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Enhanced mechanical properties of TiN-graphene composites rapidly sintered by high-frequency induction heating
Ceramics International xx (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Enhanced mechanical properties of TiN-graphene composites rapidly sintered by high-frequency induction heating ⁎
In-Jin Shon
Division of Advanced Materials Engineering and the Research Center of Advanced Materials Development, Engineering College, Chonbuk National University, 561-756, Korea
A R T I C L E I N F O
A BS T RAC T
Keywords: Nano-materials Sintering Hardness Fracture toughness Graphene
The current concern about the TiN focuses on its low fracture toughness below the ductile-brittle transition temperature despite of many attractive properties of TiN. To improve its mechanical properties, the approach generally utilized has been the addition of a second phase to form composites and to make nanostructured materials. In this respect, graphene was evaluated as the reinforcing agent of TiN ceramics using novel sintering method (high-frequency induction heated sintering method). Highly dense nanostructured TiN and TiNgraphene composites were obtained within one min at 1400 °C. The effect of graphene on the grain size and the mechanical properties (hardness and fracture toughness) of TiN -graphene composites was evaluated.
1. Introduction Titanium nitride (TiN) is an attractive ceramics due to its high hardness, high melting point, thermodynamic stability, and low density. Therefore, it has been used extensively in cutting tools and coating materials. However, as in the case of many ceramic materials, the current concern about this material focuses on their low fracture toughness below the ductile-brittle transition temperature [1]. To improve its mechanical properties, the approach generally utilized has been the addition of a second phase to form composites and to make nanostructured materials. The fracture toughness of composite can be improved by crack deflection and dividing [2,3]. Nanostructured materials have received much attention as advanced engineering materials with enhanced mechanical properties [4,5]. Since they possess a high strength and hardness as well as excellent ductility and toughness, they have garnered more attention recently. In recent days, TiN nanopowders have been fabricated by the wire explosion [6], combustion synthesis [1] and high energy milling [7]. The sintering temperature of high energy mechanically milled powder is lower than that of unmilled powder due to the increased reactivity, internal and surface energies, and surface area of the milled powder, which contribute to its so-called mechanical activation [7–9]. Even though the initial particle size is less than 100 nm, the grain size increases rapidly up to several μm or larger during pulsed electric current sintering [6]. So, controlling grain growth during the sintering process is one of the keys to the commercial success of nanostructured
⁎
materials. In this regard, to sinter nanostructured materials, a second phase such as graphene has been added to inhibit grain growth using the novel high-frequency induction heated sintering (HFIHS) technique which has been shown to be effective in the sintering of nanostructured materials in very short times (within 1 min) [10–12]. Graphene has been considered as an ideal second phase to improve the mechanical, electrical and thermal properties of metals [13], ceramics [14] and polymers [15] due to its unique combination of electrical, thermal and mechanical properties [16–18]. We present here the results of the sintering of TiN-graphene composites by a high-frequency induction heated sintering with simultaneous application of induced current and 80 MPa pressure. The goal of this study was to produce dense and nanocrystalline TiN and TiN-graphene composites in very short sintering times ( < 1 min). The effect of graphene on the mechanical properties (hardness and fracture toughness), microstructures and relative densities of TiN -graphene composites was also evaluated. 2. Experimental procedures TiN powders with a grain size of −400 mesh and 99.7% purity was supplied by Alfa. Graphene (XG-Science. Graphene grade C-750) with the thickness, and length of 2 nm and < 2 µm, respectively was used as the additive. Powders of four compositions corresponding to TiN, TiN1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene were prepared by weighting and milled in a high-energy ball mill
Corresponding author. E-mail address: [email protected].
