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Fabrication of tungsten carbide–diamond composites using SiC-coated diamond
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
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Fabrication of tungsten carbide–diamond composites using SiC-coated diamond
T
⁎
Mettaya Kitiwana,b, , Takashi Gotoa,c a
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Physics, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, 1 Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand c State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten carbide Diamond Silicon carbide Chemical vapor deposition Spark plasma sintering
Tungsten carbide (WC) and SiC-coated diamond composites were prepared by spark plasma sintering at 1473–1873 K for 300 s under 130 MPa under vacuum. The diamond particle surface was coated with silicon carbide (SiC) via rotary chemical vapor deposition to improve the interfacial bonding of the WC–diamond composites. The relative density of the WC–20 vol% diamond (SiC) composite increased from 61% to 94% with increasing sintering temperature. Raman spectroscopic analysis showed that the diamond-to-graphite transition did not occur at any of the investigated sintering temperatures. The WC–20 vol% diamond composite sintered at 1773–1873 K exhibited high hardness (30.5 GPa) and fracture toughness (12.3 MPa m1/2). The high hardness resulted from the SiC coating functioning as an interlayer to improve the bonding between the diamond and WC body. The improvement in fracture toughness was attributed to the presence of diamond, which effectively blocks crack propagation and promotes crack deflection.
1. Introduction Tungsten carbide with a cobalt binder (WC/Co), which is also referred to as cemented carbide, has been extensively used in cutting tools owing to its high hardness, high toughness, and excellent wear resistance [1–3]. The Co binder is usually used to reduce sintering temperature and enhance interparticle bonding. WC/Co has a high fracture toughness due to the ductile Co phase [1–5]. However, the presence of the Co phase inevitably decreases the hardness and wear resistance of WC/Co cutting tools [5,6]. Moreover, the Co binder degrades the oxidation and corrosion resistance of WC/Co, thereby reducing tool life [7,8]. Numerous researchers have investigated binderless WC to improve its corrosion resistance. In the absence of a binder, higher sintering temperature and pressure are required to achieve a dense WC body. Various pressure-assisted sintering methods, including hot pressing [9–11], hot-isostatic pressing (HIP) [12,13], and spark plasma sintering (SPS) [14–30], have been used to consolidate binderless WC. Recently, SPS has attracted intensive attention because of its rapid heating rate and ability to sinter at relatively lower temperatures compared with other sintering methods. These advantages effectively minimize the grain growth of nanocrystalline WC, resulting in a substantial increase
in hardness [1]. However, hardness generally has an inverse relationship with fracture toughness; thus, improvement in hardness is accompanied by a decrease in fracture toughness [31,32]. The addition of ultrahard particles such as cubic boron nitride (cBN) [33,34] and diamond [35–38] to WC and WC/Co bodies can potentially increase their hardness and wear resistance. In addition, an improvement in fracture toughness has been attributed to such a second phase. Martinez et al. [33] studied the effect of cBN addition on the microstructure and properties of cBN–WC/Co composites fabricated by HIP at 1373–1473 K. The composites containing 30 vol% cBN exhibited a high hardness of 21.1–24.5 GPa and a substantial increase in fracture toughness to 10.5–15.4 MPa m1/2. Similarly, Wang et al. [34] reported that a 30 vol% cBN–WC/Co composite sintered by SPS at 1473 K exhibited a high hardness of 18.3 GPa and a high fracture toughness of 15.6 MPa m1/2. The toughening mechanisms were indicated to be crack deflection and bridging. Miyamoto et al. [35] prepared SiC-coated diamond by vapor-phase reaction followed by SPS with WC/Co at 1493 K. In case of 20 vol% SiC-coated diamond, the hardness was 15.5 GPa and the fracture toughness was 16.3 MPa m1/2. Later, Moriguchi et al. [36] reported that using diamonds with a larger particle size and a higher SPS temperature of 1573 K improved the hardness and fracture toughness to 17.5 GPa and 18.0 MPa m1/2, respectively.
⁎ Corresponding author at: Department of Physics, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, 1 Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand. E-mail address: [email protected] (M. Kitiwan).
https://doi.org/10.1016/j.ijrmhm.2019.105053 Received 26 April 2019; Received in revised form 7 August 2019; Accepted 7 August 2019 Available online 08 August 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto
Michalski et al. [37] sintered 30 vol% diamond–WC/Co using pulse plasma sintering (PPS) at a lower temperature of 1373 K, resulting in a high hardness of 23 GPa. Grasso et al. [38] fabricated 22 vol% diamond–WC composites by rapid SPS at 1873 K and obtained a fully dense body that exhibited high wear resistance. Although WC–diamond composites have demonstrated promising mechanical properties, their use in cutting-tool applications remains limited because of the difficulty of the consolidation process. Diamond powder is difficult to sinter owing to its intrinsically strong covalent bonds and poor wettability. Moreover, diamond can undergo a phase transition to low-hardness graphite at approximately 1773 K under vacuum [39]. The transition of diamond begins at an even lower temperature (1373–1573 K) in the presence of Co, which is known to be highly reactive toward diamond [36,40]. A SiC coating on diamond has been reported to suppress the reaction between diamond and molten Co [35,36]. In our previous study [41–43], a SiC coating on diamond powder, prepared by rotary chemical vapor deposition (CVD), effectively improved the wettability of diamond particles toward SiO2 particles and enabled their consolidation into dense diamond–SiO2 composites. Results showed that the composite with the highest diamond content (54 vol%) exhibited the highest hardness (36 GPa) among the investigated diamond–SiO2 composites. In this study, SiC-coated diamond composites were combined with WC and the resultant mixtures were sintered by SPS. The effects of the sintering temperature on the phase composition, relative density, microstructure, and mechanical properties of the WC–diamond composites were investigated. In addition, the diamond toughening mechanism was also discussed.
