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TaB2 composite in spark plasma sintering
Materials Chemistry and Physics 126 (2011) 459–462
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Densification process of TaC/TaB2 composite in spark plasma sintering夽 Limeng Liu a,b,∗ , Feng Ye a , Xiulan He a , Yu Zhou a a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 14 September 2010 Accepted 12 January 2011 Keywords: Carbides Sintering Microstructure Mechanical properties
a b s t r a c t A TaC composite with ∼11 wt% in situ TaB2 was fabricated by spark plasma sintering at 1600–1900 ◦ C. The densification process was studied by analysis of the densifying shrinkage of the powder compacts. Three distinct stages for densification were determined. The starting powder mixture of TaC, B4 C and Ta completed reaction to form the desired TaC and TaB2 at temperature <1583 ◦ C. At around ∼1750 ◦ C, the TaC/TaB2 material significantly improved relative density to ∼95% with rapid grain growth. The final densification took place very rapidly at 1900 ◦ C by releasing a high pressure vapor. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
In this study, certain Ta was added to reaction (1) to modified it by
Tantalum carbide is an important ultra-high temperature ceramic (UHTC) for potential applications at >2000 ◦ C. Common practices to improve densification of this high covalent ceramic include using nanostarting powders [1–3], eliminating oxide impurities [3–6], complex powder processing [1,3,7], pinning grain growth [5,8], sintering at very high temperatures [4,5], and applying rapid heating rates [9–11]. One problem associated with processing tantalum carbide ceramic is the difficulty to remove the last few percent residual pores. These residual pores are entrapped and limit the density that a dense tantalum carbide ceramic can reach. Although there are many literatures on densifying tantalum carbide, the elimination process of these entrapped pores and the mechanisms are not clear. Recently, Talmy et al. [5] pressureless fabricated a TaC matrix composite by the following reaction: 16TaC + B4 C → 14TaC + 2TaB2 + 3C.
(1)
The dispersed TaB2 and C particles improved densification by grain boundaries pinning and oxygen reducing. However, the significant C concentration left in the TaC composites could degrade the mechanical properties and oxidation resistance of the composite.
夽 This work was financially supported by China Postdoctoral Science Foundation under Grant No. 20090450957. ∗ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Xidazhi Street 92, Harbin 150001, Heilongjiang, People’s Republic of China. Tel.: +86 451 86413921; fax: +86 451 86413921. E-mail addresses: [email protected], [email protected] (L. Liu). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.038
16TaC + B4 C + 3Ta → 17TaC + 2TaB2 .
(2)
If this modified reaction (2) is complete, ∼11 wt% TaB2 particles will be introduced into the TaCy matrix composite. Spark plasma sintering (SPS) was used to prepare this composite. The SPS densification process, particular the elimination of the residual entrapped pores at the last sintering stage was concerned. 2. Experimental The starting powders were TaC, Ta (Ningxia Orient Tantalum Industry Co., Ltd., China), and B4 C (Jingangzuan Boron Carbide Co. Ltd., China) with mean particle size 1.25 m, 0.7 m, and 2.5 m, respectively. The TaC content in the TaC powder was higher than 99%. The main impurities were ∼0.3 wt% Nb, 0.1 wt% Fe, 0.20 wt% O, 0.15 wt% free carbon, 0.05 wt% N, and Al, Ca, K, Na, Ti with a total amount <0.05 wt%. The B4 C was >95 wt% pure with major impurities of free carbon, free boron, and trace B2 O3 . 60 g TaC, B4 C and Ta in molar ratio of 16:1:3 were mixed in a steel cylinder lined by PVC using 300 g ZrO2 balls as mixing media. Batches of 13 g mixed powders were loaded into a graphite die and SPS for 5 min at 1600, 1700, 1800, or 1900 ◦ C in a vacuum. The sintering process was the same as detailed in a previous paper [10]. The densification process was thoroughly recorded by the accurate SPS data acquiring system. For example, the Pilani gauge (PT-9P, 1 × 105 to 0.13 Pa, Diavac Limited) used to measure the SPS chamber vacuum has a 0.5 Pa resolution for vacuum range of 1–50 Pa. The ram displacement detector (Digicollar, Mutoh Engineering Inc.) secures a 0.01 mm detect accuracy. Given the negligible elastic deformation of the Ø120 mm ram pillar in comparison with the large sintering shrinkage of the Ø20 mm sample under the 12.6 kN uniaxial SPS load (40 MPa) used in this study, the recorded ram displacement can be approximately ascribed to densification of the samples. Theoretical density for the composites was determined by the law of mixtures. Bulk densities were measured by the Archimedes method. Relative density and open porosity was calculated. Phase compositions were analyzed by X-ray diffraction (XRD). The lattice parameter a for the cubic TaCy was calculated by refining the XRD. The C/Ta ratio in the TaCy was determined according to the equation
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Fig. 2. SPS ram displacement and chamber vacuum as functions of the sintering temperatures, showing the densification process of the TaC/TaB2 composites. Fig. 1. Typical X-ray diffraction patterns of the TaC/TaB2 composites spark plasma sintered at 1600–1900 ◦ C.
