Simultaneous synthesis and consolidation of chromium carbides (Cr3C2, Cr7C3 and Cr23C6) by pulsed electric-current pressure sintering

Materials Science and Engineering A 399 (2005) 154–160

Simultaneous synthesis and consolidation of chromium carbides (Cr3C2, Cr7C3 and Cr23C6) by pulsed electric-current pressure sintering Ken Hirota ∗ , Kenichi Mitani, Masaru Yoshinaka, Osamu Yamaguchi Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Kyo-Tanabe, Kyoto 610-0321, Japan Received 25 October 2004; received in revised form 17 January 2005; accepted 25 February 2005

Abstract Chromium carbides (Cr3 C2 , Cr7 C3 and Cr23 C6 ) have been synthesized and consolidated simultaneously from mixtures of Cr and amorphous carbon powders by pulsed electric-current pressure sintering (PECPS). Dense ceramics thus obtained were composed of chromium carbides with a small amount of Cr2 O3 , which originates from a trace amount of oxygen adsorbed on the as-received starting Cr powder. Synthesis and consolidation processes, which were observed from their shrinkage curves during PECPS, have been examined using X-ray diffraction (XRD) and scanning electron microscopy for the powder compacts. Cr3 C2 ceramics sintered at 1300 ◦ C for 10 min under 30 MPa have a 98.9% of theoretical density and fine structures with a 3.6 ␮m grain size. They exhibit excellent mechanical properties: a bending strength σ b of 690 MPa, a Vickers hardness Hv of 18.9 GPa and a fracture toughness KIC of 7.1 MPa m1/2 . © 2005 Elsevier B.V. All rights reserved. Keywords: Chromium carbides; Simultaneous synthesis and consolidation; Pulsed electric-current pressure sintering; Microstructural development; Mechanical properties

1. Introduction Three chromium carbides (Cr3 C2 , Cr7 C3 and Cr23 C6 ) were reported as stable phases in the Cr–C system [1]. Among them, Cr3 C2 has the highest melting-point (Tm ∼ 1810 ◦ C), exhibiting high-temperature oxidation resistance, good chemical stability and high hardness [2]. Therefore, its electrical, thermal and mechanical properties have been investigated from a viewpoint of high-temperature materials [3]. On the other hand, little attention has been paid to the latter two compounds that have lower melting-points (Tm (Cr7 C3 ) ∼ 1765 ◦ C and Tm (Cr23 C6 ) ∼1576 ◦ C) [1]. One of the reasons is that it is very difficult to prepare singlephase chromium carbide powders and they exhibit poor sinterability. Unique process for fabricating high-temperature materials, i.e., self-propagating high-temperature synthesis (SHS) [4] cannot be applied due to their low adiabatic temperatures (Tad ∼ 672 ◦ C (Cr3 C2 ), Tad ∼ 663 ◦ C (Cr7 C3 ) and ∗

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Tad ∼ 473 ◦ C (Cr23 C6 )) [5]. Because it has been empirically cleared that the materials with Tad < 1530 ◦ C (1800 K) cannot be synthesized by SHS [6]. Up to now, Cr3 C2 powder has been prepared by: (1) a solid-state reaction of Cr/C mixture at 1400–1800 ◦ C for long duration time (20–40 h) under reductive atmosphere [7], (2) combination of mechanical alloying (20–100 h) and sequent thermal treatment at 1200 ◦ C [8] and (3) thermit process of the 3Cr2 O3 + 4C + 9Mg reaction [6]. Most of this powder commercially available is produced by the solidstate reaction; its carbon content (∼13.14 mass%) is lower than the theoretical content (13.345 mass%). As a result, using this powder hot-pressed (HP: 1300 ◦ C/1 h/40 MPa) [9] and hot-isostatically pressed (HIP: 1330 ◦ C/1 h/150 MPa) Cr3 C2 ceramics [10] were composed of Cr3 C2 and Cr7 C3 ; single-phase ceramics have not been obtained. The former hot-pressed ceramics with a 99.0% of theoretical density exhibited a Vickers hardness Hv of 18.0 GPa [10]. A combined use of a vacuum pre-sintering (1500 ◦ C/1 h) and a sequent HIP treatment (1330 ◦ C/1 h/150 MPa) resulted in the composite materials (99.8% of theoretical density) with a bending strength of σ b = 440 MPa and a hardness of Hv = 17 GPa [10].

