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Influence of coarse TiCN content on the morphology and mechanical properties of ultrafine TiCN-based cermets
Materials Science & Engineering A 682 (2017) 648–655
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of coarse TiCN content on the morphology and mechanical properties of ultrafine TiCN-based cermets
crossmark
⁎
Huiwen Xionga, Yu Wena, Xueping Gana, Zhiyou Lia, , Liyuan Chaib a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China School of Metallurgy and Environment, Central South University, Changsha 410083, China
A R T I C L E I N F O
A BS T RAC T
Keywords: TiCN Ultrafine cermet Duel grain structure Long time milling Mechanical properties
In this study, TiCN-based cermets with duel grain structure were prepared by long time milling of the microsized raw powders. The effects of coarse TiCN content on the microstructure and mechanical properties of ultrafine TiCN-based cermets were investigated. Results showed that the addition of coarse TiCN powders in ultrafine composite mixture could increase the compact density, but also resulted in increased closed pores in cermets. Coarse TiCN particles would accelerate the dissolution and precipitation reaction of the fine powders, forming more bright cores or thick inner rims on the fine TiCN grains. Grain size of the cermets gradually increased with higher coarse TiCN content. When the coarse content was more than 30%, abnormal growth of TiCN grains occurred via the coalescence of TiCN particles. Mechanical properties of the cermets were firstly increased within 10 or 20% coarse TiCN, then decreased sharply with further increasing the coarse powders. Highest strength (1940 MPa) and toughness (11.67 MPa·m1/2) were achieved with 10% coarse TiCN. Coarse ceramic grains with thick rims exhibit brittleness, deteriorating the final properties of the cermets.
1. Introduction
Therefore, to a certain extent, replacing part of the ultrafine powders by coarse particles can save energy and also decline the total impurities of the final mixture. Commonly, blending the coarse and fine powders can fabricate the duel grain structure. Cemented carbides with a bimodal grain size distribution show attractive performance, as fine grains provide high hardness and coarse grains ensure good toughness [17]. It has been reported that coarse plate-like WC grains dispersed in ultrafine WC-Co matrix can improve the final toughness via crack deflection and bridging mechanism [18]. Recently, coarse plate WC grains in TiCbased cermets shows the highest KIC value of 16.56 MPa·m1/2, which are fabricated via high energy milling and subsequently sintering of TiW-C mixture [19]. Our previous work has been shown that coarse TiC whisker-type grains in ultrafine TiC-based cermets are beneficial for improved properties [16]. Therefore, TiCN-based cermets with duel grain structure should be promising materials for satisfied mechanical properties. However, to date, works on the TiCN-based cermets with bimodal grain size have not been reported. In this study, commercial microsized TiCN powders were used as the starting materials. Ultrafine TiCN-based composite powders were fabricated by long time ball milling. Moreover, TiCN-based cermets with duel grain structure were prepared by different time milling of the starting TiCN powders and subsequent vacuum sintering. The effects of
TiCN-based cermets exhibit high hardness, good wear resistance and chemical stability, which are widely used as high-speed cutting tools [1,2]. However, the strength and toughness of cermets are usually inferior to the WC-Co system, which greatly limits their further applications [3,4]. Compared to the microsized cermets, ultrafine TiCN-based cermets have attracted more attention, due to the improved hardness and toughness [5]. Many studied have been carried to fabricate nano-modified or ultrafine TiCN-based cermets for harsher applications [6–8]. Firstly, nanosized or ultrafine composite powders should be obtained by various methods like carbothermal reduction synthesis [9], SHS or MSR [10,11], mechanical alloying [12], ect. Secondly, the compact samples are prepared via different sintering process include vacuum sintering, HIP, SPS and so on [13–15]. Obviously, high energy consumption and high requirement of the sintering procedure are put forward to meet the satisfied properties of these ultrafine composites. Moreover, careful handling of fine composite powders is needed during the preservation and transferring, in order to reduce the impurity content and oxygen absorption [16]. High oxygen content of ultrafine mixture will decrease the wettability of Ni on TiCN particles, and also result in more closed pores in the sintered cermets [14].
⁎
Corresponding author. E-mail address: [email protected] (Z. Li).
http://dx.doi.org/10.1016/j.msea.2016.11.085 Received 3 November 2016; Received in revised form 23 November 2016; Accepted 24 November 2016 Available online 25 November 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 682 (2017) 648–655
