown that graphene forms a mechanical bond with the aluminum matrix, which improves the hardness and strength of the composite compared to pure aluminum [20,21]. However, the incorporation of GNPs into the aluminum matrix also poses some challenges. One of them is the formation of Al4C3 carbides at the interface between the GNPs and the aluminum during the composite production. Al4C3 is a brittle and thermodynamically unstable compound that can degrade the mechanical properties of the aluminum composites by weakening the interfacial bonding and causing stress concentration. Studies have confirmed the occurrence of an interfacial reaction in the C/Al composites, leading to the formation of an Al4C3 phase [20,21]. The interfacial reaction in such composites recently gained significant attention as a critical factor affecting both the interfacial characteristics and mechanical properties [22,23,24], with studies demonstrating that it enhances the load transfer efficiency [25,26,27]. Notably, a high load transfer efficiency has been observed at the interface with Al4C3 as a result of the anchor effect of Al4C3 and the change of the interfacial bonding state from mechanical bonding to strong chemical bonding. Therefore, it is important to understand how Al4C3 affects the tribological properties of aluminum GNPs composites. Studies have shown that adding 3 wt.% of graphene to the aluminum matrix composites can reduce the wear rate by 34% and the coefficient of friction (CF) by 25% but incorporating a high load of 5 wt.% of graphene can result in agglomeration and a higher wear rate [28]. To understand the tribological performance of Al composites containing 1.0 wt.% GNPs, Khorshid et al. [29] observed the effects of the load and sliding speed. They observed that the wear rate increased and the CF decreased at high loads, while increasing the sliding speed generally decreased the CF as expected. Jiangshan Zhang et al. [30] produced an Al–graphene nanocomposite and observed that 1 vol.% of reinforcement reduced the tribological properties, such as the CF by up to 39.1% and the wear rate by up to 85.0% when compared to the matrix material. Ghazaly et al. [31] synthesized aluminum/graphene composites using the powder metallurgy technique with varying weight percentages of graphene (0.5, 3 and 5 wt.%). They found that the self-lubricating composite reinforced with 3 wt.% graphene exhibited the best tribological properties under dry wear test conditions compared to the unreinforced and graphene-reinforced composites with different weight percentages. The formation of a layer that protects against wear and reduces friction (tribolayer) between the contact surfaces explains these excellent tribological properties [28,29,31]. The morphology of graphene in the tribo-layer of a bulk composite material is hard to observe because of its 2D nature, and the common methods for its characterization are scanning electron microscopy and optical microscopy, as reported in [28,29,32,33].
While previous studies [28,29,30,31] provided valuable insights into the effect of graphene on the tribological properties of aluminum-based nanocomposites, none have investigated the combined effect of GNPs and Al4C3 on the tribological performance of these composites. Therefore, the present study aims to bridge this gap in the literature by investigating the combined effect of GNPs and Al4C3 on the tribological performance of aluminum-based nanocomposites fabricated using the powder metallurgy method. Specifically, the objectives of this study are as follows.
  • To identify the optimal concentration of GNPs for minimizing the coefficient of friction and the mass wear of the nanocomposites, both annealed and non-annealed.
  • To examine the role of Al4C3, which forms in the annealed Al/GNPs nanocomposite, on the tribological performance of the nanocomposites.
In addition, the microstructure of the composites was characterized using a range of techniques, including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). A pin-on-disk test was used to measure the mass wear and the coefficient of friction under dry sliding friction conditions.
This investigation builds upon a previous study in which Al-based composites reinforced with GNPs were fabricated using the powder metallurgy method, followed by heat treatment. The microstructure, mechanical properties and microhardness of the composites during tensile testing were investigated in the prior study [16].

2. Materials and Methods

2.1. Materials

The materials used to prepare the composites were as follows.
-
Aluminum powder with a chemical purity of 99.5% and an average particle size of 37 µm (Figure 1a and Figure 2a).
-
Graphene nanoplatelets with a thickness of 6–8 nm and a purity of 99.5% (IoLiTec Ionic Liquids Technologies GmbH,Im Zukunftspark 9, D-74076 Heilbronn, Deutschland) (Figure 1b and Figure 2b).

2.2. Production Method

The production method was the same as the method described in [16].
-
Powder mixing in a planetary agate ball mill. The content of the graphene in the powder mixtures varied from 0.1 wt.% to 1.1 wt.% with an increment of 0.2 wt.%.
-
Hot extrusion. Seven bars of the aluminum-based nanocomposite reinforced with graphene nanoplatelets with 12 mm in diameter were produced, including one of pure aluminum.
-
Heat treatment. In order to obtain nanosized carbides at the Al/GNP interface, annealing with the following parameters was performed: heating temperature, 610 °C; holding time, 3 h; cooling ambiance, air out of the furnace.
The scheme of the production process of the Al/GNP nanocomposites is illustrated in Figure 3.

