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Effect of nano TiO2 particles on microhardness and microstructural behavior of AA7068 metal matrix composites
Author’s Accepted Manuscript Effect of Nano TiO 2 Particles on Microhardness and Microstructural Behaviour of AA7068 Metal Matrix Composites K. John Joshua, S.J. Vijay, D. Philip Selvaraj www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32134-5 https://doi.org/10.1016/j.ceramint.2018.08.077 CERI19112
To appear in: Ceramics International Received date: 4 June 2018 Revised date: 24 July 2018 Accepted date: 7 August 2018 Cite this article as: K. John Joshua, S.J. Vijay and D. Philip Selvaraj, Effect of Nano TiO 2 Particles on Microhardness and Microstructural Behaviour of AA7068 Metal Matrix Composites, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Nano TiO2 Particles on Microhardness and Microstructural Behaviour of AA7068 Metal Matrix Composites 1*
2*
K. John Joshua , S.J. Vijay , D. Philip Selvaraj
2
1
Research Scholar, Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore-641114, Tamil Nadu, India. 2
Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore-641114, Tamil Nadu, India.
* Corresponding authors: K. John Joshua Tel: +91-9894341750; Email: [email protected] S. J. Vijay Tel: +91-9944516658; Email: [email protected]
Abstract Present work deals with the fabrication of AA7068 composites reinforced with different percentages of Nano TiO2 (3, 6 and 9 wt.%) using powder metallurgy technique along with as sintered AA7068 material. The pressure applied for compacting the composites was 318 MPa and sintered at o
560 C in a sintering furnace for one hour. Vickers micro hardness test have been conducted for finding the microhardness. Optical Microscopy was carried out to analyze the microstructural behavior. The worn surface was investigated by the Field Emission Scanning Electron Microscope (FESEM) after the wear test. The microhardness was found to increase by increasing the weight percentage of TiO2 particles. It was observed to be a maximum of 68 VHN by adding 9% Nano TiO2 particles. Abrasion, oxidation and delamination were the dominant wear mechanisms present in the composites. Energy Dispersive Spectroscopy analysis was done to confirm the presence of the TiO2 particles.
Keywords: Compacting, Sintering, Powder metallurgy, FESEM, EDS 1. Introduction Metal matrix composites (MMCs) that incorporate ceramic particles as a reinforcement enhances the wear resistance, elasticity and hardness of pure metals. In past few years, researchers has focused attention on the aluminum alloys, due to their low density, high strength-to-weight ratio, improved resistance to corrosion and mechanical property [1, 2]. AA7068 alloy provides high mechanical strength and offers better bulk mechanical properties [3]. AA7068 is the strongest aluminum alloy which offers high
strength, good workability, high resistance to corrosion and so on [4]. Titanium dioxide (TiO2) has properties like chemical stability, good optical transparency, high refractive index, low cost, and nontoxicity. TiO2 has been widely used as in environmental applications, building materials such as tiles, concrete, paints and glasses. TiO2 nanoparticles finds its use in industrial and medical applications. TiO2– Al2O3 nanocomposite materials provides better wear and corrosion due to their thermal, chemical and mechanical stability. TiO2–Al2O3 structures are also used in various applications such as catalysis, solar cells, photocatalytic and self-cleaning [5, 6]. TiO2 is inexpensive and provides better wear resistance, mechanical and thermal properties [7]. The presence of TiO2 in an Al2O3 coating has enhanced its mechanical behavior [8]. The toughness of Ti/TiO2 nanocomposite samples has improved considerably [9]. The hardness and wear performance of Ni–TiO2 nanocomposite coating are improved by increasing of TiO2 wt.% in coating [10]. Al6061–TiO2 composites exhibits higher hardness, lower wear coefficient when compared with the matrix alloy [11]. The Ni–P–TiO2 nanocomposite coatings show improved hardness and wear resistance as compared to that of Ni–P alloy coatings [12]. Other researchers show that the hardness has been improved by the addition of increase in TiO2 content [13-16]. Stir casting and powder metallurgy are two widespread methods used for product manufacturing. Powder metallurgy method is the most suitable method for making MMCs which has the advantage of low processing temperature and has better distribution of reinforcement particles in the matrix [17,18]. The fabricated AA7068/TiO2 composites with various compositions such as 3%, 6% and 9% reinforcement in Al matrix are characterized for their microhardness and metallurgical properties and their results are reported. The results of the composites are also compared with as sintered AA7068.