H. Xiong et al.
Zhuzhou cemented carbide limited company. Table 1 displays the nominal composition of the samples. From sample A to F, the coarse TiCN content of the TiCN powders was 0%, 10%, 20%, 30%, 50% and 100%, respectively. As for ultrafine cermets, the weighted powder was mixed by ball milling for 96 h. Meanwhile, the coarse system was ball milled for 24 h. Fig. 1 shows the procedure of preparing the cermets with duel grain structure. To investigate the effect of coarse TiCN on the ultrafine system, partial TiCN particles were added at the last 24 h of the milling process. During the milling, the ball to powder ratio was 12:1 and the rolling speed was 60r/min. After the milling, the mixture was put into a vacuum drying box. The drying was conducted at 80 ℃ for 12 h. Then, the composite powders were seized through 200 meshes before the uniaxial pressing under 200 MPa. The green samples were degreased at 350 ℃ for 90 min, then hold at 800 ℃ for 30 min in H2. Finally, these samples were transferred to the furnace for vacuum sintering at 1470 ℃ for 1 h. Carbon，Nitrogen and oxygen contents of the composite powders were determined using a Carbon/Nitrogen analyzer (Leco, WC-200AC, USA). Sintered samples were characterized by X-ray diffraction (XRD, Hitachi RAX-10 A-X, Japan). The XRD patterns were numerically analyzed with the use of MDI JADE (Version 6.0, Materials data Ltd., USA). The microstructure of polished specimens (finished with 1 µm diamond paste) were analyzed by environmental scanning electron microscopy (ESEM, NOVATM NanoSEM 230, Holland) coupled with EDXA. The densities of the sintered cermets were measured by the Archimedes method. The particle grain size distribution was acquired was measured by lineal intercept method based on SEM images. The transverse rupture test was conducted using a three-point bending instrument based on Instron3369 according to ISO 3327. The Rockwell hardness (HRA) was determined using an AR-600 Rockwell hardness. A Vicker Indenter was used to measure the toughness. The fracture toughness was evaluated by a direct crack measurement method with a 10 kg load and 15 s of load time on the polished surface of the sample using the expression derived by Shetty et al. [20].
Table 1 Chemical composition of the experimental materials (Unit: g). Samples
A B C D E F
TiCN 1st added
2nd addition
51 45.9 40.8 35.7 25.5 –
– 5.1 10.2 15.3 25.5 51
WC
Mo2C
TaC
Ni
Co
19 19 19 19 19 19
7 7 7 7 7 7
5 5 5 5 5 5
10 10 10 10 10 10
8 8 8 8 8 8
Fig. 1. A diagram of the fabrication process of the TiCN-based cermet.
3. Results and discussion 3.1. Density and metallographic observation Fig. 2 shows the density of the green compacts and sintered samples with varied coarse and ultrafine TiCN ratios. Ultrafine composite has the lowest compact density of 3.64 g/cm3, which increases to 3.91 g/cm3 with adding 10% coarse TiCN powders. Composite mixture with finer grain size owns lower green density, owing to the larger friction between particles during pressing. Highest green density (4.03 g/cm3) is achieved for samples E with 50% coarse TiCN content. Appropriate arrangement of coarse and fine particles can lead to a high density compact, as smaller particles will occupy the interspace formed by the bigger ones. The sintered density of the cermets increases from 6.95 to 7.14 g/cm3 with more fine TiCN content. Meanwhile, sample A has the highest sintering shrinkage. During the milling, certain amount of WC-Co contamination from the hardmetal balls exists in the final mixture. To a certain content, higher ultrafine TiCN powders means higher W and Co content, which leads to a higher sintered density. Fig. 3 shows the composite powders of Samples A and E. Uniformly dispersed composite powders are observed after 96 h milling. The milled particles have a size of around 200 nm. Moreover, a certain amount of microsized TiCN partcles can be seen in powders with 50% coarse TiCN content. Metallography is helpful for the study of the sintered density and phase distribution, while different phases are not easy to figure out in a BSE-SEM mode for cermets with high secondary carbides addition [5]. Fig. 4 shows the metallographic photos. Ceramic grains embedded homogeneously in the binder for samples milled for 96 h. As for cermet F, the binder is slightly gathered, forming small binder pools (Fig. 4f). Twenty-four hours blending is insufficient for a uniform distribution.
Fig. 2. Density and shrinkage of TiCN-based cermets with different ultrafine TiCN content.
coarse TiCN content on the microstructure and mechanical properties of ultrafine cermets were investigated. 2. Experiments Details of the raw materials used in this study have been already described in [16]. TiCN powders have a size of 2.5 µm, purchasing from 649
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H. Xiong et al.
Fig. 3. SEM images of composite powders of samples A (ultrafine) and E (50% coarse).: Ultrafine TiCN-based composite powders, (b): Composite powders with 50% coarse TiCN.
Fig. 4. Metallographic photos (1500×) of the TiCN-based cermets. (a) Smaple A, (b) Sample B, (c) Sample C, (d) Sample D, (e) Sample E and (f) Sample F. Insets are the enlarged views of the dashed squares.
grain size of the secondary added TiCN powders, low milling intensity was adopted. Therefore, it is hardly to obtain a uniform distribution of the coarse TiCN particles, especially for samples D and E. Compared to the ultrafine matrix, the decreased capillary forces of the coarse particles will shorten the migration of the liquid, leading increased closed pores after sintering.
The wettablity of Nickel on TiCN grains is not well enough, and low capillary force can shorten the diffusion distance of the binder, leading an uneven microstructure. All cermets show a highly dense body after vacuum sintering, except for samples D and E. Large amount of coarse TiCN particles seem harmful for achieving full dense and several round closed pores are left at the later sintering stage. To ensure the coarse 650
Materials Science & Engineering A 682 (2017) 648–655
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Fig. 5. SEM images of TiCN-based cermets. (a) Sample A (Ultrafine), (b) Sample F (Microsized), (c) Sample B(10% coarse), (d) Sample C (20% coarse), (e) Sample D (30% coarse) and (f) Sample E (50% coarse). a1, b1, c1, d1, e1 and f1 are the enlarged views in the dashed squares of (a)–(f). The red bars are 1 µm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
diffusion rate, while the thick outer rims are quickly precipitated during the liquid sintering. Big ceramic grains with the size of 800 nm are obtained for the ultrafine cermet (Fig. 5a). Compared to the composite powder (around 200 nm) in Fig. 3a, the sintered TiCN grains grow up for nearly three times bigger. According to several studies [6,21], nanosized TiCN particles mainly grow by attachment mechanism before the rim phases appear. Formation of the surrounding phases can effectively hinder the growth of TiCN cores, as the undissolved black cores are separated from the matrix. With increasing the coarse TiCN content, more coarse TiCN grains are observed.
