Synergistic Effect of B₄C and Multi-Walled CNT on Enhancing the Tribological Performance of Aluminum A383 Hybrid Composites

Abstract

The requirement for high-performance and energy-saving materials motivated the researchers to develop novel composite materials. This investigation focuses on utilizing aluminum alloy (A383) as the matrix material to produce hybrid metal matrix composites (HMMCs) incorporating boron carbide (B₄C) and multi-walled carbon nanotube (MWCNT) through a cost-effective stir casting technique. The synthesis of HMMCs involved varying the weight fractions of B₄C (2%, 4%, and 6%) and MWCNT (0.5%, 1%, and 1.5%). The metallographic study was carried out by field emission scanning electron microscopy (FESEM) mapped with EDS analysis. The results indicated a uniform dispersion and robust interfacial interaction between aluminum and the reinforced particles, significantly enhancing the mechanical properties. Micro-hardness and wear characteristics of the fabricated HMMCs were investigated using Vickers microhardness testing and the pin-on-disc tribometer setup. The disc is made of hardened chromium alloy EN 31 steel of hardness 62 HRC. The applied load was varied as 10N, 20N, 30N with a constant sliding speed of 1.5 m/s for different sliding distances. The micro-hardness value of composites reinforced with 1.5 wt% MWCNT and 6 wt% B₄C improved by 61% compared to the base alloy. Additionally, the wear resistance of the composite material improved with increasing reinforcement content. Incorporating 1.5% CNT and 6% B₄C as reinforcements results in the composite experiencing about a 40% reduction in wear loss compared to the unreinforced aluminum alloy matrix. Furthermore, the volumetric wear loss of the HMMCs was critically analyzed with respect to different applied loads and sliding distances. This research underscores the positive impact of varying the reinforcement content on the mechanical and wear properties of aluminum alloy-based hybrid metal matrix composites.

1. Introduction

The production of lightweight and high-strength materials has consistently been a requirement of industries in order to attain performance as well as economic advantages. The demand for the development of sustainable materials with a goal of less resource consumption with minimal waste has gained eminence in the transport industry. Recognizing the limitations of single-material solutions, industries are swiftly embracing composites for their excellent mechanical properties and cost-effectiveness. This shift towards innovative, sustainable solutions reflects industry’s response to advancing technology and escalating material demands towards composite materials [1]. Aluminum metal matrix composites (MMCs) have been employed in the aerospace and automobile industries over the past few decades due to their excellent specific strength, high stiffness, lightweight, good wear-corrosion resistance, etc. [2,3].

Reinforcement of ceramic carbide particles in the aluminum MMCs notably improved the mechanical, electrical, tribological, and thermal properties. Some common reinforcement such as alumina (Al₂O₃) [4], silicon carbide (SiC) [5], tungsten carbide (WC) [6], boron carbide (B₄C) [7], titanium carbide (TiC) [8], etc. have been employed as reinforcement particles to enhance the properties of aluminum-based MMCs. Alumina particles reinforced with AA8011 alloy composites were fabricated and the wear properties were found to be improved with the addition of more alumina particles [9]. Additionally, B₄C stands out as a favored reinforcement material for synthesizing aluminum metal matrix composites, owing to its outstanding hardness, which is nearly equivalent to that of diamond, and its elevated elastic modulus. [10]. With incorporation in aluminum alloy, it provides numerous applications from bulletproof vests to blasting/cutting instruments, brake liners, anti-ballistic armored plates, etc. [11]. Baradeswaran et al. [12] studied the tribological behavior of B₄C reinforced AA7075 composite, where the wear resistance of the aluminum alloy was significantly enhanced due to the addition of boron carbide particulates. Similarly, Minqiang et al. [13] pointed out the enhancement in mechanical properties of AA6061-based MMC with micro- and nano-sized B₄C particulates.