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In addition, a new process named as “Gas-Pressurized Combustion Synthesis (GPCS)” produced dense sintered ceramics (99.5% of theoretical density) which exhibited a strength of σ b = 625 MPa and a hardness of Hv = 18 GPa [11]. On the other hand, the latter two powder preparation processes lead to the formation of composite powders consisting of Cr3 C2 , Cr7 C3 and other by-products. Therefore, until now mechanical properties of dense chromium carbides except for Cr3 C2 ceramics have not been evaluated. Pulsed electric-current pressure sintering (PECPS), or spark plasma sintering (SPS), is a newly developed process, which can consolidate powder compact by applying an onoff dc electric pulse under uniaxial pressure [12]. During the sintering, a spark discharge at the narrow gaps between particles induces a high-temperature state locally. When a stacked powder exhibits a high electrical conductivity, the dc pulse passes through the powder compact, generating (1) spark plasma, (2) spark impact pressure, (3) Joule heat and (4) an electric field among the particles. From this reason, PECPS or SPS processes have been expected to provide “a reaction field” at which field synthesis and sintering of metal carbide might be occurred simultaneously. However, the formation and consolidation of chromium carbides using this method has not yet been reported. In the present study, PECPS was utilized to prepare three kinds of chromium carbide compounds directly from the elemental powder mixtures with the corresponding stoichiometric composition; dense ceramics consisting of homogeneous fine grains with the almost single-phase were obtained. Their mechanical properties, such as bending strength σ b , Vickers hardness Hv and fracture toughness KIC , were measured and reported for the first time.

2. Experimental procedure Metal Cr powder (Nippon New Metal Co. Ltd., Osaka, Japan, average particle size Ps of ∼2 ␮m, purity of 99.36%, Fig. 1(a)) and amorphous carbon (Mitsubishi Chemical Co. Ltd., Tokyo, Japan, Ps of ∼30 nm, purity of 98.5%) powder were used as starting materials. Before mixing, the latter powder was pre-heated for 2 h at 950 ◦ C under reduced pressure to eliminate a small amount of oxygen adsorbed on the surface of carbon particles without particle-sintering during the heat-treatment. These powders weighed in molar ratios of Cr/C = 3:2, 7:3 and 23:6 were mixed completely with a planetary ball mill using partially stabilized zirconia (PSZ) balls (2–3 mm␾ in diameter) and methyl-alcohol for 2 h at rotating speed of 300 rpm (centrifugal force about ∼11 gravitational acceleration g). The mixed powders dried in an ambient atmosphere were put into a carbon mold (40 mm␾ –20 mm␾ –40 mmh ) and heated at 100 ◦ C/min in a vacuum (∼10−2 Pa) under an uniaxial pressure of 30 MPa applying dc electric-current (on/off pulse interval = 12:2). Soaking time at the highest temperature (up to ∼1350 ◦ C) which temperature varies corresponding to each chromium

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Fig. 1. (a) SEM photograph of Cr power and (b) TEM photograph of amorphous carbon powder pre-heated at 950 ◦ C for 2 h under reduced pressure.

carbide compound was 10 min and the same cooling rate about 50 ◦ C/min was adopted. Heating temperature was measured at the center position about 5 mm far from the inside wall of carbon mold using a Pt–Rh thermocouple (B-type, 0.5 mm␾ ). Bulk densities of mixed elemental powder compacts with each composition obtained under 30 MPa were 2.38 Mg/m3 (Cr3 C2 : ∼46.3% of theoretical), 2.97 Mg/m3 (Cr7 C3 : ∼52.4%) and 3.32 Mg/m3 (Cr23 C6 : ∼54.0%); the theoretical densities were calculated using the values of 7.20 Mg/m3 (Cr) [13] and 1.8 Mg/m3 (amorphous carbon) [14]. With increasing Cr content, relative density increased gradually from ∼46.5 to ∼54%, which might be explained in terms of deformable metal Cr particles under uniaxial pressure. To study the formation process of chromium carbides during PECPS: (i) shrinkage profiles of the powder compacts were taken in the same direction of applying pressure, (ii) crystalline phase developments during heating were investigated by X-ray diffraction (XRD) analysis (Cu K␣1 radiation with a monochromator) and (iii) microstructural observation by scanning electron microscopy (SEM) was performed using the samples quenched from the desired temperatures. Spectrochemical analysis (Horiba Co. Ltd., Kyoto, Japan, EMGA 620 W/C), i.e., non-dispersive infrared (IR) absorption using inert gas fusion in an impulse furnace was

