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Spark plasma sintering of Co–WC cubic boron nitride composites
Materials Letters 63 (2009) 1041–1043
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Spark plasma sintering of Co–WC cubic boron nitride composites Bilge Yaman a,⁎, Hasan Mandal b a b
Department of Metallurgical and Materials Engineering, Eskisehir Osmangazi University, Meselik Campus, TR-26480, Eskisehir, Turkey Department of Materials Science and Engineering, Anadolu University, Iki Eylul Campus, TR-26555, Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 17 November 2008 Accepted 30 January 2009 Available online 12 February 2009 Keywords: Tungsten carbide WC–6Co–cBN composites Spark plasma sintering
a b s t r a c t 25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC–6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC–6Co–cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction WC–Co cemented carbides have been used as cutting tools, dies and wear-resistant parts owing to their high hardness and excellent wear resistance as well as their retention of room temperature hardness at elevated temperatures [1–5]. Morphologically, they consist of a high volume fraction of hard WC phase embedded within a soft and tough Co binder phase [2]. Increasing the volume fraction of Co improves the fracture toughness at the expense of hardness and wear resistance [4]. WC–Co hard materials can be densified by liquid phase sintering and the mechanical properties of these materials depend on the composition and microstructure [2]. The use of conventional methods of powder consolidation often results in grain growth. It is essential to minimize grain growth through careful control of consolidation parameters, particularly temperature and sintering time [3]. Spark plasma sintering (SPS) is an effective sintering technique that allows densification of ceramic powders at a relatively lower temperature with short holding time owing to rapid heating and cooling rates [6–10]. With SPS, the microstructure and grain growth can be controlled while maintaining the desired properties of the material [7]. A survey of the relevant literature indicates that the consolidation of tungsten carbide powders have been widely investigated [1,7,11,12]. However, only a few studies regarding to SPS sintering of WC–Co–cBN composites can be found [13]. The cBN is a highly attractive material for different applications. It is the second hardest one of all known materials; it has a high abrasive wear resistance, in contrast to ⁎ Corresponding author. Tel.: +90 222 2393750/3187; fax: +90 222 2213918. E-mail addresses: [email protected] (B. Yaman), [email protected] (H. Mandal). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.086
diamond, it does not react with ferrous materials and is a highly potential material for cutting tool industry [14]. According to fundamental properties of cubic boron nitride and tungsten carbide, it is thought that consolidation of these materials may offer excellent properties to the relevant composites. On the other hand, it is not easy to obtain fully dense cBN materials, due to its strong covalent bonding and low diffusion coefficient of B and N. Moreover, cBN transform into hexagonal BN (hBN) at high temperatures [15]. The aim of the work was to determine the sintering conditions and behavior of WC–Co–cBN composites by SPS in order to achieve higher thermal and mechanical properties and further investigate the effects of cBN addition on densification behaviour and final properties. 2. Experimental procedure Commercial tungsten carbide (WC) powder containing 6 wt.% cobalt (Co), (Boehlerit GmbH & CO KG) and cubic boron nitride (cBN) (Zibo ShineSo Chemical Material Co. Ltd) with average particle sizes of 0.8 and 5 µm respectively, were used as starting materials. The composite powders with a WC–6Co:cBN volume ratio of 75:25, were prepared by wet milling in isopropyl alcohol for 4 h. The isopropyl alcohol was removed by a rotary evaporator and sieved through 25 µm mesh screen to break up the soft agglomerates. The powders were poured into a 20 mm inner diameter graphite die directly and uniaxially pre-pressed under pressure of 2 MPa. The SPS was carried out in FCT HP D 25/I (FCT System GmbH) under vacuum at different temperatures in the range of 1100–1300 °C under pressure of 25– 75 MPa for the time varying from 5 to 7.5 min. Heating rate was maintained as 100 °C/min for all sintering trials. The densities of the sintered samples were measured according to Archimedes method. For mechanical tests, hardness and fracture toughness were measured
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B. Yaman, H. Mandal / Materials Letters 63 (2009) 1041–1043
Table 1 Sintering conditions, theoretical densities (%) and mechanical properties of the investigated composites Composition WC–6Co WC–6Co WC–6Co WC–6Co–25cBN WC–6Co–25cBN WC–6Co–25cBN WC–6Co–25cBN
Sintering temp.
