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Synthesis and characterization of high purity Mo2Ti2AlC3 ceramic
Journal of Alloys and Compounds 815 (2020) 152485
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Synthesis and characterization of high purity Mo2Ti2AlC3 ceramic Shuai Fu, Yunlong Liu, Haiwen Zhang, Salvatore Grasso, Chunfeng Hu* Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2019 Received in revised form 19 September 2019 Accepted 27 September 2019 Available online 28 September 2019
High purity Mo2Ti2AlC3 was synthesized by heating mixed elemental powders of Mo, Ti, Al, and C. Dense bulks Mo2Ti2AlC3 were consolidated up to 99%RD by hot pressing at 1400 C for 1 h under 40 MPa. Microstructural analysis, mechanical characterization, and physical properties were reported. Similar to other MAX phases, typical layered structure was observed for Mo2Ti2AlC3. Vickers hardness of 4.81 GPa, elastic modulus of 374.15 GPa, flexural strength of 452 MPa, fracture toughness of 8.4 MPa,m1/2, and compressive strength of 1145 MPa were comparable with other MAX phases. The electrical conductivity measured in the temperature range of 300e600 K had a metallic character, decreasing from 0.41 106 to 0.38 106 U1,m1. The thermal conductivity in the temperature range of 300e1273 K decreased from 6.82 to 6.05 W/m,K. The thermal expansion coefficient measured in the temperature range of 350 e1100 K was 11.3 106 K1. © 2019 Elsevier B.V. All rights reserved.
Keywords: Mo2Ti2AlC3 Synthesis Microstructure Properties
1. Introduction MAX phases are ternary compounds with nano-layered structure and chemical formula of Mnþ1AXn, where M is a transition metal, A is a IIIAeIVA group element, X is carbon or nitrogen, and n ¼ 1e3. There is a growing research interest on these compounds because of their unique properties bridging ceramics to metals [1e10]. Their hexagonal crystal structure with mixed covalent and metallic bonds explains the metallic electrical conductivity and high Young’s modulus. The weakly bonded basal planes contribute to a good thermal shock resistance, graphite-like machinability, and excellent damage tolerance. Al-containing MAX phases, such as Ti3AlC2 [11e13] and Cr2AlC [14e16], have outstanding oxidation resistance resulting from the formation of an dense Al2O3erich passivating scale at temperatures exceeding 1300 C. Aiming to modify the properties of MAX phases (M0 ,M’’)nþ1AlCn, researchers have explored a wide range of chemical compositions. In 2008, mixing M elements was proposed to achieve intermediate properties of MAX phases. For example, in VeCreAleC system MAX phase combined the good high temperature stiffness of V-based MAX phase with the hot corrosion resistance given by Cr [17,18]. Guided by ab initio calculations [19,20], Zhou et al. [21] synthesized
* Corresponding author. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China. E-mail address: [email protected] (C. Hu). https://doi.org/10.1016/j.jallcom.2019.152485 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and identified three (M0 ,M’’)nþ1AlCn phases, including (V0.5Cr0.5)3AlC2, (V0.5Cr0.5)4AlC3, and (V0.5Cr0.5)5Al2C3. This research suggests mixing M elements of MAX phases could: i) overcome thermodynamic instability as in the case of ZreAleC system stabilized by adding Ti [3]; ii) the possibility to tune properties by adjusting the relative fractions. In 2010, a new compound in TieNbeAleC quaternary system was identified by Zheng et al. [22]. (Ti0.5Nb0.5)5AlC4 was the first 514 type MAX phase. In 2013, Ingason et al. [23] synthesized a magnetic MAX phase (Cr0.75Mn0.25)2GeC film. Since then, several new magnetic MAX phase with mixing M elements have been discovered, including (V,Mn)2AlC [24], (V,Mn)3GaC2 [25], (Cr,Mn)2AlC [26], (Cr,Mn)2GeC [27], (Cr,Mn)2GaC [28], and (Mo,Mn)2GaC [29]. Anasori et al. [30,31] identified Mo2TiAlC2 and Mo2Ti2AlC3 MAX phases in 2015. The stacking sequence was MoeTieMoe AleMoeTieMo for Mo2TiAlC2 and MoeTieTieMoeAleMoe TieTieMo for Mo2Ti2AlC3 with Mo layers directly bonded to Al layers, and the carbon atoms occupied the octahedral sites between the transition metal layers. However, the low relative density of Mo2Ti2AlC3 sample, limited to 84%, did not allow reliable measurements of properties. Mo-based (M ¼ Mo) MAX phases have been predicted computationally. However, most of them were not synthesized. Mo2Ti2AlC3 [30], Mo2TiAlC2 [31], Mo2ScAlC2 [32], and Mo2GaC [33] are the only four Mo containing MAX phases produced experimentally. In this work, we synthesized the dense bulk Mo2Ti2AlC3