The size of the reinforcement particles greatly determines the overall properties of the aluminum composites. By lowering the size of the reinforcing particles from a micro-scale to a nanoscale significantly enhances the mechanical properties of the composites. Recent advancements in carbonaceous nanomaterials underscore the importance of studying the properties of carbon nano-tubes (CNTs) within AMMCs for their engineering applications [14]. Over the past few years, numerous researchers have investigated the contribution of carbon nanotubes (CNTs) in enhancing the strength of aluminum hybrid metal matrix composites. The high aspect ratio with excellent mechanical properties makes them most suitable for reinforcing material in composites. The combined effect of TiC and CNT on aluminum MMCs was investigated and it was found that the mechanical and wear properties were improved for the composites [15]. Moreover, the addition of the multi-walled CNT primarily restricted the loss of ductility of the MMCs. Similarly, the incorporation of CNT into alumina-reinforced aluminum composites effectively exhibited excellent tensile strength as a result of an increase in the interfacial bonding and restriction crack initiation due to CNT [16]. Zhang et al. [17] studied the reinforcing effect of SiC and CNT toward the increase in mechanical strength of aluminum hybrid composites. Moreover, the pinning effect of CNT with interfacial bonding resulted in an improvement in the mechanical behavior of Al/SiC composites.

It is crucial to note that the addition of B₄C alone strengthened the aluminum alloy, however, a significant amount of loss in ductility and toughness of the composites was observed as mechanical properties inversely affect ductility. Therefore, the addition of CNT as the secondary reinforcement is useful for restricting the loss in toughness while further strengthening the composites. Moreover, the addition of CNTs often results in the formation of agglomeration and causes decreased wettability as a result of weak Van der Waals forces interaction. Thus, the combination of dual reinforcements such as micro and nano reinforcement particles efficiently strengthened the aluminum composites. Saba et al. [18] studied the effect of micro-phased TiC and nano-phased CNT reinforced aluminum composites where it was observed that the tensile strength was considerably enhanced without loss in ductility. Earlier investigations show a very limited number of utilizations of aluminum A383 alloy in composite usage, despite its notable wear and corrosion resistance, exceptional mechanical properties, machinability, and castability. The current study focuses on manufacturing hybrid composites by incorporating micro-sized B₄C and nano-sized CNT into the A383 alloy, examining their microstructural characteristics and wear properties through a low-cost stir casting process. The analysis of the microstructure had the aim to understand the dispersion of the reinforcement phases and their impact on the interfacial strength and wear properties of the A383-B₄C-CNT hybrid composites.

2. Materials and Methods

For this research, we procured commercially available aluminum A383 non-heat treatable alloy ingot from Parshwamani Metals, Mumbai, India. The chemical composition of the received aluminum A383 alloy is provided in

Table 1. Chemical composition of aluminum A383 alloy.

Elements	Si	Cu	Mg	Zn	Fe	Mn	Al
wt.%	10.13	3.02	0.26	0.58	0.55	0.32	Remaining

below.

As reinforcement materials in this research, MWCNT and B₄C were utilized. Reinforced material B₄C powder particles were obtained from Parshwamani Metals, Mumbai, India, greater than 99% pure and with an average particle diameter of 20–40 microns. Ad-Nano Technologies Pvt. Ltd., Karnataka, India, supplied multi-walled CNTs with a diameter of 10–15 nm and a length of 5 μm. These CNTs have a purity of greater than 98% and an average diameter of 10–15 nm. The morphological analysis of reinforcement powder particles such as with SEM analysis was undertaken and is shown in Figure 1a,b. Multiple concentric single-walled CNTs compose a CNT with multiple walls. CNTs are a more competitive reinforcing material than ceramic particles as their coefficient of thermal expansion (CTE) is nearly zero.

