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SiCp Nanocomposites Synthesized by High Energy Ball Milling and Spark Plasma Sintering
J. Mater. Sci. Technol., 2012, 28(11), 969–975.
Nanoindentation and Wear Characteristics of Al 5083/SiCp Nanocomposites Synthesized by High Energy Ball Milling and Spark Plasma Sintering Sivaiah Bathula, Saravanan M and Ajay Dhar† Metals and Alloys Group, Materials Physics and Engineering Division CSIR-National Physical Laboratory, Dr. K.S. Krishanan Marg, New Delhi – 110012, India [Manuscript received June 30, 2012, in revised form September 28, 2012]
Al 5083/10 wt% SiCp nano composites have been synthesized by means of high energy ball milling followed by spark plasma sintering (SPS). Nano composites produced via this method exhibited near-theoretical density while retaining the nano-grained features. X-ray diffraction (XRD) analysis indicated that the crystalline size of the ball milled Al 5083 matrix was observed to be ∼25 nm and it was coarsened up to ∼30 nm after SPS. Nano indentation results of nano composites demonstrated a high hardness of ∼280 HV with an elastic modulus of 126 GPa. Wear and friction characteristics with addition of SiCp reinforcement exhibited significant improvement in terms of coefficient of friction and specific wear rate to that of nano structured Al 5083 alloy. The reduction in specific wear rate in the nanocomposite was mainly due to the change of wear mechanism from adhesive to abrasive wear with the addition of SiCp which resulted in high hardness associated with nano-grained microstructure. KEY WORDS: Metal matrix nano composites; Friction/wear; Mechanical properties; High resolution transmission electron microscopy; Spark plasma sintering; Nanoindentation
1. Introduction Metal matrix composites (MMCs) are materials that incorporate micron-sized fibres/particulates into the matrix of a standard alloy, which results in drastic improvement in properties such as mechanical strength, toughness, wear etc.[1] . Aluminium (Al) based composites are important due to their light weight and high strength for manufacturing automobile parts such as brake rotors for high speed train, automotive braking systems and many other applications[2] . MMCs reinforced with nanoparticulates show drastic improvement in their mechanical properties[3] due to high surface to volume ratio. Al based nanocomposites are required to pos† Corresponding author. Ph.D.; Tel.: +91 11 45609455; Fax: +91 11 45609310; E-mail address: [email protected] (A. Dhar).
sess adequate strength without sacrificing their wear properties to exploit their use for structural applications in the place of their micro-sized counterparts and also the interface between the matrix and reinforcement is substantially altered in these nanocomposites. There are a few existing methods for the production of MMCs such as liquid metallurgy, powder metallurgy and spray forming[4] . However, it is well known that the retention of nanoscale features in liquid metallurgy route or spray forming is rather difficult due to their inherent solidification mechanisms. Powder metallurgy employing ball milling[5] , is a technique for the production of nanocomposite powders in bulk quantity with effective dispersion of nanoparticles in the matrix. It may be noted that conventional sintering of nanocomposites with longer duration of sintering cycles leads to excess grain growth in the material, due to the excess free energy associ-
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ated with the large amount of grain boundary area of the nanostructured matrix, which thermodynamically favours grain growth. In the present study, the ball milled powders are consolidated and sintered using spark plasma sintering technique (SPS). SPS is one of the most successful technique for consolidation and sintering of nanocomposite powders for obtaining near theoretical densification while retaining the nano-grained structure which directly influences the mechanical and wear properties[6,7] . This technique works mainly based on the electric spark discharge, where a high energy pulsated current momentarily generates spark plasma between the particles resulting in highly localized temperatures[8] . It is recognized that the heat generated at the point contacts of the particles vaporize contaminants and break the oxide layer on the surface of Al 5083 particles prior to neck formation[9] . The rapid heating rates and shorter sintering cycles during SPS with simultaneous application of load avoid grain growth in nanocomposites thus resulting in a highly dense product. There have been several studies on the synthesis of MMCs and Al/SiCp is one of the most well studied material due to its potential applications, which are described elsewhere[10] . Although there are sufficient studies available in the literature on the wear behaviour of MMCs, with micron-sized particulates[11] , only few studies on nanostructured MMCs are reported. The wear properties in nanocomposites are important as their small grain size associated with the nanoparticulates dispersion greatly improves the wear characteristics[12] . Similarly, a lot of data on microhardness has been reported on a variety of MMCs[13] but there are hardly any measurements available on their nanoindentation measurement results. In the present study, we report the nanoindentation results and wear mechanism of Al 5083/SiCp nanocomposites synthesized employing high-energy ball milling followed by consolidation and sintering using spark plasma sintering. Also these results have been correlated with the microstructure of the nanocomposites by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM).
