Experimental investigation of coconut shell charcoal and boron carbide/zirconium dioxide in aluminium 7075 matrix composites under varying operating conditions: a comparative analysis

This study focused on investigating the influence of varying weight percentages of coconut shell charcoal (CSC) on the physical, mechanical, and wear properties of aluminum 7075 (Al-7075) matrix composites reinforced by boron carbide (B4C) and zirconium dioxide (ZrO2). Throughout the study, these composites were prepared with a constant 3 wt% B4C/ZrO2. The developed composites were then subjected to the tribological analysis using a pin-on-disc apparatus under a constant sliding speed (2000 m), a sliding distance (4 m s−1), and varying operating temperatures (room temperature, 150, 200, and 250 °C). The worn-out surfaces of the composites were examined using a magnified scanning electron microscope (SEM) to investigate the microstructural analysis and wear behavior. The composites containing 3% B4C/ZrO2 and 3% CSC exhibited the highest hardness and wear resistance among the studied composites. The incorporation of CSC increased the density and porosity of the composites up to a 3 wt%, but at 4 wt%, a decrease was observed. It is worth noting that the B-series samples had slightly lower hardness values compared to the Z-series samples. Regarding the effect of temperature, the wear rate decreased as the temperature increased. The Al-7075/ZrO2/CSC composite demonstrated improved wear resistance and coefficient of friction (COF) compared to the Al-7075/B4C/CSC composite, with respective enhancements of 19.30% and 42.19%. The analysis of variance (ANOVA) confirmed the significant impact of CSC weight fraction on wear for both composites, whereas only the Al-7075/ZrO2/CSC composites showed significance for COF. The SEM analysis revealed that the addition of CSC resulted in a uniform distribution of particles within the matrix, leading to improved wear resistance. Therefore, this study demonstrated that the addition of CSC influenced the density, porosity, hardness, wear resistance, and COF in the Al-7075 matrix composites. Optimal performance was achieved with a 3 wt% CSC for Al-7075/ZrO2/CSC at 250 °C. The composites developed in this study, comprising boron carbide (B4C)- and zirconium dioxide (ZrO2)-reinforced aluminum 7075 (Al-7075) matrix with varying weight percentages of CSC, have several potential applications such as in the fields of automotive, aerospace, defense, and industrial machinery.