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and its microstructural, thermo-physical, electrical, and mechanical properties were systematically investigated. 2. Experimental procedure The starting powders were Mo (300 mesh, 99%), Ti (200 mesh, 99.5%), Al (300 mesh, 99%), and graphite (1500 mesh, 99%). The synthesis process is detailed in Refs. [30,31]. Powders were dry mixed in a rotary milling machine for 20 h. After sieving, the powder mixture was placed in an alumina crucible and heated in a tube furnace (BR-17MT, Brother Corp., China). The synthesis was carried out under the inert argon flow (99.9%). The synthesized Mo2Ti2AlC3 powder was used for consolidation of dense bulks. Powder was poured into a graphite die with a diameter of 20 mm. Hot pressing was carried out using a furnace (ZT-50-24Y, Chenhua Corp., China) in vacuum (10 Pa). The temperature was raised to 1400 C at a rate of 10 C/min, and the dwelling time was one hour followed by natural cooling to room temperature. The applied pressure was 40 MPa. The sintering temperature of 1400 C was set following preliminary tests because that a lower temperature resulted in porous materials while higher temperature contributed to the formation of carbides. After sintering, the sample surface was grinded off to remove graphite contamination. For metallographic inspection, the sample was polished down to 1.0 mm diamond grits. The density of sample was measured using Archimedes’ method. A D8 Bruker X-ray diffractometer (XRD) (PA, Almelo, Holland) with Cu Ka (l ¼ 1.54178 Å) was used to determine the phase composition of the sample. Vickers hardness was measured using a micro-hardness tester (HVS-50, Wheng Corp., China) under a force of 9.8 N on the polished surface. Five points were tested. A scanning electron microscope (FEI, Hillaboro Corp., US) equipped with an energy dispersive X-ray spectroscopy (EDS) was used for morphological observation. The elastic modulus was determined using the polished bar of 3 5 19 mm3 by the resonance method using a Grindo Sonic Mk5i (J.W. Lemmens N.V., Belgium), and at least 6 readings were taken for the sample. The flexural strength, fracture toughness, and
compressive strength were measured using an universal testing machine (YCe100KN, Yice Corp., China). For each test, five samples were adopted. The sample sizes used to test three-point bending strength, fracture toughness, and compressive strength were 1.5 2 18 mm3, 2 4 18 mm3, and 2 2 4 mm3 respectively. The testing span of the three-point bending strength and fracture toughness was 16 mm. The cross-head speed was 0.05 mm min1. The fractured surface and the polished etched surface (etched by strong mixed acids of HNO3: HF: H2O ¼ 1 : 1: 1, volumetric fraction) were observed by SEM. Electrical conductivity was measured with a custom designed equipment Namicro-III thermoelectric measurement system (Namicro-III, Joule Yacht Corp., China) from 300 to 600 K using four probes, and the sample size was 3 3 16 mm3. Thermal conductivity was measured by Hot Disk method (TPS2500, Hot Disk Corp., Sweden) and the sample used for testing was a cylinder with 20 mm diameter and 5 mm height. Thermal expansion coefficient was measured using a Thermal Expansion Analyser (DIL402C, Netzsch Corp., Germany) in the temperature range of 350e1100 K under argon flow, the heating rate was set at 1.5 C/ min, and the size of sample was 4 4 3 mm3. 3. Results and discussion In order to clearly describe the properties of Mo2Ti2AlC3, in this section, we compared the properties with Ta4AlC3 which is one of the few known 413 phase. The comparison of Ti3AlC2 gave some insights on the chemical composition, while the comparison with Ta4AlC3 suggested structural induced effects. 3.1. Phase composition and microstructure Fig. 1 shows X-ray diffraction (XRD) patterns of synthesized Mo2Ti2AlC3 powder and dense bulk prepared by hot pressing at 1400 C. It is seen that within the sensitivity limits of the XRD method there are no impurities for both powder and sintered sample (Fig. 1(a) and (b)). In previous work done by Anasori et al. [30], the impurities (3 wt% MoC and 4 wt% Mo3Al) in their samples
Fig. 1. X-ray diffraction (XRD) patterns of (a) synthesized pure Mo2Ti2AlC3 powder and (b) dense Mo2Ti2AlC3 ceramics fabricated by hot-pressing at 1400 C.
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Fig. 2. Scanning electron microscope (SEM) micrographs of dense Mo2Ti2AlC3 ceramics: (a) chemically etched polished surface and (b) fractured surface.
might have affected the physical and mechanical properties. The polished etched surface is shown in Fig. 2(a). Plate-like morphology of Mo2Ti2AlC3 grains with random orientation is apparent. Based on the EDS analysis, the atomic ratio of Mo, Ti, and Al was 1.86 : 1.77: 1. The average grain size was about 5 mm in length and 1.3 mm in width. In the fractured surface of Fig. 2(b), almost no pore is visible confirming the near full density. The layered structure of Mo2Ti2AlC3 is a clear signature of the MAX phase structure. 3.2. Electrical and thermal properties The electrical conductivity of dense Mo2Ti2AlC3 sample in the temperature range of 300e600 K is shown in Fig. 3. The electrical conductivity decreases with increasing temperature, reducing from 0.41 106 to 0.38 106 U1,m1. This trend is similar to other 413 MAX phase, like Ta4AlC3 [34], suggesting a metal-like conductivity. As the temperature increases, the thermally activated movement of metal ions hinders the flow of free electrons. Anasori et al. [30] reported the electrical resistivity of Mo2Ti2AlC3 in the temperature range of 10e300 K. Taking as their reference value of 0.12 106 U1,m1 at 300 K, we report a higher electrical conductivity of 0.41 106 U1,m1. Such difference might be attributed to the relative density and presence on undesired impurities.