2. Experimental Procedure 2.1 Composite Preparation Figure 1 shows the FESEM images of powders used in this research work. The AA7068/TiO2 composite samples were produced through powder metallurgy route. The composition of AA7068 powders were shown in the Table 1. Shimadzu electronic weighing machine-ATY 224 model with an accuracy of 0.0001g was used for weighing all the powders accurately and milled in a ball mill for 40 hours. 2% stearic acid was added to have proper bonding between the particles [19]. Reduction of reinforcing
particle size and matrix grain size to submicron level was possible by ball milling. Distribution of reinforcing particles during ball milling depend on ball milling parameters and on initial particle sizes [20]. The average particle size of AA7068 powders and Nano TiO2 powders were calculated using particle size analyzer which were shown in Table 2. The powders were cold compacted at a pressure of 318 MPa using a Universal Testing Machine (UTM) to produce green compacts of size 20 mm diameter and 30±2 mm height. The green compacts of the samples were shown in the fig. 2. The green compacts were o
sintered at 560 C for one hour in a sintering furnace. Similar procedure was used to fabricate the composites of AA7068 reinforced with Nano TiO2 with 3, 6, 9 wt.% and plain AA7068 samples. The samples were allowed to cool at room temperature after sintering. The sintered composites and as sintered AA7068 were machined to 10 mm diameter for testing purpose. The samples were ground using belt grinder with abrasive papers of grit sizes 600, 800 and 1200. They were finally polished using 2 µm diamond paste and then with 0.5 µm diamond paste in a twin disc polisher. The samples were etched with Keller’s reagent for 10 seconds to remove impurities on the surface and expose the grains. Optical microscopy analysis was done to study microstructural behavior of the samples. The samples were tested for their wear behavior at 5 N load, sliding velocity of 1.2 m/s and sliding distance of 2.5 km to study the dominant wear behavior. Subsequently the worn surface and the wear debris collected were investigated by the Field Emission Scanning Electron Microscope (FESEM). Energy Dispersive Spectroscopy (EDS) analysis was carried out to confirm the presence of combinations of the worn surface. Table 1. Composition of AA7068 (% by weight) Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Zr
Al
Content
0.12
0.15
2
0.1
3
0.05
8
0.01
0.1
Bal
Table 2. Average particle size Powders
Average Particle size
AA7068
1.785 µm
TiO2
2.585 nm
The hardness test was conducted using a Vickers microhardness machine. The results were presented in Figure 5. Three spots were identified and Vickers microhardness was measured and the average values were taken for all the samples. 3. Results and Discussion 3.1 Microstructural analysis Figure 3(a-d) shows the optical micrographs of AA7068 reinforced with Nano TiO2 particles. Fig. 3a shows the micrograph of AA7068 base matrix without reinforcement. It consists of more number of uniformly distributed spherical shaped pores. This is due to the application of low pressure (318 MPa) during the compacting process [3]. It is observed from the fig 3(b-d) that TiO2 particles are uniformly distributed throughout the matrix. The hard TiO2 reinforcement provides strong bonding with the matrix alloy which enabled uniform load transfer from the base matrix to the reinforcement [7]. Uniform dispersion of Nano TiO2 particles effectively prevent the formation of large grains and grain growth. Smaller sized particle has more pinning effect compared to those of large sized particles [28]. The composites reinforced with TiO2 are denser compared to the matrix alloy. This is due to the addition of fine nano-TiO2 particles mixed with the matrix material, uniform mixing of particles and proper application of compaction pressure. Fine grain size improves hardness and reduces wear rate. Porosity formation is highly probable in sintering method compared to stir casting technique [17]. The presence of porosity is due to the result of poor mixing of powders or insufficient compaction and sintering conditions [27]. 3.2 EDS Analysis EDS analysis is used to determine the elemental analysis at different regions [12]. Figure 4(a-d) Illustrates the EDS pattern for AA7068 reinforced with Nano TiO2 particles. It is noticed that the highest peak belongs to aluminum for all patterns from the EDS analysis. 7XXX series alloys are age-hardenable high strength aluminum alloys, which has zinc as its major element next to aluminum [4,29,30]. The EDS elemental analysis confirms the maximum amount of zinc present next to aluminum. The EDS pattern in fig 4(b-d) confirms the existence of Nano TiO2 reinforcement particles. All the EDS patterns confirm the presence of oxide layer. Oxidation occurs due to the frictional heat generation during sliding surfaces.
The oxide layer acts as protective layer which reduces the wear loss by minimizing the effective area of contact between the mating surfaces [7].
3.3 Microhardness measurement Figure 5 indicates the effect of Nano TiO2 on microhardness for the composite samples. The graph shows that as the Nano TiO2 content increases the microhardness also increases. Vickers hardness number increases from 33 VHN (as sintered AA7068) to 45 VHN (3 wt% TiO2), indicating an improvement of 36.3%. The maximum hardness is found to be 68 VHN for an addition of 9% Nano TiO2 reinforcement particles. The incorporation of hard ceramic particles into an aluminum matrix results in increase in the bulk hardness of the material [31]. The hardness of an aluminum based composite depends on many factors such as grain size, dislocation density, nano sized reinforcement particles, heat input [28], porosity [31] and so on depending on the fabrication techniques used. The hardness value is inversely proportional to the grain size. Larger grain size reduces the hardness and smaller grain size increases the hardness.
Larger proportions of porosity minimizes the hardness and lower proportions of porosity
increases the hardness of the composite. Hardness increases due to a better distribution of the TiO 2 nanoparticles and grain refinement in the base matrix [28]. During sliding, an increase in the Nano TiO2 content, increases the dislocation density there by restricting the plastic deformation. Uniform distribution of the reinforcement particles in the matrix and the higher hardness values of TiO2 are the reasons for the increase in the microhardness of the composites [7].
3.4 Interpretation of worn surface Figure 6(a-h) shows the FESEM images of worn surfaces of AA7068 reinforced with TiO2. Figure 6a and 6b illustrates the FESEM images of worn surfaces of as sintered AA7068 at lower magnification and at higher magnification respectively. Wear mechanisms such as abrasion, delamination and oxidation were predominant mechanisms in the sample. Figure 6b shows large number of deep and shallow narrow plastic grooves present on the worn surface. Grooves are present paralleling to the sliding direction which is the characteristic of abrasive wear [23]. The presence of deep grooves and plastic flow increases the
shear stress required to peel off the wearing surface and contributes to delamination wear mechanism. Deep grooves present are due to the formation of plastic deformation and soft aluminum asperities developed by the frictional heat and shear stress [24]. Few pits are found in fig 6b which is due to the pulling out of the particles of the matrix material. Wider Grooves and scratching becomes more severe at the higher speeds of 1–5 m/s and at higher loads. Such feature is the characteristics of severe abrasion, where hard asperities on the steel counter face or hard pulled out particles in between the contacting surfaces, which causes wear debris [25]. The EDS pattern shown in fig. 4(a) confirms the presence of oxide layer. Oxidation occurs during the sliding of surfaces due to frictional heat generation. The wear loss is reduced due to the protective oxide layer, which minimizes the effective area of contact between the mating surfaces [7].
