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Abnormal thermal shock behavior in electrical conductivity of Ti2SnC
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
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Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Original Research
Abnormal thermal shock behavior in electrical conductivity of Ti2SnC☆ ⁎
Linquan Zhang, Shibo Li , Xiaodong Chen, Yang Zhou Center of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ti2SnC Abnormal thermal shock behavior Recovery Mechanism Crack healing
Some ternary carbide and nitride ceramics have been demonstrated to exhibit abnormal thermal shock behavior in mechanical properties. However, the influence of thermal shock on other properties is not clear. This work reports on the influence of thermal shock on electrical conductivity of Ti2SnC as a representative member of ternary carbides. Abnormal change in electrical conductivity was first demonstrated during quenching Ti2SnC in water at 500–800 °C. The residual electrical conductivity of the quenched Ti2SnC gradually decreased with increasing temperature, but abnormally increased after quenching at 600 °C. The microstructure of surface cracks was characterized. The main mechanism for the abnormal electrical conductivity recovery is that some narrow branching cracks are filled by metallic Sn precipitating from Ti2SnC.
1. Introduction Ternary carbides and nitrides belonging to the family of so-called MAX phase materials (M denotes an early transition metal, A is a mostly IIIA or IVA group element, and X is either C or N) possess a combination of ceramic and metallic properties both at room and high temperatures. The MAX phase compounds have high strength, yet are relatively ductile and damage tolerant. They are electrically and thermally conductive, and also resistant to oxidation, corrosion and thermal shock [1–4]. In addition, they have crack healing ability [5–9]. Furthermore, some MAX phases have been demonstrated to exhibit abnormal thermal shock behavior in mechanical properties [10–13]. For example, Ti3SiC2 as a member of MAX materials was first found with abnormal thermal shock behavior in 1996 [10], i.e. the residual strength of the as-quenched samples gradually decreases without catastrophic failure with increasing temperature and then unbelievably increases after quenching from even higher temperatures. Since then, other MAX phases have been discovered with strength recovery after quenching from certain temperatures [2,11–13]. The main mechanism for such unique thermal shock behavior has been demonstrated that the thermal shock induced cracks are instantly healed by the formation of reactants well adhering to the crack faces during quenching [12,13]. So far most investigations have focused on abnormal thermal shock behavior in mechanical properties, but there is little information on changes in the electrical property of MAX materials after thermal shock. Ti2SnC is one of the most attractive materials in the MAX family because it has damage tolerance, self lubricity, crack healability and
high electrical conductivity (4–14 × 106 Ω−1 m−1) [1,14–16]. These attractive properties make Ti2SnC promising for use as electrical contact materials (pantographs and brushes), bipolar plates, electronic contact films and a novel reinforcement for polymers, metals and ceramics. For example, Cu composites reinforced with Ti2SnC exhibited higher strength and comparable electrical conductivity as compared to Cu composites reinforced with other phase particles [17]. In this work, the influence of thermal shock on electrical conductivity of Ti2SnC was investigated through water quenching method in the temperature range of 500–800 °C. The changes in the electrical conductivity of Ti2SnC after thermal shock test were first demonstrated. 2. Experimental details Ti (average particle size: 48 µm, > 99% purity), Sn (average particle size: 75 µm, > 99% purity) and C (graphite, average particle size: 45 µm, > 99% purity) powders were used as starting materials. The powders of Ti, Sn and C with a molar ratio of 2:1:1 were dry-mixed for 10 h. The mixture were put into a graphite die coated with boron nitride and then hot-pressed at 1250 °C with 30 MPa for 1 h in vacuum to prepare Ti2SnC. Rectangular bars with the dimensions of 4 mm × 3 mm × 36 mm were cut from the synthesized Ti2SnC samples. The bars were ground with 1200-grit SiC paper and then polished to 0.25 µm by diamond paste. After polish, the bars were cleaned thoroughly in an ultrasonic bath with ethanol. The thermal shock test was performed by quenching the bars from
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.pnsc.2017.06.008 Received 25 January 2017; Received in revised form 6 June 2017; Accepted 6 June 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Zhang, L., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.06.008