Pressure
Holding time
Specimen thickness
Theoretical density
KIC
HV10
(°C)
(MPa)
(min)
(mm)
(%)
(MPam1/2)
(GPa)
1100 1150 1200 1250 1300 1300 1300
25 50 50 75 75 75 75
5 5 5 5 5 5 7.5
4 4 4 4 4 5.5 5.5
64.62 86.47 99.86 99.40 99.63 99.39 99.52
– – 6.68 ± 0.5 10.90 ± 0.8 11.60 ± 0.7 8.43 ± 0.9 10.97 ± 0.5
– – 18.59 ± 0.2 17.52 ± 0.6 20.59 ± 0.8 18.47 ± 0.8 21.23 ± 0.4
using a Vickers indenter on the polished surface with a 10 kg load. The phase analyses were conducted employing an X-ray diffractometer (Rigaku Rint series). The microstructures were studied by using
scanning electron microscope (Zeiss Supra 50 VP). Determination of the thermal diffusivity of sintered samples was carried out by a laser flash apparatus (LFA 457-Netzsch). 3. Results and discussion
Fig. 1. Fracture surface of WC–6Co–25cBN sample sintered at 1300 °C, 75 MPa, 7.5 min. at different magnifications.
In order to determine the densification behaviour of WC–6Co materials, firstly sintering studies were carried out without cBN addition in the temperature range of 1100–1200 °C. After obtaining dense WC–6Co materials, cBN containing WC–6Co materials were studied. The sintering conditions and densification results are given in Table 1. Nearly fully dense (99.9%) tungsten carbide samples were obtained at 1200 °C, under a pressure of 50 MPa for 5 min. The temperature and pressure were further increased when 25 vol.% cBN added to achieve full densification and therefore, 99.6% theoretical density was achieved for 4 mm thick WC–Co–cBN samples by densification at 1300 °C, for 5 min sintering time, under pressure of 75 MPa. Due to the lower thermal stability of cBN, the temperature was kept constant and the holding time was extended to 7.5 min to reach higher density values for the thick samples. The microstructure of WC–Co–cBN sintered at 1300 °C, under 75 MPa for 7.5 min is given in Fig. 1. The sintered material was observed to be dense, apart from a small amount of porosity which can be attributed to the grain pull-out of hard cBN particles during sample preparation. It can be observed from the SEM image that sharply cornered cBN particles were homogeneously distributed and have good bonding with WC matrix due to Co binder and no agglomerations of Co were encountered. The thermal stability of cBN was confirmed by XRD and compared with the patterns of WC– 6Co (Fig. 2). The low intensity of cBN peak can be explained by the strong mass-absorption coefficient of WC–Co compared to cBN [16]. The measured hardness and fracture toughness values of all the sintered composites are also presented in Table 1. The hardness and toughness values are significantly increased by the incorporation of cBN.
Fig. 2. XRD patterns of sintered WC–6Co at 1200 °C, under 50 MPa, for 5 min and sintered WC–6Co–25cBN composite at 1300 °C, under 75 MPa, for 7.5 min (the positions of hBN peaks are indicated by (⁎)).
B. Yaman, H. Mandal / Materials Letters 63 (2009) 1041–1043
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full densification of WC–Co–cBN powders was achieved by SPS. According to XRD and SEM analysis, there was no indication that cBN transformed to hBN. The addition of 25% cBN significantly increased the mechanical and thermal properties of the material. cBN incorporated WC–Co can be suggested to be a potential composite for wear-resistant applications due to its improved mechanical and thermal properties compared to WC–Co.
References
Fig. 3. Thermal diffusivities of WC–Co and WC–Co–25cBN composites.