3.2. Microstructure and composition Fig. 5 shows the SEM images of TiCN-based cermets with different coarse TiCN content. Under the BSE-SEM mode, the contrast of the phase is sensitive to the average atomic number. The black core is the undissolved TiCN grain, which is covered by bright inner rim and grey outer rim [1,5]. The two rims have the same fcc structure as the TiCN grains, which are gradually formed via the dissolution-precipitation process. Typical core-rim structure can be seen for all cermets. The irregular inner rims are formed at the solid state sintering with low 651
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Meanwhile, the feature of dual grain structure is obvious with 30% and 50% coarse TiCN content. It is noteworthy that more bright core-grey rim phases can be observed in samples with partial coarse TiCN addition. And also, ultrafine grains with thick inner rims appear in Fig. 5d. However, only a few bright cores or irregular thin inner rims are seen in ultrafine or coarse TiCN-based cermets. As is known, the bright cores are formed by solid state diffusion of heavy elements into ultrafine TiCN particles [6]. After the long time milling, ultrafine TiCN grains have much higher surface area and defect density than the secondary added coarse grains. Compared to the ultrafine system, heavy elements (W, Mo, and Ta) will firstly diffuse into fine TiCN grains with the addition of coarse TiCN powders. Moreover, according to the Ostwald ripening mechanism, coarse TiCN particles will break the equilibrium of the ultrafine system, accelerating the dissolution and precipitation of ultrafine TiCN grains. Therefore, more ultrafine TiCN grains gradually evolve into bright cores or small cores with thick inner rims. Meanwhile, irregular inner rims are formed on the big particles at the solid state reaction. After the liquid phase appears at high temperature, coarse particles will prefer to grow up, by means of deposited thick outer rims (Fig. 5e1). Fig. 6 displays the XRD patterns of the TiCN cermets. All samples are composed of (Ti, M) CN (M=W, Mo and Ta) and Nickel-based binder. When W, Mo and Ta replace Ti atoms in TiCN or Ni in Nickel binder, the lattice parameter of TiCN or Ni will declines or increases, respectively. Meanwhile, diffraction will shift to low angles with an expanded fcc lattice. With increasing the coarse TiCN content, the (200) peak of TiCN slightly shift to a lower angle, indicating less amount of heavy elements in TiCN. As for the binder phase, high coarse TiCN content shows a high angle. Table 2 shows the carbon, nitrogen and oxygen content of the composite powders. Oxygen content of the composite powder increases with more long milled TiCN powders. Certain amount of carbon will be consumed, leading high solid solute in Nickel after sintering. The grain size distribution is shown in Fig. 7. As for sample A, the largest numbers of the ceramic grains have a grain size of 0.2–0.4 µm. After the addition of coarse TiCN grains, grain size of the ceramic grains increase. When with 10 or 20% coarse TiCN particles, most of
Fig. 6. XRD patterns of the TiCN-based cermets with different coarse TiCN content.
Table 2 Carbon, nitrogen and oxygen content of the composite powders. Samples
Carbon (wt%)
Nitrogen (wt%)
Oxygen (wt%)
A B C D E F
8.53 8.57 8.51 8.63 8.67 8.62
5.58 5.72 5.54 5.55 5.65 5.77
2.08 1.92 1.96 1.72 1.63 1.08
Fig. 7. Grain size distribution of samples sintered at 1470 ℃. (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D, (e) Sample E, (f) Sample F.
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Fig. 8. Mechanical properties of the TiCN-based cermets with different coarse TiCN content. TRS, (b) HRA, (c) Fracture toughness.
Fig. 9 shows the crack deflections of the cermets under 10 kg load. When coarse TiCN content is no more than 20%, the fracture mode is mainly intergranular fracture. Crack goes along the ultrafine grains, making the crack propagation path tortuous. As for the coarse grains, the fracture mechanism turns into transgranular. With further increasing the coarse TiCN content, more microsized grains are dispersed in the ultrafine matrix. The crack propagation path seems more smooth and straight, indicating decreased toughness. Coarse grains, usually with thick rims exhibit brittle and can be easily passed through by cracks. Fig. 10 shows crack deflection and fracture morphology of TiCN-based cermets with 10% and 30% coarse TiCN content. As for sample B, many bright cores can be observed in Fig. 10(a) and the fracture mode is intergranular fracture. As for the coarse grain in Fig. 10(c), smooth crack propagation path and transgranular type fracture can be clearly observed. Compared to the fine grains, thicker rims are formed on these coarse grains by ripening growth mechanism. As is known, certain internal stress exists at the interface between the core and rim. Therefore, the abnormal growth of TiCN grains can deteriorates the mechanical properties of cermets.
