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Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique
Materials Today: Proceedings xxx (xxxx) xxx
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Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique A.M. Okoro a,⇑, S.S. Lephuthing a, M.A. Awotunde a, O.E. Falodun a, R. Machaka b, P.A. Olubambi a a b
Centre for Nanoengineering and Tribocorrosion, Department of Metallurgy, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, South Africa Council for Scientific and Industrial Research, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 15 September 2019 Received in revised form 5 November 2019 Accepted 26 December 2019 Available online xxxx Keywords: Nanomechanical properties Nanoindentation Titanium alloy Multiwall carbon nanotubes Spark plasma sintering
a b s t r a c t In this study, nanoindentation tests were carried out to investigate the nanomechanical behaviours of Ti6Al4V and its nanocomposites comprising of multiwall carbon nanotubes (MWCNTs) fabricated via spark plasma sintering. Prior to the nanoindentation tests, the fabricated alloy and the nanocomposites comprising of 0.5 and 1.0 wt% of MWCNTs were observed under a scanning electron microscope (SEM) to reveal the morphology and the presence of the MWCNTs in the nanocomposites. The nanoindentation study was conducted using a Berkovich diamond indenter with a load of 75 mN to ascertain the nanohardness, reduced elastic modulus, plasticity index and elastic recovery index of the sintered materials. The SEM results revealed the dispersibility of the nanotubes in the titanium-based matrix and the presence of lamellar structures in the fabricated alloy. Furthermore, it was observed that the nanohardness and elastic modulus of the sintered materials ranges from 3.4–5.7 GPa and 27.6–38.9 GPa, which improved with the MWCNTs content. Also, the elastic recovery index increased with MWCNTs content in the nanocomposites. While the sintered Ti6Al4V displayed the maximum plasticity during the nanoindentation study. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
1. Introduction Recent progress in materials science has unveiled the possibilities of assessing the mechanical behaviours of various materials at a micro and nanoscale range. These possibilities have been actualized by the application of an innovative mechanical testing technique called nanoindentation. Although, nanoindentation technique has been in existence since the mid-1970s for the testing of a small volume of materials [1]. However, the research breakthrough by Oliver and Pharr method [2] using the Berkovich nanoindenter in investigating the hardness and elastic modulus of six distinct materials have given the technique unprecedented attention in recent years. This mechanical testing method has been successfully employed to ⇑ Corresponding author. E-mail address: [email protected] (A.M. Okoro).
explore the mechanical behaviours of a wide range of materials namely ceramics, composites, alloys, coated surfaces, and thinfilm to mention but a few [3]. The widespread applications of this technique are traceable to its effectiveness, and its capability to probe small materials without hampering with their microstructures [4]. Additionally, it can probe the grains and grain boundaries by assessing the mechanical properties of those regions of the materials. In the past years, different research adventures have been carried out using destructive testing techniques to assess the mechanical properties such as tensile, hardness and fracture toughness of a wide range of materials. However, it usually leads to the destruction of the microstructure without actualizing results of higher accuracy. Hence, in a bid to understand the effects of multiwall carbon nanotubes (MWCNTs) addition on the nanomechanical properties of Ti6Al4V matrix without hampering the microstructure of the
https://doi.org/10.1016/j.matpr.2019.12.285 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
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A.M. Okoro et al. / Materials Today: Proceedings xxx (xxxx) xxx
nanocomposites, nanoindentation method was used in this study. Being cognizant of the reinforcing potentials of MWCNTs in enhancing the mechanical properties of numerous metal matrix composites [5,6], it is worthwhile to assess the nanomechanical behaviours of MWCNTs reinforced Ti6Al4V nanocomposites, especially at the micro and nanoscale range. Therefore, in this study, MWCNTs was incorporated into Ti6Al4V matrix via shift speed ball milling (SSBM) and fabricated using the spark plasma sintering (SPS) technique. The nanomechanical behaviours of the sintered alloy and nanocomposites were investigated via the nanoindentation technique without destroying the microstructure of the sintered materials.