Prior to consolidation and sintering, the ball milled nanocomposite powder was handled in glove box (Mbraun Inert Gas System, GmBH, Germany) to minimize oxidation and other atmospheric contamination. Crystallite size measurements were carried out by using an X-ray diffractometer (model: Miniflex II, maker: Rigaku). In order to calculate the final crystallite size of milled powders, full width half maxima (FWHM) values were subtracted for instrumental broadening and Kα2 corrections. Nanocomposite powders were subsequently consolidated and sintered under vacuum of ∼4 Pa using SPS (model: SPS 725, maker: SPS Syntax Inc., Japan) at a pressure of 50 MPa, at 300 ◦ C for 3 min. using graphite die and punches. The heating rate was maintained at 300 ◦ C per min. The surface morphology of ball milled powders and sintered pellets were studied by scanning electron microscopy (SEM model: EVO MA10, maker: Zeiss). High resolution transmission microscopy (HR-TEM, Model: Tecnai G2F 30 STWIN, FEG source, electron accelerating voltage 300 kV) was carried out on milled powders and SPS nanocomposites to study the dispersion of different nanoconstituents and the matrix-reinforcement interface. Micro-hardness measurements were carried out using a Vickers micro-hardness tester (model: FM-e7) with a load of 100 g for 10 s. Subsequently, elastic modulus and hardness measurements were performed using nanoindentation technique (M/s Fischer-Cripps Laboratories Pvt. Limited, Australia) at a load of 50 mN. The mechanical properties were derived from the measured load-penetration depth curves under loading/unloading through a standard data analysis software. Wear and friction measurements were evaluated using a pin-on-disc sliding wear machine (DucomTR20LE) as per ASTM Standard G99. These tests were conducted for sintered samples against a polished EN-31 steel disc keeping a sliding speed of 1 m/s and 2 m/s. All the tests were conducted at a load of 9.81 N and 49.05 N with a sliding distance of 600 m. The density of composites was measured by means of conventional Archimedes principle.
2. Experimental
3.1 SEM and XRD analysis
Al 5083 alloy (purity: 99.9%, particle size: ∼15 μm, density: 2.66 g/cm3 , maker: valimet Inc., USA) was ball milled along with 10 wt% of SiCp nanoparticles of ∼20 nm in size (density: 3.21 g/cm3 ) in a Fritsch make Pulverisette-4 ball mill for 15 h. The vial of the ball-mill and the milling media were made of stainless steel. The ball-to-powder weight ratio was maintained at 20:1 and milling was done in an argon atmosphere with a maximum milling speed of 400 r/min. In order to prevent the re-welding and promote the fracturing of powder particles, 2 wt% of stearic acid was added as a process control agent.
The as-received elemental powders at the starting stage of milling were nearly spherical in shape with an average size of 15 μm. Fig. 1(a) and (b) shows that the ball-milled powders exhibit facetted morphology with the particle size between ∼50 to 60 μm and the spark plasma sintered nanocomposite shows homogeneous dispersion of SiCp within the Al 5083 alloy matrix. This increase in the particle size could be explained on the basis of welding of particles at the first instant followed by fracture at the second stage[14,15] . Further milling leads to an increase in the stored energy of the system up to a critical value and beyond this,
3. Results and Discussion
S. Bathula et al.: J. Mater. Sci. Technol., 2012, 28(11), 969–975.
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milling . The average crystallite size and lattice strain of the matrix were found to be ∼25 nm and 0.43% respectively, after high energy ball milling for 15 h and it was coarsened up to ∼30 nm after SPS. Subsequently, ball milled nanocomposite powders were consolidated and sintered by SPS technique, which resulted in nanocomposites with neartheoretical density while preserving the nanoscale features in the nanocomposites[18,19] . The HR-TEM images as shown in Fig. 3(a), reveals fine grained microstructure features in nanocomposites powders after 15 h ball milling and Fig. 3(b) shows the nanoscale features with large density of nano-grained network even after spark plasma sintering at 500 ◦ C. The average crystallite size was evaluated from HR-TEM measurement and is in agreement with the XRD results, although a few small crystallites are also visible. Further analysis on this nanocomposite[20] elucidated that a good interface was evolved around the SiCp in the α-Al matrix. Fig. 3(b) further suggests that apart from the good interface, there has been no visible porosity delineated at the interface around SiCp , even at lattice scale. Fig. 1 High resolution SEM images of Al 5083/SiCp (a) nano-powders after 15 h of ball milling and (b) after SPS process, showing homogeneous distribution of SiCp in the Al 5083 matrix
Fig. 2 XRD peak profile of (a) un-milled Al 5083 alloy, (b) Al 5083/SiCp nanocomposite powders ball milled for 15 h and (c) Al 5083/SiCp nanocomposite after SPS consolidation process
reduction of energy in terms of forming sub-grains and high angle grain boundaries over the entire volume of milling powder leading to nanostructure. Xray diffraction analysis (XRD) was carried out and Williamson-Hall method[16] was used to calculate the crystallite size and lattice strain of the Al 5083 matrix. The XRD peak profile shown in Fig. 2(b) indicates an increase in peak broadening with a sharp decrease in peak intensity after ball milling, which is mainly due to the reduction in the crystallite size. The increased peak broadening could also be attributed to crystalline defects such as dislocations and stacking faults generated by impact and shear forces during