the ceramic grains have a size of 0.4–0.6 µm. And the big grains show a diameter less than 2.0 µm. With increasing the content of coarse powders to 30 or 50%, grain size of the highest proportion goes up to 0.6–0.8 µm. Meanwhile, the coarse grains have a size of 3.6 and 4.8 µm, much higher than samples B and C. Coarse TiCN particles have a great impact on the dissolution of ultrafine TiCN powders. Grains with small size firstly dissolved, leading coarse grains grow up. Growth of the black core phase is considered by the coalescence of TiCN particles at the solid state sintering. Complete rims on the TiCN cores will hinder the growth of black cores during the liquid sintering. High coarse TiCN content increases the possibility of the coarse particles contact. Therefore, grains with size larger than 3.5 µm are found in samples D and E. 3.3. Mechanical properties Fig. 8 shows the mechanical properties of the TiCN-based cermets. Hardness of the ultrafine cermet first rise from 91.8 to 92.3, then decrease to 90.6 for sample F. The hardness of the cermets is mainly provided by the ceramic grains. With the addition of coarse TiCN powders, more bright cores or thicker inner rims on the ultrafine grains are formed in Fig. 5. These bright (Ti, W, Mo, Ta) (C, N) solid solution is rich in heavy elements and owns higher hardness than the grey rims in ultrafine system. Therefore, higher hardness is achieved by adding coarse particles. Moreover, large grains in cermets are formed in high coarse TiCN content, which decrease the hardness by Hall-petch formula. The TRS and KIC display the same trend with increasing the coarse TiCN content, first increase then decline sharply with more than 30% coarse TiCN addition. Sample B shows the best strength and toughness of 1940 MPa and 11.67 MPa·m1/2.
4. Conclusions Ultrafine TiCN-based cermets have been prepared by long time milling of the commercial microsized mixture. The effects of coarse TiCN content on the microsturcture and mechanical properties of ultrafine TiCN-based cermets have been studied. Conclusions are summarized as follows: (1) TiCN-based cermets with duel grain structure can be prepared via different milling time of the TiCN powders. Matching of the coarse 653
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Fig. 9. SEM images of crack deflections of the TiCN-based cermets under 10 kg load. (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D, (e) Sample E, (f) Sample F.
and fine particles improved the compact density, which had a negative effect for obtaining the full density. (2) Coarse TiCN powders would accelerate the dissolution and precipitation process of the fine particles, resulting in more bright cores or thick inner rims on the fine TiCN grains. No coarse grains (less than 2 µm) were observed with the addition of 10–20% coarse powders. With further increasing the coarse TiCN content to 30% or higher, abnormal growth of the TiCN grains were formed by solid-state attachment.
(3) Mechanical properties of the ultrafine TiCN-based cermets were firstly increased with the addition of coarse TiCN powders, and then decreased sharply when with more than 30% coarse TiCN content. Highest strength (1940 MPa) and toughness (11.67 MPa· m1/2) were achieved with 10% coarse TiCN. Brittleness of the coarse grains with thick rims greatly deteriorated the final properties with high coarse TiCN addition.
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Materials Science & Engineering A 682 (2017) 648–655
H. Xiong et al.
Fig. 10. Crack deflection and fracture morphology of Samples B and D. (a) Crack deflection of sample B, (b) Fracture image of sample B, (c) Crack deflection of sample D, (b) Fracture image of sample D. [9] J.H. Xiang, Z.P. Xie, Y. Huang, H.N. Xiao, Synthesis of TiCN ultrafine powders by carbothermal reduction of TiO2 derived from sol-gel process, J. Eur. Ceram. Soc. 20 (2000) 933–938. [10] P. Mossino, Some aspects in sel-propagating high-temperature synthesis, Ceram. Int. 30 (2004) 311–332. [11] J.M. Córdoba, M.J. Sayagués, M.D. Alcalá, F.J. Gotor, Monophasic TiyNb1−yCxN1−x nanopowders obtained at room temperature by MSR, J. Mater. Chem. 17 (2007) 650–653. [12] S. Park, Y.J. Kang, H.J. Kwon, S. Kang, Synthesis of (Ti, M1, M2)(C, N)-Ni nanocrystalline powders, Int. J. Refract. Met. Hard Mater. 24 (2006) 115–121. [13] S. Park, S. Kang, Toughened ultrafine (Ti, W)(C, N)-Ni cermet, Scr. Mater. 52 (2005) 129–133. [14] J. Xiong, Z.X. Guo, M. Yang, B.L. Shen, Preparation of ultra-fine TiC0.7N0.3-based cermet, Int. J. Refract. Met. Hard Mater. 26 (2008) 212–219. [15] Y. Zheng, S.X. Wang, M. You, H.Y. Tan, W.H. Xiong, Fabrication of nanocomposite TiCN-based cermet by spark plasma sintering, Mater. Chem. Phys. 92 (2005) 64–70. [16] H.W. Xiong, Z.Y. Li, K.C. Zhou, TiC whisker reinforced ultra-fine TiC-based cermets: microstructure and mechanical properties, Ceram. Int. 42 (2016) 6858–6867. [17] K. Liu, L. Zhu, R. Tu, Hardening and toughening mechanisms of cemented carbides with plate-like WC grains prepared by seeding, Mater. Trans. 52 (2011) 699-3. [18] S. Kinoshita, M. Kobayashi, K. hayashi, High temperature strength of WC-Co base cemented carbide having highly oriented plate-like trian-gular prismatic WC grains, J. Jpn. Soc. Powder Metall. 49 (2002) 299-5. [19] H. Kwon, C. Suh, W. Kim, Preparation of a highly toughened (Ti,W)C-20Ni cermet through in situ formation of solid solution and WC whiskers, Ceram. Int. 41 (2015) 4223–4226. [20] W.D. Schubert, H. Neumeister, G. Kinger, B. Lux, Hardness to toughness relationship of fine-grained WC-Co hard metals, Int. J. Refract. Met. Hard Mater. 16 (1998) 133–142. [21] S.Y. Ahn, S.H. 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.
