Tribological, mechanical and thermal response of diamond micro-particles reinforced copper matrix composites fabricated by powder metallurgy

Copper/Diamond composites have gained a lot of attention in recent years due to their excellent thermal conductivity and their potential for use in high-power electronic devices. The current work targets on an experimental investigation of the tribological,mechanical, and thermal behaviour of copper diamond composite by using reinforced micro-diamond particles. Copper matrix composites with varying weight percentages of diamond particles were produced with the aid of the powder metallurgy. The wear tests were carried out on Pin-on-Disc wear test machine as per ASTM G99. The doping of an optimum amount of diamond particles (1% wt.) improved the overall wear performance by 51% under a normal load of 80 N. The doping had also showed a significant improvement in hardness by 26% and thermal conductivity by 1%. The primary wear mechanisms of Cu-Diamond composites appear to be a combination of brittle fracture, fragmentation of diamond-reinforced particles and ploughing in the Cu-alloy matrix.


Introduction
The characteristics of the metal matrix are directly affected by the ingredients as well as the production procedures [1]. Copper composites for their exceptional thermal and electrical properties have wide applications in the electrical and powerplant sectors. Particulate reinforced copper matrix is mostly suitable for the construction of brushes, bearing bushes, and contact wires in electrical sliding contacts. The inclusion of non-metallic second-phase particles in metal matrix composites can significantly increase mechanical characteristics and wear resistance. Cu matrix composites, in particular, have emerged as one of the most promising alternatives for thermal management materials as a result of their appealing thermal conductivity and low Coefficient of Thermal Expansion (CTE) [2].
To attain enhanced mechanical and physical qualities, carbon compounds such as graphene, graphite, carbon nanotubes (CNTs), graphene nanosheets (GNS), and diamond are typically combined with copper. The hardness, melting point, thermal conductivity, and wear resistance of diamonds are well known. Cu components always face major wear and erosion during routine service. Due to their numerous shortcomings, such as poor wear-resistance, strength, and hardness, which significantly limits their service time and their application. Contrarily, diamond is known to have incredibly high hardness, making it an outstanding wearresistant material. Diamond micro particles reinforced copper matrix composite is a cutting-edge material that can benefit from both copper and diamond. While diamond reinforcements enhance the Cu matrix's performance in terms of wear resistance, Cu serves as a binder phase, contributing to machinability. The implantation of diamond particles in a copper matrix enables the interface in defining the composite's heat conductivity, CTE, and mechanical characteristics. A good interface should have strong adhesion and a low thermal boundary resistance [3]. Powder metallurgical processes are often used to create copper/diamond composites. However, traditional methods necessitate high sintering temperatures, which leads to the graphitisation of the diamond. The primary shear zone, deformation zone, and worn-out flanks of the three main sources of heat generation during machining and mechanical alloying. This heat has an impact on surface characteristics [4]. The amount of contamination in the diamond and the presence of metallic elements help to determine the temperature of graphitisation [5].
To increase the thermophysical performance of Cu/Diamond composites, however, two major concerns must be addressed: Inconsistent diamond particle dispersion in copper matrix and low interface bond strength between two phases. The low tendency to wet the interface bonding of diamond and copper aids in the reduction of binding strength while increasing thermal resistance. Furthermore, an additional phase is frequently used as an interface layer to increase wettability by avoiding direct contact between diamond and copper. Adding Zr, Cr, B, or Ti to the copper matrix increases interface bonding by permitting the binding elements to disperse through the matrix and react with the surface of diamond powders. The coating of diamond with a carbide layer allows for improving the thermal properties of the composite and thereby avoid the unfavourable impacts of additional elements on the copper matrix [6]. The sintering of Cu/diamond composites is found to possess considerable challenge as no chemical interaction is observed between Cu/diamond interface due to its non-wettability of copper on diamond [7]. The observations reported in the literature shows a relatively weak bond between pure copper matrix and the diamond powders in the final fabricated composite. The use of atomized copper alloy with optimum quantity of chromium to improve the bonding in Cu/diamond interface by a fine nano-sized Cr 3 C 2 layer facilitates the enhancement in the thermo-physical characteristics and bonding strength of the fabricated composites [8]. However, by adding a carbide producing element to the copper, such as chromium or boron, a suitable link between diamond and copper can be created [9]. These elements diffuse to the diamond's surface during the sintering process, forming an interface. This creates a strong link between the composite's constituents, lowering the heat resistance [10]. Hence, Xiuhui L et al conducted a research using ball-milling as a solution based on the afore mentioned shortcomings [11].
Numerous researchers have demonstrated the development of the carbide phases Cr 3 C 2 or Cr 7 C 3 at the interface between diamond and the surrounding copper matrix for chromium addition or coating. The impact of diamond particle size and distribution on tribological, mechanical, and thermal properties has not been comprehensively investigated in any of these investigations. These improved copper diamond composites are quite strong with a minimal coefficient of expansion, and exhibit a lower heat resistance at the interface. The interfacial phase can fill the gap between the diamond and the metallic bonding matrix in addition to improving the composite material's mechanical stability. This interfacial layer can improve the thermal conductivity of the composite material by decreasing the interfacial thermal resistance [1].
The goal of this study is to look at the wear behaviour and also analyze thermal and mechanical response of copper-diamond composites made via powder metallurgy. The composites' wear characteristics was evaluated utilising a pin-on-disk wear tester in dry sliding settings. The present study gives a detailed insight to the wear process of copper-diamond composites considering the morphologies of the worn surface. Additionally, the present work describes the characteristics discovered by a more critical characterization of the interfaces, in addition to the thermal and mechanical analysis.