Introduction
Metal-matrix composites (MMCs) have gained significant attention across various industrial sectors due to their exceptional mechanical properties, including high strength, stiffness, and wear resistance.These qualities make them highly suitable for high-performance applications.In particular, aluminum-based MMCs have attracted special interest due to their high specific strength, low density, and excellent machinability.Moreover, the incorporation of ceramic reinforcements such as silicon carbide (SiC) [1], alumina (Al 2 O 3 ) [2], boron nitride (BN) [3], titania (TiO 2 ), boron carbide (B 4 C) [4], and zirconium dioxide (ZrO 2 ) [3] has been proven to considerably enhance the mechanical and wear properties of Al MMCs.
Aluminum alloys are commonly used in various applications, with different series designated by numerical identifiers such as 2000, 5000, 6000, and 7000.While each series possesses unique properties and applications, the 7xxx series is often regarded as one of the top choices.This series is known for its excellent strength-toweight ratio, making it highly desirable in industries requiring lightweight materials with high strength [5].Zinc serves as the primary alloying element in this series, along with copper, magnesium, and occasionally other elements [6].These alloying elements contribute to the remarkable strength characteristics of the 7xxx series.Among the key alloys in this series, 7075 stands out for its exceptional strength and toughness.It finds widespread use in aerospace applications, particularly for aircraft structures, wings, fuselages, and critical components [7].The high strength of 7075 renders it suitable for demanding applications that necessitate reliable performance under extreme conditions [8].While the 7xxx series alloys offer impressive strength, it is important to note that they may not exhibit the same level of corrosion resistance as some other aluminum series [9].However, the inclusion of ceramic particles can mitigate corrosion while also enhancing strength.
Ashok Kumar et al [10] conducted a study on aluminum alloys reinforced with crushed rock sand and SiC and investigated various weight proportions ranging from 0% to 6%.The researchers used response surface methodology based on a central composite design to determine and optimize the weight percentage of reinforcements, taking into account the mechanical properties [11].The results indicated that the composite with 3%wt SiC showed superior mechanical properties, exhibiting a maximum density of 2.766 g cm −3 .Similarly, Robinson Smart et al [12] investigated the wear and mechanical properties of Al-7075 MMCs reinforced with tantalum carbide (TiC), silicon nitride (Si 3 N 4 ), and titanium (Ti).Using the stir casting process, the authors produced five composites with varying weight percentages of reinforcements.Wear performance was assessed using a pin-on-disc apparatus to evaluate wear resistance at both ambient and high temperatures, based on the coefficient of friction (COF) and wear rate.The findings showed that incorporating TiC, Si 3 N 4 , and Ti reinforcements into Al-7075 resulted in improved mechanical properties.The theoretical density was calculated to be 3.11 g cm −3 whereas the experimental density was measured at 3.09 g cm −3 , both slightly higher than those of the base alloy.In addition, it showed better tribological properties for MMCs containing 1 wt% TaC, 8 wt% Si 3 N 4 , and 2 wt% Ti.
Among the various ceramic particles used as reinforcements in MMCs, B 4 C and ZrO 2 have emerged as wellestablished ceramic reinforcements.These two materials have shown their efficacy in enhancing the mechanical properties of MMCs and are widely recognized for their unique characteristics and benefits.Boron carbide (B 4 C) is known for its exceptional hardness, high strength-to-weight ratio, and improved wear resistance [4,13].Also, ZrO 2 (zirconia) possesses remarkable thermal and chemical stability, making it suitable for applications involving high temperatures or aggressive environments.In addition, ZrO 2 has a unique property known as 'transformation toughening,' where it undergoes a phase transformation under stress, absorbing energy and preventing crack propagation [13,14].This property further enhances the mechanical strength and durability of the composite material.
Bellamkonda et al [15] conducted a study to evaluate the corrosion and wear behavior of Al-7075 MMCs reinforced with ZrO 2 particles using stir casting technique with varying weight percentages ranging from 0 to 4 wt%.The results revealed that ZrO 2 particles were uniformly distributed, which improved the wear resistance at 2 and 4 wt% reinforcement levels compared to the base alloy.In addition, the compression and impact strength of the MMCs increased with the addition of reinforcement from 0 to 4 wt%.Notably, at a 4 wt% reinforcement level, the MMCs showed higher corrosion resistance compared to Al-7075.
Researchers have been exploring various materials to enhance the properties of MMCs beyond traditional ceramic reinforcements.Alongside ceramics, agricultural residues such as rice husk ash, coconut husks ash, coconut shell ash (CSA), wheat straw ash, fly ash, bagasse, breadfruit seed, and egg shell have emerged as promising alternatives for reinforcing MMCs.These residues, which are by-products of agricultural processes, offer several advantages such as cost-effectiveness, easy availability, and eco-friendliness.Using agricultural residues as reinforcements in MMCs, researchers aim to not only improve the material properties but also contribute to sustainable development and reduce the environmental impact.Among these, coconut shell charcoal (CSC) is proven to be an excellent additional reinforcement in MMCs.It is an abundant and low-cost biomass material that has gained increasing attention in recent years.CSC has several desirable properties, such as high carbon content, low ash content, and high surface area, which make it a promising reinforcement material [16].
Raju et al [17] conducted a study on MMCs reinforced with CSA particles at ratios of 5%, 10%, and 15%.The researchers investigated the impact of CSA particles on the physical, mechanical, and wear properties of the prepared Al-CSA composite.Tribological analysis revealed a wear measurement of 260 μm, a specific wear rate of 1.43 × 10 -4 mm 3 /Nm, and a COF of 0.275 under constant parametric conditions (load: 30 N, CSA ratio: 15%, sliding distance: 1500 m, sliding speed: 1.5 m s −1 ).The findings showed a significant enhancement in the strength and wear resistance of the composite as the volume percentage of CSA increased compared to that of the base alloy.
Similarly, in a study by Arunkumar and Senthil Kumar [1], two combinations of reinforcements (Al 2 O 3 /SiC and Al 2 O 3 /eggshell powder) were used to fabricate Al-7075 MMCs.The weight proportions of SiC and eggshell powder were varied to investigate their impact on the composite properties.The composite containing 6% eggshell powder exhibited the highest surface hardness and lower density compared to synthetic materials.Moreover, the composites with 3% SiC and 6% eggshell powder showed good wear resistance.As the proportion of hard particles increased, the wear mechanism shifted toward abrasion, resulting in higher metal loss.The hybrid composite with 6% Al 2 O 3 and 6% SiC showed adhesive morphology and minimal wear, according to the results.Hence, it was concluded that adjusting the proportion of hard particles or increasing the quantity of eggshell powder could achieve the desired sliding wear behavior.Likewise, Gupta et al [8] used the stir casting method to create Al-7075 MMCs by incorporating different weight percentages of rice husk ash and carbonized eggshells.The combined weight percentage of both reinforcements was maintained at a constant value of 5 wt%.The findings indicated that the hybrid composite with 3.75 wt% rice husk ash and 1.25 wt% carbonized eggshells showed the highest tensile and compression strength.
In an extensive study, Kar and Surekha [18] investigated the effect of inclusion of red mud and TiC on the friction and wear properties of Al-7075 MMCs.The researchers used two types of reinforcement, namely red mud (industrial waste) and TiC, in Al-7075 using a liquid metallurgy method.Different levels of red mud and TiC particles were added to aluminum in increments of 3 wt%.The fabricated MMCs were then tested for dry sliding wear and friction characteristics using a pin-on-disc wear and friction monitor, both with and without heat treatment.The results showed that the developed samples exhibited enhanced specific strength and wear resistance compared to the base alloy.In addition, the Al-7075/TiC composite showed an 8% improvement in the COF and a 19% reduction in wear rate compared to the Al-7075/red mud composite.
Extensive literature suggests that the addition of CSC can improve the mechanical and tribological properties of Al-7075-B 4 C-ZrO 2 composites.The optimal B 4 C/ZrO 2 content varies depending on the specific experimental conditions and fabrication methods used, but generally falls within the range of 3-4 wt%.Similarly, the optimal amount of reinforcement for Coconut shell charcoal (CSC) is typically between 2-8 wt%.Incorporating this amount into composites can lead to notable enhancements in their mechanical and tribological properties as well as their machinability [10,16,[19][20][21].The improvement in properties is attributed to the formation of a uniform distribution of CSC particles in the matrix, which enhances the hardness and wear resistance of the composites.Similarly, the fabrication and testing will have some critical impact on the properties of the MMCs.From literature, it can be said that the conventional stir casting method is a costeffective and widely used method for fabricating MMCs, and the pin-on-disc wear test is a commonly used method for evaluating the wear behavior of MMCs.Scanning electron microscopy (SEM) is an essential tool to investigate the microstructural changes and wear mechanisms that occur during wear tests.
On the basis of recent literature reviews, it is clear that no previous research has explored the hybrid combination of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC.Furthermore, there is a lack of studies focusing on optimizing the content of CSC and the processing conditions to attain the most favorable mechanical and wear properties for specific applications.Therefore, this study aimed to investigate the effect of varying weight percentages of CSC on the mechanical and wear properties of Al-7075 MMCs reinforced with B 4 C/ZrO 2 .The study focused on investigating the effect of CSC on the wear behavior of the composites under different operating conditions, such as temperature and sliding speed.The results of this study will contribute to a better understanding of the potential use of CSC as a reinforcement material in MMCs and provide insights into the effect of operating conditions on the wear properties of MMCs.