2. Materials and method 2.1. Dispersing MWCNTs in Ti6Al4V matrix and consolidation of the mixed powders The MWCNTs acquired from Nanocyl Belgium with 99.7%, 9.5 nm thickness and 1.5 mm long was incorporated into prealloyed Ti6Al4V matrix. The titanium alloy used as the matrix material was supplied by TLS Technik GmbH & Co. Germany and it has an average particle size of 25 mm. The homogeneity of the powders was achieved by incorporating 0.5 and 1.0 wt% of MWCNTs into the Ti6Al4V matrix using the SSBM technique employed by Okoro et al., [7] to disperse MWCNTs in Ti6Al4V powders. Furthermore, the admixed powders were cold compacted under a force of 10 KN and then sintered via SPS with the following sintering parameters; the temperature of 1000 °C, the pressure of 50 MPa, heating rate, 100 °C/min and holding time of 10 min. Immediately after the consolidation process, the samples were sandblasted to remove impurities from the surface and metallographic preparations were conducted for easy microstructural analysis. 2.2. Microstructural analysis of the fabricated materials The metallographic preparations were carried out using the advanced laboratory solutions of metallographic procedure for titanium alloys to achieve samples of a flat and shiny surface. Afterwards, microscopic characterization was conducted under a field emission scanning electron microscope (Carl Zeiss Sigma) to disclose the surface morphology of the fabricated materials and the assess the presence and dispersibility of MWCNTs in the Ti6Al4V matrix. 2.3. Nanoindentation study of the fabricated materials In a bid to access the nanomechanical behaviours of the fabricated materials, nanoindentation method was employed by using ultra nanoindenter Switzerland which employs the Berkovich diamond indenter to assess the nanomechanical properties of the consolidated materials. Furthermore, this method was employed to assess the load-depth curve, nanohardness, reduced elastic modulus, plasticity index, elastic recovery index of the fabricated materials. In order to obtain accurate test results, the equipment was calibrated with a reference fused silica sample. During the nanoindentation, 75 mN load was applied on the surface of prepared samples at a loading rate of 10 mN/s in accordance with ISO 14577 [8]. Meanwhile, over 15 indentations were made on various points on the samples to achieve results of high accuracy and a pulse time of 1000 s was used before unloading the samples and the data was acquired and plotted. The image of the samples used for the nanoindentation test is shown in Fig. 1(a–c).
Additionally, the nanohardness and reduced elastic modulus of the sintered materials were evaluated from the loading and unloading of the samples using Oliver and Pharr methods which are presented in Eqs. (1) and (2);
Hn ¼
Pm Ac
1 1 v 2s 1 v 2i ¼ þ Er Es Ei
ð1Þ
ð2Þ
From Eq. (1), Hn represents the nanohardness at the maximum load Pm while Ac ; is the contact area of the Berkovich indenter on the sample. Similarly, Er, Es, Ei in Eq. (2) represents the reduced elastic modulus, the elastic modulus of the sample, and elastic modulus of the Berkovich indenter respectively [9]. While vi and vs are the Poisson ratio of the sample and indenter. 3. Results and discussion 3.1. SEM analysis of the fabricated materials The dispersibility of the MWCNTs in the sintered nanocomposites and surface morphology of the fabricated alloy was characterized under SEM and the results are shown in Fig. 2. The surface morphology of the fabricated Ti6Al4V is shown in Fig. 2(a), which reveals the alpha lamellar structures of the titanium alloy. It was noticed that no evidence of pores or void were seen on the sintered alloy which affirmed the effectiveness of the sintering technique. Meanwhile, the formed lamellar structures on the SEM micrograph of the sintered alloy could be attributed to the sluggish cooling process employed after heating the material above the alpha-beta transformation temperature. Correspondingly, the micrograph in Fig. 2(b) reveals the surface morphology of the fabricated nanocomposites containing 0.5 wt% of MWCNTs, where the presence of nanotubes was observed across the phases in the Ti6Al4V matrix. A similar SEM micrograph was observed in Fig. 2(c), which reveals the surface morphology of the fabricated nanocomposite grade containing 1.0 wt% of MWCNTs. 3.2. Nanomechanical behaviours of the fabricated materials The nanomechanical behaviours evaluated from the nanoindentation studies are nanohardness, reduced elastic modulus, elastic recovery index and plasticity index. 3.2.1. Load-depth and displacement-time plot of the fabricated materials The load-depth and displacement-time curve of the fabricated materials are presented in Fig. 3. These curves help to evaluate the deformation behaviours of the materials under nanoindentation loading and unloading. The load-displacement curve of the fabricated materials is presented in Fig. 3(a), which depict the behaviour of the fabricated materials under nanoindentation. From the load-displacement curve of the sintered titanium alloy, it was observed that the alloy undergoes significant plastic deformation. Similarly, this was accompanied by the load-displacement curve of the fabricated 0.5 wt% MWCNTs nanocomposite grade where a lesser plastic deformation behaviour was seen. However, the fabricated nanocomposite grade with the highest amount of reinforcement (1.0 wt% MWCNTs) showed the maximum resistance to plastic deformation. From all indications, the incorporation of MWCNTs into the Ti6Al4V matrix poses some significant resistance to dislocation motion which was observed in the nanocomposite grades. This further indicates that the sintered nanocomposites
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
A.M. Okoro et al. / Materials Today: Proceedings xxx (xxxx) xxx
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Fig. 1. The image of the fabricated and mounted Pristine Ti6Al4V (a), 0.5 wt% MWCNT reinforced Ti6Al4V (b) and 1.0 wt% MWCNT reinforced Ti6Al4V.
