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Synthesis and thermal and electrical properties of bulk Cr2AlC
Scripta Materialia 54 (2006) 841–846 www.actamat-journals.com
Synthesis and thermal and electrical properties of bulk Cr2AlC Wubian Tian a
a,b
, Peiling Wang a,*, Guojun Zhang a, Yanmei Kan a, Yongxiang Li a, Dongsheng Yan a
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China b Graduate School of Chinese Academy of Sciences, China Received 15 July 2005; received in revised form 4 November 2005; accepted 10 November 2005 Available online 29 November 2005
Abstract Cr2AlC ceramics were fabricated by hot-pressing using Cr, Al and C powders as starting materials. The phase assemblages of the samples consisted of Cr2AlC, as a major crystalline phase, together with a very small amount of Cr7C3 and an unknown phase. Its thermal and electrical as well as mechanical properties were determined. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cr2AlC; Hot-pressing; Ceramics; Electrical properties; Thermal conductivity
1. Introduction Nowotny reported the work that he and his coauthors performed on the syntheses of a great numbers of carbides and nitrides in 1970 [1]. These compounds belong to a large class of solids with the general formula Mn+1AXn, where n = 1, 2, 3, M is an early transition metal, A is an A-group (mostly IIIA or IVA) element, and X is C or N. Among the M3AX2 compounds, there are now lots of papers dealing with the synthesis of Ti3SiC2, and the characterization of its electrical, thermal and oxidation behavior, since Ti3SiC2 is a remarkable material that combines many of the best attributes of both metals and ceramics [2–6]. In contrast, research work on M2AX compounds, which has more than thirty so-called H- or Ha¨gg phases, is scarce. It is noted however, that the fabrication of single phase, bulk dense samples of Ti3SiC2 has proved to be much more elusive [7]. As a consequence, little was known about Ti3SiC2, and much of what was known has been shown to be incorrect [7], until the great progress in synthesis was made by
*
Corresponding author. Tel.: +86 21 5241 2324; fax: +86 21 5241 3122. E-mail address: [email protected] (P. Wang).
BarsoumÕs group in 1996 [2]. The research approach to Ti3SiC2 revealed the importance of the successfully fabricating a pure phase to develop a new material. For M2AX phases, no systematic study is available in the literature [8], and the work has mainly focused on Ti2AlC, Nb2AlC and the solid solutions [9,11–17], although it has been known that M2AX phases are good thermal and electrical conductors [10,12], and they exhibit a high modulus but are relatively soft and readily machinable [11,13,16]. Recently, Sun and co-workers performed theoretical studies of the bulk modulus of M2AlC, where M = Ti, V, Cr, by means of ab initio total energy calculations [8]. The results have shown that the bulk modulus of M2AlC increases as Ti is substituted with V and Cr by 19% and 36%, respectively, which is associated with an extensive increase in the M–Al and M–C bond energy [8]. Lately, the same authors used the same method to calculate the elastic properties of M2AlC, with M = Ti, V, Cr, Nb and Ta, and reported that Cr2AlC possessed the highest bulk, Shear and YoungÕs modulus among the five hexagonal phases [18]. These results encouraged us to study the synthesis, elastic and electrical properties of Cr2AlC. In this paper, the fabrication of bulk polycrystalline and dense samples of Cr2AlC is reported. The thermal
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.11.009
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and electric as well as mechanical properties of sintered samples are also described. 2. Experimental procedure The starting materials used in the present work were chromium (200 mesh, 99.95%, Shanghai Chemical Reagent Company of National Medicine Group), aluminum (100– 200 mesh, 99.95%, Shanghai Chemical Reagent Company of National Medicine Group) and graphite (3200 mesh, 99%, Shanghai Colloid Chemical Plant). The powders weighed according to the designed composition (Cr:Al:C = 2:1.1:1) were milled in absolute alcohol for 24 h. Pellets of dried powders were hot-pressed at 1400 C for 1 h under 20 MPa in an argon atmosphere. The densities of sintered samples were measured by the Archimedes principle. Phase assemblages were determined based on X-ray diffraction (XRD) patterns from diffractometer. The lattice parameters of Cr2AlC phase were determined by XRD using a Guinier–Ha¨gg camera with Cu Ka1 radiation and Si as an internal standard. Microstructural observation of fracture surfaces of the sample was performed under a field emissive scanning electron microscope (SEM, JEOL JSE-6700F, Japan). SEM and energy dispersive spectroscopy (EDS) analyses of polished surface