- Email: [email protected]
Microstructural and mechanical characterization of high energy ball milled and sintered WC–10 wt%Co–xTaC nano powders
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructural and mechanical characterization of high energy ball milled and sintered WC–10 wt%Co–xTaC nano powders M. Mahmoodan a, H. Aliakbarzadeh a,b, R. Gholamipour c,* a
Azad University-Saveh Centre, Saveh, Iran Maham Research Center, Tehran, Iran c Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815/3538, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 22 January 2009 Accepted 18 February 2009
Keywords: WC–10 wt%Co Ball to powder weight ratio High energy ball milling TaC Sintering
a b s t r a c t Ultra fine tungsten carbide and cobalt powders were milled by high energy planetary ball mill at different ball to powder weight ratios (BPR) to produce particles of WC–10 wt%Co hard metal in nanometer scale size. Microstructural characterizations by TEM show that the particle size of tungsten carbide was achieved to 32 nm after milling at 15 BPR during 10 h. In order to reduce the WC grain growth during the sintering process, tantalum carbide was added to the hard metal as a WC grain growth inhibitor. The nano hard metal powders were compacted at 200 MPa pressure and sintered at 1370–1450 °C temperatures in a high purity hydrogen atmosphere. The results show that the addition of 0.6 wt% of TaC p improves the hardness and fracture toughness from 1493 HV30 and 11.8 MPa m (for TaC free sample) p to 1614 HV30 and 13.7 MPa m, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction WC–Co hard metals are widely used as different cutting tools because of their high hardness, suitable wear-resistance, good fracture resistance and high temperature strength [1,2]. The physical and mechanical properties of WC–Co composites depend on many factors such as alloy composition, purity, homogeneity of the structure, particle size of the initial powders and the use of an adequate grain growth inhibitor [3]. Also, the decrease of grain size of tungsten carbide to nanometer scale increases the strength and toughness simultaneously [1,3]. It is well known that the properties of the milled powders such as the particle size distribution depend on the milling conditions as well as the final product properties. High energy ball milling is a simple and efficient method to produce the nanostructured WC–Co powders [4,5]. Several carbides such as TaC, VC, Cr3C2 and TiC are added to WC-base hard metals to inhibit grain growth during sintering and their effects on the mechanical and microstructural properties of cemented carbide with cobalt binder has been investigated [6– 9]. These additives reduce the growth rate of WC grains by lowering the solubility of tungsten carbide in Co-rich phase during liquid phase sintering. Higher solubility of an inhibitor in Co liquid phase causes less solubility of WC in it and this leads to final finer grain structure [8,10]. TaC is one of the best grain growth inhibitors. Morton et al. [9] showed that TaC as unlike as VC is unaffected * Corresponding author. Tel./fax: +98 2188826692. E-mail address: [email protected] (R. Gholamipour). 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.02.001
by the carbon content of WC to control the grain growth process in WC–Co composites. In this paper, the effect of ball to powder weight ratios (BPRs) as an important parameter on the particle size of WC in high energy ball milling of WC–10 wt%Co powders have been characterized and the effects of various amounts of TaC on the mechanical properties and microstructure of the sintered WC–10 wt%Co composite were also studied. 2. Experimental WC (99.5%), Co (99.9%) and TaC (99.5%) with average particle size of 0.52, 0.48 and 0.47 lm were used respectively as starting materials (Fig. 1). TaC with (0.3, 0.6 and 1.0 wt%) as an inhibitor was added to WC–10 wt%Co nano powders. WC–10 wt%Co powders and balls (7, 10 and 15 mm in diameter) mixed at ethanol was sealed in WC hard metal vial in a glove box containing inert atmosphere of argon. The process of milling was continued by 10 h on a high energy planetary ball mill (Retsch PM400-MA type) with the selected rotation velocity of 200 rpm and 1:3 rotation ratios. The powders were milled at different BPRs (5, 15, 25 and 35). After milling, the powders dried in a vacuum oven at 100 °C for 24 h. Then the nano powders were compacted in a cylindrical die at pressure of 200 MPa to reach green bulk near to 10 mm at height and diameter. The sintering was carried out at the range of 1370– 1450 °C temperature for 1 h in a high purity hydrogen atmosphere. The powders were dripped onto Cu grid covered with carbon after being shocked by ultrasonic waves in ethanol for TEM
802
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
Fig. 1. SEM images of (a) WC, (b) Co and (c) TaC starting powders.