The 20 vol% SiC-coated diamond powder was gently mixed with WC in a mortar with a small amount of ethanol. The mixed powder was dried and sieved through a 200-mesh sieve. The powder mixture was placed in a 10-mm-inner-diameter graphite die, and the sintering was carried out by SPS (SPS-210LX; Fuji Electronic Industrial, Japan) under vacuum. The sintering temperature ranged from 1473 K to 1873 K, the heating rate was 100 K min−1, and the holding time was 300 s. A constant uniaxial pressure of 130 MPa was applied to the specimen throughout the sintering process. A sample of pure WC and a sample of WC–diamond with uncoated diamond powder were also sintered for comparison. Hereafter, the abbreviation WC–diamond (SiC) is used to denote the sintered specimen of WC containing 20 vol% SiC-coated diamond, while the abbreviation WC–diamond (uncoated) is used to denote the sintered specimen of WC containing 20 vol% uncoated diamond. The relative density of the sintered specimens was measured by the Archimedes method using a theoretical density of 15.7 and 3.5 g cm−3 for WC and diamond, respectively. The phase composition was examined by X-ray diffraction (XRD) on a diffractometer (RAD-2C, Rigaku, Japan) equipped with a Cu Kα X-ray source. The diamond-tographite phase transition was investigated using Raman spectroscopy with a laser wavelength of 532 nm (NRS5100, Jasco, Japan). The microstructure of the polished surface was observed by a scanning electron microscopy (SEM; S–3400, Hitachi, Japan). The Vickers hardness and fracture toughness were measured using a microhardness tester (HM-221, Mitutoyo, Tokyo, Japan) with a load of 19.6 N. Note that each reported value is the average of the data obtained from five indentation points. The Vickers hardness (HV) was calculated using the following equation:
2. Material and methods
P HV = 1.854 × 10−9 ⎛ 2 ⎞ ⎝d ⎠
Diamond powder (average particle size of 7 μm, Element Six, Luxembourg) and WC (average particle size of 0.6 μm, Kojundo, Japan) were used in this study. The diamond powder was coated with SiC by rotary CVD using C6H18Si2 (hexamethyldisilane, Shin-Etsu Chemical, Japan) as a precursor. The precursor was preheated at 303 K and carried into the reaction chamber by Ar gas flowing at 1.7 × 10−7 m3 s−1. The deposition temperature was 973 K, the deposition time was 10.8 ks, the total pressure in the chamber was 400 Pa, and the reactor rotating speed was 20 rpm. Following the coating procedure, the characteristics of the coating layer on the diamond powder were investigated by transmission electron microscopy (TEM; EM–002B, TOP–CON, Japan); Fig. 1 shows the corresponding micrograph. The SiC layer was uniformly deposited onto the diamond surface, with an average thickness of 80 nm. The layer deposited onto diamond particles contained Si and C, as confirmed by energy-dispersive X-ray spectroscopy (EDX).
where P is the applied load and d is the average of the two diagonal lengths of the Vickers indentation. The Shetty equation [44], which has been used by many researchers to evaluate the fracture toughness (KIc) of WC and WC composites, was used to calculate fracture toughness in this study:
P 1/2 KIc = 0.0889(HV )1/2 ⎛ ⎞ ⎝ 4a ⎠
(1)
(2)
where a is the mean radial crack length measured from the tip of the indentation. 3. Results and discussion 3.1. Phase composition, relative density, and microstructure Fig. 2 shows the XRD patterns of WC–diamond (SiC) sintered at 1473–1873 K. The XRD patterns show an intense peak of WC and a lowintensity peak of diamond. A small peak of W was observed at 1473 K. This phase could be the product of tungsten oxide (WO2 and WO3) impurities decomposed during sintering under the reducing atmosphere. At higher temperatures, this excess W can react with carbon from the graphite die and form WC [22,45]. No graphite peak was observed in the XRD pattern, indicating that the diamond-to-graphite transition did not occur during sintering at moderate pressure. However, the peak intensity of the diamond phase was small. Because the strongly diffracting WC might interfere with the detection of diamond and graphite, the phase transition was difficult to identify by XRD. Therefore, Raman spectroscopy was used to determine whether the diamond-to-graphite phase transition occurred. The phase transition of diamond was examined by Raman spectroscopy, which can detect various allotropes of carbon. Fig. 3 shows the Raman spectra of WC–diamond (SiC) sintered at 1473–1873 K. The Raman spectra show only a sharp line at around 1333 cm−1 corresponding to the sp3 carbon bonds of diamond [46]. No signal of
Fig. 1. TEM image of SiC-coated diamond prepared by rotary CVD. 2
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto
of pure WC and WC–diamond (uncoated). The relative density of WC–diamond (SiC) increased from 61% to 94% with increasing sintering temperature. At 1673–1773 K, the density of WC–diamond (SiC) was comparable to that of pure WC. However, at 1883 K, the density of pure WC reached 98%, whereas that of WC–diamond (SiC) was 94%. The densification behaviors of binderless WC subjected to SPS are different depending on the particle size of the sample and the pressure and temperature at which the sintering is conducted. A comparison of the results of previous studies involving WC powders with the submicron particle size range (0.2–0.6 μm) reveals that fully dense WC bodies could be obtained by SPS at moderate pressures (20–80 MPa) and at temperatures between 1773 and 1973 K [14–17,19–21,27,30,47], which is consistent with the results of the present study. However, introducing the second phase appears to reduce the sinterability of the WC composite, particularly in the case of adding a powder with a low self-diffusion coefficient, such as diamond. Herein, when uncoated diamond was used, the composite obtained at 1773 K exhibited a relative density (85%) lower than that of pure WC (91%). The density of the composite was improved by using SiC-coated diamond (90%). The SiC coating on the diamond powder might function as an intermediate layer to improve the bonding between diamond and WC. A SiC interlayer on a WC/Co substrate has been reported to exhibit excellent adhesion of a diamond film deposited by CVD [48,49]. Dense-sinteredbody WC has also been obtained by adding a small amount of SiC [50]. In the present study, although the reaction mechanism between WC and SiC layer has not yet been clarified, the SiC coating was chemically compatible with WC and enhanced the sinterability of the diamond powder. Fig. 5 shows backscattered-electron (BSE) SEM images of the polished surface of WC–diamond (SiC) composites sintered at 1573–1873 K. The SiC-coated diamond particles (dark-contrast phase) were uniformly distributed in the WC body. The WC–diamond (SiC) composites exhibited poor densification at 1573 K, whereas a welldensified microstructure as well as a good bonding between the diamond and WC were obtained at 1673–1773 K. However, fine cracks were observed at the interface between diamond and WC at a sintering temperature of 1873 K. The large difference between the thermal expansion coefficient of WC (5.2 × 10−6 K−1) and that of diamond (1.0 × 10−6 K−1) [51] causes tensile tangential and compressive radial stress in the surrounding WC after the sample cools from high temperature [52]. This residual stress could cause microcracks at the interface between the diamond and WC. The higher the sintering temperature, the greater the expected thermal expansion mismatch.