The aluminum hybrid composites comprised of B₄C and MWCNT were manufactured utilizing a low-cost mechanized stir casting technique. In the beginning, aluminum A383 alloy was slowly melted in an electric furnace up to a temperature of 750 °C. After the aluminum alloy has completely melted, preheated (at 400 °C) B₄C and MWCNT wrapped in aluminum foil were added separately to the melt. Preheating of the reinforcement particles was necessary to remove any moisture presence and minimize the chilling effect. Further, preheating is also necessary as the elevated temperature process induces dislocations and porosity in the vicinity of the reinforcing particles. A small amount of magnesium (10 gm) billet is added to the aluminum melt to improve the wettability during the processing. For good dispersion of the reinforcement particles, a mechanized stirring is required at 450 rpm for 15 min after the addition of the reinforcement particulates. The literature explained that maintaining the optimum stirring speed at 450 rpm resulted in a homogenous distribution of the low-density particles in fabricating aluminum composites [19]. Furthermore, a steady supply of argon gas was consistently introduced into the heating chamber to mitigate any potential reactions between molten aluminum and atmospheric gases. After ensuring thorough mixing of the CNT, the molten material was subsequently poured into a preheated metallic mold to produce the composites, following the removal of any slag residue from the surface of the molten material. It is to be noted that the mold (150 mm × 100 mm × 6 mm) is made up of hardened steel that can withstand high temperatures. Figure 2 shows a schematic diagram illustrating the experimental setup used for the synthesis of the composites. The detailed composition of the produced composite is depicted in

Table 2. Composition of the hybrid MMC specimen.

MMC Specimen	B4C (%)	CNT (%)
C0	0	0
C1	2	0.5
C2	4	0.5
C3	6	0.5
C4	2	1
C5	4	1
C6	6	1
C7	2	1.5
C8	4	1.5
C9	6	1.5

. For each composition, three specimens were prepared for different testing.

The microstructural evaluation of hybrid composites was explored by using FESEM with EDS. The specimen for microstructural evaluation was polished using emery paper of grades 400, 600, 800, 1000, 1200, and 1500. Final cotton polishing was accomplished with diamond paste and spray. Further, the polished samples were chemically etched using an etchant Keller’s reagent (95% distilled water, 2.5% HNO3, 1.5% HCl, 1% HF). The microhardness test of the produced composites was performed using a semi-automatic Vickers hardness testing machine with a load of 500 gf and a dwell time of 10 s. The hardness value was evaluated at five separate spots on the surface of the specimen, and the average value was taken into account to evaluate the final hardness value of the specimen.

Furthermore, the DUCOM pin on disc tribometer was utilized to examine the composite’s wear behavior. The pin-on-disc tribometer used for the wear study in this research was the following make: DUCOM, TR-20LE, Bangalore, India). In accordance with the ASTM G99-04 standard [20], rectangular specimens measuring 30 mm × 6 mm × 6 mm were machined from the cast product. Before the wear testing, the pins were cleaned with acetone and polished to remove any unwanted particles. The counter disc is composed of EN31-hardened steel and was meticulously cleaned before the start of the testing. The disc is made of hardened chromium alloy EN 31 steel of hardness 62 HRC. The initial roughness of the composite disc is kept at less than 0.05 μm. On the tribometer, the height loss of the specimen due to sliding wear is continuously recorded. By multiplying the height loss while sliding by the specimen’s cross-sectional area, the volumetric wear loss can be calculated. The study of wear rate is determined by the ratio of volumetric wear loss to the sliding distance. Similarly, the tribometer continuously records the frictional force generated by the sliding operation. The ratio of frictional force to applied normal force during the wear test will provide the coefficient of friction of the mating parts between the aluminum hybrid composite and the counter disc material. The input parameters such as applied load (10 N, 20 N, 30 N), and sliding distance up to 1000 m were varied for different compositions of the composite specimen. The sliding speed was kept constant at 1.5 m/s in this wear study. All the tribological tests were repeated three times to arrive at a conclusion for the final value. A detail of the research flow chart is presented in Figure 3.

