1 Introduction

The increasing need to reduce the energy consumption may be realized through maximizing the strength to weight ratio. Non-ferrous alloys of aluminum, magnesium, etc., meet the requirement of high specific strength to weight ratio and hence are widely researched material in the current era (Ref 1,2,3,4,5,6,7,8). The uncomplicated and facile processing of aluminum alloys makes it one of the market leaders for numerous applications in various industries like aerospace, defense, space, automotives, electronics, etc. Specific applications of aluminum alloys includes but are not limited to brake disks and drums, cargo bay struts in space shuttle, automobile drive shafts, electrical conductors, heat sinks, etc. (Ref 9,10,11,12). However, the pertaining difficulties like poor abrasive wear resistance causes a hindrance to its practical applications. Recently, aluminum metal matrix composites reinforced with discontinuous particulate phases are well known for their favorable properties like low density, corrosion resistance, wear resistance, and ductility, which will eventually help in the cost-effectiveness of the developed product (Ref 9). The amalgamation of particulate phase which possess superior properties such as high strength, more ductility, high modulus, and hardness into metal matrix produce intermediate alloy bearing properties of both matrix as well as reinforcement (Ref 13,14,15,16). Various refractory particulate materials, like silicon carbide (SiC), boron carbide (B4C), zirconium dioxide (ZrO2), alumina (Al2O3), graphite, etc., are the most widely utilized to prepare aluminum metal matrix nanocomposites (AMMNC) (Ref 9, 10, 17,18,19).

Among the various grades of aluminum alloys, AA2014 is one of the most widely utilized alloy in the 2000 series. It is a copper based aluminum alloy possessing excellent strength even at high temperatures owing to the intermetallic compounds formed during precipitation strengthening. This alloy exhibits potential applications in a variety of aerospace structural applications. Researchers have explored the applicability of AA2014 as the matrix material for fabrication of AMMNCs and observed favorable results in terms of mechanical and tribological properties (Ref 20,21,22,23).

The breakthrough in the domain of nanotechnology has enabled the availability of a plethora of materials in the nanoscale with improved structural and functional properties. The usage of reinforcements in the nanoscale for metal matrix composites have garnered considerable interest in this domain of research (Ref 7, 10, 16, 18, 24,25,26). Graphite is the one of the excellent reinforcement particles due to its extravagant mechanical, chemical, electrical and thermal properties. Graphite is a layered hexagonal structure which in the nano-layered structure is referred to as graphene and it provides excellent mechanical properties like Young’s modulus (approximately 1 TPa) and the fracture strength (~125 GPa). Further, graphite possesses excellent thermal conductivity (5000 W/mK), and large theoretical surface area which are utilized for improving the matrix properties of the nanocomposites (Ref 27). Graphite nano reinforcement is sufficiently robust improving the matrix material properties (Ref 27). The weak interatomic bonding across planes in graphite makes it an excellent self-lubricating material, and hence AMMNCs reinforced with graphite nanosheets provide the suitable lubricating properties to improve the sliding tribological properties (Ref 28). The AMMNCs with micro- and nano-sized graphite layered structure are potential candidates for high performance applications such as auto mobile, aerospace, and electronic device industries.

Various processing methodologies are utilized for fabrication of AMMNCs such as high energy ball milling (Ref 22) spray deposition, powder metallurgy (solid state method) (Ref 29) and stir casting process (liquid state method) (Ref 21, 25). Stir casting method for fabrication of MMNC is an extremely attractive process due to its simplicity, flexibility, and cost-effectiveness for large batches of production (Ref 30, 31). In this regard, stir casting is one of the most prominently utilized production method for AMMNCs (Ref 21, 25, 26, 32). Several parameters such as speed, depth and time of stirring have prominent influence on the properties of the AMMNCs (Ref 32). However, stir casting has certain disadvantages—the most notable one being the formation of pores, and non-uniform distribution of the reinforcement phase in the matrix. To overcome these difficulties, researchers have utilized combined stir and ultrasonic casting for the fabrication of composites (Ref 32, 33). As a result, improvement in the uniform distribution of reinforcement could be achieved to certain level, but the porosity issue still could not be answered. Squeeze casting is another method by which AMMNCs are produced (Ref 16, 34, 35). Recently, researchers have explored the possibility of stir casting amalgamated squeeze casting to solve problems of porosity (Ref 18, 26, 34). However, only a few researchers have utilized a combination of stir-ultrasonic-squeeze casting method to process AMMNCs (Ref 8, 33, 36). With this combined technique, the uniform distribution of the reinforced phase along with reduction in porosity of the composites are expected to be achieved.

