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Microstructure and mechanical properties of (Ti,W)C–Ni cermet prepared using a nano-sized TiC–WC powder mixture
Journal of Alloys and Compounds 639 (2015) 21–26
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure and mechanical properties of (Ti,W)C–Ni cermet prepared using a nano-sized TiC–WC powder mixture Hanjung Kwon, Chang-Yul Suh, Wonbaek Kim ⇑ Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 January 2015 Received in revised form 10 March 2015 Accepted 14 March 2015 Available online 20 March 2015 Keywords: Composite materials Mechanical alloying Sintering Mechanical properties Electron emission spectroscopies
a b s t r a c t The microstructure and mechanical properties of a (Ti,W)C–Ni cermet, which was prepared from a mixture of nano-sized TiC–WC powder, were investigated in comparison with those of a (Ti,W)(CN)–Ni cermet, which was also prepared from a mixture of nano-sized powder, and previously reported to have high toughness values. The mixture of nano-sized powders was prepared by milling commercial Ti and W powders at various molar ratios (9:1, 8:2, 7:3, 6:4), with graphite. The mixture was isothermally sintered with Ni powder at 1500 °C for 1 h. EDS analysis revealed that the solid–solution phase in the (Ti,W)C–Ni cermet contained considerably more W (44.6 at%) than in the (Ti,W)(CN)–Ni cermet (30.6 at%). This was attributed to the higher affinity between TiC and WC, than between Ti(CN) and WC. The fracture toughness (max. 15.2 MPa m1/2) of the (Ti,W)C–Ni cermet was superior to that of the (Ti,W)(CN)–Ni cermet (max. 12.1 MPa m1/2). The toughness enhancement is attributed to the increase of W content in the solid–solution phase, together with the higher volume fraction of Ni-binder phase. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Ti-based cermet is used as a material for cutting tools because of its outstanding hardness, high resistance to wear, and excellent chemical stability [1]. However, the relatively lower toughness of Ti-based cermet (compared to WC–Co cermet) limits its application as a cutting-tool material to specific milling operations [1–11]. At present, Ti-based cermet cutting tools are typically used for high-speed milling, semi-finishing, and finishing work for both carbon and stainless steels [2]. Generally, Ti-based cermet contains other carbides (Mo2C, WC, VC, NbC, and TaC) to improve its properties [1,3,5]. The cutting performance of Ti-based cermet was improved by adding WC due to the excellent wear properties of TiC/Ti(CN), combined with the extreme elastic modulus and excellent heat conductivity of WC [4–9]. In addition, WC in Ti-based cermet serves to increase the density by improving the wetting and sinterability and decreases the particle growth rate [5–8]. However, the fracture toughness of Ti-based cermet with WC, is still inferior to that of WC–Co, and as a result, Ti-based cermet is not as widely used as cuttingtool material as WC–Co.
⇑ Corresponding author. Tel.: +82 42 868 3623; fax: +82 42 868 3415. E-mail address: [email protected] (W. Kim). http://dx.doi.org/10.1016/j.jallcom.2015.03.115 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
Ultrafine or nano-sized powders have also been used to improve the fracture toughness of Ti-based cermet. A significant improvement in the mechanical properties of (Ti,W)(CN)–Ni cermet was reported from using ultrafine or nano-sized powders instead of conventional micron-sized powders (for cermet from micron-sized powders: Hv 9–11 GPa, KIC 6–7 MPa m1/2; cermet from ultrafine powders: Hv 14–15 GPa, KIC 8–10 MPa m1/2; cermet from nano-sized powders: Hv 12–13 GPa, KIC 10–12 MPa m1/2) [6–8]. In particular, the cermet from a mixture of nano-sized powders (nano-pmix) provides significantly improved toughness by eliminating Ti(CN) cores. More specifically, the use of nanosized Ti(CN) facilitates the complete dissolution of Ti(CN), resulting in more homogeneous nucleation and growth of the (Ti,W)(CN) solid solution than in cermet prepared from micron-sized powders (micron-pmix) or ultrafine powders (uf-pmix). The cermet prepared from micron-pmix and uf-pmix usually exhibit maximum saturation of W content in the composition of the solid–solution phase of the (Ti,W)(CN)–Ni cermet, although the values vary from sample to sample. On the other hand, the cermet from nano-pmix does not appear to have a saturation point in the composition of the solid–solution phase. It is well known that the toughness value in a Ti-based cermet increases with increasing of W content of the solid–solution phase [10,11]. Therefore, there appears to be room for further improvement of the fracture toughness when nano-powder-mixes are used, because the W content in the solid–solution phase is not limited.
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The reason for the unsaturated W in the composition of the solid–solution phase of cermet from nano-pmix of Ti(CN)–WC, may be due to the low affinity between W and N [12,13]. Thus, when the nano-pmix of Ti(CN) dissolves completely in the Ni matrix during liquid-phase sintering, nitrogen atoms from the Ti(CN) impede the solubility of W in the solid–solution phase. Therefore, it can be posited that the W concentration in the solid–solution phase of cermet from a nano-pmix of TiC–WC powders could be higher than that of cermet from a nano-pmix of Ti(CN)–WC powders. As a result, the fracture toughness of the (Ti,W)C–Ni cermet might be higher than that of the (Ti,W)(CN)–Ni cermet. In this study, a cermet from the nano-pmix of TiC–WC was fabricated to obtain a cermet with improved toughness, and then compared to a cermet from the nano-pmix of Ti(CN)–WC. First, a nano-pmix of TiC–WC, with a uniform distribution of TiC and WC, was prepared by high-energy milling and subsequent heat treatment. Second, changes in the microstructure and mechanical properties of a (Ti,W)C–Ni cermet prepared using a nano-pmix of TiC–WC were investigated as a function of WC content. Finally, the results were compared with previously reported (Ti,W)(CN)–Ni cermet prepared from a nano-pmix of Ti(CN)–WC.
