1 Introduction

The main focus for technological advancement is to improve the properties of components and for these, novel materials are required. Pure metal cannot be abundantly used for industrial applications, because it doesn’t meet the demand for conflicting properties [1, 2]. To overcome these shortcomings, alloys are chosen as they possess remarkable properties like high tensile strength, high corrosion resistance, excellent hardness and reasonable wear resistance over pure metals [3, 4]. Continuous drive in various industrial fields, particularly in automotive industry towards higher wear resistance, weight reduction and improved crash performance at an economically viable cost, has resulted in usage of new material called advanced materials [5]. Metal matrix composites (MMCs) are one such materials having properties like high strength-to-weight ratio, high stiffness-to-density ratio, better fatigue resistance, lower co-efficient of thermal expansion and better wear resistance [6]. Copper MMCs are extensively used in several applications such as welding electrodes, plasma interactive components in fusion power system, high speed motors, high performance switches, machine guide ways, electrometers and bearings [7, 8]. Copper MMCs reinforced with varying volume percentage (5, 10 and 15) of Al2O3 (alumina) particles (size ~ 5.71 µm) have been investigated and it was found that hardness of the composite was maximum at Cu–15 vol% Al2O3 [9]. Copper/15–60vol% TiB2 (titanium diboride) composite having the particle size of 3–7 µm fabricated by powder metallurgy technique was studied and the result revealed that the composite had higher hardness, higher compressive yield strength, higher wear resistance and scratch resistance compared to unreinforced alloy [10]. Dry sliding wear behavior of Cu/Ni3Al (nickel aluminide) composite containing varying wt% of 5, 10 and 15 with an average size of 63 µm fabricated by powder metallurgy route was investigated and it was found that wear resistance of the composite decreased with increasing wt% of Ni3Al particles [11].

Continuous efforts have been made by the researchers to find a new novel material called functionally graded materials (FGM). These materials are fabricated by different techniques such as powder metallurgy, centrifugal casting, vapour deposition technique and solid freeform process [12]. Among these techniques, centrifugal casting route is most economical and amenable for large scale production [13]. The mechanical and tribological properties of the composite fabricated by centrifugal casting route is strongly dependent on solidification time, size, shape and volume fraction of the reinforcement particles [14]. The solidification time of the centrifugal casting depends on speed of rotation of the mold and it increases with increase in volume fraction of the particles [15]. The mechanical and wear properties of functionally graded copper composite reinforced with 15 vol% of NbC (niobium carbide) (size < 180 µm) particle fabricated by ball milling technique was investigated. From this investigation, it was observed that the mechanical and wear properties of the composite were superior to that of conventional pure copper [16].

The dry sliding wear of copper alloy (phosphorous bronze) reinforced with 5 wt% of (silicon carbide) SiC particles with the average size of 50–100 µm fabricated by powder metallurgy technique was analyzed and the result revealed that wear resistance of the composite increased with increase in sliding speed and load [17]. The wear resistance of copper/Al2O3 (Alumina) (2.5, 7.5 and 12.5 wt%) composite fabricated by powder metallurgy technique was studied and it was found that load was the most influential parameter on wear rate of the composite [18]. The effect of applied load and sliding velocity on wear rate of pure copper and aluminium were investigated and it was found that wear rate of both metals increased with increase in load and sliding velocity [19].

From the literature survey, it is inferred that the dry sliding wear behavior of functionally graded copper composite fabricated through centrifugal casting technique has not been explored fully. Hence the present research is to fabricate the functionally graded Cu–10Sn/10SiC composite using such technique and to analyze the dry sliding wear behavior using Taguchi’s design of experiment (DOE).

2 Experimental Details

Copper has excellent load bearing and wear resistance properties that makes it the material of choice for this investigation. Copper is used in many applications such as electrical wire, condenser tube, springs and automotive parts. Tin (10 wt%) was added as an alloying element to copper since it highly improves mechanical and tribological properties as compared to other alloying elements. The application of tin metal includes the production of electronic valve, capacitor electrode, bushes and bearings. The density of the matrix was determined to be 8.731 g/cm3. SiC particle was reinforced into copper alloy (Cu–10 wt% Sn) in order to develop a combination of properties. The grain size of SiC particle incorporated in copper alloy was 36 µm. Scanning electron microscope (SEM) image of SiC particle is shown in Fig. 1. SiC has a property such a low density (3.21 g/cm3), high strength, superior hardness and low co-efficient of thermal expansion as compared to other reinforcements. It is used in various applications such as car brake, clutch, bulletproof vests and semiconductor electronic devices.