Figure 6c and 6d illustrates the FESEM images of worn surfaces of AA7068 reinforced with 3% of nano TiO2 composite at lower magnification and at higher magnification respectively. Adhesion, abrasion and delamination are the dominant wear mechanisms present in the sample. Shallow and narrow grooves are present in the sample which encourages abrasive wear mechanism. This indicates the occurrence of micro-cutting and micro-ploughing effect of the counter face which are signs of ductile fracture [12]. The worn surface shows few cavities which are clearly visible in fig 6d. Cavities of rough regions indicate adhesive wear and smooth regions of fine grooves or ploughing suggest abrasive wear [21].
Figure 6e, 6f and 6g, 6h illustrates the FESEM images of worn surfaces of AA7068 reinforced with 6% and 9% of nano TiO2 particles respectively. Both worn surfaces shows similar type of wear pattern. Mild abrasion and oxidation are the dominant mechanisms present in the sample. Restriction of grooves along the sliding surface can be attributed to the high dislocation density of the deformed planes and high content of nano TiO2 (9%). The presence of TiO2 offers extreme hardness to the composites and reduces loss of material. Evidence of the oxide layer and the adhesive compacted particles on the worn surface obviously restricts severe plastic deformation thereby lowers the wear loss [7]. The EDS pattern shown in fig 4 (b-d) confirms the presence of nano TiO2 particles and the oxide layer present in the composite.
3.5 Examination of Wear Debris Fig. 7(a–d) shows the FESEM images of wear debris collected for as sintered AA7068 and the composites. Fig 7a shows the bigger deep groove shaped sheets and irregular shaped particles. During dry sliding, larger sized wear debris is formed due to large scale plastic deformation. Presence of grooves can be attributed to abrasive wear mechanism, which is due to the hard asperities of the hard counterface or hard particles which exists between the pin and disk surfaces. This can be attributed to a combination of delamination and abrasion wear mechanism [22]. Fig 7b and 7c shows large irregular shaped debris particles. The cutting action of the hard asperities of the counterface creates such kind of debris. The large sized debris is probably due to the presence of deep grooves in the worn surface. Presence of large grooves confirms delamination wear mechanism [26]. Fig 7d shows fine loose particles of wear debris. The image further shows reduced mean size of particles for an addition of 9% TiO2 compared to other composite combinations. The addition of Nano TiO2 particles in the composite minimizes the plastic deformation due to increase in dislocation density. Hence the hardness of the composite is increased and reduces the particle size. The amount of hard TiO2 phases dictates the size and morphology of wear debris [7].
4. Conclusions ·
AA7068/TiO2 metal matrix composite was successfully fabricated using powder metallurgy technique with improved hardness and better bonding.
·
Optical microscopic images, represented a uniform dispersion of reinforcement particles in the aluminum matrix.
·
The microhardness of the composites increased with increase in TiO2 particles and was found to be having a maximum of 68 VHN for an addition of 9% Nano TiO2 particles.
·
Abrasion, oxidation and delamination were found to be the dominant wear mechanisms composites.
in the
Acknowledgment
The authors would like to thank Mr. J. Deva Manoharan, Centre for Research in Metallurgy, Karunya Institute of Technology and Sciences for providing his valuable guidance and support.
References [1] E. Kim G. Cho, J. Lee, Y. Jung Y. Yoo, S. Seo, Fabrication and mechanical properties of metal matrix composite with homogeneously dispersed ceramic particles, Ceram. Int. 39 (2013) 6503–6508. [2] S.K. Patel, B. Kuriachen, N. Kumar and R. Nateriya, The slurry abrasive wear behavior and microstructural analysis of A2024-SiC-ZrSiO4 metal matrix composite, Ceram. Int. 44(6) (2018) 64266432. [3] A. Azimi, A. Shokuhfar, O. Nejadseyfi O, H. Fallahdoost and S. Salehi, Optimizing consolidation behaviour of Al 7068–TiC nanocomposites using Taguchi statistical analysis Trans. Nonferrous. Met. Soc. China. 25 (2015) 2499−2508. [4] M. Madhusudhan, G.J. Naveen and K. Mahesha, Mechanical Characterization of AA7068-ZrO2 reinforced Metal Matrix Composites, Mater Today: Proceedings. 4 (2017) 3122–3130. [5] A.J. Haider, R.H. Al-Anbari, G.R. Kadhim and C.T. Salame, Exploring potential environmental applications of TiO2 nanoparticles, Energy Procedia. 119 (2017) 332-345. [6] U.A.O. Arıer and F.Z. Tepehan, Influence of Al2O3:TiO2 ratio on the structural and optical properties of TiO2–Al2O3 nano-composite films produced by sol gel method, Compos. Part B. 58 (2014) 147–151. [7] V.K.C. Antony and J. Selwinrajadurai, Influence of rutile (TiO2) content on wear and microhardness characteristics of aluminium-based hybrid composites synthesized by powder metallurgy, Trans. Nonferrous. Met. Soc. China. 26 (2016) 63−73. [8] M.J. Ghazali, S.M. Forghani, N. Hassanuddin, A. Muchtar and R. Daud, Comparative wear study of plasma sprayed TiO2 and Al2O3–TiO2 on mild steels, Tribol. Int. 93 (2016) 681–686. [9] J. Moradgholi, A. Monshi, K. Farmanesh, M.R. Toroghinejad, M.R. Loghman-Estarki, Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and