a(y) = 4.3007 + 0.1563y [12]. Microstructure and fracture surfaces were observed by scanning electron microscopy (SEM). The samples were sectioned, ground, and were polished to 1 m diamond finish. Vickers hardness, flexure strength and fracture toughness were tested by the same methods used in a previous study [10].
3. Results and discussion Typical XRD spectra, phase assembles and the calculated C/Ta ratios for cubic TaCy in the TaC/TaB2 composites SPS for 5 min at different temperatures were shown in Fig. 1 and Table 1. Fig. 1 exhibits predominant cubic TaCy and minor hexagonal TaB2 . No other crystalline phases (if present), were revealed in the XRD spectra to the detecting limit of XRD. The TaB2 concentration approximately reached the equilibrium predication by reaction (2). This meant the B4 C particle had completely decomposed, and B and C were incorporated into the Ta lattice to form TaCy and TaB2 , despite of some B and C loss by reducing oxide impurities [4,5]. The TaC in the starting powder had regulated its stoichiometry and contributed part of the TaCy contents. Table 1 shows C/Ta ratios for the cubic TaCy in different materials. The C/Ta values increased with SPS temperature from 1600 ◦ C to 1900 ◦ C. C further dissolving into the TaCy lattice at higher temperatures was accounted for the C/Ta ratio increase. In addition, elemental impurities such as Ti, Cr, Fe incorporation into TaCy lattice may also extend the TaCy lattice, resulting in an overestimation of the C/Ta ratio. However, the largest C/Ta = 0.974 calculated for the 1900 ◦ C material was still lower than the C/Ta = 1 predication by reaction (2). This was due to and also indicated some C loss by forming CO [4,5]. The hexagonal tantalum diboride may also form a substoichiometric TaB2−x instead of stoichiometric TaB2 . In this study however, the stoichiometry of this hexagonal tantalum diboride phase was not determined due to lack of necessary references. The relative densities of the materials were significantly improved by increasing the SPS temperature from 1700 to 1800 ◦ C or over (Table 1). The materials SPS at 1600 and 1700 ◦ C had relative density of 70.3% and ∼75.2%, with open porosity of 25.7% and 17.9%, respectively. When the SPS was increased to 1800 ◦ C or above, the open pores were completely closed. Relative density reached 95.8% and 98.3% for the 1800 ◦ C and 1900 ◦ C material, respectively. Fig. 2 shows the densification process of the TaC/TaB2 compact. Before densification initiation at 1351 ◦ C and after densification completion at 1900 ◦ C, the thermal expansion of the graphite punches plus the samples produced the discrete down-ward peaks in the displacement rate curve. Three distinct densification stages were recognized. The first two stages covered temperature ranges of ∼1351–1583 ◦ C and 1675–1863 ◦ C. The third stage only took
place immediately after the SPS temperature approached 1900 ◦ C and lasted for only ∼30 s. After the thermal expansion of the graphite punches (40 mm in height) was subtracted, sample density as a function at any realtime temperatures can be calculated according to the shrinkage curve shown in Fig. 2. Consider the facts the powder compact before sintering had a ∼45.0% relative density and the final density after hold at 1900 ◦ C was 98.3%, the intermediate density were estimated to be 66.1%, 73.9% and 93.6% for temperature points of 1600, 1700 and 1800 ◦ C. These values were in rough consistency with the measured densities for the materials sintered at different temperatures shown in Table 1. An on-going densification during the 5 min hold accounted for the slight higher values for the sintered materials. The microstructures of the materials SPS at different temperatures were illustrated in Fig. 3. The dark regions as those marked by arrows in