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A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing sintering
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International Journal of Refractory Metals & Hard Materials 26 (2008) 251–255 www.elsevier.com/locate/IJRMHM
A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing sintering Zhuhui Qiao a
a,b
, Xianfeng Ma a,*, Wei Zhao a,b, Huaguo Tang Shuguang Cai a,b, Bo Zhao a,b
a,b
,
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate School of Chinese Academy of Sciences, Beijing, PR China Received 23 January 2007; accepted 13 April 2007
Abstract Novel cemented carbides (W0.4Al0.6)C0.5–Co with different cobalt contents were prepared by mechanical alloying and hot-pressing technique. Hot-pressing technique as a common technique was performed to fabricate the bulk bodies of the hard alloys. The novel cemented carbides have good mechanical properties compared with WC–Co. The density and operation cost of the novel material were much lower than the WC–Co system. It was easy to process submicroscale sintering with the novel materials and obtain the rounded particles in the bulk materials. There is almost no g-phase in the (W0.4Al0.6)C0.5–Co cemented carbides system although the carbon deficient obtains the astonishing value of 50%. 2007 Elsevier Ltd. All rights reserved. Keywords: Novel cemented hard alloy; Mechanical alloying; Hot-pressing sintering; Mechanical properties; g-phase
1. Introduction Tungsten carbide/cobalt (WC–Co) cemented carbides or cermets, characterized by their high hardness and strength, were used where materials with high wear resistance and toughness are required [1]. However, pure WC has many inherent shortcomings, such as room-temperature brittleness, high density and high operating costs. Therefore, in recent years, many studies have been focused on how to improve the physical and chemical properties of WC, as well as to reduce its high operating cost. In this sense, one first approach is partial substitution of WC by other non-oxide compounds, such as, Ti(C,N), MoC, Cr2C3, and VC [2,3], which results in a lower density while maintaining the high hardness and wear resistance. The second alternative is to modify the binder component, such as, Co, Fe, and Ni to improve the corrosion resistance and/or *
Corresponding author. Tel.: +86 431 5262220; fax: +86 431 5685653. E-mail address: [email protected] (X. Ma).
0263-4368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.04.002
mechanical strength [4–7]. Upadhayaya has reported that dissolving some metals into the WC system, such as molybdenum, tantalum in order to improve the properties of WC [8]. However, to date no work has been done on the solid solution of aluminum in WC. Aluminum is more ductile and lighter than tungsten, so dissolving aluminum into the lattice of WC to form a solid solution is expected to enhance the bend strength of WC and reduce its density. In addition, aluminum is inexpensive compared with tungsten, thus operating cost of ternary carbide Al–W–C is surely less than that of WC. Because of the very little mutual solid solubility (<13 at.%) and the very large difference between the melting points of tungsten (3683 K) and aluminum (933 K), it is very difficult to prepare Al–W–C ternary system by melting or by other equilibrium methods. But non-equilibrium processes [9], such as sputter-deposition [10–12] and mechanical alloying [13,14] (MA) technique, can solve this problem. In our previous paper [14,15], we reported that (W1 xAlx)Cy (x = 0.1–0.86, y = 0.5–1.0) powder could be