3. Results and Discussion

3.1. Microstructural Analysis

Figure 4 depicts the morphological analysis, including FESEM and EDS spectrum analysis of casted aluminum A383 alloy composites. The SEM images reveal a homogeneous dispersion of B₄C particles within the composite. This uniform distribution of B₄C is primarily due to the effective mechanical stirring during the melting process. Achieving uniform dispersion of reinforcement particles is essential for enhancing the mechanical and tribological properties of the composites. Additionally, the composites exhibited no casting defects, indicating strong bonding between the matrix and reinforcement interface. SEM spectra in Figure 4a–c confirmed the absence of B₄C particle clusters or accumulation in the composites, even with higher content, validating the effectiveness of the casting process. It is important to note that the lack of particle agglomeration indicates robust bonding at the aluminum–B₄C interface, likely bolstering the performance of the composites. Further, the higher magnification spectra also revealed that the CNT fibers are very much intact with the boron carbide particles visible in Figure 4c. Moreover, B₄C and alloying elements of A383 alloy were identified through EDX analysis of Al/B₄C composites, as illustrated in Figure 4. The peaks corresponding to B and C in the EDX spectrum confirmed the presence of B₄C particles within the composites.

3.2. Elemental Mapping

Figure 5 shows the elemental analysis of cast aluminum alloy MMCs (Al + 1% MWCNT + 6% B₄C). The elemental mapping of the fabricated composites also shows the presence of major elements of the hybrid composites and the distribution of the main alloying elements in it. Uniform distribution of B₄C and MWCNT reinforcement particles shows also that there is no agglomeration of any reinforcement particles present. It is observed that the boron and carbon elements are dispersed homogeneously as evident from the elemental mapping.

Similarly, elemental analyses of cast aluminum alloy MMCs (Al + 1.5% MWCNT + 6% B₄C) are shown in Figure 6. Uniform distribution of B₄C reinforcement particles is shown in a similar manner to other compositions. Also, at a few sites, composite, agglomeration of MWCNT particles could be visible. During the incorporation of MWCNTs into the aluminum melt, certain particles seemed to rise to the surface because of changes in density and surface tension. Nevertheless, through prolonged stirring and additional stirring time, these particles dispersed and remained suspended within the melt. Optimal stirring speed facilitated the uniform dispersion of the reinforcement particles, with vortex formation induced by the stirring ensuring homogeneous distribution.

3.3. Microhardness Behavior

The microhardness test was carried out with the help of a Vickers microhardness testing machine at 500 gf load and dwell time of 10 s. Figure 7 depicts the variation in microhardness with a variation in weight percentage of B₄C and MWCNT in the composites. From the image, it can be viewed that the hardness increases as the percentage of weight increases of B₄C as well as MWCNT reinforcement particles. In comparison to the base alloy, the 1.5 weight percentage of MWCNT and 6 weight percentage of B₄C reinforced composites gave a 61% enhancement in the value of microhardness as can be seen in the figure. The improvement in hardness is attributed to the higher hardness characteristic of B₄C and MWCNT and its inclusion in the aluminum matrix. Also, the grain refinement present, because of the thermal conductivity of the MWCNT particle, is exceptional [16]. In addition, the higher hardness value of the composites indicates that the hard and rigid ceramic in the composites increased the resistance to indentation. In addition, reinforcement (B₄C and MWCNT) present in the aluminum matrix provides a greater obstacle to the plastic deformation of the composite material [21]. It is important to note that the agglomerated particles present may fail to interact efficiently with the matrix, resulting in localized stress points that diminish the composite material’s hardness. Additionally, agglomeration can create voids, further weakening the material and decreasing its hardness. The increase in the hardness can be attributed to the absence of agglomeration or cluster as seen on the SEM micrographs.

3.4. Tribological Analysis

The incorporation of ceramic reinforcement within the soft matrix phase led to enhancements in wear resistance properties. Additionally, the wear characteristics of the materials were influenced by factors such as sliding distance, sliding speed, and applied load. The wear test was conducted on a pin-on-disc tribometer for a 1000 m sliding distance at a speed of 1.5 m/s with applied loads of 10 N, 20 N, and 30 N. As the sliding distance increases, there is a gradual increase in material loss due to wear. The volume wear loss of the composites at 10 N applied load for the progressive sliding distance is shown in Figure 8. The pure aluminum matrix alloy exhibits the highest volumetric wear loss, attributed to its heightened plastic deformation on the surface under the specified load condition. As the amount of reinforcement was increased, there was a notable reduction in volumetric wear loss, suggesting an enhancement in the wear resistance of the composites against the plate’s counter surface. This improvement can be attributed to the integration of rigid B₄C particles and CNT within the pliable aluminum alloy, which effectively reduced the actual contact area between the abrasive mating parts, thus significantly improving wear resistance. Moreover, the presence of tough abrasive particles serves to impede the plastic deformation and material flow of the composite surface. Specifically, when incorporating 1.5% CNT and 6% B₄C as reinforcement, the composite demonstrates a notable reduction of approximately 40% in wear loss compared to the unreinforced aluminum alloy matrix.

