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Spark plasma sintering of TiN–TiB2 composites
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ScienceDirect Journal of the European Ceramic Society 34 (2014) 197–203
Spark plasma sintering of TiN–TiB2 composites Mettaya Kitiwan, Akihiko Ito ∗ , Takashi Goto Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 30 April 2013; received in revised form 20 August 2013; accepted 26 August 2013 Available online 19 September 2013
Abstract TiN–TiB2 composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB2 content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB2 composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB2 . Shrinkage of the TiN–70 vol% TiB2 composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB2 composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB2 became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB2 composite containing 50–60 vol% TiB2 above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB2 composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m1/2 , respectively. © 2013 Elsevier Ltd. All rights reserved. Keywords: Titanium nitride; Titanium diboride; Spark plasma sintering
1. Introduction Titanium nitride (TiN) has excellent wear resistance, high hardness, and high chemical stability.1 Titanium diboride (TiB2 ) also exhibits high hardness and elastic modulus values and high thermal and electrical conductivity.2 Therefore, TiN–TiB2 composites are attractive for structural application such as cutting tools, jet engine parts, and armor plates. However, because of high melting point and low self-diffusion coefficient, monolithic TiN and TiB2 are difficult to consolidate. TiN–TiB2 composites have been produced by the direct mixing of TiN and TiB2 powders and subsequently densified by conventional pressureless sintering3,4 or hot pressing.5–7 Spark plasma sintering (SPS) offers advantages for rapid preparation of dense bodies with limited grain growth, leading to superior mechanical properties. During SPS processes, a pulsed direct current can be supplied to a conductive die and specimen. The specimen is uniformly heated inside and out. TiN–TiB2 composite is potentially consolidated by SPS as same as other ceramic composites that cannot be fully densified by conventional sintering processes.8 Reactive SPS of TiN–TiB2
∗ Corresponding author at: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 215 2106; fax: +81 22 215 2107. E-mail address: [email protected] (A. Ito).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.08.034
composites with various component ratio using TiH2 , BN, B and TiN was previously reported.9–11 However, because of a highly exothermic reaction, a fully dense body could not be obtained. Dense TiN–TiB2 composites were obtained by hot pressing at high temperatures; however, prolonged sintering time was required, which caused grain growth and resulted in degradation of mechanical properties. 5 The eutectic temperature of a composite is much lower than the melting temperature of either component; thus, a eutectic composite can be sintered at a lower temperature. We have previously demonstrated that SiC–ZrB2 composites can be well sintered and that they exhibit a local eutectic texture even below the eutectic temperature12 . TiN–TiB2 is a eutectic system, and it is expected to be easily sintered because of its eutectic nature. In this study, TiN–TiB2 composites were prepared by SPS at high temperatures below a eutectic temperature using TiN and TiB2 powders, and the relationship among the sintering conditions, microstructure and mechanical properties was investigated. 2. Experimental procedure TiN (particle size 1.2–1.8 m; Wako Pure Chemical, Osaka, Japan) and TiB2 (particle size 2–3 m; Kojundo Chemical Laboratory, Sakado, Japan) powders were used as starting materials. The powders were manually mixed in an agate mortar and then passed through a 200-mesh sieve. The mixed powder was filled
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into a graphite die (inner diameter of 10 mm) and sintered using a spark plasma sintering (SPS) apparatus (SPS-210LX, Fuji Electronic Industrial, Kawasaki, Japan) at a temperature between 1773 and 2573 K with a heating rate of 1.67 K s−1 under a vacuum. A constant uniaxial pressure of 10 MPa or 100 MPa was applied continuously during sintering. The temperature at the die surface was measured by a pyrometer (Chino, Tokyo, Japan). The graphite die wall thickness is 10 mm, and a small hole of 5 mm in depth was drilled on the graphite die. The infrared pyrometer was set to detect temperature at the hole position which close to specimen; therefore, the sintering temperature could be close to the specimen temperature. The thickness of the specimens after sintering was approximately 3 mm. The linear shrinkage of the specimens during sintering was monitored by the displacement of a punch rod. The density values were measured using the Archimedes method, and the relative density was calculated from the theoretical density of TiN (5.4 g cm−3 ) and TiB2 (4.5 g cm−3 ). The phase and lattice parameters were examined by X-ray diffractometry (XRD; Ultima VI, Rigaku, Tokyo, Japan) with Cu-K␣ radiation. Microstructures were observed using a scanning electron microscope (SEM; S-3400, Hitachi, Tokyo, Japan), and the elemental compositions were analyzed using an electron probe micro analyzer (EPMA; JXA-8530F, JEOL, Japan). A microhardness tester (HM-221, Mitutoyo, Tokyo, Japan) was used to determine the Vickers microhardness and fracture toughness at a load of 0.98 and 4.9 N, respectively. The Vickers hardness (Hv ) was calculated from Eq. (1): P (1) Hv = 1854 d2 where P is the applied load, and d is the average value of the two diagonal lengths for Vickers indentation. The fracture toughness (KIC ) was calculated from Eq. (2): 1/2 P E (2) KIC = 0.018 H c3/2
Fig. 1. Effect of sintering temperature on displacement of TiN and TiB2 .