of the sample were performed under an electron probe microanalyzer (JXA-8100F, Japan) equipped with EDS (Oxford INCA energy). The sample was etched in HF acid for 2 min before SEM observation. The Vickers hardness was determined by indentation using a Vickers diamond indenter and a load of 50 N for 10 s (Akashi). The flexural strength was determined via three-point bending with dimensions of 2.9 mm · 2.6 mm · 35 mm. The electrical resistances and their temperature dependencies, in the temperature range of 77–270 K, were measured with a 4-probe technique. The heat capacity, Cp, was measured using a differential scanning calorimeter (DSC-2C) under nitrogen at a heating rate of 10 K/min, in which the sample had a cylindrical shape with 5.5 mm in diameter and 0.8 mm in thickness. The coefficient of temperature conductivity, a, was measured using a laser-flash technique with a cylindrical sample (B10 mm · 2.5 mm). The thermal conductivity, k (W/m K), was calculated by k ¼ d C p a 101
dynamic YoungÕs modulus is determined using the resonant frequency in the flexural mode of vibration. The dynamic YoungÕs modulus and dynamic Shear modulus were used to compute the PoissonÕs ratio. 3. Results and discussion 3.1. Sample characterization and mechanical properties The XRD pattern of the sample hot-pressed at 1400 C for 1 h is shown in Fig. 1. It is found that the phase assemblage of sintered sample consisted of the Cr2AlC phase, as the major crystalline phase, together with a very small amount of Cr7C3 and a trace amount of an unknown phase, and the amount of the Cr2AlC phase reached as high as 95 wt.%. As the first stage, a stoichiometric composition Cr2AlC was tried for synthesis of the phase. The results showed that the sintered sample contained Cr2AlC and Cr7C3 with weight ratios of 90–10, revealing the lack of aluminum in the composition. An excess of Al was therefore used for the composition in the following experiments. It was found, however, that as well as the excess amount of aluminum, sintering temperature, holding time and atmosphere affected the amount of Cr2AlC phase formed in the sample and bulk densities of the materials. The investigation of the reaction process and mechanism for the Cr2AlC material are proceeding. The lattice parameters of Cr2AlC were determined by Guinier–Ha¨gg film data and related programs as described in the experimental section. The cell dimensions obtained ˚ and c = 12.814(3) A ˚, for the Cr2AlC phase, a = 2.863(4) A ˚ and c = 12.82 A ˚ are very close to the values of a = 2.86 A that were reported in the JCPDS card. The measured density of sintered sample was 5.21 g/cm3, which was around 98% of the theoretical density calculated based on the densities of Cr2AlC (5.229 g/cm3, JCPDS 29-0017) and Cr7C3 (6.877, JCPDS 36-1482).
ð1Þ
where d, Cp and a are, respectively, the density (g/cm3), the heat capacity (J/kg K) and the coefficient of temperature conductivity (cm2/s). The coefficient of thermal expansion (K1) was measured by means of thermal dilatometer (NETZSCH DIL 402C, Germany) in the temperature range 30–1200 C at a heating rate of 5 C/min in air. According to the ASTM standard (test method for YoungÕs modulus, Shear modulus and PoissonÕs ratio for advanced ceramics by impulse excitation of vibration), a
Fig. 1. XRD pattern of the sample hot-pressed at 1400 C for 1 h.
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Table 1 Atomic ratios of Cr, Al and C of gray and white grains in BSE micrograph by EDS analysis
Gray grains White grains
Cr
Al
C
Cr/Al
48.43 63.90
23.69
27.88 34.01
2.04
Cr/C 1.88
film spattered on the surface of the sample that is necessary for SEM observation. The hardness of the sample is around 3.5 GPa, a value that is similar to Ti3SiC2 [2] and other M2AX materials [11,13,16]. It was found, however, that no indentation cracks were observed and it was therefore hard to estimate Fig. 2. SEM micrograph of fracture surface of sample.
A typical SEM micrograph of the fracture surface of the sample is shown in Fig. 2. Several areas with cleavage planes are apparent, confirming the layered nature of Cr2AlC. Scattered electron (SE) and back scattered electron (BSE) SEM micrographs of polished surface of the material in the same area are shown in Fig. 3(a) and (b), respectively. It is noted that the grains of the sample are hexagonal shaped. The EDS patterns of grains of gray and white color are shown in Fig. 3(c) and (d), respectively. The average atomic ratio of Cr/Al of the gray grains that was calculated from EDS analysis to be around 2.04, listed in Table 1, corresponds to the Cr2AlC phase. It is noted, however, that the amount of C is higher than the values in formula Cr2AlC and Cr7C3, which is caused by the C
Fig. 4. Optical micrograph of the damage region of sample around the indentation.
