5th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th –14 th , 2014, IIT Guwahati, Assam, India

DEVELOPMENT AND TRIBOLOGICAL PERFORMANCE OF NANO SiC PARTICLES ON THE AA 2024 HYBRID COMPOSITES WITH THE ADDITION OF NANO GRAPHITE 1* S.Vinoth kumar , K. Manisekar2, P.Ravindran 3 1* Department of Mechanical Engineering, National Engineering College, Kovilpatti- 628503. Tamilnadu, India. Email:[email protected] 2 Centre for Manufacturing Sciences, National Engineering College, Kovilpatti 628503. Tamilnadu, India. Email:[email protected] 3 Department of Mechanical Engineering, St.Mother Theresa Engineering College, Thoothukudi -628102, Tamilnadu, India. Email:[email protected]

ABSTRACT In this experimental study, hybrid nano composites of AA 2024 aluminum alloy matrix reinforced with high weight fractions of nanometric SiC particles ranging from 5 to 10 wt % and 5% of Graphite particles were produced by blend– press–sinter methodology. Consolidation was done at 700Mpa using uniaxial pressing. Sintering procedure was done at 530°C for 60 min. The mechanical properties of the sintered specimens were evaluated by hardness and porosity measurements. The wear behaviour of these composite materials was investigated under dry sliding conditions. The sintered samples have been characterized by OM and XRD. Wear mechanisms are discussed based on worn surface morphology and wear debris morphology. The hardness and wear resistance of the hybrid nano composites were increased considerably by increasing the reinforcement content. The results of the tests revealed that the SiC-reinforced hybrid nano composites exhibited a lower wear loss compared to the unreinforced alloy and Al–Gr composites. The nano composite with 5 wt. % Gr and 10 wt. % SiC showed the greatest improvement in tribological performance. It confirms that graphite particle was an effective solid lubricant for matrix composite applied in dry sliding wear condition. Key words : Powder metallurgy, Mechanical milling, Wear and hybrid nano composites.

1 Introduction (AA 2024) as a heat- sometimes compromise the values of its physical and treatable metal has received a great deal of attention tribological properties. Thus, it is essential to identify for past decades, due to their superiority in strength ways to retain the advantageous influence of SiC to weight ratio and other mechanical properties. while simultaneously attending to the problems of Ravindran et al. (2013) investigated that the use of machining SiC-reinforced composites. Graphite AA 2024, has been growing gradually in various particulates are well suited for this application, as technological fields such as automobile and their addition improves the machinability and wear aerospace sectors. The widely used reinforced resistance of Al–SiC composites. Rajaram et al. materials for this aluminium alloy are carbide (2010) suggested that Al–SiC composites reinforced (SiC), aluminium oxide (Al 2O3), carbide with graphite particulates are known as Al–SiC–Gr (TiC) ,boron carbide (B 4C)and graphite(Gr) in the hybrid composites. Dharmalingam et al. (2011) have form of particles or whiskers. The fabrication of identified that hybridization of reinforcements has Metal Matrix Composites (MMCs) can be achieved gained great importance in enhancing the properties by the accumulation of reinforcement phase to the of MMCs. Additions of silicon carbide (SiC) matrix. Suresha et al. (2010) have studied that certain improve both strength and wear resistance of suitable methods are powder metallurgy, spray composites, but high amount of SiC makes atomization and co-deposition, plasma spraying , stir machining difficult and composites become brittle. casting and squeeze casting. Furthermore, the use of Thus, graphite can be advantageously used as a a single reinforcement in an aluminium matrix may second reinforcement to overcome the problem of

