Investigate the Mechanical and Tribological Characteristics of Cu- Al₂O₃- Gr Composite via Stir Route

Abstract

The present work examined Cu-Al₂O₃-Gr composites produced using stir casting and analyzed their mechanical properties. Additionally, the wear performance of these composites was evaluated concerning different reinforcement ratios. Three samples were analysed Pure Copper, 96% Cu, 2% Al₂O₃, 2% Gr, 95% Cu, 3% Al₂O₃, 2% Gr and 92% Cu, 3% Al₂O₃, 5% Gr. The incorporation of Al₂O₃ and Gr reinforcements significantly enhanced the hardness and tribological properties of these metal composites in comparison to pure copper. The potential enhancements in the lubricating and load-bearing characteristics of graphene in Sample 4, comprising 5% graphene, led to improved performance. An analysis of the microstructure of a worn surface from the study examining the distribution of reinforcement and its correlation with mechanical parameters. Wear studies were conducted under various situations to evaluate tribological performance. The results suggest potential uses of Cu-Al₂O₃-Gr composites in domains requiring strong abrasive wear resistance and enhanced mechanical properties, aiding in the determination of optimal reinforcing quantities for material development in demanding industrial settings.

Keywords:
Tribological; Graphene; Composite; XRD Analysis; Wear Analysis

1. Introduction

Over the last few decades metal matrix composites (MMCs) have gained profound recognition as materials to produce hybrid capabilities, maintaining the beneficial properties of metals together with those of other unprocessed reinforcements such as ceramics. Composites based on copper have emerged as significant materials with potential use in various applications that need excellent thermal and electrical conductivity properties together with high mechanical and tribological properties¹. Copper matrix composites with reinforcing phases of alumina (Al₂O₃) and graphene (Gr) have been developed as a novel approach to overcome the disadvantages of pure copper by utilizing the intrinsic properties of these distinctive reinforcing constituents. Although it has good electrical and thermal conductivity, copper frequently lacks the required wear resistance and mechanical strength for use in some applications. To address these limitations, a number of strengthening materials and fabrication processes have been studied. Many attempts are made to study Al₂O₃ as a reinforcement for such composites since Alumina is both hard and thermally stable, it has a good chemical resistance. With its remarkable mechanical, thermal, and lubricating characteristics, graphene has become an effective candidate for copper matrix composites in order to improve the performance. Stir casting is a liquid-state manufacturing technology that has gained popularity for fabricating metal matrix composites owing to its simplicity, cost-effectiveness, and large-scale production capability. In this method, reinforcing particles are mechanically stirred in a molten metal matrix followed by controlled solidification. Though stir casting has been widely used for aluminum matrix composites, its application in copper-based systems poses unique challenges and offers opportunities.