TiC WC W2 C W
(d) 6:4
(c) 7:3
(b) 8:2
(a) 9:1
20
30
40
50
60
70
80
90
2θ Fig. 1. X-ray diffraction patterns of the Ti–W–C powder mixture after milling for 20 h: The molar ratios of Ti and W were (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
TiC WC
2. Experimental procedure A nano-pmix of TiC–WC was synthesized using commercial Ti (Tasco, 325 mesh, 99.5%) and W (TaeguTec, <10 lm, >99%) powders. The powders were mixed with graphite (Alfa, 325 mesh, 99.8%) to attain the nano-pmix of TiC–WC (molar ratios of Ti:W = 9:1, 8:2, 7:3, and 6:4). The powders were subjected to high-energy milling using a planetary mill (Model Pulverisette 5, Fritsch, Germany). Tungsten carbide balls were mixed with Ti, W, and graphite at a ball-to-powder weight ratio of 20:1. A stainless steel bowl was used, and milling was conducted at a speed of 250 RPM for 20 h under Ar gas. The milled powder was heat-treated at 1200 °C for 1 h to complete the carbide-formation reaction. The phases in the milled and heat-treated powder were analyzed using X-ray diffraction (XRD) analysis (SmartLab, Rigaku, Japan). Monochromatized Cu Ka radiation (k = 1.5418 Å) was employed during the analyses, and Si was used as a standard to calibrate the diffractometer. The morphology of the powders was examined using a fieldemission transmission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). A (Ti,W)C–Ni cermet was prepared by mixing a nano-pmix of TiC–WC with Ni powder (Sigma–Aldrich, <1 lm, 99%) by horizontal ball milling. The weight ratio of the TiC–WC mixed powder, and the Ni powder, was maintained at 8:2. The mixed powder was then compacted into a disc under a pressure of 125 MPa. The mixture was sintered at 1500 °C for 1 h. Then, the sintered specimens were ground and their microstructure observed via a field-emission scanning electron microscope (Quanta 650F, FEI, Oregon, USA). In order to determine the regional compositions of the phases (e.g., core, rim, and binder), an energy dispersive X-ray analysis (EDS) was performed with a field-emission transmission electron microscope. Each data point was an average of 10 points or more from several particles. Vickers hardness was measured with an indenter load of 30 kg, and fracture toughness was calculated using the expression derived by Shetty et al. [14].
3. Results and discussion 3.1. Preparation of mixture of nano-sized TiC–WC powders Fig. 1 is a series of X-ray diffraction patterns illustrating the phase evolution during high-energy milling of Ti + W + C mixtures to achieve various molar ratios of Ti and W. When high-energy milling was applied to the mixtures for 20 h, pure Ti in the raw material reacted preferentially with C, forming a TiC phase with a B1 (NaCl-like) structure at all Ti/W ratios. This observation demonstrates the higher stability of TiC compared to that of WC. The standard Gibbs free energy of formation of TiC is 173.12 kJ/mole at 1000 K while that of WC is 35.81 kJ/mole [15]. When W was the smallest (molar ratio 9:1), small WC peaks were observed. When the molar ratio was increased to 8:2 and 7:3, a W2C phase was also formed. When the molar ratio was the highest (6:4), metallic W peaks appeared along with broadened
(d) 6:4
(c) 7:3
(b) 8:2
(a) 9:1
20
30
40
50
60
70
80
90
2θ Fig. 2. X-ray diffraction patterns of the milled Ti–W–C mixture after heat treatment at 1200 °C for 1 h: The milled/heat-treated powders of Ti, W, and C: molar ratios of Ti and W are (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
TiC peaks. It may be that the applied mechanical energy during milling may not be sufficient to complete the WC formation. However, the reaction was completed by heat treatment afterwards. Fig. 2 shows the XRD patterns of the powders after heat treatment at 1200 °C for 1 h. It clearly shows that the mixture was fully converted to TiC and WC. One thing to be noted here is that the TiC peaks are significantly broadened as the molar ratio increases. The crystallite sizes of the synthesized phases were calculated using the Halder–Wagner method [16]. The crystallite sizes of the synthesized powders range from 10.4 nm to 28.3 nm (Table 1). Fig. 3 shows the TEM micrograph of the mixture of TiC–WC powders wherein the molar ratio of Ti and W was 6:4. It can be seen that the calculated crystallite size is comparable to the actual size.
Table 1 Crystallite sizes of the nano-sized TiC–WC powder mixtures estimated by the Halder– Wagner method. Molar ratio Ti and W in TiC– WC mixture (Ti:W)
9:1
8:2
7:3
6:4
Average crystallite size
28.3 ± 2.4
12.8 ± 2.2
12.0 ± 2.0
10.4 ± 1.9
H. Kwon et al. / Journal of Alloys and Compounds 639 (2015) 21–26
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Fig. 3. TEM micrographs of synthesized TiC–WC mixture: The molar ratio of Ti and W was 6:4. (a) Bright field image and (b) EDS image.
(a) 9:1
(b) 8:2
(c) 7:3
(d) 6:4
Fig. 4. FE-SEM micrographs of the (Ti,W)C–Ni cermets prepared using the nano-sized TiC–WC powder mixture: The molar ratios of Ti and W were (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
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(a)
(b)
S.S.
Outer S.S.
TiC Inner S.S.
Ni
Ni
(c)
(d)
Ni
Inner S.S. Outer S.S.
Ni
S.S.
WC Fig. 5. Transmission electron micrographs of (Ti,W)C–Ni cermets prepared using the nano-sized TiC–WC powder mixture: The molar ratios of Ti and W are (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
85.0
40
Volume fraction (%)
W concentration (at.%)
50
30 20 TiC-WC-Ni cermet from nanosized mixture
10
Ti(CN)-WC-Ni cermet from nanosized mixture [8]
82.5 TiC-WC-Ni cermet from nanosized mixture Ti(CN)-WC-Ni cermet from nanosized mixture
80.0
77.5
0 0.00
0.25
0.50
0.75
1.00
Molar ratio of W/Ti Fig. 6. Effect of the W/Ti ratio on W concentration in the solid–solution phase of (Ti,W)C–Ni cermet. For comparison, the data from previous study on (Ti,W)(CN)–Ni cermet [8] is included.