2.3. Characterization Methods

A Ducom TR-20 rotary (pin/ball-on-disk) tribometer (Ducom Instruments Pvt. Ltd., Bangalore, India) was used to measure the wear properties of all the specimens. The test specimens were machined from the tested material using a lathe to obtain a spherical tip geometry with a height of 20 mm and a diameter of 12 mm. The machining process was carefully designed to minimize the potential for any structural flaws that could affect the wear mechanisms. Dry wear tests were performed on them using a pin-on-disk system with the following parameters: load = 30 N, linear velocity = 0.9 m∙s−1 and sliding distance = 540 m.
The tribometer’s data acquisition system calculated the coefficient of friction (CF). The counter disk was composed of EN-31 steel with a hardness of 62 HRC, a diameter of 140 mm and a surface roughness of 1.6 Ra. The composition of the counterbody consisted of the following concentrations, wt.%: C 0.90–1.20; Si 0.10–0.35; Mn 0.30–0.75; Cr 1.00–1.60; Si 0.20; and Fe–rest.
The microstructure was observed using a Hirox SH-5500P scanning electron microscope (SEM) (Hirox Japan Co Ltd., Tokyo, Japan). A BRUKER QUANTAX 100 Advanced X-ray energy dispersive spectroscopy (EDS) system (BRUKER, Kontich, Belgium) was used for the chemical micro-analysis.
For the TEM and HRTEM analyses, a JEOL JEM-2100 (200 kV) with an Oxford Instruments X-MAX N 80 T EDS detector and a GatanOrius SC1000 CCD camera (11 MP) was employed. The specimens were prepared by cutting ultrathin sections with a Leica EM UC7 ultramicrotome, Danaher Corporation, for the electron microscopy. ImageJ open source software (version 1.8.0, developed by Wayne Rasband at the National Institutes of Health, Bethesda, MD, USA) was used to measure the lattice fringe spacing from the HRTEM micrographs by analyzing the parameters of the reciprocal space in the digital images.