Acknowledgments The authors are grateful to the Ph.D. Programs Foundation of Ministry of Education of China (Nos. 20110162130003 and 20110162110044) for financial support. We are also grateful to the Project supported by State Key Laboratory of Powder Metallurgy of Central South University. References [1] Y. Peng, H.Z. Miao, Z.J. Peng, Development of TiCN-based cermets: mechanical properties and wear mechanism, Int. J. Refract. Met. Hard Mater. 39 (2013) 78–89. [2] Y. Deng, X.Q. Jiang, Y.H. Zhang, H. Chen, M.J. Tu, L. Deng, J.P. Zou, The effect of Co particle structures on the mechanical properties and microstructure of TiCNbased cermets, Mater. Sci. Eng. A 675 (2016) 164–170. [3] A. Demoly, W. Lenguar, C. Veitsh, K. Rabitsh, Effect of submicron Ti(C, N) on the microstructure and the mechanical properties of Ti(C,N)-based cermets, Int. J. Refract Met. Hard Mater. 29 (2011) 716–723. [4] Y.J. Zhao, Y. Zheng, W. Zhou, J.J. Zhang, Q. Huang, W.H. Xiong, Effect of carbon addition on the densification behavior, microstructure evolution and mechanical properties of Ti(C, N)-based cermets, Ceram. Int. 42 (2016) 5487–5496. [5] J. Jung, S. Kang, Effect of ultra-fine powders on the microstructure of Ti(C, N)xWC-Ni cermets, Acta Mater. 52 (2004) 1379–1383. [6] N. Liu, W. Yin, L. Zhu, Effect of TiC/TiN powder size on microstructure and properties of Ti(C,N)-based cermets, Mater. Sci. Eng. A 445 (2007) 707–716. [7] N. Liu, C. Sheng, H.D. Yang, Cutting performances, mechanical property and microstructure of ultra-fine grade Ti(C, N)-based cermets, Int. J. Refract. Met. Hard Mater. 24 (2006) 445–452. [8] Y. Liu, Y.Z. Jin, H.J. Yu, J.W. Ye, Ultrafine (Ti, M)(C, N)-based cermets with optimal mechanical properties, Int. J. Refract. Met. Hard Mater. 29 (2011) 104–107.
655
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of coarse TiCN content on the morphology and mechanical properties of ultrafine TiCN-based cermets
crossmark
⁎
Huiwen Xionga, Yu Wena, Xueping Gana, Zhiyou Lia, , Liyuan Chaib a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China School of Metallurgy and Environment, Central South University, Changsha 410083, China
A R T I C L E I N F O
A BS T RAC T
Keywords: TiCN Ultrafine cermet Duel grain structure Long time milling Mechanical properties
In this study, TiCN-based cermets with duel grain structure were prepared by long time milling of the microsized raw powders. The effects of coarse TiCN content on the microstructure and mechanical properties of ultrafine TiCN-based cermets were investigated. Results showed that the addition of coarse TiCN powders in ultrafine composite mixture could increase the compact density, but also resulted in increased closed pores in cermets. Coarse TiCN particles would accelerate the dissolution and precipitation reaction of the fine powders, forming more bright cores or thick inner rims on the fine TiCN grains. Grain size of the cermets gradually increased with higher coarse TiCN content. When the coarse content was more than 30%, abnormal growth of TiCN grains occurred via the coalescence of TiCN particles. Mechanical properties of the cermets were firstly increased within 10 or 20% coarse TiCN, then decreased sharply with further increasing the coarse powders. Highest strength (1940 MPa) and toughness (11.67 MPa·m1/2) were achieved with 10% coarse TiCN. Coarse ceramic grains with thick rims exhibit brittleness, deteriorating the final properties of the cermets.