Fig. 3. (a) SE SEM micrograph and (b) BSE SEM micrograph of polished surface of the sample in the same area; (c) EDS pattern of grain with gray color and (d) EDS pattern of grain with white color, shown in (b).
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the length of the microcracks from indentation during the determination of hardness, so as to calculate the toughness of the material. The damage region around the indentation is shown in Fig. 4. The figure is remarkably similar to that of Ti3SiC2 [19], in which instead of sinking into the surface, the indentations accumulate as a result of a combination of
grain pullout, breakup and buckling, extending to a limited distance and absorbing most of damage energy. The sintered material was very easy to machine as expected. Holes were drillable using a commonly available high-speed tool steel drill bit without lubrication. This machinable property is also similar to that of Ti3SiC2. 3.2. Thermal and electrical properties The average thermal expansion coefficient of the sample in the range of 30–1200 C was 1.33 · 105 (K1); this is close to, but a little higher than, that of Ti3Al1.1C1.8 (9.0 · 106 (K1)) measured from 25 to 1200 C [20]. The coefficient of temperature conductivity of sample was determined in the range of 200–400 C. It is found that the coefficients of temperature conductivity decreased with the increase of temperature, as shown in Fig. 5(a). The specific heat capacity of the sample increases linearly from 50 to 450 C, as shown in Fig. 5(b). The solid line in Fig. 5(b) represents a third-order polynomial fit to the Debye model. The heat capacity at 25 C, namely, 590 J/(kg K) obtained from Fig. 5(b), is larger than that of Ti2AlC (581 J/(kg K)) [12] and Cr (460 J/(kg K)). According to Eq. (1), the thermal conductivities of the sample, k, between 200 and 400 C can be calculated. It is found that the thermal conductivity decreases slightly with the increase in temperature, as shown in Fig. 5(c). The least-squares fit of the data is shown as a straight solid line in Fig. 5(c). That is k ¼ 18:6 0:0023T The thermal conductivity of the sample at 200 C is 17.5 W/(m K), shown in Table 2, which is in the range of 15–45 W/(m K) for M2AlC (M = Nb, (Ti, Nb) and Ti) at room temperature and its variation with temperature is very similar to that of Ti3SiC2 and Ti2AlC [12]. The temperature dependence of resistivity of Cr2AlC sample is shown in Fig. 6. It can be seen that the resistivi-
Fig. 5. Temperature dependence of (a) coefficient of temperature conductivity, (b) specific heat capacity and (c) thermal conductivity of Cr2AlC sample.
Fig. 6. Temperature dependence of resistivity of Cr2AlC sample hotpressed at 1400 C for 1 h.
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Table 2 Some properties of Cr2AlC in comparison to Ti2AlC, Ti3Al1.1C1.8 and Ti3SiC2 Property
Value Cr2AlC
Ti2AlC [9]
Ti3Al1.1C1.8 [20]
Ti3SiC2 [20]
Elementary cell ˚) a (A ˚) c (A Theoretical density (g/cm3) Measured density (g/cm3) Coefficient of thermal expansion (K1) Special heat capacity at 25 C J/(kg K) Thermal conductivity at 200 C W/(m K) Electrical conductivity (S/m) Temperature coefficient of resistivity, b (K1) YoungÕs modulus (GPa) Shear modulus (GPa) PoissonÕs ratio Flexural strength (MPa) Hardness (GPa)
Hexagonal 2.863(4) 12.814(3) 5.229 5.21 1.33 · 105 590 17.5 1.4 · 106 0.0028 278 116 0.153 378 3.5
Hexagonal 3.051 13.637 4.113 –a 8.2 · 106 579 41.7 [12] 2.7 · 106 0.0035 [12] –a –a –a –a 4.5
Hexagonal 3.0654 18.487 4.247 4.2 9.0 · 106 –a –a 2.9 · 106 0.0031 297 124 0.2 375 ± 15 3.5
Hexagonal 3.0665 17.671 4.531 4.5 9.2 · 106 588 [2] 43 (at room temp.) [2] 4.5 · 106 0.004 333 139 0.2 260 ± 20 [2] 4 [2,20]
a
Not reported.