3.2. Microstructure of (Ti,W)C–Ni cermet prepared using a mixture of nano sized TiC–WC powders Fig. 4 shows SEM/BSE (backscattered electron) images of the (Ti,W)C–Ni cermet prepared from the nano-pmix of TiC–WC after sintering at 1500 °C for 1 h. In all cases, the TiC core/solid–solution rim structure, which is typical in the case of (Ti,W)C–Ni cermet
75.0 0.00
0.25
0.50
0.75
1.00
Molar ratio of W/Ti Fig. 7. Effect of the W/Ti ratio on the volume fraction of the solid solution phase. For comparison, the data from previous study on (Ti,W)(CN)–Ni cermet [8] is included.
prepared from conventional micron-pmix, was not observed. The core/rim structure is formed because the TiC phase is more stable than WC; thus, the dissolution rate of WC is significantly higher than TiC (about 2–5 times) [6]. Consequently, partially dissolved TiC and completely dissolved WC precipitate heterogeneously around the undissolved TiC (core), creating a (Ti,W)C
H. Kwon et al. / Journal of Alloys and Compounds 639 (2015) 21–26
30
Volume fraction (%)
TiC-WC-Ni cermet from nanosized mixture Ti(CN)-WC-Ni cermet from nanosized mixture [8]
25
20
15
10 0.00
0.25
0.50
0.75
1.00
Molar ratio of W/Ti Fig. 8. Effect of the W/Ti ratio on the volume fraction of the binder phase.
14
22
12
20
Hv (Gpa)
8
Hv - TiC-WC-Ni cermet from nanosized mixture
18
KIC - TiC-WC-Ni cermet from nanosized mixture Hv - Ti(CN)-WC-Ni cermet from nanosized mixture [8] KIC - Ti(CN)-WC-Ni cermet from nanosized mixture [8]
16
6
14
4
12
2
10
0 0.00
0.25
0.50
0.75
KIC (MPa∙m1/2)
10
8 1.00
Molar ratio of W/Ti Fig. 9. Effect of the W/Ti ratio on the micro-hardness and fracture toughness of (Ti,W)C–Ni cermet. For comparison, the data from previous study on (Ti,W)(CN)–Ni cermet [8] is included.
solid–solution phase (rim) during liquid-phase sintering. However, when TiC is very fine, it can dissolve quickly, resulting in very scarce or even no undissolved TiC in the microstructure [8]. Accordingly, the volume fraction of undissolved TiC core was very low, even when the molar ratio of Ti and W was 9:1. This suggests that nano-sized TiC and WC particles dissolved mostly at the stage of Ni melting because of the TiC-size effect, forming a (Ti,W)C solid–solution phase. Numerous fine WC whiskers were precipitated in the microstructure when the molar ratio of Ti and W was 6:4. Fig. 5 shows TEM micrographs of (Ti,W)C–Ni cermet prepared in this study. When the molar ratio of Ti and W was 9:1, undissolved TiC remained in a solid–solution phase as seen in Fig. 5(a). As W content in the cermet increased, the undissolved TiC diminished and only the solid–solution phases existed. It is known that TiC/(Ti,W)(CN)–Ni cermet is composed of two kinds of solid–solution phases, inner rim and outer rim [5,6]. The inner rim is believed to form during the heating stage from the onset temperature for liquid formation (1300 °C), whereas the outer rim forms at the sintering temperature [6]. Thus, the inner solid–solution phase and the outer solid–solution phase, which are seen in the microstructures of sintered bodies prepared using the nano-pmix of TiC–WC, correspond to the inner rim and outer rim of the typical TiC/(Ti,W)(CN)–Ni cermet, respectively.
25
In the sample where the molar ratio of Ti and W was 7:3, it was difficult to distinguish between the inner solid–solution and outer solid–solution because of the comparatively small difference in the composition of the solid–solution phases. As mentioned above, Fig. 6 shows the W concentration in the solid–solution phase of the (Ti,W)C–Ni cermet prepared in this study. For comparison, the previously reported results of a (Ti,W)(CN)–Ni cermet prepared using nano-pmix Ti(CN), are plotted along with those from [8]. With more WC, the W concentration in the solid–solution phase increases due to the increase in input WC content. It is seen that W concentration in the solid–solution phase of the (Ti,W)C–Ni cermet is significantly higher than that of the (Ti,W)(CN)–Ni cermet. This difference may be explained rather simply by the thermodynamic factor. W is known to have a low affinity for nitrogen [12,13]. Therefore, the absence of N in the solid–solution phase of the TiC–WC–Ni cermet is expected to increase the solubility of W in comparison to the case of (Ti,W)(CN)–Ni cermet. Figs. 7 and 8 show the volume fractions of the solid–solution and Ni-binder phases, respectively. The increased W concentration in the solid–solution phase of the (Ti,W)C–Ni cermet makes the solid–solution phase more dense, and as a result, the volume fraction of the solid–solution phase becomes lower compared to that of the (Ti,W)(CN)–Ni cermet. Similarly, the volume fraction of the Ni binder in the (Ti,W)C–Ni cermet becomes higher than that in the (Ti,W)(CN)–Ni cermet because of the relatively decreased volume fraction of solid–solution phase in the former. 