Moreover, the inclusion of B₄C and CNT reinforcements in the matrix demonstrates a notable enhancement in wear resistance. This improvement stems from the heightened hardness and a greater resistance to plastic deformation. These augmented properties play a pivotal role in bolstering wear resistance and reducing the likelihood of thermal wear [22]. Moreover, the increased content of reinforcement leads to a decrease in interparticle spacing among the ceramic particles, thereby exposing a greater number of B₄C particles to the counter surface and subsequently enhancing the resistance to plastic deformation. Furthermore, the successful casting process resulted in the creation of a thermodynamically stable, clean B₄C interface with CNT fibers characterized by strong bonding and favorable wettability at the interface. These benign effects complement the overall enhancement in wear resistance. As a result of persistent sliding between the material and the counter surface, wear grooves appeared on the test specimen. Debris generated during this process accumulated within these wear grooves on the pin surface. Simultaneously, the elevation in temperature at the contact points facilitated the formation of a protective oxide layer as shown in Figure 9. This layer acted as a barrier, mitigating direct metallic contact and subsequently a reduction in the wear rate. It was observed that following the formation of this oxide layer, the wear rate declined as the sliding distance increased. However, it is of note that excessive temperature increases could potentially lead to plastic deformation of the pin surface.

Figure 10 illustrates the impact of the applied load on volumetric wear loss at a sliding distance of 1000 m. Wear resistance of the composite mainly depends on a few factors such as high hardness and better mechanical properties of the reinforcement particles, load bearing capacity, reduced contact area, and the formation of a solid lubricant tribo-layer of MWCNTs. According to the graph, there is a noticeable decrease in material wear loss with the addition of B₄C and CNT ceramic particles under specific loads. Additionally, the wear loss graph exhibits greater rigidity when the load shifts from 20 N to 30 N compared to 10 N to 20 N over a consistent sliding distance. This disparity may stem from increased heat generation at higher loads, facilitating the formation of oxide layers and subsequently enhancing wear resistance. Both the base alloy and the composites display a similar pattern under each loading condition. The matrix alloy exhibits the highest wear loss due to its surface undergoing more plastic deformation under high-loading conditions. The composites, however, show a decrease in abrasive wear loss with an increase in the percentage of reinforcements such as B₄C and CNT, indicating an improvement in wear resistance. This is because the incorporation of hard B₄C particles into the soft aluminum alloy significantly enhances its hardness by reducing the actual contact area between the two abrasive mating parts. Also, as the load increases, friction between the mating surfaces increases due to this heat generation at the mating surfaces, and because of this the contact area between the mating surfaces increases, which in turn increases the wear loss significantly [23].

Continuous friction between the sample and the abrasive contact surface induces material wear, resulting in the formation of wear grooves and small scratches due to plastic deformation. This process is often accompanied by thermal softening and melting. The morphology of the wear surfaces in the aluminum alloy and reinforced composites was investigated using FESEM and is presented in Figure 11. Observations revealed different mechanisms such as delamination, grooving, oxide formation, and wear debris on both the base alloy and composites. Material wear occurs as the specimen pin slides against the abrasive surface, causing plastic deformation due to the hardness of the abrasive. This deformation results in the removal of surface materials, initiates crack formation, and promotes parallel surface crack propagation. Wear grooves develop on the surface as a result of the ploughing action caused by wear debris during the delamination process of the contact surface. When the applied load increases during wear, the size of the wear debris also increases, consequently leading to the formation of larger and deeper wear grooves. Previous research noted that the wear grooves in materials typically align parallel to the sliding direction [24]. The size of these grooves is influenced by both the applied load and the sliding speed, as abrasive particles remove material fragments either in the form of powder or strips. Additionally, an increase in the reinforcement content results in the transformation of large grooves into finer ones, indicating an improvement in the wear resistance of the composite material [25,26].