3. Results and Discussion

3.1. Morphological Analysis of the Worn Surfaces

The wear mechanisms of the Al nanocomposites reinforced with graphene nanoplatelets and annealed with nanosized Al4C3 can be complex and depend on various factors, such as the composition of the nanocomposites, the testing conditions and the nature of the contact between the material and the counterbody. The SEM images can provide insight into the mechanisms responsible for the observed wear. For example, the images may reveal the presence of wear debris, cracks, scratches, or other features on the worn surface, which can indicate the type of wear mechanism that occurred. The EDS analysis can also provide complementary information by identifying the elemental composition of the worn surface, which can help determine the mechanisms responsible for the wear.
Figure 4a–f presents the SEM images of the test specimen after the pin-on-disk tests, with the distinct zones indicated by the markers. These images were captured following the wear experiments conducted under the following conditions: a linear speed of 0.9 m/s, a sliding distance of 540 m and a load of 30 N. The results of the EDS analysis are shown in Table 1, Table 2, Table 3, Table 4, Table 5 and Table 6 and Figure 5a–f.
Figure 4a shows the worn surface of the Al with 0.1 wt.% GNPs, which exhibited a smooth appearance with shallow grooves and some debris. The EDS analysis (Table 1 and Figure 5a) indicated that zone 1 had a high carbon content of 82.74%, which suggested that GNPs were embedded in the surface and acted as solid lubricants, reducing the friction and wear. Zone 2 had a low C content and a high Al content of 97.70%, which indicated that it was part of the contact zone.
Figure 4b shows the worn surface of the Al with 0.5 wt.% GNPs, which exhibited a rougher appearance with deeper grooves and more debris. The EDS analysis (Table 2 and Figure 5b) indicated that zone 1 had a high C content of 77.57%, which suggested that GNPs were also embedded in the surface, but less effectively than in Figure 4a. Zone 2 had a low C content and a high Fe content of 43.17%, which indicated that it was mainly composed of iron from the counterface, forming a tribofilm that increased the friction and wear.
Figure 4c shows the worn surface of the Al with 1.1 wt.% GNPs, which exhibited a very rough appearance with severe grooves, large debris and cracks. The EDS analysis (Table 3 and Figure 5c) indicated that zone 1 had a very high C content of 87.34%, which suggested that GNPs were agglomerated on the surface, forming abrasive particles that increased the friction and wear. Zone 2 had a moderate C content of 74.09%, which suggested that GNPs were partially embedded in the surface, but not enough to provide lubrication. Zone 3 had a low C content and a high Fe content of 24.91%, which indicated that it was also mainly composed of iron oxide from the counterface.
Figure 4d shows the worn surface of the annealed Al with 0.1 wt.% GNPs, which exhibited a similar appearance to Figure 1a, but with more debris and some cracks. The EDS analysis (Table 4 and Figure 5d) indicated that zone 1 had a high C content of 82.36%, which suggested that GNPs were embedded in the surface and acted as solid lubricants, similar to Figure 1a. Zone 2 had a low C content and a high Fe content of 37.63%, which indicated that it was mainly composed of iron oxide from the counterface, similar to Figure 4b.
Figure 4e shows the worn surface of the annealed Al with 0.5 wt.% GNPs, which exhibited a smoother appearance than the same material with a different GNP concentration than in Figure 4d (0.1 wt.%) and Figure 4f (1.1 wt.%). The EDS analysis (Table 5 and Figure 5e) indicated that zone 1 had a high C content of 71.82%, which suggested that GNPs were embedded in the surface and acted as solid lubricants, similar to Figure 4a,d. Zone 2 had a moderate C content of 62.50%, which suggested that GNPs were partially embedded in the surface, but less effectively than in zone 1. Zone 3 had a low C content and a high Al content of 85.89%, which indicated that it was mainly composed of the matrix material.
Figure 4f shows the worn surface of the annealed Al with 1.1 wt.% GNPs, which exhibited a rougher appearance than Figure 4c, but with more debris. The EDS analysis (Table 6 and Figure 5f) indicated that zone 1 had a high C content of 72.30%, which suggested that GNPs were agglomerated on the surface, similar to Figure 4c. Zone 2 had a moderate C content of 52.46%, which suggested that GNPs were partially embedded in the surface, but less effectively than in zone 1. Zone 3 had a low C content and a high Fe content of 21.31%, which indicated that it was also mainly composed of iron oxide from the counterface. The cracks in the annealed Al with 1.1 wt.% GNPs indicated that the material had a low ductility and a high brittleness due to the presence of nanosized Al4C3 particles, which were formed during the annealing process. These particles also contributed to the wear resistance of the material by hindering the plastic deformation and the crack propagation. However, the high GNP content also led to the formation of abrasive particles and tribofilms that increased the friction and wear.
The EDS of Al4C3 may be influenced by the Fe debris that can cause extra X-ray peaks of the Fe element and change the surface structure of Al4C3. The surface changes that occur when Fe atoms bind to the face of Al4C3 that has C atoms can lower the nucleation ability of the Al4C3 particles for heterogeneous solidification [34]. Therefore, the employed XRD, TEM and HRTEM techniques were employed.
The wear mechanisms of Al/GNPs composites vary based on the number of GNPs added to the matrix, as observed in the SEM images. In the composite with 0.1 wt.% GNPs (Figure 4a), the wear mechanism was primarily a mild abrasive wear. The surface appeared smooth with shallow grooves and some debris. The GNPs were embedded in the surface and acted as solid lubricants, effectively reducing the friction and wear. In the composite with 0.5 wt.% GNPs (Figure 4b), the wear mechanism was mainly a moderate abrasive wear and an adhesive wear. The surface appeared rougher with deeper grooves and more debris. The GNPs were less effectively embedded in the surface, and it was possible that some iron from the counterface formed a tribofilm that increased the friction and wear. For the composite with 1.1 wt.% GNPs (Figure 4c), the wear mechanism was primarily a severe abrasive wear and a delamination wear. The surface appeared very rough with severe grooves, large debris and cracks. The GNPs were agglomerated on the surface, forming abrasive particles that increased the friction and wear. Although the GNPs were partially embedded in the surface, they did not provide enough lubrication. Additionally, some iron from the counterface contributed to the wear.
The wear mechanisms of the annealed Al/GNPs composites also depend on the number of GNPs added to the matrix and the presence of Al4C3, as indicated by the SEM images. For the annealed composite with 0.1 wt.% GNPs (Figure 4d), the wear mechanism was primarily a moderate abrasive wear and a slight adhesive wear. The surface exhibited grooves along the sliding direction and more damage than the composite with 0.1 wt.% GNPs without Al4C3 (Figure 4a). However, the GNPs were embedded in the surface and acted as solid lubricants, reducing the friction and wear. For the annealed composite with 0.5 wt.% GNPs (Figure 3e), the wear mechanism was primarily a severe abrasive wear and an adhesive wear. The surface displayed slightly deeper grooves and more damage than the annealed composite with 0.1 wt.% GNPs with Al4C3 (Figure 4d). For the annealed composite with 1.1 wt.% GNPs (Figure 4f), the wear mechanism was primarily a severe abrasive wear and a delamination wear, as seen by the surface’s severe grooves, large debris and cracks. The GNPs were agglomerated on the surface, forming abrasive particles that increased the friction and wear. Although the GNPs were partially embedded in the surface, they did not provide enough lubrication and some iron from the counterface contributed to the wear.
Comparing the wear surfaces of the composites to the different content of GNPs, it can be seen that adding more GNPs to Al did not necessarily improve its wear resistance, but rather had an optimal amount that balanced between lubrication and abrasion. According to Wu et al. [19], this optimal amount was around 0.3 wt.% for Al–GNPs. However, incorporating a high fraction of 5 wt.% of graphene resulted in agglomeration and a higher wear rate, as shown by Ghazaly et al. [28]. To understand the effect of the GNP content on the microstructure of Al composites, Zhang et al. [30] produced an Al/GNSs nanocomposite and observed that 1 vol.% of reinforcement significantly reduced the size of the Al grains and improved the distribution of the GNPs in the Al matrix.