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synthesized by mechanical alloying and solid state reaction. However, the mechanical properties of (W0.4Al0.6)C0.5–Co composite cemented carbide have not been reported as a result of bulk alloys being not obtained. Because (W1 xAlx)Cy (x = 0.1–0.86, y = 0.5–1.0) has the same hexagonal structure as WC [18], so we chose cobalt as the binder which is a good binder for WC sintering. One aim of this work is to fabricate the (W0.4Al0.6)C0.5–Co alloy bulk bodies. Hot-pressing (HP) as a common technique is a suitable method to facilitate the sintering of (W0.4Al0.6)C0.5– Co hard alloy. Additionally, the thermal stability, mechanical properties and the microstructures of (W0.4Al0.6)C0.5– Co bulk bodies were also tested. 2. Experimental procedures Elemental powders of tungsten (0.98 lm, 99.8% purity), aluminum(3.59 lm, 99.5% purity), cobalt (4.5 lm, 99.7%) and carbon (<30 lm, 99% purity) were used as raw materials. The (W0.4Al0.6)C0.5 powders were prepared by mechanical alloying [14,15]. The (W0.4Al0.6)C0.5–Co powders with different cobalt contents (5.16%, 10.1%, 13.3%, and 16.4% by volume; 5.3%, 10.3%, 13.6%, and 16.7% by weight) were used for this study. All the raw materials of (W0.4Al0.6)C0.5–Co powders had carbon deficient, because it is reported that vacancies have a profound influence on the physical properties of materials [16]. The (W0.4Al0.6)C0.5 powders and cobalt powders were enclosed in assembling graphite dice under argon atmosphere, and then were sintered in an inductive hot-pressing vacuum furnace with the following cycle: (a) heated from room temperature to sintering temperature (1400– 1500 C) with a heating rate of about 120 C/min; (b) kept the sintering temperature for the desired duration (10– 20 min); (c) cooled down from the sintering temperature to 600 C at about 400 C/min, and then furnace cooled from 600 C to room temperature. The pressure in the die was kept as 40 MPa and the vacuum degree was 80 Pa in the furnace. After hot-pressing sintering, the bulk specimens were ground and polished. The specimens were investigated by X-ray diffraction (XRD), environment scanning electron microscopy (ESEM) and energy dispersive analysis of X-ray (EDAX). XRD analyses were performed on a Rigaku D/max-IIB X-ray ˚ ), diffractometer with a Cu Ka radiation (k = 1.5406 A operating at 40 kV and 20 mA. The scanning speed was 4 min 1. The microstructures of fracture surfaces were examined using environment scanning electron microscope (ESEM, Philips, XL30) with a link EDAX system for local chemical composition analyses. The densities of the sintered specimens were determined by the Archimedes water immersion method. Microhardnesses of the hard alloy bulk bodies were measured by the Vickers microhardness tester (FM-700, Japan) with a load of 300 gf and dwell time of 15 s. The transverse strengths were measured by three-point bending test. Bending tests were performed on an Instron model 1125 test
machine at a crosshead speed of 2 mm/min; the gap length of bending test was 30 mm, bending specimens (4 · 3 · 38 mm) were cut from the hot-pressed alloy bulk bodies. All the reported data were the average of at least three test results. 3. Results and discussion 3.1. X-ray diffractometry and thermal stability Fig. 1 shows the XRD pattern of (W0.4Al0.6)C0.5– 10.1 vol%Co bulk alloy obtained at 1450 C and 40 MPa for 18 min. The peaks of (W0.4Al0.6)C0.5 phase are still stable and clear. Al is still dissolved in W lattice after being sintered at 1450 C and 40 MPa for 18 min by reason of finding no peaks of Al and/or aluminous compounds in the XRD pattern. Only cobalt peaks are found in Fig. 1, and no other compounds of cobalt were formed during the sintering process. This suggested that the novel solid solution (W0.4Al0.6)C0.5 has an excellent thermal stability even up to 1450 C during hot-pressing sintering. From Fig. 1 it is also concluded that the (W0.4Al0.6)C0.5–Co hard alloy was not obviously contaminated by oxygen and/or other elements during ball milling and hot-pressing. 3.2. Structures and mechanical properties The (W0.4Al0.6)C0.5 has hexagonal WC-type structure, and its theoretical density is 8.647 g cm 3, which is much lower than WC (15.63 g cm 3). Table 1 shows the relative density of (W0.4Al0.6)C0.5–Co bulk specimens with different cobalt contents. The relative density of the specimens reached over 98%. This indicates that cobalt is a good binder for (W0.4Al0.6)C0.5 and hot-pressing is a suitable technology for sintering (W0.4Al0.6)C0.5 bulk materials. With the various cobalt contents, the hardness values of (W0.4Al0.6)C0.5–Co hard alloy are 23.07 GPa, 20.43 GPa,
Fig. 1. XRD patterns for the (W0.4Al0.6)C0.5–10.1 vol%Co bulk alloy obtained at 1450 C and 40 MPa for 18 min.