Fig. 2. SEM micrograph of fabricated Ti6Al4V (a), nanocomposites containing 0.5 wt% of MWCNTS (b) and 1.0 wt% of MWCNTs (b).
Fig. 3. Load-depth (a) and displacement-time (b) curve of the fabricated Ti6Al4V and MWCNTs-Ti6Al4V nanocomposites.
exhibit some stiffening behaviour resulting from the addition of MWCNTs in the titanium-based matrix. Furthermore, the depth-time curve shown in Fig. 3(b) assist in affirming the extent of plastic deformation the sintered materials undergo during the nanoindentation test. From the depth-time curve of the sintered Ti6Al4V shown in Fig. 3(b), which depicts the displacement in relation with time during the nanoindentation, it was seen that the Ti6Al4V experienced the highest displacement with a depth of 1446 nm. This was followed by the nanocomposite grade containing 0.5 wt% of MWCNTs, which had a depth of 1301 nm. Additionally, the 1.0 wt% MWCNTs reinforced titanium alloy grade showed the least displacement during the nanoindentation with a depth of 1249 nm. These results implied that the nanocomposites grade showed significant hindrance to the indentation load during the test. Also, the resistance to the indentation load increased with an increase in MWCNTs content in the titanium alloy matrix which supports the fact that the incorporation of nanotubes into the titanium alloy strengthen the resulting material [10].
Fig. 4. Nanohardness and reduced elastic modulus of the fabricated materials.
3.2.2. Nanohardness and reduced elastic modulus of the fabricated material The chart in Fig. 4 shows the nanohardness and reduced elastic modulus of the fabricated materials. From the nanohardness results, it was seen that the nanohardness improved with the
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
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alloy strengthened the nanocomposites. Although, the plasticity index declined with the addition of nanotubes into the nanocomposite system. 4. Conclusion
Fig. 5. Plasticity index and Elastic recovery index of the fabricated materials.
The nanomechanical behaviours of fabricated Ti6Al4V and its nanocomposites containing MWCNTs were assessed by nanoindentation test. The results from the investigations revealed the significant improvement of the elastic recovery index, reduced elastic modulus and nanohardness by the incorporation of MWCNTs into the nanocomposite system. However, the sintered alloy experienced the least hindrance to dislocation movements during the test. Declaration of Competing Interest
incorporation of MWCNTs into the Ti6Al4V matrix. The fabricated material comprising of 1.0 wt% MWCNTs displayed the maximum nanohardness of 5.7 GPa, which amounts to 67.6% increased, this was followed by the fabricated material containing 0.5 wt% MWCNTs with a nanohardness of 4.0 GPa that correspond to 17.6% improvement. However, the sintered titanium alloy exhibited the least nanohardness of 3.4 GPa. These results further implied that the incorporation of MWCNTs into the Ti6Al4V matrix augmented the nanomechanical properties of the fabricated materials by posing hindrances to the dislocation movement during the test. The improvement in nanohardness could be attributed to the dispersion strengthening and the pinning effect by the nanotubes in the Ti6Al4V matrix [11,12]. Additionally, the stiffness of the fabricated materials was accessed by evaluating the reduced elastic modulus and the results are shown in Fig. 4, it was seen that the elastic modulus was enhanced by the incorporation of MWCNTs into the Ti6Al4V matrix. The fabricated material containing 1.0 wt% of MWCNTs showed the best reduced elastic modulus of 38.9 GP, which amounts to 40.9% increased. This was accompanied by 0.5 wt% MWCNTs reinforced Ti6Al4V matrix with an elastic modulus of 29.6 GPa, which corresponds to 7.2% increased. However, the sintered titanium alloy exhibited the lowest reduced elastic modulus of 27.6 GPa. These results also indicated that the incorporation of nanotubes into the Ti6Al4V matrix stiffened the fabricated materials by inhibiting the dislocation movement during the test [13].