3.2. XRD and TEM Analysis

The X-ray diffraction pattern of a specimen containing 1.1 wt.% GNPs before and after the heat treatment (HT) is shown in Figure 6. The pattern exhibited intense peaks of aluminum, a low-intensity peak of graphene and a broad band of the amorphous phase. A small peak at approximately 54° 2θ in the HT sample indicated the presence of Al4C3. However, the peak was very low intensity and was within the instrument resolution, so the XRD analysis did not provide reliable evidence for the existence of the carbide phase. This did not exclude the possibility that a trace number of nano-carbides was present in the sample. To investigate the sample further, the TEM and HRTEM techniques were employed.
The HRTEM analyses provided the strongest evidence for the formation of nanosized A4C3 after the HT. Figure 7a shows a crystalline carbide particle (approximately 50 nm in longitude and about 7 nm in diameter) located at the grain boundary of the matrix. The observed carbide corresponded to COD #96-154-0875 (c = 2.498000 nm). The registered plane was (0012) with an interplanar distance of 0.208 nm. It was suggested that the graphene fully reacted with the aluminum to produce the carbide, since there was no evidence of the presence of graphene. Figure 7b shows the Fourier filtered image of the clear interface of the well bonded to the Al matrix Al4C3 carbide. The measured interplanar distance here was 0.416 nm, corresponding to the plane (006) of the carbide.