Materials
Copper/Diamond composites and copper samples were made by powder metallurgy route, where copper as the base metal and a sufficient mix of diamond powder is used since the goal is to create a hybrid reinforcement. Commercially available copper powder of 30 micron and 99.7% purity supplied from sigma-Aldrich and synthetic diamond powders of particle size 10 micron were applied as feedstock materials where diamond content varied from 1%wt to 5%wt.

Preparation of copper/diamond composite
Dong et al [12] suggested that ball milling can lead to particle size reduction down to the nanoscale because of the brittle nature of the reinforcement material. Copper and diamond particles are chosen in a balanced proportion, using a planetary micro mill and tungsten carbide balls in a tungsten carbide vial, high energy milling of copper powder is done for size reduction for 4 h at 300 rpm. The proportion of balls to powder is 10:1. The planetary ball mill is used to increase high energy milling inside the tungsten carbide vial by strongly centrifugally altering the Cu powder size. Cu particles are continuously impacted, welded, fractured, and reweld during the milling process. By lowering thickness to 5 μm during 4 h of milling at 300 rpm, copper powders are cold-welded, resulting in gigantic metal particles of increased size. It is more likely that diamonds will disperse in metal powders if the metal particle size is reduced. Figure 1 shows a poor interfacial connection between the copper and the diamonds is indicated by the presence of large cracks at the boundaries and the majority of the diamond particles being exposed on the fracture surfaces. The literature survey implies that the use of Cr-coating can efficiently enhance the interfacial bonding between the particles and the matrix, which in turn makes it easier to densify composite materials as a whole [13]. However, the addition of higher content of diamond particles and reinforcement can make the densification more complicated because there is no suitable matrix that can fill in the spaces created by the adjacent diamond particles, there is a very low relative density when the particle concentration is exceeded.
The powders achieved after the mixing process in the planetary mill are then cold pressed at 2 × 10 5 kPa − 2.5 × 10 5 kPa, then sintered at 1023K-1123K for 2 h in a vacuum environment and extruded at 973K-1023K to create a cylindrical specimen of 15 mm diameter and 50 mm in length.
The sintered samples were analysed by TEM and SEM to determine whether graphitization of the diamond particles in the composite will occur. Additionally, XRD was used to examine the diamond particles that were recovered from the sintered samples. The sintered samples were broken up, and the powders were dissolved in 10% HNO 3 solution to remove the copper and copper oxide, and then in 10% HCl solution to remove the copper oxide. The deposit was then employed for the XRD analysis after being baked at 50°C in an oven after filtering through filter paper.
The samples of the copper/diamond composite were initially washed with isopropyl alcohol before being ultrasonically cleaned for 15 min using a 230 VAC, 50 Hz numerically controlled ultrasonic cleaner (MEDICA INSTRUMENT MFG.CO.). Its objective was to remove contaminants from the sample's surface and any leftover impurities from the cavity, lowering the density calculation error and obtaining improved surface images.
Using Archimedes' principle and a very thin layer of vaseline coated to prevent water infiltration, the density of the composites was determined. The relative densities of the samples were calculated using the theoretical densities of pure copper (8.96 g cm −3 ) and diamond (3.52 g cm −3 ).