http://dx.doi.org/10.1016/j.ceramint.2016.09.169 Received 7 September 2016; Received in revised form 23 September 2016; Accepted 23 September 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Shon, I., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.169
Ceramics International xx (xxxx) xxxx–xxxx
I.-J. Shon
into the high-frequency induced heated sintering (HFIHS) system. A schematic diagram of this system is shown in Ref. [19]. The HFIHS apparatus includes a 15 kW power supply which provides an induced current through the die and sample, and a 50kN uniaxial press. The system was first evacuated and a uniaxial pressure of 80 MPa was applied. An induced current was then activated and maintained until the densification rate was negligible, as indicated by the real-time output of the shrinkage of the sample. Sample shrinkage was measured in real time by a linear gauge measuring the vertical displacement. Temperature was measured by a pyrometer focused on the surface of the graphite die. The heating rates were approximately 1400 K minute−1 during the process. At the end of the process, the induced current was turned off and the sample was allowed to cool to room temperature. The entire process of densification using the HFIHS technique consists of four major control stages: chamber evacuation, pressure application, power application, and cooling off. The process was carried out under a vacuum of 10 Pa. The relative densities of the sintered samples were measured by the Archimedes method. Microstructural information was obtained from fracture surface of product samples. Compositional and microstructural analyses of the samples were carried out through X-ray diffraction (XRD) with Cu Kα radiation, and field-emission scanning electron microscopy (FE-SEM) with EDS. Vickers hardness was measured by performing indentations at a load of 10 kgf with a dwell time of 15 s. The grain size of the TiN was calculated from the full width at halfmaximum (FWHM) of the diffraction peak by Suryanarayana and Grant Norton's formula [20]. The fracture toughness of sintered specimen was calculated from the lengths of these cracks produced around the indentation by means of the expression [21]:
Fig. 1. XRD patterns of TiN powder + x vol% graphene powder milled for 10 h: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
(Pulverisette-5 planetary mill) at 250 rpm for 10 h. WC-10Co balls with 10 mm in diameter were used in a sealed cylindrical stainless steel vial under an argon atmosphere. The weight ratio of balls-to-powder was 30:1. The milled powders were placed in a graphite die (outside diameter, 35 mm; inside diameter, 10 mm; height, 40 mm) and then introduced
Fig. 2. Plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ for TiN in TiN powder+x wt% graphene powder milled for 10 h: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
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Fig. 3. FE-SEM images and EDS of TiN powder+x vol% graphene powder milled for 10 h: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
KIC = 0.023(c / a )−3/2.Hv.a1/2
(1)
after milling. The broadening of TiN peaks is due to crystallite refinement and strain induced during the milling. Plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ in Suryanarayana and Grant Norton's formula [20] for TiN particle size measurements is shown in Fig. 2. The average grain sizes of the TiN in TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene are calculated as about 50, 31, 19 and 18 nm, respectively. Fig. 3 shows the FESEM images of TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene powders after milling for 10 h. The powders
where c is the trace length of the crack measured from the center of the indentation, a is one half of the average length of the two indent diagonals, and Hv is the hardness. 3. Results and discussion Fig. 1 shows X-ray diffraction patterns of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene powders 3
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Fig. 5 shows the XRD patterns of TiN, TiN-1 vol% graphene, TiN3 vol% graphene and TiN-5 vol% graphene after sintering. In all cases, only TiN peaks are detected. Again, their grain sizes were calculated by the plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ in Suryanarayana and Grant Norton's formula [20] as shown in Fig. 6. The average grain sizes of TiN were about 199, 164, 127 and 84 nm for TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene, respectively. This means that the grain size of TiN decreases as the graphene content increases. This indicates that graphene may block the grain growth of TiN during sintering. Fig. 7 shows the FE-SEM images of the fracture surfaces of the samples after sintering at 1400 °C. The grain refining effect of graphene can also be confirmed by the fracture surface. It can be seen that the crack propagation mode for the TiN and TiN-1 vol% graphene samples is inter-granular as the cracks propagates along grain boundaries. In general, cracks propagate through the grain boundaries of ceramic materials because they are weaker than the grains. However, when the amount of graphene increased to 3 vol% and 5 vol%, trans-granular type fracture appears to occur locally. Fig. 7(c) and (d) are the fracture surface of TiN containing 3 vol% and 5 vol% graphene. Obviously, the grain size is finer and the fracture surface appears to be the mixture of trans-granular and inter-granular type failure. In any case, it should be mentioned that TiN ceramics having nanostructure could be obtained by HFIHS even for pure TiN without graphene. This retention of ultra-fine grain structure can be attributed