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DEVELOPMENT AND TRIBOLOGICAL PERFORMANCE OF NANO SiC PARTICLES ON THE AA 2024 HYBRID COMPOSITES WITH THE ADDITION OF NANO GRAPHITE machining difficulties and brittle nature of SiC Table 3 gives the mechanical property of the hybrid reinforced composites, resulting in what is known nano composites. The mixing of the powder were as hybrid composites . This has been an performed in a planetary ball mill using tungsten incentive for increasing attention towards particulate balls with a diameter of 10 mm and a ball to powder reinforced aluminium alloy composites for weight ratio of 1 0:1. The mixed powders were tribological applications . Kok and Oz din (2007) pressed in a uniaxial press at 700 MPa to form green investigated the effect of Al2O3 particle c ontent and compacts . Before each run, die wall lubrication was size on the wear behavior of Al 2O3 particle performed manually using stearate. The green reinforced 2024 aluminium alloy c omposites compacts were sintered at a closely regulated fabricated by vortex method. Surapp a et al. (1982) temperature of 530°C for 60 minutes, as suggested by have studied the influence of 5 vol.% Al2O3 Yamagushi et al. (1997). The sintered composites particles addition on the wear resisttance of hyper were solution t reated at 540°C in a muffle furnace for eutectic Al–Si alloys. Earlier Straffelini et al. (1997) 120 min and water quenched; then, they were have reported on the influence of the matrix hardness naturally aged for 72 h. The wear specimens were o n d r y sliding wear behaviour of Al 6061/Al2O3 manufactured with a diameter of 8 mm and a height composites, Yu et al. (1997) have studied the effects of 30 mm. The ends of the specimens were of applied load and temperature on the dry sliding sequentially polished with abras ive paper of grades behaviour of Al 6061/SiC composites, Liang et al. 600, 800 and 1000. The of the composite (1995) have identified that the MMCs containing specimens was determined using a high precision SiC particles exhibit improved wear resistance. digital electronic weighing balance with an accuracy Rajaram et al. (2010) have studied that aluminium of 0.0001 mg by using Archimedes' principle . The hybrid composites reinforced with SiC, and graphite porosity of the composites was evaluat ed using the (Gr) particles have considerable attenti oon due to their theoretical and the experimental obtained high wear resistance combined with a low friction from each sample. coefficient. Thus, the objective of this work is to investigate the effect of the reinforce particles on the mechanical properties and wear behaviour of AA 2024–SiC-Gr hybrid nano composites.

2 Experimental setup and procedures In the present experimental investigation the base material used is Aluminium 2024 (AA 2024) (supplied by The Metal Powder Company Ltd., India, 99.8% purity) and details of its composition is given in Table 1. The matrix powder was sieved and a sieve Figure 1 (a) Morphology of as-received aluminium fraction of average sizes of 30 µm was used. This powder (b) Morphology of as-received nano SiC matrix was chosen because it provides good Powder resistance and excellent combination of strength and damage tolerance at el evated and Table 1 Chemical composition of the matrix alloy cryogenic temperatures. Commercially available SiC and Gr nano-sized particles (supplied by Xuzhou Jiechuang New Material Technology Co. Ltd., China, Element Cu Mg Fe Mn Si Cr Zn Al 99.9% purity) of average sizes of 100 nm and 50 nm as-received particles were used as the rein forcing materials. Prior to processing, all the elemental Content % 4 1.8 0.5 0.3 0.5 0.3 0.2 Balance 0 powders were dried at 110 C in an oven for 1 h. An SEM micrograph of the as-received aluminium and Table 2 Details of reinforcements SiC particles are shown in Fig. 1a and b. To carry out the study, four types of nano composites pr epared Grain size Density are: AA 2024, AA 2024/5 wt. % Gr, AA 2024/5 wt. Reinforcement % Gr/5 wt. % SiC and AA 2024/5 wt. % Gr/10 wt. % (nm) (g/cm3) SiC hybrid nano-composites. The P/M process route SiC 100 3.22 was used to fabricate the composites. Table 2 Gr 40-50 2.26 provides the details of the SiC and graphite particulate s, which were used as reinforcements.

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5th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th –14 th , 2014, IIT Guwahati, Assam, India

Table 3 Mechanical properties of the samples

Actual Theoretical Sample Hardness Porosity Composition (Wt. % ) density density No. (BHN) (% ) (g/cm3) (g/cm3)