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
L. Zhang et al.
desired high temperatures (Td, 525–825 °C) in ambient temperature water (Ta, 25 °C). The bars were placed in an air furnace and held at the designed temperatures for 10 min, and then rapidly removed from the furnace and immediately dropped into water. The quenched bars were used to determine the residual electrical conductivity as a function of the quenching temperature (ΔT), ΔT = Td -Ta. The initial and the residual electrical conductivities were measured with an Aglient 4338B microhmmeter with an accuracy of ± 0.4%. Electrical conductivity measurement was carried out at room temperature of 25 °C to avoid effect of temperature on electrical conductivity. For each average value of electrical conductivity, at least three rectangular bars were tested. A ZEISS EVO 18 scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS) system was used to observe the microstructures of the original and quenched specimens. Oxides formed on the specimens were slightly polished off using 4000grit SiC papers to observe the surface cracks. The filling phase in the local crack space was characterized by electron backscatter diffraction (EBSD, Oxford Instruments INCA, Oxford, UK). Thermal stresses generated by thermal shock were simulated using the commercial software ProCAST®2009. 3. Results and discussion Electrical conductivity as a function of quenching temperature is described in Fig. 1. In the quenching temperature range of 500–800 °C, the electrical conductivity almost decreased gradually with increasing temperature, whereas it abnormally increased to 4.18 × 106 Ω−1 m−1 after quenching at 600 °C. But the recovery value was still lower than the initial electrical conductivity of 4.34 × 106 Ω−1 m−1. Our previous work proved that crack healing is responsible for the strength recovery of the quenched Cr2AlC MAX material [12,13], as is a possible case for the abnormal change in electrical conductivity of Ti2SnC. Hence we observed the quenched samples on which surface scale was slightly polished. The morphologies of surface cracks induced by thermal shock at 600 °C and 700 °C are shown in Fig. 2(a) and (b). The crack propagation paths exhibited crack deflection and grain pullout. The crack width became larger with increasing temperature. After quenching at 600 °C, some areas and narrow branching cracks filled with white products (denoted by arrows) were detected around the wider crack-damaged zones, as shown in Fig. 2(a). Fig. 2(c) demonstrates that the white phase in the small cracks is metallic Sn, confirmed by the EDS (inset in Fig. 2(c)). The filling phase in the narrow branching cracks was further confirmed to be β-Sn by the EBSD measurement (Fig. 2(d)). However, the branching cracks filled by Sn were seldom detected in the 700 °C-quenched samples (Fig. 2(b)).
Fig. 2. Back-scattered SEM micrographs of polished surfaces of Ti2SnC after quenching from (a) 600 °C and (b) 700 °C; (c) A high magnification image taken from (a). The inset in (c) is an EDS spectrum for metallic Sn; (d) EBSD micrograph of Sn distributing in a small crack after quenching from 600 °C.
Fig. 1. Electrical conductivity as a function of quenching temperature.
2
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Fig. 3. Surface morphologies of quenched samples. (a) Optical image of samples quenched from 600 °C (left) and from 700 °C (right). (b) Second electron SEM image showing Sn beads and whiskers on the quenched surface. (c) EDS spectra for Sn and SnO2.
Fig. 4. (a) Thermal stresses generated at different quenching temperatures; (b) Distribution of thermal stresses on the samples during quenching at ΔT=600 °C.