It is well known that tungsten carbides find wide applications as cutting tools. However, it is important to improve thermal properties in order to remove heat flux generated between tool and work piece during machining, which leads to chemical wear and tool failures if thermal conductivity of the tool is not satisfactory. The thermal diffusivity of the sintered samples was also measured from room temperature to 600 °C and the results are given in Fig. 3. As seen from the figure, thermal diffusivity of cBN incorporated WC–Co sample is much higher.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14]
4. Conclusions The sintering behavior and mechanical properties of WC–Co and WC–Co–cBN composites sintered by SPS were investigated. Almost
[15] [16]
Huang B, Chen LD, Bai SQ. Scr Mater 2006;54:441–5. Kim HC, Shon IJ, Yoon JK, Doh JM. Int J Refract Met Hard Mater 2007;25:46–52. Kim HC, Yoon JK, Doh JM, Ko IY, Shon IJ. Mater Sci Eng, A 2006;435–436:717–24. Kim HC, Jeong IK, Shon IJ, Ko IY, Doh JM. Int J Refract Met Hard Mater 2007;25:336–40. Michalski A, Siemiaszko D. Int J Refract Met Hard Mater 2007;25:153–8. Zhang Y, Wang L, Jiang W, Chen L, Bai G. J Euro Ceram Soc 2006;26:3393–7. Jia CC, Tang H, Mei X, Yin FZ, Qu XH. Mater Lett 2005;59:2566–9. Chae JH, Kim KH, Choa YH, Matsushita J, Yoon JW, Shim KB. J Alloys Compd 2006;413:259–64. Chaim R. Mater Sci Eng 2007;A 443:25–32. Shen Z, Nygren M. Key Eng Mater 2002;206–213:2155–8. Sivaprahasam D, Chandrasekar SB, Sundaresan R. Int J Refract Met Hard Mater 2007;25:144–52. Cha SI, Hong SH, Kim BK. Mater Sci Eng 2003;A351:31–8. Echeberria J, Martinez V, Sanchez JM, Bourgeois L, Barbier G, Hennicke J. In: Kneringer G, et al, editor. Proceeding 16th Plansee Seminar, vol. 2. Reutte, Austria: Plansee; 2005. p. 434–48. Keunecke M, Wiemann E, Weigel K, Park ST, Bewilogua K. Thin Solid Films 2006;515:967–72. Hotta M, Goto T. Mater Sci Forum 2007;561–565:599–602. Martinez V, Echeberria J. J Am Ceram Soc 2007;90:415–24.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Spark plasma sintering of Co–WC cubic boron nitride composites Bilge Yaman a,⁎, Hasan Mandal b a b
Department of Metallurgical and Materials Engineering, Eskisehir Osmangazi University, Meselik Campus, TR-26480, Eskisehir, Turkey Department of Materials Science and Engineering, Anadolu University, Iki Eylul Campus, TR-26555, Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 17 November 2008 Accepted 30 January 2009 Available online 12 February 2009 Keywords: Tungsten carbide WC–6Co–cBN composites Spark plasma sintering
a b s t r a c t 25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC–6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC–6Co–cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction WC–Co cemented carbides have been used as cutting tools, dies and wear-resistant parts owing to their high hardness and excellent wear resistance as well as their retention of room temperature hardness at elevated temperatures [1–5]. Morphologically, they consist of a high volume fraction of hard WC phase embedded within a soft and tough Co binder phase [2]. Increasing the volume fraction of Co improves the fracture toughness at the expense of hardness and wear resistance [4]. WC–Co hard materials can be densified by liquid phase sintering and the mechanical properties of these materials depend on the composition and microstructure [2]. The use of conventional methods of powder consolidation often results in grain growth. It is essential to minimize grain growth through careful control of consolidation parameters, particularly temperature and sintering time [3]. Spark plasma sintering (SPS) is an effective sintering technique that allows densification of ceramic powders at a relatively lower temperature with short holding time owing to