TiB2 at 973–2473 K and the time dependence of isothermal displacement at 2473 K up to 60 s. The displacement of TiN–TiB2 composites similarly began at 1373 K, gradually decreased with increasing temperature from 1373 to 2473 K, and further slightly decreased during dwell period at 2473 K. However, densification rates depended on composition. The displacement of TiN–70 vol% TiB2 was the most significant at 1573–2473 K. Fig. 3 depicts a phase diagram of the TiN1−x –TiB2 system at 10 MPa in an N2 –Ar atmosphere.16 TiN1−x –TiB2 is a binary eutectic system whose eutectic temperature and composition are 2873 K and 64 vol% TiB2 , respectively, at x = 0.04 of TiN1−x . TiN1−x has a wide range of non-stoichiometry, and the eutectic temperature and eutectic composition decrease with increasing x in TiN1−x . TiN–70 vol% TiB2 is close to the eutectic composition of the TiN–TiB2 binary system. This suggests that the
where E is Young’s modulus (Young’s moduli of TiN and TiB2 are 260 and 500 GPa,13,14 respectively, and Young’s modulus of composites was obtained by a rule of mixture) and c is the half lengths of the crack formed around the corners of indentation.15 3. Results and discussion Densification behavior (i.e., displacement of the specimen) of monolithic TiN and TiB2 and TiN–TiB2 composites was evaluated from the displacement of the graphite punch. The effect of sintering temperature on displacement of monolithic TiN and TiB2 is shown in Fig. 1. The displacement of TiN and TiB2 gradually decreased between 973 and 1373 K and then rapidly decreased from 1373 to 2473 K. At temperatures over 2373 K, the displacement of TiN remained constant, whereas that of TiB2 decreased by a reaction with the carbon from graphite sheet. The XRD peak of TiB2 and TiC were identified at the surface sintered TiB2 specimen. Fig. 2 shows the effect of sintering temperature on the displacement of TiN–TiB2 composites containing 10–90 vol%
Fig. 2. Effect of sintering temperature on the displacement of TiN–TiB2 composites containing 10–90 vol% TiB2 at 973–2473 K and the time dependence of isothermal displacement at 2473 K up to 60 s.
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Fig. 3. Phase diagram of the TiN1−x –TiB2 system at 10 MPa in an N2 –Ar atmosphere.16
eutectic composition could be easily densified as reported for a SiC–ZrB2 composite.12 Fig. 4 shows the effect of the TiB2 content on relative densities of TiN–TiB2 composites. At 1773 K, the relative densities of TiN–TiB2 composites containing 50–60 vol% TiB2 (close to the eutectic composition) were maximal. At 1973 K, the TiN–TiB2 composites containing 10–80 vol% TiB2 exhibited relative densities higher than 95%. At 2373 K, the TiN–TiB2 composites containing 10–80 vol% TiB2 exhibited densities between 96.0% and 99.3%. At 2473 K, the TiN–TiB2 composites containing 20–30 vol% TiB2 showed the highest density of 99.7%. The relative density of TiN–TiB2 composite containing 60 vol% TiB2
Fig. 4. Effect of the TiB2 content on relative densities of TiN–TiB2 composites sintered between 1773 and 2573 K.
Fig. 5. XRD patterns of TiN–TiB2 composites sintered at 2573 K containing (a) 30 (b) 50 and (c) 70 vol% TiB2 , dashed-line indicated the peak position from JCPDS cards.