Cracks filled by Sn should play a major role in the improvement of the residual electrical conductivity. The electrical conductivity of Sn is about 10×106 Ω−1 m−1 [18], more than twice that of Ti2SnC. Due to the high electrical conductivity of Sn the filled small cracks exhibit a low resistance to electron transport which may explain the abnormal recovery of electrical conductivity after quenching at 600 °C. The contribution of Sn to the recovery mechanical and electrical properties has also been confirmed by the healing treatment of Sn-containing MAX phases [16,19]. Based on the above analysis, it is clear the abnormal recovery in electrical conductivity after quenching at 600 °C is the healing of narrow branching cracks by metallic Sn. However, it is necessary to understand why more Sn presents in the 600 °C-quenched samples and to explain why Sn mainly fills the narrow cracks rather than the wide cracks. Some literatures reported that temperature has a strong influence on Sn precipitation from Ti2SnC [20–22]. Sn easily de-intercalates
from the crystal structure of Ti2SnC at lower temperatures due to its low melting temperature of 232 °C and low migration energy of 0.66 eV [23]. The metallic Sn was also detected in Ti2SnC after oxidation at 600 °C for 20 h [21]. In the present study, quenching Ti2SnC in water in the temperature range of 500–800 °C causes the precipitation of Sn from Ti2SnC but cannot trigger the oxidation of Sn under a lower oxygen partial pressure in water. It has been reported that a low oxygen partial pressure may be sufficient to trigger oxidation of Ti from Ti2SnC but too small for Sn to form SnO2 during the oxidation and healing treatment of Ti2SnC even at 800 °C [16,19]. The higher the quenching temperature is, the more Sn de-intercalates from Ti2SnC. Elemental Sn quickly diffuses along small surface cracks and grain boundaries, filling the small defects within a short quenching period. However, the influence of the quenching temperature on the loss of Sn and the degree of damage should not be neglected. With increasing temperature, the loss of Sn and the degree of damage become serious. Upon quenching at 500 °C, the thermal shock induced damage is not severe 3
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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and little Sn yields to fill narrow cracks, causing a modest decrease in the electrical conductivity. At 600 °C, the precipitation of Sn to fill the narrow cracks may be predominant over the loss of Sn, resulting in a recovery of the electrical conductivity. In contrast, at the temperatures above 700 °C, the loss of Sn and the degree of damage become severe, causing a further decrease in electrical conductivity. The loss of Sn at higher quenching temperatures has been confirmed by experimental evidence as shown in Fig. 3. Fig. 3(a) shows the surface morphologies of the 600 °C- and 700 °C- quenched samples. Few Sn beads were found on the 600 °C-quenched sample, whereas lots of Sn beads on the 700 °C-quenched sample were detected. Most interestingly, some Sn whiskers were detected on the 700 °C-quenched sample after storing in air for 3 days (Fig. 3(b)). Some Sn beads and whiskers are covered by SnO2, confirmed by the EDS (Fig. 3(c)). The self-growth of Sn whiskers has been extensively studied, which will not be further addressed here. To explain the reason why Sn mainly fills the narrow branching cracks, the effect of thermal shock-induced tensile stress on Sn distribution should be taken into consideration. During quenching, the surfaces of the samples cool down faster than the core. The tensile stress mainly generates on the surface. The simulated results show that the thermal stress increases with quenching temperatures (Fig. 4(a)). The maximum tensile stress generated at ΔT=800 °C is up to 413 MPa at a quenching time of 0.129 s Fig. 4(b) demonstrates that the higher thermal stress mainly concentrates in the edge areas. The higher thermal stress induces the formation of surface cracks. Once a crack initiates from the sample edges, the stress around its tip is so high and becomes the driving force to push the free Sn atoms forward along the crack propagation direction. Previous literatures reported that inner stress as the driving force accelerated the diffusion of Sn and Ga atoms in the Sn- and Ga-containing MAX bulks and made them spontaneous growth even at room temperature [24–27]. In addition, the quenching temperature is another effective accelerator to promote the diffusion of Sn atoms. The high mobility of Sn driven by thermal shock stresses and temperature seems to favor the transport of Sn into defects around the crack tip, and even cause the loss of Sn. Upon quenching at 600 °C, as a main crack passes through Sn-filling regions, the wider crack space is no longer filled by Sn due to the fast quenching rate. This may interpret the appearance of Sn in the narrow cracks rather than the wide cracks.