1. Introduction
Therefore, to a certain extent, replacing part of the ultrafine powders by coarse particles can save energy and also decline the total impurities of the final mixture. Commonly, blending the coarse and fine powders can fabricate the duel grain structure. Cemented carbides with a bimodal grain size distribution show attractive performance, as fine grains provide high hardness and coarse grains ensure good toughness [17]. It has been reported that coarse plate-like WC grains dispersed in ultrafine WC-Co matrix can improve the final toughness via crack deflection and bridging mechanism [18]. Recently, coarse plate WC grains in TiCbased cermets shows the highest KIC value of 16.56 MPa·m1/2, which are fabricated via high energy milling and subsequently sintering of TiW-C mixture [19]. Our previous work has been shown that coarse TiC whisker-type grains in ultrafine TiC-based cermets are beneficial for improved properties [16]. Therefore, TiCN-based cermets with duel grain structure should be promising materials for satisfied mechanical properties. However, to date, works on the TiCN-based cermets with bimodal grain size have not been reported. In this study, commercial microsized TiCN powders were used as the starting materials. Ultrafine TiCN-based composite powders were fabricated by long time ball milling. Moreover, TiCN-based cermets with duel grain structure were prepared by different time milling of the starting TiCN powders and subsequent vacuum sintering. The effects of
TiCN-based cermets exhibit high hardness, good wear resistance and chemical stability, which are widely used as high-speed cutting tools [1,2]. However, the strength and toughness of cermets are usually inferior to the WC-Co system, which greatly limits their further applications [3,4]. Compared to the microsized cermets, ultrafine TiCN-based cermets have attracted more attention, due to the improved hardness and toughness [5]. Many studied have been carried to fabricate nano-modified or ultrafine TiCN-based cermets for harsher applications [6–8]. Firstly, nanosized or ultrafine composite powders should be obtained by various methods like carbothermal reduction synthesis [9], SHS or MSR [10,11], mechanical alloying [12], ect. Secondly, the compact samples are prepared via different sintering process include vacuum sintering, HIP, SPS and so on [13–15]. Obviously, high energy consumption and high requirement of the sintering procedure are put forward to meet the satisfied properties of these ultrafine composites. Moreover, careful handling of fine composite powders is needed during the preservation and transferring, in order to reduce the impurity content and oxygen absorption [16]. High oxygen content of ultrafine mixture will decrease the wettability of Ni on TiCN particles, and also result in more closed pores in the sintered cermets [14].
⁎
Corresponding author. E-mail address: [email protected] (Z. Li).
http://dx.doi.org/10.1016/j.msea.2016.11.085 Received 3 November 2016; Received in revised form 23 November 2016; Accepted 24 November 2016 Available online 25 November 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 682 (2017) 648–655
H. Xiong et al.
Zhuzhou cemented carbide limited company. Table 1 displays the nominal composition of the samples. From sample A to F, the coarse TiCN content of the TiCN powders was 0%, 10%, 20%, 30%, 50% and 100%, respectively. As for ultrafine cermets, the weighted powder was mixed by ball milling for 96 h. Meanwhile, the coarse system was ball milled for 24 h. Fig. 1 shows the procedure of preparing the cermets with duel grain structure. To investigate the effect of coarse TiCN on the ultrafine system, partial TiCN particles were added at the last 24 h of the milling process. During the milling, the ball to powder ratio was 12:1 and the rolling speed was 60r/min. After the milling, the mixture was put into a vacuum drying box. The drying was conducted at 80 ℃ for 12 h. Then, the composite powders were seized through 200 meshes before the uniaxial pressing under 200 MPa. The green samples were degreased at 350 ℃ for 90 min, then hold at 800 ℃ for 30 min in H2. Finally, these samples were transferred to the furnace for vacuum sintering at 1470 ℃ for 1 h. Carbon，Nitrogen and oxygen contents of the composite powders were determined using a Carbon/Nitrogen analyzer (Leco, WC-200AC, USA). Sintered samples were characterized by X-ray diffraction (XRD, Hitachi RAX-10 A-X, Japan). The XRD patterns were numerically analyzed with the use of MDI JADE (Version 6.0, Materials data Ltd., USA). The microstructure of polished specimens (finished with 1 µm diamond paste) were analyzed by environmental scanning electron microscopy (ESEM, NOVATM NanoSEM 230, Holland) coupled with EDXA. The densities of the sintered cermets were measured by the Archimedes method. The particle grain size distribution was acquired was measured by lineal intercept method based on SEM images. The transverse rupture test was conducted using a three-point bending instrument based on Instron3369 according to ISO 3327. The Rockwell hardness (HRA) was determined using an AR-600 Rockwell hardness. A Vicker Indenter was used to measure the toughness. The fracture toughness was evaluated by a direct crack measurement method with a 10 kg load and 15 s of load time on the polished surface of the sample using the expression derived by Shetty et al. [20].
Table 1 Chemical composition of the experimental materials (Unit: g). Samples
A B C D E F
TiCN 1st added
2nd addition
51 45.9 40.8 35.7 25.5 –
– 5.1 10.2 15.3 25.5 51
WC
Mo2C
TaC
Ni
Co
19 19 19 19 19 19
7 7 7 7 7 7
5 5 5 5 5 5
10 10 10 10 10 10
8 8 8 8 8 8
Fig. 1. A diagram of the fabrication process of the TiCN-based cermet.
3. Results and discussion 3.1. Density and metallographic observation Fig. 2 shows the density of the green compacts and sintered samples with varied coarse and ultrafine TiCN ratios. Ultrafine composite has the lowest compact density of 3.64 g/cm3, which increases to 3.91 g/cm3 with adding 10% coarse TiCN powders. Composite mixture with finer grain size owns lower green density, owing to the larger friction between particles during pressing. Highest green density (4.03 g/cm3) is achieved for samples E with 50% coarse TiCN content. Appropriate arrangement of coarse and fine particles can lead to a high density compact, as smaller particles will occupy the interspace formed by the bigger ones. The sintered density of the cermets increases from 6.95 to 7.14 g/cm3 with more fine TiCN content. Meanwhile, sample A has the highest sintering shrinkage. During the milling, certain amount of WC-Co contamination from the hardmetal balls exists in the final mixture. To a certain content, higher ultrafine TiCN powders means higher W and Co content, which leads to a higher sintered density. Fig. 3 shows the composite powders of Samples A and E. Uniformly dispersed composite powders are observed after 96 h milling. The milled particles have a size of around 200 nm. Moreover, a certain amount of microsized TiCN partcles can be seen in powders with 50% coarse TiCN content. Metallography is helpful for the study of the sintered density and phase distribution, while different phases are not easy to figure out in a BSE-SEM mode for cermets with high secondary carbides addition [5]. Fig. 4 shows the metallographic photos. Ceramic grains embedded homogeneously in the binder for samples milled for 96 h. As for cermet F, the binder is slightly gathered, forming small binder pools (Fig. 4f). Twenty-four hours blending is insufficient for a uniform distribution.