Stir casting has proved to be an effective process for preparing copper based composites, as several high quality studies. This studied the microstructure, mechanical and tribological properties of stir cast AA2024/Ni metal matrix composites. The incorporation of Ni powder improved precipitation profile and mechanical properties². The Composite material fabricated using stir casting and has a uniform distribution of reinforcing particles and improved corrosion resistance³. With the introduction of aluminum oxide (Al₂O₃) to copper matrix composites, there is a significant positive impact on mechanical and tribological properties. It has been shown the improvement of wettability between matrix and reinforcing materials will help to achieve the best composite properties⁴. It was reported that the incorporation of Al₂O₃ particles led to an increase in the hardness, tensile strength, and wear resistance of copper-based composites. Internal oxidation generated Cu-Al₂O₃ composites exhibiting increasing hardness and wear resistance as the Al₂O₃ content increased⁵. As a promising new reinforcing second-phase material, graphene offers significant improvement in metal matrix composites. Due to its two-dimensional framework and excellent mechanical properties, graphene is a favourable candidate for strengthening and enhancing the wear resistance of copper-based materials⁶. Investigated on the mechanical property and wear behaviour of graphene reinforced Al matrix composites. This study suggested that sliding initiated the formation of a lubricating tribolayer coating and then that the combination of aluminum oxide on graphene acted as an abrasive in the composite leading to greater hardness and wear resistance benefits⁷,⁸. Combination of Al₂O₃ and graphene as hybrid reinforcements offers a new route to achieve a synergistically improved property of copper matrix composites. This hybrid reinforcement approach was designed to take advantage of the superior hardness and thermal stability of Al₂O₃ along with the lubricating and load-bearing characteristics of graphene. We have previously looked at similar hybrid methodologies like this are explored in aluminum matrix systems with promising results. One study comparing the mechanical properties of Al₂O₃-decorated reduced graphene oxide (Al₂O₃/RGO) reinforced aluminum matrix composites showed prominent improvements in tensile strength and hardness⁹. Wear properties of metal matrix composites play a critical rôle in their use under tribological conditions. The addition of ceramic particles such as Al₂O₃ has improved the hardness of the composite and its load-carrying capacity, thus also enhancing wear resistance. Graphene not only has a good self-lubrication ability but can also form a protected tribofilm, which provides better abrasion resistance in a tribological system. The synergistic effects of multiple reinforcements on wear behavior are being studied, where each system contributes to superior wear resistance than a single reinforcement system¹⁰,¹¹. The mechanical properties of Cu-Al₂O₃-Gr composites will rely on factors such as the volume fraction of reinforcements, size and distribution of particles and the bonding at the matrix-reinforcements interface¹²,¹³. In fact, increased addition of Al₂O₃ particles increases hardness and aesthetics, as well as tensile strength (TS) but also decreases ductility. Graphene, when dispersed properly, enhances strength by mechanisms like load transfer and dislocation strengthening. There is a significant difficulty in uniformly dispersing graphene in metal matrices, which often necessitates special processing or even surface modifications. The Cu-Al₂O₃-Gr composites verify numerous advantages and difficulties of the stir casting procedure. However, while this is a low-cost method for the large-scale preparation of nanocomposites, achieving a uniform distribution of reinforcements, especially nanoscale fillers such as graphene, is challenging. The final microstructure and properties of the composite are significantly affected by the stirring velocity, temperature, and the order of addition of the reinforcements. Improving these process parameters is critical to meeting the necessary mechanical and tribological properties. Cu-Al₂O₃-Gr composites have a large scope of application in different sectors including automotive, aerospace and electronics. These materials are appealing for use in components facing high wear and mechanical stress conditions, e.g. bearings, bushings and electrical contacts, due to their increased wear resistance and mechanical properties. Copper's remarkable thermal and electrical conductivity, even in formless matrix composites, along with superior mechanical properties, could lead to new applications in thermal management systems and advanced electrical interconnects¹⁴,¹⁵.

Thus, stir casting of Cu-Al₂O₃-Gr composites is a successful method for producing materials with improved mechanical and tribological properties in addition to corresponding benefits. Al₂O₃-graphene synergy can have a significant combined effect on hardness, wear resistance and energy saving also leading to a wide range of applications as compared to pure copper or some other reinforcing system. Further study is needed to succinctly elucidate the interrelationships among the matrix and reinforcements, improve processing parameters, and examine the complete gamut of applications for such unique materials.

2. Experimental Methodology

2.1. Material selection and composite preparation

Copper alloy was used as reference material to study wear characteristics of composites of cubic aluminium oxide and graphite in this study. The matrix material is an alloy of pure copper. The Al₂O₃ is used for Corrosion resistance and improve the mechanical properties like Hardness, Fracture toughness and Flexural Strength (Figure 1).

Figure 1
Graphene Powder.

Alumina (Al₂O₃) particles with a density of 3.95 g/cm3 and graphene (Gr) with a density of 2.26 g/cm3 were used to strengthen reinforcement. Likewise, Cu with Al₂O₃ and Gr reinforcements improves the wettability of copper and probably contributes to the improvement of mechanical properties as well as the improvement of temperature and voltage resistance shown in Figure 2.

Figure 2
Al₂O₃ Powder.

The stir casting method was used in the preparation of the composites due to its efficacy in manufacturing metal matrix composites (MMCs) through improved interfacial adhesion of the metal and the reinforcement. Use this technology has advantages such as large-scale production scalability, simplicity, and design flexibility. Three different composite samples were designed with different compositions (Table 1).

Table 1
Composite sample preparation.