peratures, but abnormally increases after quenching at 600 °C. The main mechanism for the electrical conductivity recovery is the filling of the narrow branching cracks by metallic Sn. The mechanism demonstrated here can be applied to other MAX phases containing soft metals (such as Sn, Ga and In) if they exhibit abnormal thermal shock behavior in electrical conductivity. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant nos. 51372015 and 51472024, and Beijing Government Funds for the Constructive Project of Central Universities. The authors also thank Dr. Guoping Bei, at University of Erlangen-Nürnberg, Germany, for EBSD analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
4. Conclusions
[24]
The influence of thermal shock on the electrical conductivity of Ti2SnC has been investigated via water quenching method in the temperature range of 500–800 °C. The electrical conductivity of the quenched Ti2SnC samples gradually decreases with increasing tem-
[25] [26] [27]
4
M.W. Barsoum, Prog. Solid State Chem. 28 (2000) 201–281. M.W. Barsoum, M. Radovic, Annu. Rev. Mater. Res. 41 (2011) 195–227. Z.M. Sun, Int. Mater. Rev. 56 (2011) 143–166. J.Y. Wang, Y.C. Zhou, Annu. Rev. Mater. Res. 39 (10) (2009) 1–29. G.M. Song, Y.T. Pei, W.G. Sloof, S.B. Li, J.Th.M. de Hosson, S. van der Zwaag, Scr. Mater. 58 (2008) 13–16. S.B. Li, G.M. Song, C. Kwakernaak, S. van der Zwaag, W.G. Sloof, J. Eur. Ceram. Soc. 32 (2012) 1813–1820. H.J. Yang, Y.T. Pei, J.C. Rao, J.Th.M. de Hosson, J. Mater. Chem. 22 (2012) 8304–8313. S.B. Li, L.O. Xiao, G.M. Song, W.G. Sloof, S. van der Zwaag, J. Am. Ceram. Soc. 96 (2013) 892–899. G.P. Bei, B.J. Pedimonte, T. Fey, P. Greil, J. Am. Ceram. Soc. 96 (2013) 1359–1362. M.W. Barsoum, T. El-Raghy, J. Am. Ceram. Soc. 79 (1996) 1953–1956. H.B. Zhang, Y.C. Zhou, Y.W. Bao, M.S. Li, J. Mater. Res. 21 (2006) 2401–2407. S.B. Li, H.L. Li, Y. Zhou, H.X. Zhai, J. Eur. Ceram. Soc. 34 (2014) 1083–1088. H.L. Li, S.B. Li, Y. Zhou, Mater. Sci. Eng. A 607 (2014) 525–529. M.W. Barsoum, G. Yaroschuk, S. Tyagi, Scr. Mater. 37 (1997) 1583–1591. Y.C. Zhou, H.Y. Dong, X.H. Wang, C.K. Yan, Mater. Res. Innov. 6 (2002) 219–225. S.B. Li, G.P. Bei, X.D. Chen, L.Q. Zhang, Y. Zhou, M. Mačković, E. Spiecker, P. Greil, J. Eur. Ceram. Soc. 36 (2016) 25–32. J.Y. Wu, Y.C. Zhou, C.K. Yan, Z. Metallkd. 96 (2005) 847–852. T. El-Raghy, S. Chakraborty, M.W. Barsoum, J. Eur. Ceram. Soc. 20 (2000) 2619–2625. G.P. Bei, B.J. Pedimonte, M. Pezoldt, J. Ast, T. Fey, M. Goeken, P. Greil, J. Am. Ceram. Soc. 98 (2015) 1604–1610. H.Y. Dong, C.K. Yan, S.Q. Chen, Y.C. Zhou, J. Mater. Chem. 11 (2001) 1402–1407. Y.C. Zhou, H.Y. Dong, X.H. Wang, Oxid. Met. 61 (2004) 365–377. J. Zhang, B. Liu, J.Y. Wang, Y.C. Zhou, J. Mater. Res. 24 (2009) 39–49. B. Liu, J.Y. Wang, J. Zhang, J.M. Wang, F.Z. Li, Y.C. Zhou, Appl. Phys. Lett. 94 (2009) 181906–181908. S.B. Li, G.P. Bei, H.X. Zhai, Z.L. Zhang, Y. Zhou, C.W. Li, J. Mater. Res. 22 (2007) 3226–3232. M.W. Barsoum, L. Farber, Science 284 (1999) 937–939. M.W. Barsoum, E.N. Hoffman, R.D. Doherty, S. Gupta, A. Zavalingos, Phys. Rev. Lett. 93 (2004) (206104-206104). Z.M. Sun, S. Gupta, H. Ye, M.W. Barsoum, J. Mater. Res. 20 (2005) 2618–2621.