3.2 Nanoindentation measurements Microhardness was measured on Al 5083/SiCp nanocomposite and nanostructured Al 5083 alloy samples of spark plasma sintered employing a microhardness tester under identical conditions. The results show the microhardness values of ∼280 and 148 HV for both samples, respectively. Fig. 4 shows a typical load-penetration depth curve obtained in a nanoindentation test and the resulting peak indentation depth includes elastic and plastic deformation. Nanoindentation results, as shown in Fig. 4, were also in good agreement with microhardness values measured on Al 5083/SiCp nanocomposite[21,22] . Measured hardness and elastic modulus values of Al 5083 alloy after milling for 15 h and SPS found to be 148 HV and 78 GPa, respectively and were enhanced to ∼280 HV and 126 GPa for nanocomposite of Al 5083/10 wt% SiCp . Significant enhancement in the hardness observed for Al 5083/SiCp nanocomposites were due to its nano-grained structure which is maintained even after SPS and dislocation pileups related to that of Orowan strengthening at grain boundaries[23,24] . 3.3 Coefficient of friction Fig. 5 shows the variation of coefficient of friction (COF) of nanostructured Al 5083 alloy and Al 5083/SiCp nanocomposite under applied loads of 9.81 and 49.05 N under a sliding speed of 2 m/s. The value of COF was found to be ∼0.22 at a load of 9.81 N, and it increased to 0.37 at a load of 49.04 N for nanostructured Al5083 alloy. This increase in COF for nanostructured Al 5083 alloy was accompanied with an in-
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Fig. 3 HR-TEM images of Al 5083/SiCp (a) nanocomposite ball milled for 15 h (b) spark plasma sintered. The inset shows the nano-grained microstructure with networked dislocations
Fig. 4 Nanoindentation measurements of nanostructured Al 5083 alloy and Al 5083/SiCp nanocomposite
crease in specific wear rate as shown in Fig. 6. The Al 5083/SiCp nanocomposite shows comparatively lower COF value (Fig. 5(d)) as compared to that of nanostructured Al 5083 alloy at higher loads (Fig. 5(b)). This has been attributed to the fine grained microstructure with large density dislocations network and also the well defined interface between matrixinterface that has been observed employing HR-TEM studies and has been discussed earlier in this paper. These results are in good agreement with similar studies reported for Al and Al/B4 C nanocomposites[25] . 3.4 Wear behaviour Fig. 6 shows the specific wear rates of nanostructured Al5083 alloy and Al5083/SiCp composite under the sliding speeds of 1 m/s and 2 m/s with 9.81 and 49.05 N applied loads. In general, for all the samples, it has been observed that the wear rate increases with an increase in the applied load. However, this increase is more significant in the case of nanostructured Al5083 alloy samples in comparison to the nanocomposites. In the case of nanostructured Al 5083 alloy at a load of 49.05 N, extensive
specific wear rate was observed which is due to localized plastic deformation on the friction surface of the base material. However, this increased specific wear rate for nanostructured alloy was mainly due to the rapid transition of wear mechanism from abrasive wear to a combination of adhesive wear and fatigue wear, as explained in later part of the paper. In Al5083/SiCp nanocomposite the application of higher applied load of 49.05 N resulted in lower specific wear rate and COF as compared to that of nanostructured Al 5083 alloy. Considerably, lower specific wear rate has been observed for Al5083/SiCp nanocomposites at 9.81 N and 48.05 N applied loads. This may be due to the change in wear mechanism from adhesive wear to abrasive wear with addition of SiCp associated with nano-grained microstructure after SPS which resulted in higher hardness and strength values, as discussed earlier. This mechanism was also reported by other researchers[26] who observed that the increase in reinforcement of alumina fibers to the aluminium matrix led to a significant decrease in wear rate. 3.5 Wear mechanism Fig. 7(a) and (b) shows the microstructures of the worn surface of nanostructured Al5083 alloy at an applied load of 9.81 and 49.05 N with a sliding speed of 2 m/s. The worn surface revealed continuous grooves along with sub-crack formation (fatigue crack) at lower applied load of 9.81 N. This indicates that the wear mechanism of nanostructured Al 5083 alloy at lower load is removal of material with abrasion. However, for higher applied load of 49.05 N, the worn surface shows severely deformed structure with plastic yielding which resulted in large portions of material being removed or pulled out of the surface[27] . This may also be due to localized melting on the sliding surface at higher loads due to rise in temperature. This therefore, suggests that the wear mechanism is a combination of abrasive and delamination of nano-
S. Bathula et al.: J. Mater. Sci. Technol., 2012, 28(11), 969–975.
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Fig. 5 Coefficient of friction with sliding distance at load 9.81 and 49.05 N at sliding speed of 2 m/s
Fig. 6 Comparison of specific wear rates with applied loads for nanostructured Al 5083 alloy with Al 5083/SiCp nanocomposite
structured Al5083 alloy at higher applied load of 49.05 N. Similar observations have also been reported for AZ61 Mg alloy at higher applied load[28] . Fig. 7(c) and (d) shows the worn surface microstructures of Al5083/SiCp nanocomposite at an applied load of 9.81 and 49.05 N with a sliding speed of 2 m/s. For lower applied load of 9.81 N, worn surface of Al 5083/SiCp nanocomposite reveals continuous
grooves and initiation of some cracks on the surfaces. This may be due to homogeneous distribution of SiC particulates in nanostructured matrix associated with networked dislocations which results in improved wear resistance. This suggest that abrasive type of wear is the dominant mechanism in Al 5083/SiCp nanocomposite at lower applied load of 9.81 N. At higher loads of 49.05 N, it has been observed that the abrasive groove formation with less delamination of surface layers resulting in improved wear resistance. The absence of surface porosity, due to near-theoretical density obtained after SPS may be inferred that the interface between Al5083 matrix and SiCp shows more wear resistance resulting in lower specific wear rate and considerably lower COF. Further, it may also noted that the nano-grained microstructure imparts high wear resistance and delays the severe wear rate regime as reported by other researchers on Cu-TiB2 nanocomposites[29] . SPS which leads to high hardness and high strength of the nanocomposites, synthesized in the present study, resulted in the formation of strong interfacial bond between the SiC and Al5083 matrix, which in turn improved the load transfer from the matrix to the hard particles. Similar wear mechanism was also reported earlier on Al/SiC composite[30] .