Z. Qiao et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 251–255
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Table 1 The experimental density and relative density of (W0.4Al0.6)C0.5–Co bulk specimens with different cobalt contents Specimen specification
Theoretical density (g/cm 3)
Experimental density (g/cm 3)
Relative density (%)
(W0.4Al0.6)C0.5–5.16 vol%Co (W0.4Al0.6)C0.5–10.1 vol%Co (W0.4Al0.6)C0.5–13.3 vol%Co (W0.4Al0.6)C0.5–16.4 vol%Co
8.658 8.668 8.675 8.682
8.493 8.540 8.571 8.604
98.1 98.5 98.8 99.1
17.69 GPa, 14.98 GPa, while the cobalt contents are 5.16%, 10.1%, 13.1%, and 16.4% by volume. Fig. 2 shows the microhardnesses of the (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC– Co hard alloy. It shows that the microhardnesses of (W0.4Al0.6)C0.5–Co are much higher than that of WC–Co (5.16%, 10.1%, 13.3%, and 16.4% by volume; 3%, 6%, 8%, and 10% by weight) hard alloy with same cobalt contents by volume. This indicated that the high microhardnesses of the (W0.4Al0.6)C0.5–Co bulk bodies were likely due to the ultrafine particles and/or the carbon deficient. Fig. 3 shows the bending strength of (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy. As the cobalt contents increased from 5.16% to 16.4% by volume, the bending strength increased from 1121 MPa to 1775 MPa. The bending strengths of (W0.4Al0.6)C0.5–Co bulk bodies are a little lower than that of WC–Co hard alloy. We hypothesized that the dissolved aluminum and/or the slight porosity in the bulk bodies decrease the strength. The microstructures of various specimens were observed by environment scanning electron microscope (ESEM) as shown in Fig. 4. Fig. 4a shows a fractured specimen with cobalt content of 5.17 vol%, Fig. 4b shows a fractured specimen with cobalt content of 16.4 vol%, Fig. 4c and d shows a fractured specimen with cobalt content of 13.3 vol%. It was found that the (W0.4Al0.6)C0.5 particle size in the sintered dense specimens was less than 500 nm (about 200–300 nm) and (W0.4Al0.6)C0.5 particles near
Fig. 2. The microhardness of (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy is shown.
Fig. 3. The bending strength of (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy is shown.
pores appeared rounded. This result shows that (W0.4Al0.6)C0.5–Co cemented hard alloy is greatly different from WC–Co cemented carbides, in which clear coarsening of WC particle could not be prevented, even though it was sintered by the spark plasma sintering process [17]. In hardmetals industry, it is considered that nanostructured cemented carbides as an important branch of nanocrystalline materials could offer new opportunities for achieving superior hardness and toughness combinations. However, attempts to fabricate nanostructured WC–Co hard alloys have been frustrated by the inability to retain a nanoscale grain size in the fully sintered bulk alloys, since the presence of an unusually high surface and interface area provides a strong driving force for grain growth. So, in order to control the grain growth in nanostructured WC– Co composites, one of the key opperations is to add some inhibitors, such as Cr2C3 and VC, but the second-phase additive will become the center of cracking and decrease the strength of the hard alloy. In (W0.4Al0.6)C0.5, Al was dissolved into W lattice, there is no addition of inhibitors, but submicro grain size can be fully obtained in the fully sintered bulk alloy. Schubert and coworkers have reported that WC hardmetals with rounded particles have superior properties [18]. Fig. 4, it can be noticed that the shape of the (W0.4Al0.6)C0.5 grains other than prismatic grains are rounded, the rounded grains do not contain the sharp edge, which form local tensile stress concentrations on loading of the composite. So, the rounded (W0.4Al0.6)C0.5 grains can
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Fig. 4. (a) A fractured specimen with cobalt content of 5.17 vol%, (b) a fractured specimen with cobalt content of 16.4 vol%, (c,d) a fractured specimen with cobalt content of 3.3 vol%.