Al-7075
Al-7075, a highly popular and widely used alloy, belongs to the 7xxx series of aluminum alloys known for their exceptional properties.The Al-7075 alloy was procured from Bharat Aerospace Metals, Mumbai, India.The composition of Al-7075 is shown in table 1.The table provides valuable information about the elemental makeup and proportions that contribute to the alloy's remarkable strength, durability, and other desired properties.The density of Al-7075 matrix is 2.81 g/c.c.

Boron carbide (B 4 C)
Boron carbide powder (size: <10 μm), composed of boron and carbon, is known for its exceptional hardness.It was procured from Bharat Aerospace Metals, Mumbai, India with the density of 2.52 g/c.c.It is often used in applications that require high abrasion resistance, such as cutting tools, nozzles, and armor plating [23].It has a very high melting point and is chemically inert (figure 1(a)).

Zirconia (ZrO 2 )
Zirconia (size: 30 μm) is a ceramic material that is commonly used in various industrial applications.The material was sourced from Bharat Aerospace Metals in Mumbai, India, with a density of 5.89 g /c .c .It exhibits high strength, hardness, and resistance to wear and corrosion.It also has a high melting point and low thermal conductivity (figure 1(b)).

Coconut shell charcoal (CSC)
Initially, mature coconut shells are gathered and external dirt or impurities are removed by gently rubbing them.The shells are then cracked into smaller pieces using a machete and carbonized mechanically in a controlled environment with limited oxygen.Mechanical furnaces with enclosed containers are used to carbonize the coconut shells, subjecting them to high heat in the absence of oxygen.This method enables faster and more controlled carbonization.Once the carbonization process is complete, the coconut shells are cooled down.Cooling can be achieved naturally by letting them cool in the open air or by using water to expedite the process.After cooling, the resulting charcoal is collected.The collected charcoal is crushed and ball-milled to reduce particle size and obtain a powdered form.The powder is then sieved using a sieve shaker for 30 min to achieve a particle size within the range of 30-40 μm, as shown in figure 1(c).The density of CSC can vary based on the specific composition and manufacturing process.By this type of manufacturing process, the density is found to be 2.32 g cc −1 .

Methods
The fabrication of the Al-7075 MMCs was done using SwamEquip Bottom Pouring Stir Casting Machine, as shown in figure 2. This technique was chosen among various types of processing techniques due to its ability to minimize oxide formation on the molten surface, leading to reduced porosity in the resulting castings and widely acceptable in many industries [24,25].Initially, small pieces of wrought Al-7075 were placed in a crucible for melting.Subsequently, the mold, with a diameter of 100 mm, was preheated at 420 °C for 45 min to eliminate any moisture, ensure even solidification, and enhance its compatibility with the molten material.Following this, the alloy was heated at 760 °C to transform it into a liquid state.The necessary quantity of preheated reinforcements (B 4 C, ZrO 2 , and CSC) specified in table 2 was added to the molten alloy while continuously stirring at 300 rpm for 5-10 min.In addition, a small amount of magnesium was introduced to improve the bonding between the matrix and the reinforcements.By enhancing wettability, the incorporation of reinforcements into the matrix is facilitated by reducing the surface tension of the molten alloy, increasing the surface energy of solid particles, and decreasing the interfacial energy at the solid/liquid boundary.To prevent oxidation, continuous supply of argon gas was maintained at a 1.5 L per minute within the molten chamber  during the stirring process.Once the stirring process was completed, the molten alloy with the reinforcements was poured into the mild steel die with a diameter of 30 mm and a length of 300 mm to cast these Al-7075 MMCs.The crucible was subsequently cleaned to eliminate any slag or impurities, preparing it for subsequent procedures.The mold was removed from its chamber and allowed to cool in ambient air.Finally, the mold was opened to reveal the finished casting, as illustrated in figure 2.