3.3. Wear Behavior

Figure 8 presents a comparison of the coefficient of the friction results for the aluminum-based nanocomposites reinforced with GNPs at weight percentages ranging from 0 to 1.1% and annealed aluminum-based nanocomposites reinforced with both GNPs and nanosized Al4C3 at the same weight percentages. The coefficient of friction is an important parameter for determining the tribological behavior of a material. A lower coefficient of friction indicates reduced resistance to sliding, which can result in less wear and damage to the material. The results presented in Figure 8 allowed for a direct comparison of the tribological properties of the two different types of nanocomposites. The addition of GNPs to Al had a significant effect on the CF. As the concentration of GNPs increased, the CF also increased. This was particularly evident when comparing 0.1 wt.% GNP to 0.5 wt.% GNP, where the CF increased by approx. 44%. However, further increasing the GNP concentration to 1.1 wt.% led to a 15% decrease in the CF compared to 0.5 wt.%, which may be due to the agglomeration of GNPs at higher concentrations.
When Al4C3 was added to the Al/GNPs composite, the CF generally increased compared to the Al/GNPs composite without Al4C3. The annealed Al/GNPs with 0.1 wt.% GNPs and Al4C3 had a CF that was approx. 38% higher than the Al with 0.1 wt.% GNPs. However, the effect of Al4C3 on the CF appeared to diminish as the concentration of GNPs increased. For instance, the CF of the annealed Al with 0.5 wt.% GNPs and Al4C3 was only marginally higher than that of the Al with 0.5 wt.% GNP. The CF of the annealed Al with 1.1 wt.% GNPs with Al4C3 was 8% lower than the same composite with 0.5 wt.% GNPs, which was contrary to the trend observed for the other GNPs concentrations. This may have been due to the agglomeration of GNPs at higher concentrations and the formation of weak interfacial bonding between the Al matrix and GNPs.
While adding GNPs to Al increased the CF due to their high surface roughness and interlocking tendency with the matrix, which increased the frictional resistance and contact area between the sliding surfaces, the agglomeration of GNPs at higher concentrations decreased the CF by reducing the uniform distribution and dispersion of GNPs in the matrix, which resulted in less effective reinforcement and lubrication of the composite. However, the effect of Al4C3 to the annealed Al/GNPs composite increased the CF since Al4C3 is a hard and brittle phase that can increase the abrasive wear and plowing effect of the composite, leading to a higher frictional force and heat generation. As shown in Figure 8, adding GNPs to Al increased the CF by approximately 44% when comparing 0.1 wt.% GNPs to 0.5 wt.% GNP, while adding Al4C3 to the Al/GNP composite with 0.1 wt.% GNPs increased the CF by approximately 38% compared to the Al with 0.1 wt.% GNP. These results suggested that the optimal concentration of GNPs for minimizing the CF of Al-based nanocomposites was 0.1 wt.%, which can enhance their tribological performance and durability. However, adding Al4C3 to the annealed Al/GNPs composite had a positive effect on the CF at low GNPs concentrations, but a negative effect at high GNPs concentrations.
After reviewing the studies [19,28,29,31], some similarities and differences were found in the tribological test parameters and results. One similarity was that all the studies investigated the tribological properties of aluminum-based composites with the addition of different concentrations of graphene. Moreover, they all used dry sliding wear tests to evaluate the tribological performance of the composites under the different sliding conditions, such as the normal load, sliding speed and sliding distance. The results showed that the tribological behavior of the composites was affected by the concentration of the nano-reinforcements and the sliding conditions.
However, there were also some differences in the tribological test parameters and results among the studies. For example, Wu et al. [19] found that the optimal wear parameters for GNPs (0.5 wt.%)/AlSi10Mg composites were a load of 30 N and sliding speed of 0.9 m/s, whereas Khorshid et al. [29] investigated the effects of normal loads of 5, 10, 15 N and sliding speeds of 0.14, 0.29, 0.4 m/s and indicated that the CF decreased at high loads of 15 N, while increasing the sliding speed to 0.4 m/s. Moreover, the effect of the concentration of the nano-reinforcements on the tribological properties varied among the studies. For instance, Ghazaly et al. [28,31] observed that adding 3 wt.% of graphene reduced the wear rate by 34% and the CF by 25% but incorporating a higher concentration of 5 wt.% of graphene resulted in agglomeration and a higher wear rate. In contrast, the current study found that the addition of GNPs to Al-based nanocomposites increased the CF by approximately 44% when comparing the 0.1 wt.% GNP to 0.5 wt.% GNP. However, the CF decreased by 15% when the GNPs concentration increased to 1.1 wt.%.
Overall, the similarities and differences in the tribological test parameters and results among the studies indicated that the tribological behavior of the composites was affected by various factors, such as the concentration of the nano-reinforcements, the sliding conditions and the synthesis and processing techniques. Therefore, it is important to carefully select the optimal combination of these factors to achieve the desired tribological performance of the composites in the specific applications.
Figure 9 presents the mass wear of Al with GNPs (0–1.1 wt.%) and the annealed Al with GNPs (0–1.1 wt.%) at a 30 N load, 0.9 m∙s−1 sliding speed and 540 m sliding distance under dry sliding friction conditions at room temperature. The mass wear increased as the concentration of GNPs increased. When comparing the Al with 0.1 wt.% GNP to 0.5 wt.% GNP, the mass wear showed a 46% increase. Further increasing the GNP concentration to 1.1 wt.% led to a 202% increase in the mass wear result, which may have been due to the agglomeration of GNPs at higher concentrations. By comparing all the concentrations of the annealed Al/GNP (0.1–1.1 wt.%) composite with the presence of Al4C3, the mass wear increased with the increase in the GNPs concentrations. The annealed Al/GNPs with 0.1 wt.% GNPs and Al4C3 had a mass wear that was approximately 38% higher than that of the Al with 0.1 wt.% GNPs. However, the effect of Al4C3 on the mass wear tended to diminish as the concentration of GNPs increased. For instance, the mass wear of the annealed Al with 0.5 wt.% GNPs and Al4C3 was 27% higher than that of the Al with 0.5 wt.% GNPs. The mass wear of the annealed Al with 1.1 wt.% GNPs with Al4C3 was 52% lower than the Al composite with 1.1 wt.% GNPs. This may have been due to the agglomeration of GNPs at higher concentrations and the formation of weak interfacial bonding between the Al matrix and GNPs.

