Introduction

Carbon nanotubes (CNT) were discovered by Iijima (Ref 1), and this has since led to research in the field of CNT and their applications in various industries. The excellent mechanical and tribological properties of CNT made them reinforcements in composites to enhance the properties. Extensive research on polymer-based CNT-reinforced composites has been carried out (Ref 2, 3). However, the research has been focused on carbon nanotubes-reinforced aluminum (Al-CNT) composites to improve their properties, and the first paper in this field is published by Kuzumaki et al. (Ref 4). It is noticed that Al-CNT composites are majorly prepared by powder metallurgy (PM) methods (Ref 5,6,7,8,9,10,11,12,13,14,15,16). The primary advantage of the preparation of Al-CNT composites by PM methods is the use of lower temperatures than melting temperature of the samples. In the case of Al-CNT composites, major research is based on pure Al composites (Ref 4, 6,7,8,9,10,11,12,13,14,15). The Al-CNT composites prepared by powder metallurgy usually fail to achieve full density due to the porosities formed by insufficient sintering, and therefore display lower mechanical performances (Ref 10, 14). Hence, in several studies, secondary processing of Al-CNT composites has been performed to improve their densification and mechanical properties (Ref 5, 6, 8,9,10, 12,13,14,15,16). Many publications in the field of Al-CNT had used hot extrusion as a secondary processing after powder metallurgy to improve the properties of composites (Ref 5, 6, 9, 12,13,14, 16,17,18,19). During hot extrusion, composite billets experience a combination of compressive and shear forces. Hot extrusion of Al-CNT composites is usually performed at a temperature between 400 and 500 °C (Ref 5, 9, 11,12,13,14, 16,17,18). Grain refinement and densification during the hot extrusion enhance the properties of the carbon nanotubes-reinforced Al-based composites.

It is known that pure Al is very soft, is ductile, has good formability and is not suitable for most of the engineering applications due to its low strength. Alloy elements are generally added to Al in order to improve its mechanical performance. The material properties are mainly dependent on the kind of elements which are added to the primary element. The major alloying elements used in Al are Cu, Mg, Si, Mn, Zn and Li. Due to their lower density and better mechanical properties, aluminum alloys are widely used in aerospace industries and automobile structures (Ref 20). Research investigations also focused on Al alloy-based composites reinforced with CNT to improve the properties of the composites (Ref 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35). Laha et al. (Ref 21) conducted post-sintering treatment at 400 °C in Al-Si-CNT composites fabricated by plasma spraying and high-velocity oxyfuel spraying and achieved improved densification, and consequently, they obtained enhanced mechanical properties. It is inferred from the literature that mechanical properties of Al alloy-based CNT-reinforced composites were enhanced after secondary processing. Mechanical properties of Al alloy-based CNT-reinforced composites are studied; however, few researches have been carried out on tribological behavior of CNT-reinforced Al alloy composites (Ref 36,37,38,39,40,41,42). Wear properties of Al alloy-based composites are important in several automobile applications. Graphite- and Saffil-reinforced cylinder liners are already in use in Honda cars (Ref 43). Zhou et al. (Ref 36) successfully investigated the tribological behavior of multiwalled carbon nanotubes (MWCNT)-reinforced Al-Mg alloy composites and reported that wear performance of the composites is enhanced with an increased amount of CNT. Bakshi et al. (Ref 37, 38) reported that the presence of CNT enhanced the macroscale wear resistance of Al-Si alloy composites. In their studies, eutectic Al-Si 5 wt.% CNT coating showed better lubrication and a lower wear volume and coefficient of friction (COF) due to smaller-sized CNT clusters but the 10 wt.% CNT composite showed increased wear volume similar to Al-Si alloy matrix due to the presence of large CNT agglomerations as well as Al4C3. Pérez-Bustamante et al. (Ref 39) have investigated the wear behavior of up to 5 wt.% MWCNT-reinforced AA2024 composites and reported that the wear resistance of CNT-Al alloy composites was enhanced with the increase in CNT content. Many publications on Al-CNT composites focused on the room-temperature wear behavior and are not explored high-temperature wear behavior.