titanium/SiC composites manufactured by accumulative roll bonding (ARB) process, Ceram. Int. 43(10) (2017) 7701-7709. [10] P. Baghery, M. Farzam, A.B. Mousavi and M. Hosseini, Ni–TiO2 nanocomposite coating with high resistance to corrosion and wear, Surface & Coatings Technol. 204 (2010) 3804–3810. [11] C.S. Ramesh, A.R. Anwar Khan, N. Ravikumar and P. Savanprabhu, Prediction of wear coefficient of Al6061–TiO2 composites, Wear. 259 (2005) 602–608. [12] P. Makkar, R.C. Agarwala and V. Agarwala, Wear characteristics of mechanically milled TiO2 nanoparticles incorporated in electroless Ni–P coatings, Adv. Powder Technol. 25 (2014) 1653–60. [13] G. Elango and B.K. Raghunath, Tribological Behavior of Hybrid (LM25Al + SiC+ TiO2) Metal Matrix Composites, Proc. Eng. 64 (2013) 671–680. [14] S. Yizhou, T. Haijun, L. Yuebin, Z. Xiaofei, W. Tao, T. Jie and P. Lei, Fabrication and Wear Resistance of TiO2/Al2O3 Coatings by Micro-arc Oxidation, Rare Metal Mat. Eng. 46 (2017) 23-27. [15] H. Luo, P. Song, A. Khan, J. Feng and J.S. Lu, Alternant phase distribution and wear mechanical properties of an Al2O3-40 wt%TiO2 composite coating, Ceram. Int. 43(9) (2017) 7295-7304. [16] R. Younes, M.A. Bradai, A. Sadeddine, Y. Mouadji, A. Bilek and A. Benabbas, Effect of TiO2 and ZrO2 reinforcements on properties of Al2O3 coatings fabricated by thermal flame spraying, Trans. Nonferrous. Met. Soc. China. 26(5) (2016) 1345-1352. [17] H. Abdizadeh, R. Ebrahimifard and M.A. Baghchesara, Investigation of microstructure and mechanical properties of Nano MgO reinforced Al composites manufactured by stir casting and powder metallurgy methods: A comparative study, Compos. Part B. 56 (2014) 217–221. [18] T. Varol, A. Canakci and S. Ozsahin, Artificial neural network modeling to effect of reinforcement properties on the physical and mechanical properties of Al2024–B4C composites produced by powder metallurgy, Compos. Part B. 54 (2013) 224–233. [19] P.C. Angelo and R. Subramanian, Powder Metallurgy: Science, Technology and Application, First ed. 2008, Prentice Hall: India. [20] J. Corrochano, M. Lieblich and J. Ibanez, The effect of ball milling on the microstructure of powder metallurgy aluminium matrix composites reinforced with MoSi2 intermetallic particles, Compos. Part A. 42 (2011) 1093–1099.
[21] Shabani, M. Ostad and A. Mazahery A, Prediction of wear properties in A356 matrix composite reinforced with B4C particulates, Synthet. Metal. 161(13) (2011) 1226-1231. [22] A. Alizadeh and E.T. Nassaj, Wear Behavior of Nanostructured Al and Al–B4C Nanocomposites Produced by Mechanical Milling and Hot Extrusion, Tribol. Lett. 44 (2011) 59–66. [23] Y. Yao, L. Jiang, G. Fu and L. Chen, Wear behavior and mechanism of B4C reinforced Mg-matrix composites fabricated by metal-assisted pressureless infiltration technique, Trans. Nonferrous. Met. Soc. China. 25 (2015) 2543−2548. [24] B.A. Kumar, N. Murugan and I. Dinaharan, Dry sliding wear behavior of stir cast AA6061-T6/AlNp composite, Trans. Nonferrous. Met. Soc. China. 24 (2014) 2785−2795. [25] M. Uthayakumar, S. Aravindan and K. Rajkumar, Wear performance of Al–SiC–B4C hybrid composites under dry sliding conditions, Mater. Des. 47 (2013) 456–464. [26] A. Kurs¸ E. Bayraktar and M.H. Enginsoy, Experimental and numerical study of alumina reinforced aluminum matrix composites: Processing, microstructural aspects and properties, Compos. Part B. 90 (2016) 302-314. [27] H. Izadi, A. Nolting, C. Munro, D.B. Bishop, K.P. Plucknett and A.P. Gerlich, Friction stir processing of Al/SiC composites fabricated by powder metallurgy, J. Mater. Process. Technol. 213 (2013) 1900– 1907. [28] S.S. Mirjavadi, M. Alipour, S. Emamian, S. Kord, A.M.S. Hamouda, P.G. Koppad, R. Keshavamurthy, Influence of TiO2 nanoparticles incorporation to friction stir welded 5083 aluminum alloy on the microstructure, mechanical properties and wear resistance, J. Alloys. Comp. 712 (2017) 795-803. [29] P.A. Rometsch, Y. Zhang and S. Knight, Heat treatment of 7xxx series aluminium alloys—Some recent developments, Trans. Nonferrous Met. Soc. China. 24 (2014) 2003−2017. [30] M. Dixit, R.S. Mishra and K.K. Sankaran, Structure–property correlations in Al 7050 and Al 7055 highstrength aluminum alloys, Mater. Sci. Eng A. 478 (2008) 163–172. [31] A.A. Hamida, P.K. Ghosh, S.C. Jain, and S. Ray, The influence of porosity and particles content on dry sliding wear of cast in situ Al(Ti)–Al2O3(TiO2) composite, Wear. 265 (2008) 14–26.
FIGURE CAPTIONS Figure No.
Figure Captions
1
FESEM image of powders as received a) AA7068 after milling b) Nano TiO2
2
Compacted samples of AA7068 reinforced with Nano TiO2
3
4 5
Optical Micrographs of the processed samples (a) As sintered AA7068 (b) AA7068-3%TiO2 (c) AA7068-6%TiO2 and (d) AA7068-9%TiO2 EDS patterns of worn surface (a) As sintered AA7068 (b) AA7068-3%TiO2 (c) AA70686%TiO2 and (d) AA7068-9%TiO2 Vickers microhardness of AA7068 with increase in TiO2 Reinforcement FESEM micrograph of the worn surface (a) As sintered AA7068 (c) AA7068-3%TiO2 (e)
6
AA7068-6%TiO2 and (g) AA7068-9%TiO2 at lower magnification and (b) As sintered AA7068 (d) AA7068-3% TiO2 (f) AA7068-6% TiO2 and (h) AA7068-9% TiO2 at higher magnification respectively
7
FESEM images of wear debris (a) As sintered AA7068 (b) AA7068-3%TiO2 (c) AA70686%TiO2 and (d) AA7068-9%TiO2
TABLE CAPTIONS Table No.