Fig. 1
figure 1

SEM image of SiC particle

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Functionally graded alloy (Cu–10Sn) and composite (Cu–10Sn/10SiC) was fabricated using stir casting technique followed by horizontal centrifugal casting route. Initially, solid copper (6 kg) was taken in the graphite crucible and heated to a temperature of 1100 °C using electric resistance furnace (Fig. 2). After melting the copper, tin (10 wt%) in the form of small ingots were slowly dissolved into the copper melt. The melting was carried out under argon gas atmosphere in order to prevent undesirable chemical reactions. SiC particles were preheated to 250 °C in order to remove the impurities during solidification. Then SiC particles were gradually added into the copper alloy melt. During the addition of SiC particles, the melt was continuously stirred at 200 rpm for 10 min for attaining uniform dispersion of the reinforcement particles in the liquid alloy. The graphite crucible containing the melt was taken out from the furnace and poured into the preheated (350 °C) horizontal centrifugal mold rotating at 1000 rpm (Fig. 3). Then the melt was allowed to solidify and was ejected from the die. The dimension of obtained solidified hollow casting was Øout100 × Øin70 × 100 mm (Fig. 4). The same procedure was repeated for the fabrication of functionally graded copper alloy.

Fig. 2
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Copper melting furnace

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Fig. 3
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Centrifugal casting setup

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Fig. 4
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Cast specimen a full section and b cut section

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In order to observe the presence of phases, microstructural investigation was conducted on the cast specimen using (Zeiss Axiovert 25 CA, Model: EP-IMET-3) inverted metallurgical microscope. The microstructures of the composite as well as unreinforced alloy were observed at different radial distances (1, 15 and 30 mm) from the outer periphery of the casting. Initially, the small section of the cast portion (30 × 30 × 10 mm) was cut using an abrasive cutter and the test pieces were flattened by linisher polisher. Test pieces were polished by different grits of emery paper such as 1/0, 2/0 and 3/0 to obtain scratch free surface. Fine polishing was done using fine grinder with addition of alumina powder (50 µm) to achieve mirror like finish on the surface. Finally, specimens were etched with a solution of 2 g FeCl3, 5 ml HCl, 30 ml water and 60 ml methanol for obtaining the microstructural features. The concentration of reinforcement particles along the radial distance of composite was analysed using Image J software.

Hardness of both composite and unreinforced alloy was measured at different radial distances (1, 5, 8, 15, 20 and 30 mm) from outer periphery of casting using Vicker’s micro (Make: MITUTOYO, Model: MVK-H11) hardness tester and test procedure was followed according to ASTM B721-91 standard. The test was performed at the load of 0.5 kg with a dwell time of 15 s and the mean value out of five readings was reported.

Tensile test specimens were prepared from cast according to the ASTM E8 standard for tensile tests. Tensile tests were performed on both composite and unreinforced alloy at outer (1–15 mm) and inner zones (16–30 mm) using universal testing machine (UTM) (Make: Tinius Olsen, Model: H25KT).The tests were carried out at a strain rate of 2 mm/min.

The dry sliding wear experiments at inner region of composite were conducted at ambient atmosphere (~ 30 °C, 22% humidity) as per the ASTM G 99 standard using pin-on-disc tribometer. It consists of a rotating disc made up of hardened steel (EN 31) having a hardness of 65 HRC and that acts as the counterface on which the specimen slides. The specimens were machined from the cast composite to the required dimension of 12 × 12 × 30 mm. A track diameter of 80 mm was selected for this assessment. A lever-arm device was used to hold the test specimen which allowed the sample to get pressed against the rotating disc on application of load. For each test, the disc was ground with 2/0 grit emery paper in order to establish fresh contact between the specimen and counterface material. The samples were properly cleaned with suitable non-film forming cleaning agents in order to remove foreign material and dirt particles. The specimens were weighed before and after the test using digital electronic weigh balance with an accuracy of 0.1 mg. Then the wear rate of composite was calculated through weight loss method.