For the electrical resistivity, we obtained the expression:
r(mU,m) ¼ r0(1 - b△T) ¼ 2.41(1e0.000324 (300 e T)) with a coefficient of determination, r2, of 0.98. Where r0 is the electrical resistivity at 300 K (mU m), b is the temperature coefficient of resistivity (K1), and T is the absolute temperature (K). The temperature coefficient of resistivity was 0.324 103 K1. The thermal conductivity of bulk Mo2Ti2AlC3 measured in the range of 300e1273 K is displayed in Fig. 4. A least-square fit of the data of thermal conductivity yields the relationship:
l ¼ 7.01464e
0.000792441T
with a coefficient of determination, r2, of 0.98. This trend is similar to the one reported for Ta4AlC3 [34] but opposite to Ti4AlN3 [35] where thermal conductivity increases with the increment of temperature. The measured thermal conductivity of Mo2Ti2AlC3 at 300 K was 6.82 W,(m K)1, which is well below those of Ta4AlC3 (38.4 W,(m K)1) and Ti3AlC2 (40 W,(m K)1). The total thermal conductivity includes both electron and phonon contributions. Because Mo2Ti2AlC3 and Ti4AlN3 are similar in structure and belong to 413 phase, we can speculate that the
Fig. 3. Electrical conductivity and electrical resistivity of bulk Mo2Ti2AlC3 as a function of temperature of 300e600 K.
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Fig. 4. Temperature dependence of thermal conductivity of Mo2Ti2AlC3 measured in 300e1273 K.
value of lelectron can be calculated using the WiedmanneFranz Law (lelectron ¼ L0sT, where s is the electrical conductivity at temperature T, and L0 ¼ 2.45 108 W U K2) [35], was 3.01 W,(m K)1 at 300 K. Therefore, the lphonon was 3.81 W,(m K)1 at 300 K. Fig. 5 gives an overview for the thermal conductivity and electrical resistivity for twenty two MAX phases. The thermal conductivity of Mo2Ti2AlC3 is the lowest one while its electrical conductivity is the second lowest among all the listed MAX phases. This can be explained when considering: (1) The Debye model
evaluation of lattice thermal conductivity [36], thermal conductivity is proportional to the mean phonon free path (the lattice vibration). The crystal structure of 413 phase reflects this and this is in agreement with reduced thermal conductivity of 13.5 W,(m K)1 for Nb4AlC3 and 12 W,(m K)1 for Ti4AlN3. (2) ordered Mo and Ti layers suppress thermal and electrical conductivities. The thermal expansion coefficient of Mo2Ti2AlC3 measured in the temperature range of 350e1100 K is shown in Fig. 6. The average thermal expansion coefficient in the range of 350e1100 K
Fig. 5. Summary of thermal conductivity and electrical resistivity of 22 MAX phases [37].
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Fig. 6. Temperature dependence of bulk dilatometric expansion of Mo2Ti2AlC3 during heating in Ar in the temperature range of 350e1100 K.
was 11.3 106 K1. The value is higher than those of Ti3AlC2 (9.0 106 K1) [38] and Ta4AlC3 (8.2 106 K1) [34]. 3.3. Mechanical properties Table 1 summarizes the physical and mechanical properties of Mo2Ti2AlC3 compared to other Al-containing MAX phases (Ti3AlC2 and Ta4AlC3). The measured density of Mo2Ti2AlC3 was 6.15 g/m3 equivalent to a relative density of 99.3% when assuming a theoretical density of 6.2 g/m3 [30]. The average Vickers hardness was 4.81 ± 0.15 GPa, which is higher than that of Ti3AlC2 (3.5 GPa) [39] but lower than that of Ta4AlC3 (5.1 GPa) [34]. The elastic modulus of Mo2Ti2AlC3 was 374.15 ± 0.16 GPa, which is close to the calculated value (367 GPa) [30], higher than those of Ti3AlC2 (297 GPa) [39] and Ta4AlC3 (324 GPa) [34]. Anasori et al. speculated that the high elastic modulus of Mo2Ti2AlC3 could be attributed to the alter of bond chemistry, improving the elastic properties with respect to their end members [30]. The flexural strength was 452 ± 17 MPa, which is much higher than both of Ti3AlC2 (340 MPa) [39] and Ta4AlC3 (372 MPa) [34]. The high flexural strength resulted from the high relative density and fine grain size (e.g. 5 mm). For example, the flexural strength reported for fine grained Ti2AlC (<10 mm) was
606 MPa, which is larger than 275 MPa for coarse grained (z50 mm) ceramic [40]. Similarly, the compressive strength of Mo2Ti2AlC3 was 1145 ± 57 MPa, which is higher than the ones reported for Ti3AlC2 (764 MPa) [39] and Ta4AlC3 (821 MPa) [34]. The fracture toughness of Mo2Ti2AlC3 was 8.4 ± 0.4 MPa,m1/2, which is slightly higher than those of Ti3AlC2 (7.2 MPa m1/2) [39] and Ta4AlC3 (7.7 MPa,m1/2) [34], but lower than that of a MAX phase solid solution (Nb0.85,Zr0.15)4AlC3 (10.1 MPa,m1/2). That is because of the addition of Zr enhancing the intrinsic ductility of (Nb0.85,Zr0.15)4AlC3, which greatly contributes to the increase of fracture toughness [41]. The damage tolerance was evaluated by the indentation on the polished surface. Fig. 7 shows a SEM image of a Vickers indent under a load of 9.8 N. No cracks propagated at the indent diagonals. Instead, several energy-dispersive modes including delamination, transgranular and intergranular fractures were activated. Therefore, the damage induced by an indentation can be effectively limited in a small region, protecting the integrity of whole bulk. 4. Conclusions High purity Mo2Ti2AlC3 powder was successfully synthesized
Table 1 Comparative analysis of physical and mechanical properties of Mo2Ti2AlC3, Ti3AlC2, and Ta4AlC3. Properties
Mo2Ti2AlC3 [this work]
Mo2Ti2AlC3 [30]
Ti3AlC2 [39]
Ta4AlC3 [34]
Molecular weight (g/mol) Density (g/cm3) Relative density (%) Elastic modulus (GPa) Vickers hardness (GPa) Flexural strength (MPa) Fracture toughness (MPa·m1/2) Compressive strength (MPa) Electrical conductivity (£ 106 U¡1,m¡1) (300 K) Thermal expansion coefficient (£ 10¡6 K¡1) Thermal conductivity (W,(m,K)¡1) (300 K)
350.63 6.15 99.3 374.15 ± 0.16 4.81 ± 0.15 452 ± 17 8.4 ± 0.4 1145 ± 57 0.41 11.3 ± 0.7 6.82
350.63 5.21 84 367(cal.) / / / / 0.12 / /
194.61 4.21 99 297 3.5 340 7.2 764 3.48 9.0 40
786.81 13.18 99 324 5.1 ± 0.1 372 ± 20 7.7 ± 0.5 821 ± 97 2.59 8.4 ± 0.3 38.4
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Fig. 7. SEM micrograph of Vickers indent (9.8 N) on the polished surface of dense Mo2Ti2AlC3 ceramics.