4. Conclusions

In summary, this study investigated the influence of graphene nanoplatelets on the tribological behavior of aluminum-based nanocomposites, both annealed after extrusion and non-annealed. Additionally, the role of Al4C3 that forms in the annealed Al/GNPs nanocomposite on the tribological properties was also investigated.
  • The results showed that the addition of GNPs to aluminum-based nanocomposites increased the coefficient of friction (CF) by approximately 44% when comparing 0.1 wt.% GNP to 0.5 wt.% GNP. However, the CF decreased by 15% when the GNPs concentration increased to 1.1 wt.%. The optimal concentration of GNPs for minimizing the CF of Al-based nanocomposites was 0.1 wt.%.
  • The presence of Al4C3 in the annealed Al/GNPs composite had a positive effect on the CF at low GNP concentrations, with a 38% increase at 0.1 wt.% GNPs. However, this effect diminished as the concentration of GNPs increased.
  • The findings also indicated that the mass wear of the Al-based nanocomposites increased with the concentration of GNPs. When comparing the Al with 0.1 wt.% GNP to 0.5 wt.% GNP, it showed a 46% increase in the mass wear. Further increasing the GNPs concentration to 1.1 wt.% led to a 202% increase in the mass wear, which may have been due to the agglomeration of GNPs at higher concentrations.
  • By comparing all the concentrations of the annealed Al/GNPs (0.1–1.1 wt.%) composite with the presence of Al4C3, an increase in the mass wear was observed corresponding to the increase in the GNPs concentrations. The annealed Al/GNPs with 0.1 wt.% GNP and Al4C3 had a mass wear that was approximately 38% higher than that of Al with 0.1 wt.% GNPs. However, the effect of Al4C3 on the mass wear tended to diminish as the concentration of GNPs increased.
  • The mass wear of the annealed Al with 0.5 wt.% GNPs and Al4C3 was 27% higher than that of Al with 0.5 wt.% GNPs. The mass wear of the annealed Al with 1.1 wt.% GNPs with Al4C3 was 52% lower than the Al composite with 1.1 wt.% GNPs. This may have been due to the agglomeration of GNPs at higher concentrations and the formation of weak interfacial bonding between the Al matrix and GNPs.
Overall, these results provided important insights into the tribological properties of Al-based nanocomposites and can help guide the development of more effective and durable materials for various applications. While the study focused on the tribological properties of aluminum-based nanocomposites with graphene nanoplatelets, both annealed after extrusion and non-annealed, the results are preliminary and not intended for specific practical application. However, the findings suggest that the addition of these nanoparticles can significantly improve the wear resistance and reduce the friction coefficient of the material. Further studies can explore the potential use of these nanocomposites in industries such as aerospace, automotive and manufacturing, where the performance of materials under high stress and wear is critical. Additionally, the optimization of the manufacturing process and the use of different nanoparticle concentrations can lead to even more significant improvements in the tribological properties of the material.

5. Future Scope

  • Investigating the effect of the different processing techniques on the tribological properties of Al-based nanocomposites, such as the varying extrusion parameters or annealing temperatures.
  • Investigate the effect of GNPs on the mechanical and wear behavior of the Al/Al2O3 composites.
  • Conducting in-depth studies to understand the mechanisms behind the observed changes in the tribological behavior with varying GNP concentrations, including the role of agglomeration, interfacial bonding and other factors.

Author Contributions

Conceptualization, M.K. and R.L.; methodology, M.K., R.L., V.P., Y.M. and D.N.; formal analysis, M.K., R.L., V.P., Y.M. and D.N.; investigation, M.K., R.L., V.P., Y.M. and D.N.; writing—original draft preparation, M.K.; writing—review and editing, R.L.; visualization, all authors; funding acquisition, R.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the BULGARIAN NATIONAL SCIENCE FUND, Project KΠ–06–H57/17 “Fabrication of aluminum–graphene nanocomposites by powder metallurgical method and investigation of their nano-, microstructure, mechanical and tribological properties”.

Data Availability Statement

Not applicable.