AA7075 is a very important 7xxx alloy with Zn and Mg as major alloying elements. It has tremendous applications in automobile and aerospace industries, and the improved wear properties of this alloy can be utilized for several applications. Most of the automobile components require excellent mechanical and wear performance in their service. Being a part of the carbon family, carbon nanotubes have a lubricant nature to improve the wear and friction behavior of the composites. The present investigation focuses on influence of CNT on mechanical and tribological behavior of AA7075 composites prepared through powder metallurgy followed by hot extrusion. The dry sliding wear behavior of various amounts of CNT-reinforced AA7075 composites has been investigated at room temperature for different applied loads. Further, the study explores the tribological behavior of AA7075-CNT composites at high temperatures. The wear surfaces are characterized to investigate the wear mechanism of composites. To the best of our knowledge, this is the first investigation on the wear studies of Al 7xxx-CNT composites up to 400 °C.

Experimental Details

Materials Used

The AA7075 matrix used in this present work was prepared from blending of elemental powders of Zn, Mg, Cu, Si and Al. The elemental powders used in this study are procured from Sigma-Aldrich. The elemental powder characteristics and the composition of matrix alloy are given in Table 1. Multiwalled carbon nanotubes are prepared by modified arc discharge method in an open air with Ni as a metal catalyst, and as-synthesized CNT-containing soot is purified to remove the impurities and other carbon structures, in different steps. The detailed preparation and purification procedure of CNT is provided elsewhere (Ref 44).

Table 1 Elemental characteristics of powders used for the AA7075 and chemical composition of AA7075 matrix used to prepare the composites
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Mixing and Cold Compaction of Powders

The elemental powders were taken in the proportion mentioned for AA7075 matrix in a cylindrical shear mixer. Various amounts of CNT reinforcement (0.5, 1.0, 1.5, 2.0 wt.%) were added to the alloy powders individually and mixed for 1 h. The powder mixers were compacted at 400 MPa using a universal testing machine (UTM, 60T, FIE model) for 4-5 min. Zinc stearate was used as a die wall lubricant while compacting the powders. The dimensions of the compacted samples are 30 mm in diameter and 12-14 mm in height.

Conventional Sintering and Hot Extrusion of the Samples

The cold compacted samples were painted using a protective coating (alumina paint) to avoid the oxidation during sintering. The alumina paint is applied on the compacted samples using the paint brush. The thickness of alumina paint is 100 µm, and it is uniform throughput the material. After drying the coating, compacts were sintered at 600 °C for 2 h in a muffle furnace (Hindfurr, Chennai) in an open air. After sintering, the samples are polished using emery papers to remove the alumina paint, and then, the sintered samples are extruded. Hence, alumina paint will not affect the properties of the extruded samples. The sintered samples were extruded with an extrusion ratio of 1:16 using UTM. Prior to extrusion, the samples were heated up to 450 °C and kept for 90-min soaking time.

Microhardness and Tensile Properties of the Samples

Microhardness values of the samples were measured by Vickers microhardness tester using diamond indenter with 300 g load and dwell time 20 s, as per ASTM E384 standards. The reported values of hardness are average of 10 indentations at different locations on the samples. The tensile properties of hot-extruded samples were measured using a “Hounsfield” tensile testing machine with maximum operated load of 100 kN. The standard tensile test specimens are prepared using machining. Circular cross section of dog-bone-shaped specimens with 20 mm gauge length and 4 mm gauge diameter was used for the tensile tests according to ASTM E8 standards. These tests were carried at a constant crosshead speed of 1 mm/min. The obtained tensile strength, yield strength and elongation of hot-extruded samples are an average of three tensile tests carried. After samples are broken, fracture surface was characterized using scanning electron microscopy (SEM, Quanta 400, operating voltage 30 kV).