Contents lists available at ScienceDirect
HOSTED BY
Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Original Research
Abnormal thermal shock behavior in electrical conductivity of Ti2SnC☆ ⁎
Linquan Zhang, Shibo Li , Xiaodong Chen, Yang Zhou Center of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ti2SnC Abnormal thermal shock behavior Recovery Mechanism Crack healing
Some ternary carbide and nitride ceramics have been demonstrated to exhibit abnormal thermal shock behavior in mechanical properties. However, the influence of thermal shock on other properties is not clear. This work reports on the influence of thermal shock on electrical conductivity of Ti2SnC as a representative member of ternary carbides. Abnormal change in electrical conductivity was first demonstrated during quenching Ti2SnC in water at 500–800 °C. The residual electrical conductivity of the quenched Ti2SnC gradually decreased with increasing temperature, but abnormally increased after quenching at 600 °C. The microstructure of surface cracks was characterized. The main mechanism for the abnormal electrical conductivity recovery is that some narrow branching cracks are filled by metallic Sn precipitating from Ti2SnC.
1. Introduction Ternary carbides and nitrides belonging to the family of so-called MAX phase materials (M denotes an early transition metal, A is a mostly IIIA or IVA group element, and X is either C or N) possess a combination of ceramic and metallic properties both at room and high temperatures. The MAX phase compounds have high strength, yet are relatively ductile and damage tolerant. They are electrically and thermally conductive, and also resistant to oxidation, corrosion and thermal shock [1–4]. In addition, they have crack healing ability [5–9]. Furthermore, some MAX phases have been demonstrated to exhibit abnormal thermal shock behavior in mechanical properties [10–13]. For example, Ti3SiC2 as a member of MAX materials was first found with abnormal thermal shock behavior in 1996 [10], i.e. the residual strength of the as-quenched samples gradually decreases without catastrophic failure with increasing temperature and then unbelievably increases after quenching from even higher temperatures. Since then, other MAX phases have been discovered with strength recovery after quenching from certain temperatures [2,11–13]. The main mechanism for such unique thermal shock behavior has been demonstrated that the thermal shock induced cracks are instantly healed by the formation of reactants well adhering to the crack faces during quenching [12,13]. So far most investigations have focused on abnormal thermal shock behavior in mechanical properties, but there is little information on changes in the electrical property of MAX materials after thermal shock. Ti2SnC is one of the most attractive materials in the MAX family because it has damage tolerance, self lubricity, crack healability and
high electrical conductivity (4–14 × 106 Ω−1 m−1) [1,14–16]. These attractive properties make Ti2SnC promising for use as electrical contact materials (pantographs and brushes), bipolar plates, electronic contact films and a novel reinforcement for polymers, metals and ceramics. For example, Cu composites reinforced with Ti2SnC exhibited higher strength and comparable electrical conductivity as compared to Cu composites reinforced with other phase particles [17]. In this work, the influence of thermal shock on electrical conductivity of Ti2SnC was investigated through water quenching method in the temperature range of 500–800 °C. The changes in the electrical conductivity of Ti2SnC after thermal shock test were first demonstrated. 2. Experimental details Ti (average particle size: 48 µm, > 99% purity), Sn (average particle size: 75 µm, > 99% purity) and C (graphite, average particle size: 45 µm, > 99% purity) powders were used as starting materials. The powders of Ti, Sn and C with a molar ratio of 2:1:1 were dry-mixed for 10 h. The mixture were put into a graphite die coated with boron nitride and then hot-pressed at 1250 °C with 30 MPa for 1 h in vacuum to prepare Ti2SnC. Rectangular bars with the dimensions of 4 mm × 3 mm × 36 mm were cut from the synthesized Ti2SnC samples. The bars were ground with 1200-grit SiC paper and then polished to 0.25 µm by diamond paste. After polish, the bars were cleaned thoroughly in an ultrasonic bath with ethanol. The thermal shock test was performed by quenching the bars from
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.pnsc.2017.06.008 Received 25 January 2017; Received in revised form 6 June 2017; Accepted 6 June 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Zhang, L., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.06.008