3.2.3. Elastic recovery and plasticity index of the fabricated materials The elastic recovery and plasticity index were used to ascertain the nanomechanical behaviours of materials under nanoindentation load. The elastic recovery index provides the details about the energy liberated from the material under mechanical loading [14], while the plasticity index gives information about the intrinsic plastic behaviour of materials [15]. Furthermore, the elastic recovery and plasticity index of the fabricated materials are shown in Fig. 5. From the chart in Fig. 5, it was noticed that the elastic recovery index of fabricated alloy and nanocomposites comprising of MWCNTs improved significantly from 7.3% to 57.4%, with the increased of weight percent of the MWCNTs from 0.5 to 1.0 respectively. While the plasticity index declined from 25.8% to 46.6% with the increased in weight fractions of MWCNTs in the titanium alloy. Also, it was noticed that the fabricated Ti6Al4V displayed the highest plasticity index which implies that the alloy experienced the least hindrances to plastic deformation owing to its higher ductility when compared with the fabricated nanocomposites. From all indications, the nanomechanical properties tested improved with the additions of MWCNTs in the Ti6Al4V matrix, which further revealed that the incorporation of the nanotubes into the titanium
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was funded by the National Research Foundation of South Africa in partnership with the World Academy of Science (NRF-TWAS) and the Global Excellence and Stature of the University of Johannesburg, South Africa. References [1] B. Poon, D. Rittel, G. Ravichandran, An analysis of nanoindentation in linearly elastic solids, Int. J. Solids Struct. 45 (2008) 6018–6033. [2] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [3] K.D. Sattler, Handbook of Nanophysics: Functional Nanomaterials, CRC Press, 2010. [4] H. Attar, S. Ehtemam-Haghighi, D. Kent, I.V. Okulov, H. Wendrock, M. Bӧnisch, A.S. Volegov, M. Calin, J. Eckert, M.S. Dargusch, Nanoindentation and wear properties of Ti and Ti-TiB composite materials produced by selective laser melting, Mater. Sci. Eng. A 688 (2017) 20–26. [5] K.S. Munir, Y. Zheng, D. Zhang, J. Lin, Y. Li, C. Wen, Improving the strengthening efficiency of carbon nanotubes in titanium metal matrix composites, Mater. Sci. Eng. A 696 (2017) 10–25. [6] A.M. Okoro, S.S. Lephuthing, S.R. Oke, O.E. Falodun, M.A. Awotunde, P.A. Olubambi, A review of spark plasma sintering of carbon nanotubes reinforced titanium-based nanocomposites: fabrication, densification, and mechanical properties, JOM (2018). [7] A.M. Okoro, R. Machaka, S.S. Lephuthing, M.A. Awotunde, S.R. Oke, O.E. Falodun, P.A. Olubambi, Dispersion characteristics, interfacial bonding and nanostructural evolution of MWCNTS in Ti6Al4V powders prepared by shift speed ball milling technique, J. Alloys Compd. (2019). [8] J. Nohava, N.X. Randall, N. Conté, Novel ultra nanoindentation method with extremely low thermal drift: principle and experimental results, J. Mater. Res. 24 (2009) 873–882. [9] B.B. Medeiros, M.M. Medeiros, J. Fornell, J. Sort, M.D. Baró, A.M.J. Junior, Nanoindentation response of Cu–Ti based metallic glasses: comparison between as-cast, relaxed and devitrified states, J. Non. Cryst. Solids 425 (2015) 103–109. [10] D. Lin, M. Saei, S. Suslov, S. Jin, G.J. Cheng, Super-strengthening and stabilizing with carbon nanotube harnessed high density nanotwins in metals by shock loading, Sci. Rep. 5 (2015) 15405. [11] F. Mokdad, D.L. Chen, Z.Y. Liu, B.L. Xiao, D.R. Ni, Z.Y. Ma, Deformation and strengthening mechanisms of a carbon nanotube reinforced aluminum composite, Carbon N. Y. 104 (2016) 64–77. [12] S. Li, B. Sun, H. Imai, T. Mimoto, K. Kondoh, Powder metallurgy titanium metal matrix composites reinforced with carbon nanotubes and graphite, Compos. Part A Appl. Sci. Manuf. 48 (2013) 57–66. [13] M. Rashad, F. Pan, J. Zhang, M. Asif, Use of high energy ball milling to study the role of graphene nanoplatelets and carbon nanotubes reinforced magnesium alloy, J. Alloys Compd. 646 (2015) 223–232. [14] J. Musil, F. Kunc, H. Zeman, H. Polakova, Relationships between hardness, Young’s modulus and elastic recovery in hard nanocomposite coatings, Surf. Coat. Technol. 154 (2002) 304–313. [15] E. Zhang, H. Chen, F. Shen, Biocorrosion properties and blood and cell compatibility of pure iron as a biodegradable biomaterial, J. Mater. Sci. Mater. Med. 21 (2010) 2151–2163.