Table Captions
1
Composition of AA7068 (% by weight)
2
Average particle size
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
PII: DOI: Reference:
S0272-8842(18)32134-5 https://doi.org/10.1016/j.ceramint.2018.08.077 CERI19112
To appear in: Ceramics International Received date: 4 June 2018 Revised date: 24 July 2018 Accepted date: 7 August 2018 Cite this article as: K. John Joshua, S.J. Vijay and D. Philip Selvaraj, Effect of Nano TiO 2 Particles on Microhardness and Microstructural Behaviour of AA7068 Metal Matrix Composites, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Nano TiO2 Particles on Microhardness and Microstructural Behaviour of AA7068 Metal Matrix Composites 1*
2*
K. John Joshua , S.J. Vijay , D. Philip Selvaraj
2
1
Research Scholar, Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore-641114, Tamil Nadu, India. 2
Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore-641114, Tamil Nadu, India.
* Corresponding authors: K. John Joshua Tel: +91-9894341750; Email: [email protected] S. J. Vijay Tel: +91-9944516658; Email: [email protected]
Abstract Present work deals with the fabrication of AA7068 composites reinforced with different percentages of Nano TiO2 (3, 6 and 9 wt.%) using powder metallurgy technique along with as sintered AA7068 material. The pressure applied for compacting the composites was 318 MPa and sintered at o
560 C in a sintering furnace for one hour. Vickers micro hardness test have been conducted for finding the microhardness. Optical Microscopy was carried out to analyze the microstructural behavior. The worn surface was investigated by the Field Emission Scanning Electron Microscope (FESEM) after the wear test. The microhardness was found to increase by increasing the weight percentage of TiO2 particles. It was observed to be a maximum of 68 VHN by adding 9% Nano TiO2 particles. Abrasion, oxidation and delamination were the dominant wear mechanisms present in the composites. Energy Dispersive Spectroscopy analysis was done to confirm the presence of the TiO2 particles.
Keywords: Compacting, Sintering, Powder metallurgy, FESEM, EDS 1. Introduction Metal matrix composites (MMCs) that incorporate ceramic particles as a reinforcement enhances the wear resistance, elasticity and hardness of pure metals. In past few years, researchers has focused attention on the aluminum alloys, due to their low density, high strength-to-weight ratio, improved resistance to corrosion and mechanical property [1, 2]. AA7068 alloy provides high mechanical strength and offers better bulk mechanical properties [3]. AA7068 is the strongest aluminum alloy which offers high
strength, good workability, high resistance to corrosion and so on [4]. Titanium dioxide (TiO2) has properties like chemical stability, good optical transparency, high refractive index, low cost, and nontoxicity. TiO2 has been widely used as in environmental applications, building materials such as tiles, concrete, paints and glasses. TiO2 nanoparticles finds its use in industrial and medical applications. TiO2– Al2O3 nanocomposite materials provides better wear and corrosion due to their thermal, chemical and mechanical stability. TiO2–Al2O3 structures are also used in various applications such as catalysis, solar cells, photocatalytic and self-cleaning [5, 6]. TiO2 is inexpensive and provides better wear resistance, mechanical and thermal properties [7]. The presence of TiO2 in an Al2O3 coating has enhanced its mechanical behavior [8]. The toughness of Ti/TiO2 nanocomposite samples has improved considerably [9]. The hardness and wear performance of Ni–TiO2 nanocomposite coating are improved by increasing of TiO2 wt.% in coating [10]. Al6061–TiO2 composites exhibits higher hardness, lower wear coefficient when compared with the matrix alloy [11]. The Ni–P–TiO2 nanocomposite coatings show improved hardness and wear resistance as compared to that of Ni–P alloy coatings [12]. Other researchers show that the hardness has been improved by the addition of increase in TiO2 content [13-16]. Stir casting and powder metallurgy are two widespread methods used for product manufacturing. Powder metallurgy method is the most suitable method for making MMCs which has the advantage of low processing temperature and has better distribution of reinforcement particles in the matrix [17,18]. The fabricated AA7068/TiO2 composites with various compositions such as 3%, 6% and 9% reinforcement in Al matrix are characterized for their microhardness and metallurgical properties and their results are reported. The results of the composites are also compared with as sintered AA7068.