to the high heating rate, the low sintering temperature and the relatively short exposure to the high temperature. The composites were fairly dense and relative densities of 100%, 100%, 99% and 98% were obtained for TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene, respectively. It is clearly demonstrated that the method is effective to consolidate nano-structured TiN and TiNgraphene composites. The effect of the current on enhanced sintering has been explained by past heating due to Joule heating at contacts points, the presence of plasma in pores separating powder particles, the enhancement of wettability under the electric field and the fast mass transport due to electromigration [22–27]. Several investigators have studied on consolidation of TiN by pulsed current activated sintering. TiN with 98% relative density and 10 µm grain size can achieved for heating time of 20 min at 1800 °C [1]. Kim et al. [6] has reported that even though the initial particle size of TiN is about 25 nm, the grain size increases rapidly up to 7.1 µm during pulsed electric current sintering at 1600 °C. And the relative density of the TiN was about 97%. Compared with above the studies, nanostructured TiN with higher density can be obtained within shorter time using mechanically milled powder TiN and high-frequency induction heated sintering method. The role of mechanical milling on the enhanced consolidation has been explained by the increased reactivity, diffusion route at contact points, internal and surface energies, and surface area of the milled powder, which contribute to its so-called mechanical activation [7–9]. Vickers hardness measurements were performed on polished sections of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene samples using a 10 kgf load and 15 s dwell time. Indentions with 10 kgf load produced median cracks around the indentation from which fracture toughness can be calculated. The lengths of these cracks permit the estimation of the fracture toughness values using the method of Niihara et al. [28]. Fig. 8 shows the effect of graphene addition on the hardness and fracture toughness of TiNgraphene composites. The Vickers hardness and the fracture toughness values of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene samples were 2100 kg/mm2, 2.5 MPa m1/2 and 2190 kg/mm2, 2.9 MPa m1/2 and 2300 kg/mm2, 3.2 MPa m1/2 and 2450 kg/mm2, 3.5 MPa m1/2, respectively. All values represent the average of five measurements. The results suggest that both hardness and fracture toughness increased simultaneously. To understand the effect of graphene on the mechanical properties, two factors may be
Fig. 4. Variations of temperature and shrinkage displacement with heating time during the sintering of TiN and TiN-graphene samples by HFIHS.
Fig. 5. XRD patterns of TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
consist of nanopowders and show clusters of very fine particles. In EDS, Ti and N peaks are only detected. The impurity peaks of Fe and W usually introduced during milling process from the ball and/or container could not be identified... The shrinkage displacement-time (temperature) curve provides a useful information on the consolidation behavior. The shrinkage records of TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene compacts are shown in Fig. 4. In all cases, the thermal expansion of the compacts occurs up to 1000 °C. Afterwards, shrinkage displacement rapidly increases to 1300 °C at which the consolidation terminates. The shrinkage curve suggests that the consolidation terminates in one minute. 4
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Fig. 6. Plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ for TiN in TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
Fig. 7. FE-SEM images of fracture surface of TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
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to three additional cracks propagating radially from the indentation. The toughness of TiN-graphen composite may be that the graphene may deter the crack propagation. The effect of graphene on fracture toughness of Al2O3-graphene composites and Si3N4-graphene composites was investigated [29–31]. The fracture toughness of the composite increased with addition of graphene due to crack deflection and crack bridging. In this study, crack deflection and crack bridging were also observed in Fig. 10. The enhanced fracture toughness of TiNgraphene composite is believed that graphene in the composite may deter the crack propagation..
4. Conclusions Nanopowder of TiN was fabricated by high-energy ball milling. Nanostructured TiN and TiN-graphene composites with near fulldensity could be achieved within one min. Nanostructured TiN with higher density can be obtained within shorter time at lower temperature using mechanically milled powder and high-frequency induction heated sintering method. The grain size of TiN was reduced remarkably by the addition of graphene. The Vickers hardness and the fracture toughness values of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene samples were 2100 kg/mm2, 2.5 MPa m1/2 and 2190 kg/mm2, 2.9 MPa m1/2 and 2300 kg/mm2, 3.2 MPa m1/2 and 2450 kg/mm2, 3.5 MPa m1/2, respectively. The addition of graphene to TiN simultaneously improved the fracture toughness and hardness of TiN-graphene composite due to refinement of TiN and deterring crack propagation by graphene. This study demonstrates that the graphene can be an effective reinforcing agent for improved hardness and fracture toughness of TiN composites.
Fig. 8. The variation of hardness and fracture toughness of TiN with addition of graphene.