S. Bathula et al.: J. Mater. Sci. Technol., 2012, 28(11), 969–975.
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Fig. 7 Worn surface of (a) and (b) nanostructured Al5083 alloy and (c) and (d) Al 5083/SiCp
4. Conclusion Al 5083/10 wt% SiCp nanocomposites were synthesized by employing high energy ball milling followed by spark plasma sintering (SPS), which resulted in nanocomposites with near-theoretical density while preserving the nanoscale features of the ball milled nano-powders. The crystallite size of the ball milled Al 5083 matrix was observed to be ∼25 nm which coarsened up to 30 nm after rapid consolidation and sintering using SPS. HR-TEM investigations of Al5083/SiCp have shown a homogeneous distribution of SiCp in the Al 5083 alloy matrix with no evidence of voids or cracks. Nanoindentation measurements of these nanocomposites exhibited much higher hardness which was attributed to the homogeneous dispersion of reinforcements and nano-grained microstructure even after SPS. Wear studies show that the specific wear rate of Al 5083/SiCp nanocomposite is considerably lower than that of nanostructured Al 5083 alloy. Nanostructured Al5083 alloys, on the other hand, exhibited a combination of abrasive and delamination wear mechanisms in contrast to Al5083/SiCp which exhibited wear mechanism of adhesive wear to abrasive wear. The near-theoretical density, nanograined microstructure and homogeneous dispersion of reinforcement in the matrix resulted in very low specific wear rate of the Al 5083/SiCp nanocomposites.
Acknowledgements This work was supported by Council of Scientific and Industrial Research (CSIR) under its network project
(CSIR-NWP-51) entitled “Nanostructured Advanced Materials (NAM)”. The authors are grateful to the Director, CSIR-NPL for his support and facilities provided. The technical support rendered by Mr. Radhey Shyam, Mr. Naval Kishor Upadhyay, Mr. K.N. Sood, Dr. Sushil Kumar and Dr. D. Haranath is gratefully acknowledged. REFERENCES [1 ] J.R. Davis: ASM Specialty Handbook, Aluminium and Aluminium Alloys, ASM International, 1993. [2 ] C. Cayron, C. Hausmann, P.A. Buffat and O. Beffort: J. Mater. Sci. Lett., 1999, 18, 1671. [3 ] E.T. Thostenson, C.Y. Li and T.W. Chou: Compos. Sci. Technol., 2005, 65, 491. [4 ] Y.Y. Chen and D.D.L. Chung: J. Mater. Sci., 1996, 31, 407. [5 ] C. Suryanarayana: Prog. Mater. Sci., 2011, 46, 1. [6 ] R. Orru, R. Licheri, A.M. Locci, A. Cincotti and G. Cao: Mater. Sci. Eng. R, 2003, 63, 127. [7 ] G.D. Zhan, J. Kuntz, J. Wan, J. Garay and A.K. Mukherjee: Scripta Mater., 2002, 47(1), 737. [8 ] H. Kwon, M. Estili, K. Takagi, T. Miyazaki and A. Kawasaki: Carbon, 2009, 47, 570. [9 ] M.J. Yang, D.M. Zhang, X.F. Gu and L.M. Zhang: Mater. Chem. Phys., 2006, 99, 170. [10] A.P. Sannino and H.J. Rack: Wear, 1995, 189(1–2), 1. [11] I.A. Ibrahim, F.A. Mohamed and E.J. Lavernia: J. Mater. Sci., 1991, 26(5), 1137. [12] K.T. Kim, S.I. Cha and S.H. Hong: Mater. Sci. Eng. A, 2007, 449–451, 46. [13] G.B. Veeresh Kumar, C.S.P. Rao and N. Selvaraj: J. Min. Mater. Charac. Eng., 2011, 10, 59. [14] F.L. Zhang, C.Y. Wang and M. Zhu: Scripta Mater., 2003, 49, 1123.
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[15] J.H. Yu, J.S. Lee, Y.H. Choa and H. Hofmann: J. Mater. Sci. Technol., 2010, 26, 333. [16] B.D. Cullity and S.R. Stock: Elements of X-Ray Diffraction, 3rd edn, Prentice-Hall Inc., 2001, 167. [17] R.M. Daly, Khitouni, A.W. Kolsi and N. Njah: Phys. Stat. Sol. (c), 2006, 3, 3325. [18] S. Bathula, R.C. Anandani, A. Dhar and A.K. Srivastava: Adv. Mater. Res., 2012, 410, 224. [19] J.C. Chen, J. Feng, B. Xiao, K.H. Zhang, Y.P. Du, Z.J. Hong and R. Zhou: J. Mater. Sci. Technol., 2010, 26, 49. [20] S. Bathula, R.C. Anandani, A. Dhar and A.K. Srivastava: Mater. Sci. Eng. A, 2012, 545, 97. [21] W.C. Oliver and G.M. Pharr: J. Mater. Res., 2004, 19, 3.