samples preparation. TEM observations were carried out by a PHILIPS-CM200 FEG at 200 kV. The SEIFERT-3003 with Cu Ka radiation (k = 0.154 nm) was used for XRD to determine the crystallite size, being obtained by Williamson-Hall method [11]. The formula is Bcosh = Kk/D + 4 e sinh where B (radians) is the full width at half maximum (FWHM) of the XRD peaks, k (nm) is the wavelength of the X-ray, K is the Scherrer constant (K = 0.9), e is the lattice strain, h (radian) is the Bragg angle and D (nm) is crystallite size. B can be calculated by Gaussian X-ray profiles as B2 = Bm2Bs2 where Bm is the FWHM for the measured sample and Bs is the FWHM of the standard sample. Here, the silicon (Si) was taken as the standard sample. The sintered samples grinded and polished by 3 and 1 lm diamond pastes. The hardness and fracture toughness was measured by Vickers hardness with 30 kg load and Palmqvist equation, respectively [12]. Also, the samples were etched by Murakami solution for 1 min. Scanning electron microscopy was carried out by PHILIPS XL-30 to observe microstructures of the bulk samples. 3. Results and discussions 3.1. WC–10Co nano powders characterizations X-ray diffraction patterns of WC–10 wt%Co powders prepared with different BPRs have been shown in Fig. 2. All main WC peaks of samples with different BPRs are present, but the peaks sharpness reduces as a result of the increase of BPR. It means that the grain sizes decrease that is due to the increase of milling energy. Fig. 3 shows the results being achieved by Williamson-Hall method analysis of XRD data. It is clear that the increase of BPR from 5 to 15 causes a high decrease of crystallite size, whereas the strains are steady. By the increase of BPR to 35, the lattice strain enhances dramatically, but the variations of crystallite size are negligible. Fig. 4 indicates the nanoparticles morphologies of WC–10 wt%Co after milling at different BPRs based on TEM observations. It is
Fig. 2. X-ray diffraction patterns of WC–10 wt%Co powders at various BPRs (5, 15, 25 and 35).
obvious that the milling with 5 BPR has not produced enough energy to reduce the WC particles to nanometer scale size. Fig. 5 shows the average particle and crystallite size of 15, 25 and 35 BPRs. TEM observation confirms the XRD data. TEM results also illustrate that the variations of the particle size achieved by 15, 25 and 35 BPRs of powders milling are not so much different, though the standard deviations decrease. On the other hand, the statistical calculations of grain size and standard deviation values
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
803
Fig. 5. Comparison of average particle and crystallite size of WC at 15, 25 and 35 BPRs by TEM and XRD data. Fig. 3. Average crystallite size and strain of WC–10 wt%Co powder versus BPRs analyzed by Williamson-Hall method.