considered. One is the effect of grain refinement of TiN. The other would be the role of graphene on the crack propagation to affect the toughness. It is generally accepted that the grain refinement may increase the hardness according to the Hall-Patch type strengthening effect. This may be the case since the addition of graphene refined the grain size of TiN significantly in this study. Vickers hardness indentations in the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene samples are shown in Fig. 9. They show typically one
Fig. 9. Vickers hardness indentation in TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
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[9] M.K. Beyer, H. Clausen-Schaumann, The mechanical activation of covalent bonds, Chem. Rev. 105 (2005) 2921–2944. [10] B.R. Kang, I.J. Shon, Properties and rapid consolidation of nanostructured 4Cr3ZrO2 composite by pulsed current activated sintering, Korean J. Met. Mater. 53 (2015) 320–325. [11] Bo-Ram Kang, Jin-kook Yoon, Kyung-Tae Hong, In-Jin Shon, Mechanical properties and rapid low-temperature consolidation of nanocrystalline Cu-ZrO2 composites by pulsed current activated heating, Met. Mater. Int. 21 (2015) 698–703. [12] In-Jin Shon, Hyun-Su Kang, Jung-Mann Doh, Jin-Kook Yoon, Pulsed current activated synthesis and rapid consolidation of a nanostructured Mg2Al4Si5O18 and its mechanical properties, Met. Mater. Int. 21 (2015) 345–349. [13] M.C. Liu, C.L. Chen, J. Hu, X.L. Wu, X.K. Wang, Synthesis of magnetite/graphene oxide composite and application for cobalt(II) removal, J. Phys. Chem. C 115 (51) (2011) 25234–25240. [14] Lv Zhang, Weiwei Liu, Chunguang Yue, Taihua Zhang, Pei Li, Zhanwen Xing, Yao Chen, A tough graphene nanosheet/hydroxyapatite composite with improved in vitro biocompatibility, Carbon 61 (2013) 105–115. [15] Long-Cheng Tang, Yan-Jun Wan, Dong Yan, Yong-Bing Pei, Li Zhao, Yi-Bao Li, Lian-Bin Wu, Jian-Xiong Jiang, Guo-Qiao Lai, The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites, Cabon 60 (2013) 16–27. [16] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [17] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (5887) (2008) 385–388. [18] A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008) 902–907. [19] In-Jin Shon, Mechanochemical synthesis and fast consolidation of a nanostructured CoTi-ZrO2 composite by high-frequency induction heating, Ceram. Int. 42 (2016) 13314–13318. [20] C. Suryanarayana, M. Grant Norton, X-ray Diffraction a Practical Approach, Plenum Press, New York, 1998. [21] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of KIC of brittle solids by the indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [22] Z. Shen, M. Johnsson, Z. Zhao, M. Nygren, Spark plasma sintering of alumina, J. Am. Ceram. Soc. 85 (2002) 1921–1927. [23] J.E. Garay, U. Anselmi-Tamburini, Z.A. Munir, S.C. Glade, P. Asoka- Kumar, Electric current enhanced defect mobility in Ni3Ti intermetallic electric current enhanced defect mobility in Ni3Ti intermetallics, Appl. Phys. Lett. 85 (2004) 573–575. [24] J.R. Friedman, J.E. Garay, U. Anselmi-Tamburini, Z.A. Munir, Modified interfacial reactions in Ag–Zn multilayers under the influence of high DC currents, Intermetallics 12 (2004) 589–597. [25] J.E. Garay, J.E. Garay, U. Anselmi-Tamburini, Z.A. Munir, Enhanced growth of intermetallic phases in the Ni–Ti system by current effects, Acta Mater. 51 (2003) 4487–4495. [26] Marco Rishi Raj, Cologna, John S.C. Francis, Influence of externally imposed and internally generated electrical fields on grain growth, diffusional creep, sintering and related phenomena in ceramics, J. Am. Ceram. Soc. 94 (7) (2011) 1941–1965. [27] Yan Gu, Ping Shen, Nan-Nan Yang, Kang-Zhan Cao, Effects of direct current on the wetting behavior and interfacial morphology between molten Sn and Cu substrate, J. Alloy. Compd. 586 (2014) 80–86. [28] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of KIC of brittle solids by the indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [29] Wonbaek Kim, Hyun-Su Oh, In-Jin Shon, The effect of graphene reinforcement on the mechanical properties of Al2O3 ceramics rapidly sintered by high-frequency induction heating, Int. J. Refract. Met. Hard Mater. 48 (2015) 376–381. [30] L.S. Walker, V.R. Marotto, M.A. Rafiee, N. Koratkar, E.L. Corral, Toughening in Graphene Ceramic Composites, ACS Nano 5 (2011) 3182–3190. [31] L. Kvetková, A. Duszová, P. Hvizdoš, J. Dusza, P. Kun, C. Balázsi, Fracture toughness and toughening mechanisms in graphene platelet reinforced Si3N4 composites, Scr. Mater. 66 (2012) 793–796.
Fig. 10. Crack propagation in TiN+5 vol% graphene composite sintered at 1400 °C.