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[22] G.F. Wang and K.F. Zhang, J. Mater. Sci. Technol., 2010, 26, 625. [23] D.J. Lloyd: Int. Mater. Rev., 1994, 39, 1. [24] Z.Y. Han, H.K. Li, G.Q. Lin and C. Dong: J. Mater. Sci. Technol., 2010, 26, 967 [25] A. Alizadeh and E. Taheri-Nassaj: Tribo. Lett., 2011, 44, 59. [26] D.S. Xiong and S.R. Shirong: Wear, 2001, 250, 242. [27] R.L. Deuis, C. Subramanian and J.M. Yellup: Compos. Sci. Technol., 1997, 57, 415. [28] A.W. El-Morsy: Mater. Sci. Eng. A, 2008, 473, 330. [29] J.P. Tu, W. Rong, S.Y. Guo and Y.Z. Yang: Wear, 2003, 255, 832. [30] T. Ma, H. Yamaura, D.A. Koss and R.C. Voigt: Mater. Sci. Eng. A, 2003, 360, 116.
Nanoindentation and Wear Characteristics of Al 5083/SiCp Nanocomposites Synthesized by High Energy Ball Milling and Spark Plasma Sintering Sivaiah Bathula, Saravanan M and Ajay Dhar† Metals and Alloys Group, Materials Physics and Engineering Division CSIR-National Physical Laboratory, Dr. K.S. Krishanan Marg, New Delhi – 110012, India [Manuscript received June 30, 2012, in revised form September 28, 2012]
Al 5083/10 wt% SiCp nano composites have been synthesized by means of high energy ball milling followed by spark plasma sintering (SPS). Nano composites produced via this method exhibited near-theoretical density while retaining the nano-grained features. X-ray diffraction (XRD) analysis indicated that the crystalline size of the ball milled Al 5083 matrix was observed to be ∼25 nm and it was coarsened up to ∼30 nm after SPS. Nano indentation results of nano composites demonstrated a high hardness of ∼280 HV with an elastic modulus of 126 GPa. Wear and friction characteristics with addition of SiCp reinforcement exhibited significant improvement in terms of coefficient of friction and specific wear rate to that of nano structured Al 5083 alloy. The reduction in specific wear rate in the nanocomposite was mainly due to the change of wear mechanism from adhesive to abrasive wear with the addition of SiCp which resulted in high hardness associated with nano-grained microstructure. KEY WORDS: Metal matrix nano composites; Friction/wear; Mechanical properties; High resolution transmission electron microscopy; Spark plasma sintering; Nanoindentation
1. Introduction Metal matrix composites (MMCs) are materials that incorporate micron-sized fibres/particulates into the matrix of a standard alloy, which results in drastic improvement in properties such as mechanical strength, toughness, wear etc.[1] . Aluminium (Al) based composites are important due to their light weight and high strength for manufacturing automobile parts such as brake rotors for high speed train, automotive braking systems and many other applications[2] . MMCs reinforced with nanoparticulates show drastic improvement in their mechanical properties[3] due to high surface to volume ratio. Al based nanocomposites are required to pos† Corresponding author. Ph.D.; Tel.: +91 11 45609455; Fax: +91 11 45609310; E-mail address: [email protected] (A. Dhar).
sess adequate strength without sacrificing their wear properties to exploit their use for structural applications in the place of their micro-sized counterparts and also the interface between the matrix and reinforcement is substantially altered in these nanocomposites. There are a few existing methods for the production of MMCs such as liquid metallurgy, powder metallurgy and spray forming[4] . However, it is well known that the retention of nanoscale features in liquid metallurgy route or spray forming is rather difficult due to their inherent solidification mechanisms. Powder metallurgy employing ball milling[5] , is a technique for the production of nanocomposite powders in bulk quantity with effective dispersion of nanoparticles in the matrix. It may be noted that conventional sintering of nanocomposites with longer duration of sintering cycles leads to excess grain growth in the material, due to the excess free energy associ-
970
S. Bathula et al.: J. Mater. Sci. Technol., 2012, 28(11), 969–975.
ated with the large amount of grain boundary area of the nanostructured matrix, which thermodynamically favours grain growth. In the present study, the ball milled powders are consolidated and sintered using spark plasma sintering technique (SPS). SPS is one of the most successful technique for consolidation and sintering of nanocomposite powders for obtaining near theoretical densification while retaining the nano-grained structure which directly influences the mechanical and wear properties[6,7] . This technique works mainly based on the electric spark discharge, where a high energy pulsated current momentarily generates spark plasma between the particles resulting in highly localized temperatures[8] . It is recognized that the heat generated at the point contacts of the particles vaporize contaminants and break the oxide layer on the surface of Al 5083 particles prior to neck formation[9] . The rapid heating rates and shorter sintering cycles during SPS with simultaneous application of load avoid grain growth in nanocomposites thus resulting in a highly dense product. There have been several studies on the synthesis of MMCs and Al/SiCp is one of the most well studied material due to its potential applications, which are described elsewhere[10] . Although there are sufficient studies available in the literature on the wear behaviour of MMCs, with micron-sized particulates[11] , only few studies on nanostructured MMCs are reported. The wear properties in nanocomposites are important as their small grain size associated with the nanoparticulates dispersion greatly improves the wear characteristics[12] . Similarly, a lot of data on microhardness has been reported on a variety of MMCs[13] but there are hardly any measurements available on their nanoindentation measurement results. In the present study, we report the nanoindentation results and wear mechanism of Al 5083/SiCp nanocomposites synthesized employing high-energy ball milling followed by consolidation and sintering using spark plasma sintering. Also these results have been correlated with the microstructure of the nanocomposites by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM).