3.3. Mechanical properties of the (Ti,W)C–Ni cermet prepared using a mixture of nano-sized TiC–WC powders Fig. 9 shows the hardness and fracture toughness of the (Ti,W)C–Ni cermet prepared using the nano-pmix of TiC–WC, in this study. For comparison, the data from the previous study on (Ti,W)(CN)–Ni cermet [8] is also included. For both (Ti,W)C–Ni and (Ti,W)(CN)–Ni, the fracture toughness was found to increase with the amount of W in the cermet. In addition, the facture toughness of (Ti,W)C–Ni is significantly higher than that of (Ti,W)(CN)–Ni. Therefore, it is thought that the improved toughness of the (Ti,W)C–Ni cermet compared to that of the (Ti,W)(CN)–Ni cermet can be attributed to the increased W concentration of the solid–solution phase in the (Ti,W)C–Ni cermet. This appears to be reasonable considering that the solid–solution phase is tougher than TiC or Ti(CN) due to solid–solution strengthening [8,10,11,17–19]. In addition, the increased volume fraction of the Ni binder as shown in Fig. 8 may also contribute to the improved toughness of the (Ti,W)C–Ni cermet. 4. Conclusions In this study, a mixture of nano-sized TiC–WC powders was joined by high-energy milling of a mixture of Ti, W, and C powders, and subsequent heat treatment at 1200 °C. Ti reacted with C during high-energy milling, while WC formed after heat treatment. It was posited that the higher stability of the TiC phase (than WC) was the cause of the reaction sequence. The size of the TiC-crystallites in the synthesized powder was in a range of 10.4–28.3 nm. The reduced crystallite size of TiC facilitates dissolution of TiC particles during liquid-phase sintering of the (Ti,W)C–Ni cermet prepared using the mixture of nano-sized TiC–WC powders. The solid–solution phase in the (Ti,W)C–Ni cermet prepared using the nano-sized TiC–WC-powder mixture contains more W (44.6 at%) than in the (Ti,W)(CN)–Ni cermet (30.6 at%). It is believed that the improved toughness of the (Ti,W)C–Ni cermet in this study (compared to that of the (Ti,W)(CN)–Ni cermet) can be attributed to the increased W
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concentration of the solid–solution phase, and the increased volume fraction of the Ni binder. Acknowledgement This work was supported by a Grant-in-Aid from the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning (GP2012-019). References [1] P. Ettmayer, Hard metals and cermets, Annu. Rev. Mater. Sci. 19 (1989) 145– 164. [2] E.B. Clark, B. Roebuck, Extending the application areas for titanium carbonitride cermets, Int. J. Refract. Met. Hard Mater. 11 (1992) 23–33. [3] H. Suzuki, K. Hayashi, O. Terada, The two-phase region in TiC–Mo–30%Ni alloys, J. Jpn. Inst. Met. 35 (1971) 146–150. [4] S.Y. Ahn, S.W. Kim, S. Kang, Microstructure of Ti(CN)–WC–NbC–Ni cermets, J. Am. Ceram. Soc. 84 (2001) 843–849. [5] S. Kim, K.H. Min, S. Kang, Rim structure in Ti(C0.7N0.3)–WC–Ni cermets, J. Am. Ceram. Soc. 86 (2003) 1761–1766. [6] S.Y. Ahn, S. Kang, Formation of core/rim structures in Ti(C, N)–WC–Ni cermets via a dissolution and precipitation process, J. Am. Ceram. Soc. 83 (2000) 1489– 1494.
[7] J. Jung, S. Kang, Effect of ultra-fine powders on the microstructure of Ti(CN)– xWC–Ni cermets, Acta Mater. 52 (2004) 1379–1386. [8] J. Jung, S. Kang, Effect of nano-size powders on the microstructure of Ti(C,N)– xWC–Ni cermets, J. Am. Ceram. Soc. 90 (2007) 2178–2183. [9] H. Kwon, S. Kang, Microstructure and mechanical properties of TiC–WC–(Ti, W)C–Ni cermets, Mater. Sci. Eng. A 520 (2009) 75–79. [10] S. Park, S. Kang, Toughened ultrafine (Ti, W)(CN)–Ni cermets, Scripta Mater. 52 (2005) 129–133. [11] J. Jung, S. Kang, Sintered (Ti, W)C carbides, Scripta Mater. 56 (2007) 561–564. [12] A. Doi, T. Nomura, M. Tobioka, K. Takahashi, A. Hara, Thermodynamic evaluation of equilibrium nitrogen pressure and WC separation in Ti–W–C– N system carbonitride, in: H. Bildstein, H.M. Ortner (Eds.), 11th International Plansee Seminar’85, Plansee AG, Reutte, 1985, pp. 825–843. [13] J. Jung, Ph.D. Thesis, Department of Materials Science and Engineering, Seoul National University, Seoul, Korea, 2006. [14] D.K. Shetty, I.G. Wright, P.N. Mincer, A.H. Clauer, Indentation fracture of WC– Co cermets, J. Mater. Sci. 20 (1985) 1873–1882. [15] I. Barin, Thermochemical Data of Pure Substances, VCH Verlagsgesellschaft mbH, Weinheim, 1989. [16] N.C. Halder, C.N.J. Wagner, Separation of particle size and lattice strain in integral breadth measurements, Acta Cryst. 20 (1966) 312–313. [17] H. Kwon, J. Kim, S.A. Jung, C.Y. Suh, D.S. Kil, K.M. Roh, W. Kim, (Ti, W)C–Ni cermet prepared by high-energy ball milling and subsequent carbothermal reduction of TiO2–Ti–WO3–C mixture, Ceram. Int. 40 (2014) 7607–7611. [18] H. Kwon, S.A. Jung, C.Y. Suh, K.M. Roh, W. Kim, Mechanical properties of (Ti,V)C–Ni composite prepared using ultrafine solid–solution (Ti,V)C phase, Ceram. Int. 40 (2014) 12579–12583. [19] H. Kwon, S.A. Jung, Solid solution cermet: (Ti,Nb)(CN)–Ni cermet, J. Nanosci. Nanotechnol. 14 (2014) 8823–8827.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure and mechanical properties of (Ti,W)C–Ni cermet prepared using a nano-sized TiC–WC powder mixture Hanjung Kwon, Chang-Yul Suh, Wonbaek Kim ⇑ Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 January 2015 Received in revised form 10 March 2015 Accepted 14 March 2015 Available online 20 March 2015 Keywords: Composite materials Mechanical alloying Sintering Mechanical properties Electron emission spectroscopies