ties of the sample, q, increase linearly with the increase in temperature from 75 to 269 K and the temperature coefficient of resistivity is 0.0028 K1, which is very close to that of Ti3Al1.1C1.8, at 0.0031 K1 [20]. The electrical conductivity at room temperature calculated by the extrapolated value of resistivity is 1.4 · 106 X1 m1, which is slightly lower than that of Ti3Al1.1C1.8 (2.9 · 106 X1 m1) [20]. The YoungÕs modulus and Shear modulus of Cr2AlC, 278 and 116 GPa, respectively, are also very comparable to that of Ti3Al1.1C1.8. Some properties of Cr2AlC are summarized in Table 2, together with those of Ti3Al1.1C1.8 for comparison. 4. Conclusion Cr2AlC ceramics were prepared by hot-pressing using the proper ratio of chromium, aluminum and graphite powders at 1400 C for 1 h; the density was around 98% of the theoretical value. The sintered material consisted of Cr2AlC and Cr7C3 with a weight ratio of 95–5, together with a trace amount of an unknown phase. SEM micrographs of fractured surfaces leave no doubt as to its layered nature. The EDS results are in good agreement with the XRD analysis. The sample has comparable properties with Ti3Al1.1C1.8. It is relatively soft (Vickers hardness of 3.5 GPa) and elastically stiff (YoungÕs modulus of 278 GPa, Shear modulus of 116 GPa). The thermal conductivity decreases slightly with the increase in temperature in the range of 200–400 C. The electrical conductivity decreased linearly with the increase in temperature from 75 to 269 K, and the extrapolated value at room temperature was 1.4 · 106 X1 m1. The coefficient of thermal expansion in the temperature range 30–1200 C, the thermal conductivity at 200 C and the heat capacity at 25 C, were 1.33 · 105 (K1), 17.5 W/(m K), and 590 J/ (kg K), respectively.
It should be pointed out that mechanical and thermal as well as electric properties of Cr2AlC reported here might not reach the maximum values since some synthesis parameters (composition, temperature, holding time and atmosphere, etc.) could be further optimized in the process of understanding the reaction mechanism of the material. Anyway, it is noted that the higher value of the heat capacity of Cr2AlC compared to Ti3SiC2 and other MAX compounds would be beneficial for its use in the heat insulation field. Acknowledgments This financial support from the Science and Technology Commission of Shanghai was highly appreciated (Contract No. 04JC14076). We thank Professors Jef Vleugels and Omer Van der Biest, Dept. of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Belgium, for carrying out the YoungÕs modulus measurements. Thanks also to Professor Tonggeng Xi, Shanghai Institute of Ceramics, for the helpful discussion of the thermal properties of the sample. References [1] Nowotny H. Prog Solid State Chem 1970;2:27. [2] Barsoum MW, El-Raghy T. J Am Ceram Soc 1996;79:1953. [3] El-Raghy T, Barsoum MW, Zavaliangos A, Kalidindi S. J Am Ceram Soc 1999;82:2849. [4] El-Raghy T, Barsoum MW, Zavaliangos A, Kalidindi S. J Am Ceram Soc 1999;82:2855. [5] Sun ZM, Zhou YC, Li S. Acta Mater 2001;49:4347. [6] Barsoum MW, El-Raghy T. J Phys Chem Solids 1999;60:429. [7] Barsoum MW, Brodkin D, El-Raghy T. Scripta Metall Mater 1997;36:535. [8] Sun Z, Ahuja R, Li S, Schneider Jochen M. Appl Phys Lett 2003;83:899. [9] Barsoum MW, Ali M, El-Raghy T. Metall Mater Trans 2000;31A:1857.
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[10] Barsoum MW, Golczewski J, Seifert HJ, Aldinger F. J Alloys Compd 2002;340:173. [11] Salama I, El-Raghy T, Barsoum MW. J Alloys Compd 2002;347:271. [12] Barsoum MW, Salama I, El-Raghy T, Golczewski J, Porter WD, Wang H, et al. Metall Mater Trans 2002;33A:2775. [13] Ramaseshan R, Kakitsuji A, Seshadri SK, Nair NG, Mabuchi H, Tsuda H, et al. Intermetallics 1999;7:571. [14] Zhou A, Wang CA, Ge Z, Wu L. J Mater Sci Lett 2001;20:1971.