Prior to consolidation and sintering, the ball milled nanocomposite powder was handled in glove box (Mbraun Inert Gas System, GmBH, Germany) to minimize oxidation and other atmospheric contamination. Crystallite size measurements were carried out by using an X-ray diffractometer (model: Miniflex II, maker: Rigaku). In order to calculate the final crystallite size of milled powders, full width half maxima (FWHM) values were subtracted for instrumental broadening and Kα2 corrections. Nanocomposite powders were subsequently consolidated and sintered under vacuum of ∼4 Pa using SPS (model: SPS 725, maker: SPS Syntax Inc., Japan) at a pressure of 50 MPa, at 300 ◦ C for 3 min. using graphite die and punches. The heating rate was maintained at 300 ◦ C per min. The surface morphology of ball milled powders and sintered pellets were studied by scanning electron microscopy (SEM model: EVO MA10, maker: Zeiss). High resolution transmission microscopy (HR-TEM, Model: Tecnai G2F 30 STWIN, FEG source, electron accelerating voltage 300 kV) was carried out on milled powders and SPS nanocomposites to study the dispersion of different nanoconstituents and the matrix-reinforcement interface. Micro-hardness measurements were carried out using a Vickers micro-hardness tester (model: FM-e7) with a load of 100 g for 10 s. Subsequently, elastic modulus and hardness measurements were performed using nanoindentation technique (M/s Fischer-Cripps Laboratories Pvt. Limited, Australia) at a load of 50 mN. The mechanical properties were derived from the measured load-penetration depth curves under loading/unloading through a standard data analysis software. Wear and friction measurements were evaluated using a pin-on-disc sliding wear machine (DucomTR20LE) as per ASTM Standard G99. These tests were conducted for sintered samples against a polished EN-31 steel disc keeping a sliding speed of 1 m/s and 2 m/s. All the tests were conducted at a load of 9.81 N and 49.05 N with a sliding distance of 600 m. The density of composites was measured by means of conventional Archimedes principle.
2. Experimental
3.1 SEM and XRD analysis
Al 5083 alloy (purity: 99.9%, particle size: ∼15 μm, density: 2.66 g/cm3 , maker: valimet Inc., USA) was ball milled along with 10 wt% of SiCp nanoparticles of ∼20 nm in size (density: 3.21 g/cm3 ) in a Fritsch make Pulverisette-4 ball mill for 15 h. The vial of the ball-mill and the milling media were made of stainless steel. The ball-to-powder weight ratio was maintained at 20:1 and milling was done in an argon atmosphere with a maximum milling speed of 400 r/min. In order to prevent the re-welding and promote the fracturing of powder particles, 2 wt% of stearic acid was added as a process control agent.
The as-received elemental powders at the starting stage of milling were nearly spherical in shape with an average size of 15 μm. Fig. 1(a) and (b) shows that the ball-milled powders exhibit facetted morphology with the particle size between ∼50 to 60 μm and the spark plasma sintered nanocomposite shows homogeneous dispersion of SiCp within the Al 5083 alloy matrix. This increase in the particle size could be explained on the basis of welding of particles at the first instant followed by fracture at the second stage[14,15] . Further milling leads to an increase in the stored energy of the system up to a critical value and beyond this,
3. Results and Discussion
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[17]
milling . The average crystallite size and lattice strain of the matrix were found to be ∼25 nm and 0.43% respectively, after high energy ball milling for 15 h and it was coarsened up to ∼30 nm after SPS. Subsequently, ball milled nanocomposite powders were consolidated and sintered by SPS technique, which resulted in nanocomposites with neartheoretical density while preserving the nanoscale features in the nanocomposites[18,19] . The HR-TEM images as shown in Fig. 3(a), reveals fine grained microstructure features in nanocomposites powders after 15 h ball milling and Fig. 3(b) shows the nanoscale features with large density of nano-grained network even after spark plasma sintering at 500 ◦ C. The average crystallite size was evaluated from HR-TEM measurement and is in agreement with the XRD results, although a few small crystallites are also visible. Further analysis on this nanocomposite[20] elucidated that a good interface was evolved around the SiCp in the α-Al matrix. Fig. 3(b) further suggests that apart from the good interface, there has been no visible porosity delineated at the interface around SiCp , even at lattice scale. Fig. 1 High resolution SEM images of Al 5083/SiCp (a) nano-powders after 15 h of ball milling and (b) after SPS process, showing homogeneous distribution of SiCp in the Al 5083 matrix
Fig. 2 XRD peak profile of (a) un-milled Al 5083 alloy, (b) Al 5083/SiCp nanocomposite powders ball milled for 15 h and (c) Al 5083/SiCp nanocomposite after SPS consolidation process
reduction of energy in terms of forming sub-grains and high angle grain boundaries over the entire volume of milling powder leading to nanostructure. Xray diffraction analysis (XRD) was carried out and Williamson-Hall method[16] was used to calculate the crystallite size and lattice strain of the Al 5083 matrix. The XRD peak profile shown in Fig. 2(b) indicates an increase in peak broadening with a sharp decrease in peak intensity after ball milling, which is mainly due to the reduction in the crystallite size. The increased peak broadening could also be attributed to crystalline defects such as dislocations and stacking faults generated by impact and shear forces during