Acknowledgements This research was supported by Basic Science Research Program though the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01056600) and this work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20164030201070). References [1] Santiago Suarez-Vazquez, Makoto Nanko, Preparation of dense TiN1–X (X=0–0.4) by pulsed electric current sintering: densification and mechanical behavior, Int. J. Refract. Met. Hard Mater. 44 (2014) 54–59. [2] So-Mang Kwon, Seok-Jae Lee, In-Jin Shon, Enhanced properties of nanostructured ZrO2–graphene composites rapidly sintered via high-frequency induction heating, Ceram. Int. 41 (2015) 835–842. [3] In-Jin Shon, Hyoung-Gon Jo, Byung-Su Kim, Jin-Kook Yoon, Kyung-Tae Hong, Mechanical properties of nanostructured TiC-FeAl hard materialsn, Korean J. Met. Mater. 53 (2015) 474–479. [4] M. Sherif El-Eskandarany, Structure and properties of nanocrystalline TiC fulldensity bulk alloy consolidated from mechanically reacted powders, J. Alloy. Compd. 305 (2000) 225–238. [5] Seung-Jin Oh, Byung SuKim, Jin-Kook Yoon, Kyung-Tae Hong, In-Jin Shon, Enhanced mechanical properties and consolidation of the ultra-fine WC–Al2O3 composites using pulsed current activated heating, Ceram. Int. 42 (2016) 9304–9310. [6] Wonbaek Kim, Je-Shin Park, Sung-Wook Cho, Na-Ri Kim, In-Yong Ko, InJin Shon, Properties and rapid consolidation of binderless titanium nitride by pulsed current activated sintering, J. Ceram. Process. Res. 11 (2010) 627–630. [7] So-Mang Kwon, Na-Ra Park, Jae-Won Shin, Se-Hoon Oh, Byung-Su Kim, InJin Shon, Mechanical Properties and Sintering of Nanostructured Ti-TiC Composites, Korean J. Met. Mater. 53 (2015) 555–562. [8] F. Charlot, E. Gaffet, B. Zeghmati, F. Bernard, J.C. Liepce, Mechanically activated synthesis studied by X-ray diffraction in the Fe–Al system, Mater. Sci. Eng. A 262 (1999) 279–288.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Enhanced mechanical properties of TiN-graphene composites rapidly sintered by high-frequency induction heating ⁎
In-Jin Shon
Division of Advanced Materials Engineering and the Research Center of Advanced Materials Development, Engineering College, Chonbuk National University, 561-756, Korea
A R T I C L E I N F O
A BS T RAC T
Keywords: Nano-materials Sintering Hardness Fracture toughness Graphene
The current concern about the TiN focuses on its low fracture toughness below the ductile-brittle transition temperature despite of many attractive properties of TiN. To improve its mechanical properties, the approach generally utilized has been the addition of a second phase to form composites and to make nanostructured materials. In this respect, graphene was evaluated as the reinforcing agent of TiN ceramics using novel sintering method (high-frequency induction heated sintering method). Highly dense nanostructured TiN and TiNgraphene composites were obtained within one min at 1400 °C. The effect of graphene on the grain size and the mechanical properties (hardness and fracture toughness) of TiN -graphene composites was evaluated.
1. Introduction Titanium nitride (TiN) is an attractive ceramics due to its high hardness, high melting point, thermodynamic stability, and low density. Therefore, it has been used extensively in cutting tools and coating materials. However, as in the case of many ceramic materials, the current concern about this material focuses on their low fracture toughness below the ductile-brittle transition temperature [1]. To improve its mechanical properties, the approach generally utilized has been the addition of a second phase to form composites and to make nanostructured materials. The fracture toughness of composite can be improved by crack deflection and dividing [2,3]. Nanostructured materials have received much attention as advanced engineering materials with enhanced mechanical properties [4,5]. Since they possess a high strength and hardness as well as excellent ductility and toughness, they have garnered more attention recently. In recent days, TiN nanopowders have been fabricated by the wire explosion [6], combustion synthesis [1] and high energy milling [7]. The sintering temperature of high energy mechanically milled powder is lower than that of unmilled powder due to the increased reactivity, internal and surface energies, and surface area of the milled powder, which contribute to its so-called mechanical activation [7–9]. Even though the initial particle size is less than 100 nm, the grain size increases rapidly up to several μm or larger during pulsed electric current sintering [6]. So, controlling grain growth during the sintering process is one of the keys to the commercial success of nanostructured
⁎
materials. In this regard, to sinter nanostructured materials, a second phase such as graphene has been added to inhibit grain growth using the novel high-frequency induction heated sintering (HFIHS) technique which has been shown to be effective in the sintering of nanostructured materials in very short times (within 1 min) [10–12]. Graphene has been considered as an ideal second phase to improve the mechanical, electrical and thermal properties of metals [13], ceramics [14] and polymers [15] due to its unique combination of electrical, thermal and mechanical properties [16–18]. We present here the results of the sintering of TiN-graphene composites by a high-frequency induction heated sintering with simultaneous application of induced current and 80 MPa pressure. The goal of this study was to produce dense and nanocrystalline TiN and TiN-graphene composites in very short sintering times ( < 1 min). The effect of graphene on the mechanical properties (hardness and fracture toughness), microstructures and relative densities of TiN -graphene composites was also evaluated. 2. Experimental procedures TiN powders with a grain size of −400 mesh and 99.7% purity was supplied by Alfa. Graphene (XG-Science. Graphene grade C-750) with the thickness, and length of 2 nm and < 2 µm, respectively was used as the additive. Powders of four compositions corresponding to TiN, TiN1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene were prepared by weighting and milled in a high-energy ball mill
Corresponding author. E-mail address: [email protected].