reduce the sensitivity to crack due to, the reason that the novel (W0.4Al0.6)C0.5 solid solution has a good mechanical strength during the dissolving of aluminum. In addition, it should be noted that there is almost no g-phase in the (W0.4Al0.6)C0.5–Co cemented carbides system which is a complex compound of WC and Co. This is a very particular and exciting phenomenon different from WC–Co cemented carbides system. It is well known that vacancies in carbides can enhance their hardness. But no one tried to prepare WC–Co cemented carbides with carbon deficient, because it is very easy to form g-phase when WC– Co cemented carbides deficient in carbon and the g-phase can decrease the mechanical strength of WC–Co cemented carbides.
Considering its superior hardness, lower density, and the simple producing technology, (W0.4Al0.6)C0.5–Co hard alloys are expected to be new cemented carbides with lower operating costs and can replace the standard materials for cutting tools, wear parts, electrode materials, etc. Acknowledgements This work was supported by the National Natural Science Foundation of China with program: 50371080, and was also supported by the project of Science and Technology Development program (20030508) of Jilin Province, China. References
4. Conclusions Novel cemented carbides materials (W0.4Al0.6)C0.5–Co with different cobalt contents were prepared by mechanical alloying and hot-press. The new material has an excellent thermal stability and good mechanical properties. The aluminum dissolved can overcome many shortcomings of the normal WC–Co cemented carbides. It is easier to process nanosintering compared with WC–Co. The most particular and exciting phenomenon is that no g-phase will be formed during (W0.4Al0.6)C0.5–Co sintering although the carbon deficient obtains the astonishing value of 50%. This novel property can give us more choice to design and gain new materials that we need. And it also has superior mechanical properties, higher hardness, and lower density compared with the normal hard alloy (WC).
[1] Leo J, Prakash. Application of fine grained tungsten carbide based cemented carbides. Int J Refract Met Hard Mater 1995;13:257–64. [2] Bhaumik SK, Upadhyaya GS, Vaidya ML. Microstructure and mechanical properties of WC–TiN–Co and WC–TiN–Mo2C–Co(Ni) cemented carbides. Ceram Int 1992;18:327–36. [3] Viatte T, Bolognini S, Cutard T, Feusier G, Mari D, Benoit W. Investigation into the potential of a composite combining toughness and plastic deformation resistance. Int J Refract Met Hard Mater 1999;17:79–89. [4] Gille G, Bredthauer J, Gries B, Mende B, Heinrich W. Advanced and new grades of WC and binder powder – their properties and application. Int J Refract Met Hard Mater 2000;18:87–102. [5] Uhhrenius B, Pastor H, Pauty E. On the composition of Fe–Ni–Co– WC-based cemented carbides. Int J Refract Met Hard Mater 1997;15:139–49. [6] Kursawe S, Pott P, Sockel HG, Wolf W. On the influence of binder content and binder composition on the mechanical properties of hardmetals. Int J Refract Met Hard Mater 2001;19:335–40.