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employed to measure the content of oxygen in starting materials and PECPS-products. Bulk densities of sintered ceramics were measured by Archimedes’s method. After phase identification, test bars for mechanical-property measurements were cut from the ceramics with a diamond cutting-blade and then polished their four sides to mirror surface with a diamond paste (nominal particle size, 1–3 ␮m). Three-point bending strength σ b was measured under the conditions of cross-head speed of 0.5 mm/min and an 8 mm span length. Vickers hardness Hv and fracture toughness KIC were evaluated; measuring conditions for Hv were that an applying load of 19.6 N and a duration time of 15 s, and the indentation fracture method (IF) with Niihara’s equation [15] were adopted to determine the KIC values.

3. Results and discussion Preliminary experiments revealed that a powder mixture of Cr and fine graphite (Ps ∼ 5 ␮m, 99.7% purity) with the molar ratio of Cr/C = 3:2 consolidated by PECPS resulted in grain-oriented ((0 k 0)-plane) Cr3 C2 [16] ceramics. Because of little orientation in other Cr7 C3 and Cr23 C6 ceramics, this orientation was thought to be induced by its high volume ratio of hexagonal graphite [17] powder having much particleorientation of c-axis perpendicular to the uniaxial pressing direction and a topotactic reaction between graphite and Cr powders during PECPS. The grain-oriented Cr3 C2 ceramics exhibited poor mechanical properties (σ b : 400 MPa, Hv : 14 GPa and KIC : 4.6 MPa m1/2 ), although their high density (>98% of theoretical) and small grain sizes (∼10 ␮m), in comparison with those (σ b : 440 MPa, Hv : 17 GPa and KIC :

Fig. 2. (a) A shrinkage profile of an elemental powder compact (Cr/C = 3:2 molar ratio) and (b) heating temperature as a function of soaking time.

4.4 MPa m1/2 ) of randomly grain-oriented Cr3 C2 fabricated by hot isostatic pressing. Therefore, in the present study, the amorphous nanometer-sized carbon powder (∼30 nm, Fig. 1(b)) after the heat-treatment of 950 ◦ C was utilized as a starting material to prevent the grain-orientation and to achieve high density. Among three chromium carbides, Cr3 C2 with the highest melting-point (1810 ◦ C) was first investigated as one of Cr–C compounds. The formation process, microstructural development and mechanical properties of dense sintered compacts have been described. 3.1. Formation process of Cr3 C2 during pulsed electric-current pressure sintering The shrinkage of powder compact was in situ monitored during PECPS process. Fig. 2 shows the displacement profile

Fig. 3. SEM photographs for compacted powder samples heated to: (a) 500 ◦ C, (b) 700 ◦ C, (c) 950 ◦ C, (d) 1050 ◦ C and (e) 1150 ◦ C in a vacuum.

K. Hirota et al. / Materials Science and Engineering A 399 (2005) 154–160

Fig. 4. XRD patterns of pulverized samples obtained by PECPS at: (i) 500 ◦ C, (ii) 700 ◦ C and (iii) 1150 ◦ C from a mixture of elemental powders (Cr/C = 3:2 molar ratio).

of Z-axis parallel to the pressing direction as a function of soaking time. The shrinkage curve was divided into five stepregions: (i) room temperature (RT) ∼500 ◦ C, (ii) 500–700 ◦ C, (iii) 700–950 ◦ C, (iv) 950–1050 ◦ C and (v) 1050–1200 ◦ C. Fig. 3 shows SEM photographs of fracture surfaces for samples obtained at each heating region: (a) 500 ◦ C, (b) 700 ◦ C, (c) 950 ◦ C, (d) 1050 ◦ C and (e) 1150 ◦ C. Representative XRD patterns for samples obtained at various temperatures were presented in Fig. 4(i–iii). Sample obtained at 500 ◦ C was a mixture of Cr and amorphous carbon powders (Fig. 4(i)). In the 700 ◦ C-heated sample, in addition to Cr, a small amount of chromium carbides (Cr7 C3 [18] and