Fig. 2. XRD patterns of WC–diamond (SiC) sintered at 1473–1873 K.
Fig. 3. Raman at1473–1873 K.
spectra
of
WC–diamond
(SiC)
composite
sintered
graphite at 1357 cm−1 (D band) or 1580 cm−1 (G band) was observed [46]. Therefore, the Raman spectroscopic analysis confirmed that no phase transition from diamond to graphite occurred in the WC–diamond (SiC) composites sintered at 1473–1873 K. Fig. 4 shows the effect of sintering temperature on the relative density of WC–diamond (SiC) and a comparison of the relative densities
3.2. Mechanical properties Fig. 6 shows the effect of sintering temperature on the Vickers hardness of pure WC, WC–diamond (SiC), and WC–diamond (uncoated). The Vickers hardness of pure WC increased from 12.0 to 29.4 GPa with increasing temperature from 1673 K to 1873 K, whereas that of WC–diamond (SiC) increased from 8.5 to 30.5 GPa with increasing temperature from 1573 K to 1873 K. The temperature dependence of the hardness of both WC and WC–diamond (SiC) is related to the density of the sintered bodies. Specifically, an increase in relative density substantially increases the Vickers hardness. However, the hardness of WC–diamond (SiC) was overall higher than that of pure WC, despite their similar densities. Its superior hardness was derived from the presence of diamond in the composite. At 1773 K, the Vickers hardness of WC–diamond (SiC) was approximately 34% higher than that of WC–diamond (uncoated), which was attributed to the higher relative density of the WC–diamond (SiC). This result implies that the SiC coating on the diamond surface effectively improved the adhesion strength between the diamond powder and the WC body, thus resulting in the greater hardness. Fig. 7 shows the effect of sintering temperature on the fracture toughness of pure WC, WC–diamond (SiC), and WC–diamond
Fig. 4. Effect of sintering temperature on the relative density of WC–diamond (SiC), and a comparison of the relative densities of pure WC and WC–diamond (uncoated). 3
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto
Fig. 5. BSE–SEM images of the polished surface of WC–diamond (SiC) composites sintered at (a) 1573 K, (b) 1673 K, (c) 1773 K, and (d) 1873 K.
Fig. 6. Effect of sintering temperature on the Vickers hardness of pure WC, WC–diamond (SiC), and WC–diamond (uncoated).
Fig. 7. Effect of sintering temperature on the fracture toughness of pure WC, WC–diamond (SiC), and WC–diamond (uncoated).
(uncoated). Pure WC exhibited similar fracture toughness in the range of 6.9–7.3 MPa m1/2 at 1673–1873 K. The fracture toughness was substantially enhanced by the presence of diamond. The fracture toughness of WC–diamond (SiC) increased from 5.5 to 12.3 MPa m1/2 with increasing temperature from 1573 K to 1773 K and then slightly decreased to 11.7 MPa m1/2 at 1873 K. As evident in Fig. 8(a) and (b), the crack length through the pure WC body was straight and much longer than that through the WC–diamond (SiC) composite body. For the WC–diamond (SiC) composite, when the crack propagated toward large diamond particles, the crack tip stopped at a diamond particle, as indicated by the arrows in Fig. 8(c) and (d). Because of the high elastic modulus of diamond, a high propagation energy is necessary for a crack
to pass across diamond; consequently, crack propagation can be blocked by diamond particles. In addition, crack deflection was observed when the crack met small diamond particles. Furthermore, some of the crack propagation paths were also shortened by the compressive radial stress field around the diamond. Thus, the toughening mechanisms in WC–diamond composites, which include crack blocking and crack deflection, are strongly related to the presence of diamond particles. The relationships between Vickers hardness and fracture toughness (obtained via indentation fracture toughness method) reported in the literature for WC/Co, binderless WC, and WC–diamond (SiC)
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International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto
Fig. 8. SEM micrographs of crack propagation of (a) WC and (c) WC–diamond (SiC) sintered at 1773 K; (b) and (d) are higher-magnification micrographs of (a) and (c), respectively.