Taguchi’s DOE is a powerful tool used to analyze the design parameters. This technique offers high quality product design with minimum number of experiments at low cost and time compared to other conventional approaches. It is also possible to optimize the best combination of factors with less deviation. Hence, this method was used in the present investigation to study the influence of process parameters (load, sliding distance and sliding velocity) on wear rate. Table 1 indicates the selected process parameters and values assigned to their corresponding levels. The L27 orthogonal array was chosen for this experimentation according to the degree of freedom.

Table 1 Levels of the variables used in the experiment
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3 Results and Discussion

Microstructural evaluation, mechanical properties (hardness and tensile strength) and dry sliding wear behaviour have been discussed in the following subsections.

3.1 Metallographic Examination

The microstructure of functionally graded unreinforced copper alloy and its composite is shown in Figs. 5 and 6 respectively. Functionally graded unreinforced copper alloy at both inner (Fig. 5a) and outer regions (Fig. 5b) show a typical dendritic structure without cracks and other defects. The structure differences between the inner and outer regions are dendritic size and grain size. The outer region has coarse dendrites and at inner region, the dendrites are very fine in size. This may be attributed to preheating and slow cooling of the mould. In microstructure of copper composite (Fig. 6a–c), dark phase region denotes hard SiC particles and the remaining white phase regions indicate copper alloy matrix. The radial distance of 1 mm from outer periphery of the composite (Fig. 6a) surface shows lesser volume percentage (6%) of SiC particles. At the radial distance of 15 mm from outer periphery (Fig. 6b), the gradient structure transition is observed over matrix and also the volume percentage of SiC particles is found to be higher (15%) than at 1 mm radial distance of the composite. The microstructure (Fig. 6c) observed at the radial distance of 30 mm from outer periphery reveals higher volume percentage (40%) of reinforcement particles distributed over matrix without significant clustering or isolation. With the increase of radial distance from outer periphery, the volume fraction of reinforcement particles increases progressively. The inner periphery of the composite has more SiC particles as a result of density difference between the SiC particles and copper matrix and also the centrifugal force that is created due to rotation of the mould. A similar result is observed in centrifugally cast Copper-graphite composite [20].

Fig. 5
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Microstructure of unreinforced alloy a outer periphery and b inner periphery

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Fig. 6
figure 6

Graded microstructures in composite specimen at different radial distance from outer periphery a 1 mm, b 15 mm and c 30 mm

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3.2 Hardness

The hardness results obtained at different radial distances (1, 5, 8, 15, 20 and 30 mm) from the outer periphery of the composite and unreinforced functionally graded alloy is shown in Fig. 7. From the figure, it is evident that the hardness of the composite is higher compared to unreinforced alloy. At the radial distance of 1 mm from outer periphery, the hardness of the composite is observed to be 116 HV, which is 38.8% higher than that of unreinforced Cu alloy. During solidification, the incorporation of second phase particles in Cu alloy matrix introduces local strain and limits the grain growth which ends up with fine grain size. Fine grain structure has large number of grain boundary areas, which significantly impede the dislocation motion when plastic deformation take place and as a result, hardness of the composite increases towards the inner periphery. A similar result is observed in Mg–Cu based composite [21]. The formation of gradient structure (Fig. 6) due to varying reinforcement concentration also enhances the hardness of composite. At the radial distance of 30 mm from the outer periphery, the composite has maximum hardness value of 205 HV due to the higher volume fraction of reinforcement particles and is 41% higher than that of the unreinforced alloy. The higher volume fraction of reinforcement particles results in greater resistance to indentation and eventually lead to better hardness. The hardness also increases as a result of formation of dislocation loop over second phase particles. When the dislocation experiences shear strength, it moves and bow around the reinforcement particles, leaving behind a dislocation loop. This loop has stress field which interfere with the dislocation movement and as a result, the hardness is found to be high. The hardness of the unreinforced alloy increases towards inner periphery due to the occurrence of substitutional solid solution strengthening by increased solute atom.