1 AA 2024 2.848 2.905 82 1.962

AA2024/5%Gr nano 2 2.805 2.873 80 2.366 composite

AA2024/5%Gr/5% SiC 3 2.851 2.917 83 2.262 hybrid nano composite Figure 3 Hardness and Porosity results for the AA2024/5%Gr/10% SiC 4 2.867 2.926 84 2.016 prepared nano composites. hybrid nano composite 3.2 Characterization of nano composites 3. Results and discussion Sintered specimens were prepared using 3.1 Measurement of Mechanical properties standard hand polishing papers of 240, 600, 800 1000 and 1500-grit silicon carbide. The specimens were The experimental density obtained by the then finish-polished using 1 µm diamond paste Archimedes’ principle exhibited that the density of suspended in distilled water to obtain mirror-like newly developed aluminium based materials surface finish. To expose the micro structural decreases with the addition of Gr. Further with the features, the polished specimens were etched with addition of Sic, the density of composites increases. Keller etching solution (1 ml hydrofluoric acid, 1.5 (see Table 3). This can be attributed to the addition of ml hydrochloric acid, and 2.5 ml nitric acid in 95 ml higher density reinforcements of SiC. Obtained distilled water). The etch–polish–etch procedures results also revealed that the experimental values were used to attain excellent microstructure. were relatively close to the theoretical values. From Microstructural characterization conducted on the Fig. 2 it can be observed that the densities of hybrid composites specimens showed uniform composites are higher than that of their base matrix. reinforcement distribution (see Fig. 4). These The hardness of the composites was evaluated using microstructure analyses show the presence of Gr and a Brinell hardness tester. Table 3 shows the SiC in each hybrid nano composites. Fig. 4a shows mechanical properties of the samples. Fig. 3 also the microstructure of sintered pure AA 2024 matrix. shows the variation of hardness and porosity of the It shows a good chemical bonding among the Al composite with increased SiC content. The particles where the Al particles join together to increase in hardness of hybrid nano composite is due construct a solid structure was evidently seen in Fig. to the following reasons (i) high hardness of SiC 4a. Figs. 4b–d shows micrographs of sintered reinforcement particles. (ii) Uniform distribution of preforms of aluminium with various percentages of SiC in the nano composites. Porosity of the materials nano Gr and nano SiC particles. These graphite and was calculated using the theoretical and the SiC particles are uniformly distributed throughout the experimental densities obtained from each sample. It AA 2024 matrix phase. The absence of cracks can can be understood from Fig. 3 that the porosity of the also be observed from the micrographs. The X-ray composites was decreases with the increase in weight diffraction (XRD) results for the prepared nano percent of SiC reinforcements. composites are shown in Fig. 5. The diffraction patterns of the composites revealed various peaks corresponding to the face centered cubic (FCC) phase of Al. Al peaks, SiC peaks and graphite(C) peaks were indexed using Xpert high score software. The absence of other peaks of the alloying matrix elements can be attributed to the restriction of the filtered X-ray to detect phases with less amount of weight fractions. These results indicate the presence of aluminium (in the largest peaks), and the presence Figure 2 Density results for the prepared nano of silicon carbide particles and carbon is indicated by composite minor peaks. A clearly visible carbon peak can also be observed in the hybrid nano composites. The increase in the intensity of the Silicon carbide peaks

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DEVELOPMENT AND TRIBOLOGICAL PERFORMANCE OF NANO SiC PARTICLES ON THE AA 2024 HYBRID COMPOSITES WITH THE ADDITION OF NANO GRAPHITE with the increasing SiC content of the composite is Figure 6 shows the variation of wear loss of evident. These patterns show that reinforcement hybrid nano composites with increase in SiC content. particles are well distributed in the aluminum matrix. It is confirmed that the wear loss decreases as the The XRD pattern confirmed the presence of amount of SiC addition in the nano composites aluminium, Gr (C) and SiC particles in the hybrid increases. Ravindran et al. (2013) have identified that nano com posite. The XRD facilities were taken from the high w ear resistance of nano composite materials Central Electrochemical Research Institute, is due to the solid lubrication of graphite particles and Karaikudi, Tamil Nadu, India, using HITACHI SU - SiC particle, which are released during sliding and 6600. form a lubricant layer at the contact surfaces . It can be seen that the wear loss of the AA 2024 -5 wt. % Gr-10 wt. % SiC nano composite is about 2 times lower than that for the Gr free composites. Therefore, nano composites with higher SiC content would have lower wear loss in this respect. This is also evident from the small size of the wear debris particl es as determined by SEM analysis (refer Fig. 8). Moreover, composites show better wear resistance in comparison to the matrix alloy.

3.3.2 Effect of sliding distance Figure 4 Optical micrographs of the of the produced nano composites (a) AA 2024 (b) AA The variation of wear loss with applied load 2024/5 wt. % Gr composite (c) AA 2024/5 wt. % at a fixed sliding distance of 2500 m is shown in Fig. Gr/5 wt. % SiC hybrid composite (d) AA 2024/5 7. It can be seen that the wear loss of the matrix alloy wt. % Gr/10 wt. % SiC hybrid composite. and composites increased non-linearly with applied load. For instance, the wear losses of the hybri d composites increased when the applied load raised from 5 to 20 N. Beyond 20N (i.e. at an applied load of 25N) a drastic decrease in wear loss of hybrid nano composites was noticed. This effect was caused by the solid lubrication of graphite particles and high amount of SiC particle, which are released and form a lubricant layer at the contact surfaces when sliding distance reached 2500m. Under all of the studied loads and for a given sliding distance, AA 2024/5 wt. % Gr/10 wt. % SiC composites confirmed t he lowest wear loss. Figs. 7 clearly indicates that wear loss decreases with an increase in reinforcement for all Figure 5 XRD results for the prepared nano the loads studied. The above-mentioned observations composites. clearly indicate that the processing route plays an important role in determining the wear performance 3.3 Sliding wear behaviour of the nano composites. 3.3.1 Addition of SiC content on wear