http://dx.doi.org/10.1016/j.ceramint.2016.09.169 Received 7 September 2016; Received in revised form 23 September 2016; Accepted 23 September 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Shon, I., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.169
Ceramics International xx (xxxx) xxxx–xxxx
I.-J. Shon
into the high-frequency induced heated sintering (HFIHS) system. A schematic diagram of this system is shown in Ref. [19]. The HFIHS apparatus includes a 15 kW power supply which provides an induced current through the die and sample, and a 50kN uniaxial press. The system was first evacuated and a uniaxial pressure of 80 MPa was applied. An induced current was then activated and maintained until the densification rate was negligible, as indicated by the real-time output of the shrinkage of the sample. Sample shrinkage was measured in real time by a linear gauge measuring the vertical displacement. Temperature was measured by a pyrometer focused on the surface of the graphite die. The heating rates were approximately 1400 K minute−1 during the process. At the end of the process, the induced current was turned off and the sample was allowed to cool to room temperature. The entire process of densification using the HFIHS technique consists of four major control stages: chamber evacuation, pressure application, power application, and cooling off. The process was carried out under a vacuum of 10 Pa. The relative densities of the sintered samples were measured by the Archimedes method. Microstructural information was obtained from fracture surface of product samples. Compositional and microstructural analyses of the samples were carried out through X-ray diffraction (XRD) with Cu Kα radiation, and field-emission scanning electron microscopy (FE-SEM) with EDS. Vickers hardness was measured by performing indentations at a load of 10 kgf with a dwell time of 15 s. The grain size of the TiN was calculated from the full width at halfmaximum (FWHM) of the diffraction peak by Suryanarayana and Grant Norton's formula [20]. The fracture toughness of sintered specimen was calculated from the lengths of these cracks produced around the indentation by means of the expression [21]:
Fig. 1. XRD patterns of TiN powder + x vol% graphene powder milled for 10 h: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
(Pulverisette-5 planetary mill) at 250 rpm for 10 h. WC-10Co balls with 10 mm in diameter were used in a sealed cylindrical stainless steel vial under an argon atmosphere. The weight ratio of balls-to-powder was 30:1. The milled powders were placed in a graphite die (outside diameter, 35 mm; inside diameter, 10 mm; height, 40 mm) and then introduced
Fig. 2. Plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ for TiN in TiN powder+x wt% graphene powder milled for 10 h: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
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Fig. 3. FE-SEM images and EDS of TiN powder+x vol% graphene powder milled for 10 h: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
KIC = 0.023(c / a )−3/2.Hv.a1/2
(1)
after milling. The broadening of TiN peaks is due to crystallite refinement and strain induced during the milling. Plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ in Suryanarayana and Grant Norton's formula [20] for TiN particle size measurements is shown in Fig. 2. The average grain sizes of the TiN in TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene are calculated as about 50, 31, 19 and 18 nm, respectively. Fig. 3 shows the FESEM images of TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene powders after milling for 10 h. The powders
where c is the trace length of the crack measured from the center of the indentation, a is one half of the average length of the two indent diagonals, and Hv is the hardness. 3. Results and discussion Fig. 1 shows X-ray diffraction patterns of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene powders 3
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Fig. 5 shows the XRD patterns of TiN, TiN-1 vol% graphene, TiN3 vol% graphene and TiN-5 vol% graphene after sintering. In all cases, only TiN peaks are detected. Again, their grain sizes were calculated by the plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ in Suryanarayana and Grant Norton's formula [20] as shown in Fig. 6. The average grain sizes of TiN were about 199, 164, 127 and 84 nm for TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene, respectively. This means that the grain size of TiN decreases as the graphene content increases. This indicates that graphene may block the grain growth of TiN during sintering. Fig. 7 shows the FE-SEM images of the fracture surfaces of the samples after sintering at 1400 °C. The grain refining effect of graphene can also be confirmed by the fracture surface. It can be seen that the crack propagation mode for the TiN and TiN-1 vol% graphene samples is inter-granular as the cracks propagates along grain boundaries. In general, cracks propagate through the grain boundaries of ceramic materials because they are weaker than