Metallurgical properties
A comprehensive study on the qualitative elemental composition and the microstructural features of the fabricated composites has been conducted using ultra-high-resolution TEM with an EDS attachment. Many changes in particle microstructure occur when metallic powders are ball milled in an inert environment using a high planetary mill. Powders are caught between colliding balls and/or balls and vials are subjected to high pressures during ball milling. As a result, powders undergo severe plastic deformation that surpasses their mechanical strength, which is followed by a temperature increase [14].
The microstructural characteristics and qualitative elements content of the composites formed were also studied in depth using a SEM coupled with an EDS.

Mechanical properties
However, the mechanical behaviour of the powder determines the properties of as-milled powders. During milling, a balance between coalescence and fragmentation is attained, resulting in a relatively steady average particle size. A pin-on-disk tribometer helps conducting dry sliding wear testing. The evaluated samples were cylindrical pins with a diameter of 15 mm and a length of 50 mm. Pure copper samples are manufactured for comparative purposes. The composites are then subjected to a series of grinding and polishing process to achieve metallographic finish before the wear test. Before employing a 230 VAC, 50 Hz numerically controlled ultrasonic cleaner to perform a 15-minute ultrasonically cleaning process, the samples of the copper/diamond composite were first cleaned with isopropyl alcohol (MEDICA INSTRUMENT MFG.CO.). Its goal was to reduce the density calculation error and generate better surface images by removing contaminants from the sample's surface and any lingering impurities from the cavity. On a sophisticated pin-on-disk tribometer (TR-20LE-CHM400), the CDCs (test samples) were moved against a harder counter-body of GCr15 type bearing steel with a hardness of HRC60 for real-time wear testing in compliance with ASTM G99 standard. The disc specimens are cylinders with an inner diameter of 138 mm, an outer diameter of 160 mm, and a breadth of 30 mm. The pin samples were pre-worn prior to wear test to attain contact surface between the pins and the disc.
Equation (1) was used to determine the wear rate value from the weight loss data.
where P is the applied load (N), S is the sliding distance (m), δm is the weigh loss (g), ρ is the density (g mm −3 ) and W is the specific wear rate (mm 3 Nm) −1 .
Copper/Diamond composites and copper samples were put on the wear disc, and sliding tests with various loads and distances as process parameters were performed. The tests were run at a sliding speed of 100 m s −1 for loads between 50 and 100 N and over a range of 2 to 10 km in sliding distance.
The sliding distance for all specimens in the friction coefficient test was 1000 m, and the pin-on-disk tribometer was used to automatically calculate the friction coefficients of the sample from their centres and surrounding areas.
Using a Vickers hardness instrument, microhardness was assessed with a steady load of 100 gm and a dwell duration of 15 s. With a 400-times magnification optical microscope, the indent diagonals were measured.

Thermal properties
The coefficient of thermal expansion (CTE) of copper/diamond composites can be adapted to be close to that of semiconductor chip materials (4-6 ppm/K), suggesting that copper/diamond composites can be used as the viable contender for next-generation heat sink materials in sophisticated electrical gadgets. Synthetic diamond used as a metal matrix composite reinforcement has the highest thermal conductivity in nature, up to 2200 W/ (mK) [15].
The Thermal Transport Option (TTO) of a Physical Property Measurement System (PPMS) was used to test the thermal conductivities of the composite materials at 26°C. To guarantee a one-directional heat flow, samples in the form of rods with uniform cross sectional areas were employed. (2) can be used to determine the sample's thermal conductivities,k in steady state. Where δt is the difference in temperature recorded by two thermocouples attached to the sample, A is the cross-sectional area, q is steady-state heat flow in the sample, and l is the distance between the thermocouples. The thermal conductivity obtained by experimental method is compared with the The maxwell model for the effective thermal conductivity suggested by Hasselman and Johnson [16].
Due to attributes like composition, diamond purity, and particle size significantly affect the thermal characteristics of the composite, it is challenging to directly compare the observed thermal conductivity with values given in literature. However, the interfacial thermal conductance may be computed by using the experimental value of thermal conductivity in a suitable mathematical model. In the literature, the Hasselmann-Johnson model has frequently been used to estimate the interfacial thermal conductivity of different composites [17].
A highly conductive metal matrix, like copper, is projected to be significantly affected by the addition of finely split diamond particles. The copper matrix's interaction with the diamond particles, however, has a crucial role in the transport characteristics. These characteristics of composite materials are greatly dependent on the integrity of the interface and chemical bonding, which are both crucial [18]. The current study allows determining the impact of the weight percentage of diamond and its effective dispersion in copper test specimens on their tribological, mechanical and thermal properties.