Fig. 2. Density and shrinkage of TiCN-based cermets with different ultrafine TiCN content.
coarse TiCN content on the microstructure and mechanical properties of ultrafine cermets were investigated. 2. Experiments Details of the raw materials used in this study have been already described in [16]. TiCN powders have a size of 2.5 µm, purchasing from 649
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Fig. 3. SEM images of composite powders of samples A (ultrafine) and E (50% coarse).: Ultrafine TiCN-based composite powders, (b): Composite powders with 50% coarse TiCN.
Fig. 4. Metallographic photos (1500×) of the TiCN-based cermets. (a) Smaple A, (b) Sample B, (c) Sample C, (d) Sample D, (e) Sample E and (f) Sample F. Insets are the enlarged views of the dashed squares.
grain size of the secondary added TiCN powders, low milling intensity was adopted. Therefore, it is hardly to obtain a uniform distribution of the coarse TiCN particles, especially for samples D and E. Compared to the ultrafine matrix, the decreased capillary forces of the coarse particles will shorten the migration of the liquid, leading increased closed pores after sintering.
The wettablity of Nickel on TiCN grains is not well enough, and low capillary force can shorten the diffusion distance of the binder, leading an uneven microstructure. All cermets show a highly dense body after vacuum sintering, except for samples D and E. Large amount of coarse TiCN particles seem harmful for achieving full dense and several round closed pores are left at the later sintering stage. To ensure the coarse 650
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Fig. 5. SEM images of TiCN-based cermets. (a) Sample A (Ultrafine), (b) Sample F (Microsized), (c) Sample B(10% coarse), (d) Sample C (20% coarse), (e) Sample D (30% coarse) and (f) Sample E (50% coarse). a1, b1, c1, d1, e1 and f1 are the enlarged views in the dashed squares of (a)–(f). The red bars are 1 µm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
diffusion rate, while the thick outer rims are quickly precipitated during the liquid sintering. Big ceramic grains with the size of 800 nm are obtained for the ultrafine cermet (Fig. 5a). Compared to the composite powder (around 200 nm) in Fig. 3a, the sintered TiCN grains grow up for nearly three times bigger. According to several studies [6,21], nanosized TiCN particles mainly grow by attachment mechanism before the rim phases appear. Formation of the surrounding phases can effectively hinder the growth of TiCN cores, as the undissolved black cores are separated from the matrix. With increasing the coarse TiCN content, more coarse TiCN grains are observed.
3.2. Microstructure and composition Fig. 5 shows the SEM images of TiCN-based cermets with different coarse TiCN content. Under the BSE-SEM mode, the contrast of the phase is sensitive to the average atomic number. The black core is the undissolved TiCN grain, which is covered by bright inner rim and grey outer rim [1,5]. The two rims have the same fcc structure as the TiCN grains, which are gradually formed via the dissolution-precipitation process. Typical core-rim structure can be seen for all cermets. The irregular inner rims are formed at the solid state sintering with low 651
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Meanwhile, the feature of dual grain structure is obvious with 30% and 50% coarse TiCN content. It is noteworthy that more bright core-grey rim phases can be observed in samples with partial coarse TiCN addition. And also, ultrafine grains with thick inner rims appear in Fig. 5d. However, only a few bright cores or irregular thin inner rims are seen in ultrafine or coarse TiCN-based cermets. As is known, the bright cores are formed by solid state diffusion of heavy elements into ultrafine TiCN particles [6]. After the long time milling, ultrafine TiCN grains have much higher surface area and defect density than the secondary added coarse grains. Compared to the ultrafine system, heavy elements (W, Mo, and Ta) will firstly diffuse into fine TiCN grains with the addition of coarse TiCN powders. Moreover, according to the Ostwald ripening mechanism, coarse TiCN particles will break the equilibrium of the ultrafine system, accelerating the dissolution and precipitation of ultrafine TiCN grains. Therefore, more ultrafine TiCN grains gradually evolve into bright cores or small cores with thick inner rims. Meanwhile, irregular inner rims are formed on the big particles at the solid state reaction. After the liquid phase appears at high temperature, coarse particles will prefer to grow up, by means of deposited thick outer rims (Fig. 5e1). Fig. 6 displays the XRD patterns of the TiCN cermets. All samples are composed of (Ti, M) CN (M=W, Mo and Ta) and Nickel-based binder. When W, Mo and Ta replace Ti atoms in TiCN or Ni in Nickel binder, the lattice parameter of TiCN or Ni will declines or increases, respectively. Meanwhile, diffraction will shift to low angles with an expanded fcc lattice. With increasing the coarse TiCN content, the (200) peak of TiCN slightly shift to a lower angle, indicating less amount of heavy elements in TiCN. As for the binder phase, high coarse TiCN content shows a high angle. Table 2 shows the carbon, nitrogen and oxygen content of the composite powders. Oxygen content of the composite powder increases with more long milled TiCN powders. Certain amount of carbon will be consumed, leading high solid solute in Nickel after sintering. The grain size distribution is shown in Fig. 7. As for sample A, the largest numbers of the ceramic grains have a grain size of 0.2–0.4 µm. After the addition of coarse TiCN grains, grain size of the ceramic grains increase. When with 10 or 20% coarse TiCN particles, most of
Fig. 6. XRD patterns of the TiCN-based cermets with different coarse TiCN content.