rapid heating and cooling rates [6–10]. With SPS, the microstructure and grain growth can be controlled while maintaining the desired properties of the material [7]. A survey of the relevant literature indicates that the consolidation of tungsten carbide powders have been widely investigated [1,7,11,12]. However, only a few studies regarding to SPS sintering of WC–Co–cBN composites can be found [13]. The cBN is a highly attractive material for different applications. It is the second hardest one of all known materials; it has a high abrasive wear resistance, in contrast to ⁎ Corresponding author. Tel.: +90 222 2393750/3187; fax: +90 222 2213918. E-mail addresses: [email protected] (B. Yaman), [email protected] (H. Mandal). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.086
diamond, it does not react with ferrous materials and is a highly potential material for cutting tool industry [14]. According to fundamental properties of cubic boron nitride and tungsten carbide, it is thought that consolidation of these materials may offer excellent properties to the relevant composites. On the other hand, it is not easy to obtain fully dense cBN materials, due to its strong covalent bonding and low diffusion coefficient of B and N. Moreover, cBN transform into hexagonal BN (hBN) at high temperatures [15]. The aim of the work was to determine the sintering conditions and behavior of WC–Co–cBN composites by SPS in order to achieve higher thermal and mechanical properties and further investigate the effects of cBN addition on densification behaviour and final properties. 2. Experimental procedure Commercial tungsten carbide (WC) powder containing 6 wt.% cobalt (Co), (Boehlerit GmbH & CO KG) and cubic boron nitride (cBN) (Zibo ShineSo Chemical Material Co. Ltd) with average particle sizes of 0.8 and 5 µm respectively, were used as starting materials. The composite powders with a WC–6Co:cBN volume ratio of 75:25, were prepared by wet milling in isopropyl alcohol for 4 h. The isopropyl alcohol was removed by a rotary evaporator and sieved through 25 µm mesh screen to break up the soft agglomerates. The powders were poured into a 20 mm inner diameter graphite die directly and uniaxially pre-pressed under pressure of 2 MPa. The SPS was carried out in FCT HP D 25/I (FCT System GmbH) under vacuum at different temperatures in the range of 1100–1300 °C under pressure of 25– 75 MPa for the time varying from 5 to 7.5 min. Heating rate was maintained as 100 °C/min for all sintering trials. The densities of the sintered samples were measured according to Archimedes method. For mechanical tests, hardness and fracture toughness were measured
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Table 1 Sintering conditions, theoretical densities (%) and mechanical properties of the investigated composites Composition WC–6Co WC–6Co WC–6Co WC–6Co–25cBN WC–6Co–25cBN WC–6Co–25cBN WC–6Co–25cBN
Sintering temp.
Pressure
Holding time
Specimen thickness
Theoretical density
KIC
HV10
(°C)
(MPa)
(min)
(mm)
(%)
(MPam1/2)
(GPa)
1100 1150 1200 1250 1300 1300 1300
25 50 50 75 75 75 75
5 5 5 5 5 5 7.5
4 4 4 4 4 5.5 5.5
64.62 86.47 99.86 99.40 99.63 99.39 99.52
– – 6.68 ± 0.5 10.90 ± 0.8 11.60 ± 0.7 8.43 ± 0.9 10.97 ± 0.5
– – 18.59 ± 0.2 17.52 ± 0.6 20.59 ± 0.8 18.47 ± 0.8 21.23 ± 0.4
using a Vickers indenter on the polished surface with a 10 kg load. The phase analyses were conducted employing an X-ray diffractometer (Rigaku Rint series). The microstructures were studied by using
scanning electron microscope (Zeiss Supra 50 VP). Determination of the thermal diffusivity of sintered samples was carried out by a laser flash apparatus (LFA 457-Netzsch). 3. Results and discussion
Fig. 1. Fracture surface of WC–6Co–25cBN sample sintered at 1300 °C, 75 MPa, 7.5 min. at different magnifications.