show that more than 99% of particles (prepared with BRPs of 15, 25 or 35) are less than 100 nm. It is mentioned that the increase of BPR will be raised the rate of the abrasion of milling media and introduces the uncertainty on the composition [8] and increases the lattice strain of the samples. Therefore the 15 BPR at 10 h in high energy ball mill is suitable to achieve nanometer scale size particles of WC–10 wt%Co powders. Based on TEM and XRD results, since particles and crystallites sizes are nearly equal, it gives rise that after proper milling each particle is a single crystal. 3.2. Sintering behaviour of WC–10Co–xTaC nano powders Hardness measurements of WC–10 wt%Co–xTaC (x = 0, 0.3, 0.6 and 1.0 wt%) bulk nanocomposite at various sintering temperatures
(1370, 1410 and 1450 °C) have been presented in Fig. 6. It is shown that the hardness of the sintered specimens improved with the increase of TaC content (x) of the samples up to 0.6 wt%. This is due to the effect of TaC as a tungsten carbide grain growth inhibitor. According to Hall-Petch relation, the strength of materials increases with decrease of grain size [13]. Fig. 7 shows the backscattered scanning electron microscopy of microstructures of all sintered samples at 1410 °C temperature and Table 1 summarizes the images processing data. As can be seen from Table 1 in TaC free sintered sample, the grains grow dramatically, however, grain growth inhibition occurs by TaC addition up to 0.6 wt% and this is why the hardness improves. As TaC is introduced to the system, it solutes to liquid phase (Co) during sintering and limited the solution of WC in the phase, thus kinetic of WC grains growth decreases as a result of TaC addition. The increase of TaC content more than 0.6 wt% decreases the hardness in all sintering temperatures. This is probably
Fig. 4. Bright field TEM images of WC–10 wt%Co powders after milling at (a) 5, (b) 15, (c) 25 and (d) 35 BPRs.
804
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805 Table 1 Fracture toughness and grain size of WC–10 wt%Co–x wt%TaC samples prepared at 1410 °C sintering temperature. %TaC
0
0.3
0.6
1.0
p Fracture toughness (MPa m) Grain size (micron)
11.8 0.9 ± 0.7
15.6 0.6 ± 0.3
13.7 0.5 ± 0.3
15.9 0.7 ± 0.3
Table 2 Comparison of mechanical properties of sintered WC–10 wt%Co sample in this study with other reported values.
Fig. 6. The hardness of WC–10 wt%Co–x wt%TaC bulk samples at various sintering temperatures.
due to the saturation of TaC in the liquid phase and the remaining of TaC in the microstructure causes the decrease of hardness because of its lower hardness than that of WC. Fracture toughness value of a material describes its ability to withstand crack propagation. In WC–Co systems, fracture toughness depends on cobalt content as well as WC grain size [12]. The size of WC grains (measured by intercept linear method) and fracture toughness values of the WC–10 wt%Co–xTaC bulk samples (sintered at 1410 °C) have been listed in Table 1. There is a significant improvement of toughness in TaC containing sample as compared to that of TaC free WC–Co composite. The best fracture toughness among the samples can be achieved by choosing a suitable sintering temperature and an optimum addition of TaC which is 1410 °C and 0.6 wt%, respectively. This condition is confirmed by the results of WC grain size and hardness measurements. The hardness and fracture toughness values of WC–10 wt%Co sintered samples obtained by this work and others [12,14,15] have been listed in Table 2. As can be seen, fracture toughness of
Ref.
Hardness (kg/mm2)
Fracture toughness p (MPa m)
Grain size (micron)
[12] [14] [15] This work
1541 1680 1600 1614
11 7 10 14
0.5 0.4 0.5 0.5
sintered WC–10 wt%Co obtained in this work, with a relatively high hardness, enhances by contrast to the others. It is probably due to properly distribution of Co phase around the WC grain boundaries, as a result of applying an optimized sintering conditions and an appropriate performance of TaC as a grain growth inhibitor during sintering. 4. Conclusion 1. Based on TEM and XRD results, high energy ball milling can efficiently refine the microstructure of WC–10Co powders to a nanometer scale size. 2. Five BPR at 10 h milling is not suitable to produce WC nanoparticles. 3. After 10 h milling at more than 15 BPR, the particle size of WC is reduced to less than 100 nm.
Fig. 7. Backscattered electron SEM images of (a) WC–10 wt%Co, (b) WC–10 wt%Co–0.3 wt%TaC, (c) WC–10 wt%Co–0.6 wt%TaC and (d) WC–10 wt%Co–1.0 wt%TaC at 1410 °C.