2. Experimental Procedure 2.1 Composite Preparation Figure 1 shows the FESEM images of powders used in this research work. The AA7068/TiO2 composite samples were produced through powder metallurgy route. The composition of AA7068 powders were shown in the Table 1. Shimadzu electronic weighing machine-ATY 224 model with an accuracy of 0.0001g was used for weighing all the powders accurately and milled in a ball mill for 40 hours. 2% stearic acid was added to have proper bonding between the particles [19]. Reduction of reinforcing
particle size and matrix grain size to submicron level was possible by ball milling. Distribution of reinforcing particles during ball milling depend on ball milling parameters and on initial particle sizes [20]. The average particle size of AA7068 powders and Nano TiO2 powders were calculated using particle size analyzer which were shown in Table 2. The powders were cold compacted at a pressure of 318 MPa using a Universal Testing Machine (UTM) to produce green compacts of size 20 mm diameter and 30±2 mm height. The green compacts of the samples were shown in the fig. 2. The green compacts were o
sintered at 560 C for one hour in a sintering furnace. Similar procedure was used to fabricate the composites of AA7068 reinforced with Nano TiO2 with 3, 6, 9 wt.% and plain AA7068 samples. The samples were allowed to cool at room temperature after sintering. The sintered composites and as sintered AA7068 were machined to 10 mm diameter for testing purpose. The samples were ground using belt grinder with abrasive papers of grit sizes 600, 800 and 1200. They were finally polished using 2 µm diamond paste and then with 0.5 µm diamond paste in a twin disc polisher. The samples were etched with Keller’s reagent for 10 seconds to remove impurities on the surface and expose the grains. Optical microscopy analysis was done to study microstructural behavior of the samples. The samples were tested for their wear behavior at 5 N load, sliding velocity of 1.2 m/s and sliding distance of 2.5 km to study the dominant wear behavior. Subsequently the worn surface and the wear debris collected were investigated by the Field Emission Scanning Electron Microscope (FESEM). Energy Dispersive Spectroscopy (EDS) analysis was carried out to confirm the presence of combinations of the worn surface. Table 1. Composition of AA7068 (% by weight) Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Zr
Al
Content
0.12
0.15
2
0.1
3
0.05
8
0.01
0.1
Bal
Table 2. Average particle size Powders
Average Particle size
AA7068
1.785 µm
TiO2
2.585 nm
The hardness test was conducted using a Vickers microhardness machine. The results were presented in Figure 5. Three spots were identified and Vickers microhardness was measured and the average values were taken for all the samples. 3. Results and Discussion 3.1 Microstructural analysis Figure 3(a-d) shows the optical micrographs of AA7068 reinforced with Nano TiO2 particles. Fig. 3a shows the micrograph of AA7068 base matrix without reinforcement. It consists of more number of uniformly distributed spherical shaped pores. This is due to the application of low pressure (318 MPa) during the compacting process [3]. It is observed from the fig 3(b-d) that TiO2 particles are uniformly distributed throughout the matrix. The hard TiO2 reinforcement provides strong bonding with the matrix alloy which enabled uniform load transfer from the base matrix to the reinforcement [7]. Uniform dispersion of Nano TiO2 particles effectively prevent the formation of large grains and grain growth. Smaller sized particle has more pinning effect compared to those of large sized particles [28]. The composites reinforced with TiO2 are denser compared to the matrix alloy. This is due to the addition of fine nano-TiO2 particles mixed with the matrix material, uniform mixing of particles and proper application of compaction pressure. Fine grain size improves hardness and reduces wear rate. Porosity formation is highly probable in sintering method compared to stir casting technique [17]. The presence of porosity is due to the result of poor mixing of powders or insufficient compaction and sintering conditions [27]. 3.2 EDS Analysis EDS analysis is used to determine the elemental analysis at different regions [12]. Figure 4(a-d) Illustrates the EDS pattern for AA7068 reinforced with Nano TiO2 particles. It is noticed that the highest peak belongs to aluminum for all patterns from the EDS analysis. 7XXX series alloys are age-hardenable high strength aluminum alloys, which has zinc as its major element next to aluminum [4,29,30]. The EDS elemental analysis confirms the maximum amount of zinc present next to aluminum. The EDS pattern in fig 4(b-d) confirms the existence of Nano TiO2 reinforcement particles. All the EDS patterns confirm the presence of oxide layer. Oxidation occurs due to the frictional heat generation during sliding surfaces.
The oxide layer acts as protective layer which reduces the wear loss by minimizing the effective area of contact between the mating surfaces [7].
3.3 Microhardness measurement Figure 5 indicates the effect of Nano TiO2 on microhardness for the composite samples. The graph shows that as the Nano TiO2 content increases the microhardness also increases. Vickers hardness number increases from 33 VHN (as sintered AA7068) to 45 VHN (3 wt% TiO2), indicating an improvement of 36.3%. The maximum hardness is found to be 68 VHN for an addition of 9% Nano TiO2 reinforcement particles. The incorporation of hard ceramic particles into an aluminum matrix results in increase in the bulk hardness of the material [31]. The hardness of an aluminum based composite depends on many factors such as grain size, dislocation density, nano sized reinforcement particles, heat input [28], porosity [31] and so on depending on the fabrication techniques used. The hardness value is inversely proportional to the grain size. Larger grain size reduces the hardness and smaller grain size increases the hardness.
Larger proportions of porosity minimizes the hardness and lower proportions of porosity
increases the hardness of the composite. Hardness increases due to a better distribution of the TiO 2 nanoparticles and grain refinement in the base matrix [28]. During sliding, an increase in the Nano TiO2 content, increases the dislocation density there by restricting the plastic deformation. Uniform distribution of the reinforcement particles in the matrix and the higher hardness values of TiO2 are the reasons for the increase in the microhardness of the composites [7].
3.4 Interpretation of worn surface Figure 6(a-h) shows the FESEM images of worn surfaces of AA7068 reinforced with TiO2. Figure 6a and 6b illustrates the FESEM images of worn surfaces of as sintered AA7068 at lower magnification and at higher magnification respectively. Wear mechanisms such as abrasion, delamination and oxidation were predominant mechanisms in the sample. Figure 6b shows large number of deep and shallow narrow plastic grooves present on the worn surface. Grooves are present paralleling to the sliding direction which is the characteristic of abrasive wear [23]. The presence of deep grooves and plastic flow increases the
shear stress required to peel off the wearing surface and contributes to delamination wear mechanism. Deep grooves present are due to the formation of plastic deformation and soft aluminum asperities developed by the frictional heat and shear stress [24]. Few pits are found in fig 6b which is due to the pulling out of the particles of the matrix material. Wider Grooves and scratching becomes more severe at the higher speeds of 1–5 m/s and at higher loads. Such feature is the characteristics of severe abrasion, where hard asperities on the steel counter face or hard pulled out particles in between the contacting surfaces, which causes wear debris [25]. The EDS pattern shown in fig. 4(a) confirms the presence of oxide layer. Oxidation occurs during the sliding of surfaces due to frictional heat generation. The wear loss is reduced due to the protective oxide layer, which minimizes the effective area of contact between the mating surfaces [7].
Figure 6c and 6d illustrates the FESEM images of worn surfaces of AA7068 reinforced with 3% of nano TiO2 composite at lower magnification and at higher magnification respectively. Adhesion, abrasion and delamination are the dominant wear mechanisms present in the sample. Shallow and narrow grooves are present in the sample which encourages abrasive wear mechanism. This indicates the occurrence of micro-cutting and micro-ploughing effect of the counter face which are signs of ductile fracture [12]. The worn surface shows few cavities which are clearly visible in fig 6d. Cavities of rough regions indicate adhesive wear and smooth regions of fine grooves or ploughing suggest abrasive wear [21].