Pin-on-disk Dry Sliding Wear Test of the Samples for Various Parameters

A pin-on-disk wear test apparatus equipped with a furnace fabricated by M/s Wear and Friction Tech., Chennai, India, was used in this work to investigate the wear behavior of hot-extruded samples. The size of the cylindrical pin was 10 mm in diameter and 25 mm in height according to ASTM G99-95a standards. The alloy or composites were used as the test pin material. The counter disk used was hardened chromium steel (EN-32) with 55 mm outside diameter and 10 mm thickness. The rotating diameter of the disk sample is about 36 mm. The hardness of the EN-32 steel is 55 HRC. Prior to wear test, the test pin and counter disk samples were cleaned with acetone to ensure that the tests were carried out under normal dry sliding conditions.

A digital balance with a precession of 0.0001 g was used to measure the weight of the tested pin before and after each test. The wear rate is calculated from the weight loss measurements. Three samples were tested at each condition to ensure the accuracy of properties measured. The friction torque was recorded during each experiment, and COF was calculated. The sliding speed and distance were fixed at 500 rpm and 850 m, respectively, for 15 min.

Pin-on-disk test was conducted for various applied loads such as 5, 10, 15 and 20 N at room temperature. The samples tested were AA7075 and various amounts of MWCNT (0.5, 1.0, 1.5, 2.0 wt.%)-reinforced AA7075 composites. Pin-on-disk test was also carried out for the alloy and composites at fixed load 10 N for 100, 200, 300 and 400 °C. After wear test, worn surfaces of alloy and composite pins tested were characterized using Quanta 200 SEM attached with energy-dispersive spectroscopy (EDS) to investigate the wear mechanism of AA7075-CNT composites.

Characterization of Carbon Nanotubes, AA7075 and AA7075-CNT Composites

The purified CNT and the hot-extruded composites were characterized using transmission electron microscopy (TEM, Philips) operated at 120 kV. Thermal stability of the as-synthesized CNT was determined using thermogravimetric analysis (TGA, Netzsch-Gerätebau GmbH-STA 409 PC Luxx). The measurements were taken in air, at a constant air flow of 60 mL min−1 from 30 to 1000 °C at a scan rate of 10 °C min−1. Raman spectroscopy was carried out using a Horiba Yvon HR-800 UV Laser Raman Spectrometer, using a He-Ne laser with a wavelength of 633 nm to characterize the purified carbon nanotubes. The surface morphology of the samples is characterized using Quanta 200 SEM and optical microscopy (Leica, Chennai). To reveal the structure of the samples using SEM and optical microscopy, the samples were polished using various grades (1/0, 2/0, 3/0, 4/0 and 1000 grits) of emery papers followed by alumina polishing. Then, the samples are polished using diamond of 0.3 µm followed by the use of etchant, modified Keller’s reagent which contains 2 mL HF + 3 mL HCl + 20 mL HNO3 + 175 mL H2O.

Results and Discussion

Characterization of Carbon Nanotubes and AA7075-CNT Composites

TEM image of purified CNT is shown in Fig. 1(a), and it is observed that the diameter of the CNT synthesized is in the range of 20-26 nm, but their length is around a few microns. Figure 1(b) shows the high-resolution TEM image of the purified carbon nanotubes, and it indicates that the CNT are multiwalled in nature. The number of walls in MWCNT varies depending on the processing conditions (Ref 45). The maximum walls found in the as-synthesized MWCNT in the present investigation are 32. Figure 1(c) shows the Raman spectrum of purified CNT. The D-peak at 1354 cm−1 is an indication of defects associated with the formation of CNT, and G-peak at 1566 cm−1 indicates the graphitic nature of CNT. It is observed from the Raman analysis that the ratio between defect and graphitic intensities (ID/IG) is around 26%, which indicates that the defects are less in the arc discharge synthesized CNT after purification. Figure 1(d) shows the TGA curve of as-synthesized CNT, and it is observed that CNT are thermally stable up to 680 °C in an open air. Above 680 °C, weight of the raw soot CNT is decreased gradually. XRD is a versatile characterization technique to confirm the formation of phases in the processed samples. Figure 2 shows the XRD pattern of AA7075-CNT composites with an increase in CNT amount. The MgZn2 is found from the XRD analysis. But, carbon and Al4C3 peaks are not observed from the XRD pattern of the composites. This may be due to the lower amount of CNT used in composites which is difficult to detect by XRD. Relative density of AA7075 and its composites were measured using Archimedes method, and the density values after compaction, sintering and hot extrusion are given in Table 2. It is found from Table 2 that the relative density of the composites is decreased with increasing CNT amount. It is also found from Table 2 that relative density of the composites after hot extrusion is increased compared with the green compacts and sintered samples as well. The maximum density of green compact composites is 80.12%, and the sintered compact has maximum density 86.2%. However, maximum density of hot-extruded composites is 99.4%. This proves the enhancement in density of materials after secondary processing.