Z. Qiao et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 251–255 [7] Hanyaloglu C, Aksakal B, Bolton JD. Production and indentation analysis of WC/Fe–Mn as an alternative to cobalt-bonded hardmetals. Mater Charact 2001;47:315–22. [8] Upadhayaya GS. Sintered Metallic and Ceramic Materials. New York: Wiley; 2000. [9] Tonejc A. Phase transformations in Al-rich Al–W alloys rapidly quenched from the melt. J Mater Sci 1972;7:1292–8. [10] Radic N, Tonejc A, Milum M, Pervant P, Ivkov J, Stubicar M. Preparation and structure of AlW thin films. Thin Solid Films 1998;317:96–9. [11] Metikos-Hukovic M, Radic N, Grubac Z, Tonejc A. The corrosion behavior of sputter-deposited aluminum–tungsten alloys. Electrochim Acta 2002;47:2387–97. [12] Belov NA, Alabin AN, Eskin DG. Improving the properties of coldrolled Al–6%Ni sheets by alloying and heat treatment. Scripta Mater 2004;50:89–94.
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[13] Mao SX, McMinn NA, Wu NQ. Processing and mechanical behaviour of TiAl/NiAl intermetallic composites produced by cryogenic mechanical alloying. Mater Sci Eng A 2003;363:275–89. [14] Tang HG, Ma XF, Zhao W, Hong RJ. Preparation of W–Al alloys by mechanical alloying. J Alloy Compd 2002;347:228–30. [15] Yan JM, Ma XF, Zhao W, Tang HG, Zhu CJ. Synthesis, crystal structure, and density of (W1 xAlx)C. J Solid State Chem 2004;177:2265–70. [16] Toth LE. Transition Metal Carbides and Nitrides. New York: Academic Press; 1971. [17] Cha SI, Hong SH, Kim BK. Spark plasma sintering behavior of nanocrystalline WC–10Co cemented carbide powders. Mater Sci Eng A 2003;351:31–8. [18] Herber RP, Schubert WD, Lux B. Hardmetals with ‘‘rounded’’ WC grains. Int J Refract Met Hard Mater 2006;24:360–4.
International Journal of Refractory Metals & Hard Materials 26 (2008) 251–255 www.elsevier.com/locate/IJRMHM
A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing sintering Zhuhui Qiao a
a,b
, Xianfeng Ma a,*, Wei Zhao a,b, Huaguo Tang Shuguang Cai a,b, Bo Zhao a,b
a,b
,
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate School of Chinese Academy of Sciences, Beijing, PR China Received 23 January 2007; accepted 13 April 2007
Abstract Novel cemented carbides (W0.4Al0.6)C0.5–Co with different cobalt contents were prepared by mechanical alloying and hot-pressing technique. Hot-pressing technique as a common technique was performed to fabricate the bulk bodies of the hard alloys. The novel cemented carbides have good mechanical properties compared with WC–Co. The density and operation cost of the novel material were much lower than the WC–Co system. It was easy to process submicroscale sintering with the novel materials and obtain the rounded particles in the bulk materials. There is almost no g-phase in the (W0.4Al0.6)C0.5–Co cemented carbides system although the carbon deficient obtains the astonishing value of 50%. 2007 Elsevier Ltd. All rights reserved. Keywords: Novel cemented hard alloy; Mechanical alloying; Hot-pressing sintering; Mechanical properties; g-phase
1. Introduction Tungsten carbide/cobalt (WC–Co) cemented carbides or cermets, characterized by their high hardness and strength, were used where materials with high wear resistance and toughness are required [1]. However, pure WC has many inherent shortcomings, such as room-temperature brittleness, high density and high operating costs. Therefore, in recent years, many studies have been focused on how to improve the physical and chemical properties of WC, as well as to reduce its high operating cost. In this sense, one first approach is partial substitution of WC by other non-oxide compounds, such as, Ti(C,N), MoC, Cr2C3, and VC [2,3], which results in a lower density while maintaining the high hardness and wear resistance. The second alternative is to modify the binder component, such as, Co, Fe, and Ni to improve the corrosion resistance and/or *
Corresponding author. Tel.: +86 431 5262220; fax: +86 431 5685653. E-mail address: [email protected] (X. Ma).