Sample	Composition
1	Cu
2	96% Cu, 2% Al2O3, 2% Gr
3	95% Cu, 3% Al2O3, 2% Gr
4	92% Cu, 3% Al2O3, 5% Gr

The process of fabrication started with the melting of pure copper in a crucible that was housed inside of a furnace. The temperature at which copper melts is 1085 degrees Celsius. After the copper alloy reached its melting point, preheated particles of Gr and Al₂O₃, maintained at 350 degrees Celsius, were introduced into the crucible. It was essential to warm the reinforcements in order to avoid a thermal imbalance with the matrix throughout the process. The mixture was continually agitated at a steady speed of 150 revolutions per minute for twenty to twenty-five minutes in order to guarantee that the reinforcements were distributed evenly throughout the matrix. For the purpose of preventing the mixture from oxidizing and hydrogen gas from building up in the cast, an atmosphere of argon was created and maintained. In order to avoid any thermal imbalances, the melt was then poured into a permanent die casting mold that measured 150 millimeters in length and 15 millimeters in diameter. The mold had been maintained at 450 degrees Celsius before casting. Once it had reached the desired consistency, the cast material was extracted from the mold. This procedure was repeated in order to produce the three composite samples, each of which had a different distribution of reinforcement.

Several specimens were created for a variety of tests in order to evaluate the wear properties of the Cu- Al₂O₃-Gr composites shown in Figure 3. In order to conduct the microstructure examination, samples were cut from the stir cast component, ground using a bench grinder, and polished with emery sheets of varied grit sizes (1000, 2000, and 3000). To accomplish the goal of achieving a smooth surface, the final polishing was performed using a disc while alumina was present. Before being examined with an inverted metallurgical microscope, the specimens were etched with FeCl₂ to prepare them for observation.

Figure 3
Prepared specimens.

For durability testing, pin-on-disc wear test specimens were made in line with ASTM G99 guidelines. The pins measured thirty millimeters in length and eight millimeter in width. The wear tests were performed under dry sliding conditions with help from a pin-on-disc tribometer. Different weights (10N, 20N, and 30N) and sliding distances (1000m, 2000m, and 3000m) were utilized in the experiments conducted at room temperature; the sliding velocity was kept constant during the whole operation.

3. Results and Discussion

The prepared specimens and performed testing processes helped one to thoroughly investigate the mechanical qualities and wear characteristics of Cu- Al₂O₃-Gr composites, generated by the stir casting technique. This work exposed the great possibilities of these materials for uses demanding extraordinary mechanical performance and wear resistance.

3.1. Wear analysis

Analyzing the wear properties of the composites was done using the weight loss method. The specimens were weighed both before and after each test using a highly precise electronic scale with a 0.1 mg accuracy. One used the following equation to ascertain the wear rate:

$\begin{array}{l}\text{Wear rate}=\left(\text{Initial weight}-\text{Final weight}\right)/\\ \left(\text{Sliding distance}\times \text{Applied load}\right)\end{array}$

The wear rate (mm³/m) of pure copper and Cu- Al₂O₃-Gr composites with different reinforcement proportions is depicted in Figure 4 through the use of a bar chart. In terms of wear resistance, pure copper has the highest wear rate, which is approximately 0.23 millimeters per square meter. While the addition of 2% Al₂O₃ and 2% Gr results in a substantial reduction in the wear rate, raising the amount of Al₂O₃ to 3% results in an even greater improvement in wear resistance due to the increased hardness. By exhibiting the lowest wear rate (about 0.1 mm³/m), the composite material that contains 92% copper, 3% aluminum oxide, and 5% grit displays exceptional performance. This can be attributable to the synergistic effects of the load-bearing capacity of alumina and the lubricating qualities of graphene, which reduce friction and material loss.

Figure 4
Wear rate of composite materials.

3.2. Worn surface morphology

FESEM image 5 that have been provided illustrate the worn surfaces of Cu- Al₂O₃-Gr composites with different reinforcement compositions after they have been subjected to wear testing. These images emphasize the impact that the amount of alumina (Al₂O₃) and graphene (Gr) content has on the wear resistance of the composites. As a result of abrasive wear, this image 1 displays noticeable grooves and scratches, which indicates that the material has a moderate level of wear resistance. When 2% Al₂O₃ is added, the hardness of the material is increased, and 2% Gr is added to provide some lubrication. On the other hand, the reinforcing content is not adequate to considerably minimize the amount of material loss, which results in surface degradation that is evident. When compared to Image 1, the surface of Image 2 displays grooves that are both narrower and shallower, which indicates that the surface is more resistant to wear. The composite's load-bearing capacity and hardness are both improved as a result of the increased Al₂O₃ content, which is three percent (Figure 5).