using Mo, Ti, Al, and graphite powders as initial materials and dense bulk Mo2Ti2AlC3 ceramics were consolidated up to 99.3%RD by hot pressing at 1400 C under 40 MPa. It was found that dense Mo2Ti2AlC3 had excellent mechanical properties with low Vickers hardness of 4.81 GPa, high elastic modulus of 374.15 GPa, high flexural strength of 452 MPa, high fracture toughness of 8.4 MPa,m1/2, and high compressive strength of 1145 MPa. Also, it exhibited the high electrical conductivity of 0.41 106 U1,m1 and low thermal conductivity of 6.82 W (m K)1 at 300 K. Its thermal expansion coefficient was measured as 11.3 106 K1, which is high enough to match some metals. Present results endow Mo2Ti2AlC3 a good candidate used in high temperature fields as bulk ceramics or coating on metals. Acknowledgements This work is supported by Thousand Talents Program of Sichuan Province, the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (17kffk01), Outstanding Young Scientific and Technical Talents in Sichuan Province (2019JDJQ0009), and the Natural Sciences Foundation of China (No. 51741208). The authors are grateful to Prof. Qinghui Jiang from Huazhong University of Science and Technology for the help in the measurement of electrical conductivity and Dr. Tatarko (Institute of Inorganic Chemistry, Slovakia) for the help in the measurement of the elastic modulus. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152485. References [1] C. Lai, A. Petruhins, J. Lu, M. Farle, L. Hultman, P. Eklund, J. Rosen, Thermally induced substitutional reaction of Fe into Mo2GaC thin films, Mater. Res. Lett. 5 (2017) 533e539. https://doi.org/10.1080/21663831.2017.1343207. [2] X. Li, B. Liang, Z. Li, Combustion synthesis of Ti2SC, Int. J. Mater. Res. 104 (2013) 1038e1040. https://doi.org/10.3139/146.110972. [3] B. Tunca, T. Lapauw, O.M. Karakulina, M. Batuk, T. Cabioc’h, J. Hadermann, R. Delville, K. Lambrinou, J. Vleugels, Synthesis of MAX phases in the Zr-Ti-AlC system, Inorg. Chem. 56 (2017) 3489e3498. https://doi.org/10.1021/acs. inorgchem.6b03057.
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S. Fu et al. / Journal of Alloys and Compounds 815 (2020) 152485 (2015) 2e7. https://doi.org/10.1063/1.4926611. [30] B. Anasori, M. Dahlqvist, J. Halim, E.J. Moon, J. Lu, B.C. Hosler, E.N. Caspi, n, M.W. Barsoum, Experimental and S.J. May, L. Hultman, P. Eklund, J. Rose theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3, J. Appl. Phys. 118 (2015), 094304. https://doi.org/10.1063/1. 4929640. [31] B. Anasori, J. Halim, J. Lu, C.A. Voigt, L. Hultman, M.W. Barsoum, Mo2TiAlC2 : a new ordered layered ternary carbide, Scr. Mater. 101 (2015) 5e7. https://doi. org/10.1016/j.scriptamat.2014.12.024. [32] R. Meshkian, Q. Tao, M. Dahlqvist, J. Lu, L. Hultman, J. Rosen, Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo2ScAlC2, and its two-dimensional derivate Mo2ScC2 MXene, Acta Mater. 125 (2017) 476e480. https://doi.org/10.1016/j.actamat.2016.12.008. [33] C. Hu, C. Li, J. Halim, S. Kota, D.J. Tallman, M.W. Michel, On the rapid synthesis of the ternary Mo2GaC, J. Am. Ceram. Soc. 98 (2015) 2713e2715. https://doi. org/10.1111/jace.13743. [34] C. Hu, Z. Lin, L. He, Y. Bao, J. Wang, M. Li, Y. Zhou, Physical and mechanical properties of bulk Ta4AlC3 ceramic prepared by an in situ reaction synthesis/ hot-pressing method, J. Am. Ceram. Soc. 90 (2007) 2542e2548. https://doi. org/10.1111/j.1551-2916.2007.01804.x. [35] M.W. Barsoum, C.J. Rawn, T. El-Raghy, A.T. Procopio, W.D. Porter, H. Wang, C.R. Hubbard, Thermal properties of Ti4AlN3, J. Appl. Phys. 87 (2000) 8407e8414. https://doi.org/10.1063/1.373555.