Fig. 3c were residual pores. TaB2 particles could not be recognized from TaC in the SEM images. The slight imaging contrast difference was due to different grain orientations. Starting B4 C particles with coarse diameter of ∼2.5 m were not observed, in agreement with the completion of reaction (2) indicated by XRD in Fig. 1. According to the grain growth and pore elimination behavior, the microstructures in Fig. 3 also suggested a three-stage densification for the TaC/TaB2 composite. The microstructure suggested termination of the first stage should be modified up to 1700 ◦ C because Fig. 3a and b showed comparable grain sizes in the 1600 ◦ C and 1700 ◦ C materials. At the low temperatures grain growth was inhibited. The average sizes of the TaCy and TaB2 grains for both materials were ∼1.2 m, simulating the starting TaC and Ta particles. Formation of TaCy and TaB2 by C and B diffusion into the Ta particle and stoichiometry modification of TaC to TaCy did not change the grain sizes noticeably. When the SPS temperature was increased to 1800 ◦ C, sufficient densification was observed, accompanied by rapid grains growth and entrapped residual pores. The average grain size was 5.2 m, significantly larger than that in the 1600 ◦ C and 1700 ◦ C materials. The dispersed in situ TaB2 grains did not effectively pin the TaCy grains from rapid growing [13]. The pores indicated by the arrows in Fig. 3c were isolated at the grain corners. Such pores were very difficult to eliminate and therefore further densification was postponed to the higher temperature of 1900 ◦ C. When the temperature was increased from 1800 ◦ C to 1900 ◦ C, 60% of these residual pores were expulsed out with no further grain growth as shown in Fig. 3d. The three-stage densification of the TaC/TaB2 composite was accompanied by observable vacuum deviation in the SPS chamber. The vacuum deviation in Fig. 2 marked by v1 and v2 suggested gaseous products in accompany with the first and third stage. The v1 was correlated to oxygen elimination by forming BO and
L. Liu et al. / Materials Chemistry and Physics 126 (2011) 459–462
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Table 1 Relative density, phase assemble and mechanical properties of the TaC/TaB2 composites. Sintering temperature
Density (%)
Phase assemble
C/Ta in TaCy
Vickers hardness (GPa)
1600 ◦ C 1700 ◦ C 1800 ◦ C 1900 ◦ C
70.3 75.2 95.8 98.3
TaCy , TaB2 TaCy , TaB2 TaCy , TaB2 TaCy , TaB2
0.933 0.964 0.970 0.974
13.2 16.1 21.8 23.9
CO [4,5]. The v2 indicated a volatile product that appeared till high temperature of 1900 ◦ C was reached (this was confirmed by a parallel blank test). The appearance of this vapor suggested evaporation–condensation in the residual pores. It was presumable that this evaporation inhibited further densification to reach 100% density. Determining the chemical constitution of the vapor was of obvious importance but unfortunately out of the SPS equipment’s ability. However, the vapor pressure confined in the residual pores can be roughly estimated as follows. Pc = 1 Pa (the subscribe c means SPS chamber. Actually 1–2 Pa pressure increase was observed with continuously vacuuming) of gas pressure increase in the 450 mm × 500 mm SPS chamber at 473K (a reasonable estimation from the die radiation at 1900 ◦ C) needed n = (PVc /RTc ) = 2.022 × 10−5 mol gas added. For relative density increase from 0.958 to 0.983, the 0.947 × 10−6 m3 sample (13 g with relative density of 0.958) reduced a volume of 0.02368 × 10−6 m3 which equaled to the Vs of the gas that was expulsed out into the SPS chamber. This meant the gas confined in the residual pores had a least pressure of 15.43 MPa (Ps = nRTs /Vs , the subscribe s means sample). This rough estimation should give us a quantitative view about the resistance for further densification.