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Cr3 C2 [16]) and a trace amount of graphite [17] were detected (Fig. 4(ii)). The former Cr7 C3 was synthesized first due to its lower free formation energy (−143 kJ/mol) [19], reflecting its lower melting-point (Tm ∼ 1765 ◦ C [1]) than the latter Cr3 C2 (−78 kJ/mol [19], Tm ∼ 1810 ◦ C [1]). Although among chromium carbides, Cr23 C6 has the lowest free formation energy (−396 kJ/mol [19]), however, the formation of this compound during PECPS was not recognized by XRD. With regard to the formation of graphite from amorphous carbon, our preliminary experiments revealed that graphite phase was not detected in the carbon powder heated with a conventional vacuum electric furnace up to 1000 ◦ C. Therefore, pre-heating temperature for amorphous carbon was determined to be 950 ◦ C. However, in the mixtures of both Cr/C = 7:3 and 23:6 molar ratios, crystallized graphite was observed after PECPS treatments at lower temperatures (700–750 ◦ C), as shown in Fig. 5(i and ii). These results suggested that the amorphous carbon with a much smaller particle size (∼30 nm␾ ) tightly interposed between the larger metal Cr particles (∼2 ␮m␾ ) might transform (crystallize) to graphite at low temperatures under activated PECPS treatment. In the step-region (iii), a main compound changed from Cr to Cr3 C2 , and chromium oxide (Cr2 O3 ) [20] began to form. The presence of Cr2 O3 might be explained in terms of the reaction between Cr and a trace amount of oxygen adsorbed on the surface of metal particles nevertheless the strong reductive PECPS atmosphere. As for the formation of Cr3 C2 and Cr2 O3 , Yamaguchi [21] reported that Cr3 C2 phase began to appear in the compacted mixture of Cr and Cr2 O3 powders embedded in a carbon powder at 1100 ◦ C. He explained this phenomena in terms of the difference in free energy of formation between Cr3 C2 and Cr2 O3 related to an equation (2/3)Cr2 O3 (s) + (8/9)C (s) = (4/9)Cr3 C2 (s) + O2 (g) + (G◦ ), here (G◦ ) value is negative, for example, −570 kJ at 700 ◦ C [22]. In the present study, adoption of both an amorphous nanometer sized carbon powder and an activating heat-treatment (PECPS) in a vacuum (∼10−2 Pa) might enhance the formation of Cr3 C2 at lower temperatures. On the other hand, based on the equilibrium phase diagram for the system Cr–C–O [23], at temperature between 1100 and 1350 ◦ C, Cr2 O3 is thought to be more stable than Cr3 C2 under such a reductive atmosphere as − log(PCO2 /PCO ) ≤ 6.0, i.e., PO2 ≤ 10−7.5 Pa [24]. Therefore, in the present study Cr2 O3 phase was hardly disappeared in the specimen heated for 10 min at 1350 ◦ C by the PECPS treatment due to much shorter soaking time and a weak reduction atmosphere (PO2 ∼ 2 × 10−3 Pa). In the step-region (iv), a sharp shrinkage, indicating a large amount of densification, was observed in Fig. 2, without crystalline phase changes from the step-region (iii). At the last stage, a main product became Cr3 C2 with a small amount of Cr2 O3 (Fig. 4(iii)). Crystalline phase development of the elemental powder mixed compacts with the Cr/C = 3:2 molar ratio was summarized in Table 1(i). Here, a chemical reaction between Cr7 C3 and Cr in the step-regions (ii) and (iv) might

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Fig. 5. XRD patterns of pulverized samples obtained from the mixtures of elemental powders; (i) Cr/C = 7:3 molar ratio and (ii) Cr/C = 23:6 molar ratio by PECPS at: (a) 500 ◦ C, (b) 750 ◦ C, (c) 900 ◦ C and (d) 1150 ◦ C.

be presented as follows: 3Cr 7 C3 + 5Cr → 7Cr 3 C2 3.2. Microstructure of Cr3 C2 ceramics From XRD analysis, mass% of Cr3 C2 and Cr2 O3 in the Cr3 C2 ceramics fabricated by PECPS was evaluated based on XRD calibration curves. It was confirmed that a small amount (2.5–3.5 mass%) of Cr2 O3 still remained in the Cr3 C2 ceramics. From these results, almost single-phase Cr3 C2 ceramics could be fabricated by PECPS. Theoretical density of Cr3 C2 ceramics with various Cr3 C2 /Cr2 O3 ratios was calculated using theoretical densities of Dx (Cr3 C2 ) = 6.66 Mg/m3 [16] and Dx (Cr2 O3 ) = 5.231 Mg/m3 [20]. Bulk densities of Cr3 C2 ceramics and relative densities determined from the theoretical density were presented in Table 1(ii). Densities increased from 6.23 Mg/m3 (94.3%) to 6.57 Mg/m3 (99.1%) with increasing heating temperature. Fig. 6 shows the representative fracture surfaces of Cr3 C2 ceramics fabricated by PECPS at various temperatures: (a) 1200 ◦ C, (b) 1250 ◦ C, (c) 1300 ◦ C