Fig. 1
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Characterization of multiwalled carbon nanotubes; (a, b) High-resolution transmission electron microscopy, (c) Raman spectroscopy and (d) thermogravimetric analysis

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Fig. 2
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XRD analysis of AA7075 and AA7075-CNT composites

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Table 2 Relative density values of AA7075 and its composites after compaction, sintering and hot extrusion processes
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Figure 3 illustrates the SEM micrographs of AA7075-CNT composites after sintering and hot extrusion. It is observed from Fig. 3 that the density of the samples is increased with the process sequence, i.e., from sintering to hot extrusion. The applied load enables better density, while compaction and temperature are the major influencing parameters in the case of the sintering. But, in hot extrusion, both the load and temperature are dominant factors to get the full density of the samples. The density of the hot-extruded composites is reduced with an increase in CNT content as shown in Fig. 3(b)-(d). Several voids and agglomerations are found, while 2.0 wt.% CNT are added into the matrix (Fig. 3d). The addition of secondary particles will be in between the matrix particles and hence cause difficulty while consolidating the composites. Consequently, density of the AA7075-CNT composites will decrease with the addition of CNT and reduces with an increased amount of CNT in the composites. Figure 4(a)-(f) shows the optical micrographs of hot-extruded AA7075 and AA7075-CNT composites. The grain size of the matrix is decreased with an increased amount of CNT in the composites. The grain size of hot-extruded AA7075 is 45-84 µm, and the lowest grain size is 12-28 µm for 2 wt.% CNT-AA7075 composites. The grain size values of hot-extruded samples are also given in Table 2. Figure 5(a), (b) shows TEM images of hot-extruded AA7075-1 wt.% CNT, and Fig. 5(c), (d) shows AA7075-2 wt.% CNT composites. Carbon nanotubes are uniformly distributed in AA7075-1 wt.% CNT as shown in Fig. 5(a), (b), and (c) shows the agglomerations of CNT. The formation of Al4C3 was observed for 2 wt.% CNT-AA7075 composites as shown in Fig. 5(d). However, from XRD analysis of these composites, Al4C3 was not found. This means the amount of Al4C3 formation was very less, and it could be due to the formation of agglomerated CNT in composites.

Fig. 3
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(a) SEM images of sintered AA7075-1 wt.% CNT composite, (b-d) SEM images of hot-extruded samples: (b) AA7075, (c) AA7075-1 wt.% CNT and (d) AA7075-2 wt.% CNT

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Fig. 4
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Optical microscopy images of hot-extruded (a) AA7075 and (b-f) various amounts of CNT-reinforced AA7075 composites: (b, c) 0.5 wt.%, (d) 1 wt.%, (e) 1.5 wt.% and (f) 2 wt.%

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Fig. 5
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TEM images of AA7075-CNT composites; (a, b) 1 wt.% CNT and (c, d) 2 wt.% CNT; inset of (d) is selected area electron diffraction pattern of Al4C3

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Mechanical Properties of AA7075 and CNT-Reinforced AA7075 Composites