Łopacin´ski M, Puszynski J, Lis J. J Am Ceram Soc 2001;84:3051. Mei Bingchu, Miyamoto Yoshinari. Mater Chem Phys 2002;75:291. Khoptiar Y, Gotman I. Mater Lett 2002;57:72. Sun Z, Li S, Ahuja R, Schneider Jochen M. Solid State Commun 2004;129:589. [19] Ganguly A, Zhen T, Barsoum MW. J Alloys Compd 2004;376:287. [20] Tzenov Nikolay V, Barsoum Michel W. J Am Ceram Soc 2000;83:825. [15] [16] [17] [18]
Synthesis and thermal and electrical properties of bulk Cr2AlC Wubian Tian a
a,b
, Peiling Wang a,*, Guojun Zhang a, Yanmei Kan a, Yongxiang Li a, Dongsheng Yan a
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China b Graduate School of Chinese Academy of Sciences, China Received 15 July 2005; received in revised form 4 November 2005; accepted 10 November 2005 Available online 29 November 2005
Abstract Cr2AlC ceramics were fabricated by hot-pressing using Cr, Al and C powders as starting materials. The phase assemblages of the samples consisted of Cr2AlC, as a major crystalline phase, together with a very small amount of Cr7C3 and an unknown phase. Its thermal and electrical as well as mechanical properties were determined. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cr2AlC; Hot-pressing; Ceramics; Electrical properties; Thermal conductivity
1. Introduction Nowotny reported the work that he and his coauthors performed on the syntheses of a great numbers of carbides and nitrides in 1970 [1]. These compounds belong to a large class of solids with the general formula Mn+1AXn, where n = 1, 2, 3, M is an early transition metal, A is an A-group (mostly IIIA or IVA) element, and X is C or N. Among the M3AX2 compounds, there are now lots of papers dealing with the synthesis of Ti3SiC2, and the characterization of its electrical, thermal and oxidation behavior, since Ti3SiC2 is a remarkable material that combines many of the best attributes of both metals and ceramics [2–6]. In contrast, research work on M2AX compounds, which has more than thirty so-called H- or Ha¨gg phases, is scarce. It is noted however, that the fabrication of single phase, bulk dense samples of Ti3SiC2 has proved to be much more elusive [7]. As a consequence, little was known about Ti3SiC2, and much of what was known has been shown to be incorrect [7], until the great progress in synthesis was made by
*
Corresponding author. Tel.: +86 21 5241 2324; fax: +86 21 5241 3122. E-mail address: [email protected] (P. Wang).
BarsoumÕs group in 1996 [2]. The research approach to Ti3SiC2 revealed the importance of the successfully fabricating a pure phase to develop a new material. For M2AX phases, no systematic study is available in the literature [8], and the work has mainly focused on Ti2AlC, Nb2AlC and the solid solutions [9,11–17], although it has been known that M2AX phases are good thermal and electrical conductors [10,12], and they exhibit a high modulus but are relatively soft and readily machinable [11,13,16]. Recently, Sun and co-workers performed theoretical studies of the bulk modulus of M2AlC, where M = Ti, V, Cr, by means of ab initio total energy calculations [8]. The results have shown that the bulk modulus of M2AlC increases as Ti is substituted with V and Cr by 19% and 36%, respectively, which is associated with an extensive increase in the M–Al and M–C bond energy [8]. Lately, the same authors used the same method to calculate the elastic properties of M2AlC, with M = Ti, V, Cr, Nb and Ta, and reported that Cr2AlC possessed the highest bulk, Shear and YoungÕs modulus among the five hexagonal phases [18]. These results encouraged us to study the synthesis, elastic and electrical properties of Cr2AlC. In this paper, the fabrication of bulk polycrystalline and dense samples of Cr2AlC is reported. The thermal
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.11.009
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W. Tian et al. / Scripta Materialia 54 (2006) 841–846