exhibited a substantial toughness of 16.3–18.0 MPa m1/2; however, the hardness was limited (15.5–18.0 GPa) because of the presence of the Co binder [35,36]. In the present study, the WC–diamond (SiC) composites showed a combination of high hardness of 27.6–30.5 GPa and high fracture toughness of 11.7–12.3 MPa m1/2. The addition of SiC-coated diamond, which bonded well with the WC, improved the hardness of the WC–diamond (SiC) composite. The uniform distribution of SiCcoated diamond in the WC body effectively prevented crack propagation and promoted crack deflection, thus contributing to the high fracture toughness. The WC–diamond (SiC) composites fabricated in this study exhibited high hardness comparable to that of binderless WC, whereas their fracture toughness was similar to that of WC/Co. 4. Conclusions In this study, densified WC–diamond (SiC) composites with 20 vol% SiC-coated diamond were consolidated by SPS at 1773–1873 K for 300 s at 130 MPa under vacuum. No diamond-to-graphite phase transition was observed. The relative densities of WC–diamond (SiC) were 90% and 94% at 1773 K and 1873 K, respectively. The coating of the SiC layer on diamond powder promoted bonding between the diamond and the WC body, resulting in improved mechanical properties in comparison to pure WC and WC-diamond (uncoated). The hardness and high fracture toughness achieved at 1773 K were 27.6 GPa and 12.3 MPa m1/ 2 , respectively, whereas those at 1873 K were 30.5 GPa and 11.7 MPa m1/2.
Fig. 9. Relationship between Vickers hardness and fracture toughness (obtained via indentation fracture toughness method) of WC/Co, binderless WC, and WC–diamond (SiC) composites fabricated using SPS and reported in the literature.
composites fabricated using SPS are presented in Fig. 9. Notably, fracture toughness is inversely proportional to hardness. As expected, a high hardness (18.6–34.0 GPa) was obtained with binderless WC bodies. However, in most cases, the reported fracture toughness is lower than 8 MPa m1/2 [15–21,24,26–30]. Huang et al. [17] reported a substantially high fracture toughness of 9–15 MPa m1/2 for binderless WC; however, the authors did not clarify the reason for this high value. WC/Co cemented carbides have been reported to exhibit a moderate hardness of 14.5–22.5 GPa and high fracture toughness in the range of 10.5–15.3 MPa m1/2 [5,18,30,53–57]. The Co binder plays an important role in the high fracture toughness of WC/Co bodies. The toughening effects are explained by the shielding of the crack tip from the external stress field caused by plastic deformation of the binder phase and by the crack bridging caused by the interaction of the metallic binder [3]. The addition of the second phase usually results in higher fracture toughness of composites. The WC/Co–diamond (SiC)
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Met. Hard Mater. 25 (2007) 46–52. [19] L. Girardini, M. Zadra, F. Casari, A. Molinari, SPS, binderless WC powders, and the problem of sub carbide, Met. Powder Rep. 63 (2008) 18–22. [20] S.G. Huang, K. Vanmeensel, O. Van der Biest, J. Vleugels, Binderless WC and WC–VC materials obtained by pulsed electric current sintering, Int. J. Refract. Met. Hard Mater. 26 (2008) 41–47. [21] X. Liu, T. Lin, H. Shao, Z. Guo, J. Luo, J. Hao, Consolidation and properties of ultrafine binderless cemented carbide by spark plasma sintering, Rare Metals 27 (2008) 320–323. [22] J. Zhao, T. Holland, C. Unuvar, Z.A. Munir, Sparking plasma sintering of nanometric tungsten carbide, Int. J. Refract. Met. Hard Mater. 27 (2009) 130–139. [23] S. Grasso, Y. Sakka, G. Maizza, C. Hu, Pressure effect on the homogeneity of spark plasma-sintered tungsten carbide powder, J. Am. Ceram. Soc. 92 (2009) 2418–2421. [24] I.J. Shon, B.R. Kim, J.M. Doh, J.K. Yoon, K.D. Woo, Properties and rapid consolidation of ultra-hard tungsten carbide, J. Alloys Compd. 489 (2010) L4–L8. [25] A.K.N. Kumar, M. Watabe, K. Kurokawa, The sintering kinetics of ultrafine tungsten carbide powders, Ceram. Int. 37 (2011) 2643–2654. [26] S. Grasso, J. Poetschke, V. Richter, G. Maizza, Y. Sakka, M.J. Reece, Low-temperature spark plasma sintering of pure nano WC powder, J. Am. Ceram. Soc. 96 (2013) 1702–1705. [27] X. Ren, Z. Peng, C. Wang, H. Miao, Influence of nano-sized La2O3 addition on the sintering behavior and mechanical properties of WC–La2O3 composites, Ceram. Int. 41 (2015) 14811–14818. [28] V.N. Chuvil’deev, Y.V. Blagoveshchenskiy, A.V. Nokhrin, M.S. Boldin, N.V. Sakharov, N.V. Isaeva, S.V. Shotin, O.A. Belkin, A.A. Popov, E.S. Smirnova, E.A. Lantsev, Spark plasma sintering of tungsten carbide nanopowders obtained through DC arc plasma synthesis, J. Alloys Compd. 708 (2017) 547–561. [29] W. Tang, L. Zhang, J. Zhu, Y. Chen, W. Tian, T. Liu, Effect of direct current patterns on densification and mechanical properties of binderless tungsten carbides fabricated by the spark plasma sintering system, Int. J. Refract. Met. Hard Mater. 64 (2017) 90–97. [30] W. Tang, L. Zhang, Y. Chen, H. Zhang, L. Zhou, Corrosion and strength degradation behaviors of binderless WC material and WC–Co hardmetal in alkaline solution: a comparative investigation, Int. J. Refract. Met. Hard Mater. 68 (2017) 1–8. [31] D.G.F. O’Quigley, S. Luyckx, M.N. James, New results on the relationship between hardness and fracture toughness of WC-Co hardmetal, Mater. Sci. Eng. A 209 (1996) 228–230. [32] W.D. Schubert, H. Neumeister, G. Kinger, B. Lux, Hardness to toughness relationship of fine-grained WC-Co hardmetals, Int. J. Refract. Met. Hard Mater. 16 (1998) 133–142. [33] V. Martínez, J. Echeberria, Hot isostatic pressing of cubic boron nitride-Tungsten carbide/cobalt (cBN?WC/co) composites: effect of cBN particle size and some processing parameters on their microstructure and properties, J. Am. Ceram. Soc. 90 (2007) 415–424.