Figure 7 Wear loss versus applied load of nano Figure 6 Variation of the wear loss of AA 2024 AA 2024 and hybrid composites at a constant and hybrid composites with weight percentage of sliding distance of 2500 m. SiC.

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5th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th –14 th , 2014, IIT Guwahati, Assam, India

3.4 Wear debris analysis matrix (Fig. 9 a) clearly exhibits the presence of deep permanent grooves, micro cutting, grain pullouts and Wear debris collected during the wear test to fracture of t he oxide debris, which may have caused show the information abou t the wear mechanism of the increase in wear loss. The wear grooves with the composites. SEM images of the collected debris irregular shape were observed in various deepness. generated from AA 2024 base alloy and hybrid nano Some surface materials were lifted and sheared off in composites at 20 N load are shown in Figs. 8 (a –d). It the wear process. This morphology shows that the is clear from Fig. 8 a and b that the wear debris for AA 2024 matrix has undergone significant severe the AA 2024 nano composite without SiC addition plastic deformation. However, the worn surfaces of can be very large. However, the wear debris of the the other composites (Figs. 9b and 9c) exhibit finer hybrid composite consists of a combination of fine grooves and slight plastic deformation at the edges of and coarse powders with irregular shapes. The the grooves. As the SiC weight fraction increases the generation of this kind of debris can be attributed to surface m orphologies also have been changed. The an abrasive micro-cutt ing effect. As shown in Fig. 8 c surfaces also appears to be smooth because of the and d, the composite with 10 wt. % of SiC has graphite reinforcement content. The worn surface smaller strip debris than the composite with 5 wt. % morphology of the Al–5%Gr composite is shown in of SiC. The debris become smaller as the amount of Fig. 9 b. The worn surface of AA 2024 –5%Gr SiC addition increases because the SiC particles composite was smooth with less grooves due to the scattered in the alumini um matrix can minimize the lubrication effect of graphite. The general wear mean size of the wear particles. Graphite particles pattern seems to be ploughing out of the material assist in the formation and retention of the solid from the surface. It should be noted that the surface lubricating film on the composite sliding surface roughness of AA 2024–5%Gr –10%SiC hybrid which prevent metal-to-metal contact and maintain composites was lower than that of the A A 2024– wear behaviour within the mild regime. This decrease 5%Gr composite. Fig. 9d shows very small size in the size of the debris is consistent with those microcrystal of graphite particles with smaller reported by other researchers named Liang et al. amounts of oxide particles at the worn surface. Also (1995) . Therefore, the morphology and size of the these loose particles are tightly packed with wear debris is dictated by the amount of graphite and themselves. It forms an adherent film over the contact silicon carbide in the composite. These results surfaces. Finally it leads to decrease of plastic confirm that the high content SiC composites are deformation of pin surface of AA 2024 –5%Gr– clearly superior to the base alloy and hence provide 10%SiC hybrid composites. The fine graphite grains the best wear resistance to the composites. were mixed with the other wear debris and then a smooth Gr-rich tribolayer was formed on the worn surface. Ravind ran et al. (2013) investigated the probability of severe wear in the Al –5%Gr–10%SiC composites is low, and abrasion and delamination are the dominant wear mechanisms .

Figure 8 SEM micrographs of the collected debris from (a) AA 2024 (b) AA 2024/5 w t. % Gr composite (c) AA 2024/5 wt. % Gr/5 wt. % SiC hybrid composite (d) AA 2024/5 wt. % Gr/10 wt. % SiC hybrid composite.