Friction occurring between the pin and the abrasive surface results in heat generation, causing the formation of oxide on their contact surface. As the pin continuously slides on the abrasive disc surface, peaks and valleys form on the pin’s surface. These valleys accumulate wear debris, which, under the influence of heat, transforms into oxide, creating a protective layer on the contact surface of the sample. This protective layer serves to mitigate direct contact between the rotating disc and the pin sample, consequently reducing wear loss. A significant quantity of oxide particles adhere to the worn surface, resulting in the formation of an oxide layer that contributes to reducing the material’s wear loss [27]. The structure of aluminum-based alloy exhibited significant delamination, indicating adhesive wear. This was particularly evident in the A383 alloy, which, due to its ductile nature, struggled to withstand the heat generated during continuous sliding. Consequently, the alloy experienced substantial delamination due to plastic deformation. Conversely, when hard and stiff particulates were introduced, as in the case of composites, the resistance to plastic deformation improved.

Consequently, the delamination characteristic was replaced by a ploughing mechanism. This mechanism facilitated the production of deep grooves parallel to the sliding direction along the contact surface of the aluminum metal matrix composites (MMCs). The rough and uneven ploughing action had the capability to extract large particles from the contact interfaces along the sliding direction, resulting in the formation of large grooves [28]. These grooves, which were more pronounced with increased sliding distances, revealed the abrasive nature of the wear. With higher concentrations of hard and rigid reinforcements, the excellent bonding between particles acted as a barrier against sliding, leading to reduced wear and minimal or absent delamination, indicating enhanced wear resistance. Abrasive wear, attributed to hard asperities on sliding surfaces, was consistently observed across different weight percentages in the composites. This ploughing action, resulting from two-body or three-body abrasions, caused the formation of deep parallel grooves on the composite surface [29]. The dominance of delamination in unreinforced aluminum alloy indicates its adhesive wear nature, while the presence of ploughing action in composites signifies abrasive wear behavior.

Additionally, Figure 12 illustrates the surface morphologies of the wear debris observed during the wear test. The wear debris exhibited a combination of fine particles and larger flake debris, as evident from the SEM micrographs. The formation of wear debris can be attributed primarily to several factors: first, the shearing of asperities at the interfaces of the pin and disc, facilitated by localized high temperatures; second, the breakdown of the thin tribo-layer (mechanically mixed layer) due to continual rubbing; and third, the disintegration of the harder reinforcement phase from the composites. The presence of large-sized flake debris is indicative of the delamination wear mechanism. Notably, micro-cracks were discernible within the flake debris, suggesting an additional mechanism contributing to delamination [30]. Furthermore, the continuous sliding motion resulted in the fragmentation of these large flakes into smaller debris. Particularly under higher loads, this process led to the generation of fractured debris, as depicted in the micrographs.

4. Conclusions

Aluminum-based metallic matrix composite involving copper silicon alloy (AA383) as a matrix with different combinations of reinforcements was fabricated successfully, and the following observations were made:

• The microstructural study of the composites with different reinforcements shows a homogenous distribution of the reinforcements in the matrix made by the stir casting technique.
• The micro-hardness value of the composites improves with an addition of reinforcement, and the composite with 6 wt% of B₄C and 1.5 wt% of MWCNT as reinforcements shows a 61% higher microhardness value than that of the base material alloy.
• The tribological analysis of the composites shows that the addition of reinforcement particles such as B₄C and MWCNT increases the wear resistance of the material and decreases the 40% volumetric wear loss as compared to the base matrix alloy.
• Volumetric wear loss of the composites decreases as the applied load increases and at 30N loading condition, minimum wear loss was found. The prevalence of delamination in unreinforced aluminum alloy suggests adhesive wear characteristics, whereas the occurrence of ploughing in composites indicates abrasive wear behavior.