Mettaya Kitiwan is a researcher at Department of Physics, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand. She received her B.S. degree in Materials Science, M.S. degree in Ceramic Technology and Ph.D. in Materials Processing. Previously, she was an assistant professor at Institute for Materials Research (IMR), Tohoku University, Japan. Prior to IMR, she was a researcher at National Metal and Materials Technology Center, Thailand. Her research areas are spark plasma sintering of advanced ceramics, microwave processing of material and materials for hydrogen separation membrane.
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International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto Takashi Goto is a professor of Tohoku University, Japan and Wuhan University of Technology, China. He received a B.S., M.S. and Doctor degree in Materials Science from Tohoku University. His research interests are manufacturing of ceramic materials by spark plasma sintering, chemical vapor deposition and melt growth. He is a fellow of American Ceramic Society and the Ceramic Society of Japan, and an academicians of the World Academy of Ceramics and the Asia Pacific Academy of Materials. He is a general editor/editor-in-chief of Journal of Asian Ceramic Societies and an associate editor of Materials Letters.
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Fabrication of tungsten carbide–diamond composites using SiC-coated diamond
T
⁎
Mettaya Kitiwana,b, , Takashi Gotoa,c a
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Physics, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, 1 Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand c State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten carbide Diamond Silicon carbide Chemical vapor deposition Spark plasma sintering
Tungsten carbide (WC) and SiC-coated diamond composites were prepared by spark plasma sintering at 1473–1873 K for 300 s under 130 MPa under vacuum. The diamond particle surface was coated with silicon carbide (SiC) via rotary chemical vapor deposition to improve the interfacial bonding of the WC–diamond composites. The relative density of the WC–20 vol% diamond (SiC) composite increased from 61% to 94% with increasing sintering temperature. Raman spectroscopic analysis showed that the diamond-to-graphite transition did not occur at any of the investigated sintering temperatures. The WC–20 vol% diamond composite sintered at 1773–1873 K exhibited high hardness (30.5 GPa) and fracture toughness (12.3 MPa m1/2). The high hardness resulted from the SiC coating functioning as an interlayer to improve the bonding between the diamond and WC body. The improvement in fracture toughness was attributed to the presence of diamond, which effectively blocks crack propagation and promotes crack deflection.
1. Introduction Tungsten carbide with a cobalt binder (WC/Co), which is also referred to as cemented carbide, has been extensively used in cutting tools owing to its high hardness, high toughness, and excellent wear resistance [1–3]. The Co binder is usually used to reduce sintering temperature and enhance interparticle bonding. WC/Co has a high fracture toughness due to the ductile Co phase [1–5]. However, the presence of the Co phase inevitably decreases the hardness and wear resistance of WC/Co cutting tools [5,6]. Moreover, the Co binder degrades the oxidation and corrosion resistance of WC/Co, thereby reducing tool life [7,8]. Numerous researchers have investigated binderless WC to improve its corrosion resistance. In the absence of a binder, higher sintering temperature and pressure are required to achieve a dense WC body. Various pressure-assisted sintering methods, including hot pressing [9–11], hot-isostatic pressing (HIP) [12,13], and spark plasma sintering (SPS) [14–30], have been used to consolidate binderless WC. Recently, SPS has attracted intensive attention because of its rapid heating rate and ability to sinter at relatively lower temperatures compared with other sintering methods. These advantages effectively minimize the grain growth of nanocrystalline WC, resulting in a substantial increase
in hardness [1]. However, hardness generally has an inverse relationship with fracture toughness; thus, improvement in hardness is accompanied by a decrease in fracture toughness [31,32]. The addition of ultrahard particles such as cubic boron nitride (cBN) [33,34] and diamond [35–38] to WC and WC/Co bodies can potentially increase their hardness and wear resistance. In addition, an improvement in fracture toughness has been attributed to such a second phase. Martinez et al. [33] studied the effect of cBN addition on the microstructure and properties of cBN–WC/Co composites fabricated by HIP at 1373–1473 K. The composites containing 30 vol% cBN exhibited a high hardness of 21.1–24.5 GPa and a substantial increase in fracture toughness to 10.5–15.4 MPa m1/2. Similarly, Wang et al. [34] reported that a 30 vol% cBN–WC/Co composite sintered by SPS at 1473 K exhibited a high hardness of 18.3 GPa and a high fracture toughness of 15.6 MPa m1/2. The toughening mechanisms were indicated to be crack deflection and bridging. Miyamoto et al. [35] prepared SiC-coated diamond by vapor-phase reaction followed by SPS with WC/Co at 1493 K. In case of 20 vol% SiC-coated diamond, the hardness was 15.5 GPa and the fracture toughness was 16.3 MPa m1/2. Later, Moriguchi et al. [36] reported that using diamonds with a larger particle size and a higher SPS temperature of 1573 K improved the hardness and fracture toughness to 17.5 GPa and 18.0 MPa m1/2, respectively.