Figure 6e, 6f and 6g, 6h illustrates the FESEM images of worn surfaces of AA7068 reinforced with 6% and 9% of nano TiO2 particles respectively. Both worn surfaces shows similar type of wear pattern. Mild abrasion and oxidation are the dominant mechanisms present in the sample. Restriction of grooves along the sliding surface can be attributed to the high dislocation density of the deformed planes and high content of nano TiO2 (9%). The presence of TiO2 offers extreme hardness to the composites and reduces loss of material. Evidence of the oxide layer and the adhesive compacted particles on the worn surface obviously restricts severe plastic deformation thereby lowers the wear loss [7]. The EDS pattern shown in fig 4 (b-d) confirms the presence of nano TiO2 particles and the oxide layer present in the composite.
3.5 Examination of Wear Debris Fig. 7(a–d) shows the FESEM images of wear debris collected for as sintered AA7068 and the composites. Fig 7a shows the bigger deep groove shaped sheets and irregular shaped particles. During dry sliding, larger sized wear debris is formed due to large scale plastic deformation. Presence of grooves can be attributed to abrasive wear mechanism, which is due to the hard asperities of the hard counterface or hard particles which exists between the pin and disk surfaces. This can be attributed to a combination of delamination and abrasion wear mechanism [22]. Fig 7b and 7c shows large irregular shaped debris particles. The cutting action of the hard asperities of the counterface creates such kind of debris. The large sized debris is probably due to the presence of deep grooves in the worn surface. Presence of large grooves confirms delamination wear mechanism [26]. Fig 7d shows fine loose particles of wear debris. The image further shows reduced mean size of particles for an addition of 9% TiO2 compared to other composite combinations. The addition of Nano TiO2 particles in the composite minimizes the plastic deformation due to increase in dislocation density. Hence the hardness of the composite is increased and reduces the particle size. The amount of hard TiO2 phases dictates the size and morphology of wear debris [7].
4. Conclusions ·
AA7068/TiO2 metal matrix composite was successfully fabricated using powder metallurgy technique with improved hardness and better bonding.
·
Optical microscopic images, represented a uniform dispersion of reinforcement particles in the aluminum matrix.
·
The microhardness of the composites increased with increase in TiO2 particles and was found to be having a maximum of 68 VHN for an addition of 9% Nano TiO2 particles.
·
Abrasion, oxidation and delamination were found to be the dominant wear mechanisms composites.
in the
Acknowledgment
The authors would like to thank Mr. J. Deva Manoharan, Centre for Research in Metallurgy, Karunya Institute of Technology and Sciences for providing his valuable guidance and support.
References [1] E. Kim G. Cho, J. Lee, Y. Jung Y. Yoo, S. Seo, Fabrication and mechanical properties of metal matrix composite with homogeneously dispersed ceramic particles, Ceram. Int. 39 (2013) 6503–6508. [2] S.K. Patel, B. Kuriachen, N. Kumar and R. Nateriya, The slurry abrasive wear behavior and microstructural analysis of A2024-SiC-ZrSiO4 metal matrix composite, Ceram. Int. 44(6) (2018) 64266432. [3] A. Azimi, A. Shokuhfar, O. Nejadseyfi O, H. Fallahdoost and S. Salehi, Optimizing consolidation behaviour of Al 7068–TiC nanocomposites using Taguchi statistical analysis Trans. Nonferrous. Met. Soc. China. 25 (2015) 2499−2508. [4] M. Madhusudhan, G.J. Naveen and K. Mahesha, Mechanical Characterization of AA7068-ZrO2 reinforced Metal Matrix Composites, Mater Today: Proceedings. 4 (2017) 3122–3130. [5] A.J. Haider, R.H. Al-Anbari, G.R. Kadhim and C.T. Salame, Exploring potential environmental applications of TiO2 nanoparticles, Energy Procedia. 119 (2017) 332-345. [6] U.A.O. Arıer and F.Z. Tepehan, Influence of Al2O3:TiO2 ratio on the structural and optical properties of TiO2–Al2O3 nano-composite films produced by sol gel method, Compos. Part B. 58 (2014) 147–151. [7] V.K.C. Antony and J. Selwinrajadurai, Influence of rutile (TiO2) content on wear and microhardness characteristics of aluminium-based hybrid composites synthesized by powder metallurgy, Trans. Nonferrous. Met. Soc. China. 26 (2016) 63−73. [8] M.J. Ghazali, S.M. Forghani, N. Hassanuddin, A. Muchtar and R. Daud, Comparative wear study of plasma sprayed TiO2 and Al2O3–TiO2 on mild steels, Tribol. Int. 93 (2016) 681–686. [9] J. Moradgholi, A. Monshi, K. Farmanesh, M.R. Toroghinejad, M.R. Loghman-Estarki, Comparison of microstructure, toughness, mechanical properties and work hardening of titanium/TiO2 and