± ± ± ±
0.3 0.4 0.3 0.3
Flexure strength (MPa) 206 253 397 419
± ± ± ±
29 43 51 46
Fracture toughness (MPa m1/2 ) 2.8 3.2 3.9 4.5
± ± ± ±
0.7 0.6 0.7 0.8
Both Zhang et al. [8] and Talmy et al. [5] processed TaC–TaB2 composites via reaction forming and densification by hot-pressing and pressureless sintering, respectively. However, densification process was not detailed due to absence of necessary information as that was shown in Fig. 2 in this study. Here we would like to bring up the following as a very brief discussion. (1) XRD analysis and Fig. 2 indicated TaC, B4 C and Ta starting powders had completed the reaction to form the targeted products TaCy and TaB2 , and simultaneously oxide impurities were reduced by B4 C below 1583 ◦ C. The resultant cubic TaCy and hexagonal TaB2 underwent a particle rearrangement to reach relative density of 70.3% with high open porosity of 25.7%, ready for better sintering at higher temperatures. (2) In the temperature range 1675–1863 ◦ C, the relative density increased significantly from 70% to ∼95%, which was presumably promoted by a liquid from metallic impurities such as Ti, Cr, and Fe [3]. Few hundreds ppm metallic impurities present in the starting powders could cause remarkable densification and grain growth [3]. Although there are open publications subjected to the beneficial effects of nanostarting powders on densification [1,2], the authors of this paper agreed with Sautereau and Mocellin [3] on that powder granulometry showed no significant influences
Fig. 3. Microstructures of TaC/TaB2 composites SPS at different temperatures: (a) 1600 ◦ C, (b) 1700 ◦ C, (c) 1800 ◦ C, and (d) 1900 ◦ C.
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on the microstructure with metallic impurities at presence. (3) The difficulty of eliminating the last few percent residual pores in the TaC ceramics was associated with a 15 MPa vapor. Therefore, almost all the issues in cubic TaCy ceramic production such as synthesis, microstructure control, and maintaining high temperature mechanical properties demand exquisitely pure starting powders [3,14], besides processing nanopowders. In addition, this clarification of the densifying information should be useful to guide the tailoring of a heating run for tantalum carbide ceramic fabrication. A temperature hold at say 1800 ◦ C so that all the volatile gases can be outgassed before the temperature is increased to 1900 ◦ C may be beneficial. Such study is on-going and the results will be published elsewhere. 4. Conclusion The densification process of the TaC/TaB2 composite with ∼11 wt% in situ TaB2 was composed of three stages: TaCy and
TaB2 phase formation, densification with rapid grain growth, and the elimination of the entrapped pores. The reason for the limited density of cubic TaCy was the evaporation–condensification taking place in the residual pores at high temperatures over 1900 ◦ C. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