Microstructure of Cu-diamond composites
The exceptional hardness of the diamond particles restricts the microstructure observation using traditional metallographic techniques, such as optical light microscopy and polishing. This applies for micro-sized diamond particles, but in case of nano-sized diamond particles reinforced Cu-composites, the polishing process can be done to some extent.
The micro-diamond particles were observed to be scattered equally in the Cu-matrix, as shown in figures 2(a) and (b). On the other hand, in the nano-particle reinforced composite, however, a rare amount of diamond particle colonies can be seen (figures 2(c) and (d)), showing that a negligible amount of nano-sized diamond particles had agglomerated. It should be mentioned that in their as-received state, the diamond powders in nano-size, were already accumulated to a great extent. In conjunction with the accumulation stated above, there is a clear indication of finely distributed nano-diamond particles in the Cu-matrix, and the micrograph shows the copper alloy matrix and diamond powder interface as illustrated in figures 2(e) and (f). The morphology of the nanoparticles ranges from long and irregular to cuboidal, and the faceting of diamond particles has been seen in some cases (figures 2(g) and (h)). The morphology shows amorphous phases in the proximity of certain clusters of diamond in the nanoparticle Cu-diamond composite.
The results from x-ray diffraction (figure 3) revealed three significant peaks at 43.206°, 50.321°, and 73.964°. These peaks corresponded to the Cu planes (111), (220), and (311). At 2ϴ = 43.206°, the diamond-doped composite showed a significant peak. The higher intensity of peaks implies the existence of crystal orientation [19]. The interplanar distances corresponding to the three peaks were 0.209, 0.181 and 0.128 nm respectively, which are in agreement with the standard values of diamond [20].

Hardness and density
The presence of these particles reduces grain size while also increasing surface composite hardness [21]. The hardness and density of unreinforced copper alloys and diamond-reinforced copper composites are listed in table 1. The hardness of the composites rose with increasing weight percentage before decreasing with further increase in volume fraction due to the impacts of reinforcement and porosity [22]. The composite with 1%wt diamond particles exhibits the highest hardness value. Further addition of diamond particles into the copper matrix resulted in a substantial decrease in hardness. The reason for lowering hardness is the presence of diamond particles that increases the migration of the dislocation by increasing the number of contacts between the diamond and the copper matrix. In contrast, a higher stress concentration would happen in the material because diamond and copper have different coefficients of thermal expansion. These stresses would then be released during the cooling process after sintering, leading to the generation of strain, known as thermal mismatch strain. These tensions will release dislocation loops when they relax [23]. The sintering temperature has a negligible influence on the hardness of the composite material.
When the weight fraction of diamond powder was smaller, the density of the composites stayed almost the same, but when the weight percentage of the diamond was larger, the density decreased dramatically. The table 1 shows that, for both pure copper and copper/diamond composites with varying %wt, the density of sintered copper/diamond composites at 750°C was lowest. The density of composites reached its highest point when the sintering temperature was increased to 800 C. The density of composites increased initially with temperature before declining, a pattern that is consistent with the findings in [23]. The interior of the composite becomes denser and the interface condition is optimized with the progressive rise in temperature, which is the key factor.