a b s t r a c t The microstructure and mechanical properties of a (Ti,W)C–Ni cermet, which was prepared from a mixture of nano-sized TiC–WC powder, were investigated in comparison with those of a (Ti,W)(CN)–Ni cermet, which was also prepared from a mixture of nano-sized powder, and previously reported to have high toughness values. The mixture of nano-sized powders was prepared by milling commercial Ti and W powders at various molar ratios (9:1, 8:2, 7:3, 6:4), with graphite. The mixture was isothermally sintered with Ni powder at 1500 °C for 1 h. EDS analysis revealed that the solid–solution phase in the (Ti,W)C–Ni cermet contained considerably more W (44.6 at%) than in the (Ti,W)(CN)–Ni cermet (30.6 at%). This was attributed to the higher affinity between TiC and WC, than between Ti(CN) and WC. The fracture toughness (max. 15.2 MPa m1/2) of the (Ti,W)C–Ni cermet was superior to that of the (Ti,W)(CN)–Ni cermet (max. 12.1 MPa m1/2). The toughness enhancement is attributed to the increase of W content in the solid–solution phase, together with the higher volume fraction of Ni-binder phase. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Ti-based cermet is used as a material for cutting tools because of its outstanding hardness, high resistance to wear, and excellent chemical stability [1]. However, the relatively lower toughness of Ti-based cermet (compared to WC–Co cermet) limits its application as a cutting-tool material to specific milling operations [1–11]. At present, Ti-based cermet cutting tools are typically used for high-speed milling, semi-finishing, and finishing work for both carbon and stainless steels [2]. Generally, Ti-based cermet contains other carbides (Mo2C, WC, VC, NbC, and TaC) to improve its properties [1,3,5]. The cutting performance of Ti-based cermet was improved by adding WC due to the excellent wear properties of TiC/Ti(CN), combined with the extreme elastic modulus and excellent heat conductivity of WC [4–9]. In addition, WC in Ti-based cermet serves to increase the density by improving the wetting and sinterability and decreases the particle growth rate [5–8]. However, the fracture toughness of Ti-based cermet with WC, is still inferior to that of WC–Co, and as a result, Ti-based cermet is not as widely used as cuttingtool material as WC–Co.
⇑ Corresponding author. Tel.: +82 42 868 3623; fax: +82 42 868 3415. E-mail address: [email protected] (W. Kim). http://dx.doi.org/10.1016/j.jallcom.2015.03.115 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
Ultrafine or nano-sized powders have also been used to improve the fracture toughness of Ti-based cermet. A significant improvement in the mechanical properties of (Ti,W)(CN)–Ni cermet was reported from using ultrafine or nano-sized powders instead of conventional micron-sized powders (for cermet from micron-sized powders: Hv 9–11 GPa, KIC 6–7 MPa m1/2; cermet from ultrafine powders: Hv 14–15 GPa, KIC 8–10 MPa m1/2; cermet from nano-sized powders: Hv 12–13 GPa, KIC 10–12 MPa m1/2) [6–8]. In particular, the cermet from a mixture of nano-sized powders (nano-pmix) provides significantly improved toughness by eliminating Ti(CN) cores. More specifically, the use of nanosized Ti(CN) facilitates the complete dissolution of Ti(CN), resulting in more homogeneous nucleation and growth of the (Ti,W)(CN) solid solution than in cermet prepared from micron-sized powders (micron-pmix) or ultrafine powders (uf-pmix). The cermet prepared from micron-pmix and uf-pmix usually exhibit maximum saturation of W content in the composition of the solid–solution phase of the (Ti,W)(CN)–Ni cermet, although the values vary from sample to sample. On the other hand, the cermet from nano-pmix does not appear to have a saturation point in the composition of the solid–solution phase. It is well known that the toughness value in a Ti-based cermet increases with increasing of W content of the solid–solution phase [10,11]. Therefore, there appears to be room for further improvement of the fracture toughness when nano-powder-mixes are used, because the W content in the solid–solution phase is not limited.
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The reason for the unsaturated W in the composition of the solid–solution phase of cermet from nano-pmix of Ti(CN)–WC, may be due to the low affinity between W and N [12,13]. Thus, when the nano-pmix of Ti(CN) dissolves completely in the Ni matrix during liquid-phase sintering, nitrogen atoms from the Ti(CN) impede the solubility of W in the solid–solution phase. Therefore, it can be posited that the W concentration in the solid–solution phase of cermet from a nano-pmix of TiC–WC powders could be higher than that of cermet from a nano-pmix of Ti(CN)–WC powders. As a result, the fracture toughness of the (Ti,W)C–Ni cermet might be higher than that of the (Ti,W)(CN)–Ni cermet. In this study, a cermet from the nano-pmix of TiC–WC was fabricated to obtain a cermet with improved toughness, and then compared to a cermet from the nano-pmix of Ti(CN)–WC. First, a nano-pmix of TiC–WC, with a uniform distribution of TiC and WC, was prepared by high-energy milling and subsequent heat treatment. Second, changes in the microstructure and mechanical properties of a (Ti,W)C–Ni cermet prepared using a nano-pmix of TiC–WC were investigated as a function of WC content. Finally, the results were compared with previously reported (Ti,W)(CN)–Ni cermet prepared from a nano-pmix of Ti(CN)–WC.