decreased to 96.1–97.6% owing to the formation of hexagonal boron nitride (hBN), as discussed later. The relative densities of TiN–TiB2 composites observed were higher than those reported by Moriyama et al.4 They fabricated TiN–TiB2 composite by pressureless sintering at 2373 K and obtained the highest density of 87% for 23–64 vol% TiB2 . Reactive SPS was utilized to consolidate TiN–TiB2 composites, and the maximum density was 96.5–97.8%.9–11 Fig. 5 shows the XRD patterns of TiN–TiB2 composites containing 30–70 vol% TiB2 at 2573 K. The peaks in the XRD patterns of the sintered specimens were identified as cubic TiN (JCPDS #38-1420) and hexagonal TiB2 (JCPDS #75-1045) phases. Shifts in the TiN and TiB2 peaks were observed, and the lattice parameter changed depending on the composition, as reported in our previous study.17 The TiN lattice parameter increased from 0.4243 to 0.4250 nm with increasing TiB2 content because of the formation of a solid solution in the TiN unit cell, caused by the diffusion of B atoms from TiB2 while the depletion of B resulted in the decrease of the c-axis length of TiB2 from 0.3230 to 0.3227 nm.17 Fig. 6 shows the SEM images of polished surfaces of TiN–70 vol% TiB2 at 1773–2573 K. The light gray and dark gray areas were identified as TiN and TiB2 , respectively. At 1773 K (Fig. 6(a)), the TiN grain size was 1–2 m, similar to that of the starting powder. The grain shape was spherical or granular with a large amount of pores, indicating insufficient
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Fig. 6. SEM images of TiN–70 vol% TiB2 composites at 1773 K under 100 MPa for 300 s (a), 1973 K under 100 MPa for 300 s (b), 2373 K under 10 MPa for 300 s (c) and 2573 K under 10 MPa without holding time (d). Light gray and dark gray areas were identified as TiN and TiB2 , respectively.
Fig. 7. A schematic of microstructure for TiN–70 vol% TiB2 composites sintered at 2373 K. White and gray areas correspond TiN and TiB2 , respectively.
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Fig. 8. SEM images of polished surface of TiN–TiB2 composites sintered at 2473 K under 10 MPa for 60 s with the TiB2 content of 30 vol% (a), 60 vol% (b) and 90 vol% (c). Light gray and dark gray areas were identified as TiN and TiB2 , respectively. Fracture surface of TiN–60 vol% TiB2 sintered at 2373 K under 10 MPa for 60 s (d).
densification. At 1973–2373 K (Fig. 6(b) and (c)), the TiN grain size was partially increased by grain growth, while some grains were flattened, and the grain size became much smaller than that of the starting powder. Grain growth with a polygonal shape is commonly observed at such high sintering temperatures. This is contradictory to the observed shape in this study. Fig. 7 illustrates a schematic of microstructure for Fig. 6(c). TiN grains were found in a complex texture in TiN–TiB2 composites. A large pulsed current applied in SPS process could have potential field effects at particle contacts, i.e., high local temperature gradients18 and enhancement of mass transport,19 which would result in a rapid consolidation. The complex texture containing smaller TiN grains might be associated with the earlier stage of eutectic nature of TiN–TiB2 system where the structure starts to form lamella pattern. A similar texture was formed in ZrB2 –SiC eutectic system in the composite sintered by SPS12 . Fig. 8 shows the polished surfaces of TiN–TiB2 composites containing 30–90 vol% TiB2 sintered at 2473 K and the fracture surface of TiN–60 vol% TiB2 sintered at 2373 K. The TiN and TiB2 phases were uniformly distributed. The rectangular pores
with thicknesses of 0.2–0.8 m and lengths of 1–3 m were observed, as indicated by the white arrows in Fig. 8(b). The fracture surface of TiN–60 vol% TiB2 showed the presence of an hBN phase, as indicated by the white arrows in Fig. 8(d). hBN was removed during polishing, leaving rectangular pores. The hBN formation was observed in the TiN–TiB2 composites containing 50–60 vol% TiB2 . This could have caused the slight decrease in density. Fig. 9 shows the TEM image of the hBN phase in the TiN–50 vol% TiB2 composite sintered at 2473 K. The hBN phase was detected, as shown by white arrows in Fig. 9(a). hBN formation was confirmed by the selected area electron diffraction pattern in Fig. 9(b), which was indexed along [2 1 0] zone axis of hBN. TiN would lose N and TiB2 would lose B at high temperature when TiN and TiB2 coexist.17 hBN formation can be explained by reactions (3)–(5): TiN → TiN1−x + x1/2N2
(3)
TiB2 → TiB2−y + yB
(4)
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Fig. 9. TEM image of the hBN phase in the TiN–50 vol% TiB2 composite sintered at 2473 K (a) and selected area electron diffraction pattern of hBN phase (b).
Δ1/2N2 + ΔB → ΔBN(Δ = min(x, y))
(5)
Amount of BN formation (Δ) depends on the content of TiN or TiB2 , whichever is lesser. Therefore, hBN formation occurred significantly at an intermediate composition, namely 50–60 vol% TiB2 . Fig. 10 displays the effects of composition and sintering temperature on the hardness and fracture toughness of TiN–TiB2 composites. The Vickers hardness increased with temperature corresponding to an increase in relative density. The hardness of the TiN–TiB2 composites sintered between 1973 and 2473 K ranged between 11.3 and 26.3 GPa and exhibited a maximum at 70–80 vol% TiB2 . The fracture toughness had two maxima of 4.9 and 4.5 MPa m1/2 for 10 and 80 vol% TiB2 , respectively. Fig. 11 shows the SEM image of the Vickers indentation and cracks of TiN–70% vol% TiB2 composites sintered at 2473 K.