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[36] C. Jeong, S. Datta, M. Lundstrom, Full dispersion versus Debye model evaluation of lattice thermal conductivity with a Landauer approach, J. Appl. Phys. 109 (2011), 073718. https://doi.org/10.1063/1.3567111. [37] M.W. Barsoum, MAX Phase (Properties of Machinable Ternary Carbides and Nitrides), vol. 110, 2013, pp. 158e159. [38] Y.W. Bao, X.H. Wang, H.B. Zhang, Y.C. Zhou, Thermal shock behavior of Ti3AlC2 from between 200 C and 1300 C, J. Eur. Ceram. Soc. 25 (2005) 3367e3374. https://doi.org/10.1016/j.jeurceramsoc.2004.08.026. [39] X. Wang, Y. Zhou, Microstructure and properties of Ti3AlC2 prepared by the solideliquid reaction synthesis and simultaneous in-situ hot pressing process, Acta Mater. 50 (2002) 3143e3151. https://doi.org/10.1016/S1359-6454(02) 00117-9. [40] Y. Bai, X. He, R. Wang, Y. Sun, C. Zhu, S. Wang, G. Chen, High temperature physical and mechanical properties of large-scale Ti2AlC bulk synthesized by self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing, J. Eur. Ceram. Soc. 33 (2013) 2435e2445. https://doi.org/10. 1016/j.jeurceramsoc.2013.04.014. [41] T. Lapauw, D. Tytko, K. Vanmeensel, S. Huang, P. Choi, D. Raabe, E.N. Caspi, O. Ozeri, M. Baben, J.M. Schneider, K. Lambrinou, J. Vleugels, (Nbx,Zr1x)4AlC3 MAX phase solid solutions: processing, mechanical properties, and Density Functional Theory calculations, Inorg. Chem. 55 (2016) 5445. https://doi.org/ 10.1021/acs.inorgchem.6b00484.
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Synthesis and characterization of high purity Mo2Ti2AlC3 ceramic Shuai Fu, Yunlong Liu, Haiwen Zhang, Salvatore Grasso, Chunfeng Hu* Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2019 Received in revised form 19 September 2019 Accepted 27 September 2019 Available online 28 September 2019
High purity Mo2Ti2AlC3 was synthesized by heating mixed elemental powders of Mo, Ti, Al, and C. Dense bulks Mo2Ti2AlC3 were consolidated up to 99%RD by hot pressing at 1400 C for 1 h under 40 MPa. Microstructural analysis, mechanical characterization, and physical properties were reported. Similar to other MAX phases, typical layered structure was observed for Mo2Ti2AlC3. Vickers hardness of 4.81 GPa, elastic modulus of 374.15 GPa, flexural strength of 452 MPa, fracture toughness of 8.4 MPa,m1/2, and compressive strength of 1145 MPa were comparable with other MAX phases. The electrical conductivity measured in the temperature range of 300e600 K had a metallic character, decreasing from 0.41 106 to 0.38 106 U1,m1. The thermal conductivity in the temperature range of 300e1273 K decreased from 6.82 to 6.05 W/m,K. The thermal expansion coefficient measured in the temperature range of 350 e1100 K was 11.3 106 K1. © 2019 Elsevier B.V. All rights reserved.
Keywords: Mo2Ti2AlC3 Synthesis Microstructure Properties
1. Introduction MAX phases are ternary compounds with nano-layered structure and chemical formula of Mnþ1AXn, where M is a transition metal, A is a IIIAeIVA group element, X is carbon or nitrogen, and n ¼ 1e3. There is a growing research interest on these compounds because of their unique properties bridging ceramics to metals [1e10]. Their hexagonal crystal structure with mixed covalent and metallic bonds explains the metallic electrical conductivity and high Young’s modulus. The weakly bonded basal planes contribute to a good thermal shock resistance, graphite-like machinability, and excellent damage tolerance. Al-containing MAX phases, such as Ti3AlC2 [11e13] and Cr2AlC [14e16], have outstanding oxidation resistance resulting from the formation of an dense Al2O3erich passivating scale at temperatures exceeding 1300 C. Aiming to modify the properties of MAX phases (M0 ,M’’)nþ1AlCn, researchers have explored a wide range of chemical compositions. In 2008, mixing M elements was proposed to achieve intermediate properties of MAX phases. For example, in VeCreAleC system MAX phase combined the good high temperature stiffness of V-based MAX phase with the hot corrosion resistance given by Cr [17,18]. Guided by ab initio calculations [19,20], Zhou et al. [21] synthesized
* Corresponding author. School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, China. E-mail address: [email protected] (C. Hu). https://doi.org/10.1016/j.jallcom.2019.152485 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and identified three (M0 ,M’’)nþ1AlCn phases, including (V0.5Cr0.5)3AlC2, (V0.5Cr0.5)4AlC3, and (V0.5Cr0.5)5Al2C3. This research suggests mixing M elements of MAX phases could: i) overcome thermodynamic instability as in the case of ZreAleC system stabilized by adding Ti [3]; ii) the possibility to tune properties by adjusting the relative fractions. In 2010, a new compound in TieNbeAleC quaternary system was identified by Zheng et al. [22]. (Ti0.5Nb0.5)5AlC4 was the first 514 type MAX phase. In 2013, Ingason et al. [23] synthesized a magnetic MAX phase (Cr0.75Mn0.25)2GeC film. Since then, several new magnetic MAX phase with mixing M elements have been discovered, including (V,Mn)2AlC [24], (V,Mn)3GaC2 [25], (Cr,Mn)2AlC [26], (Cr,Mn)2GeC [27], (Cr,Mn)2GaC [28], and (Mo,Mn)2GaC [29]. Anasori et al. [30,31] identified Mo2TiAlC2 and Mo2Ti2AlC3 MAX phases in 2015. The stacking sequence was MoeTieMoe AleMoeTieMo for Mo2TiAlC2 and MoeTieTieMoeAleMoe TieTieMo for Mo2Ti2AlC3 with Mo layers directly bonded to Al layers, and the carbon atoms occupied the octahedral sites between the transition metal layers. However, the low relative density of Mo2Ti2AlC3 sample, limited to 84%, did not allow reliable measurements of properties. Mo-based (M ¼ Mo) MAX phases have been predicted computationally. However, most of them were not synthesized. Mo2Ti2AlC3 [30], Mo2TiAlC2 [31], Mo2ScAlC2 [32], and Mo2GaC [33] are the only four Mo containing MAX phases produced experimentally. In this work, we synthesized the dense bulk Mo2Ti2AlC3
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and its microstructural, thermo-physical, electrical, and mechanical properties were systematically investigated. 2. Experimental procedure The starting powders were Mo (300 mesh, 99%), Ti (200 mesh, 99.5%), Al (300 mesh, 99%), and graphite (1500 mesh, 99%). The synthesis process is detailed in Refs. [30,31]. Powders were dry mixed in a rotary milling machine for 20 h. After sieving, the powder mixture was placed in an alumina crucible and heated in a tube furnace (BR-17MT, Brother Corp., China). The synthesis was carried out under the inert argon flow (99.9%). The synthesized Mo2Ti2AlC3 powder was used for consolidation of dense bulks. Powder was poured into a graphite die with a diameter of 20 mm. Hot pressing was carried out using a furnace (ZT-50-24Y, Chenhua Corp., China) in vacuum (10 Pa). The temperature was raised to 1400 C at a rate of 10 C/min, and the dwelling time was one hour followed by natural cooling to room temperature. The applied pressure was 40 MPa. The sintering temperature of 1400 C was set following preliminary tests because that a lower temperature resulted in porous materials while higher temperature contributed to the formation of carbides. After sintering, the sample surface was grinded off to remove graphite contamination. For metallographic inspection, the sample was polished down to 1.0 mm diamond grits. The density of sample was measured using Archimedes’ method. A D8 Bruker X-ray diffractometer (XRD) (PA, Almelo, Holland) with Cu Ka (l ¼ 1.54178 Å) was used to determine the phase composition of the sample. Vickers hardness was measured using a micro-hardness tester (HVS-50, Wheng Corp., China) under a force of 9.8 N on the polished surface. Five points were tested. A scanning electron microscope (FEI, Hillaboro Corp., US) equipped with an energy dispersive X-ray spectroscopy (EDS) was used for morphological observation. The elastic modulus was determined using the polished bar of 3 5 19 mm3 by the resonance method using a Grindo Sonic Mk5i (J.W. Lemmens N.V., Belgium), and at least 6 readings were taken for the sample. The flexural strength, fracture toughness, and
compressive strength were measured using an universal testing machine (YCe100KN, Yice Corp., China). For each test, five samples were adopted. The sample sizes used to test three-point bending strength, fracture toughness, and compressive strength were 1.5 2 18 mm3, 2 4 18 mm3, and 2 2 4 mm3 respectively. The testing span of the three-point bending strength and fracture toughness was 16 mm. The cross-head speed was 0.05 mm min1. The fractured surface and the polished etched surface (etched by strong mixed acids of HNO3: HF: H2O ¼ 1 : 1: 1, volumetric fraction) were observed by SEM. Electrical conductivity was measured with a custom designed equipment Namicro-III thermoelectric measurement system (Namicro-III, Joule Yacht Corp., China) from 300 to 600 K using four probes, and the sample size was 3 3 16 mm3. Thermal conductivity was measured by Hot Disk method (TPS2500, Hot Disk Corp., Sweden) and the sample used for testing was a cylinder with 20 mm diameter and 5 mm height. Thermal expansion coefficient was measured using a Thermal Expansion Analyser (DIL402C, Netzsch Corp., Germany) in the temperature range of 350e1100 K under argon flow, the heating rate was set at 1.5 C/ min, and the size of sample was 4 4 3 mm3. 3. Results and discussion In order to clearly describe the properties of Mo2Ti2AlC3, in this section, we compared the properties with Ta4AlC3 which is one of the few known 413 phase. The comparison of Ti3AlC2 gave some insights on the chemical composition, while the comparison with Ta4AlC3 suggested structural induced effects. 3.1. Phase composition and microstructure Fig. 1 shows X-ray diffraction (XRD) patterns of synthesized Mo2Ti2AlC3 powder and dense bulk prepared by hot pressing at 1400 C. It is seen that within the sensitivity limits of the XRD method there are no impurities for both powder and sintered sample (Fig. 1(a) and (b)). In previous work done by Anasori et al. [30], the impurities (3 wt% MoC and 4 wt% Mo3Al) in their samples
Fig. 1. X-ray diffraction (XRD) patterns of (a) synthesized pure Mo2Ti2AlC3 powder and (b) dense Mo2Ti2AlC3 ceramics fabricated by hot-pressing at 1400 C.