Density and porosity measurement
The density of MMCs is a crucial factor in determining their properties, and it is influenced by the relative proportions of the reinforcing and matrix materials.To measure the experimental density of the MMC samples, the Archimedes' theory was used, as shown in equation (1).This method involves weighing the sample in air and in a fluid with a known density during the density calculation process.
where ρ exp is the experimental density of MMCs in g/c.c., m 1 is the mass of MMCs measured in air, m 2 is the mass of MMCs measured in distilled water, and ρ w is the density of distilled water (1 g cm −3 ) Theoretical density of composites can be determined through the rule of mixture, which takes into account the density of the matrix-metal and the density of reinforcements, considering their respective volume fractions, as depicted in equation (2).
where ρ the is the theoretical density of MMCs in g/c.c., V Al-7075 is the volume fraction of Al-7075, V B4C/ZrO2 is the volume fraction of B 4 C or ZrO 2 , V CSC is the volume fraction of CSC, ρ Al-7075 is the density of Al-7075, ρ B4C/ZrO2 is the density of B 4 C or ZrO 2 , and ρ CSC is the density of CSC.The relation between porosity and density for each developed composite was calculated as per equation The experimental density, calculated theoretical density, and porosity of the developed Al-7075 MMCs are shown in figure 3.
Figure 3 presents data on four Al-7075 MMCs samples (B1, B2, B3, and B4) with varying compositions of aluminum alloy (Al-7075), boron carbide (B 4 C), and CSC.In this study, the theoretical density values ranged from 2.6938 g cm −3 for sample B4 to 2.6946 g cm −3 for sample B1.The difference between the highest and lowest theoretical density was only 0.0008 g cm −3 , indicating that the theoretical density remains relatively consistent across the samples.However, the experimental density values for the samples ranged from 2.6167 g cm −3 for sample B3 to 2.6711 g cm −3 for sample B1.The difference between the highest and lowest experimental density was 0.0544 g cm −3 .These values indicated that there was a small variation in the experimental density among the samples, with sample B1 having the highest experimental density and sample B3 having the lowest.In this study, the porosity values ranged from 0.87% for sample B1 to 2.87% for sample B3.The difference between the highest and lowest porosity was 2.00%.These values indicated that there was a slight variation in porosity among the samples, with sample B3 having the highest porosity and sample B1 having the lowest.The fluctuations in theoretical density and porosity could be ascribed to variations in the composition of CSC within the mixture.When the proportion of CSC increased, there was a gradual and slight decrease in the theoretical density, whereas the opposite trend was observed for porosity.It increased until reaching 3 wt% CSC, but for 4 wt%, it decreased.Comparing the samples, it can be observed that as the percentage of CSC increases from B1 to B4, the experimental density decreases until reaching 3 wt% CSC, but for 4 wt%, it increases a little.As the CSC reinforcements increase, the theoretical densities show a corresponding decrease, resulting in the following percentage variations from B1: 0.0148% for B2, 0.0296% for B3, and 0.0443% for B4 respectively.
A similar trend was observed for Al-7075/ZrO 2 /CSC composites, as the theoretical density values ranged from 2.7894 g cm −3 for sample Z1 to 2.7886 g cm −3 for sample Z4.The difference between the highest and lowest theoretical density was only 0.0008 g cm −13 , indicating that the theoretical density remained relatively consistent across the different samples.It is to note that, when the CSC reinforcements increase, the theoretical densities decrease, leading to percentage variations from Z1 of 0.0143% (Z2), 0.0214% (Z3), and 0.0285% (Z4), respectively.However, the experimental density values for the samples ranged from 2.7463 g cm −3 for sample Z1 to 2.7324 g cm −3 for sample Z4.The difference between the highest and lowest experimental density was 0.0139 g cm −3 .These values indicated that there was a slight variation in the experimental density among the samples, with sample Z1 having the highest experimental density and sample Z4 having the lowest.As the CSC reinforcements increase, the experimental densities gradually decrease, except for the 4 wt% composition where they increase.The percentage variation from Z1 for each composition is as follows: Z2 shows a decrease of 1.0566%, Z3 exhibits a decrease of 1.2739%, but Z4 shows an increase of 0.5555%.
In addition, the porosity values ranged from 1.5451% for sample Z1 to 2.7718% for sample Z3.The difference between the highest and lowest porosity values was 1.2267%.These values indicated that there was a noticeable variation in the porosity among the samples.Furthermore, the porosity shows variation with the addition of CSC.Sample Z1, with no CSC, had the lowest porosity of 1.5451%.As the percentage of CSC increased in samples Z2, Z3, and Z4, the porosity increased to 2.5708%, 2.7718%, and 2.0153%, respectively.This indicates that the addition of CSC contributes to an increase in porosity.The observed trend in porosity with varying percentages of CSC (from 0 wt% to 3 wt% shows an increase and at 4 wt% shows a decrease) can be attributed to a combination of particle arrangement, interparticle spacing, microstructural effects, particle agglomeration, disruption of the binding matrix, and interactions between the CSC particles and the matrix.These factors collectively influence the distribution of pores and voids within the material, resulting in the observed changes in porosity.
At lower percentages of CSC (0 wt% to 3 wt%), the matrix is primarily composed of a Al-7075 with fewer CSC particles.These particles may be dispersed more evenly, and their introduction might not significantly hinder the overall packing of the matrix.The porosity might increase due to the spaces left between the CSC particles and the matrix.In addition, the lower amount of CSC, the spacing between the particles might be relatively larger.This spacing can contribute to increased porosity as there are larger voids or gaps between particles that can be considered as pores.At higher percentages of CSC (4 wt%), the relatively larger concentration of CSC particles can lead to agglomeration or clustering.These clusters might reduce the interparticle spacing and increase the density of the material.This densification can decrease overall porosity.As the CSC reinforcements increase, the porosity gradually increases, except for the 4 wt% composition where it decreases.The percentage variation from Z1 for each composition is as follows: Z2 shows a 66.7805% increase, Z3 exhibits a 79.7031% increase, and Z4 demonstrates a 30.4063% increase from Z1, but not from Z2 and Z3, respectively.Therefore, these findings reveal that the experimental density and porosity of the samples show slight variations based on the composition of CSC whereas theoretical density remains relatively consistent across the samples.
In summary, the theoretical density, experimental density, and porosity of the both Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC samples showed slight variations based on the composition of CSC.The theoretical density remained relatively consistent across the samples.These findings suggest that the addition of CSC affects the density and porosity of the material, which could have implications for its tribological properties and performance in specific applications.