The present study focusses on the fabrication of AMMNC utilizing 1.5 wt.% and 3 wt.% nano-graphite as the reinforced phase. The composition of the reinforced phase is limited to 3 wt.% as the wettability of nanoparticles into the matrix decreases with increasing the wt.% of reinforcement particles (Ref 23). Liquid state stir-casting process amalgamated with ultrasonic agitation is utilized to prepare the nanocomposites. In the next step, the nanocomposites are subjected to squeeze casting. The fabrication of AMMNCs by a combination of stir-ultrasonic-squeeze casting has been rarely reported in the literature and is the focus of this study. This combined method is selected to ensure homogenous cast structure with minimum porosity achieved through squeezing operation. The processed AMMNCs will be characterized to analyze the phase, morphology, mechanical and tribological properties to understand the behavior of the processing on the developed nanocomposites.

2 Experimental Details

The present work primarily focusses on the preparation and assessment of physical, mechanical and tribological properties of AMMNCs fabricated by a combination of stir-ultrasonic-squeeze casting technique. The experimental details are elaborated in the following sub-sections.

  1. A.

    Raw Materials

AA2014 utilized as the matrix material was procured from Alcast India, Ranchi, India and was used in the as-received condition. The chemical composition of the Al-alloy as supplied by the manufacturer is shown in Table 1. Graphite nanoparticles were purchased from Nano Research Laboratory, Jamshedpur, India. The average size of the nanoparticles as mentioned by the manufacturers was 30-50 nm.

  1. B.

    Casting Setup

Table 1 Chemical composition of AA2014 as supplied by the manufacturer
Full size table

The casting process was carried out utilizing a Stir casting setup procured from SwamEquip, Chennai, India. The test setup consisted of a bottom pouring electrical resistance heating furnace attached with a mechanical retractable stirrer (shown in Fig. 1a). An additional ultrasonic probe (shown in Fig. 1b)—made from a high melting point material—was utilized for ultrasonic agitation. The squeeze casting attachment was connected to the bottom of the furnace by a heated path furnace as shown in Fig. 1.

Fig. 1
figure 1

Photographs of the casting setup showing the (a) front view and (b) side view of the equipment

Full size image

AA2014 was utilized as the matrix material and a total mass of 1.5 kg of the alloy was cut into small pieces. Magnesium (1 wt.% of the matrix alloy) was added to improve wettability of the reinforced phase into the matrix. The matrix alloy was melted in a stainless steel crucible within the controlled furnace of the casting setup by heating to 760 °C. The nano-graphite reinforcement was measured depending on the mass fraction and preheated separately in a muffle furnace. After complete melting of the matrix phase, the preheated nano-graphite was added to the melt in the form of small packets. Immediately after addition of nano-graphite, mechanical stirring was carried out at 450 rpm for a stipulated duration of time. This was subsequently followed by ultrasonic agitation of the melt slurry at a designated frequency (20 kHz) for a duration of 2 minutes. Once both the steps of mechanical and ultrasonic stirring were completed, the melt slurry was poured through the bottom gate of the stir casting setup into the squeeze casting mold through a path furnace. The path furnace acted as the guideway between the controlled furnace and the squeeze casting attachment and was heated to 700 °C to maintain the fluidity of the slurry. Permanent cylindrical mold made of die steel was used in the squeeze casting operation. It was heated to 300 °C prior to pouring the slurry. The squeeze casting process of the slurry was carried out with the help of a hydraulic piston operated at 30 MPa pressure, till solidification of the slurry was completed. The solidified casting was taken out of the squeeze casting setup and allowed to cool to room temperature prior to any further processing of the material.