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Fig. 2. Scanning electron microscope (SEM) micrographs of dense Mo2Ti2AlC3 ceramics: (a) chemically etched polished surface and (b) fractured surface.
might have affected the physical and mechanical properties. The polished etched surface is shown in Fig. 2(a). Plate-like morphology of Mo2Ti2AlC3 grains with random orientation is apparent. Based on the EDS analysis, the atomic ratio of Mo, Ti, and Al was 1.86 : 1.77: 1. The average grain size was about 5 mm in length and 1.3 mm in width. In the fractured surface of Fig. 2(b), almost no pore is visible confirming the near full density. The layered structure of Mo2Ti2AlC3 is a clear signature of the MAX phase structure. 3.2. Electrical and thermal properties The electrical conductivity of dense Mo2Ti2AlC3 sample in the temperature range of 300e600 K is shown in Fig. 3. The electrical conductivity decreases with increasing temperature, reducing from 0.41 106 to 0.38 106 U1,m1. This trend is similar to other 413 MAX phase, like Ta4AlC3 [34], suggesting a metal-like conductivity. As the temperature increases, the thermally activated movement of metal ions hinders the flow of free electrons. Anasori et al. [30] reported the electrical resistivity of Mo2Ti2AlC3 in the temperature range of 10e300 K. Taking as their reference value of 0.12 106 U1,m1 at 300 K, we report a higher electrical conductivity of 0.41 106 U1,m1. Such difference might be attributed to the relative density and presence on undesired impurities.
For the electrical resistivity, we obtained the expression:
r(mU,m) ¼ r0(1 - b△T) ¼ 2.41(1e0.000324 (300 e T)) with a coefficient of determination, r2, of 0.98. Where r0 is the electrical resistivity at 300 K (mU m), b is the temperature coefficient of resistivity (K1), and T is the absolute temperature (K). The temperature coefficient of resistivity was 0.324 103 K1. The thermal conductivity of bulk Mo2Ti2AlC3 measured in the range of 300e1273 K is displayed in Fig. 4. A least-square fit of the data of thermal conductivity yields the relationship:
l ¼ 7.01464e
0.000792441T
with a coefficient of determination, r2, of 0.98. This trend is similar to the one reported for Ta4AlC3 [34] but opposite to Ti4AlN3 [35] where thermal conductivity increases with the increment of temperature. The measured thermal conductivity of Mo2Ti2AlC3 at 300 K was 6.82 W,(m K)1, which is well below those of Ta4AlC3 (38.4 W,(m K)1) and Ti3AlC2 (40 W,(m K)1). The total thermal conductivity includes both electron and phonon contributions. Because Mo2Ti2AlC3 and Ti4AlN3 are similar in structure and belong to 413 phase, we can speculate that the
Fig. 3. Electrical conductivity and electrical resistivity of bulk Mo2Ti2AlC3 as a function of temperature of 300e600 K.
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Fig. 4. Temperature dependence of thermal conductivity of Mo2Ti2AlC3 measured in 300e1273 K.
value of lelectron can be calculated using the WiedmanneFranz Law (lelectron ¼ L0sT, where s is the electrical conductivity at temperature T, and L0 ¼ 2.45 108 W U K2) [35], was 3.01 W,(m K)1 at 300 K. Therefore, the lphonon was 3.81 W,(m K)1 at 300 K. Fig. 5 gives an overview for the thermal conductivity and electrical resistivity for twenty two MAX phases. The thermal conductivity of Mo2Ti2AlC3 is the lowest one while its electrical conductivity is the second lowest among all the listed MAX phases. This can be explained when considering: (1) The Debye model
evaluation of lattice thermal conductivity [36], thermal conductivity is proportional to the mean phonon free path (the lattice vibration). The crystal structure of 413 phase reflects this and this is in agreement with reduced thermal conductivity of 13.5 W,(m K)1 for Nb4AlC3 and 12 W,(m K)1 for Ti4AlN3. (2) ordered Mo and Ti layers suppress thermal and electrical conductivities. The thermal expansion coefficient of Mo2Ti2AlC3 measured in the temperature range of 350e1100 K is shown in Fig. 6. The average thermal expansion coefficient in the range of 350e1100 K
Fig. 5. Summary of thermal conductivity and electrical resistivity of 22 MAX phases [37].
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Fig. 6. Temperature dependence of bulk dilatometric expansion of Mo2Ti2AlC3 during heating in Ar in the temperature range of 350e1100 K.