Table 2 Carbon, nitrogen and oxygen content of the composite powders. Samples
Carbon (wt%)
Nitrogen (wt%)
Oxygen (wt%)
A B C D E F
8.53 8.57 8.51 8.63 8.67 8.62
5.58 5.72 5.54 5.55 5.65 5.77
2.08 1.92 1.96 1.72 1.63 1.08
Fig. 7. Grain size distribution of samples sintered at 1470 ℃. (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D, (e) Sample E, (f) Sample F.
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Fig. 8. Mechanical properties of the TiCN-based cermets with different coarse TiCN content. TRS, (b) HRA, (c) Fracture toughness.
Fig. 9 shows the crack deflections of the cermets under 10 kg load. When coarse TiCN content is no more than 20%, the fracture mode is mainly intergranular fracture. Crack goes along the ultrafine grains, making the crack propagation path tortuous. As for the coarse grains, the fracture mechanism turns into transgranular. With further increasing the coarse TiCN content, more microsized grains are dispersed in the ultrafine matrix. The crack propagation path seems more smooth and straight, indicating decreased toughness. Coarse grains, usually with thick rims exhibit brittle and can be easily passed through by cracks. Fig. 10 shows crack deflection and fracture morphology of TiCN-based cermets with 10% and 30% coarse TiCN content. As for sample B, many bright cores can be observed in Fig. 10(a) and the fracture mode is intergranular fracture. As for the coarse grain in Fig. 10(c), smooth crack propagation path and transgranular type fracture can be clearly observed. Compared to the fine grains, thicker rims are formed on these coarse grains by ripening growth mechanism. As is known, certain internal stress exists at the interface between the core and rim. Therefore, the abnormal growth of TiCN grains can deteriorates the mechanical properties of cermets.
the ceramic grains have a size of 0.4–0.6 µm. And the big grains show a diameter less than 2.0 µm. With increasing the content of coarse powders to 30 or 50%, grain size of the highest proportion goes up to 0.6–0.8 µm. Meanwhile, the coarse grains have a size of 3.6 and 4.8 µm, much higher than samples B and C. Coarse TiCN particles have a great impact on the dissolution of ultrafine TiCN powders. Grains with small size firstly dissolved, leading coarse grains grow up. Growth of the black core phase is considered by the coalescence of TiCN particles at the solid state sintering. Complete rims on the TiCN cores will hinder the growth of black cores during the liquid sintering. High coarse TiCN content increases the possibility of the coarse particles contact. Therefore, grains with size larger than 3.5 µm are found in samples D and E. 3.3. Mechanical properties Fig. 8 shows the mechanical properties of the TiCN-based cermets. Hardness of the ultrafine cermet first rise from 91.8 to 92.3, then decrease to 90.6 for sample F. The hardness of the cermets is mainly provided by the ceramic grains. With the addition of coarse TiCN powders, more bright cores or thicker inner rims on the ultrafine grains are formed in Fig. 5. These bright (Ti, W, Mo, Ta) (C, N) solid solution is rich in heavy elements and owns higher hardness than the grey rims in ultrafine system. Therefore, higher hardness is achieved by adding coarse particles. Moreover, large grains in cermets are formed in high coarse TiCN content, which decrease the hardness by Hall-petch formula. The TRS and KIC display the same trend with increasing the coarse TiCN content, first increase then decline sharply with more than 30% coarse TiCN addition. Sample B shows the best strength and toughness of 1940 MPa and 11.67 MPa·m1/2.
4. Conclusions Ultrafine TiCN-based cermets have been prepared by long time milling of the commercial microsized mixture. The effects of coarse TiCN content on the microsturcture and mechanical properties of ultrafine TiCN-based cermets have been studied. Conclusions are summarized as follows: (1) TiCN-based cermets with duel grain structure can be prepared via different milling time of the TiCN powders. Matching of the coarse 653
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Fig. 9. SEM images of crack deflections of the TiCN-based cermets under 10 kg load. (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D, (e) Sample E, (f) Sample F.
and fine particles improved the compact density, which had a negative effect for obtaining the full density. (2) Coarse TiCN powders would accelerate the dissolution and precipitation process of the fine particles, resulting in more bright cores or thick inner rims on the fine TiCN grains. No coarse grains (less than 2 µm) were observed with the addition of 10–20% coarse powders. With further increasing the coarse TiCN content to 30% or higher, abnormal growth of the TiCN grains were formed by solid-state attachment.
(3) Mechanical properties of the ultrafine TiCN-based cermets were firstly increased with the addition of coarse TiCN powders, and then decreased sharply when with more than 30% coarse TiCN content. Highest strength (1940 MPa) and toughness (11.67 MPa· m1/2) were achieved with 10% coarse TiCN. Brittleness of the coarse grains with thick rims greatly deteriorated the final properties with high coarse TiCN addition.
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