Fig. 7
figure 7

Hardness of functionally graded alloy and composite at different radial distance

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3.3 Tensile Test

Tensile test results of both functionally graded alloy and its composite at inner and outer zones are shown in Table 2. The refinement of grain size produces high tensile strength at inner zone than that at outer zone of unreinforced alloy. For composite, strength of the outer zone is found to be less as a result of less volume percentage of reinforcement particles in matrix. The dislocation is able to move relatively over longer distance, as the obstacles for movement of dislocation is very less. Thus, due to lower resistance to deformation, outer zone of the composite specimen fails at low tensile strength. At inner zone, the composite has high tensile strength (248 MPa) due to the presence of higher volume fraction of reinforcement particles. The dislocation pinning effect provided by reinforcement particles to matrix is higher thereby resulting in delayed nucleation of voids in the specimen. Therefore, the strength required for occurrence of plastic deformation increases and consequently provide the material with higher tensile strength.

Table 2 Tensile test result for unreinforced alloy and composite
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3.3.1 Fractography

Fractured surface morphologies for inner and outer zones of functionally graded alloy are shown in Fig. 8. From Fig. 8a, it is observed that, the failure of inner zone of alloy is not completely in ductile mode due to the grain refinement and hence, the specimen fails over limited ductility. On the other hand, the outer zone (Fig. 8b) almost fails by ductile mode, since the grain size is large and dislocation can move easily without any obstacles. Also, the presence of inclusion act as stress concentration, thereby resulting in initiation of cracks whose propagation was torturous. Hence, the fractured surface is observed to be in fibrous nature, which is one of the characteristic appearance of ductile fracture.

Fig. 8
figure 8

Fractographs of copper alloy a inner zone and b outer zone

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The fractured surface morphologies of functionally graded composite at inner and outer zones are shown in Fig. 9a, b respectively. The fractograph of inner zone (Fig. 9a) of composite reveals quasi-cleavage mode, where, the specimen fails due to brittle nature. This is markedly attributed to the high volume fraction of reinforcement particles. The cracks initially propagate intergranularly and occur along the grain boundary which results in high tensile strength. However, at the outer zone (Fig. 9b), the volume fraction of reinforcement particle is less, which results in initiation and coalescence of voids and produces a dimple morphology, which is the characteristic appearance of ductile mode of failure.

Fig. 9
figure 9

Fractograph of functionally graded composite a inner zone and b outer zone

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3.4 Wear Behavior

The microstructural and mechanical properties results are found better at the inner surface of the composite and hence, only this surface is considered for wear analyses. The resulting wear rate of the composite specimen by conducting 27 experiments obtained from L27 orthogonal array is shown in Table 3.

Table 3 Taguchi DOE-process parameter and measured wear rate
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3.5 Signal-to-Noise (S/N) Ratio Analysis

S/N ratio is the ratio of mean to the standard deviation and it is used to evaluate the effect of process parameters such as load, sliding velocity and sliding distance on wear rate of the composite. “smaller-the-better” characteristic is subjected to this analysis and this help to attain better results with less deviation. The rank for the parameters based on the delta value is shown in Table 4. Delta statistic is the difference between the highest and the lowest average of each factor. From the S/N ratio analysis, load can be found to be the most influential parameter on wear rate followed by sliding distance and sliding velocity.

Table 4 S/N ratio response table
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3.6 Parameters' Influence on Wear Rate of Composite

The S/N ratio and the mean effect plots are shown in Figs. 10 and 11 respectively. From the S/N ratio plot, it is evident that the optimum value of parameters enhancing the wear resistance of the composite are load of 10 N, sliding velocity of 3 m/s and sliding distance 500 mm.

Fig. 10
figure 10

S/N ratio plot

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Fig. 11
figure 11

Wear rate main effects plot

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3.7 Effect of Applied Load

The influence of applied load on wear rate of the composite is shown in Fig. 11a. The trend follows the fact that the wear rate of composite is quite sensitive to the applied load and is observed that, the wear rate of composite increases as load is increased from 10 to 30 N. At a lower load of 10 N, wear rate of the composite is less due to work hardening tendency of the matrix in the presence of hard SiC particles, where the load transfer between the matrix and reinforcement particles is almost homogeneous. Also, very thin amount of oxide film is formed on the mating surface which assists in obtaining mild wear at lower load. As the applied load increases from 10 to 20 N, wear rate of composite is found to be increased linearly due to high pressure contact between the mating surfaces and this lead to faster removal of the formed oxide film resulting in higher wear rate. A similar result was observed in previous work on Cu/WC (53 µm) metal matrix composite [22]. At higher load of 30 N, severe plastic deformation on the subsurface layer occurs due to higher penetration of counterface material. Hence it is concluded that the dominant wear mechanism at lower load is abrasive wear, while at higher load, it is adhesion and delamination wear. The wear rate of this composite at a high load of 30 N with constant sliding velocity of 2 m/s and at constant sliding distance of 1000 m is observed to be 77% less than that of Cu–10 vol%TiC–10 vol%Graphite hybrid composite fabricated by powder metallurgy technique [23].