Vickers microhardness test
To begin the Vickers microhardness test, the Al-7075 MMCs were cut into appropriate dimensions and polished properly to remove contaminants on their surface.The load range used adhered to the specifications outlined in ASTM Standard E-384, which typically falls between 1 and 100 kg, as shown in figure 4. The angle between the opposing faces of the pyramid was fixed at 136°whereas the chosen load was applied for a predetermined dwell time (typically 10-15 s) to generate the indentation on the sample surface.The diagonal lengths (d 1 and d 2 ) of the resulting Vickers indentation were measured using an optical microscope or an automated image analysis system with a precision of at least 0.01 mm.Then, the Vickers microhardness was calculated using the formula shown in equation (4) [27].
where the applied load is in kg and the diagonal lengths Figure 5 shows the results of multiple trials carried out on various samples comprising different compositions of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC.The top-ranking sample is B1, which exhibits an average hardness value of 133.7 with 0 wt% CSC reinforcement.As the CSC content increases from 0% to 4%, the average values of the samples fluctuate.Among the B-series samples, B2 has the lowest average value of 110.08 whereas B4 has the highest average value of 132.16.However, within the Z-series samples, sample Z2 with 2% CSC exhibits an average value of 144.18, which is the highest among the Z-series samples.This indicates a contrasting trend compared to the B-series samples.In addition, samples Z1, Z3, and Z4, with different proportions of ZrO 2 and CSC, have average hardness values of 140.72, 133.92, and 140.16, respectively.Comparing the B-series and Z-series samples, it is evident that the Z-series samples generally have higher average values.However, the differences between the two series are relatively small, suggesting that CSC as an additional reinforcement with ZrO 2 has a notable impact on the average hardness values.

Tribological test
The study used the Taguchi method to design the wear experiments.L16 orthogonal arrays, based on the factors and their levels presented in table 3, were used.This method used for minimizing the number of experimental runs while identifying influential factors [28].Specifically, this research focused on the wt% of CSC reinforcements and temperature (°C) as the selected factors and levels.From various pieces of literature, the optimum factors held constant throughout the study were the wt% of B 4 C/ZrO 2 (3%), load (30 N), speed (4 m s −1 ), and sliding distance (2000 m) [29,30].The objective of the study was to examine how these factors affected wear and the COF.To evaluate the tribological properties of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC, a pin-on-disc apparatus was used following the ASTM Standard G99, as shown in figure 6.The disc specimen had a diameter of 100 mm and a thickness of 6 mm whereas the counter pin, made of standard EN31, had a diameter of 10 mm and a height of 40 mm, according to the ASTM Standard G99.During the wear experiments, the EN31 pin was pressed against the composite disc specimen, with the disc rotating at a speed of 764 rpm and a constant load of 30 N applied.The wear measurements were directly recorded from the   Wear behavior exhibits variation with temperature due to alterations in material properties and the implications of thermal expansion.
Temperature settings can also replicate real-world scenarios, encompassing high-temperature environments or even cryogenic conditions.

Load (N)
The applied load plays a pivotal role in wear testing, as it delineates the stress and contact pressure amid materials.Diverse loads offer insights into how materials react under varying stress conditions.

30
Speed (m/s) Sliding speed tangibly influences the wear rate.Elevated speeds can amplify friction and wear due to augmented contact stresses and temperatures.Varied speeds enable the emulation of diverse operational conditions.

4
Constant sliding distance (m) Maintaining a uniform sliding distance assures that wear tests unfold within controlled and replicable parameters.This practice facilitates the observation of material wear and the accumulation of wear debris over a predetermined distance.

2000
Constant wt% of B 4 C/ZrO 2 (%) Upholding a consistent weight percentage of the reinforcement material guarantees that the sole variable under examination is the composite's composition.This meticulous approach aids in isolating the effects of reinforcement on wear behavior.
3 apparatus or graph during the experiments.A maximum of three experiments were carried out for each parameter set to obtain representative data.The difference in weight between the specimens before and after each wear experiment was calculated as the weight loss.