Fig. 10. Effects of composition and sintering temperature on hardness (a) and fracture toughness (b) of TiN–TiB2 composites.
The crack tended to propagate through TiN grains, while it was deflected along the TiB2 grain, as indicated by the white arrows. This crack deflection could have caused high fracture toughness. It is common that a higher hardness leads to lower fracture toughness. However, in this study, TiN–TiB2 with 80 vol% TiB2 had the highest hardness of 26.3 GPa and high fracture toughness of 4.5 MPa m1/2 . Fig. 12 depicts the relationship between the hardness and fracture toughness of TiN–TiB2 composites sintered at 2473 K compared to values from the literature. Bellosi et al.6 fabricated TiN–20 vol% TiB2 by hot pressing at 1473 K and reported a hardness 13.7 GPa at a load of 4.9 N and fracture toughness of and 3.8 MPa m1/2 from chevron-notch method. Moriyama et al.4 produced TiN–TiB2 by pressureless sintering at 2373 K, and a maximum hardness of 22.8 GPa at a load of 9.8 N was achieved for TiN–64 vol% TiB2 , and the fracture toughness was 4.1 MPa m1/2 from single-edge notched beam method. Khobta
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exhibited relative densities over 95%. Fully dense TiN–TiB2 composites of 99.7% were obtained at 2573 K at 20–30 vol% TiB2 . The TiN–70 vol% TiB2 composite exhibited the highest shrinkage during sintering. The TiN–80 vol% TiB2 composite sintered at 2473 K had the highest hardness and high fracture toughness of 26.3 GPa and 4.5 MPa m1/2 , respectively. The SPS optimum sintering temperature of 2473 K can prepare fully densified TiN–TiB2 composites, and results in the highest mechanical properties. References
Fig. 11. SEM image of the Vickers indentation and cracks of TiN–70 vol% TiB2 composites sintered at 2473 K.
Fig. 12. Relationship between the hardness and fracture toughness of TiN–TiB2 composites sintered at 2473 K compared to values from the literatures. (HP = hot pressing, PS = pressureless sintering, RSPS = reactive spark plasma sintering). Indentation loads for hardness: 4.9 N for squares,6 9.8 N for rhombuses4 and 0.98 N for triangles.9
et al.9 prepared TiN–TiB2 composites by reactive SPS, where the hardness of TiN–83 vol% TiB2 was 24.7 GPa at a load of 0.98 N and fracture toughness 5.2 MPa m1/2 from indentation fracture method. For the same TiB2 content, the TiN–TiB2 composites prepared by SPS showed a higher hardness. 4. Conclusions TiN–TiB2 composites were fabricated by SPS at 1773–2573 K. Above 2373 K, the TiN–TiB2 composites
1. Kuwahara H, Mazaki N, Takahashi M, Watanabe T, Yang X, Aizawa T. Mechanical properties of bulk sintered titanium nitride ceramics. Mater Sci Eng 2001;319–321:687–91. 2. Munro RG. Material properties of titanium diboride. J Res Natl Inst Stand Technol 2000;105(5):709–20. 3. Shobu K, Watanabe T, Enomoto Y, Umeda K, Tsuya Y. Frictional properties of sintered TiN–TiB2 and Ti(CN)–TiB2 ceramics at high temperature. J Am Ceram Soc 1987;70(5):C103–4. 4. Moriyama M, Aoki H, Kamata K. Mechanical and electrical properties of pressureless sintered TiN–TiB2 system. J Ceram Soc Jpn 1995;103(8):844–9. 5. Shobu K, Watanabe T. Hot pressing of TiN–TiB2 system. J Jpn Soc Powder Powder Metall 1985;32(6):215–8. 6. Bellosi A, Monteverde F. Microstructure and properties of titanium nitride and titanium diboride-based composites. Key Eng Mater 2000;175176:139–48. 7. Gu M, Huang C, Zou B, Liu B. Effect of (Ni, Mo) and TiN on the microstructure and mechanical properties of TiB2 ceramic tool materials. Mater Sci Eng 2006;433(1-2):39–44. 8. Bellosi A, Monteverde F, Sciti D. Fast densification of ultra-hightemperature ceramics by spark plasma sintering. Int J Appl Ceram Technol 2006;3(1):32–40. 9. Khobta I, Petukhov O, Vasylkiv O, Sakka Y, Ragulya A. Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering. J Alloys Compd 2011;509(5):1601–6. 10. Petukhov AS, Khobta IV, Ragulya AV, Derevyanko AV, Raichenko AI, Isaeva LP, et al. Reactive electric–discharge sintering of TiN–TiB2 . Powder Metall Met Ceram 2007;46(11–12):525–32. 11. Lee JW, Munir ZA, Shibuya M, Ohyanagi M. Synthesis of dense TiB2 –TiN nanocrystalline composites through mechanical and field activation. J Am Ceram Soc 2004;84(6):1209–16. 12. Akin I, Hotta M, Sahin FC, Yucel O, Goller G, Goto T. Microstructure and densification of ZrB2 –SiC composites prepared by spark plasma sintering. J Eur Ceram Soc 2009;29(11):2379–85. 