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique A.M. Okoro a,⇑, S.S. Lephuthing a, M.A. Awotunde a, O.E. Falodun a, R. Machaka b, P.A. Olubambi a a b
Centre for Nanoengineering and Tribocorrosion, Department of Metallurgy, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, South Africa Council for Scientific and Industrial Research, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 15 September 2019 Received in revised form 5 November 2019 Accepted 26 December 2019 Available online xxxx Keywords: Nanomechanical properties Nanoindentation Titanium alloy Multiwall carbon nanotubes Spark plasma sintering
a b s t r a c t In this study, nanoindentation tests were carried out to investigate the nanomechanical behaviours of Ti6Al4V and its nanocomposites comprising of multiwall carbon nanotubes (MWCNTs) fabricated via spark plasma sintering. Prior to the nanoindentation tests, the fabricated alloy and the nanocomposites comprising of 0.5 and 1.0 wt% of MWCNTs were observed under a scanning electron microscope (SEM) to reveal the morphology and the presence of the MWCNTs in the nanocomposites. The nanoindentation study was conducted using a Berkovich diamond indenter with a load of 75 mN to ascertain the nanohardness, reduced elastic modulus, plasticity index and elastic recovery index of the sintered materials. The SEM results revealed the dispersibility of the nanotubes in the titanium-based matrix and the presence of lamellar structures in the fabricated alloy. Furthermore, it was observed that the nanohardness and elastic modulus of the sintered materials ranges from 3.4–5.7 GPa and 27.6–38.9 GPa, which improved with the MWCNTs content. Also, the elastic recovery index increased with MWCNTs content in the nanocomposites. While the sintered Ti6Al4V displayed the maximum plasticity during the nanoindentation study. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
1. Introduction Recent progress in materials science has unveiled the possibilities of assessing the mechanical behaviours of various materials at a micro and nanoscale range. These possibilities have been actualized by the application of an innovative mechanical testing technique called nanoindentation. Although, nanoindentation technique has been in existence since the mid-1970s for the testing of a small volume of materials [1]. However, the research breakthrough by Oliver and Pharr method [2] using the Berkovich nanoindenter in investigating the hardness and elastic modulus of six distinct materials have given the technique unprecedented attention in recent years. This mechanical testing method has been successfully employed to ⇑ Corresponding author. E-mail address: [email protected] (A.M. Okoro).
explore the mechanical behaviours of a wide range of materials namely ceramics, composites, alloys, coated surfaces, and thinfilm to mention but a few [3]. The widespread applications of this technique are traceable to its effectiveness, and its capability to probe small materials without hampering with their microstructures [4]. Additionally, it can probe the grains and grain boundaries by assessing the mechanical properties of those regions of the materials. In the past years, different research adventures have been carried out using destructive testing techniques to assess the mechanical properties such as tensile, hardness and fracture toughness of a wide range of materials. However, it usually leads to the destruction of the microstructure without actualizing results of higher accuracy. Hence, in a bid to understand the effects of multiwall carbon nanotubes (MWCNTs) addition on the nanomechanical properties of Ti6Al4V matrix without hampering the microstructure of the
https://doi.org/10.1016/j.matpr.2019.12.285 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 2nd International Conference on Recent Advances in Materials & Manufacturing Technologies.
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
2
A.M. Okoro et al. / Materials Today: Proceedings xxx (xxxx) xxx
nanocomposites, nanoindentation method was used in this study. Being cognizant of the reinforcing potentials of MWCNTs in enhancing the mechanical properties of numerous metal matrix composites [5,6], it is worthwhile to assess the nanomechanical behaviours of MWCNTs reinforced Ti6Al4V nanocomposites, especially at the micro and nanoscale range. Therefore, in this study, MWCNTs was incorporated into Ti6Al4V matrix via shift speed ball milling (SSBM) and fabricated using the spark plasma sintering (SPS) technique. The nanomechanical behaviours of the sintered alloy and nanocomposites were investigated via the nanoindentation technique without destroying the microstructure of the sintered materials.