TiC WC W2 C W
(d) 6:4
(c) 7:3
(b) 8:2
(a) 9:1
20
30
40
50
60
70
80
90
2θ Fig. 1. X-ray diffraction patterns of the Ti–W–C powder mixture after milling for 20 h: The molar ratios of Ti and W were (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
TiC WC
2. Experimental procedure A nano-pmix of TiC–WC was synthesized using commercial Ti (Tasco, 325 mesh, 99.5%) and W (TaeguTec, <10 lm, >99%) powders. The powders were mixed with graphite (Alfa, 325 mesh, 99.8%) to attain the nano-pmix of TiC–WC (molar ratios of Ti:W = 9:1, 8:2, 7:3, and 6:4). The powders were subjected to high-energy milling using a planetary mill (Model Pulverisette 5, Fritsch, Germany). Tungsten carbide balls were mixed with Ti, W, and graphite at a ball-to-powder weight ratio of 20:1. A stainless steel bowl was used, and milling was conducted at a speed of 250 RPM for 20 h under Ar gas. The milled powder was heat-treated at 1200 °C for 1 h to complete the carbide-formation reaction. The phases in the milled and heat-treated powder were analyzed using X-ray diffraction (XRD) analysis (SmartLab, Rigaku, Japan). Monochromatized Cu Ka radiation (k = 1.5418 Å) was employed during the analyses, and Si was used as a standard to calibrate the diffractometer. The morphology of the powders was examined using a fieldemission transmission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). A (Ti,W)C–Ni cermet was prepared by mixing a nano-pmix of TiC–WC with Ni powder (Sigma–Aldrich, <1 lm, 99%) by horizontal ball milling. The weight ratio of the TiC–WC mixed powder, and the Ni powder, was maintained at 8:2. The mixed powder was then compacted into a disc under a pressure of 125 MPa. The mixture was sintered at 1500 °C for 1 h. Then, the sintered specimens were ground and their microstructure observed via a field-emission scanning electron microscope (Quanta 650F, FEI, Oregon, USA). In order to determine the regional compositions of the phases (e.g., core, rim, and binder), an energy dispersive X-ray analysis (EDS) was performed with a field-emission transmission electron microscope. Each data point was an average of 10 points or more from several particles. Vickers hardness was measured with an indenter load of 30 kg, and fracture toughness was calculated using the expression derived by Shetty et al. [14].
3. Results and discussion 3.1. Preparation of mixture of nano-sized TiC–WC powders Fig. 1 is a series of X-ray diffraction patterns illustrating the phase evolution during high-energy milling of Ti + W + C mixtures to achieve various molar ratios of Ti and W. When high-energy milling was applied to the mixtures for 20 h, pure Ti in the raw material reacted preferentially with C, forming a TiC phase with a B1 (NaCl-like) structure at all Ti/W ratios. This observation demonstrates the higher stability of TiC compared to that of WC. The standard Gibbs free energy of formation of TiC is 173.12 kJ/mole at 1000 K while that of WC is 35.81 kJ/mole [15]. When W was the smallest (molar ratio 9:1), small WC peaks were observed. When the molar ratio was increased to 8:2 and 7:3, a W2C phase was also formed. When the molar ratio was the highest (6:4), metallic W peaks appeared along with broadened
(d) 6:4
(c) 7:3
(b) 8:2
(a) 9:1
20
30
40
50
60
70
80
90
2θ Fig. 2. X-ray diffraction patterns of the milled Ti–W–C mixture after heat treatment at 1200 °C for 1 h: The milled/heat-treated powders of Ti, W, and C: molar ratios of Ti and W are (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
TiC peaks. It may be that the applied mechanical energy during milling may not be sufficient to complete the WC formation. However, the reaction was completed by heat treatment afterwards. Fig. 2 shows the XRD patterns of the powders after heat treatment at 1200 °C for 1 h. It clearly shows that the mixture was fully converted to TiC and WC. One thing to be noted here is that the TiC peaks are significantly broadened as the molar ratio increases. The crystallite sizes of the synthesized phases were calculated using the Halder–Wagner method [16]. The crystallite sizes of the synthesized powders range from 10.4 nm to 28.3 nm (Table 1). Fig. 3 shows the TEM micrograph of the mixture of TiC–WC powders wherein the molar ratio of Ti and W was 6:4. It can be seen that the calculated crystallite size is comparable to the actual size.
Table 1 Crystallite sizes of the nano-sized TiC–WC powder mixtures estimated by the Halder– Wagner method. Molar ratio Ti and W in TiC– WC mixture (Ti:W)
9:1
8:2
7:3
6:4
Average crystallite size
28.3 ± 2.4
12.8 ± 2.2
12.0 ± 2.0
10.4 ± 1.9
H. Kwon et al. / Journal of Alloys and Compounds 639 (2015) 21–26
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Fig. 3. TEM micrographs of synthesized TiC–WC mixture: The molar ratio of Ti and W was 6:4. (a) Bright field image and (b) EDS image.
(a) 9:1
(b) 8:2
(c) 7:3
(d) 6:4
Fig. 4. FE-SEM micrographs of the (Ti,W)C–Ni cermets prepared using the nano-sized TiC–WC powder mixture: The molar ratios of Ti and W were (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
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H. Kwon et al. / Journal of Alloys and Compounds 639 (2015) 21–26
(a)
(b)
S.S.
Outer S.S.
TiC Inner S.S.
Ni
Ni
(c)
(d)
Ni
Inner S.S. Outer S.S.
Ni
S.S.
WC Fig. 5. Transmission electron micrographs of (Ti,W)C–Ni cermets prepared using the nano-sized TiC–WC powder mixture: The molar ratios of Ti and W are (a) 9:1, (b) 8:2, (c) 7:3, and (d) 6:4.