and electric as well as mechanical properties of sintered samples are also described. 2. Experimental procedure The starting materials used in the present work were chromium (200 mesh, 99.95%, Shanghai Chemical Reagent Company of National Medicine Group), aluminum (100– 200 mesh, 99.95%, Shanghai Chemical Reagent Company of National Medicine Group) and graphite (3200 mesh, 99%, Shanghai Colloid Chemical Plant). The powders weighed according to the designed composition (Cr:Al:C = 2:1.1:1) were milled in absolute alcohol for 24 h. Pellets of dried powders were hot-pressed at 1400 C for 1 h under 20 MPa in an argon atmosphere. The densities of sintered samples were measured by the Archimedes principle. Phase assemblages were determined based on X-ray diffraction (XRD) patterns from diffractometer. The lattice parameters of Cr2AlC phase were determined by XRD using a Guinier–Ha¨gg camera with Cu Ka1 radiation and Si as an internal standard. Microstructural observation of fracture surfaces of the sample was performed under a field emissive scanning electron microscope (SEM, JEOL JSE-6700F, Japan). SEM and energy dispersive spectroscopy (EDS) analyses of polished surface of the sample were performed under an electron probe microanalyzer (JXA-8100F, Japan) equipped with EDS (Oxford INCA energy). The sample was etched in HF acid for 2 min before SEM observation. The Vickers hardness was determined by indentation using a Vickers diamond indenter and a load of 50 N for 10 s (Akashi). The flexural strength was determined via three-point bending with dimensions of 2.9 mm · 2.6 mm · 35 mm. The electrical resistances and their temperature dependencies, in the temperature range of 77–270 K, were measured with a 4-probe technique. The heat capacity, Cp, was measured using a differential scanning calorimeter (DSC-2C) under nitrogen at a heating rate of 10 K/min, in which the sample had a cylindrical shape with 5.5 mm in diameter and 0.8 mm in thickness. The coefficient of temperature conductivity, a, was measured using a laser-flash technique with a cylindrical sample (B10 mm · 2.5 mm). The thermal conductivity, k (W/m K), was calculated by k ¼ d C p a 101
dynamic YoungÕs modulus is determined using the resonant frequency in the flexural mode of vibration. The dynamic YoungÕs modulus and dynamic Shear modulus were used to compute the PoissonÕs ratio. 3. Results and discussion 3.1. Sample characterization and mechanical properties The XRD pattern of the sample hot-pressed at 1400 C for 1 h is shown in Fig. 1. It is found that the phase assemblage of sintered sample consisted of the Cr2AlC phase, as the major crystalline phase, together with a very small amount of Cr7C3 and a trace amount of an unknown phase, and the amount of the Cr2AlC phase reached as high as 95 wt.%. As the first stage, a stoichiometric composition Cr2AlC was tried for synthesis of the phase. The results showed that the sintered sample contained Cr2AlC and Cr7C3 with weight ratios of 90–10, revealing the lack of aluminum in the composition. An excess of Al was therefore used for the composition in the following experiments. It was found, however, that as well as the excess amount of aluminum, sintering temperature, holding time and atmosphere affected the amount of Cr2AlC phase formed in the sample and bulk densities of the materials. The investigation of the reaction process and mechanism for the Cr2AlC material are proceeding. The lattice parameters of Cr2AlC were determined by Guinier–Ha¨gg film data and related programs as described in the experimental section. The cell dimensions obtained ˚ and c = 12.814(3) A ˚, for the Cr2AlC phase, a = 2.863(4) A ˚ and c = 12.82 A ˚ are very close to the values of a = 2.86 A that were reported in the JCPDS card. The measured density of sintered sample was 5.21 g/cm3, which was around 98% of the theoretical density calculated based on the densities of Cr2AlC (5.229 g/cm3, JCPDS 29-0017) and Cr7C3 (6.877, JCPDS 36-1482).
ð1Þ
where d, Cp and a are, respectively, the density (g/cm3), the heat capacity (J/kg K) and the coefficient of temperature conductivity (cm2/s). The coefficient of thermal expansion (K1) was measured by means of thermal dilatometer (NETZSCH DIL 402C, Germany) in the temperature range 30–1200 C at a heating rate of 5 C/min in air. According to the ASTM standard (test method for YoungÕs modulus, Shear modulus and PoissonÕs ratio for advanced ceramics by impulse excitation of vibration), a
Fig. 1. XRD pattern of the sample hot-pressed at 1400 C for 1 h.
W. Tian et al. / Scripta Materialia 54 (2006) 841–846
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Table 1 Atomic ratios of Cr, Al and C of gray and white grains in BSE micrograph by EDS analysis
Gray grains White grains
Cr
Al
C
Cr/Al
48.43 63.90
23.69
27.88 34.01
2.04
Cr/C 1.88
film spattered on the surface of the sample that is necessary for SEM observation. The hardness of the sample is around 3.5 GPa, a value that is similar to Ti3SiC2 [2] and other M2AX materials [11,13,16]. It was found, however, that no indentation cracks were observed and it was therefore hard to estimate Fig. 2. SEM micrograph of fracture surface of sample.