the grains. However, when the amount of graphene increased to 3 vol% and 5 vol%, trans-granular type fracture appears to occur locally. Fig. 7(c) and (d) are the fracture surface of TiN containing 3 vol% and 5 vol% graphene. Obviously, the grain size is finer and the fracture surface appears to be the mixture of trans-granular and inter-granular type failure. In any case, it should be mentioned that TiN ceramics having nanostructure could be obtained by HFIHS even for pure TiN without graphene. This retention of ultra-fine grain structure can be attributed to the high heating rate, the low sintering temperature and the relatively short exposure to the high temperature. The composites were fairly dense and relative densities of 100%, 100%, 99% and 98% were obtained for TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene, respectively. It is clearly demonstrated that the method is effective to consolidate nano-structured TiN and TiNgraphene composites. The effect of the current on enhanced sintering has been explained by past heating due to Joule heating at contacts points, the presence of plasma in pores separating powder particles, the enhancement of wettability under the electric field and the fast mass transport due to electromigration [22–27]. Several investigators have studied on consolidation of TiN by pulsed current activated sintering. TiN with 98% relative density and 10 µm grain size can achieved for heating time of 20 min at 1800 °C [1]. Kim et al. [6] has reported that even though the initial particle size of TiN is about 25 nm, the grain size increases rapidly up to 7.1 µm during pulsed electric current sintering at 1600 °C. And the relative density of the TiN was about 97%. Compared with above the studies, nanostructured TiN with higher density can be obtained within shorter time using mechanically milled powder TiN and high-frequency induction heated sintering method. The role of mechanical milling on the enhanced consolidation has been explained by the increased reactivity, diffusion route at contact points, internal and surface energies, and surface area of the milled powder, which contribute to its so-called mechanical activation [7–9]. Vickers hardness measurements were performed on polished sections of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene samples using a 10 kgf load and 15 s dwell time. Indentions with 10 kgf load produced median cracks around the indentation from which fracture toughness can be calculated. The lengths of these cracks permit the estimation of the fracture toughness values using the method of Niihara et al. [28]. Fig. 8 shows the effect of graphene addition on the hardness and fracture toughness of TiNgraphene composites. The Vickers hardness and the fracture toughness values of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene samples were 2100 kg/mm2, 2.5 MPa m1/2 and 2190 kg/mm2, 2.9 MPa m1/2 and 2300 kg/mm2, 3.2 MPa m1/2 and 2450 kg/mm2, 3.5 MPa m1/2, respectively. All values represent the average of five measurements. The results suggest that both hardness and fracture toughness increased simultaneously. To understand the effect of graphene on the mechanical properties, two factors may be
Fig. 4. Variations of temperature and shrinkage displacement with heating time during the sintering of TiN and TiN-graphene samples by HFIHS.
Fig. 5. XRD patterns of TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
consist of nanopowders and show clusters of very fine particles. In EDS, Ti and N peaks are only detected. The impurity peaks of Fe and W usually introduced during milling process from the ball and/or container could not be identified... The shrinkage displacement-time (temperature) curve provides a useful information on the consolidation behavior. The shrinkage records of TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene compacts are shown in Fig. 4. In all cases, the thermal expansion of the compacts occurs up to 1000 °C. Afterwards, shrinkage displacement rapidly increases to 1300 °C at which the consolidation terminates. The shrinkage curve suggests that the consolidation terminates in one minute. 4
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Fig. 6. Plot of Br (Bcrystalline+Bstrain) cosθ versus sinθ for TiN in TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
Fig. 7. FE-SEM images of fracture surface of TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
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to three additional cracks propagating radially from the indentation. The toughness of TiN-graphen composite may be that the graphene may deter the crack propagation. The effect of graphene on fracture toughness of Al2O3-graphene composites and Si3N4-graphene composites was investigated [29–31]. The fracture toughness of the composite increased with addition of graphene due to crack deflection and crack bridging. In this study, crack deflection and crack bridging were also observed in Fig. 10. The enhanced fracture toughness of TiNgraphene composite is believed that graphene in the composite may deter the crack propagation..