2. Materials and method 2.1. Dispersing MWCNTs in Ti6Al4V matrix and consolidation of the mixed powders The MWCNTs acquired from Nanocyl Belgium with 99.7%, 9.5 nm thickness and 1.5 mm long was incorporated into prealloyed Ti6Al4V matrix. The titanium alloy used as the matrix material was supplied by TLS Technik GmbH & Co. Germany and it has an average particle size of 25 mm. The homogeneity of the powders was achieved by incorporating 0.5 and 1.0 wt% of MWCNTs into the Ti6Al4V matrix using the SSBM technique employed by Okoro et al., [7] to disperse MWCNTs in Ti6Al4V powders. Furthermore, the admixed powders were cold compacted under a force of 10 KN and then sintered via SPS with the following sintering parameters; the temperature of 1000 °C, the pressure of 50 MPa, heating rate, 100 °C/min and holding time of 10 min. Immediately after the consolidation process, the samples were sandblasted to remove impurities from the surface and metallographic preparations were conducted for easy microstructural analysis. 2.2. Microstructural analysis of the fabricated materials The metallographic preparations were carried out using the advanced laboratory solutions of metallographic procedure for titanium alloys to achieve samples of a flat and shiny surface. Afterwards, microscopic characterization was conducted under a field emission scanning electron microscope (Carl Zeiss Sigma) to disclose the surface morphology of the fabricated materials and the assess the presence and dispersibility of MWCNTs in the Ti6Al4V matrix. 2.3. Nanoindentation study of the fabricated materials In a bid to access the nanomechanical behaviours of the fabricated materials, nanoindentation method was employed by using ultra nanoindenter Switzerland which employs the Berkovich diamond indenter to assess the nanomechanical properties of the consolidated materials. Furthermore, this method was employed to assess the load-depth curve, nanohardness, reduced elastic modulus, plasticity index, elastic recovery index of the fabricated materials. In order to obtain accurate test results, the equipment was calibrated with a reference fused silica sample. During the nanoindentation, 75 mN load was applied on the surface of prepared samples at a loading rate of 10 mN/s in accordance with ISO 14577 [8]. Meanwhile, over 15 indentations were made on various points on the samples to achieve results of high accuracy and a pulse time of 1000 s was used before unloading the samples and the data was acquired and plotted. The image of the samples used for the nanoindentation test is shown in Fig. 1(a–c).
Additionally, the nanohardness and reduced elastic modulus of the sintered materials were evaluated from the loading and unloading of the samples using Oliver and Pharr methods which are presented in Eqs. (1) and (2);
Hn ¼
Pm Ac
1 1 v 2s 1 v 2i ¼ þ Er Es Ei
ð1Þ
ð2Þ
From Eq. (1), Hn represents the nanohardness at the maximum load Pm while Ac ; is the contact area of the Berkovich indenter on the sample. Similarly, Er, Es, Ei in Eq. (2) represents the reduced elastic modulus, the elastic modulus of the sample, and elastic modulus of the Berkovich indenter respectively [9]. While vi and vs are the Poisson ratio of the sample and indenter. 3. Results and discussion 3.1. SEM analysis of the fabricated materials The dispersibility of the MWCNTs in the sintered nanocomposites and surface morphology of the fabricated alloy was characterized under SEM and the results are shown in Fig. 2. The surface morphology of the fabricated Ti6Al4V is shown in Fig. 2(a), which reveals the alpha lamellar structures of the titanium alloy. It was noticed that no evidence of pores or void were seen on the sintered alloy which affirmed the effectiveness of the sintering technique. Meanwhile, the formed lamellar structures on the SEM micrograph of the sintered alloy could be attributed to the sluggish cooling process employed after heating the material above the alpha-beta transformation temperature. Correspondingly, the micrograph in Fig. 2(b) reveals the surface morphology of the fabricated nanocomposites containing 0.5 wt% of MWCNTs, where the presence of nanotubes was observed across the phases in the Ti6Al4V matrix. A similar SEM micrograph was observed in Fig. 2(c), which reveals the surface morphology of the fabricated nanocomposite grade containing 1.0 wt% of MWCNTs. 3.2. Nanomechanical behaviours of the fabricated materials The nanomechanical behaviours evaluated from the nanoindentation studies are nanohardness, reduced elastic modulus, elastic recovery index and plasticity index. 3.2.1. Load-depth and displacement-time plot of the fabricated materials The load-depth and displacement-time curve of the fabricated materials are presented in Fig. 3. These curves help to evaluate the deformation behaviours of the materials under nanoindentation loading and unloading. The load-displacement curve of the fabricated materials is presented in Fig. 3(a), which depict the behaviour of the fabricated materials under nanoindentation. From the load-displacement curve of the sintered titanium alloy, it was observed that the alloy undergoes significant plastic deformation. Similarly, this was accompanied by the load-displacement curve of the fabricated 0.5 wt% MWCNTs nanocomposite grade where a lesser plastic deformation behaviour was seen. However, the fabricated nanocomposite grade with the highest amount of reinforcement (1.0 wt% MWCNTs) showed the maximum resistance to plastic deformation. From all indications, the incorporation of MWCNTs into the Ti6Al4V matrix poses some significant resistance to dislocation motion which was observed in the nanocomposite grades. This further indicates that the sintered nanocomposites
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
A.M. Okoro et al. / Materials Today: Proceedings xxx (xxxx) xxx
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Fig. 1. The image of the fabricated and mounted Pristine Ti6Al4V (a), 0.5 wt% MWCNT reinforced Ti6Al4V (b) and 1.0 wt% MWCNT reinforced Ti6Al4V.