In order to determine the densification behaviour of WC–6Co materials, firstly sintering studies were carried out without cBN addition in the temperature range of 1100–1200 °C. After obtaining dense WC–6Co materials, cBN containing WC–6Co materials were studied. The sintering conditions and densification results are given in Table 1. Nearly fully dense (99.9%) tungsten carbide samples were obtained at 1200 °C, under a pressure of 50 MPa for 5 min. The temperature and pressure were further increased when 25 vol.% cBN added to achieve full densification and therefore, 99.6% theoretical density was achieved for 4 mm thick WC–Co–cBN samples by densification at 1300 °C, for 5 min sintering time, under pressure of 75 MPa. Due to the lower thermal stability of cBN, the temperature was kept constant and the holding time was extended to 7.5 min to reach higher density values for the thick samples. The microstructure of WC–Co–cBN sintered at 1300 °C, under 75 MPa for 7.5 min is given in Fig. 1. The sintered material was observed to be dense, apart from a small amount of porosity which can be attributed to the grain pull-out of hard cBN particles during sample preparation. It can be observed from the SEM image that sharply cornered cBN particles were homogeneously distributed and have good bonding with WC matrix due to Co binder and no agglomerations of Co were encountered. The thermal stability of cBN was confirmed by XRD and compared with the patterns of WC– 6Co (Fig. 2). The low intensity of cBN peak can be explained by the strong mass-absorption coefficient of WC–Co compared to cBN [16]. The measured hardness and fracture toughness values of all the sintered composites are also presented in Table 1. The hardness and toughness values are significantly increased by the incorporation of cBN.
Fig. 2. XRD patterns of sintered WC–6Co at 1200 °C, under 50 MPa, for 5 min and sintered WC–6Co–25cBN composite at 1300 °C, under 75 MPa, for 7.5 min (the positions of hBN peaks are indicated by (⁎)).
B. Yaman, H. Mandal / Materials Letters 63 (2009) 1041–1043
1043
full densification of WC–Co–cBN powders was achieved by SPS. According to XRD and SEM analysis, there was no indication that cBN transformed to hBN. The addition of 25% cBN significantly increased the mechanical and thermal properties of the material. cBN incorporated WC–Co can be suggested to be a potential composite for wear-resistant applications due to its improved mechanical and thermal properties compared to WC–Co.
References
Fig. 3. Thermal diffusivities of WC–Co and WC–Co–25cBN composites.
It is well known that tungsten carbides find wide applications as cutting tools. However, it is important to improve thermal properties in order to remove heat flux generated between tool and work piece during machining, which leads to chemical wear and tool failures if thermal conductivity of the tool is not satisfactory. The thermal diffusivity of the sintered samples was also measured from room temperature to 600 °C and the results are given in Fig. 3. As seen from the figure, thermal diffusivity of cBN incorporated WC–Co sample is much higher.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14]
4. Conclusions The sintering behavior and mechanical properties of WC–Co and WC–Co–cBN composites sintered by SPS were investigated. Almost
[15] [16]
Huang B, Chen LD, Bai SQ. Scr Mater 2006;54:441–5. Kim HC, Shon IJ, Yoon JK, Doh JM. Int J Refract Met Hard Mater 2007;25:46–52. Kim HC, Yoon JK, Doh JM, Ko IY, Shon IJ. Mater Sci Eng, A 2006;435–436:717–24. Kim HC, Jeong IK, Shon IJ, Ko IY, Doh JM. Int J Refract Met Hard Mater 2007;25:336–40. Michalski A, Siemiaszko D. Int J Refract Met Hard Mater 2007;25:153–8. Zhang Y, Wang L, Jiang W, Chen L, Bai G. J Euro Ceram Soc 2006;26:3393–7. Jia CC, Tang H, Mei X, Yin FZ, Qu XH. Mater Lett 2005;59:2566–9. Chae JH, Kim KH, Choa YH, Matsushita J, Yoon JW, Shim KB. J Alloys Compd 2006;413:259–64. Chaim R. Mater Sci Eng 2007;A 443:25–32. Shen Z, Nygren M. Key Eng Mater 2002;206–213:2155–8. Sivaprahasam D, Chandrasekar SB, Sundaresan R. Int J Refract Met Hard Mater 2007;25:144–52. Cha SI, Hong SH, Kim BK. Mater Sci Eng 2003;A351:31–8. Echeberria J, Martinez V, Sanchez JM, Bourgeois L, Barbier G, Hennicke J. In: Kneringer G, et al, editor. Proceeding 16th Plansee Seminar, vol. 2. Reutte, Austria: Plansee; 2005. p. 434–48. Keunecke M, Wiemann E, Weigel K, Park ST, Bewilogua K. Thin Solid Films 2006;515:967–72. Hotta M, Goto T. Mater Sci Forum 2007;561–565:599–602. Martinez V, Echeberria J. J Am Ceram Soc 2007;90:415–24.