  1. C.

    Measurement of Physical and Mechanical Properties

The experimental density and porosity of the nanocomposites were measured utilizing the Archimedes method. To measure the density and porosity, small cube-shaped samples were machined from nanocomposites. The samples were weighed first in air and then in water. The theoretical density (\({\rho }_{\mathrm{th}}\)) was also calculated with the help of the rule of mixture. The experimental density (\({\rho }_{\mathrm{exp}}\)) and porosity (\(\varphi\)) of the nanocomposites were evaluated utilizing the Archimedes principle with following formula:

$${\rho }_{\mathrm{exp}}=\frac{{m}_{\mathrm{air}}}{({m}_{\mathrm{air}}-{m}_{\mathrm{liquid}})}\times {\rho }_{\mathrm{liquid}}$$
(1)
$$\varphi =\left(1-\frac{{\rho }_{\mathrm{exp}}}{{\rho }_{\mathrm{th}}}\right)\times 100...$$
(2)

where \({m}_{\mathrm{air}}\), \({m}_{\mathrm{liquid}}\) is the mass of the nanocomposites in the dry and suspended conditions, respectively and \({\rho }_{\mathrm{liquid}}\) is the density of the liquid (water).

Tensile test specimens of the composites were prepared according to the ASTM standard E8 (dimensions shown in Fig. 2). The tensile tests were carried out using a Universal Testing Machine (INSTRON 8801K1403) at room temperature at a strain rate of 1 mm/min. The samples of tensile test were cut to require dimensions with the help of wire-cut electro discharge (wire EDM) machine. Three samples of each composition were tested to obtain accurate results.

Fig. 2
figure 2

Tensile test specimen dimensions

Full size image

A Brinell hardness tester was used for determining hardness of the nanocomposites. The applied load and dwell times for the hardness measurement were 1000 kgf and 12 s, respectively. The diameter of the indenter ball was 10 mm. Each sample was tested at an average of three times to determine the average hardness of the specimen.

  1. D.

    Analysis of Microstructure of Nanocomposites

The microstructure of the developed nanocomposites was investigated under an optical microscope (model: Axioscope, make: Carl Zeiss) and field emission scanning electron microscope (FESEM) (model: Sigma 300, make: Carl Zeiss). Metallographic polishing and etching of the nanocomposites were carried out to observe the microstructure and phase distribution of the samples. A small cuboid shaped section was cut from the cast ingot by wire-EDM machine. The samples were polished utilizing different grades of emery paper following which they were washed and fine polished with velvet cloth and diamond paste. The fine polished samples were then washed in acetone to remove the dirt and excess oil, dried and etched with Kellers reagent to reveal the microstructure under the microscope.

  1. E.

    Wear Test

A multi-function tribometer (model: TR-705, make: Ducom, USA) was utilized to carry out ball-on-plate linear reciprocation tests. Sliding reciprocation of the nanocomposite plates (dimensions 20 mm × 20 mm × 5 mm) were carried out against SS 440C balls (6 mm diameter) to generate the wear scars. The experimental conditions utilized for the wear tests were as follows: normal load = 10, 20 N, stroke length = 10 mm, frequency = 10 Hz, duration = 30 minutes. The wear tests were carried out to determine the coefficient of friction and wear rate for the nanocomposites with varying mass fraction of the reinforced nanoparticles. The matrix material was also examined as a benchmark for comparison.

3 Results and Discussion

The major objectives of the present investigation are to assess the microstructure, phase distribution, physical, mechanical and tribological properties of AMMNCs reinforced with nano-graphite processed through combined stir-ultrasonic-squeeze casting method. The findings of the present study are discussed in detail in the following sub-sections:

  1. A.

    Physical properties of the AMMNCs

The density and porosity of the fabricated AMMNCs have been measured following the Archimedes’ principle given by Eq 1 and 2, respectively. The variation of the density and porosity of the nanocomposites is shown in Fig. 3(a) and (b), respectively. The experimental density is observed to vary marginally in comparison with the theoretical density for the developed AMMNCs. The porosity of the nanocomposites varies within 3-4% which is commendable in terms of the utilized squeeze casting technology.

  1. B.

    Microstructure of the Nanocomposites

Fig. 3
figure 3

Variation of (a) density and (b) porosity of AMMNCs with varying mass fraction of nano-graphite reinforcement