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
4. TaC is an effective grain growth inhibitor of tungsten carbide in WC–10Co during sintering and improves the hardness and fracture toughness of the bulk samples. 5. Addition of 0.6 wt% TaC has the most effect on hardness of WC– 10 wt%Co sintered samples. 6. The best results achieved at 1410 °C for WC–10Co–0.6TaC with p 1614 HV30, 13.7 MPa m and 0.5 lm average grain sizes.
References [1] Ostberg G, Buss K, Christensen M, Norgren S, Andren H, Mari D, Wahnstrom G, Reineck I. Int J Refract Met Hard Mater 2006;24:145–54. [2] Qu X, Gao J, Qin M, Lei C. Int J Refract Met Hard Mater 2005;23: 273–7.
805
[3] Sivaprahasam D, Chandrasekar SB, Sundaresan R. Int J Refract Met Hard Mater 2007;25:144–52. [4] Sun J, Zhang F, Shen J. Mater Lett 2003;57:3140–8. [5] El-Eskandarany MS, Mahday AA, Ahmed HA, Amer AH. J Alloys Compd 2000;312:315–25. [6] Hashe NG, Neethling JH, Berndt PR, Andrén H, Norgren SA. Int J Refract Met Hard Mater 2007;25:207–13. [7] Schubert WD, Bock A, Lux B. Int J Refract Met Hard Mater 1995;13:281–96. [8] Upadhyaya GS. New Jersey, USA: Noyes Publication; 1998. [9] Morton CW, Wills DJ, Stjernberg K. Int J Refract Met Hard Mater 2005;23:287–93. [10] Petersson A, Agren J. Int J Refract Met Hard Mater 2005;23:258–66. [11] Kameli P, Salamati H, Aezami A. J Appl Phys 2006;100:053914. [12] Schubert WD, Neameister H, Kinger G. Int J Refract Met Hard Mater 1998;16:133–42. [13] Cha SI, Hong SH, Ha GH, Kim BK. Scripta Mater 2001;44:1535–9. [14] Richter V, Ruthendorf MV. Int J Refract Met Hard Mater 1999;17:141–52. [15] Fang ZZ. Int J Refract Met Hard Mater 2005;23:119–27.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructural and mechanical characterization of high energy ball milled and sintered WC–10 wt%Co–xTaC nano powders M. Mahmoodan a, H. Aliakbarzadeh a,b, R. Gholamipour c,* a
Azad University-Saveh Centre, Saveh, Iran Maham Research Center, Tehran, Iran c Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815/3538, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 22 January 2009 Accepted 18 February 2009
Keywords: WC–10 wt%Co Ball to powder weight ratio High energy ball milling TaC Sintering
a b s t r a c t Ultra fine tungsten carbide and cobalt powders were milled by high energy planetary ball mill at different ball to powder weight ratios (BPR) to produce particles of WC–10 wt%Co hard metal in nanometer scale size. Microstructural characterizations by TEM show that the particle size of tungsten carbide was achieved to 32 nm after milling at 15 BPR during 10 h. In order to reduce the WC grain growth during the sintering process, tantalum carbide was added to the hard metal as a WC grain growth inhibitor. The nano hard metal powders were compacted at 200 MPa pressure and sintered at 1370–1450 °C temperatures in a high purity hydrogen atmosphere. The results show that the addition of 0.6 wt% of TaC p improves the hardness and fracture toughness from 1493 HV30 and 11.8 MPa m (for TaC free sample) p to 1614 HV30 and 13.7 MPa m, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction WC–Co hard metals are widely used as different cutting tools because of their high hardness, suitable wear-resistance, good fracture resistance and high temperature strength [1,2]. The physical and mechanical properties of WC–Co composites depend on many factors such as alloy composition, purity, homogeneity of the structure, particle size of the initial powders and the use of an adequate grain growth inhibitor [3]. Also, the decrease of grain size of tungsten carbide to nanometer scale increases the strength and toughness simultaneously [1,3]. It is well known that the properties of the milled powders such as the particle size distribution depend on the milling conditions as well as the final product properties. High energy ball milling is a simple and efficient method to produce the nanostructured WC–Co powders [4,5]. Several carbides such as TaC, VC, Cr3C2 and TiC are added to WC-base hard metals to inhibit grain growth during sintering and their effects on the mechanical and microstructural properties of cemented carbide with cobalt binder has been investigated [6– 9]. These additives reduce the growth rate of WC grains by lowering the solubility of tungsten carbide in Co-rich phase during liquid phase sintering. Higher solubility of an inhibitor in Co liquid phase causes less solubility of WC in it and this leads to final finer grain structure [8,10]. TaC is one of the best grain growth inhibitors. Morton et al. [9] showed that TaC as unlike as VC is unaffected * Corresponding author. Tel./fax: +98 2188826692. E-mail address: [email protected] (R. Gholamipour). 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.02.001