and (d) 1350 ◦ C. From direct observation on SEM photographs, average grain sizes Gs were determined using an intercept method [25]. The Gs value increased monotonously from 1.6 to 5.0 ␮m with increased sintering temperature. Between 1200 and 1300 ◦ C, a little change in microstructure, i.e., gradual grain growth and densification, was observed; however, at 1350 ◦ C, grain growth and large pores at grain boundaries were recognized. 3.3. Mechanical properties of Cr3 C2 ceramics Three-point bending strength σ b , Vickers hardness Hv and fracture toughness KIC of Cr3 C2 ceramics were evaluated (Table 1(ii)). Data present the average values obtained from five test specimens. The value of σ b increased from 630 MPa with increasing sintering temperature up to 1300 ◦ C and reached the maximum value of 690 MPa. A drop to 580 MPa at 1350 ◦ C might be explained in terms of large grain size nevertheless high relative density. The Hv and KIC showed little dependence on sintering temperature between 1200 ◦ C (18.0 GPa and 6.5 MPa m1/2 ) and 1350 ◦ C (18.3 GPa

Table 1 (i) Crystalline phase change of an elemental powder mixture with the composition of Cr/C = 3:2 molar ratio during pulsed electric-current pressure sintering (PECPS) process and (ii) characteristics of Cr3 C2 ceramics obtained at various temperatures by PECPS Temperature (◦ C)

Phases

(i) 500 700 900 1050 1150

Cr, amorphous C Cr, Cr7 C3 , Cr3 C2 , Cr2 O3 , C (graphite) Cr3 C2 Cr2 O3 , C (graphite), Cr7 C3 Cr3 C2 Cr2 O3 , C (graphite), Cr7 C3 Cr3 C2 Cr2 O3

Sintering temperature (◦ C)

Cr3 C2 /Cr2 O3 (mass%)

Theoretical densitya (Mg/m3 )

Bulk and relative densities (Mg/m3 , %)

Grain size, Gs (␮m)

Bending strength, σ b (MPa)

Vickers hardness, Hv (GPa)

Fracture toughness, KIC (MPa m1/2 )

(ii) 1150 1200 1250 1300 1350

96.45/3.55 96.71/3.29 96.80/3.20 96.61/3.39 97.53/2.47

6.61 6.61 6.61 6.61 6.63

6.23 (94.3) 6.42 (97.1) 6.51 (98.5) 6.54 (98.9) 6.57 (99.1)

1.6 3.0 3.2 3.6 5.0

630 640 660 690 580

15.1 18.0 18.5 18.9 18.3

6.1 6.5 6.8 7.1 6.3

a

Calculated using Dx (Cr3 C2 ) = 6.66 Mg/m3 (JCPDS No. 35-804) and Dx (Cr2 O3 ) = 5.231 Mg/m3 (JCPDS No. 38-1479).

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Fig. 6. SEM photographs for fracture surfaces of Cr3 C2 ceramics sintered for 10 min at: (a) 1200 ◦ C, (b) 1250 ◦ C, (c) 1300 ◦ C and (d) 1350 ◦ C by PECPS. Table 2 Characteristics of Cr7 C3 ceramics obtained at various temperatures by PECPS Sintering temperature (◦ C)

Cr7 C3 /Cr2 O3 (mass%)

Theoretical densitya (Mg/m3 )

Bulk and relative densities (Mg/m3 , %)

Grain size, Gs (␮m)

Bending strength, σ b (MPa)

Vickers hardness, Hv (GPa)

Fracture toughness, KIC (MPa m1/2 )

1150 1200 1250 1300 1350

96.45/3.55 96.57/3.43 96.44/3.56 96.77/3.23 97.41/2.59

6.819 6.821 6.818 6.824 6.834

5.75 (84.4) 6.12 (89.7) 6.77 (99.2) 6.75 (99.0) 6.78 (99.2)

– – 2.9 3.0 3.7

240 370 650 670 640

5.71 7.77 17.0 16.5 16.2

3.7 4.8 7.0 7.1 6.3

a

Calculated using Dx (Cr7 C3 ) = 6.877 Mg/m3 (JCPDS No. 36-1482) and Dx (Cr2 O3 ) = 5.231 Mg/m3 (JCPDS No. 38-1479).