13. Barsoum MW. Fundamentals of ceramics. UK: Institute of Physics Pub; 2003. 14. Munz D. Ceramics: mechanical properties, failure behaviour, materials selection. Germany: Springer; 1999. 15. Miyoshi T, Sagawa N, Sassa T. Study on fracture toughness evaluation for structural ceramics. Trans Jpn Soc Mech Eng Ser 1985;51(471): 2489–97. 16. Chupov VD, Ordan’yan SS. Reaction in the TiNx –TiB2 system. Inorg Mater 1981;17:1195–8. 17. Kitiwan M, Ito A, Goto T. B deficiency in TiB2 and B solid solution in TiN in TiN–TiB2 composites prepared by spark plasma sintering. J Eur Ceram Soc 2012;32(16):4021–4. 18. Olevsky EA, Froyen L. Impact of thermal diffusion on densification during SPS. J Am Ceram Soc 2009;92:S122–32. 19. Munir ZA, Quach DV, Ohyanagi M. Electric current activation of sintering: a review of the pulsed electric current sintering process. J Am Ceram Soc 2011;94(1):1–19.
ScienceDirect Journal of the European Ceramic Society 34 (2014) 197–203
Spark plasma sintering of TiN–TiB2 composites Mettaya Kitiwan, Akihiko Ito ∗ , Takashi Goto Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 30 April 2013; received in revised form 20 August 2013; accepted 26 August 2013 Available online 19 September 2013
Abstract TiN–TiB2 composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB2 content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB2 composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB2 . Shrinkage of the TiN–70 vol% TiB2 composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB2 composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB2 became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB2 composite containing 50–60 vol% TiB2 above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB2 composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m1/2 , respectively. © 2013 Elsevier Ltd. All rights reserved. Keywords: Titanium nitride; Titanium diboride; Spark plasma sintering
1. Introduction Titanium nitride (TiN) has excellent wear resistance, high hardness, and high chemical stability.1 Titanium diboride (TiB2 ) also exhibits high hardness and elastic modulus values and high thermal and electrical conductivity.2 Therefore, TiN–TiB2 composites are attractive for structural application such as cutting tools, jet engine parts, and armor plates. However, because of high melting point and low self-diffusion coefficient, monolithic TiN and TiB2 are difficult to consolidate. TiN–TiB2 composites have been produced by the direct mixing of TiN and TiB2 powders and subsequently densified by conventional pressureless sintering3,4 or hot pressing.5–7 Spark plasma sintering (SPS) offers advantages for rapid preparation of dense bodies with limited grain growth, leading to superior mechanical properties. During SPS processes, a pulsed direct current can be supplied to a conductive die and specimen. The specimen is uniformly heated inside and out. TiN–TiB2 composite is potentially consolidated by SPS as same as other ceramic composites that cannot be fully densified by conventional sintering processes.8 Reactive SPS of TiN–TiB2
∗ Corresponding author at: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 215 2106; fax: +81 22 215 2107. E-mail address: [email protected] (A. Ito).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.08.034
composites with various component ratio using TiH2 , BN, B and TiN was previously reported.9–11 However, because of a highly exothermic reaction, a fully dense body could not be obtained. Dense TiN–TiB2 composites were obtained by hot pressing at high temperatures; however, prolonged sintering time was required, which caused grain growth and resulted in degradation of mechanical properties. 5 The eutectic temperature of a composite is much lower than the melting temperature of either component; thus, a eutectic composite can be sintered at a lower temperature. We have previously demonstrated that SiC–ZrB2 composites can be well sintered and that they exhibit a local eutectic texture even below the eutectic temperature12 . TiN–TiB2 is a eutectic system, and it is expected to be easily sintered because of its eutectic nature. In this study, TiN–TiB2 composites were prepared by SPS at high temperatures below a eutectic temperature using TiN and TiB2 powders, and the relationship among the sintering conditions, microstructure and mechanical properties was investigated. 2. Experimental procedure TiN (particle size 1.2–1.8 m; Wako Pure Chemical, Osaka, Japan) and TiB2 (particle size 2–3 m; Kojundo Chemical Laboratory, Sakado, Japan) powders were used as starting materials. The powders were manually mixed in an agate mortar and then passed through a 200-mesh sieve. The mixed powder was filled