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
L. Zhang et al.
desired high temperatures (Td, 525–825 °C) in ambient temperature water (Ta, 25 °C). The bars were placed in an air furnace and held at the designed temperatures for 10 min, and then rapidly removed from the furnace and immediately dropped into water. The quenched bars were used to determine the residual electrical conductivity as a function of the quenching temperature (ΔT), ΔT = Td -Ta. The initial and the residual electrical conductivities were measured with an Aglient 4338B microhmmeter with an accuracy of ± 0.4%. Electrical conductivity measurement was carried out at room temperature of 25 °C to avoid effect of temperature on electrical conductivity. For each average value of electrical conductivity, at least three rectangular bars were tested. A ZEISS EVO 18 scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS) system was used to observe the microstructures of the original and quenched specimens. Oxides formed on the specimens were slightly polished off using 4000grit SiC papers to observe the surface cracks. The filling phase in the local crack space was characterized by electron backscatter diffraction (EBSD, Oxford Instruments INCA, Oxford, UK). Thermal stresses generated by thermal shock were simulated using the commercial software ProCAST®2009. 3. Results and discussion Electrical conductivity as a function of quenching temperature is described in Fig. 1. In the quenching temperature range of 500–800 °C, the electrical conductivity almost decreased gradually with increasing temperature, whereas it abnormally increased to 4.18 × 106 Ω−1 m−1 after quenching at 600 °C. But the recovery value was still lower than the initial electrical conductivity of 4.34 × 106 Ω−1 m−1. Our previous work proved that crack healing is responsible for the strength recovery of the quenched Cr2AlC MAX material [12,13], as is a possible case for the abnormal change in electrical conductivity of Ti2SnC. Hence we observed the quenched samples on which surface scale was slightly polished. The morphologies of surface cracks induced by thermal shock at 600 °C and 700 °C are shown in Fig. 2(a) and (b). The crack propagation paths exhibited crack deflection and grain pullout. The crack width became larger with increasing temperature. After quenching at 600 °C, some areas and narrow branching cracks filled with white products (denoted by arrows) were detected around the wider crack-damaged zones, as shown in Fig. 2(a). Fig. 2(c) demonstrates that the white phase in the small cracks is metallic Sn, confirmed by the EDS (inset in Fig. 2(c)). The filling phase in the narrow branching cracks was further confirmed to be β-Sn by the EBSD measurement (Fig. 2(d)). However, the branching cracks filled by Sn were seldom detected in the 700 °C-quenched samples (Fig. 2(b)).
Fig. 2. Back-scattered SEM micrographs of polished surfaces of Ti2SnC after quenching from (a) 600 °C and (b) 700 °C; (c) A high magnification image taken from (a). The inset in (c) is an EDS spectrum for metallic Sn; (d) EBSD micrograph of Sn distributing in a small crack after quenching from 600 °C.
Fig. 1. Electrical conductivity as a function of quenching temperature.
2
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Fig. 3. Surface morphologies of quenched samples. (a) Optical image of samples quenched from 600 °C (left) and from 700 °C (right). (b) Second electron SEM image showing Sn beads and whiskers on the quenched surface. (c) EDS spectra for Sn and SnO2.
Fig. 4. (a) Thermal stresses generated at different quenching temperatures; (b) Distribution of thermal stresses on the samples during quenching at ΔT=600 °C.