J.X. Liu, Y.M. Kan, G.J. Zhang, J. Am. Ceram. Soc. 93 (2010) 370. B. Kim, K. Woo, J. Doh, J. Yoon, I. Shon, Ceram. Int. 35 (2009) 3395. J. Sautereau, A. Mocellin, J. Mater. Sci. 9 (1974) 761. X. Zhang, G.E. Hilmas, W.G. Fahrenholtz, J. Am. Ceram. Soc. 90 (2007) 393. I.G. Talmy, J.A. Zaykoski, M.M. Opeka, J. Eur. Ceram. Soc. 30 (2010) 2253. X. Zhang, G.E. Hilmas, W.G. Fahrenholtz, Mater. Sci. Eng. A 501 (2009) 37. H. Xiang, Y. Xu, L. Zhang, L. Cheng, Scripta Mater. 55 (2006) 339. X. Zhang, G.E. Hilmas, W.G. Fahrenholtz, J. Am. Ceram. Soc. 91 (2008) 4129. C.L. Yeh, E.W. Liu, J. Alloys Compd. 415 (2006) 66. L. Limeng, Y. Feng, Z. Yu, Z. Zhiguo, J. Am. Ceram. Soc. 93 (2010) 2945. E. Khaleghi, Y. Lin, M. Meyers, E. Olevsky, Scripta Mater. 63 (2010) 577. A. Bowman, J. Phys. Chem. 65 (1961) 1596. K. Hackett, S. Verhoef, R.A. Cutler, D.K. Shetty, J. Am. Ceram. Soc. 92 (2009) 2404. R. Gunther, T. Klassen, B. Dickau, F. Gartner, J. Am. Ceram. Soc. 84 (2001) 1509.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Densification process of TaC/TaB2 composite in spark plasma sintering夽 Limeng Liu a,b,∗ , Feng Ye a , Xiulan He a , Yu Zhou a a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 14 September 2010 Accepted 12 January 2011 Keywords: Carbides Sintering Microstructure Mechanical properties
a b s t r a c t A TaC composite with ∼11 wt% in situ TaB2 was fabricated by spark plasma sintering at 1600–1900 ◦ C. The densification process was studied by analysis of the densifying shrinkage of the powder compacts. Three distinct stages for densification were determined. The starting powder mixture of TaC, B4 C and Ta completed reaction to form the desired TaC and TaB2 at temperature <1583 ◦ C. At around ∼1750 ◦ C, the TaC/TaB2 material significantly improved relative density to ∼95% with rapid grain growth. The final densification took place very rapidly at 1900 ◦ C by releasing a high pressure vapor. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
In this study, certain Ta was added to reaction (1) to modified it by
Tantalum carbide is an important ultra-high temperature ceramic (UHTC) for potential applications at >2000 ◦ C. Common practices to improve densification of this high covalent ceramic include using nanostarting powders [1–3], eliminating oxide impurities [3–6], complex powder processing [1,3,7], pinning grain growth [5,8], sintering at very high temperatures [4,5], and applying rapid heating rates [9–11]. One problem associated with processing tantalum carbide ceramic is the difficulty to remove the last few percent residual pores. These residual pores are entrapped and limit the density that a dense tantalum carbide ceramic can reach. Although there are many literatures on densifying tantalum carbide, the elimination process of these entrapped pores and the mechanisms are not clear. Recently, Talmy et al. [5] pressureless fabricated a TaC matrix composite by the following reaction: 16TaC + B4 C → 14TaC + 2TaB2 + 3C.
(1)
The dispersed TaB2 and C particles improved densification by grain boundaries pinning and oxygen reducing. However, the significant C concentration left in the TaC composites could degrade the mechanical properties and oxidation resistance of the composite.
夽 This work was financially supported by China Postdoctoral Science Foundation under Grant No. 20090450957. ∗ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Xidazhi Street 92, Harbin 150001, Heilongjiang, People’s Republic of China. Tel.: +86 451 86413921; fax: +86 451 86413921. E-mail addresses: [email protected], [email protected] (L. Liu). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.038
16TaC + B4 C + 3Ta → 17TaC + 2TaB2 .