Fig. 2. SEM micrograph of fabricated Ti6Al4V (a), nanocomposites containing 0.5 wt% of MWCNTS (b) and 1.0 wt% of MWCNTs (b).
Fig. 3. Load-depth (a) and displacement-time (b) curve of the fabricated Ti6Al4V and MWCNTs-Ti6Al4V nanocomposites.
exhibit some stiffening behaviour resulting from the addition of MWCNTs in the titanium-based matrix. Furthermore, the depth-time curve shown in Fig. 3(b) assist in affirming the extent of plastic deformation the sintered materials undergo during the nanoindentation test. From the depth-time curve of the sintered Ti6Al4V shown in Fig. 3(b), which depicts the displacement in relation with time during the nanoindentation, it was seen that the Ti6Al4V experienced the highest displacement with a depth of 1446 nm. This was followed by the nanocomposite grade containing 0.5 wt% of MWCNTs, which had a depth of 1301 nm. Additionally, the 1.0 wt% MWCNTs reinforced titanium alloy grade showed the least displacement during the nanoindentation with a depth of 1249 nm. These results implied that the nanocomposites grade showed significant hindrance to the indentation load during the test. Also, the resistance to the indentation load increased with an increase in MWCNTs content in the titanium alloy matrix which supports the fact that the incorporation of nanotubes into the titanium alloy strengthen the resulting material [10].
Fig. 4. Nanohardness and reduced elastic modulus of the fabricated materials.
3.2.2. Nanohardness and reduced elastic modulus of the fabricated material The chart in Fig. 4 shows the nanohardness and reduced elastic modulus of the fabricated materials. From the nanohardness results, it was seen that the nanohardness improved with the
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285
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alloy strengthened the nanocomposites. Although, the plasticity index declined with the addition of nanotubes into the nanocomposite system. 4. Conclusion
Fig. 5. Plasticity index and Elastic recovery index of the fabricated materials.
The nanomechanical behaviours of fabricated Ti6Al4V and its nanocomposites containing MWCNTs were assessed by nanoindentation test. The results from the investigations revealed the significant improvement of the elastic recovery index, reduced elastic modulus and nanohardness by the incorporation of MWCNTs into the nanocomposite system. However, the sintered alloy experienced the least hindrance to dislocation movements during the test. Declaration of Competing Interest
incorporation of MWCNTs into the Ti6Al4V matrix. The fabricated material comprising of 1.0 wt% MWCNTs displayed the maximum nanohardness of 5.7 GPa, which amounts to 67.6% increased, this was followed by the fabricated material containing 0.5 wt% MWCNTs with a nanohardness of 4.0 GPa that correspond to 17.6% improvement. However, the sintered titanium alloy exhibited the least nanohardness of 3.4 GPa. These results further implied that the incorporation of MWCNTs into the Ti6Al4V matrix augmented the nanomechanical properties of the fabricated materials by posing hindrances to the dislocation movement during the test. The improvement in nanohardness could be attributed to the dispersion strengthening and the pinning effect by the nanotubes in the Ti6Al4V matrix [11,12]. Additionally, the stiffness of the fabricated materials was accessed by evaluating the reduced elastic modulus and the results are shown in Fig. 4, it was seen that the elastic modulus was enhanced by the incorporation of MWCNTs into the Ti6Al4V matrix. The fabricated material containing 1.0 wt% of MWCNTs showed the best reduced elastic modulus of 38.9 GP, which amounts to 40.9% increased. This was accompanied by 0.5 wt% MWCNTs reinforced Ti6Al4V matrix with an elastic modulus of 29.6 GPa, which corresponds to 7.2% increased. However, the sintered titanium alloy exhibited the lowest reduced elastic modulus of 27.6 GPa. These results also indicated that the incorporation of nanotubes into the Ti6Al4V matrix stiffened the fabricated materials by inhibiting the dislocation movement during the test [13].