⁎ Corresponding author at: Department of Physics, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, 1 Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand. E-mail address: [email protected] (M. Kitiwan).
https://doi.org/10.1016/j.ijrmhm.2019.105053 Received 26 April 2019; Received in revised form 7 August 2019; Accepted 7 August 2019 Available online 08 August 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto
Michalski et al. [37] sintered 30 vol% diamond–WC/Co using pulse plasma sintering (PPS) at a lower temperature of 1373 K, resulting in a high hardness of 23 GPa. Grasso et al. [38] fabricated 22 vol% diamond–WC composites by rapid SPS at 1873 K and obtained a fully dense body that exhibited high wear resistance. Although WC–diamond composites have demonstrated promising mechanical properties, their use in cutting-tool applications remains limited because of the difficulty of the consolidation process. Diamond powder is difficult to sinter owing to its intrinsically strong covalent bonds and poor wettability. Moreover, diamond can undergo a phase transition to low-hardness graphite at approximately 1773 K under vacuum [39]. The transition of diamond begins at an even lower temperature (1373–1573 K) in the presence of Co, which is known to be highly reactive toward diamond [36,40]. A SiC coating on diamond has been reported to suppress the reaction between diamond and molten Co [35,36]. In our previous study [41–43], a SiC coating on diamond powder, prepared by rotary chemical vapor deposition (CVD), effectively improved the wettability of diamond particles toward SiO2 particles and enabled their consolidation into dense diamond–SiO2 composites. Results showed that the composite with the highest diamond content (54 vol%) exhibited the highest hardness (36 GPa) among the investigated diamond–SiO2 composites. In this study, SiC-coated diamond composites were combined with WC and the resultant mixtures were sintered by SPS. The effects of the sintering temperature on the phase composition, relative density, microstructure, and mechanical properties of the WC–diamond composites were investigated. In addition, the diamond toughening mechanism was also discussed.
The 20 vol% SiC-coated diamond powder was gently mixed with WC in a mortar with a small amount of ethanol. The mixed powder was dried and sieved through a 200-mesh sieve. The powder mixture was placed in a 10-mm-inner-diameter graphite die, and the sintering was carried out by SPS (SPS-210LX; Fuji Electronic Industrial, Japan) under vacuum. The sintering temperature ranged from 1473 K to 1873 K, the heating rate was 100 K min−1, and the holding time was 300 s. A constant uniaxial pressure of 130 MPa was applied to the specimen throughout the sintering process. A sample of pure WC and a sample of WC–diamond with uncoated diamond powder were also sintered for comparison. Hereafter, the abbreviation WC–diamond (SiC) is used to denote the sintered specimen of WC containing 20 vol% SiC-coated diamond, while the abbreviation WC–diamond (uncoated) is used to denote the sintered specimen of WC containing 20 vol% uncoated diamond. The relative density of the sintered specimens was measured by the Archimedes method using a theoretical density of 15.7 and 3.5 g cm−3 for WC and diamond, respectively. The phase composition was examined by X-ray diffraction (XRD) on a diffractometer (RAD-2C, Rigaku, Japan) equipped with a Cu Kα X-ray source. The diamond-tographite phase transition was investigated using Raman spectroscopy with a laser wavelength of 532 nm (NRS5100, Jasco, Japan). The microstructure of the polished surface was observed by a scanning electron microscopy (SEM; S–3400, Hitachi, Japan). The Vickers hardness and fracture toughness were measured using a microhardness tester (HM-221, Mitutoyo, Tokyo, Japan) with a load of 19.6 N. Note that each reported value is the average of the data obtained from five indentation points. The Vickers hardness (HV) was calculated using the following equation:
2. Material and methods
P HV = 1.854 × 10−9 ⎛ 2 ⎞ ⎝d ⎠
Diamond powder (average particle size of 7 μm, Element Six, Luxembourg) and WC (average particle size of 0.6 μm, Kojundo, Japan) were used in this study. The diamond powder was coated with SiC by rotary CVD using C6H18Si2 (hexamethyldisilane, Shin-Etsu Chemical, Japan) as a precursor. The precursor was preheated at 303 K and carried into the reaction chamber by Ar gas flowing at 1.7 × 10−7 m3 s−1. The deposition temperature was 973 K, the deposition time was 10.8 ks, the total pressure in the chamber was 400 Pa, and the reactor rotating speed was 20 rpm. Following the coating procedure, the characteristics of the coating layer on the diamond powder were investigated by transmission electron microscopy (TEM; EM–002B, TOP–CON, Japan); Fig. 1 shows the corresponding micrograph. The SiC layer was uniformly deposited onto the diamond surface, with an average thickness of 80 nm. The layer deposited onto diamond particles contained Si and C, as confirmed by energy-dispersive X-ray spectroscopy (EDX).