85.0
40
Volume fraction (%)
W concentration (at.%)
50
30 20 TiC-WC-Ni cermet from nanosized mixture
10
Ti(CN)-WC-Ni cermet from nanosized mixture [8]
82.5 TiC-WC-Ni cermet from nanosized mixture Ti(CN)-WC-Ni cermet from nanosized mixture
80.0
77.5
0 0.00
0.25
0.50
0.75
1.00
Molar ratio of W/Ti Fig. 6. Effect of the W/Ti ratio on W concentration in the solid–solution phase of (Ti,W)C–Ni cermet. For comparison, the data from previous study on (Ti,W)(CN)–Ni cermet [8] is included.
3.2. Microstructure of (Ti,W)C–Ni cermet prepared using a mixture of nano sized TiC–WC powders Fig. 4 shows SEM/BSE (backscattered electron) images of the (Ti,W)C–Ni cermet prepared from the nano-pmix of TiC–WC after sintering at 1500 °C for 1 h. In all cases, the TiC core/solid–solution rim structure, which is typical in the case of (Ti,W)C–Ni cermet
75.0 0.00
0.25
0.50
0.75
1.00
Molar ratio of W/Ti Fig. 7. Effect of the W/Ti ratio on the volume fraction of the solid solution phase. For comparison, the data from previous study on (Ti,W)(CN)–Ni cermet [8] is included.
prepared from conventional micron-pmix, was not observed. The core/rim structure is formed because the TiC phase is more stable than WC; thus, the dissolution rate of WC is significantly higher than TiC (about 2–5 times) [6]. Consequently, partially dissolved TiC and completely dissolved WC precipitate heterogeneously around the undissolved TiC (core), creating a (Ti,W)C
H. Kwon et al. / Journal of Alloys and Compounds 639 (2015) 21–26
30
Volume fraction (%)
TiC-WC-Ni cermet from nanosized mixture Ti(CN)-WC-Ni cermet from nanosized mixture [8]
25
20
15
10 0.00
0.25
0.50
0.75
1.00
Molar ratio of W/Ti Fig. 8. Effect of the W/Ti ratio on the volume fraction of the binder phase.
14
22
12
20
Hv (Gpa)
8
Hv - TiC-WC-Ni cermet from nanosized mixture
18
KIC - TiC-WC-Ni cermet from nanosized mixture Hv - Ti(CN)-WC-Ni cermet from nanosized mixture [8] KIC - Ti(CN)-WC-Ni cermet from nanosized mixture [8]
16
6
14
4
12
2
10
0 0.00
0.25
0.50
0.75
KIC (MPa∙m1/2)
10
8 1.00
Molar ratio of W/Ti Fig. 9. Effect of the W/Ti ratio on the micro-hardness and fracture toughness of (Ti,W)C–Ni cermet. For comparison, the data from previous study on (Ti,W)(CN)–Ni cermet [8] is included.
solid–solution phase (rim) during liquid-phase sintering. However, when TiC is very fine, it can dissolve quickly, resulting in very scarce or even no undissolved TiC in the microstructure [8]. Accordingly, the volume fraction of undissolved TiC core was very low, even when the molar ratio of Ti and W was 9:1. This suggests that nano-sized TiC and WC particles dissolved mostly at the stage of Ni melting because of the TiC-size effect, forming a (Ti,W)C solid–solution phase. Numerous fine WC whiskers were precipitated in the microstructure when the molar ratio of Ti and W was 6:4. Fig. 5 shows TEM micrographs of (Ti,W)C–Ni cermet prepared in this study. When the molar ratio of Ti and W was 9:1, undissolved TiC remained in a solid–solution phase as seen in Fig. 5(a). As W content in the cermet increased, the undissolved TiC diminished and only the solid–solution phases existed. It is known that TiC/(Ti,W)(CN)–Ni cermet is composed of two kinds of solid–solution phases, inner rim and outer rim [5,6]. The inner rim is believed to form during the heating stage from the onset temperature for liquid formation (1300 °C), whereas the outer rim forms at the sintering temperature [6]. Thus, the inner solid–solution phase and the outer solid–solution phase, which are seen in the microstructures of sintered bodies prepared using the nano-pmix of TiC–WC, correspond to the inner rim and outer rim of the typical TiC/(Ti,W)(CN)–Ni cermet, respectively.
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In the sample where the molar ratio of Ti and W was 7:3, it was difficult to distinguish between the inner solid–solution and outer solid–solution because of the comparatively small difference in the composition of the solid–solution phases. As mentioned above, Fig. 6 shows the W concentration in the solid–solution phase of the (Ti,W)C–Ni cermet prepared in this study. For comparison, the previously reported results of a (Ti,W)(CN)–Ni cermet prepared using nano-pmix Ti(CN), are plotted along with those from [8]. With more WC, the W concentration in the solid–solution phase increases due to the increase in input WC content. It is seen that W concentration in the solid–solution phase of the (Ti,W)C–Ni cermet is significantly higher than that of the (Ti,W)(CN)–Ni cermet. This difference may be explained rather simply by the thermodynamic factor. W is known to have a low affinity for nitrogen [12,13]. Therefore, the absence of N in the solid–solution phase of the TiC–WC–Ni cermet is expected to increase the solubility of W in comparison to the case of (Ti,W)(CN)–Ni cermet. Figs. 7 and 8 show the volume fractions of the solid–solution and Ni-binder phases, respectively. The increased W concentration in the solid–solution phase of the (Ti,W)C–Ni cermet makes the solid–solution phase more dense, and as a result, the volume fraction of the solid–solution phase becomes lower compared to that of the (Ti,W)(CN)–Ni cermet. Similarly, the volume fraction of the Ni binder in the (Ti,W)C–Ni cermet becomes higher than that in the (Ti,W)(CN)–Ni cermet because of the relatively decreased volume fraction of solid–solution phase in the former. 