Acknowledgments

We acknowledge the support of the European Regional Development Fund within the OP Science and Education for Smart Growth 2014–2020, Project CoE of the National Center of Mechatronics and Clean Technologies, BG05M2OP001-1.001-0008, for providing the necessary equipment for this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 13 00943 g001 550
Figure 1. SEM images: (a) aluminum powder with a chemical purity of 99.5% and an average particle size of 37 µm.; (b) graphene nanoplatelets with a thickness of 6–8 nm and a purity of 99.5%.
Figure 1. SEM images: (a) aluminum powder with a chemical purity of 99.5% and an average particle size of 37 µm.; (b) graphene nanoplatelets with a thickness of 6–8 nm and a purity of 99.5%.
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Metals 13 00943 g002 550
Figure 2. Cross sections of (a) aluminum powder with a chemical purity of 99.5% and an average particle size of 37 µm; and (b) graphene nanoplatelets with a thickness of 6–8 nm and a purity of 99.5%.
Figure 2. Cross sections of (a) aluminum powder with a chemical purity of 99.5% and an average particle size of 37 µm; and (b) graphene nanoplatelets with a thickness of 6–8 nm and a purity of 99.5%.
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Metals 13 00943 g003 550
Figure 3. Production process of the Al/GNP nanocomposites.
Figure 3. Production process of the Al/GNP nanocomposites.
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Metals 13 00943 g004a 550Metals 13 00943 g004b 550
Figure 4. SEM images of the worn surfaces with numbers indicating each zone of the EDS analysis (a) Al with 0.1 wt.% GNPs (left image ×1000; right image ×80); (b) Al with 0.5 wt.% GNPs (left image ×1000; right image ×80); (c) Al with 1.1 wt.% GNPs (left image ×1000; right image ×80); (d) annealed Al with 0.1 wt.% GNPs and Al4C3 (left image ×1000; right image ×80); (e) annealed Al with 0.5 wt.% GNPs and Al4C3 (left image ×1000; right image ×80); (f) annealed Al with 1.1 wt.% GNPs and Al4C3 (left image ×1000; right image ×80).
Figure 4. SEM images of the worn surfaces with numbers indicating each zone of the EDS analysis (a) Al with 0.1 wt.% GNPs (left image ×1000; right image ×80); (b) Al with 0.5 wt.% GNPs (left image ×1000; right image ×80); (c) Al with 1.1 wt.% GNPs (left image ×1000; right image ×80); (d) annealed Al with 0.1 wt.% GNPs and Al4C3 (left image ×1000; right image ×80); (e) annealed Al with 0.5 wt.% GNPs and Al4C3 (left image ×1000; right image ×80); (f) annealed Al with 1.1 wt.% GNPs and Al4C3 (left image ×1000; right image ×80).
Metals 13 00943 g004aMetals 13 00943 g004b
Metals 13 00943 g005 550
Figure 5. EDS spectra of the worn surfaces: (a) Al with 0.1 wt.% GNPs, related to Table 1, analysis 1; (b) Al with 0.5 wt.% GNPs, related to Table 2; (c) Al with 1.1 wt.% GNPs, related to Table 3; (d) annealed Al with 0.1 wt.% GNPs and Al4C3, related to Table 4; (e) annealed Al with 0.5 wt.% GNPs and Al4C3, related to Table 5; (f) annealed Al with 1.1 wt.% GNPs and Al4C3, related to Table 6.
Figure 5. EDS spectra of the worn surfaces: (a) Al with 0.1 wt.% GNPs, related to Table 1, analysis 1; (b) Al with 0.5 wt.% GNPs, related to Table 2; (c) Al with 1.1 wt.% GNPs, related to Table 3; (d) annealed Al with 0.1 wt.% GNPs and Al4C3, related to Table 4; (e) annealed Al with 0.5 wt.% GNPs and Al4C3, related to Table 5; (f) annealed Al with 1.1 wt.% GNPs and Al4C3, related to Table 6.
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Metals 13 00943 g006 550
Figure 6. XRD pattern of the sample containing 1.1 wt.% GNPs before (black curve) and after (red curve) the HT. The peaks of aluminum, graphene and the amorphous phase are labeled as Al and G. The peak of graphene corresponds to the (002) plane. The asterisks (*) denote the beta lines of aluminum. The small peak at 54° 2θ in the HT sample is a probable trace peak of Al4C3 (o). Adapted from [16].
Figure 6. XRD pattern of the sample containing 1.1 wt.% GNPs before (black curve) and after (red curve) the HT. The peaks of aluminum, graphene and the amorphous phase are labeled as Al and G. The peak of graphene corresponds to the (002) plane. The asterisks (*) denote the beta lines of aluminum. The small peak at 54° 2θ in the HT sample is a probable trace peak of Al4C3 (o). Adapted from [16].
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Metals 13 00943 g007 550
Figure 7. TEM analysis of the sample after the HT; (a) the HRTEM image and corresponding Fourier filtered image of the presented carbide phase Al4C3; (b) the Fourier filtered image of the interface of the Al matrix and the Al4C3 carbide.