3.8 Effect of Sliding Velocity

The effect of sliding velocity on the dry sliding wear rate of the composite is shown in Fig. 11b. The higher contact time between the specimen and the counter face material at low sliding velocity of 1 m/s induces higher rubbing action, resulting in high wear rate. As the sliding velocity increases from 1 to 2 m/s, there is considerable rise in interfacial temperature which enhances the material transfer between the pin and counterface and this results in the formation of stable thin film. This make the composite more resistant to wear at sliding velocity of 2 m/s. The wear rate of the composite significantly decreases as a function of sliding velocity, which is attributed to accelerated formation of oxide layer caused by increasing temperature. As the sliding velocity increases to 3 m/s, the direct contact between the composite and counterface material is prohibited by the formed oxide film resulting in minimum wear rate of composite. The decrease in wear rate of composite due to increase in sliding velocity (1-3 m/s) is observed to be 68% higher than that of Cu–10 wt%Sn/5 wt%MoS2 MMC fabricated by powder metallurgy method [24].

3.9 Effect of Sliding Distance

The influence of sliding distance on wear rate of the composite is shown in Fig. 11c. It is observed that the wear rate of the composite increases as sliding distance is increased from 500 to 1500 m. At lower sliding distance of 500 m, the contact is mostly made by high concentration of SiC reinforcement particles in the inner surface with the counterface material causing minimum wear rate of the composite. The presence of higher volume fraction of the reinforcement particles contributes to decrease in wear rate and a similar inference was reported on Cu/20 wt%SiC metal matrix composite [25]. As the sliding distance increases from 500 to 1000 m, the projected hard reinforcement particles are pulled-out from the matrix which act as third particle and slide between the mating surfaces; thereby increase in wear rate is observed. As the distance increases further to 1500 m, the load bearing capacity of reinforcement particles reduces due to less energy absorption, and consequently counterface contact happen with the matrix, resulting in higher wear rate.

3.10 Analysis of Variance and the Effect of Factors

Analysis of variance (ANOVA) was carried out with the objective of evaluating the process parameter on the total variance of the results. The impact of various parameters and their interactions were determined from this analysis. This analysis was conducted for a confidence interval of 95% and significant level of 5%. The parameter having P value less than 0.05 describes significant effect on wear rate of the composite. From Table 5, the percentage contribution of each parameter and their interactions are observed. Load (54%) is the highest influential parameter on wear rate followed by sliding distance (18.2%) and sliding velocity (3.7%). The effect of interactions between load and distance is higher (16%) as compared to other interaction parameters towards wear rate of the composite.

Table 5 Result of analysis of variance
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3.11 Linear Regression Analysis and Confirmation Experiment

The correlation between parameters such as load, sliding distance and sliding velocity are obtained by linear regression equation [1] using Minitab statistical software.

The regression equation obtained is as follows

$${\text{W(mm}}^{3} / {\text{m)}}\, = \, - \,0.00154\, + \,0.000180\,{\text{L}}\, + \,0.000006\,{\text{V}}\, + \,0.000002\,{\text{D}}\, - \,0.000011\,{\text{L}}\,*\,{\text{V}}\, - \,0.000000\,{\text{L}}\,*\,{\text{D}}\, + \,0.000000\,{\text{V}}\,*\,{\text{D}}$$
(1)

The confirmation experiment was carried out with a combination of the optimum levels other than the specified levels to compare the results with the predicted performance. Table 6 gives the level of process parameters and its regression wear rate along with experimental wear rate. It is observed that, an error percentage of confirmation experiment is less than 5% and there is a mutual agreement between the experimental and observed results.