Impact of diamond particles on wear rate
Copper samples and copper/diamond composites were placed on the wear disc, and various sliding tests where conducted by changing process parameters such as loads, sliding distance etc to investigate the effect of diamond contents, applied loads and sliding distance on the tribological behavior of the composites. The tests were conducted over a sliding distance of 2 to 10 km at a sliding speed of 100 m s −1 with loads ranging from 50 to 100 N. The track radius and rpm of disc were kept constant.
It is noted that the addition of diamond particles in an optimum amount resulted in improved wear resistance of test samples (adding 1%wt diamond particle to the copper matrix gives the superlative result) [8]. The wear depends on the applied load, temperature, speed, and reinforcement behaviour. It is evident from the outcomes that introducing diamond particles can reduce the wear rate of the sample up to a certain point while maintaining constant values for factors like track diameter, speed, and temperature. Further addition of reinforcement ended up with drastic intensification of wear rate. In the case of Cu matrix composites, the major wear mechanism was found to be ploughing wear whereas, fragmentation/cracking was observed as the major wear mechanism in diamond particle reinforcement [24]. Particle detachment/cracking and pull-out on the surface affected by the wear of the sample indicated some degree of delamination wear. The micrographs ( figure 2) show that the diamond particles, which make up a minor fraction of the overall composite, were evenly dispersed throughout the sample, with no notable grouping or isolation. As per the experiments, the diamond was found to have a uniform distribution in the Cu-alloy matrix, which leads to the high wear resistance of the diamond doped composite primarily due to the strengthening effect of the matrix which is caused by its wetting characteristics at the matrix-reinforcement interface and from milling time and applied compaction pressure. The fabricated composite of 1%wt diamond particle as reinforcement shows no signs of segregation on the Cualloy and reinforcement interface.
While studying the total wear behaviour of a diamond doped copper matrix composite, the contributions from both the matrix and reinforcing phases should be considered. The determination of precise contribution in the overall composite wear process is rather challenging process even with noticeable engagement of the reinforcement phase during wear as the interfacial characteristics of the Cu-alloy matrix and diamond particle have a substantial effect but are complex to understand. Additionally, depending on the crystallographic orientations, various wear rates and might exist within the diamond reinforcement phase [25]. Figure 4 shows how the applied load affects the wear rates of pure copper and copper-diamond composites. The wear rate of all the samples gradually rises with applied load in the load range of 50 N to 100 N, and the rise becomes significant when the load exceeds 80 N. For a range of applied normal loads, the wear rate for the copper-diamond composite with 1% wt diamond powder as reinforcement is lower than for all the other samples. In addition to increasing the material's hardness, which is a key element impacting wear resistance, the inclusion of diamond powder also protects the softer matrix during abrasive sliding. Additionally, it may effectively prevent the penetration and cutting of strong asperities into bearing steel and strengthen the copper matrix. As a result of the addition of the diamond particles upto a limit (1% wt), the composite's wear resistance is increased when compared to pure copper.  The copper matrix's surface may get microploughed and grooved as a result of the harsh asperities on the bearing steel, bringing the diamond reinforcing particles into direct touch with them. Hard asperities' cutting effectiveness is decreased by the diamond's higher hardness compared to hard asperities. Figure 4 also depicts that, 95%Cu 5%Di showed the highest wear rates at 100 N. The temperature of the composite wear surface as a function of increasing load was one of the factors examined. It was found that when the applied stress rose, the temperature of the composite wear surface increased significantly. As a result, the wear phenomena of composites at high loads can be partly attributed to a drop in the interfacial strength between the reinforcement and matrix caused by an increase in sample temperature, which causes particle pullout during wear. The inclusion of diamond particles beyond the limit led to dramatic acceleration of wear rate as the temperature of the composite wear surface increased significantly with the increase in applied load, which decreases the interfacial strength between the matrix and the reinforcement, leading to the particle pullout during wear [26]. Figure 5. depicts the wear rates of pure copper and copper-diamond composites as a function of sliding distance, normal load (80N), and sliding speed (100 m s −1 ). It illustrates that the wear rates of the samples somewhat increase with increasing sliding distance. As can also be shown, at each corresponding sliding distance, the wear rate of the copper-diamond composite with 1%wt diamond particles as reinforcement is significantly lower than that of pure copper and all other samples. The diamond's strong ceramic reinforcing, which releases less when sliding than other composites, is what gives rise to this phenomenon. The optimal amount of diamond particulate is introduced during dry sliding wear, which causes crumbling of the particles. The creation of a protective interface layer as a result of particle disintegration reduces the direct contact of sliding faces and thereby lowers wear [27].
The composite with 5% weight of diamond particles as reinforcement exhibits the highest rate of wear, as can also be shown. Friction develops with increasing sliding distance, which leads to the shattering of reinforced particles from the base matrix Cu. The increase in wear loss at the contact area is attributed to an increase in friction caused by diamond pull-out particles. Particles with pull-out reinforcement are momentarily trapped between the contact surfaces.Despite the increased interfacial strength of the hard reinforcement particles in the matrix, pull-out Diamond particles produce a reduction in wear resistance.
The wear depends on the applied load, temperature, speed, sliding distance, and reinforcement behaviour. The wear rate curves show that adding diamond particles up to a limit can lower the wear rate of the sample while maintaining other factors like temperature and track diameter as constants. Further addition of diamond particles led to a dramatic acceleration of wear rate. Ploughing wear was discovered to be the primary wear mechanism in Cu matrix composites, whereas fragmentation/cracking was revealed to be the primary wear mechanism in diamond particle reinforcement. A certain amount of delamination wear was indicated by particle detachment/cracking and pull-out on the sample's surface that was worn.
SEM micrographs of worn surfaces for pure copper and copper diamond composite samples (Cu-1% Di, Cu-3%Di, and Cu-5%Di) are shown in figures 6(a)-(d) at an applied normal load of 80 N and a sliding speed of 100 m s −1 , respectively. The density, porosity, and hardness of the compacts, as well as other factors like the applied normal wear load and sliding velocity, affect the worn surface differently for Cu and Cu/Diamond compacts. As an illustration, sliding contact at high loads and high velocities typically results in significant surface wear [28].
Because of the substantial counter body penetration in the Cu matrix, the worn surface of the Cu compact is rough, somewhat deep, and uniform. Additionally, as shown in figure 6(a), micro -cracks and shear wedges caused by plastic movement close to the subsurface region can occur. Figure 6(b) depicts the SEM image of worn surface of copper composite having 1% wt diamond particle as reinforcement. Compared to all other samples, Cu-1%Di compact is smoother, shallower, and more consistent. This is explained by the Cu-1%Di compact's greater density and hardness when compared to the pure Cu compact. The optimal quantity and effective dispersion of diamond particles are connected to the Cu-1%Di composite's high wear resistance. These particle's high load bearing capacity shields the matrix from the abrasive's damaging activity by limiting their penetration depth and acts as a more effective barrier against subsurface shear when the hardened steel counterface moves. Thus, the wear loss is significantly lower when compared to composites with a larger diamond concentration and pure copper compacts.
Micro-cracks and layer separation of the surface can arise, as seen in figures 6(c) and (d), and are analogous to the pure Cu compact. Due to the presence of hard and irregular diamond particle reinforcement and their uneven distribution, the worn surface of both samples (Cu-3%wt Di and Cu-5%wt Di) is significantly rougher than Cu-1%wt Di. Given the increased porosity, lower density, and lower hardness of the composites in comparison to the Cu-1%wt Di compacts, the deeper and non uniform wear tracks may be the result of a greater removal of Di particles from the surface due to ploughing in copper matrix composites.
The worn surface's high O content is revealed by the EDS examination of the surface. This demonstrated that pure copper suffered oxidation wear. This shows that when the specimen was exposed to sliding wear, which is characterised by adhesive wear, oxidation had taken place in the worn track as a result of friction heat. Figures 6(c) and (d) further show that during the sliding process, some diamond particles peeled off from the compact surface and served as an abrasive medium (abrasion wear), causing grooves in the worn surface.