by the carbon content of WC to control the grain growth process in WC–Co composites. In this paper, the effect of ball to powder weight ratios (BPRs) as an important parameter on the particle size of WC in high energy ball milling of WC–10 wt%Co powders have been characterized and the effects of various amounts of TaC on the mechanical properties and microstructure of the sintered WC–10 wt%Co composite were also studied. 2. Experimental WC (99.5%), Co (99.9%) and TaC (99.5%) with average particle size of 0.52, 0.48 and 0.47 lm were used respectively as starting materials (Fig. 1). TaC with (0.3, 0.6 and 1.0 wt%) as an inhibitor was added to WC–10 wt%Co nano powders. WC–10 wt%Co powders and balls (7, 10 and 15 mm in diameter) mixed at ethanol was sealed in WC hard metal vial in a glove box containing inert atmosphere of argon. The process of milling was continued by 10 h on a high energy planetary ball mill (Retsch PM400-MA type) with the selected rotation velocity of 200 rpm and 1:3 rotation ratios. The powders were milled at different BPRs (5, 15, 25 and 35). After milling, the powders dried in a vacuum oven at 100 °C for 24 h. Then the nano powders were compacted in a cylindrical die at pressure of 200 MPa to reach green bulk near to 10 mm at height and diameter. The sintering was carried out at the range of 1370– 1450 °C temperature for 1 h in a high purity hydrogen atmosphere. The powders were dripped onto Cu grid covered with carbon after being shocked by ultrasonic waves in ethanol for TEM
802
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
Fig. 1. SEM images of (a) WC, (b) Co and (c) TaC starting powders.
samples preparation. TEM observations were carried out by a PHILIPS-CM200 FEG at 200 kV. The SEIFERT-3003 with Cu Ka radiation (k = 0.154 nm) was used for XRD to determine the crystallite size, being obtained by Williamson-Hall method [11]. The formula is Bcosh = Kk/D + 4 e sinh where B (radians) is the full width at half maximum (FWHM) of the XRD peaks, k (nm) is the wavelength of the X-ray, K is the Scherrer constant (K = 0.9), e is the lattice strain, h (radian) is the Bragg angle and D (nm) is crystallite size. B can be calculated by Gaussian X-ray profiles as B2 = Bm2Bs2 where Bm is the FWHM for the measured sample and Bs is the FWHM of the standard sample. Here, the silicon (Si) was taken as the standard sample. The sintered samples grinded and polished by 3 and 1 lm diamond pastes. The hardness and fracture toughness was measured by Vickers hardness with 30 kg load and Palmqvist equation, respectively [12]. Also, the samples were etched by Murakami solution for 1 min. Scanning electron microscopy was carried out by PHILIPS XL-30 to observe microstructures of the bulk samples. 3. Results and discussions 3.1. WC–10Co nano powders characterizations X-ray diffraction patterns of WC–10 wt%Co powders prepared with different BPRs have been shown in Fig. 2. All main WC peaks of samples with different BPRs are present, but the peaks sharpness reduces as a result of the increase of BPR. It means that the grain sizes decrease that is due to the increase of milling energy. Fig. 3 shows the results being achieved by Williamson-Hall method analysis of XRD data. It is clear that the increase of BPR from 5 to 15 causes a high decrease of crystallite size, whereas the strains are steady. By the increase of BPR to 35, the lattice strain enhances dramatically, but the variations of crystallite size are negligible. Fig. 4 indicates the nanoparticles morphologies of WC–10 wt%Co after milling at different BPRs based on TEM observations. It is
Fig. 2. X-ray diffraction patterns of WC–10 wt%Co powders at various BPRs (5, 15, 25 and 35).