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into a graphite die (inner diameter of 10 mm) and sintered using a spark plasma sintering (SPS) apparatus (SPS-210LX, Fuji Electronic Industrial, Kawasaki, Japan) at a temperature between 1773 and 2573 K with a heating rate of 1.67 K s−1 under a vacuum. A constant uniaxial pressure of 10 MPa or 100 MPa was applied continuously during sintering. The temperature at the die surface was measured by a pyrometer (Chino, Tokyo, Japan). The graphite die wall thickness is 10 mm, and a small hole of 5 mm in depth was drilled on the graphite die. The infrared pyrometer was set to detect temperature at the hole position which close to specimen; therefore, the sintering temperature could be close to the specimen temperature. The thickness of the specimens after sintering was approximately 3 mm. The linear shrinkage of the specimens during sintering was monitored by the displacement of a punch rod. The density values were measured using the Archimedes method, and the relative density was calculated from the theoretical density of TiN (5.4 g cm−3 ) and TiB2 (4.5 g cm−3 ). The phase and lattice parameters were examined by X-ray diffractometry (XRD; Ultima VI, Rigaku, Tokyo, Japan) with Cu-K␣ radiation. Microstructures were observed using a scanning electron microscope (SEM; S-3400, Hitachi, Tokyo, Japan), and the elemental compositions were analyzed using an electron probe micro analyzer (EPMA; JXA-8530F, JEOL, Japan). A microhardness tester (HM-221, Mitutoyo, Tokyo, Japan) was used to determine the Vickers microhardness and fracture toughness at a load of 0.98 and 4.9 N, respectively. The Vickers hardness (Hv ) was calculated from Eq. (1): P (1) Hv = 1854 d2 where P is the applied load, and d is the average value of the two diagonal lengths for Vickers indentation. The fracture toughness (KIC ) was calculated from Eq. (2): 1/2 P E (2) KIC = 0.018 H c3/2
Fig. 1. Effect of sintering temperature on displacement of TiN and TiB2 .
TiB2 at 973–2473 K and the time dependence of isothermal displacement at 2473 K up to 60 s. The displacement of TiN–TiB2 composites similarly began at 1373 K, gradually decreased with increasing temperature from 1373 to 2473 K, and further slightly decreased during dwell period at 2473 K. However, densification rates depended on composition. The displacement of TiN–70 vol% TiB2 was the most significant at 1573–2473 K. Fig. 3 depicts a phase diagram of the TiN1−x –TiB2 system at 10 MPa in an N2 –Ar atmosphere.16 TiN1−x –TiB2 is a binary eutectic system whose eutectic temperature and composition are 2873 K and 64 vol% TiB2 , respectively, at x = 0.04 of TiN1−x . TiN1−x has a wide range of non-stoichiometry, and the eutectic temperature and eutectic composition decrease with increasing x in TiN1−x . TiN–70 vol% TiB2 is close to the eutectic composition of the TiN–TiB2 binary system. This suggests that the
where E is Young’s modulus (Young’s moduli of TiN and TiB2 are 260 and 500 GPa,13,14 respectively, and Young’s modulus of composites was obtained by a rule of mixture) and c is the half lengths of the crack formed around the corners of indentation.15 3. Results and discussion Densification behavior (i.e., displacement of the specimen) of monolithic TiN and TiB2 and TiN–TiB2 composites was evaluated from the displacement of the graphite punch. The effect of sintering temperature on displacement of monolithic TiN and TiB2 is shown in Fig. 1. The displacement of TiN and TiB2 gradually decreased between 973 and 1373 K and then rapidly decreased from 1373 to 2473 K. At temperatures over 2373 K, the displacement of TiN remained constant, whereas that of TiB2 decreased by a reaction with the carbon from graphite sheet. The XRD peak of TiB2 and TiC were identified at the surface sintered TiB2 specimen. Fig. 2 shows the effect of sintering temperature on the displacement of TiN–TiB2 composites containing 10–90 vol%
Fig. 2. Effect of sintering temperature on the displacement of TiN–TiB2 composites containing 10–90 vol% TiB2 at 973–2473 K and the time dependence of isothermal displacement at 2473 K up to 60 s.