where P is the applied load and d is the average of the two diagonal lengths of the Vickers indentation. The Shetty equation [44], which has been used by many researchers to evaluate the fracture toughness (KIc) of WC and WC composites, was used to calculate fracture toughness in this study:
P 1/2 KIc = 0.0889(HV )1/2 ⎛ ⎞ ⎝ 4a ⎠
(1)
(2)
where a is the mean radial crack length measured from the tip of the indentation. 3. Results and discussion 3.1. Phase composition, relative density, and microstructure Fig. 2 shows the XRD patterns of WC–diamond (SiC) sintered at 1473–1873 K. The XRD patterns show an intense peak of WC and a lowintensity peak of diamond. A small peak of W was observed at 1473 K. This phase could be the product of tungsten oxide (WO2 and WO3) impurities decomposed during sintering under the reducing atmosphere. At higher temperatures, this excess W can react with carbon from the graphite die and form WC [22,45]. No graphite peak was observed in the XRD pattern, indicating that the diamond-to-graphite transition did not occur during sintering at moderate pressure. However, the peak intensity of the diamond phase was small. Because the strongly diffracting WC might interfere with the detection of diamond and graphite, the phase transition was difficult to identify by XRD. Therefore, Raman spectroscopy was used to determine whether the diamond-to-graphite phase transition occurred. The phase transition of diamond was examined by Raman spectroscopy, which can detect various allotropes of carbon. Fig. 3 shows the Raman spectra of WC–diamond (SiC) sintered at 1473–1873 K. The Raman spectra show only a sharp line at around 1333 cm−1 corresponding to the sp3 carbon bonds of diamond [46]. No signal of
Fig. 1. TEM image of SiC-coated diamond prepared by rotary CVD. 2
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
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of pure WC and WC–diamond (uncoated). The relative density of WC–diamond (SiC) increased from 61% to 94% with increasing sintering temperature. At 1673–1773 K, the density of WC–diamond (SiC) was comparable to that of pure WC. However, at 1883 K, the density of pure WC reached 98%, whereas that of WC–diamond (SiC) was 94%. The densification behaviors of binderless WC subjected to SPS are different depending on the particle size of the sample and the pressure and temperature at which the sintering is conducted. A comparison of the results of previous studies involving WC powders with the submicron particle size range (0.2–0.6 μm) reveals that fully dense WC bodies could be obtained by SPS at moderate pressures (20–80 MPa) and at temperatures between 1773 and 1973 K [14–17,19–21,27,30,47], which is consistent with the results of the present study. However, introducing the second phase appears to reduce the sinterability of the WC composite, particularly in the case of adding a powder with a low self-diffusion coefficient, such as diamond. Herein, when uncoated diamond was used, the composite obtained at 1773 K exhibited a relative density (85%) lower than that of pure WC (91%). The density of the composite was improved by using SiC-coated diamond (90%). The SiC coating on the diamond powder might function as an intermediate layer to improve the bonding between diamond and WC. A SiC interlayer on a WC/Co substrate has been reported to exhibit excellent adhesion of a diamond film deposited by CVD [48,49]. Dense-sinteredbody WC has also been obtained by adding a small amount of SiC [50]. In the present study, although the reaction mechanism between WC and SiC layer has not yet been clarified, the SiC coating was chemically compatible with WC and enhanced the sinterability of the diamond powder. Fig. 5 shows backscattered-electron (BSE) SEM images of the polished surface of WC–diamond (SiC) composites sintered at 1573–1873 K. The SiC-coated diamond particles (dark-contrast phase) were uniformly distributed in the WC body. The WC–diamond (SiC) composites exhibited poor densification at 1573 K, whereas a welldensified microstructure as well as a good bonding between the diamond and WC were obtained at 1673–1773 K. However, fine cracks were observed at the interface between diamond and WC at a sintering temperature of 1873 K. The large difference between the thermal expansion coefficient of WC (5.2 × 10−6 K−1) and that of diamond (1.0 × 10−6 K−1) [51] causes tensile tangential and compressive radial stress in the surrounding WC after the sample cools from high temperature [52]. This residual stress could cause microcracks at the interface between the diamond and WC. The higher the sintering temperature, the greater the expected thermal expansion mismatch.
Fig. 2. XRD patterns of WC–diamond (SiC) sintered at 1473–1873 K.
Fig. 3. Raman at1473–1873 K.
spectra
of
WC–diamond
(SiC)
composite
sintered
graphite at 1357 cm−1 (D band) or 1580 cm−1 (G band) was observed [46]. Therefore, the Raman spectroscopic analysis confirmed that no phase transition from diamond to graphite occurred in the WC–diamond (SiC) composites sintered at 1473–1873 K. Fig. 4 shows the effect of sintering temperature on the relative density of WC–diamond (SiC) and a comparison of the relative densities
3.2. Mechanical properties Fig. 6 shows the effect of sintering temperature on the Vickers hardness of pure WC, WC–diamond (SiC), and WC–diamond (uncoated). The Vickers hardness of pure WC increased from 12.0 to 29.4 GPa with increasing temperature from 1673 K to 1873 K, whereas that of WC–diamond (SiC) increased from 8.5 to 30.5 GPa with increasing temperature from 1573 K to 1873 K. The temperature dependence of the hardness of both WC and WC–diamond (SiC) is related to the density of the sintered bodies. Specifically, an increase in relative density substantially increases the Vickers hardness. However, the hardness of WC–diamond (SiC) was overall higher than that of pure WC, despite their similar densities. Its superior hardness was derived from the presence of diamond in the composite. At 1773 K, the Vickers hardness of WC–diamond (SiC) was approximately 34% higher than that of WC–diamond (uncoated), which was attributed to the higher relative density of the WC–diamond (SiC). This result implies that the SiC coating on the diamond surface effectively improved the adhesion strength between the diamond powder and the WC body, thus resulting in the greater hardness. Fig. 7 shows the effect of sintering temperature on the fracture toughness of pure WC, WC–diamond (SiC), and WC–diamond
Fig. 4. Effect of sintering temperature on the relative density of WC–diamond (SiC), and a comparison of the relative densities of pure WC and WC–diamond (uncoated). 3
International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
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Fig. 5. BSE–SEM images of the polished surface of WC–diamond (SiC) composites sintered at (a) 1573 K, (b) 1673 K, (c) 1773 K, and (d) 1873 K.
Fig. 6. Effect of sintering temperature on the Vickers hardness of pure WC, WC–diamond (SiC), and WC–diamond (uncoated).
Fig. 7. Effect of sintering temperature on the fracture toughness of pure WC, WC–diamond (SiC), and WC–diamond (uncoated).