Cracks filled by Sn should play a major role in the improvement of the residual electrical conductivity. The electrical conductivity of Sn is about 10×106 Ω−1 m−1 [18], more than twice that of Ti2SnC. Due to the high electrical conductivity of Sn the filled small cracks exhibit a low resistance to electron transport which may explain the abnormal recovery of electrical conductivity after quenching at 600 °C. The contribution of Sn to the recovery mechanical and electrical properties has also been confirmed by the healing treatment of Sn-containing MAX phases [16,19]. Based on the above analysis, it is clear the abnormal recovery in electrical conductivity after quenching at 600 °C is the healing of narrow branching cracks by metallic Sn. However, it is necessary to understand why more Sn presents in the 600 °C-quenched samples and to explain why Sn mainly fills the narrow cracks rather than the wide cracks. Some literatures reported that temperature has a strong influence on Sn precipitation from Ti2SnC [20–22]. Sn easily de-intercalates
from the crystal structure of Ti2SnC at lower temperatures due to its low melting temperature of 232 °C and low migration energy of 0.66 eV [23]. The metallic Sn was also detected in Ti2SnC after oxidation at 600 °C for 20 h [21]. In the present study, quenching Ti2SnC in water in the temperature range of 500–800 °C causes the precipitation of Sn from Ti2SnC but cannot trigger the oxidation of Sn under a lower oxygen partial pressure in water. It has been reported that a low oxygen partial pressure may be sufficient to trigger oxidation of Ti from Ti2SnC but too small for Sn to form SnO2 during the oxidation and healing treatment of Ti2SnC even at 800 °C [16,19]. The higher the quenching temperature is, the more Sn de-intercalates from Ti2SnC. Elemental Sn quickly diffuses along small surface cracks and grain boundaries, filling the small defects within a short quenching period. However, the influence of the quenching temperature on the loss of Sn and the degree of damage should not be neglected. With increasing temperature, the loss of Sn and the degree of damage become serious. Upon quenching at 500 °C, the thermal shock induced damage is not severe 3
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and little Sn yields to fill narrow cracks, causing a modest decrease in the electrical conductivity. At 600 °C, the precipitation of Sn to fill the narrow cracks may be predominant over the loss of Sn, resulting in a recovery of the electrical conductivity. In contrast, at the temperatures above 700 °C, the loss of Sn and the degree of damage become severe, causing a further decrease in electrical conductivity. The loss of Sn at higher quenching temperatures has been confirmed by experimental evidence as shown in Fig. 3. Fig. 3(a) shows the surface morphologies of the 600 °C- and 700 °C- quenched samples. Few Sn beads were found on the 600 °C-quenched sample, whereas lots of Sn beads on the 700 °C-quenched sample were detected. Most interestingly, some Sn whiskers were detected on the 700 °C-quenched sample after storing in air for 3 days (Fig. 3(b)). Some Sn beads and whiskers are covered by SnO2, confirmed by the EDS (Fig. 3(c)). The self-growth of Sn whiskers has been extensively studied, which will not be further addressed here. To explain the reason why Sn mainly fills the narrow branching cracks, the effect of thermal shock-induced tensile stress on Sn distribution should be taken into consideration. During quenching, the surfaces of the samples cool down faster than the core. The tensile stress mainly generates on the surface. The simulated results show that the thermal stress increases with quenching temperatures (Fig. 4(a)). The maximum tensile stress generated at ΔT=800 °C is up to 413 MPa at a quenching time of 0.129 s Fig. 4(b) demonstrates that the higher thermal stress mainly concentrates in the edge areas. The higher thermal stress induces the formation of surface cracks. Once a crack initiates from the sample edges, the stress around its tip is so high and becomes the driving force to push the free Sn atoms forward along the crack propagation direction. Previous literatures reported that inner stress as the driving force accelerated the diffusion of Sn and Ga atoms in the Sn- and Ga-containing MAX bulks and made them spontaneous growth even at room temperature [24–27]. In addition, the quenching temperature is another effective accelerator to promote the diffusion of Sn atoms. The high mobility of Sn driven by thermal shock stresses and temperature seems to favor the transport of Sn into defects around the crack tip, and even cause the loss of Sn. Upon quenching at 600 °C, as a main crack passes through Sn-filling regions, the wider crack space is no longer filled by Sn due to the fast quenching rate. This may interpret the appearance of Sn in the narrow cracks rather than the wide cracks.
peratures, but abnormally increases after quenching at 600 °C. The main mechanism for the electrical conductivity recovery is the filling of the narrow branching cracks by metallic Sn. The mechanism demonstrated here can be applied to other MAX phases containing soft metals (such as Sn, Ga and In) if they exhibit abnormal thermal shock behavior in electrical conductivity. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant nos. 51372015 and 51472024, and Beijing Government Funds for the Constructive Project of Central Universities. The authors also thank Dr. Guoping Bei, at University of Erlangen-Nürnberg, Germany, for EBSD analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
4. Conclusions
[24]
The influence of thermal shock on the electrical conductivity of Ti2SnC has been investigated via water quenching method in the temperature range of 500–800 °C. The electrical conductivity of the quenched Ti2SnC samples gradually decreases with increasing tem-
[25] [26] [27]
4
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