was 11.3 106 K1. The value is higher than those of Ti3AlC2 (9.0 106 K1) [38] and Ta4AlC3 (8.2 106 K1) [34]. 3.3. Mechanical properties Table 1 summarizes the physical and mechanical properties of Mo2Ti2AlC3 compared to other Al-containing MAX phases (Ti3AlC2 and Ta4AlC3). The measured density of Mo2Ti2AlC3 was 6.15 g/m3 equivalent to a relative density of 99.3% when assuming a theoretical density of 6.2 g/m3 [30]. The average Vickers hardness was 4.81 ± 0.15 GPa, which is higher than that of Ti3AlC2 (3.5 GPa) [39] but lower than that of Ta4AlC3 (5.1 GPa) [34]. The elastic modulus of Mo2Ti2AlC3 was 374.15 ± 0.16 GPa, which is close to the calculated value (367 GPa) [30], higher than those of Ti3AlC2 (297 GPa) [39] and Ta4AlC3 (324 GPa) [34]. Anasori et al. speculated that the high elastic modulus of Mo2Ti2AlC3 could be attributed to the alter of bond chemistry, improving the elastic properties with respect to their end members [30]. The flexural strength was 452 ± 17 MPa, which is much higher than both of Ti3AlC2 (340 MPa) [39] and Ta4AlC3 (372 MPa) [34]. The high flexural strength resulted from the high relative density and fine grain size (e.g. 5 mm). For example, the flexural strength reported for fine grained Ti2AlC (<10 mm) was
606 MPa, which is larger than 275 MPa for coarse grained (z50 mm) ceramic [40]. Similarly, the compressive strength of Mo2Ti2AlC3 was 1145 ± 57 MPa, which is higher than the ones reported for Ti3AlC2 (764 MPa) [39] and Ta4AlC3 (821 MPa) [34]. The fracture toughness of Mo2Ti2AlC3 was 8.4 ± 0.4 MPa,m1/2, which is slightly higher than those of Ti3AlC2 (7.2 MPa m1/2) [39] and Ta4AlC3 (7.7 MPa,m1/2) [34], but lower than that of a MAX phase solid solution (Nb0.85,Zr0.15)4AlC3 (10.1 MPa,m1/2). That is because of the addition of Zr enhancing the intrinsic ductility of (Nb0.85,Zr0.15)4AlC3, which greatly contributes to the increase of fracture toughness [41]. The damage tolerance was evaluated by the indentation on the polished surface. Fig. 7 shows a SEM image of a Vickers indent under a load of 9.8 N. No cracks propagated at the indent diagonals. Instead, several energy-dispersive modes including delamination, transgranular and intergranular fractures were activated. Therefore, the damage induced by an indentation can be effectively limited in a small region, protecting the integrity of whole bulk. 4. Conclusions High purity Mo2Ti2AlC3 powder was successfully synthesized
Table 1 Comparative analysis of physical and mechanical properties of Mo2Ti2AlC3, Ti3AlC2, and Ta4AlC3. Properties
Mo2Ti2AlC3 [this work]
Mo2Ti2AlC3 [30]
Ti3AlC2 [39]
Ta4AlC3 [34]
Molecular weight (g/mol) Density (g/cm3) Relative density (%) Elastic modulus (GPa) Vickers hardness (GPa) Flexural strength (MPa) Fracture toughness (MPa·m1/2) Compressive strength (MPa) Electrical conductivity (£ 106 U¡1,m¡1) (300 K) Thermal expansion coefficient (£ 10¡6 K¡1) Thermal conductivity (W,(m,K)¡1) (300 K)
350.63 6.15 99.3 374.15 ± 0.16 4.81 ± 0.15 452 ± 17 8.4 ± 0.4 1145 ± 57 0.41 11.3 ± 0.7 6.82
350.63 5.21 84 367(cal.) / / / / 0.12 / /
194.61 4.21 99 297 3.5 340 7.2 764 3.48 9.0 40
786.81 13.18 99 324 5.1 ± 0.1 372 ± 20 7.7 ± 0.5 821 ± 97 2.59 8.4 ± 0.3 38.4
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Fig. 7. SEM micrograph of Vickers indent (9.8 N) on the polished surface of dense Mo2Ti2AlC3 ceramics.
using Mo, Ti, Al, and graphite powders as initial materials and dense bulk Mo2Ti2AlC3 ceramics were consolidated up to 99.3%RD by hot pressing at 1400 C under 40 MPa. It was found that dense Mo2Ti2AlC3 had excellent mechanical properties with low Vickers hardness of 4.81 GPa, high elastic modulus of 374.15 GPa, high flexural strength of 452 MPa, high fracture toughness of 8.4 MPa,m1/2, and high compressive strength of 1145 MPa. Also, it exhibited the high electrical conductivity of 0.41 106 U1,m1 and low thermal conductivity of 6.82 W (m K)1 at 300 K. Its thermal expansion coefficient was measured as 11.3 106 K1, which is high enough to match some metals. Present results endow Mo2Ti2AlC3 a good candidate used in high temperature fields as bulk ceramics or coating on metals. Acknowledgements This work is supported by Thousand Talents Program of Sichuan Province, the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (17kffk01), Outstanding Young Scientific and Technical Talents in Sichuan Province (2019JDJQ0009), and the Natural Sciences Foundation of China (No. 51741208). The authors are grateful to Prof. Qinghui Jiang from Huazhong University of Science and Technology for the help in the measurement of electrical conductivity and Dr. Tatarko (Institute of Inorganic Chemistry, Slovakia) for the help in the measurement of the elastic modulus. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152485. References [1] C. Lai, A. Petruhins, J. Lu, M. Farle, L. Hultman, P. Eklund, J. Rosen, Thermally induced substitutional reaction of Fe into Mo2GaC thin films, Mater. Res. Lett. 5 (2017) 533e539. https://doi.org/10.1080/21663831.2017.1343207. [2] X. Li, B. Liang, Z. Li, Combustion synthesis of Ti2SC, Int. J. Mater. Res. 104 (2013) 1038e1040. https://doi.org/10.3139/146.110972. [3] B. Tunca, T. Lapauw, O.M. Karakulina, M. Batuk, T. Cabioc’h, J. Hadermann, R. Delville, K. Lambrinou, J. Vleugels, Synthesis of MAX phases in the Zr-Ti-AlC system, Inorg. Chem. 56 (2017) 3489e3498. https://doi.org/10.1021/acs. inorgchem.6b03057.
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