Full size image

Optical microscopy of the as-cast AA2014 alloy is shown in Fig. 4(a) which depicts a typical dendritic structure. Elongated α-Al alloy dendritic grains with high aspect ratios are observed. The as-cast AA2014 is primarily an Al-4.4%Cu with coarse distribution of intermetallic CuAl2 precipitates along the intergranular boundaries (Ref 6). The evidence of the presence of CuAl2 precipitates have been confirmed from optical micrographs (shown as an inset of Fig. 4a) and FESEM images and EDS analysis (shown in Fig. 5 and 6, respectively). The introduction of the nano-graphite as the reinforcement considerably alters the microstructure of the AMMNCs (as evident from Fig. 4b-c). Nano-graphite is observed uniformly distributed along the grain boundaries of the alloy matrix which also considerably alters the grain size of the nanocomposites. An increase in the mass fraction of nano-graphite considerably reduces the grain size of the nanocomposites. An increase in the particulate reinforcement has led to the formation of more equiaxed grains (Fig. 4c) in comparison with the dendritic grains observed in Fig. 4(a). The grain size of the as-cast AA2014 alloy and nanocomposite samples, shown in Fig. 4(d), depicts that the grain size reduces linearly with the mass fraction of reinforced nano-graphite.

Fig. 4
figure 4

Optical microstructure of (a) as-cast AA2014 with inset showing the CuAl2 precipitates at higher magnification, (b) Al-1.5%Gr and (c) Al-3.0%Gr nanocomposites and (d) variation of grain size with weight fraction of nano-graphite reinforcement

Full size image
Fig. 5
figure 5

Field emission scanning electron micrographs of (a-c) Al-1.5%Gr and (d-f) Al-3.0%Gr AMMNCs at low (a, d), medium (b, e) and high (c, f) magnifications

Full size image
Fig. 6
figure 6

Compositional analysis from EDS attached to FESEM for (a-c) Al-1.5%Gr and (d-f) Al-3.0%Gr AMMNCs showing FESEM images (a, d) and corresponding area maps (b, e) and point compositions (c, f)

Full size image

The FESEM micrographs shown in Fig. 5 depicts the microstructure of the AMMNCs at different magnifications. The low magnification micrographs (shown in Fig. 5a and d) exhibits the uniform distribution of reinforced nano-graphite (1.5 wt.% and 3.0 wt.%, respectively) in the AA2014 matrix. The accumulation of the nano-graphite along the grain boundaries are more evident from the high magnification FESEM micrographs (Fig. 5c and f) in comparison with the optical micrographs (shown in Fig. 4). This is further corroborated from the area maps obtained through EDS (shown in Fig. 6). The evidence of carbon only along the grain boundaries (shown in Fig. 6(b) and (e)) confirms that the nano-graphite reinforced phase (peaks of carbon are evident from Fig. 6(c) and (f)) is indeed present which is responsible for the grain size reduction in the matrix alloy. An increasing amount of graphite as the reinforcement results in transformation of the grain structure of the matrix alloy to form more equiaxed grains which is confirmed from the optical micrographs. This in turn leads to the reduction in the grain size and thereby increases the strength of the AMMNCs through inverse Hall-Petch relationship (shown in Fig. 4d).

  1. C.

    Mechanical Properties of the AMMNCs

Tensile tests of the samples prepared as per ASTM standards are conducted on a universal testing machine and the elongation vs load data is utilized to calculate the true stress vs true strain of the samples. Further, the results are analyzed to calculate the yield strength, Young’s modulus, ultimate tensile strength (UTS) and percentage elongation for the as-cast alloy and AMMNCs which are exhibited in Table 2. The addition of nano-graphite reinforcement has enhanced the yield strength, Young’s modulus and UTS of the nanocomposites considerably in comparison with the AA2014 alloy material. However, the increase in mass fraction of the reinforcement has marginal effect on the yield strength and UTS of the nanocomposites (shown in Fig. 7(a) and (b), respectively). The Young’s modulus of the nanocomposites is observed to increase as the weight fraction of nano-graphite reinforced phase is increased (Fig. 7a). Similar levels of variation in UTS has been reported by Sharma et al. (Ref 37), though the UTS reported in the present study is much higher. However, it must be noted that the matrix alloy utilized by Sharma et al. (Ref 37) is different in comparison with the present matrix alloy (AA2014). The marginal variation of the yield strength and UTS for the Al-3.0%Gr nanocomposites may be attributed to the increased porosity at the higher mass fraction of the reinforced phase. In contrast, the percentage elongation (shown in Fig. 7b) of the nanocomposites is observed to decrease with the increase in the weight fraction of reinforced nano-graphite. The percentage elongation gives a measure of the ductility of the nanocomposites. The introduction of the reinforced phase restricts the ductile properties of the alloy matrix to some extent due to introduction of dislocations. This in turn also induces the formation of smaller grains which further reduces the ductility of the nanocomposites.