obvious that the milling with 5 BPR has not produced enough energy to reduce the WC particles to nanometer scale size. Fig. 5 shows the average particle and crystallite size of 15, 25 and 35 BPRs. TEM observation confirms the XRD data. TEM results also illustrate that the variations of the particle size achieved by 15, 25 and 35 BPRs of powders milling are not so much different, though the standard deviations decrease. On the other hand, the statistical calculations of grain size and standard deviation values
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
803
Fig. 5. Comparison of average particle and crystallite size of WC at 15, 25 and 35 BPRs by TEM and XRD data. Fig. 3. Average crystallite size and strain of WC–10 wt%Co powder versus BPRs analyzed by Williamson-Hall method.
show that more than 99% of particles (prepared with BRPs of 15, 25 or 35) are less than 100 nm. It is mentioned that the increase of BPR will be raised the rate of the abrasion of milling media and introduces the uncertainty on the composition [8] and increases the lattice strain of the samples. Therefore the 15 BPR at 10 h in high energy ball mill is suitable to achieve nanometer scale size particles of WC–10 wt%Co powders. Based on TEM and XRD results, since particles and crystallites sizes are nearly equal, it gives rise that after proper milling each particle is a single crystal. 3.2. Sintering behaviour of WC–10Co–xTaC nano powders Hardness measurements of WC–10 wt%Co–xTaC (x = 0, 0.3, 0.6 and 1.0 wt%) bulk nanocomposite at various sintering temperatures
(1370, 1410 and 1450 °C) have been presented in Fig. 6. It is shown that the hardness of the sintered specimens improved with the increase of TaC content (x) of the samples up to 0.6 wt%. This is due to the effect of TaC as a tungsten carbide grain growth inhibitor. According to Hall-Petch relation, the strength of materials increases with decrease of grain size [13]. Fig. 7 shows the backscattered scanning electron microscopy of microstructures of all sintered samples at 1410 °C temperature and Table 1 summarizes the images processing data. As can be seen from Table 1 in TaC free sintered sample, the grains grow dramatically, however, grain growth inhibition occurs by TaC addition up to 0.6 wt% and this is why the hardness improves. As TaC is introduced to the system, it solutes to liquid phase (Co) during sintering and limited the solution of WC in the phase, thus kinetic of WC grains growth decreases as a result of TaC addition. The increase of TaC content more than 0.6 wt% decreases the hardness in all sintering temperatures. This is probably
Fig. 4. Bright field TEM images of WC–10 wt%Co powders after milling at (a) 5, (b) 15, (c) 25 and (d) 35 BPRs.
804
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805 Table 1 Fracture toughness and grain size of WC–10 wt%Co–x wt%TaC samples prepared at 1410 °C sintering temperature. %TaC
0
0.3
0.6
1.0
p Fracture toughness (MPa m) Grain size (micron)
11.8 0.9 ± 0.7
15.6 0.6 ± 0.3
13.7 0.5 ± 0.3
15.9 0.7 ± 0.3
Table 2 Comparison of mechanical properties of sintered WC–10 wt%Co sample in this study with other reported values.