0263-4368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.04.002
mechanical strength [4–7]. Upadhayaya has reported that dissolving some metals into the WC system, such as molybdenum, tantalum in order to improve the properties of WC [8]. However, to date no work has been done on the solid solution of aluminum in WC. Aluminum is more ductile and lighter than tungsten, so dissolving aluminum into the lattice of WC to form a solid solution is expected to enhance the bend strength of WC and reduce its density. In addition, aluminum is inexpensive compared with tungsten, thus operating cost of ternary carbide Al–W–C is surely less than that of WC. Because of the very little mutual solid solubility (<13 at.%) and the very large difference between the melting points of tungsten (3683 K) and aluminum (933 K), it is very difficult to prepare Al–W–C ternary system by melting or by other equilibrium methods. But non-equilibrium processes [9], such as sputter-deposition [10–12] and mechanical alloying [13,14] (MA) technique, can solve this problem. In our previous paper [14,15], we reported that (W1 xAlx)Cy (x = 0.1–0.86, y = 0.5–1.0) powder could be
252
Z. Qiao et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 251–255
synthesized by mechanical alloying and solid state reaction. However, the mechanical properties of (W0.4Al0.6)C0.5–Co composite cemented carbide have not been reported as a result of bulk alloys being not obtained. Because (W1 xAlx)Cy (x = 0.1–0.86, y = 0.5–1.0) has the same hexagonal structure as WC [18], so we chose cobalt as the binder which is a good binder for WC sintering. One aim of this work is to fabricate the (W0.4Al0.6)C0.5–Co alloy bulk bodies. Hot-pressing (HP) as a common technique is a suitable method to facilitate the sintering of (W0.4Al0.6)C0.5– Co hard alloy. Additionally, the thermal stability, mechanical properties and the microstructures of (W0.4Al0.6)C0.5– Co bulk bodies were also tested. 2. Experimental procedures Elemental powders of tungsten (0.98 lm, 99.8% purity), aluminum(3.59 lm, 99.5% purity), cobalt (4.5 lm, 99.7%) and carbon (<30 lm, 99% purity) were used as raw materials. The (W0.4Al0.6)C0.5 powders were prepared by mechanical alloying [14,15]. The (W0.4Al0.6)C0.5–Co powders with different cobalt contents (5.16%, 10.1%, 13.3%, and 16.4% by volume; 5.3%, 10.3%, 13.6%, and 16.7% by weight) were used for this study. All the raw materials of (W0.4Al0.6)C0.5–Co powders had carbon deficient, because it is reported that vacancies have a profound influence on the physical properties of materials [16]. The (W0.4Al0.6)C0.5 powders and cobalt powders were enclosed in assembling graphite dice under argon atmosphere, and then were sintered in an inductive hot-pressing vacuum furnace with the following cycle: (a) heated from room temperature to sintering temperature (1400– 1500 C) with a heating rate of about 120 C/min; (b) kept the sintering temperature for the desired duration (10– 20 min); (c) cooled down from the sintering temperature to 600 C at about 400 C/min, and then furnace cooled from 600 C to room temperature. The pressure in the die was kept as 40 MPa and the vacuum degree was 80 Pa in the furnace. After hot-pressing sintering, the bulk specimens were ground and polished. The specimens were investigated by X-ray diffraction (XRD), environment scanning electron microscopy (ESEM) and energy dispersive analysis of X-ray (EDAX). XRD analyses were performed on a Rigaku D/max-IIB X-ray ˚ ), diffractometer with a Cu Ka radiation (k = 1.5406 A operating at 40 kV and 20 mA. The scanning speed was 4 min 1. The microstructures of fracture surfaces were examined using environment scanning electron microscope (ESEM, Philips, XL30) with a link EDAX system for local chemical composition analyses. The densities of the sintered specimens were determined by the Archimedes water immersion method. Microhardnesses of the hard alloy bulk bodies were measured by the Vickers microhardness tester (FM-700, Japan) with a load of 300 gf and dwell time of 15 s. The transverse strengths were measured by three-point bending test. Bending tests were performed on an Instron model 1125 test