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Fig. 3. Phase diagram of the TiN1−x –TiB2 system at 10 MPa in an N2 –Ar atmosphere.16
eutectic composition could be easily densified as reported for a SiC–ZrB2 composite.12 Fig. 4 shows the effect of the TiB2 content on relative densities of TiN–TiB2 composites. At 1773 K, the relative densities of TiN–TiB2 composites containing 50–60 vol% TiB2 (close to the eutectic composition) were maximal. At 1973 K, the TiN–TiB2 composites containing 10–80 vol% TiB2 exhibited relative densities higher than 95%. At 2373 K, the TiN–TiB2 composites containing 10–80 vol% TiB2 exhibited densities between 96.0% and 99.3%. At 2473 K, the TiN–TiB2 composites containing 20–30 vol% TiB2 showed the highest density of 99.7%. The relative density of TiN–TiB2 composite containing 60 vol% TiB2
Fig. 4. Effect of the TiB2 content on relative densities of TiN–TiB2 composites sintered between 1773 and 2573 K.
Fig. 5. XRD patterns of TiN–TiB2 composites sintered at 2573 K containing (a) 30 (b) 50 and (c) 70 vol% TiB2 , dashed-line indicated the peak position from JCPDS cards.
decreased to 96.1–97.6% owing to the formation of hexagonal boron nitride (hBN), as discussed later. The relative densities of TiN–TiB2 composites observed were higher than those reported by Moriyama et al.4 They fabricated TiN–TiB2 composite by pressureless sintering at 2373 K and obtained the highest density of 87% for 23–64 vol% TiB2 . Reactive SPS was utilized to consolidate TiN–TiB2 composites, and the maximum density was 96.5–97.8%.9–11 Fig. 5 shows the XRD patterns of TiN–TiB2 composites containing 30–70 vol% TiB2 at 2573 K. The peaks in the XRD patterns of the sintered specimens were identified as cubic TiN (JCPDS #38-1420) and hexagonal TiB2 (JCPDS #75-1045) phases. Shifts in the TiN and TiB2 peaks were observed, and the lattice parameter changed depending on the composition, as reported in our previous study.17 The TiN lattice parameter increased from 0.4243 to 0.4250 nm with increasing TiB2 content because of the formation of a solid solution in the TiN unit cell, caused by the diffusion of B atoms from TiB2 while the depletion of B resulted in the decrease of the c-axis length of TiB2 from 0.3230 to 0.3227 nm.17 Fig. 6 shows the SEM images of polished surfaces of TiN–70 vol% TiB2 at 1773–2573 K. The light gray and dark gray areas were identified as TiN and TiB2 , respectively. At 1773 K (Fig. 6(a)), the TiN grain size was 1–2 m, similar to that of the starting powder. The grain shape was spherical or granular with a large amount of pores, indicating insufficient
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Fig. 6. SEM images of TiN–70 vol% TiB2 composites at 1773 K under 100 MPa for 300 s (a), 1973 K under 100 MPa for 300 s (b), 2373 K under 10 MPa for 300 s (c) and 2573 K under 10 MPa without holding time (d). Light gray and dark gray areas were identified as TiN and TiB2 , respectively.
Fig. 7. A schematic of microstructure for TiN–70 vol% TiB2 composites sintered at 2373 K. White and gray areas correspond TiN and TiB2 , respectively.
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Fig. 8. SEM images of polished surface of TiN–TiB2 composites sintered at 2473 K under 10 MPa for 60 s with the TiB2 content of 30 vol% (a), 60 vol% (b) and 90 vol% (c). Light gray and dark gray areas were identified as TiN and TiB2 , respectively. Fracture surface of TiN–60 vol% TiB2 sintered at 2373 K under 10 MPa for 60 s (d).
densification. At 1973–2373 K (Fig. 6(b) and (c)), the TiN grain size was partially increased by grain growth, while some grains were flattened, and the grain size became much smaller than that of the starting powder. Grain growth with a polygonal shape is commonly observed at such high sintering temperatures. This is contradictory to the observed shape in this study. Fig. 7 illustrates a schematic of microstructure for Fig. 6(c). TiN grains were found in a complex texture in TiN–TiB2 composites. A large pulsed current applied in SPS process could have potential field effects at particle contacts, i.e., high local temperature gradients18 and enhancement of mass transport,19 which would result in a rapid consolidation. The complex texture containing smaller TiN grains might be associated with the earlier stage of eutectic nature of TiN–TiB2 system where the structure starts to form lamella pattern. A similar texture was formed in ZrB2 –SiC eutectic system in the composite sintered by SPS12 . Fig. 8 shows the polished surfaces of TiN–TiB2 composites containing 30–90 vol% TiB2 sintered at 2473 K and the fracture surface of TiN–60 vol% TiB2 sintered at 2373 K. The TiN and TiB2 phases were uniformly distributed. The rectangular pores
with thicknesses of 0.2–0.8 m and lengths of 1–3 m were observed, as indicated by the white arrows in Fig. 8(b). The fracture surface of TiN–60 vol% TiB2 showed the presence of an hBN phase, as indicated by the white arrows in Fig. 8(d). hBN was removed during polishing, leaving rectangular pores. The hBN formation was observed in the TiN–TiB2 composites containing 50–60 vol% TiB2 . This could have caused the slight decrease in density. Fig. 9 shows the TEM image of the hBN phase in the TiN–50 vol% TiB2 composite sintered at 2473 K. The hBN phase was detected, as shown by white arrows in Fig. 9(a). hBN formation was confirmed by the selected area electron diffraction pattern in Fig. 9(b), which was indexed along [2 1 0] zone axis of hBN. TiN would lose N and TiB2 would lose B at high temperature when TiN and TiB2 coexist.17 hBN formation can be explained by reactions (3)–(5): TiN → TiN1−x + x1/2N2
(3)
TiB2 → TiB2−y + yB
(4)
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Fig. 9. TEM image of the hBN phase in the TiN–50 vol% TiB2 composite sintered at 2473 K (a) and selected area electron diffraction pattern of hBN phase (b).