(2)
If this modified reaction (2) is complete, ∼11 wt% TaB2 particles will be introduced into the TaCy matrix composite. Spark plasma sintering (SPS) was used to prepare this composite. The SPS densification process, particular the elimination of the residual entrapped pores at the last sintering stage was concerned. 2. Experimental The starting powders were TaC, Ta (Ningxia Orient Tantalum Industry Co., Ltd., China), and B4 C (Jingangzuan Boron Carbide Co. Ltd., China) with mean particle size 1.25 m, 0.7 m, and 2.5 m, respectively. The TaC content in the TaC powder was higher than 99%. The main impurities were ∼0.3 wt% Nb, 0.1 wt% Fe, 0.20 wt% O, 0.15 wt% free carbon, 0.05 wt% N, and Al, Ca, K, Na, Ti with a total amount <0.05 wt%. The B4 C was >95 wt% pure with major impurities of free carbon, free boron, and trace B2 O3 . 60 g TaC, B4 C and Ta in molar ratio of 16:1:3 were mixed in a steel cylinder lined by PVC using 300 g ZrO2 balls as mixing media. Batches of 13 g mixed powders were loaded into a graphite die and SPS for 5 min at 1600, 1700, 1800, or 1900 ◦ C in a vacuum. The sintering process was the same as detailed in a previous paper [10]. The densification process was thoroughly recorded by the accurate SPS data acquiring system. For example, the Pilani gauge (PT-9P, 1 × 105 to 0.13 Pa, Diavac Limited) used to measure the SPS chamber vacuum has a 0.5 Pa resolution for vacuum range of 1–50 Pa. The ram displacement detector (Digicollar, Mutoh Engineering Inc.) secures a 0.01 mm detect accuracy. Given the negligible elastic deformation of the Ø120 mm ram pillar in comparison with the large sintering shrinkage of the Ø20 mm sample under the 12.6 kN uniaxial SPS load (40 MPa) used in this study, the recorded ram displacement can be approximately ascribed to densification of the samples. Theoretical density for the composites was determined by the law of mixtures. Bulk densities were measured by the Archimedes method. Relative density and open porosity was calculated. Phase compositions were analyzed by X-ray diffraction (XRD). The lattice parameter a for the cubic TaCy was calculated by refining the XRD. The C/Ta ratio in the TaCy was determined according to the equation
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Fig. 2. SPS ram displacement and chamber vacuum as functions of the sintering temperatures, showing the densification process of the TaC/TaB2 composites. Fig. 1. Typical X-ray diffraction patterns of the TaC/TaB2 composites spark plasma sintered at 1600–1900 ◦ C.
a(y) = 4.3007 + 0.1563y [12]. Microstructure and fracture surfaces were observed by scanning electron microscopy (SEM). The samples were sectioned, ground, and were polished to 1 m diamond finish. Vickers hardness, flexure strength and fracture toughness were tested by the same methods used in a previous study [10].
3. Results and discussion Typical XRD spectra, phase assembles and the calculated C/Ta ratios for cubic TaCy in the TaC/TaB2 composites SPS for 5 min at different temperatures were shown in Fig. 1 and Table 1. Fig. 1 exhibits predominant cubic TaCy and minor hexagonal TaB2 . No other crystalline phases (if present), were revealed in the XRD spectra to the detecting limit of XRD. The TaB2 concentration approximately reached the equilibrium predication by reaction (2). This meant the B4 C particle had completely decomposed, and B and C were incorporated into the Ta lattice to form TaCy and TaB2 , despite of some B and C loss by reducing oxide impurities [4,5]. The TaC in the starting powder had regulated its stoichiometry and contributed part of the TaCy contents. Table 1 shows C/Ta ratios for the cubic TaCy in different materials. The C/Ta values increased with SPS temperature from 1600 ◦ C to 1900 ◦ C. C further dissolving into the TaCy lattice at higher temperatures was accounted for the C/Ta ratio increase. In addition, elemental impurities such as Ti, Cr, Fe incorporation into TaCy lattice may also extend the TaCy lattice, resulting in an overestimation of the C/Ta ratio. However, the largest C/Ta = 0.974 calculated for the 1900 ◦ C material was still lower than the C/Ta = 1 predication by reaction (2). This was due to and also indicated some C loss by forming CO [4,5]. The hexagonal tantalum diboride may also form a substoichiometric TaB2−x instead of stoichiometric TaB2 . In this study however, the stoichiometry of this