(uncoated). Pure WC exhibited similar fracture toughness in the range of 6.9–7.3 MPa m1/2 at 1673–1873 K. The fracture toughness was substantially enhanced by the presence of diamond. The fracture toughness of WC–diamond (SiC) increased from 5.5 to 12.3 MPa m1/2 with increasing temperature from 1573 K to 1773 K and then slightly decreased to 11.7 MPa m1/2 at 1873 K. As evident in Fig. 8(a) and (b), the crack length through the pure WC body was straight and much longer than that through the WC–diamond (SiC) composite body. For the WC–diamond (SiC) composite, when the crack propagated toward large diamond particles, the crack tip stopped at a diamond particle, as indicated by the arrows in Fig. 8(c) and (d). Because of the high elastic modulus of diamond, a high propagation energy is necessary for a crack
to pass across diamond; consequently, crack propagation can be blocked by diamond particles. In addition, crack deflection was observed when the crack met small diamond particles. Furthermore, some of the crack propagation paths were also shortened by the compressive radial stress field around the diamond. Thus, the toughening mechanisms in WC–diamond composites, which include crack blocking and crack deflection, are strongly related to the presence of diamond particles. The relationships between Vickers hardness and fracture toughness (obtained via indentation fracture toughness method) reported in the literature for WC/Co, binderless WC, and WC–diamond (SiC)
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Fig. 8. SEM micrographs of crack propagation of (a) WC and (c) WC–diamond (SiC) sintered at 1773 K; (b) and (d) are higher-magnification micrographs of (a) and (c), respectively.
exhibited a substantial toughness of 16.3–18.0 MPa m1/2; however, the hardness was limited (15.5–18.0 GPa) because of the presence of the Co binder [35,36]. In the present study, the WC–diamond (SiC) composites showed a combination of high hardness of 27.6–30.5 GPa and high fracture toughness of 11.7–12.3 MPa m1/2. The addition of SiC-coated diamond, which bonded well with the WC, improved the hardness of the WC–diamond (SiC) composite. The uniform distribution of SiCcoated diamond in the WC body effectively prevented crack propagation and promoted crack deflection, thus contributing to the high fracture toughness. The WC–diamond (SiC) composites fabricated in this study exhibited high hardness comparable to that of binderless WC, whereas their fracture toughness was similar to that of WC/Co. 4. Conclusions In this study, densified WC–diamond (SiC) composites with 20 vol% SiC-coated diamond were consolidated by SPS at 1773–1873 K for 300 s at 130 MPa under vacuum. No diamond-to-graphite phase transition was observed. The relative densities of WC–diamond (SiC) were 90% and 94% at 1773 K and 1873 K, respectively. The coating of the SiC layer on diamond powder promoted bonding between the diamond and the WC body, resulting in improved mechanical properties in comparison to pure WC and WC-diamond (uncoated). The hardness and high fracture toughness achieved at 1773 K were 27.6 GPa and 12.3 MPa m1/ 2 , respectively, whereas those at 1873 K were 30.5 GPa and 11.7 MPa m1/2.
Fig. 9. Relationship between Vickers hardness and fracture toughness (obtained via indentation fracture toughness method) of WC/Co, binderless WC, and WC–diamond (SiC) composites fabricated using SPS and reported in the literature.
composites fabricated using SPS are presented in Fig. 9. Notably, fracture toughness is inversely proportional to hardness. As expected, a high hardness (18.6–34.0 GPa) was obtained with binderless WC bodies. However, in most cases, the reported fracture toughness is lower than 8 MPa m1/2 [15–21,24,26–30]. Huang et al. [17] reported a substantially high fracture toughness of 9–15 MPa m1/2 for binderless WC; however, the authors did not clarify the reason for this high value. WC/Co cemented carbides have been reported to exhibit a moderate hardness of 14.5–22.5 GPa and high fracture toughness in the range of 10.5–15.3 MPa m1/2 [5,18,30,53–57]. The Co binder plays an important role in the high fracture toughness of WC/Co bodies. The toughening effects are explained by the shielding of the crack tip from the external stress field caused by plastic deformation of the binder phase and by the crack bridging caused by the interaction of the metallic binder [3]. The addition of the second phase usually results in higher fracture toughness of composites. The WC/Co–diamond (SiC)
Acknowledgements This study was partially supported by a cooperative program of the CRDAM-IMR (proposal no. 17G0421), Tohoku University, and financially supported by Mitsubishi Materials. References [1] Z.Z. Fang, X. Wang, T. Ryu, K.S. Hwang, H.Y. Sohn, Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide – a review, Int. J. Refract. Met. Hard Mater. 27 (2009) 288–299.
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International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto
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Mettaya Kitiwan is a researcher at Department of Physics, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand. She received her B.S. degree in Materials Science, M.S. degree in Ceramic Technology and Ph.D. in Materials Processing. Previously, she was an assistant professor at Institute for Materials Research (IMR), Tohoku University, Japan. Prior to IMR, she was a researcher at National Metal and Materials Technology Center, Thailand. Her research areas are spark plasma sintering of advanced ceramics, microwave processing of material and materials for hydrogen separation membrane.
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International Journal of Refractory Metals & Hard Materials 85 (2019) 105053
M. Kitiwan and T. Goto Takashi Goto is a professor of Tohoku University, Japan and Wuhan University of Technology, China. He received a B.S., M.S. and Doctor degree in Materials Science from Tohoku University. His research interests are manufacturing of ceramic materials by spark plasma sintering, chemical vapor deposition and melt growth. He is a fellow of American Ceramic Society and the Ceramic Society of Japan, and an academicians of the World Academy of Ceramics and the Asia Pacific Academy of Materials. He is a general editor/editor-in-chief of Journal of Asian Ceramic Societies and an associate editor of Materials Letters.
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