Δ1/2N2 + ΔB → ΔBN(Δ = min(x, y))
(5)
Amount of BN formation (Δ) depends on the content of TiN or TiB2 , whichever is lesser. Therefore, hBN formation occurred significantly at an intermediate composition, namely 50–60 vol% TiB2 . Fig. 10 displays the effects of composition and sintering temperature on the hardness and fracture toughness of TiN–TiB2 composites. The Vickers hardness increased with temperature corresponding to an increase in relative density. The hardness of the TiN–TiB2 composites sintered between 1973 and 2473 K ranged between 11.3 and 26.3 GPa and exhibited a maximum at 70–80 vol% TiB2 . The fracture toughness had two maxima of 4.9 and 4.5 MPa m1/2 for 10 and 80 vol% TiB2 , respectively. Fig. 11 shows the SEM image of the Vickers indentation and cracks of TiN–70% vol% TiB2 composites sintered at 2473 K.
Fig. 10. Effects of composition and sintering temperature on hardness (a) and fracture toughness (b) of TiN–TiB2 composites.
The crack tended to propagate through TiN grains, while it was deflected along the TiB2 grain, as indicated by the white arrows. This crack deflection could have caused high fracture toughness. It is common that a higher hardness leads to lower fracture toughness. However, in this study, TiN–TiB2 with 80 vol% TiB2 had the highest hardness of 26.3 GPa and high fracture toughness of 4.5 MPa m1/2 . Fig. 12 depicts the relationship between the hardness and fracture toughness of TiN–TiB2 composites sintered at 2473 K compared to values from the literature. Bellosi et al.6 fabricated TiN–20 vol% TiB2 by hot pressing at 1473 K and reported a hardness 13.7 GPa at a load of 4.9 N and fracture toughness of and 3.8 MPa m1/2 from chevron-notch method. Moriyama et al.4 produced TiN–TiB2 by pressureless sintering at 2373 K, and a maximum hardness of 22.8 GPa at a load of 9.8 N was achieved for TiN–64 vol% TiB2 , and the fracture toughness was 4.1 MPa m1/2 from single-edge notched beam method. Khobta
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exhibited relative densities over 95%. Fully dense TiN–TiB2 composites of 99.7% were obtained at 2573 K at 20–30 vol% TiB2 . The TiN–70 vol% TiB2 composite exhibited the highest shrinkage during sintering. The TiN–80 vol% TiB2 composite sintered at 2473 K had the highest hardness and high fracture toughness of 26.3 GPa and 4.5 MPa m1/2 , respectively. The SPS optimum sintering temperature of 2473 K can prepare fully densified TiN–TiB2 composites, and results in the highest mechanical properties. References
Fig. 11. SEM image of the Vickers indentation and cracks of TiN–70 vol% TiB2 composites sintered at 2473 K.
Fig. 12. Relationship between the hardness and fracture toughness of TiN–TiB2 composites sintered at 2473 K compared to values from the literatures. (HP = hot pressing, PS = pressureless sintering, RSPS = reactive spark plasma sintering). Indentation loads for hardness: 4.9 N for squares,6 9.8 N for rhombuses4 and 0.98 N for triangles.9
et al.9 prepared TiN–TiB2 composites by reactive SPS, where the hardness of TiN–83 vol% TiB2 was 24.7 GPa at a load of 0.98 N and fracture toughness 5.2 MPa m1/2 from indentation fracture method. For the same TiB2 content, the TiN–TiB2 composites prepared by SPS showed a higher hardness. 4. Conclusions TiN–TiB2 composites were fabricated by SPS at 1773–2573 K. Above 2373 K, the TiN–TiB2 composites
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