Table 2 Mechanical properties of Gr reinforced AMMNCs
Full size table
Fig. 7
figure 7

Effect of varying mass fraction of nano-graphite reinforcement on the (a) yield strength and Young’s modulus, and (b) UTS and percentage elongation of AMMNCs

Full size image

The fracture surface morphologies of the tensile test samples for the base alloy and AMMNCs are shown in Fig. 8. The fractographs reveal the mechanism of failure for the alloy and the nanocomposites. In the case of AA2014 alloy, larger sized voids have been formed in the fracture surface (shown in Fig. 8a) which corresponds to a ductile nature of fracture occurring due to shearing in the form of dimples which eventually lead to void coalescence prior to fracture. Dendritic faces are pronounced in the matrix alloy depicting that fracture propagated along the intergranular dendritic regions. In comparison, cleavage morphology may be observed predominantly in the AMMNCs with small regions of dendritic faces (Fig. 8b and c). The addition of the reinforced phase reduces the formation of dimples and voids thereby promoting cleavage fracture rendering the nanocomposites brittle. A more prominent view of the cleavage faces, and vein-like structure is evident from the high magnification fractographs (Fig. 8e-f). The fracture surface of the nanocomposites may be identified as transcrystalline cleavage fracture. The strain carrying capacity of the reinforced particulate nano-graphite is much lower in comparison with the ductile matrix Al-alloy which leads to stress concentration on the particulate phase finally leading to fracture (Ref 10, 16). The cleavage facets of the fractographs observed corroborate and justify the reduced ductility (or % elongation) of the AMMNCs.

Fig. 8
figure 8

FESEM images showing the fracture surface morphology of (a, d) AA2014, (b, e) Al-1.5%Gr and (c, f) Al-3.0%Gr AMMNCs at low (a-c) and high magnifications (d-f)

Full size image

The effect of reinforced nano-graphite on the hardness of the nanocomposites is shown in Table 2. The addition of nano-graphite particulate phase increases the hardness of the nanocomposites substantially. An increase in the mass fraction of the reinforced nano-graphite further increases the hardness of the AMMNCs. The role of grain size in increasing the hardness of the nanocomposites may be confirmed from the Hall-Petch plot shown in Fig. 9. The approximately linear relationship exhibited between the grain size and hardness of the nanocomposites mark that the reduced grain size of the AMMNCs in comparison with the alloy is responsible for the increased hardness of the AMMNCs. Particulate nano-graphite is located mostly along the grain boundaries and hence the grain size accounts for the mean free intragranular movement of the dislocations. The smaller grain sizes cause the difficulty of movement of the dislocations and hence increases the hardness of the nanocomposites (Ref 4, 38, 39).

  1. D.

    Analysis Friction and Wear of the Nanocomposites

Fig. 9
figure 9

Hall-Petch plot showing the dependence of grain size on the hardness of the nanocomposites

Full size image

Linear sliding reciprocation friction tests using AA2014 alloy and AMMNCs with varying mass fraction of nano-graphite reinforcement has been carried out using a SS440C balls as the mating tribo-pair contact. Sliding reciprocation is conducted to determine the kinetic coefficient of friction and is of utmost importance for several applications like cylinder-piston contact, etc. (Ref 40, 41). Sliding reciprocation wear under dry conditions is influenced by parameters such as the applied load, frequency of oscillation and sliding distance. For the sake of simplicity of analysis of results, only the applied load has been varied in the present study for the matrix alloy and AMMNCs. Effect of time on the coefficient of friction (COF) for the nanocomposites and matrix alloy at an applied load of 20 N is shown in Fig. 10(a). After the initial running-in period, in the steady region (beyond ~ 1000 s), the COF of the nanocomposites is observed to decrease in comparison with the matrix alloy which has a prominent effect average COF value displayed in Fig. 10(b). Further it is observed from Fig. 10(a) that the increase in weight fraction of the reinforced nano-graphite in the nanocomposites reduces the COF considerably. Subsequently the average COF is observed to decrease with the addition of nano-graphite particulate phase. Nano-graphite being a self-lubricating agent substantially favors in reduction in friction for the mating tribo-pairs under dry conditions. The effect of applied load on the COF is evident from Fig. 10(b). An increase in the applied load increases the contact Hertzian pressure thereby increasing the average COF for both AA2014 and AMMNCs. However, the trend of reduced COF for the nanocomposites is still followed at all applied loads.