Figure 7. TEM analysis of the sample after the HT; (a) the HRTEM image and corresponding Fourier filtered image of the presented carbide phase Al4C3; (b) the Fourier filtered image of the interface of the Al matrix and the Al4C3 carbide.
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Metals 13 00943 g008 550
Figure 8. CF vs. sliding distance test result of Al with GNPs (0–1.1 wt.%) and the annealed Al with GNPs (0–1.1 wt.%) and Al4C3 at a 30 N load, 0.9 m∙s−1 sliding speed and 540 m sliding distance under dry sliding friction conditions at room temperature.
Figure 8. CF vs. sliding distance test result of Al with GNPs (0–1.1 wt.%) and the annealed Al with GNPs (0–1.1 wt.%) and Al4C3 at a 30 N load, 0.9 m∙s−1 sliding speed and 540 m sliding distance under dry sliding friction conditions at room temperature.
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Metals 13 00943 g009 550
Figure 9. Mass wear of Al with GNPs (0–1.1 wt.%) and the annealed Al with GNPs (0–1.1 wt.%) and Al4C3 at a 30 N load, 0.9 m∙s−1 sliding speed and 540 m sliding distance under dry sliding friction conditions at room temperature.
Figure 9. Mass wear of Al with GNPs (0–1.1 wt.%) and the annealed Al with GNPs (0–1.1 wt.%) and Al4C3 at a 30 N load, 0.9 m∙s−1 sliding speed and 540 m sliding distance under dry sliding friction conditions at room temperature.
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Table
Table 1. EDS analysis of the selected zones from Figure 4a of the worn surface of Al with 0.1 wt.% GNPs at a load of 30 N and a linear speed of 0.9 m∙s−1, mass norm, %.
Table 1. EDS analysis of the selected zones from Figure 4a of the worn surface of Al with 0.1 wt.% GNPs at a load of 30 N and a linear speed of 0.9 m∙s−1, mass norm, %.
No. AnalysisCAlFe
182.7415.850.67
2-97.702.30
Table
Table 2. EDS analysis of the selected zones from Figure 4b of the worn surface of Al with 0.5 wt.% GNPs at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
Table 2. EDS analysis of the selected zones from Figure 4b of the worn surface of Al with 0.5 wt.% GNPs at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
No. AnalysisCAlFe
177.5710.3812.04
26.9749.8643.17
Table
Table 3. EDS analysis of the selected zones from Figure 4c of the worn surface of Al with 1.1 wt.% GNPs at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
Table 3. EDS analysis of the selected zones from Figure 4c of the worn surface of Al with 1.1 wt.% GNPs at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
No. AnalysisCAlFe
187.348.953.71
274.0918.557.36
313.7061.3824.91
Table
Table 4. EDS analysis of the selected zones from Figure 4d of the worn surface of the annealed Al with 0.1 wt.% GNPs and Al4C3 at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
Table 4. EDS analysis of the selected zones from Figure 4d of the worn surface of the annealed Al with 0.1 wt.% GNPs and Al4C3 at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
No. AnalysisCAlFe
182.368.129.53
213.1549.2237.63
Table
Table 5. EDS analysis of the selected zones from Figure 4e of the worn surface of the annealed Al with 0.5 wt.% GNPs and Al4C3 at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
Table 5. EDS analysis of the selected zones from Figure 4e of the worn surface of the annealed Al with 0.5 wt.% GNPs and Al4C3 at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
No. AnalysisCAlFe
171.8226.002.18
262.5035.192.30
3-85.8914.11
Table
Table 6. EDS analysis of the selected zones from Figure 4f of the worn surface of the annealed Al with 1.1 wt.% GNPs and Al4C3 at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
Table 6. EDS analysis of the selected zones from Figure 4f of the worn surface of the annealed Al with 1.1 wt.% GNPs and Al4C3 at a load of 30 N and linear speed of 0.9 m∙s−1, mass norm, %.
No. AnalysisCAlFe
172.3016.4611.23
252.4631.175.04
312.9465.7521.31
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Kolev, M.; Lazarova, R.; Petkov, V.; Mourdjeva, Y.; Nihtianova, D. Investigating the Effects of Graphene Nanoplatelets and Al4C3 on the Tribological Performance of Aluminum-Based Nanocomposites. Metals 2023, 13, 943. https://doi.org/10.3390/met13050943

AMA Style

Kolev M, Lazarova R, Petkov V, Mourdjeva Y, Nihtianova D. Investigating the Effects of Graphene Nanoplatelets and Al4C3 on the Tribological Performance of Aluminum-Based Nanocomposites. Metals. 2023; 13(5):943. https://doi.org/10.3390/met13050943

Chicago/Turabian Style

Kolev, Mihail, Rumyana Lazarova, Veselin Petkov, Yana Mourdjeva, and Diana Nihtianova. 2023. "Investigating the Effects of Graphene Nanoplatelets and Al4C3 on the Tribological Performance of Aluminum-Based Nanocomposites" Metals 13, no. 5: 943. https://doi.org/10.3390/met13050943

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