titanium/SiC composites manufactured by accumulative roll bonding (ARB) process, Ceram. Int. 43(10) (2017) 7701-7709. [10] P. Baghery, M. Farzam, A.B. Mousavi and M. Hosseini, Ni–TiO2 nanocomposite coating with high resistance to corrosion and wear, Surface & Coatings Technol. 204 (2010) 3804–3810. [11] C.S. Ramesh, A.R. Anwar Khan, N. Ravikumar and P. Savanprabhu, Prediction of wear coefficient of Al6061–TiO2 composites, Wear. 259 (2005) 602–608. [12] P. Makkar, R.C. Agarwala and V. Agarwala, Wear characteristics of mechanically milled TiO2 nanoparticles incorporated in electroless Ni–P coatings, Adv. Powder Technol. 25 (2014) 1653–60. [13] G. Elango and B.K. Raghunath, Tribological Behavior of Hybrid (LM25Al + SiC+ TiO2) Metal Matrix Composites, Proc. Eng. 64 (2013) 671–680. [14] S. Yizhou, T. Haijun, L. Yuebin, Z. Xiaofei, W. Tao, T. Jie and P. Lei, Fabrication and Wear Resistance of TiO2/Al2O3 Coatings by Micro-arc Oxidation, Rare Metal Mat. Eng. 46 (2017) 23-27. [15] H. Luo, P. Song, A. Khan, J. Feng and J.S. Lu, Alternant phase distribution and wear mechanical properties of an Al2O3-40 wt%TiO2 composite coating, Ceram. Int. 43(9) (2017) 7295-7304. [16] R. Younes, M.A. Bradai, A. Sadeddine, Y. Mouadji, A. Bilek and A. Benabbas, Effect of TiO2 and ZrO2 reinforcements on properties of Al2O3 coatings fabricated by thermal flame spraying, Trans. Nonferrous. Met. Soc. China. 26(5) (2016) 1345-1352. [17] H. Abdizadeh, R. Ebrahimifard and M.A. Baghchesara, Investigation of microstructure and mechanical properties of Nano MgO reinforced Al composites manufactured by stir casting and powder metallurgy methods: A comparative study, Compos. Part B. 56 (2014) 217–221. [18] T. Varol, A. Canakci and S. Ozsahin, Artificial neural network modeling to effect of reinforcement properties on the physical and mechanical properties of Al2024–B4C composites produced by powder metallurgy, Compos. Part B. 54 (2013) 224–233. [19] P.C. Angelo and R. Subramanian, Powder Metallurgy: Science, Technology and Application, First ed. 2008, Prentice Hall: India. [20] J. Corrochano, M. Lieblich and J. Ibanez, The effect of ball milling on the microstructure of powder metallurgy aluminium matrix composites reinforced with MoSi2 intermetallic particles, Compos. Part A. 42 (2011) 1093–1099.
[21] Shabani, M. Ostad and A. Mazahery A, Prediction of wear properties in A356 matrix composite reinforced with B4C particulates, Synthet. Metal. 161(13) (2011) 1226-1231. [22] A. Alizadeh and E.T. Nassaj, Wear Behavior of Nanostructured Al and Al–B4C Nanocomposites Produced by Mechanical Milling and Hot Extrusion, Tribol. Lett. 44 (2011) 59–66. [23] Y. Yao, L. Jiang, G. Fu and L. Chen, Wear behavior and mechanism of B4C reinforced Mg-matrix composites fabricated by metal-assisted pressureless infiltration technique, Trans. Nonferrous. Met. Soc. China. 25 (2015) 2543−2548. [24] B.A. Kumar, N. Murugan and I. Dinaharan, Dry sliding wear behavior of stir cast AA6061-T6/AlNp composite, Trans. Nonferrous. Met. Soc. China. 24 (2014) 2785−2795. [25] M. Uthayakumar, S. Aravindan and K. Rajkumar, Wear performance of Al–SiC–B4C hybrid composites under dry sliding conditions, Mater. Des. 47 (2013) 456–464. [26] A. Kurs¸ E. Bayraktar and M.H. Enginsoy, Experimental and numerical study of alumina reinforced aluminum matrix composites: Processing, microstructural aspects and properties, Compos. Part B. 90 (2016) 302-314. [27] H. Izadi, A. Nolting, C. Munro, D.B. Bishop, K.P. Plucknett and A.P. Gerlich, Friction stir processing of Al/SiC composites fabricated by powder metallurgy, J. Mater. Process. Technol. 213 (2013) 1900– 1907. [28] S.S. Mirjavadi, M. Alipour, S. Emamian, S. Kord, A.M.S. Hamouda, P.G. Koppad, R. Keshavamurthy, Influence of TiO2 nanoparticles incorporation to friction stir welded 5083 aluminum alloy on the microstructure, mechanical properties and wear resistance, J. Alloys. Comp. 712 (2017) 795-803. [29] P.A. Rometsch, Y. Zhang and S. Knight, Heat treatment of 7xxx series aluminium alloys—Some recent developments, Trans. Nonferrous Met. Soc. China. 24 (2014) 2003−2017. [30] M. Dixit, R.S. Mishra and K.K. Sankaran, Structure–property correlations in Al 7050 and Al 7055 highstrength aluminum alloys, Mater. Sci. Eng A. 478 (2008) 163–172. [31] A.A. Hamida, P.K. Ghosh, S.C. Jain, and S. Ray, The influence of porosity and particles content on dry sliding wear of cast in situ Al(Ti)–Al2O3(TiO2) composite, Wear. 265 (2008) 14–26.
FIGURE CAPTIONS Figure No.
Figure Captions
1
FESEM image of powders as received a) AA7068 after milling b) Nano TiO2
2
Compacted samples of AA7068 reinforced with Nano TiO2
3
4 5
Optical Micrographs of the processed samples (a) As sintered AA7068 (b) AA7068-3%TiO2 (c) AA7068-6%TiO2 and (d) AA7068-9%TiO2 EDS patterns of worn surface (a) As sintered AA7068 (b) AA7068-3%TiO2 (c) AA70686%TiO2 and (d) AA7068-9%TiO2 Vickers microhardness of AA7068 with increase in TiO2 Reinforcement FESEM micrograph of the worn surface (a) As sintered AA7068 (c) AA7068-3%TiO2 (e)
6
AA7068-6%TiO2 and (g) AA7068-9%TiO2 at lower magnification and (b) As sintered AA7068 (d) AA7068-3% TiO2 (f) AA7068-6% TiO2 and (h) AA7068-9% TiO2 at higher magnification respectively
7
FESEM images of wear debris (a) As sintered AA7068 (b) AA7068-3%TiO2 (c) AA70686%TiO2 and (d) AA7068-9%TiO2
TABLE CAPTIONS Table No.
Table Captions
1
Composition of AA7068 (% by weight)
2
Average particle size
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7