Microhardness indicates the ability of the materials to withstand certain applied load. Figure 6(a) shows the variation of hardness with different amounts of CNT after sintering and hot extrusion. From Fig. 6(a), it is observed that the hardness of the composites is increased with an increased amount of CNT up to 1 wt.% and then decreased with the further addition of CNT. Figure 6(a) also indicates that the hardness after hot extrusion is higher than that of the sintered samples and this hardness enhancement is due to improvement in density of hot-extruded samples. Figure 6(b) shows the tensile properties of the hot-extruded samples. It is noticed that both the yield strength (YS) and ultimate tensile strength (UTS) of the composites are increased with CNT content up to 1 wt.% and these strengths are decreased with the further addition of CNT. The enhanced mechanical properties of the composites are attributed to uniform distribution of CNT in the composites, whereas the agglomerations of CNT cause deteriorating the properties. The yield strength of Al alloy is 149.86 MPa, whereas the YS of AA7075-2 wt.% CNT is 189.31 MPa. The highest YS of AA7075-CNT composite is from 149.86 to 215.21 MPa for 1 wt.% CNT (43.6% enhancement). The UTS of the AA7075 matrix and 2 wt.% CNT composites are 219.2 and 245.9 MPa, respectively. The maximum enhancement in the UTS of composites is 31.8% for 1 wt.% CNT-AA7075 composites (from 219.2 MPa for AA7075 to 288.9 MPa for 1 wt.% CNT). The elongation of the composites (9.8%) is not much reduced compared with AA7075 (13.2%). Deng et al. (Ref 5) used hot extrusion to prepare up to 2 wt.% CNT-reinforced AA2024 composites at 460 °C with an extrusion ratio of 25:1. They found that the tensile strength is increased up to 1 wt.% CNT addition (384-522 MPa), while it was lower than the base alloy for 2 wt.% CNT addition. Jafari et al. (Ref 46) prepared AA2024-CNT (up to 3 vol.%) composites using hot pressing at 450 and 500 °C under a pressure of 250 MPa for 30 min and reported that enhancement in the hardness and compressive strength of CNT-reinforced composites was up to 2 vol.% CNT, and these properties deteriorated with the further addition of CNT to Al alloy matrix. Esawi and El Borady (Ref 47) prepared up to 2 wt.% CNT-reinforced Al composites using cold rolling of the powders, and these were further sintered. They found agglomeration of CNT beyond 0.5 wt.% addition, and consequently, the tensile properties of the composites are increased with an increased amount of CNT up to 0.5 wt.% and then decreased with higher amount of CNT. This indicates that good CNT dispersion is necessary for higher CNT content to improve the properties of CNT-based composites. After tensile test, the fracture surfaces of the alloy and composites are shown in Fig. 7. The fracture images of AA7075 matrix indicate the dimple nature as shown in Fig. 7(a), (b). This dimple nature of alloy is good for high elongation. Figure 7(c), (d) shows the SEM fracture images of hot-extruded 1 wt.% CNT-AA7075 composites, whereas Fig. 7(e), (f) shows SEM images of fractured AA7075-2 wt.% CNT composites. These fracture images of the composites show that the surface of the sample is rough implying brittle fracture. SEM fracture images of 1 wt.% CNT-reinforced AA7075 composites as shown in Fig. 7(d) indicate that CNT fibers are pulled after tensile test. Deng et al. (Ref 5) and Kwon et al. (Ref 6) also reported the pullout and bridging of the CNT in fracture surfaces after tensile test of the composites. This pull-type mechanism of CNT fibers after tensile test helps to increase the load-bearing capacity and thus enhance the mechanical properties of the composites. The CNT are uniformly dispersed in the composites as shown in Fig. 7(d). However, agglomerated CNT are observed with the addition of 2 wt.% CNT as shown in Fig. 7(f) which are marked in circle. It is reported that ductility of the composites is reduced due to the agglomeration of the CNT (Ref 5). The strengthening factors that enhance the mechanical properties of the composites are uniform distribution of the CNT, fiber pullout and better interface between the matrix and CNT.