  1. E.

    Post-tribological Wear Surface Characterization and Analysis

Fig. 10
figure 10

(a) Variation of coefficient of friction with time and (b) average COF at different applied loads for AA2014 and AMMNCs

Full size image

The surface morphology of the wear scars is investigated thoroughly using a surface profilometer. The contour of the wear scars produced during sliding reciprocation tests are presented in Fig. 11(a). The contour of the wear scars produced corroborate with the COF observations. The depth of the scar reduces as the mass fraction of reinforced nano-graphite increases. It is also directly proportional to the applied load. As the applied load increases, the wear depth increases considerably. The wear rate under dry sliding conditions is computed from the wear volume and displayed in Fig. 11(b). The details of the calculation are presented elsewhere (Ref 42). The wear rate is observed to decrease for the AMMNCs in comparison with the matrix alloy and is also inversely proportional to the applied load. As expected, the introduction of particulate nano-graphite served to make the AMMNCs more wear resistant. Researchers have demonstrated that according to the classical Archard’s law, hardness of the material is one of the most important parameters governing the wear behavior. In addition to hardness, elastic modulus is another property which describes the wear resistance of materials (Ref 43, 44). An increased elastic deformation under specific applied load reduces the peak Hertz stress thereby imparting greater wear resistance to the material. In this regard, the hardness to elastic modules ratio (H/E represented in Table 2) bears special significance in describing the wear resistance of the nanocomposites. The hardness of the nanocomposites being substantially higher than AA2014 causes lesser wear of the AMMNCs. Additionally, the H/E ratio of the nanocomposites is observed to be higher in comparison with AA2014 alloy which further increases with higher mass fraction of reinforced nano-graphite. A higher H/E ratio for the AMMNCs justifies the lower wear rate as calculated from the wear scar contours (Fig. 11b). Similar trend of variation of wear resistance has been observed by Alam and Kumar (Ref 27) where the authors report the reduction in wear resistance when higher concentration (> 3 wt.%) of graphite is utilized as the dispersed phase. A higher concentration implies agglomeration of the dispersed phase which leads to lowering the wear resistance.

Fig. 11
figure 11

(a) Wear scar contour profiles and (b) wear rates for AA2014 and AMMNCs at different applied loads

Full size image

The mechanism of wear is investigated by analyzing the wear scar under the FESEM. For the study, the nanocomposite with Al-3.0%Gr is analyzed, and the results are depicted in Fig. 12. The mechanisms of wear are identified as formation of craters, microcrack development, adhesion, delamination (shown in Fig. 12a) and the deposition of oxide particles (Fig. 12c). The oxide particles are expected to be formed due to the high temperature developed during dry sliding of the mating tribo-pair.

Fig. 12
figure 12

Micrographs showing the mechanisms of wear for Al-3.0%Gr AMMNCs

Full size image

4 Summary and Conclusions

The present study focusses on the development of aluminum metal matrix composites reinforced with nano-graphite particulate phase by combined stir-ultrasonic-squeeze casting process and examine the mechanical and tribological properties of the same. The key findings of this work are listed below:

  • Minimal porosity (0.03% to 0.04%) is achieved for the nanocomposites processed by combined stir-ultrasonic-squeeze casting method. Uniform distribution of the particulate phase along the grain boundaries is observed which indicates that this technique can overcome one of the biggest challenges (formation of porous structure) encountered during casting.

  • The microstructure of the processed nanocomposites exhibits finer grain sizes and reinforcements distributed uniformly along the grain boundaries.

  • The developed nanocomposites exhibit improved mechanical properties in terms of higher elastic modulus, hardness and UTS which may be attributed to the reduced grain size described by the Hall-Petch relationship.

  • Sliding reciprocation wear tests exhibit reduction in average COF and wear rate for the nanocomposites in comparison with AA2014 which can be explained in terms of the mechanical properties of the nanocomposites (hardness-to-elastic modulus ratio) that increases with the increased mass fraction of the reinforced nano-graphite.

  • Analysis of the wear scar surface morphology depicts that combined phenomena of abrasion, adhesion, delamination, crater formation are the mechanisms that govern the sliding wear under reciprocating conditions.