Result and discussion
Tables 4 and 5 show the wear and COF, which are the tribological properties of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC, respectively.In addition, the effects of the weight fraction of CSC (0%, 2%, 3%, and 4%) and temperature (room temperature, 150, 200, and 250 °C) on the wear and COF by keeping B 4 C and ZrO 2 as constant (3 wt%) were examined.
Effect of weight fraction of CSC and temperature on wear and COF of Al-7075/B 4 C/CSC Figures 7 and 8 show that the weight fraction of CSC significantly impacts the wear and COF of the Al-7075/B 4 C/CSC composite.Increasing the weight fraction of CSC leads to a decrease in wear, indicating enhanced wear resistance.The relationship between the weight fraction of CSC and the COF exhibits a consistent pattern at different temperatures.The results exhibited a consistent trend where lower COF values were associated with higher wear rates, and vice versa [31].The wear rate is reduced in the B3 composition, which consists of 94% Al-7075, 3% B 4 C, and 3% CSC, owing to the effective blending of these components.Conversely, compositions with 0% and 2% wt% CSC exhibit higher wear values.In comparison, the B4 composition (4% CSC) shows a higher wear value than B3 but lower than B1 and B2.In addition, as the temperature rises, wear tends to decrease for each weight fraction of CSC, suggesting that higher temperatures may enhance wear resistance in the composite.Although there are slight variations in wear and COF among the different compositions of Al-7075/B 4 C/CSC, the lowest wear (61.8 μm) was observed at 3% CSC and 250 °C.This phenomenon can be attributed to the optimal proportion that enhances the performance of the MMCs.Overall, there is a general trend of reduced wear with increasing weight fraction of CSC in the composite.The reinforcement effect of CSC and the effect of temperature are significant factors, and analysis of variance (ANOVA) can be used to assess the significance of CSC weight fraction on wear and COF.
ANOVA was used to identify the optimal levels of input parameters, namely the weight fraction of CSC and temperature.One-way ANOVA was used to determine the significant parameters that have a substantial impact on wear and COF, as shown in figures 9 and 10, respectively.The analysis was performed using MINITAB software and the performance characteristics were evaluated based on the 'smaller is better' criterion for wear and the 'larger is better' criterion for COF.The signal-to-noise (SN) ratios for different parameters are shown in figures 9 and 10.The calculated P-value showed the level of significance for the most impactful parameter, revealing that it is below 0.05 solely for wear but higher for COF as, shown in figures 9(d) and 10(d), respectively.This implies that the performed experiments adhere to the accepted standard of 5% significance for wear, but not for COF.Furthermore, the R 2 value for wear was determined to be 90.73%,indicating its statistical significance whereas for COF it slightly deviates from the acceptable range at 84.29%.
The plots showed in figures 9   However, there is a noticeable dip at 4 wt% CSC, which is attributed to an improper dispersion rate between CSC and B 4 C reinforcement within Al-7075.Regarding temperature dependency, the wear is lower at 250 °C, following the smaller is better approach.In addition, the contour plot shown in figure 9(c) confirms that wear is reduced when the composition includes 3 wt% CSC.Furthermore, when examining the COF of Al-7075/B 4 C/CSC, the main effect and residual of the SN ratio can be observed in figures 10(a) and (b).The plots clearly illustrate a gradual increase in the COF as the weight percentage of CSC increases.However, there is a distinct dip in the COF at 4 wt% CSC, which can be attributed to an insufficient dispersion rate between the CSC and B 4 C reinforcement within the Al-7075 matrix.Regarding the influence of temperature, it is interesting to observe that the COF is higher at 250 °C, following the larger is better approach.This suggests that, in this particular scenario, a higher COF is desirable at higher temperatures.In addition, the contour plot shown in figure 10(c) provides further evidence supporting these observations.It  confirms that the COF increases when the composition contains 3 or higher weight percentage of CSC.This implies that a higher concentration of CSC has a detrimental effect on the COF, leading to an increase in friction.
Effect of weight fraction of CSC and temperature on the wear and COF of Al-7075/ZrO 2 /CSC Figures 11 and 12 illustrate that the weight fraction of CSC has a significant effect on the wear and COF of the Al-7075/ZrO 2 /CSC composite.By increasing the weight fraction of CSC, the rate of wear decreases, indicating improved resistance to wear.This result shows the pattern of Al-7075/B 4 C/CSC, as the lower COF values are associated with higher wear rates, whereas higher COF values correspond to lower wear rates.The wear decreases for the Z3 composition comprising 94% Al-7075, 3% ZrO 2 , and 3% CSC due to the proper mixing of the compositions.It also demonstrates a higher wear value for the compositions comprising 0 and 2 wt% CSC, but for Z4 (4% CSC) it exhibited a higher wear value than Z3 but lower than Z1 and Z2 samples.
Moreover, with increasing temperature, the wear tends to decrease for each weight fraction of CSC, indicating that higher temperatures may improve the wear resistance of the composite.Although there are slight variations in wear and COF among the different compositions of Al-7075/B 4 C/CSC, the lowest wear value (51.8 μm) was observed at 3% CSC and 250 °C.The interaction between ZrO 2 and CSC in Al-7075 effectively hinders the propagation of fractures during sliding wear.Strain patches are formed around the ZrO 2 and CSC particles due to thermal differences between Al-7075 and the combination of ZrO 2 and CSC particles during the solidification process.These strain patches provide resistance to fracture propagation, thereby preventing material removal and promoting strengthening through equal distribution.Similarly, at 150 and 200 °C, the wear is generally lower for all compositions, but it increases when the temperature returns to room temperature (32 °C).This is due to less thermal activation energy available for bond breaking and material removal, leading to increased friction and wear.In addition, at lower temperatures, lubrication effectiveness can be reduced, resulting in higher contact pressures and increased wear.Furthermore, the mechanical properties of materials, such as hardness and brittleness, can vary with temperature.At room temperature, certain materials may exhibit lower ductility and higher hardness, making them more susceptible to wear.Finally, environmental factors, such as humidity and the presence of corrosive agents, can contribute to increased wear rates at room temperature.Overall, there is a consistent trend of reduced wear with an increasing weight fraction of CSC in the composite.The reinforcement effect of CSC and the effect of temperature are significant factors, and the use of ANOVA can determine the significance of the CSC weight fraction on wear and COF [32].temperature.Sample Z3 still had the highest COF value, but it decreased to 0.344, indicating a reduction in the frictional resistance at this temperature.In addition, samples Z1, Z2, and Z4 showed COF values of 0.336, 0.333, and 0.352, respectively, suggesting consistent or slightly improved sliding characteristics compared to the previous temperature.At 250 °C, sample Z2 showed the highest COF value of 0.446, indicating the highest frictional resistance among the tested samples at this temperature.Sample Z1 followed closely with a COF value of 0.345, suggesting significant frictional resistance.Samples Z3 and Z4 showed COF values of 0.337 and 0.543, respectively, indicating moderate-to-high levels of frictional resistance.Finally, at room temperature, sample Z1 showed the highest COF value of 0.461, suggesting the highest frictional resistance among all the samples tested in ambient conditions.Samples Z2, Z3, and Z4 had relatively lower COF values of 0.381, 0.372, and 0.376, respectively, indicating a relatively smoother sliding behavior compared to Z1.These findings align with the principle that lower COFs are associated with higher wear rates.On the basis of the observed COF values, it can be concluded that sample Z3 outperforms the other samples across all temperatures.
Similarly to Al-7075/B 4 C/CSC, ANOVA was used to identify the optimal levels of input parameters, as shown in figures 13 and 14, respectively.The analysis was carried out using MINITAB software, and the evaluation of performance characteristics was based on the smaller is better criterion for wear and the larger is better criterion for COF.The SN ratios for different parameters can be observed in figures 13 and 14.To assess the significance of the most influential parameter, the calculated P-value was examined.The results indicated that the P-value was below 0.05 for both wear (figure 13(d)) and COF (figure 14(d)), demonstrating statistical significance.This adherence to the accepted 5% significance level for wear and COF confirms the reliability of the performed experiments.Moreover, the R 2 value, determined to be 97.87% for wear and 88.07% for COF, further underscores the statistical significance of the findings.
The plots shown in figures 13(a) and (b) provide valuable insights into the relationship between the weight percentage of CSC and the wear of Al-7075/ZrO 2 /CSC composite.It is evident that there is a gradual increase in wear as the weight percentage of CSC increases.However, an interesting observation is the noticeable dip in wear at 4 wt% CSC.This dip can be attributed to an inadequate dispersion rate between the CSC and ZrO 2 reinforcement within the Al-7075 matrix.The improper dispersion hinders the effectiveness of the reinforcement, leading to reduced wear resistance.When considering the temperature dependency, it is to be noted that the wear is lower at 250 °C, following the smaller is better approach.This suggests that higher temperatures have a positive effect on reducing wear in this composite system.Furthermore, the contour plot in figure 13(c) reinforces the finding that wear is minimized when the composition contains 3 wt% CSC.Similarly, examining the COF of the Al-7075/ZrO 2 /CSC composite, the main effect and residual of the SN ratio are shown in figures 14(a) and (b).The plots demonstrate a gradual increase in the COF as the weight percentage of CSC increases.However, a notable dip in the COF is observed at 4 wt% CSC.This dip can be attributed to the inadequate dispersion of CSC and ZrO 2 within the Al-7075 matrix, leading to an alteration in the frictional behavior.Therefore, the addition of 3% CSC with ZrO 2 in the Al-7075 matrix is considered optimal.The   presence of an optimal amount of carbon in CSC aids in reducing wear and frictional forces during sliding or rubbing contact.
Interestingly, in terms of temperature effect, the COF is higher at 250 °C, following the larger is better approach.This indicates that, in this specific case, a higher COF is desirable at higher temperatures, suggesting a different frictional mechanism at play.Moreover, the contour plot in figure 14(c) provides further confirmation of these observations, showing that the COF increases when the composition contains 3 wt% or higher CSC.This implies that a higher concentration of CSC has a detrimental effect on the COF, resulting in an increase in friction.
Comparative analysis of wear and coefficient of friction for Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC MMCs with 3% CSC We conducted a comparative analysis of wear and the COF for Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC composites.The focus was particularly on studying the effect of adding 3% CSC to each composite as it exhibited a lower wear.Comparatively, the Al-7075/ZrO 2 /CSC composite performed better than Al-7075/B 4 C/CSC.This notable difference can be attributed to the superior mechanical properties achieved by incorporating ZrO 2 in combination with CSC, which possesses high hardness, strength, and toughness.The combination of ZrO 2 and CSC in the composite resulted in the formation of strain patches that resisted fracture propagation, enhancing wear resistance.In conclusion, the addition of 3% CSC with ZrO 2 in the Al-7075 matrix was found to be optimal, as shown in figure 15.From the ribbon graph, it can be observed that the wear and COF show improvement in the Al-7075/ZrO 2 /CSC composite compared to the Al-7075/B 4 C/CSC composite, with a respective enhancement of 19.30% and 42.19%.
After the comparative analysis, a confirmatory test was also carried out for the compositions (table 6).At a temperature of 250 °C and a weight percentage of 3%, samples B3 and Z3 showed similar wear characteristics and COF values.The confirmatory results closely matched the experimental results, with low percentage errors indicating the reliability of the measurements.These findings provide valuable insights into the wear performance and COF of the investigated samples, contributing to the understanding of their suitability for potential applications in relevant temperature conditions.