3.2 Nanoindentation measurements Microhardness was measured on Al 5083/SiCp nanocomposite and nanostructured Al 5083 alloy samples of spark plasma sintered employing a microhardness tester under identical conditions. The results show the microhardness values of ∼280 and 148 HV for both samples, respectively. Fig. 4 shows a typical load-penetration depth curve obtained in a nanoindentation test and the resulting peak indentation depth includes elastic and plastic deformation. Nanoindentation results, as shown in Fig. 4, were also in good agreement with microhardness values measured on Al 5083/SiCp nanocomposite[21,22] . Measured hardness and elastic modulus values of Al 5083 alloy after milling for 15 h and SPS found to be 148 HV and 78 GPa, respectively and were enhanced to ∼280 HV and 126 GPa for nanocomposite of Al 5083/10 wt% SiCp . Significant enhancement in the hardness observed for Al 5083/SiCp nanocomposites were due to its nano-grained structure which is maintained even after SPS and dislocation pileups related to that of Orowan strengthening at grain boundaries[23,24] . 3.3 Coefficient of friction Fig. 5 shows the variation of coefficient of friction (COF) of nanostructured Al 5083 alloy and Al 5083/SiCp nanocomposite under applied loads of 9.81 and 49.05 N under a sliding speed of 2 m/s. The value of COF was found to be ∼0.22 at a load of 9.81 N, and it increased to 0.37 at a load of 49.04 N for nanostructured Al5083 alloy. This increase in COF for nanostructured Al 5083 alloy was accompanied with an in-
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Fig. 3 HR-TEM images of Al 5083/SiCp (a) nanocomposite ball milled for 15 h (b) spark plasma sintered. The inset shows the nano-grained microstructure with networked dislocations
Fig. 4 Nanoindentation measurements of nanostructured Al 5083 alloy and Al 5083/SiCp nanocomposite
crease in specific wear rate as shown in Fig. 6. The Al 5083/SiCp nanocomposite shows comparatively lower COF value (Fig. 5(d)) as compared to that of nanostructured Al 5083 alloy at higher loads (Fig. 5(b)). This has been attributed to the fine grained microstructure with large density dislocations network and also the well defined interface between matrixinterface that has been observed employing HR-TEM studies and has been discussed earlier in this paper. These results are in good agreement with similar studies reported for Al and Al/B4 C nanocomposites[25] . 3.4 Wear behaviour Fig. 6 shows the specific wear rates of nanostructured Al5083 alloy and Al5083/SiCp composite under the sliding speeds of 1 m/s and 2 m/s with 9.81 and 49.05 N applied loads. In general, for all the samples, it has been observed that the wear rate increases with an increase in the applied load. However, this increase is more significant in the case of nanostructured Al5083 alloy samples in comparison to the nanocomposites. In the case of nanostructured Al 5083 alloy at a load of 49.05 N, extensive
specific wear rate was observed which is due to localized plastic deformation on the friction surface of the base material. However, this increased specific wear rate for nanostructured alloy was mainly due to the rapid transition of wear mechanism from abrasive wear to a combination of adhesive wear and fatigue wear, as explained in later part of the paper. In Al5083/SiCp nanocomposite the application of higher applied load of 49.05 N resulted in lower specific wear rate and COF as compared to that of nanostructured Al 5083 alloy. Considerably, lower specific wear rate has been observed for Al5083/SiCp nanocomposites at 9.81 N and 48.05 N applied loads. This may be due to the change in wear mechanism from adhesive wear to abrasive wear with addition of SiCp associated with nano-grained microstructure after SPS which resulted in higher hardness and strength values, as discussed earlier. This mechanism was also reported by other researchers[26] who observed that the increase in reinforcement of alumina fibers to the aluminium matrix led to a significant decrease in wear rate. 3.5 Wear mechanism Fig. 7(a) and (b) shows the microstructures of the worn surface of nanostructured Al5083 alloy at an applied load of 9.81 and 49.05 N with a sliding speed of 2 m/s. The worn surface revealed continuous grooves along with sub-crack formation (fatigue crack) at lower applied load of 9.81 N. This indicates that the wear mechanism of nanostructured Al 5083 alloy at lower load is removal of material with abrasion. However, for higher applied load of 49.05 N, the worn surface shows severely deformed structure with plastic yielding which resulted in large portions of material being removed or pulled out of the surface[27] . This may also be due to localized melting on the sliding surface at higher loads due to rise in temperature. This therefore, suggests that the wear mechanism is a combination of abrasive and delamination of nano-
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Fig. 5 Coefficient of friction with sliding distance at load 9.81 and 49.05 N at sliding speed of 2 m/s
Fig. 6 Comparison of specific wear rates with applied loads for nanostructured Al 5083 alloy with Al 5083/SiCp nanocomposite
structured Al5083 alloy at higher applied load of 49.05 N. Similar observations have also been reported for AZ61 Mg alloy at higher applied load[28] . Fig. 7(c) and (d) shows the worn surface microstructures of Al5083/SiCp nanocomposite at an applied load of 9.81 and 49.05 N with a sliding speed of 2 m/s. For lower applied load of 9.81 N, worn surface of Al 5083/SiCp nanocomposite reveals continuous
grooves and initiation of some cracks on the surfaces. This may be due to homogeneous distribution of SiC particulates in nanostructured matrix associated with networked dislocations which results in improved wear resistance. This suggest that abrasive type of wear is the dominant mechanism in Al 5083/SiCp nanocomposite at lower applied load of 9.81 N. At higher loads of 49.05 N, it has been observed that the abrasive groove formation with less delamination of surface layers resulting in improved wear resistance. The absence of surface porosity, due to near-theoretical density obtained after SPS may be inferred that the interface between Al5083 matrix and SiCp shows more wear resistance resulting in lower specific wear rate and considerably lower COF. Further, it may also noted that the nano-grained microstructure imparts high wear resistance and delays the severe wear rate regime as reported by other researchers on Cu-TiB2 nanocomposites[29] . SPS which leads to high hardness and high strength of the nanocomposites, synthesized in the present study, resulted in the formation of strong interfacial bond between the SiC and Al5083 matrix, which in turn improved the load transfer from the matrix to the hard particles. Similar wear mechanism was also reported earlier on Al/SiC composite[30] .