Impact of diamond particles on friction
The applied pressure, track radius, and disc rpm were all maintained at the same levels. It has been discovered that adding the appropriate amount of diamond particles to test samples minimises friction. Figure 7 depicts the behaviour of the sample when subjected to frictional forces at various diamond volume fractions.
It can be observed that copper unveils large fluctuation compared with Cu 1%Di composite in figure 7(a). This fluctuation may be attributed to the intense cohesive contact between the copper pin and the rotating counter disc. Figure 7(b) shows a decrease in fluctuation with respect to a 1%wt increase in the hybrid particle dispersion. The presence of diamond particles, which take away the load during dry sliding tests, avoiding contact between cohesive copper matrixes and revolving counterpart disc, is a primary reason for this decrease in fluctuation. The introduction of self-lubricating particles scattered on the surface of copper creates a layer of self-lubrication between the rotating counter disc and the composite pin, leading to a reduction in frictional force fluctuations [27]. It is evident from the figure 7(c) and (d), with further increase of diamond particle, the coefficient of friction reduces marginally. However, inclusion of diamond particles beyond the limit, the composite's ductility may be reduced in addition to the possibility that these nanoparticles will pull away from the matrix. Even if tribological islands exist under these circumstances, the sliding against the counter body makes them easily fragmented and removed. Due to the strong adhesion between the worn surface and the stainless steel counterface material, there are significant changes in the friction coefficient that may be observed. In other words, the frequent formation and removal of tribological islands and the high adhesion between the wear surface of the nanocomposite and stainless steel counterfac material are factors in the extreme and high fluctuations in the friction coefficient of coper composites having 3% and 5% diamond particles as reinforcement [29].