Fig. 6
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(a) Microhardness of sintered and hot-extruded samples and (b) tensile properties of hot-extruded AA7075 and its composites

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Fig. 7
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SEM fracture surface images of hot-extruded (a, b) AA7075, (c, d) 1 wt.% and (e, f) 2 wt.% CNT-reinforced AA7075 composites

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Effect of Applied Load on Tribological Behavior of Hot-Extruded Samples at Room Temperature

Figure 8 shows the variation of wear rate and coefficient of friction of AA7075 and composite materials as a function of applied load. It is observed from Fig. 8(a) that the wear rate of alloy and composites increases with the applied load up to 20 N. The increase in the load from 5 to 10 N leads to small increment in the wear rate. However, with further increase in the loads beyond 10 N, the wear rate is gradually increased. As shown in Fig. 8(b), the change in COF for various loads also follows similar trend to wear rate. The maximum wear rate and coefficient of friction are observed at 20 N for all the samples. Al-Qutub et al. (Ref 40) studied the wear and friction behavior of spark plasma-sintered 0.5-2 wt.% CNT-reinforced AA6061 composites using a pin-on-disk tribometer at room temperature under applied loads ranging from 5 to 30 N. They reported that the wear rate and coefficient of friction of the composites were lower than those of AA6061 at lower loads ranging from 5 to 15 N, but with increasing loads above 15 N, wear rate and COF of the composites were higher than those of monolithic Al alloy. However, in this study the wear performance of the composites is superior to AA7075 matrix. This could be due to the quality of the multiwalled carbon nanotubes used in the present investigation. The CNT used in the present study are synthesized using arc discharge method which gives the straight and almost defect-less CNT, whereas it is noticed that Qutub et al. (Ref 40) used the chemical vapor deposition-based synthesized CNT which are curve and these types of CNT are defect. Hence, arc discharge-based CNT give the better mechanical and tribological enhancement in the composites compared with CVD-synthesized CNT-reinforced composites.

Fig. 8
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Effect of applied load at room temperature: (a) wear rate and (b) coefficient of friction

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High-Temperature Tribological Behavior of AA7075 and AA7075-CNT Composites

It is observed from the previous section that the wear rate and COF are increased with applied load and this increment is small for 5 and 10 N. It is assumed that the applied load above 10 N may cause severe damage to the samples while the test is performed at high temperatures and hence the 10 N is selected to evaluate the wear performance of the alloy and composites at high temperatures. Figure 9 shows the wear rate and COF of AA7075 and CNT-reinforced AA7075 composites as a function of test temperature up to 400 °C. The wear rate is less for AA7075-1 wt.% CNT-reinforced composites. The wear rate of the alloy and composites was increased with increasing temperature. However, the wear rate of the composites is less compared with the alloy. Figure 9(a) shows that the wear rate is decreased with an increased CNT content at different temperatures. Figure 9(b) shows the variation in coefficient of friction of composites as function of various test temperatures. The coefficient of friction is increased with increasing temperature.

Fig. 9
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Effect of high temperature at 10 N: (a) wear rate and (b) coefficient of friction

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The increase in wear rate is small with increasing temperature up to 300 °C. However, the increase in wear rate of the alloys and composites is significantly high at 400 °C. The 7xxx Al alloys mainly contain Zn and Mg as major alloying elements. They are heat treatable and can be age hardened due to the formation of MgZn2 precipitates. Generally, the solutionization temperature of Al 7xxx alloys is around 480 °C and the temperature of precipitation hardening is around 350 °C. Hence, in the present study, the matrix is not soft up to 300 °C and consequently the wear rate increment is small. On the other hand, single solid solution formation could be the reason for the softening of the alloy matrix at 400 °C and hence the wear rate is increased significantly. In general, metallic materials will be softer at their 0.5 Tm (melting temperature).

The COF of the samples at higher temperatures should be less compared with the lower temperatures since the material becomes softer. However, in this study, the coefficient of friction values are higher at high temperatures. Kumar et al. (Ref 48) studied the high-temperature wear behavior of fly ash-reinforced AA6061 composites prepared by powder metallurgy and then hot extrusion and reported that the increased COF with an increase in temperature may be due to the particle hardening effect on the subsurface at elevated temperatures.