Figure 5
Worn Surface of composite materials a) 96% Cu, 2% Al₂O₃, 2% Gr b) 95% Cu, 3% Al₂O₃, 2% Gr c) 92% Cu, 3% Al₂O₃, 5% Gr.

This results in less deformation when loads are applied. However, graphene is still present at a lesser concentration, despite the fact that it helps minimize friction. A greater level of wear resistance is demonstrated by this image 3, which shows the smoothest surface possible with a low amount of grooves and scratches. The increased graphene concentration, which is five percent, creates a protective tribolayer on the surface. This tribolayer functions as a solid lubricant and considerably reduces the amount of material loss and friction that occurs. An outstanding tribological performance is achieved as a result of the synergistic effects of 3% Al₂O₃ and 5% Gr mixture. Through the reduction of surface damage and the enhancement of lubrication, these photos demonstrate that increasing the amount of reinforcement content, particularly graphene, results in an increase in wear resistance.

3.3. XRD analysis

It is possible to acquire an understanding of the phase composition and crystalline structure of the Cu- Al₂O₃-Gr composite material by analyzing the XRD (X-ray Diffraction) pattern that is represented in the Figure 6. A graph is presented that illustrates the intensity (cps) as a function of the diffraction angle (2θ), with distinct peaks representing various crystallographic planes.

Figure 6
XRD analysis of composite.

The presence of strong and powerful peaks at particular 2θ values is indicative of a structure that has been well-crystallized. These peaks correspond to the copper (Cu) matrix, the alumina (Al₂O₃) phase, and the graphene (Gr) phase, respectively. The existence of principal peaks at 2θ ~43°, 50°, and 74° confirms copper's dominance in the composite. These peaks signify the face-centered cubic (FCC) structure of copper. Reduced peaks at lower angles (about 14° and 25°) signify the existence of graphene and aluminum oxide. The reinforcements enhance the mechanical and tribological qualities of the material. The residual intensity is presented in the lower graph, aiding in the assessment of any minor strains or irregularities in the crystalline lattice due to reinforcement additions.

3.4. Hardness

The hardness of the specimens was determined by polishing the specimens to a suitable finish. Surface indentations on the specimen were produced with a Brinell hardness tester with tungsten carbide ball indenter. The experiment was carried out by applying a 3000 kgf force for 10 to 15 seconds, and the resultant indentation was measured with a microscope. The Brinell hardness number was determined using a standard equation that incorporates the applied load, indenter diameter, and indentation diameter. Three separate trials were conducted for each composition, and the average hardness was obtained. The Figure 7 shows the reinforcement particles are gradually increase the hardness value of copper composite.

Figure 7
Hardness values for prepared composite.

4. Conclusion

• The hardness and wear resistance of the new material were significantly enhanced with the addition of Al₂O₃ and graphene (Gr) reinforcements compared to pure copper.

• Sample 4 with 5 wt% graphene indicated significant progress in wear resistance and hardness.

• The superior mechanical performance of Sample 4 might result from the synergistic lubricating and bearing of graphene.

• The reinforcement particles were homogeneously dispersed in the copper matrix and this is believed to enhance the mechanical and tribological properties of the composites.

• A tribolayer of graphene was generated on the wear marked region serving as a solid lubricant that decreased the friction between composite and counterface further ensued in higher wear resistance (Sample 4).

• X-ray diffraction (XRD) results verified that Al₂O₃and graphene were successfully deposited in the copper without serious phase transitions.

• Resulting discrete sharp peaks in the XRD pattern confirmed the presence of reinforcing phases; this hardness and wear resistance of the composite sample.

• Data Availability
  The entire dataset supporting the results of this study was published in the article itself.

5. References

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Edited by

• Associate Editor:
  Ana Sofia de Oliveira.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The entire dataset supporting the results of this study was published in the article itself.

Publication Dates

• Publication in this collection
  01 Aug 2025
• Date of issue
  2025

History

• Received
  12 Mar 2025
• Reviewed
  02 May 2025
• Accepted
  22 June 2025