Impact of diamond particles on thermal conductivity
The Maxwell theory can be used to determine the composite's overall thermal conductivity assuming a basic composition model with spherical particles scattered in a continuous matrix flow. But regardless of particle size, a composite's thermal conductivity is assessed by taking into account the volume percentage and thermal conductivity values of the particles and matrix phase [30]. The idea of an interfacial thermal barrier resistance was introduced by Hasselman and Johnson established the concept of an interfacial thermal barrier resistance and showed the particle size dependency [16]. Equation (3) was proposed by them as a mathematical formula for determining effective thermal conductivity. The factors such as wetting properties and the thermal expansion imbalance of the matrix and filler materials, the dislocation density and particle size of the material, and defects like voids generated during sintering determine the material's thermal conductivity [31]. The thermal barrier between the interface must be considered as the thermal conductivity of diamond is proportional to their size. With a small amount of addition, the diamond is spread uniformly in the copper matrix, limiting the impact on particle size and thermal expansion imbalance of the material. Diamond's huge size effectively compensates for the influence of flaws like pores and voids created in composites during sintering, which is advantageous to phonon mobility and thermal conductivity enhancement [32].
Results show slightly larger values of thermal conductivities in comparison to the theoretical values as expected by Hasselman's equation at higher fractions of the diamond. This may be due to the high tightness of the bonds between each diamond particles. The powder metallurgy approach produces diamond/copper composites with a homogeneous microstructure in which diamond particles are equally dispersed in the copper matrix. The outcomes of the experiment have been verified using the maxwell model for the effective thermal conductivity of a particulate composite.
It is evident from the figure 8, composite with a diamond content of 1 wt% has the best thermal conductivity. The copper matrix has consistent diamond dispersion with a little amount of addition, hence the impact on the material's mismatch in terms of grain size and thermal expansion is minimal. The movement of phonons and the enhancement of thermal conductivity are benefited by the large size of diamond's ability to efficiently mitigate the influence of flaws like pores and holes created in composites during sintering [32]. As the amount of diamond in the composites rises beyond a limit, the interfacial thermal resistance is further enhanced by the significant thermal expansion mismatch and limited wettability between the copper matrix and diamond. The thermal conductivity of composites with 3 wt% and 5 wt% dramatically reduces during sintering due to holes and other flaws that also contribute to blocking heat flow dispersion and transfer.
The increase in particle size, the diamond composition and the dislocation density of the composite results in the rise of nonlinear vibration between the lattice [33]. Some amount of coupling is found to exist between the lattice waves, which leads to the collision of phonons, reducing the mean free route of phonons. The coupling existing in the lattice waves rises as the dislocation density of the composites grows, the chance of phonon collision increases, and the associated mean free path decreases. The major source of thermal resistance in crystals is scattering generated by phonon collisions [31]. It's also one of the reasons for the composites' decreased thermal conductivity.

Conclusion
• Copper-diamond composites with diamond particles have dramatically improved wear resistance. Under the current wear test settings, wear is a function of the applied load, temperature, speed, and percent diamond content by weight percent. The experiment shows that adding diamond particles reduces the wear rate of the sample up to a certain point while maintaining all other variables constant, such as track diameter, load, speed, and temperature.
• The addition of reinforcement beyond the limit resulted in a dramatic increase in the rate of wear. The fragmentation/cracking of diamond particles and the ploughing wear in the Copper-alloy matrix was found to be the major wear mechanisms. On the worn surfaces of the sample, detachment/cracking and particle pull-out revealed some degree of delamination wear.
• The density of the composites remained practically constant when an ideal weight fraction of diamond powder was added to the copper matrix as reinforcement, but when the weight % of diamond was increased, the density declined drastically. Due to the effects of reinforcing and porosity, the hardness of the composites increased with increasing weight percentage before dropping with rising volume fraction.
• Wear behaviour, hardness, frictional resistance, and thermal condutivity are all improved when diamond particles are added up to a certain percentage (1 percent). Surface flaws, particle size, dislocation density, and thermal expansion imbalance between Cu and diamond leads to a considerable drop in the hardness, thermal conductivity and wear resistance of the composites when more reinforcement is added to the copper matrix.