machine at a crosshead speed of 2 mm/min; the gap length of bending test was 30 mm, bending specimens (4 · 3 · 38 mm) were cut from the hot-pressed alloy bulk bodies. All the reported data were the average of at least three test results. 3. Results and discussion 3.1. X-ray diffractometry and thermal stability Fig. 1 shows the XRD pattern of (W0.4Al0.6)C0.5– 10.1 vol%Co bulk alloy obtained at 1450 C and 40 MPa for 18 min. The peaks of (W0.4Al0.6)C0.5 phase are still stable and clear. Al is still dissolved in W lattice after being sintered at 1450 C and 40 MPa for 18 min by reason of finding no peaks of Al and/or aluminous compounds in the XRD pattern. Only cobalt peaks are found in Fig. 1, and no other compounds of cobalt were formed during the sintering process. This suggested that the novel solid solution (W0.4Al0.6)C0.5 has an excellent thermal stability even up to 1450 C during hot-pressing sintering. From Fig. 1 it is also concluded that the (W0.4Al0.6)C0.5–Co hard alloy was not obviously contaminated by oxygen and/or other elements during ball milling and hot-pressing. 3.2. Structures and mechanical properties The (W0.4Al0.6)C0.5 has hexagonal WC-type structure, and its theoretical density is 8.647 g cm 3, which is much lower than WC (15.63 g cm 3). Table 1 shows the relative density of (W0.4Al0.6)C0.5–Co bulk specimens with different cobalt contents. The relative density of the specimens reached over 98%. This indicates that cobalt is a good binder for (W0.4Al0.6)C0.5 and hot-pressing is a suitable technology for sintering (W0.4Al0.6)C0.5 bulk materials. With the various cobalt contents, the hardness values of (W0.4Al0.6)C0.5–Co hard alloy are 23.07 GPa, 20.43 GPa,
Fig. 1. XRD patterns for the (W0.4Al0.6)C0.5–10.1 vol%Co bulk alloy obtained at 1450 C and 40 MPa for 18 min.
Z. Qiao et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 251–255
253
Table 1 The experimental density and relative density of (W0.4Al0.6)C0.5–Co bulk specimens with different cobalt contents Specimen specification
Theoretical density (g/cm 3)
Experimental density (g/cm 3)
Relative density (%)
(W0.4Al0.6)C0.5–5.16 vol%Co (W0.4Al0.6)C0.5–10.1 vol%Co (W0.4Al0.6)C0.5–13.3 vol%Co (W0.4Al0.6)C0.5–16.4 vol%Co
8.658 8.668 8.675 8.682
8.493 8.540 8.571 8.604
98.1 98.5 98.8 99.1
17.69 GPa, 14.98 GPa, while the cobalt contents are 5.16%, 10.1%, 13.1%, and 16.4% by volume. Fig. 2 shows the microhardnesses of the (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC– Co hard alloy. It shows that the microhardnesses of (W0.4Al0.6)C0.5–Co are much higher than that of WC–Co (5.16%, 10.1%, 13.3%, and 16.4% by volume; 3%, 6%, 8%, and 10% by weight) hard alloy with same cobalt contents by volume. This indicated that the high microhardnesses of the (W0.4Al0.6)C0.5–Co bulk bodies were likely due to the ultrafine particles and/or the carbon deficient. Fig. 3 shows the bending strength of (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy. As the cobalt contents increased from 5.16% to 16.4% by volume, the bending strength increased from 1121 MPa to 1775 MPa. The bending strengths of (W0.4Al0.6)C0.5–Co bulk bodies are a little lower than that of WC–Co hard alloy. We hypothesized that the dissolved aluminum and/or the slight porosity in the bulk bodies decrease the strength. The microstructures of various specimens were observed by environment scanning electron microscope (ESEM) as shown in Fig. 4. Fig. 4a shows a fractured specimen with cobalt content of 5.17 vol%, Fig. 4b shows a fractured specimen with cobalt content of 16.4 vol%, Fig. 4c and d shows a fractured specimen with cobalt content of 13.3 vol%. It was found that the (W0.4Al0.6)C0.5 particle size in the sintered dense specimens was less than 500 nm (about 200–300 nm) and (W0.4Al0.6)C0.5 particles near
Fig. 2. The microhardness of (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy is shown.