and 6.3 MPa m1/2 ). However, Hv and KIC values also exhibited the same behavior as σ b ; the maximum values of 18.9 GPa and 7.1 MPa m1/2 , respectively, were attained for Cr3 C2 ceramics sintered at 1300 ◦ C. These mechanical properties were much higher than those (σ b : 400 MPa, Hv : 17 GPa and KIC : 4–5 MPa m1/2 ) reported on Cr3 C2 ceramics fabricated by hot-isostatic-pressing [10]. And even though in comparison with those (σ b : 625 MPa, Hv : 18 GPa and KIC : 5.6 MPa m1/2 ) of Cr3 C2 ceramics produced by high pressure GPCS [11], the present ceramics showed a little higher values. This might be originated from the small grain-size and high density of the present ceramics.

3.4. Characterization of other chromium carbides synthesized by PECPS Simultaneous synthesis and consolidation of other chromium carbides (Cr7 C3 and Cr23 C6 ) were carried out as the same manner as Cr3 C2 . The crystalline phase developments of mixed elemental powders (Cr/C = 7:3 and 23:6 molar ratios) were presented as XRD patterns in Fig. 5(i and ii). In these compounds, characteristic temperature-regions were divided into four steps due to mainly their low formation energies and low melting-points. Among crystalline phase development to Cr7 C3 , only Cr2 O3 was recognized

Table 3 Characteristics of Cr23 C6 ceramics obtained at various temperatures by PECPS Sintering temperature (◦ C)

Cr23 C6 /Cr2 O3 Theoretical (mass%) densitya (Mg/m3 )

Bulk and relative densities (Mg/m3 , %)

Grain size, Gs (␮m)

Bending strength, σ b (MPa)

Vickers hardness, Hv (GPa)

Fracture toughness, KIC (MPa m1/2 )

1100 1125 1150 1175

96.96/3.04 97.03/2.97 97.15/2.85 97.31/2.69

6.82 (98.8) 6.87 (99.5) 6.86 (99.4) 6.86 (99.4)

1.5 2.8 3.2 4.3

520 520 500 480

14.2 15.0 15.0 13.7

5.1 5.8 5.3 4.4

a

6.901 6.902 6.904 6.907

Calculated using Dx (Cr23 C6 ) = 6.877 Mg/m3 (JCPDS No. 35-78) and Dx (Cr2 O3 ) = 5.231 Mg/m3 (JCPDS No. 38-1479).

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as a by-product; other chromium carbides were not formed during heating. This reason could be explained as referring Cr–C phase equilibrium [1]. Some characteristics of Cr7 C3 and Cr23 C6 ceramics fabricated at various temperatures were shown in Tables 2 and 3, respectively. It should be noted that: (i) these chromium carbides have been obtained as almost single-phase materials, (ii) the mechanical properties of dense ceramics also have been first reported and (iii) the present Cr7 C3 ceramics exhibited comparable ␴b (670 MPa) and KIC (7.1 MPa m1/2 ) values with Cr3 C2 ceramics even though its lower melting-point (1765 ◦ C) than that (1810 ◦ C) of Cr3 C2 . 4. Conclusions Up to now, three kinds of chromium carbides, especially, two compounds (Cr7 C6 and Cr23 C6 ) with lower meltingpoints have attracted little attention due to much difficulty in both preparation of single-phase powders and fabrication of dense ceramics free from cracks. In the present study, by utilizing pulsed electric-current pressure sintering these compounds with the almost single-phase have been fabricated densely. Their formation process during PECPS has been investigated precisely and their mechanical properties have been evaluated for the first time. Dense Cr3 C2 and Cr7 C3 ceramics (>99% of theoretical) exhibit σ b : 670–690 MPa, Hv : 17–19 GPa and KIC : 7.1 MPa m1/2 . These values will be fundamental data utilized in the application of high-temperature materials. References [1] A.E. McHale (Ed.), Fig. 8934 in Phase Equilibria Diagrams, Phase Diagrams for Ceramists. Borides, Carbides, and Nitrides, vol. X, American Ceramic Society, Columbus, OH, 1994. [2] K. Tomomatsu, J. Matsushita, J. Adv. Sci. 11 (1999) 83–84. [3] S. Kuriyama, N. Takashima, J. Matsushita, J. Adv. Sci. 11 (1999) 87–88.

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