hexagonal tantalum diboride phase was not determined due to lack of necessary references. The relative densities of the materials were significantly improved by increasing the SPS temperature from 1700 to 1800 ◦ C or over (Table 1). The materials SPS at 1600 and 1700 ◦ C had relative density of 70.3% and ∼75.2%, with open porosity of 25.7% and 17.9%, respectively. When the SPS was increased to 1800 ◦ C or above, the open pores were completely closed. Relative density reached 95.8% and 98.3% for the 1800 ◦ C and 1900 ◦ C material, respectively. Fig. 2 shows the densification process of the TaC/TaB2 compact. Before densification initiation at 1351 ◦ C and after densification completion at 1900 ◦ C, the thermal expansion of the graphite punches plus the samples produced the discrete down-ward peaks in the displacement rate curve. Three distinct densification stages were recognized. The first two stages covered temperature ranges of ∼1351–1583 ◦ C and 1675–1863 ◦ C. The third stage only took
place immediately after the SPS temperature approached 1900 ◦ C and lasted for only ∼30 s. After the thermal expansion of the graphite punches (40 mm in height) was subtracted, sample density as a function at any realtime temperatures can be calculated according to the shrinkage curve shown in Fig. 2. Consider the facts the powder compact before sintering had a ∼45.0% relative density and the final density after hold at 1900 ◦ C was 98.3%, the intermediate density were estimated to be 66.1%, 73.9% and 93.6% for temperature points of 1600, 1700 and 1800 ◦ C. These values were in rough consistency with the measured densities for the materials sintered at different temperatures shown in Table 1. An on-going densification during the 5 min hold accounted for the slight higher values for the sintered materials. The microstructures of the materials SPS at different temperatures were illustrated in Fig. 3. The dark regions as those marked by arrows in Fig. 3c were residual pores. TaB2 particles could not be recognized from TaC in the SEM images. The slight imaging contrast difference was due to different grain orientations. Starting B4 C particles with coarse diameter of ∼2.5 m were not observed, in agreement with the completion of reaction (2) indicated by XRD in Fig. 1. According to the grain growth and pore elimination behavior, the microstructures in Fig. 3 also suggested a three-stage densification for the TaC/TaB2 composite. The microstructure suggested termination of the first stage should be modified up to 1700 ◦ C because Fig. 3a and b showed comparable grain sizes in the 1600 ◦ C and 1700 ◦ C materials. At the low temperatures grain growth was inhibited. The average sizes of the TaCy and TaB2 grains for both materials were ∼1.2 m, simulating the starting TaC and Ta particles. Formation of TaCy and TaB2 by C and B diffusion into the Ta particle and stoichiometry modification of TaC to TaCy did not change the grain sizes noticeably. When the SPS temperature was increased to 1800 ◦ C, sufficient densification was observed, accompanied by rapid grains growth and entrapped residual pores. The average grain size was 5.2 m, significantly larger than that in the 1600 ◦ C and 1700 ◦ C materials. The dispersed in situ TaB2 grains did not effectively pin the TaCy grains from rapid growing [13]. The pores indicated by the arrows in Fig. 3c were isolated at the grain corners. Such pores were very difficult to eliminate and therefore further densification was postponed to the higher temperature of 1900 ◦ C. When the temperature was increased from 1800 ◦ C to 1900 ◦ C, 60% of these residual pores were expulsed out with no further grain growth as shown in Fig. 3d. The three-stage densification of the TaC/TaB2 composite was accompanied by observable vacuum deviation in the SPS chamber. The vacuum deviation in Fig. 2 marked by v1 and v2 suggested gaseous products in accompany with the first and third stage. The v1 was correlated to oxygen elimination by forming BO and
L. Liu et al. / Materials Chemistry and Physics 126 (2011) 459–462
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Table 1 Relative density, phase assemble and mechanical properties of the TaC/TaB2 composites. Sintering temperature
Density (%)
Phase assemble
C/Ta in TaCy
Vickers hardness (GPa)
1600 ◦ C 1700 ◦ C 1800 ◦ C 1900 ◦ C
70.3 75.2 95.8 98.3
TaCy , TaB2 TaCy , TaB2 TaCy , TaB2 TaCy , TaB2