A typical SEM micrograph of the fracture surface of the sample is shown in Fig. 2. Several areas with cleavage planes are apparent, confirming the layered nature of Cr2AlC. Scattered electron (SE) and back scattered electron (BSE) SEM micrographs of polished surface of the material in the same area are shown in Fig. 3(a) and (b), respectively. It is noted that the grains of the sample are hexagonal shaped. The EDS patterns of grains of gray and white color are shown in Fig. 3(c) and (d), respectively. The average atomic ratio of Cr/Al of the gray grains that was calculated from EDS analysis to be around 2.04, listed in Table 1, corresponds to the Cr2AlC phase. It is noted, however, that the amount of C is higher than the values in formula Cr2AlC and Cr7C3, which is caused by the C
Fig. 4. Optical micrograph of the damage region of sample around the indentation.
Fig. 3. (a) SE SEM micrograph and (b) BSE SEM micrograph of polished surface of the sample in the same area; (c) EDS pattern of grain with gray color and (d) EDS pattern of grain with white color, shown in (b).
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the length of the microcracks from indentation during the determination of hardness, so as to calculate the toughness of the material. The damage region around the indentation is shown in Fig. 4. The figure is remarkably similar to that of Ti3SiC2 [19], in which instead of sinking into the surface, the indentations accumulate as a result of a combination of
grain pullout, breakup and buckling, extending to a limited distance and absorbing most of damage energy. The sintered material was very easy to machine as expected. Holes were drillable using a commonly available high-speed tool steel drill bit without lubrication. This machinable property is also similar to that of Ti3SiC2. 3.2. Thermal and electrical properties The average thermal expansion coefficient of the sample in the range of 30–1200 C was 1.33 · 105 (K1); this is close to, but a little higher than, that of Ti3Al1.1C1.8 (9.0 · 106 (K1)) measured from 25 to 1200 C [20]. The coefficient of temperature conductivity of sample was determined in the range of 200–400 C. It is found that the coefficients of temperature conductivity decreased with the increase of temperature, as shown in Fig. 5(a). The specific heat capacity of the sample increases linearly from 50 to 450 C, as shown in Fig. 5(b). The solid line in Fig. 5(b) represents a third-order polynomial fit to the Debye model. The heat capacity at 25 C, namely, 590 J/(kg K) obtained from Fig. 5(b), is larger than that of Ti2AlC (581 J/(kg K)) [12] and Cr (460 J/(kg K)). According to Eq. (1), the thermal conductivities of the sample, k, between 200 and 400 C can be calculated. It is found that the thermal conductivity decreases slightly with the increase in temperature, as shown in Fig. 5(c). The least-squares fit of the data is shown as a straight solid line in Fig. 5(c). That is k ¼ 18:6 0:0023T The thermal conductivity of the sample at 200 C is 17.5 W/(m K), shown in Table 2, which is in the range of 15–45 W/(m K) for M2AlC (M = Nb, (Ti, Nb) and Ti) at room temperature and its variation with temperature is very similar to that of Ti3SiC2 and Ti2AlC [12]. The temperature dependence of resistivity of Cr2AlC sample is shown in Fig. 6. It can be seen that the resistivi-
Fig. 5. Temperature dependence of (a) coefficient of temperature conductivity, (b) specific heat capacity and (c) thermal conductivity of Cr2AlC sample.
Fig. 6. Temperature dependence of resistivity of Cr2AlC sample hotpressed at 1400 C for 1 h.