3.5 Worn surface analysis The SEM micrographs of the worn surface Figure 9 SEM micrographs of the worn surfaces of AA 2024 matrix and the composite specimens are for (a) AA 2024 (b) AA 2024 /5 wt. % Gr shown in Figs. 9 a-d. The typical SEM micrographs composite (c) AA 2024/5 wt. % Gr/5 wt. % SiC of worn surface morphology of AA 20 24 matrix is hybrid composite (d) AA 2024 /5 wt. % Gr/10 shown in Fig. 9 a. The worn surface of the AA 2024 wt. % SiC hybrid composite.

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4 Conclusions Journal of Materials Engineering and Performance, 20, 1457-1466. From the overall results the following conclusions Rajaram, G., Kumaran, S., Srinivasa rao, T., 2010, were drawn: "Sliding Wear Behavior of Al Si/Graphite 1. The density of the nano composites are Composite", Tribology Transactions, 54,548–557. found improved than their base matrix alloy. Suresha, S., Sridhara, B.K., 2010, "Effect of addition Hardness of the composite material increased with of graphite particulates on the wear behaviour in an increase of SiC addition. The porosity content Aluminum–silicon carbide–graphite composites", decreases with increasing wt. % of SiC Materials and Design, 31, 1804–1812. reinforcement. Kok, M., Ozdin, K., 2007, "Wear resistance of 2. Characterizations like XRD and aluminum alloy and its composites reinforced by microstructural analysis were conducted on the Al2O3 particles", Journal of Material Processing specimens. A reasonably uniform distribution of Technology, 183, 301–9. reinforcing particles was observed in the Surappa, M.K., Prasad, S.V., Rohatgi, P.K., 1982, microstructural study. Good interfacial bonding "Wear and abrasion of cast Al-alumina particle between reinforcing particles and matrix was also composites", Wear, 77, 295–302. observed. Straffelini, G., Bonollo F, Tiziani A., 1997, 3. The incorporation of SiC reinforcement to "Influence of matrix hardness on the dry sliding AA 2024 matrix alloy increases the wear resistance behavior of 20 vol.% Al2O3-particulate-reinforced of the composites. The addition Graphite 6061 Al metal matrix composite", Wear, 211, 192–7. reinforcement in AA 2024 /SiC composites as a Yu, Szu Ying., Ishii, Hitoshi., Tohgo, Keiichiro., hybrid reinforcement further increases the wear Cho, Young Tae., Diao, Dongfeng., 1997, resistance of the composite. "Temperature dependence of sliding wear behavior in 4. Moreover, the tribological behaviour of the SiC whisker or SiC particulate reinforced 6061 hybrid composite depends on the hardness of the aluminum alloy composite", Wear, 213, 21–8. composite, the rate of release graphite particles, the Liang, Y.N., Ma, ZY., Li, S.Z., Li, S., Bi, J., 1995, structure of the solid lubricating film deposited on "Effect of particle size on wear behaviour of SiC the worn surface, and the structure of the wear particulate-reinforced aluminum alloy composites", debris. Journal of Material Science Letter, 1, 114–6. Ravindran, P., Manisekar, K., Vinoth kumar, S., References Rathika, P., 2013, "Investigation of microstructure Mahmoud,T.S., 2012, "Artificial neural network and mechanical properties of aluminum hybrid nano- prediction of the wear rate of powder metallurgy composites with the additions of solid lubricant", Al/Al2O3 metal matrix composites", Proceedings of Materials and Design, 51, 448-456. the Institution of Mechanical Engineers, Part L: Yamagushi, K., Takakura, N., Imatani, S., 1997, Journal of Materials Design and Applications, 226, 3- "Compaction and sintering characteristics of 15. composite metal powder", Journal of Material Ravindran, P., Manisekar, K., Rathika, P., Processing Technology, 63, 346. Narayanasamy, P., 2013, "Tribological properties of Ravindran, P., Manisekar, K., Narayanasamy, R., powder metallurgy – Processed aluminium self Narayanasamy, P., 2013, "Tribological behaviour of lubricating hybrid composites with SiC additions", powder metallurgy-processed aluminium hybrid Materials and Design, 45, 561–570. composites with the addition of graphite solid Dharmalingam, S., Subramanian, R., Somasundara lubricant". Ceramic International, 1169-1182. Vinoth, K., Anandavel, B., 2011, "Optimization of Ravindran, P., Manisekar, K., Rathika, P., Tribological Properties in Aluminum Hybrid Metal Narayanasamy, P., 2013, "Tribological properties of Matrix Composites Using Gray-Taguchi Method", powder metallurgy – Processed aluminium self lubricating hybrid composites with SiC additions", Materials and Design, 561-570.

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