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Fig. 7 Worn surface of (a) and (b) nanostructured Al5083 alloy and (c) and (d) Al 5083/SiCp
4. Conclusion Al 5083/10 wt% SiCp nanocomposites were synthesized by employing high energy ball milling followed by spark plasma sintering (SPS), which resulted in nanocomposites with near-theoretical density while preserving the nanoscale features of the ball milled nano-powders. The crystallite size of the ball milled Al 5083 matrix was observed to be ∼25 nm which coarsened up to 30 nm after rapid consolidation and sintering using SPS. HR-TEM investigations of Al5083/SiCp have shown a homogeneous distribution of SiCp in the Al 5083 alloy matrix with no evidence of voids or cracks. Nanoindentation measurements of these nanocomposites exhibited much higher hardness which was attributed to the homogeneous dispersion of reinforcements and nano-grained microstructure even after SPS. Wear studies show that the specific wear rate of Al 5083/SiCp nanocomposite is considerably lower than that of nanostructured Al 5083 alloy. Nanostructured Al5083 alloys, on the other hand, exhibited a combination of abrasive and delamination wear mechanisms in contrast to Al5083/SiCp which exhibited wear mechanism of adhesive wear to abrasive wear. The near-theoretical density, nanograined microstructure and homogeneous dispersion of reinforcement in the matrix resulted in very low specific wear rate of the Al 5083/SiCp nanocomposites.
Acknowledgements This work was supported by Council of Scientific and Industrial Research (CSIR) under its network project
(CSIR-NWP-51) entitled “Nanostructured Advanced Materials (NAM)”. The authors are grateful to the Director, CSIR-NPL for his support and facilities provided. The technical support rendered by Mr. Radhey Shyam, Mr. Naval Kishor Upadhyay, Mr. K.N. Sood, Dr. Sushil Kumar and Dr. D. Haranath is gratefully acknowledged. REFERENCES [1 ] J.R. Davis: ASM Specialty Handbook, Aluminium and Aluminium Alloys, ASM International, 1993. [2 ] C. Cayron, C. Hausmann, P.A. Buffat and O. Beffort: J. Mater. Sci. Lett., 1999, 18, 1671. [3 ] E.T. Thostenson, C.Y. Li and T.W. Chou: Compos. Sci. Technol., 2005, 65, 491. [4 ] Y.Y. Chen and D.D.L. Chung: J. Mater. Sci., 1996, 31, 407. [5 ] C. Suryanarayana: Prog. Mater. Sci., 2011, 46, 1. [6 ] R. Orru, R. Licheri, A.M. Locci, A. Cincotti and G. Cao: Mater. Sci. Eng. R, 2003, 63, 127. [7 ] G.D. Zhan, J. Kuntz, J. Wan, J. Garay and A.K. Mukherjee: Scripta Mater., 2002, 47(1), 737. [8 ] H. Kwon, M. Estili, K. Takagi, T. Miyazaki and A. Kawasaki: Carbon, 2009, 47, 570. [9 ] M.J. Yang, D.M. Zhang, X.F. Gu and L.M. Zhang: Mater. Chem. Phys., 2006, 99, 170. [10] A.P. Sannino and H.J. Rack: Wear, 1995, 189(1–2), 1. [11] I.A. Ibrahim, F.A. Mohamed and E.J. Lavernia: J. Mater. Sci., 1991, 26(5), 1137. [12] K.T. Kim, S.I. Cha and S.H. Hong: Mater. Sci. Eng. A, 2007, 449–451, 46. [13] G.B. Veeresh Kumar, C.S.P. Rao and N. Selvaraj: J. Min. Mater. Charac. Eng., 2011, 10, 59. [14] F.L. Zhang, C.Y. Wang and M. Zhu: Scripta Mater., 2003, 49, 1123.
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