3.2.3. Elastic recovery and plasticity index of the fabricated materials The elastic recovery and plasticity index were used to ascertain the nanomechanical behaviours of materials under nanoindentation load. The elastic recovery index provides the details about the energy liberated from the material under mechanical loading [14], while the plasticity index gives information about the intrinsic plastic behaviour of materials [15]. Furthermore, the elastic recovery and plasticity index of the fabricated materials are shown in Fig. 5. From the chart in Fig. 5, it was noticed that the elastic recovery index of fabricated alloy and nanocomposites comprising of MWCNTs improved significantly from 7.3% to 57.4%, with the increased of weight percent of the MWCNTs from 0.5 to 1.0 respectively. While the plasticity index declined from 25.8% to 46.6% with the increased in weight fractions of MWCNTs in the titanium alloy. Also, it was noticed that the fabricated Ti6Al4V displayed the highest plasticity index which implies that the alloy experienced the least hindrances to plastic deformation owing to its higher ductility when compared with the fabricated nanocomposites. From all indications, the nanomechanical properties tested improved with the additions of MWCNTs in the Ti6Al4V matrix, which further revealed that the incorporation of the nanotubes into the titanium
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was funded by the National Research Foundation of South Africa in partnership with the World Academy of Science (NRF-TWAS) and the Global Excellence and Stature of the University of Johannesburg, South Africa. References [1] B. Poon, D. Rittel, G. Ravichandran, An analysis of nanoindentation in linearly elastic solids, Int. J. Solids Struct. 45 (2008) 6018–6033. [2] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. [3] K.D. Sattler, Handbook of Nanophysics: Functional Nanomaterials, CRC Press, 2010. [4] H. Attar, S. Ehtemam-Haghighi, D. Kent, I.V. Okulov, H. Wendrock, M. Bӧnisch, A.S. Volegov, M. Calin, J. Eckert, M.S. Dargusch, Nanoindentation and wear properties of Ti and Ti-TiB composite materials produced by selective laser melting, Mater. Sci. Eng. A 688 (2017) 20–26. [5] K.S. Munir, Y. Zheng, D. Zhang, J. Lin, Y. Li, C. Wen, Improving the strengthening efficiency of carbon nanotubes in titanium metal matrix composites, Mater. Sci. Eng. A 696 (2017) 10–25. [6] A.M. Okoro, S.S. Lephuthing, S.R. Oke, O.E. Falodun, M.A. Awotunde, P.A. Olubambi, A review of spark plasma sintering of carbon nanotubes reinforced titanium-based nanocomposites: fabrication, densification, and mechanical properties, JOM (2018). [7] A.M. Okoro, R. Machaka, S.S. Lephuthing, M.A. Awotunde, S.R. Oke, O.E. Falodun, P.A. Olubambi, Dispersion characteristics, interfacial bonding and nanostructural evolution of MWCNTS in Ti6Al4V powders prepared by shift speed ball milling technique, J. Alloys Compd. (2019). [8] J. Nohava, N.X. Randall, N. Conté, Novel ultra nanoindentation method with extremely low thermal drift: principle and experimental results, J. Mater. Res. 24 (2009) 873–882. [9] B.B. Medeiros, M.M. Medeiros, J. Fornell, J. Sort, M.D. Baró, A.M.J. Junior, Nanoindentation response of Cu–Ti based metallic glasses: comparison between as-cast, relaxed and devitrified states, J. Non. Cryst. Solids 425 (2015) 103–109. [10] D. Lin, M. Saei, S. Suslov, S. Jin, G.J. Cheng, Super-strengthening and stabilizing with carbon nanotube harnessed high density nanotwins in metals by shock loading, Sci. Rep. 5 (2015) 15405. [11] F. Mokdad, D.L. Chen, Z.Y. Liu, B.L. Xiao, D.R. Ni, Z.Y. Ma, Deformation and strengthening mechanisms of a carbon nanotube reinforced aluminum composite, Carbon N. Y. 104 (2016) 64–77. [12] S. Li, B. Sun, H. Imai, T. Mimoto, K. Kondoh, Powder metallurgy titanium metal matrix composites reinforced with carbon nanotubes and graphite, Compos. Part A Appl. Sci. Manuf. 48 (2013) 57–66. [13] M. Rashad, F. Pan, J. Zhang, M. Asif, Use of high energy ball milling to study the role of graphene nanoplatelets and carbon nanotubes reinforced magnesium alloy, J. Alloys Compd. 646 (2015) 223–232. [14] J. Musil, F. Kunc, H. Zeman, H. Polakova, Relationships between hardness, Young’s modulus and elastic recovery in hard nanocomposite coatings, Surf. Coat. Technol. 154 (2002) 304–313. [15] E. Zhang, H. Chen, F. Shen, Biocorrosion properties and blood and cell compatibility of pure iron as a biodegradable biomaterial, J. Mater. Sci. Mater. Med. 21 (2010) 2151–2163.
Please cite this article as: A. M. Okoro, S. S. Lephuthing, M. A. Awotunde et al., Nanomechanical properties of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites via nanoindentation technique, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.285