Microstructural analysis
Field-emission scanning electron microscopy was used to examine the worn micrographs of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC MMCs obtained from unlubricated wear samples.The best (B3 and Z3) and worst (B1 and Z1) micrographs are shown in figure 16.It can be seen that the sample Z1 has small wear debris compared to the sample B1; this is reason why it has more wear on it.Similarly, Z3 has smooth surfaces than B3.This is due to the presence of CSC reinforcing particles within the composite, which contribute to the formation of a protective layer.During the wear process, these particles can become embedded in the opposing surface, acting as a barrier and reducing direct contact between the sliding surfaces.This can help minimize surface roughness and promote the formation of a smoother surface.The occurrence of plastic deformation and material transfer during the wear process can contribute to the development of a polished surface, as shown in figure 16(d).As the opposing surfaces slide against each other, plastic deformation can occur, resulting in the smoothing out of asperities and surface irregularities.This plastic flow and material transfer can lead to the formation of a more polished and smoother surface.Figure 16(d) shows the presence of multiple plastic deformations characterized by the formation of parallel deep grooves.Higher loads generate increased stress during rubbing, leading to the formation of cracks on the pin surface.
The worn surface analysis revealed the production of a smoother layer on the pin surface with increasing applied load.As observed in figure 17(d), the asperities on the pin surface undergo significant deformation at maximum load, resulting in the formation of a smooth contact layer.The continuous sliding process generates a substantial amount of heat between the sliding contacts, leading to the formation of a tribo-oxide layer.
The microstructure of the worn surface of the Al-7075/B 4 C/CSC composite depicted in figure 17(a) shows the formation of a mechanically mixed layer and shallow grooves, indicating significant material removal due to increased heat generation and high sliding speed.Similarly, figure 17(b) shows craters, which may damage the surface of the composite.When hard particles or debris become trapped between the sliding surfaces, they can