Effect of Carbon Nanotubes on Wear Behavior of AA7075-CNT Composites

Figure 10(a) shows the wear rate and Fig. 10(b) shows the coefficient of friction, as a function of CNT amount at various applied loads 5, 10, 15 and 20 N. It is observed from these figures that both the wear rate and COF were decreased with an increased amount of CNT up to 1 wt.% and then increased with a higher amount of CNT at all the applied loads. Figure 10(c) shows the wear rate and Fig. 10(d) shows the COF as a function of CNT amount in AA7075-CNT composites at various temperatures such as room temperature, 100, 200, 300 and 400 °C. The wear rate and coefficient of friction of composites are decreased with an increase in CNT amount at different temperatures as shown in Fig. 10(c) and (d). The minimum wear rate and COF are observed at 1 wt.% CNT in the composites. Further addition of CNT above 1 wt.% increased the wear rate of the composites at all temperatures. The presence of good bond between the matrix and CNT is a major factor that influences the wear behavior of the hot-extruded composites (Ref 39). The addition of small amount of CNT leads to the grain refinement and also the obstacles to the dislocation motion (Ref 19). Hence, hardness and consequently the wear rate and COF are decreased. However, agglomeration of the CNT could be the reason to increase the wear rate and COF of composites beyond 1 wt.% addition of CNT in the composites. Lipecka et al. (Ref 49) reported that the grain growth of S790 alloy (AlZn11Mg2Cu) is restricted with the addition of CNT and annealing above 450 °C leads to the partial transformation of CNT to Al4C3. Kim et al. (Ref 50) investigated the wear and frictional behavior of Al-CNT composites. They observed reduction in the coefficient of friction and wear rate of the composites with increasing CNT content up to 1 wt.% compared with Al sample. Choi et al. (Ref 51) have investigated wear and mechanical behavior of powder hot-rolled Al-based composites reinforced with 0-6 vol.% CNT and reported that wear rate and COF of 0-4.5 vol.% CNT-reinforced Al composites are decreased with increasing amount of CNT. However, incorporation of higher than 4.5 vol.% of CNT into the Al matrix leads to detrimental impact on wear performance. Kim et al. (Ref 50) used ultrasonication for dispersion and obtained better properties for 1 wt.% CNT composite, while Choi et al. (Ref 51) used ball milling for powder dispersion and obtained best properties for 4.5 vol.% CNT (~ 3 wt.%) composite. In our study, Turbula mixer has been used to disperse the carbon nanotubes and hence wear performance of the composites is best up to 1 wt.% of CNT addition in the composites. These results indicate that the dispersion of CNT depends on the process and its parameters and improved CNT dispersion can enhance the properties.

Fig. 10
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(a-c) Wear rate and (b-d) coefficient of friction as a function of carbon nanotubes

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Analysis of Worn Surfaces Using Scanning Electron Microscopy

The SEM analysis of wear surfaces provides the wear behavior of the samples more accurately. Figure 11 and 12 show the wear surfaces of AA7075 and AA7075-CNT composites of post-tribology test at room temperature with 10 N applied load, respectively. The comparison of worn surfaces of Fig. 11 and 12 shows that the debris formation and surface damage are more in the case of AA7075 alloy compared with the composites. This is attributed to the higher surface hardness of the composites compared with the Al alloy. Plowing is dominant mechanism of wear in room-temperature tested samples. The fragmentation and surface damage of the sample surfaces are more with an increase in the wear test temperature.

Fig. 11
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(a-d) SEM worn surfaces of AA7075 at room temperature under applied load 10 N

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Fig. 12
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(a-d) SEM worn surfaces of AA7075-CNT composites at RT under applied load 10 N