Fig. 3. The bending strength of (W0.4Al0.6)C0.5–Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy is shown.
pores appeared rounded. This result shows that (W0.4Al0.6)C0.5–Co cemented hard alloy is greatly different from WC–Co cemented carbides, in which clear coarsening of WC particle could not be prevented, even though it was sintered by the spark plasma sintering process [17]. In hardmetals industry, it is considered that nanostructured cemented carbides as an important branch of nanocrystalline materials could offer new opportunities for achieving superior hardness and toughness combinations. However, attempts to fabricate nanostructured WC–Co hard alloys have been frustrated by the inability to retain a nanoscale grain size in the fully sintered bulk alloys, since the presence of an unusually high surface and interface area provides a strong driving force for grain growth. So, in order to control the grain growth in nanostructured WC– Co composites, one of the key opperations is to add some inhibitors, such as Cr2C3 and VC, but the second-phase additive will become the center of cracking and decrease the strength of the hard alloy. In (W0.4Al0.6)C0.5, Al was dissolved into W lattice, there is no addition of inhibitors, but submicro grain size can be fully obtained in the fully sintered bulk alloy. Schubert and coworkers have reported that WC hardmetals with rounded particles have superior properties [18]. Fig. 4, it can be noticed that the shape of the (W0.4Al0.6)C0.5 grains other than prismatic grains are rounded, the rounded grains do not contain the sharp edge, which form local tensile stress concentrations on loading of the composite. So, the rounded (W0.4Al0.6)C0.5 grains can
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Fig. 4. (a) A fractured specimen with cobalt content of 5.17 vol%, (b) a fractured specimen with cobalt content of 16.4 vol%, (c,d) a fractured specimen with cobalt content of 3.3 vol%.
reduce the sensitivity to crack due to, the reason that the novel (W0.4Al0.6)C0.5 solid solution has a good mechanical strength during the dissolving of aluminum. In addition, it should be noted that there is almost no g-phase in the (W0.4Al0.6)C0.5–Co cemented carbides system which is a complex compound of WC and Co. This is a very particular and exciting phenomenon different from WC–Co cemented carbides system. It is well known that vacancies in carbides can enhance their hardness. But no one tried to prepare WC–Co cemented carbides with carbon deficient, because it is very easy to form g-phase when WC– Co cemented carbides deficient in carbon and the g-phase can decrease the mechanical strength of WC–Co cemented carbides.
Considering its superior hardness, lower density, and the simple producing technology, (W0.4Al0.6)C0.5–Co hard alloys are expected to be new cemented carbides with lower operating costs and can replace the standard materials for cutting tools, wear parts, electrode materials, etc. Acknowledgements This work was supported by the National Natural Science Foundation of China with program: 50371080, and was also supported by the project of Science and Technology Development program (20030508) of Jilin Province, China. References
4. Conclusions Novel cemented carbides materials (W0.4Al0.6)C0.5–Co with different cobalt contents were prepared by mechanical alloying and hot-press. The new material has an excellent thermal stability and good mechanical properties. The aluminum dissolved can overcome many shortcomings of the normal WC–Co cemented carbides. It is easier to process nanosintering compared with WC–Co. The most particular and exciting phenomenon is that no g-phase will be formed during (W0.4Al0.6)C0.5–Co sintering although the carbon deficient obtains the astonishing value of 50%. This novel property can give us more choice to design and gain new materials that we need. And it also has superior mechanical properties, higher hardness, and lower density compared with the normal hard alloy (WC).
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