Conclusion
On the basis of the comprehensive analysis of the physical and wear characteristics of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC, the following findings were noted:

❖
The theoretical density of Al-7075 MMCs remains consistent across different compositions whereas the experimental density and porosity show slight variations based on the percentage of CSC.

❖
When incorporating CSC into mixtures of Al-7075 and zirconia, an increase in density and porosity is observed, but this trend is only observed up to 3 wt%.However, for a 4 wt%, a decrease in the density and porosity is observed.
❖ B-series samples generally have lower average hardness values compared to Z-series samples, but the differences are relatively small.

❖
The weight fraction of CSC has a significant impact on the wear and COF of Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC MMCs.Increasing the weight fraction of CSC up to a 3 wt% improves wear resistance and reduces wear rates in both composites.Likewise, a similar trend is observed for the COF, which generally increases as the weight fractions of CSC in the composites increase.

❖
Higher temperatures tend to decrease wear rates in both Al-7075/B 4 C/CSC and Al-7075/ZrO 2 /CSC MMCs.Therefore, the optimal weight fraction of CSC for reduced wear is observed to be 3% in both composites at 250 °C.
❖ COF values vary with temperature, and lower temperatures can lead to increased wear due to reduced lubrication effectiveness and higher contact pressures.
❖ ANOVA confirms that the weight fraction of CSC has a significant impact on wear for both the MMCs.However, when it comes to the COF, only the Al-7075/ZrO 2 /CSC MMCs show a significant increase.
Overall, it was found that the inclusion of CSC in the Al-7075/ZrO 2 /CSC MMCs significantly improves their wear resistance (approximately 19.30%) compared to the Al-7075/B 4 C/CSC MMCs.Therefore, these findings should be considered when evaluating the suitability of these materials for demanding applications.

(d 1 and d 2 )
are in millimeters.The above steps were repeated for five times on multiple locations of the casted Al-7075 MMC samples to obtain a representative average microhardness value.The experimental Vickers microhardness values of the developed Al-7075 MMCs are shown in figure 5.

Figure 4 .
Figure 4. Experimental setup and samples for Vickers microhardness test.
(a) and (b), illustrating the main effect and residual of the SN ratio on the wear of Al-7075/B 4 C/CSC, clearly indicate a gradual increase in wear in relation to the weight percentage of CSC.

Figure 6 .
Figure 6.Experimental setup and samples for tribological test.

Figure 7 .
Figure 7. Effect of weight fraction of CSC and temperature on wear of Al-7075/B 4 C/CSC.

Figure 8 .
Figure 8.Effect of weight fraction of CSC and temperature on the COF of Al-7075/B 4 C/CSC.

Figure 11 .
Figure 11.Effect of weight fraction of CSC and temperature on the wear of Al-7075/ZrO 2 /CSC.

Figure 12 .
Figure 12.Effect of weight fraction of CSC and temperature on the COF of Al-7075/ZrO 2 /CSC.
act as abrasives and cause localized material removal, resulting in the formation of craters.In addition, figure 17(c) shows only the minimal ejected particles in Al-7075/ZrO 2 /CSC.Figure 17(d) reveals the presence of ploughing effects and smaller grooves, primarily caused by the stiffness of the reinforcement particulates.The smaller grooves indicate minimal material removal due to the lower pressure generated between the pin surface and counter-disc plate.
The percentage of reinforcement profoundly influences its mechanical properties, notably wear resistance.To comprehend the impact of reinforcement on wear behavior, varying percentages are tested.Different levels of reinforcement can instigate diverse wear mechanisms and variations in performance.