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Figure 13(a)-(c) shows the wear surfaces of AA7075 and Fig. 13(d)-(f) shows the wear surfaces of AA7075-1 wt.% CNT composites at 300 °C under load 10 N. The examination of the worn surfaces of the AA7075 and composites show different morphologies, and the grooves and broken particle are scattered along the sliding direction. The wear track of matrix alloys in Fig. 13(a)-(c) shows the presence of increased groove size. This type of features of the surface results an increased wear rate. However, AA7075-1 wt.% CNT composites show reduced surface damage as shown in Fig. 13(d)-(f) and the amount of debris is lower compared with AA7075. Figure 14 shows the SEM worn surface of AA7075 and Fig. 15 shows the SEM worn surface of AA7075-1 wt.% CNT composite tested at 400 °C. It is observed from these images that the surface after wear test is shallower and oxide is formed at 400 °C. Figure 14(d) and 15(d) show the EDS analysis of the AA7075 and composites, respectively. The oxides are formed during high-temperature wear test and the oxide formation is high in the case of the AA7075 compared with AA7075-CNT composites. The severe surface damage is observed after wear test of the samples at 400 °C and oxide particles are broken. The surface removal of samples during the wear test at 400 °C is like chipping. The debris of the AA7075 and AA7075-1 wt.% CNT composites is collected after wear test at room temperature and 400 °C, and the collected samples are analyzed using XRD to confirm the formation of oxide phases. Figure 16(a), (b) shows the XRD pattern of the AA7075 and AA7075-1 wt.% CNT composites tested at room temperature under 10 N, and Fig. 16(c), (d) shows the XRD pattern of AA7075 and AA7075-1 wt.% CNT composites after wear test at 400 °C. XRD analysis of the room-temperature tested samples shows the absence of oxide after wear test. However, both the alloy and composites show the presence of oxide formation after wear test at 400 °C. The oxides formed during high-temperature wear test of AA7075 which are Al2O3, MgO and ZnO, whereas only Al2O3 formed during the wear test of the AA7075-CNT composites. The uniform distribution and self-lubricating behavior of CNT make them easily slide or roll between the metal surfaces and hence coefficient of friction of the composites reduces (Ref 7). It is found that the worn surface at room temperature is deep with wide grooves and the surface at high temperature has shallower grooves that are higher in number. During high-temperature wear test, oxides of the alloy and composites are found from XRD analysis of the samples. Hence, oxidation wear and delamination could be the dominant wear mechanisms during high-temperature wear test. These oxide particles can cause abrasive wear of the alloy and composites during high-temperature wear test, since the oxide particles are harder than the Al alloy matrix (Ref 36). The addition of CNT into the alloy reduces the contact between the pin and steel counter surface and hence the oxide formation of composites is low as shown in XRD (Fig. 16d). A change in the wear mechanism from plowing to delamination, chipping and oxidation could be responsible for reduction in the wear performance of the Al alloy at high temperature.

Fig. 13
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SEM worn surface images of (a-c) AA7075 and (c-f) AA7075-CNT composites performed the wear test at 300 °C under applied load 10 N

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Fig. 14
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(a-c) SEM worn surface of AA7075 at 400 °C and (d) EDS of selected area in (c)

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Fig. 15
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(a-c) SEM worn surfaces of AA7075-CNT composites at 400 °C under 10 N and (d) EDS for selected area in (c)

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Fig. 16
figure 16

XRD analysis of wear debris collected after wear test of the alloy and composites at various temperatures: (a) AA7075 and (b) AA7075-1 wt.% CNT composite tested at room temperature, (c) AA7075 and (d) AA7075-1 wt.% CNT composite tested at 400 °C

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Summary and Conclusions

Multiwalled carbon nanotubes-reinforced AA7075 composites were prepared by powder metallurgy and hot extrusion processes, and better densification of the composites was achieved during hot extrusion. The prepared composites were characterized, and mechanical and tribological properties of the composites were determined. It is observed that the properties such as hardness, tensile and dry sliding wear performance of the composites are enhanced with the addition of CNT up to 1 wt.% and further addition of CNT leads to decrease in mechanical and tribological properties of the composites. The strengthening factors that enhance the mechanical properties of the composites are uniform distribution of the CNT, fiber pullout and better interface between the matrix and CNT. The wear rate and COF of the AA7075 and CNT-reinforced AA7075 composites are increased with an increase in applied load and test temperature. The wear rate of the alloy is very severe at 400 °C compared with the composites. The wear mechanism of the samples is transformed from plowing at initial stages to delamination, chipping and oxide wear at later stages.