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Table 2 Some properties of Cr2AlC in comparison to Ti2AlC, Ti3Al1.1C1.8 and Ti3SiC2 Property
Value Cr2AlC
Ti2AlC [9]
Ti3Al1.1C1.8 [20]
Ti3SiC2 [20]
Elementary cell ˚) a (A ˚) c (A Theoretical density (g/cm3) Measured density (g/cm3) Coefficient of thermal expansion (K1) Special heat capacity at 25 C J/(kg K) Thermal conductivity at 200 C W/(m K) Electrical conductivity (S/m) Temperature coefficient of resistivity, b (K1) YoungÕs modulus (GPa) Shear modulus (GPa) PoissonÕs ratio Flexural strength (MPa) Hardness (GPa)
Hexagonal 2.863(4) 12.814(3) 5.229 5.21 1.33 · 105 590 17.5 1.4 · 106 0.0028 278 116 0.153 378 3.5
Hexagonal 3.051 13.637 4.113 –a 8.2 · 106 579 41.7 [12] 2.7 · 106 0.0035 [12] –a –a –a –a 4.5
Hexagonal 3.0654 18.487 4.247 4.2 9.0 · 106 –a –a 2.9 · 106 0.0031 297 124 0.2 375 ± 15 3.5
Hexagonal 3.0665 17.671 4.531 4.5 9.2 · 106 588 [2] 43 (at room temp.) [2] 4.5 · 106 0.004 333 139 0.2 260 ± 20 [2] 4 [2,20]
a
Not reported.
ties of the sample, q, increase linearly with the increase in temperature from 75 to 269 K and the temperature coefficient of resistivity is 0.0028 K1, which is very close to that of Ti3Al1.1C1.8, at 0.0031 K1 [20]. The electrical conductivity at room temperature calculated by the extrapolated value of resistivity is 1.4 · 106 X1 m1, which is slightly lower than that of Ti3Al1.1C1.8 (2.9 · 106 X1 m1) [20]. The YoungÕs modulus and Shear modulus of Cr2AlC, 278 and 116 GPa, respectively, are also very comparable to that of Ti3Al1.1C1.8. Some properties of Cr2AlC are summarized in Table 2, together with those of Ti3Al1.1C1.8 for comparison. 4. Conclusion Cr2AlC ceramics were prepared by hot-pressing using the proper ratio of chromium, aluminum and graphite powders at 1400 C for 1 h; the density was around 98% of the theoretical value. The sintered material consisted of Cr2AlC and Cr7C3 with a weight ratio of 95–5, together with a trace amount of an unknown phase. SEM micrographs of fractured surfaces leave no doubt as to its layered nature. The EDS results are in good agreement with the XRD analysis. The sample has comparable properties with Ti3Al1.1C1.8. It is relatively soft (Vickers hardness of 3.5 GPa) and elastically stiff (YoungÕs modulus of 278 GPa, Shear modulus of 116 GPa). The thermal conductivity decreases slightly with the increase in temperature in the range of 200–400 C. The electrical conductivity decreased linearly with the increase in temperature from 75 to 269 K, and the extrapolated value at room temperature was 1.4 · 106 X1 m1. The coefficient of thermal expansion in the temperature range 30–1200 C, the thermal conductivity at 200 C and the heat capacity at 25 C, were 1.33 · 105 (K1), 17.5 W/(m K), and 590 J/ (kg K), respectively.
It should be pointed out that mechanical and thermal as well as electric properties of Cr2AlC reported here might not reach the maximum values since some synthesis parameters (composition, temperature, holding time and atmosphere, etc.) could be further optimized in the process of understanding the reaction mechanism of the material. Anyway, it is noted that the higher value of the heat capacity of Cr2AlC compared to Ti3SiC2 and other MAX compounds would be beneficial for its use in the heat insulation field. Acknowledgments This financial support from the Science and Technology Commission of Shanghai was highly appreciated (Contract No. 04JC14076). We thank Professors Jef Vleugels and Omer Van der Biest, Dept. of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Belgium, for carrying out the YoungÕs modulus measurements. Thanks also to Professor Tonggeng Xi, Shanghai Institute of Ceramics, for the helpful discussion of the thermal properties of the sample. References [1] Nowotny H. Prog Solid State Chem 1970;2:27. [2] Barsoum MW, El-Raghy T. J Am Ceram Soc 1996;79:1953. [3] El-Raghy T, Barsoum MW, Zavaliangos A, Kalidindi S. J Am Ceram Soc 1999;82:2849. [4] El-Raghy T, Barsoum MW, Zavaliangos A, Kalidindi S. J Am Ceram Soc 1999;82:2855. [5] Sun ZM, Zhou YC, Li S. Acta Mater 2001;49:4347. [6] Barsoum MW, El-Raghy T. J Phys Chem Solids 1999;60:429. [7] Barsoum MW, Brodkin D, El-Raghy T. Scripta Metall Mater 1997;36:535. [8] Sun Z, Ahuja R, Li S, Schneider Jochen M. Appl Phys Lett 2003;83:899. [9] Barsoum MW, Ali M, El-Raghy T. Metall Mater Trans 2000;31A:1857.
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