Table 6 Levels of process parameters and comparison of experimental and predicted results
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3.12 Scanning Electron Microscopy Analysis

Worn surfaces of the functionally graded composite characterized by SEM are shown in Fig. 12a–f. From Fig. 12a, b, the effect of applied load (10 and 30 N) on worn surfaces of the composite with constant sliding distance (1000 m) and at constant sliding velocity (2 m/s) is observed. The worn surface of composite specimen at low load of 10 N (Fig. 12a) reveals lesser wear tracks, fewer scratches and wear ploughing, which are characterized by the longitudinal grooves parallel to the sliding direction. During sliding, the reinforcement particles remain intact with the composite material resulting in minimum material removal and hence, mild wear is the dominant wear mechanism at low load. The worn surface observed at high load of 30 N (Fig. 12b), shows severe scratches and delamination due to the significant amount of plastic deformation on the composite surface. This is attributed to the deformation of matrix along the sliding direction, which removes more material in the form of layers. The severe wear is also characterized by the presence of morphologies such as adhesion, large grooves and wear patches. A similar result was observed in previous research on Cu/Ti2SnC metal matrix composite [26].

Fig. 12
figure 12

SEM images of worn specimens at different conditions a L = 10 N, D = 1000 m and V = 2 m/s, b L = 30 N, D = 1000 m and V = 2 m/s, c L = 10 N, D = 500 m and V = 1 m/s, d L = 10 N, D = 1500 m and V = 1 m/s, e L = 20 N, D = 1000 m and V = 1 m/s, f L = 20 N, D = 1000 m and V = 3 m/s

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The effect of sliding distance on worn surfaces of the composite at constant sliding velocity (1 m/s) and load (10 N) are shown in Fig. 12c, d respectively. At low sliding distance of 500 m (Fig. 12c), counterface contact is exhibited with hard SiC reinforcement particles, which maintain the structural integrity without occurrence of significant plastic deformation on the surface. Thus, the shear band formation is observed to be less, indicating lesser wear rate on the surface. At high sliding distance of 1500 m (Fig. 12d), the shear stress induced on the reinforcement particles exceed the fracture strength and thus it lead to fracture. As a result, counterface contact apparently moves towards the matrix and result in high wear rate. Hence, goughing and shear band formation are found to be the dominant wear mechanisms at high sliding distance.

The worn composite surfaces by varying sliding velocity (1 and 3 m/s) at constant sliding distance (1000 m) and constant load (20 N) are shown in Fig. 12e, f respectively. At low sliding velocity of 1 m/s (Fig. 12e), severe formation of wear debris and delamination of the composite surface are observed and this results in maximum material removal rate. This is due to high rubbing action produced between the specimen and counterface material. At high sliding velocity of 3 m/s (Fig. 12 f), material removal rate is found to be minimum due to the formation of thin Mechanically Mixed Layer (MML) over composite surface. This MML is formed due to diffusion of oxygen atom on plastically deformed regions. This formed layer protects the matrix from severe wear, indicating only lesser wear tracks and shallow grooves on the surface. A similar mechanism was observed in previous research on Cu/WS2/Graphite hybrid composite [27].

The worn surface at optimum condition of L = 10 N, D = 500 m and V = 3 m/s is shown in Fig. 13 and it is observed that the shallow scratches and minimum delamination of the composite assure lesser wear rate.

Fig. 13
figure 13

SEM image of worn surface at optimum condition (L = 10 N, D = 500 m and V = 3 m/s)

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4 Conclusion

The following conclusions are drawn from the research on mechanical and tribological properties of functionally graded copper alloy and Cu/SiC composite:

  • Microstructural examination showed the formation of gradient structure along the radial direction of the composite. The maximum volume fraction (40%) of reinforcement particles segregated towards inner region was attributed to density difference between reinforcement particle and matrix and also, the creation of centrifugal force during rotation of the mould.

  • Mechanical properties such as hardness and tensile strength were found to be better at inner region of composite compared to outer region and unreinforced alloy, due to higher concentration of reinforcement particles.

  • From ANOVA and S/N ratio analysis, it was observed that applied load (54%) was found to be the most dominant parameter on wear rate followed by sliding distance (18.2%) and sliding velocity (3.7%). Linear regression model was developed and validated with confirmation experiments.

  • Worn surface morphology on composite showed delamination, goughing, shear band formation and Mechanically Mixed Layer (MML) under different sliding conditions.

  • Hence, this fabricated functionally graded copper composite exhibited superior result at inner region and thus, it was recommended to use in automotive components such as brakes and shaft bushes.