0.933 0.964 0.970 0.974
13.2 16.1 21.8 23.9
CO [4,5]. The v2 indicated a volatile product that appeared till high temperature of 1900 ◦ C was reached (this was confirmed by a parallel blank test). The appearance of this vapor suggested evaporation–condensation in the residual pores. It was presumable that this evaporation inhibited further densification to reach 100% density. Determining the chemical constitution of the vapor was of obvious importance but unfortunately out of the SPS equipment’s ability. However, the vapor pressure confined in the residual pores can be roughly estimated as follows. Pc = 1 Pa (the subscribe c means SPS chamber. Actually 1–2 Pa pressure increase was observed with continuously vacuuming) of gas pressure increase in the 450 mm × 500 mm SPS chamber at 473K (a reasonable estimation from the die radiation at 1900 ◦ C) needed n = (PVc /RTc ) = 2.022 × 10−5 mol gas added. For relative density increase from 0.958 to 0.983, the 0.947 × 10−6 m3 sample (13 g with relative density of 0.958) reduced a volume of 0.02368 × 10−6 m3 which equaled to the Vs of the gas that was expulsed out into the SPS chamber. This meant the gas confined in the residual pores had a least pressure of 15.43 MPa (Ps = nRTs /Vs , the subscribe s means sample). This rough estimation should give us a quantitative view about the resistance for further densification.
± ± ± ±
0.3 0.4 0.3 0.3
Flexure strength (MPa) 206 253 397 419
± ± ± ±
29 43 51 46
Fracture toughness (MPa m1/2 ) 2.8 3.2 3.9 4.5
± ± ± ±
0.7 0.6 0.7 0.8
Both Zhang et al. [8] and Talmy et al. [5] processed TaC–TaB2 composites via reaction forming and densification by hot-pressing and pressureless sintering, respectively. However, densification process was not detailed due to absence of necessary information as that was shown in Fig. 2 in this study. Here we would like to bring up the following as a very brief discussion. (1) XRD analysis and Fig. 2 indicated TaC, B4 C and Ta starting powders had completed the reaction to form the targeted products TaCy and TaB2 , and simultaneously oxide impurities were reduced by B4 C below 1583 ◦ C. The resultant cubic TaCy and hexagonal TaB2 underwent a particle rearrangement to reach relative density of 70.3% with high open porosity of 25.7%, ready for better sintering at higher temperatures. (2) In the temperature range 1675–1863 ◦ C, the relative density increased significantly from 70% to ∼95%, which was presumably promoted by a liquid from metallic impurities such as Ti, Cr, and Fe [3]. Few hundreds ppm metallic impurities present in the starting powders could cause remarkable densification and grain growth [3]. Although there are open publications subjected to the beneficial effects of nanostarting powders on densification [1,2], the authors of this paper agreed with Sautereau and Mocellin [3] on that powder granulometry showed no significant influences
Fig. 3. Microstructures of TaC/TaB2 composites SPS at different temperatures: (a) 1600 ◦ C, (b) 1700 ◦ C, (c) 1800 ◦ C, and (d) 1900 ◦ C.
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on the microstructure with metallic impurities at presence. (3) The difficulty of eliminating the last few percent residual pores in the TaC ceramics was associated with a 15 MPa vapor. Therefore, almost all the issues in cubic TaCy ceramic production such as synthesis, microstructure control, and maintaining high temperature mechanical properties demand exquisitely pure starting powders [3,14], besides processing nanopowders. In addition, this clarification of the densifying information should be useful to guide the tailoring of a heating run for tantalum carbide ceramic fabrication. A temperature hold at say 1800 ◦ C so that all the volatile gases can be outgassed before the temperature is increased to 1900 ◦ C may be beneficial. Such study is on-going and the results will be published elsewhere. 4. Conclusion The densification process of the TaC/TaB2 composite with ∼11 wt% in situ TaB2 was composed of three stages: TaCy and
TaB2 phase formation, densification with rapid grain growth, and the elimination of the entrapped pores. The reason for the limited density of cubic TaCy was the evaporation–condensification taking place in the residual pores at high temperatures over 1900 ◦ C. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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