3.3. Mechanical properties of the (Ti,W)C–Ni cermet prepared using a mixture of nano-sized TiC–WC powders Fig. 9 shows the hardness and fracture toughness of the (Ti,W)C–Ni cermet prepared using the nano-pmix of TiC–WC, in this study. For comparison, the data from the previous study on (Ti,W)(CN)–Ni cermet [8] is also included. For both (Ti,W)C–Ni and (Ti,W)(CN)–Ni, the fracture toughness was found to increase with the amount of W in the cermet. In addition, the facture toughness of (Ti,W)C–Ni is significantly higher than that of (Ti,W)(CN)–Ni. Therefore, it is thought that the improved toughness of the (Ti,W)C–Ni cermet compared to that of the (Ti,W)(CN)–Ni cermet can be attributed to the increased W concentration of the solid–solution phase in the (Ti,W)C–Ni cermet. This appears to be reasonable considering that the solid–solution phase is tougher than TiC or Ti(CN) due to solid–solution strengthening [8,10,11,17–19]. In addition, the increased volume fraction of the Ni binder as shown in Fig. 8 may also contribute to the improved toughness of the (Ti,W)C–Ni cermet. 4. Conclusions In this study, a mixture of nano-sized TiC–WC powders was joined by high-energy milling of a mixture of Ti, W, and C powders, and subsequent heat treatment at 1200 °C. Ti reacted with C during high-energy milling, while WC formed after heat treatment. It was posited that the higher stability of the TiC phase (than WC) was the cause of the reaction sequence. The size of the TiC-crystallites in the synthesized powder was in a range of 10.4–28.3 nm. The reduced crystallite size of TiC facilitates dissolution of TiC particles during liquid-phase sintering of the (Ti,W)C–Ni cermet prepared using the mixture of nano-sized TiC–WC powders. The solid–solution phase in the (Ti,W)C–Ni cermet prepared using the nano-sized TiC–WC-powder mixture contains more W (44.6 at%) than in the (Ti,W)(CN)–Ni cermet (30.6 at%). It is believed that the improved toughness of the (Ti,W)C–Ni cermet in this study (compared to that of the (Ti,W)(CN)–Ni cermet) can be attributed to the increased W
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concentration of the solid–solution phase, and the increased volume fraction of the Ni binder. Acknowledgement This work was supported by a Grant-in-Aid from the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning (GP2012-019). References [1] P. Ettmayer, Hard metals and cermets, Annu. Rev. Mater. Sci. 19 (1989) 145– 164. [2] E.B. Clark, B. Roebuck, Extending the application areas for titanium carbonitride cermets, Int. J. Refract. Met. Hard Mater. 11 (1992) 23–33. [3] H. Suzuki, K. Hayashi, O. Terada, The two-phase region in TiC–Mo–30%Ni alloys, J. Jpn. Inst. Met. 35 (1971) 146–150. [4] S.Y. Ahn, S.W. Kim, S. Kang, Microstructure of Ti(CN)–WC–NbC–Ni cermets, J. Am. Ceram. Soc. 84 (2001) 843–849. [5] S. Kim, K.H. Min, S. Kang, Rim structure in Ti(C0.7N0.3)–WC–Ni cermets, J. Am. Ceram. Soc. 86 (2003) 1761–1766. [6] S.Y. Ahn, S. Kang, Formation of core/rim structures in Ti(C, N)–WC–Ni cermets via a dissolution and precipitation process, J. Am. Ceram. Soc. 83 (2000) 1489– 1494.
[7] J. Jung, S. Kang, Effect of ultra-fine powders on the microstructure of Ti(CN)– xWC–Ni cermets, Acta Mater. 52 (2004) 1379–1386. [8] J. Jung, S. Kang, Effect of nano-size powders on the microstructure of Ti(C,N)– xWC–Ni cermets, J. Am. Ceram. Soc. 90 (2007) 2178–2183. [9] H. Kwon, S. Kang, Microstructure and mechanical properties of TiC–WC–(Ti, W)C–Ni cermets, Mater. Sci. Eng. A 520 (2009) 75–79. [10] S. Park, S. Kang, Toughened ultrafine (Ti, W)(CN)–Ni cermets, Scripta Mater. 52 (2005) 129–133. [11] J. Jung, S. Kang, Sintered (Ti, W)C carbides, Scripta Mater. 56 (2007) 561–564. [12] A. Doi, T. Nomura, M. Tobioka, K. Takahashi, A. Hara, Thermodynamic evaluation of equilibrium nitrogen pressure and WC separation in Ti–W–C– N system carbonitride, in: H. Bildstein, H.M. Ortner (Eds.), 11th International Plansee Seminar’85, Plansee AG, Reutte, 1985, pp. 825–843. [13] J. Jung, Ph.D. Thesis, Department of Materials Science and Engineering, Seoul National University, Seoul, Korea, 2006. [14] D.K. Shetty, I.G. Wright, P.N. Mincer, A.H. Clauer, Indentation fracture of WC– Co cermets, J. Mater. Sci. 20 (1985) 1873–1882. [15] I. Barin, Thermochemical Data of Pure Substances, VCH Verlagsgesellschaft mbH, Weinheim, 1989. [16] N.C. Halder, C.N.J. Wagner, Separation of particle size and lattice strain in integral breadth measurements, Acta Cryst. 20 (1966) 312–313. [17] H. Kwon, J. Kim, S.A. Jung, C.Y. Suh, D.S. Kil, K.M. Roh, W. Kim, (Ti, W)C–Ni cermet prepared by high-energy ball milling and subsequent carbothermal reduction of TiO2–Ti–WO3–C mixture, Ceram. Int. 40 (2014) 7607–7611. [18] H. Kwon, S.A. Jung, C.Y. Suh, K.M. Roh, W. Kim, Mechanical properties of (Ti,V)C–Ni composite prepared using ultrafine solid–solution (Ti,V)C phase, Ceram. Int. 40 (2014) 12579–12583. [19] H. Kwon, S.A. Jung, Solid solution cermet: (Ti,Nb)(CN)–Ni cermet, J. Nanosci. Nanotechnol. 14 (2014) 8823–8827.