Fig. 6. The hardness of WC–10 wt%Co–x wt%TaC bulk samples at various sintering temperatures.
due to the saturation of TaC in the liquid phase and the remaining of TaC in the microstructure causes the decrease of hardness because of its lower hardness than that of WC. Fracture toughness value of a material describes its ability to withstand crack propagation. In WC–Co systems, fracture toughness depends on cobalt content as well as WC grain size [12]. The size of WC grains (measured by intercept linear method) and fracture toughness values of the WC–10 wt%Co–xTaC bulk samples (sintered at 1410 °C) have been listed in Table 1. There is a significant improvement of toughness in TaC containing sample as compared to that of TaC free WC–Co composite. The best fracture toughness among the samples can be achieved by choosing a suitable sintering temperature and an optimum addition of TaC which is 1410 °C and 0.6 wt%, respectively. This condition is confirmed by the results of WC grain size and hardness measurements. The hardness and fracture toughness values of WC–10 wt%Co sintered samples obtained by this work and others [12,14,15] have been listed in Table 2. As can be seen, fracture toughness of
Ref.
Hardness (kg/mm2)
Fracture toughness p (MPa m)
Grain size (micron)
[12] [14] [15] This work
1541 1680 1600 1614
11 7 10 14
0.5 0.4 0.5 0.5
sintered WC–10 wt%Co obtained in this work, with a relatively high hardness, enhances by contrast to the others. It is probably due to properly distribution of Co phase around the WC grain boundaries, as a result of applying an optimized sintering conditions and an appropriate performance of TaC as a grain growth inhibitor during sintering. 4. Conclusion 1. Based on TEM and XRD results, high energy ball milling can efficiently refine the microstructure of WC–10Co powders to a nanometer scale size. 2. Five BPR at 10 h milling is not suitable to produce WC nanoparticles. 3. After 10 h milling at more than 15 BPR, the particle size of WC is reduced to less than 100 nm.
Fig. 7. Backscattered electron SEM images of (a) WC–10 wt%Co, (b) WC–10 wt%Co–0.3 wt%TaC, (c) WC–10 wt%Co–0.6 wt%TaC and (d) WC–10 wt%Co–1.0 wt%TaC at 1410 °C.
M. Mahmoodan et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 801–805
4. TaC is an effective grain growth inhibitor of tungsten carbide in WC–10Co during sintering and improves the hardness and fracture toughness of the bulk samples. 5. Addition of 0.6 wt% TaC has the most effect on hardness of WC– 10 wt%Co sintered samples. 6. The best results achieved at 1410 °C for WC–10Co–0.6TaC with p 1614 HV30, 13.7 MPa m and 0.5 lm average grain sizes.
References [1] Ostberg G, Buss K, Christensen M, Norgren S, Andren H, Mari D, Wahnstrom G, Reineck I. Int J Refract Met Hard Mater 2006;24:145–54. [2] Qu X, Gao J, Qin M, Lei C. Int J Refract Met Hard Mater 2005;23: 273–7.
805
[3] Sivaprahasam D, Chandrasekar SB, Sundaresan R. Int J Refract Met Hard Mater 2007;25:144–52. [4] Sun J, Zhang F, Shen J. Mater Lett 2003;57:3140–8. [5] El-Eskandarany MS, Mahday AA, Ahmed HA, Amer AH. J Alloys Compd 2000;312:315–25. [6] Hashe NG, Neethling JH, Berndt PR, Andrén H, Norgren SA. Int J Refract Met Hard Mater 2007;25:207–13. [7] Schubert WD, Bock A, Lux B. Int J Refract Met Hard Mater 1995;13:281–96. [8] Upadhyaya GS. New Jersey, USA: Noyes Publication; 1998. [9] Morton CW, Wills DJ, Stjernberg K. Int J Refract Met Hard Mater 2005;23:287–93. [10] Petersson A, Agren J. Int J Refract Met Hard Mater 2005;23:258–66. [11] Kameli P, Salamati H, Aezami A. J Appl Phys 2006;100:053914. [12] Schubert WD, Neameister H, Kinger G. Int J Refract Met Hard Mater 1998;16:133–42. [13] Cha SI, Hong SH, Ha GH, Kim BK. Scripta Mater 2001;44:1535–9. [14] Richter V, Ruthendorf MV. Int J Refract Met Hard Mater 1999;17:141–52. [15] Fang ZZ. Int J Refract Met Hard Mater 2005;23:119–27.