4. Conclusions Nanopowder of TiN was fabricated by high-energy ball milling. Nanostructured TiN and TiN-graphene composites with near fulldensity could be achieved within one min. Nanostructured TiN with higher density can be obtained within shorter time at lower temperature using mechanically milled powder and high-frequency induction heated sintering method. The grain size of TiN was reduced remarkably by the addition of graphene. The Vickers hardness and the fracture toughness values of the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN-5 vol% graphene samples were 2100 kg/mm2, 2.5 MPa m1/2 and 2190 kg/mm2, 2.9 MPa m1/2 and 2300 kg/mm2, 3.2 MPa m1/2 and 2450 kg/mm2, 3.5 MPa m1/2, respectively. The addition of graphene to TiN simultaneously improved the fracture toughness and hardness of TiN-graphene composite due to refinement of TiN and deterring crack propagation by graphene. This study demonstrates that the graphene can be an effective reinforcing agent for improved hardness and fracture toughness of TiN composites.
Fig. 8. The variation of hardness and fracture toughness of TiN with addition of graphene.
considered. One is the effect of grain refinement of TiN. The other would be the role of graphene on the crack propagation to affect the toughness. It is generally accepted that the grain refinement may increase the hardness according to the Hall-Patch type strengthening effect. This may be the case since the addition of graphene refined the grain size of TiN significantly in this study. Vickers hardness indentations in the TiN, TiN-1 vol% graphene, TiN-3 vol% graphene and TiN5 vol% graphene samples are shown in Fig. 9. They show typically one
Fig. 9. Vickers hardness indentation in TiN+x vol% graphene specimens sintered at 1400 °C: (a) x=0, (b) x=1, (c) x=3 and (d) x=5.
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Fig. 10. Crack propagation in TiN+5 vol% graphene composite sintered at 1400 °C.
Acknowledgements This research was supported by Basic Science Research Program though the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01056600) and this work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20164030201070). References [1] Santiago Suarez-Vazquez, Makoto Nanko, Preparation of dense TiN1–X (X=0–0.4) by pulsed electric current sintering: densification and mechanical behavior, Int. J. Refract. Met. Hard Mater. 44 (2014) 54–59. [2] So-Mang Kwon, Seok-Jae Lee, In-Jin Shon, Enhanced properties of nanostructured ZrO2–graphene composites rapidly sintered via high-frequency induction heating, Ceram. Int. 41 (2015) 835–842. [3] In-Jin Shon, Hyoung-Gon Jo, Byung-Su Kim, Jin-Kook Yoon, Kyung-Tae Hong, Mechanical properties of nanostructured TiC-FeAl hard materialsn, Korean J. Met. Mater. 53 (2015) 474–479. [4] M. Sherif El-Eskandarany, Structure and properties of nanocrystalline TiC fulldensity bulk alloy consolidated from mechanically reacted powders, J. Alloy. Compd. 305 (2000) 225–238. [5] Seung-Jin Oh, Byung SuKim, Jin-Kook Yoon, Kyung-Tae Hong, In-Jin Shon, Enhanced mechanical properties and consolidation of the ultra-fine WC–Al2O3 composites using pulsed current activated heating, Ceram. Int. 42 (2016) 9304–9310. [6] Wonbaek Kim, Je-Shin Park, Sung-Wook Cho, Na-Ri Kim, In-Yong Ko, InJin Shon, Properties and rapid consolidation of binderless titanium nitride by pulsed current activated sintering, J. Ceram. Process. Res. 11 (2010) 627–630. [7] So-Mang Kwon, Na-Ra Park, Jae-Won Shin, Se-Hoon Oh, Byung-Su Kim, InJin Shon, Mechanical Properties and Sintering of Nanostructured Ti-TiC Composites, Korean J. Met. Mater. 53 (2015) 555–562. [8] F. Charlot, E. Gaffet, B. Zeghmati, F. Bernard, J.C. Liepce, Mechanically activated synthesis studied by X-ray diffraction in the Fe–Al system, Mater. Sci. Eng. A 262 (1999) 279–288.
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