Journal of Alloys and Compounds 456 (2008) 514–517
Fabrication, thermal stability and mechanical properties of novel (W0.5Al0.5)C0.8–Co composite prepared by mechanical alloying and hot-pressing sintering Zhuhui Qiao a,b , Xianfeng Ma a,∗ , Wei Zhao a,b , Huaguo Tang a,b , Shuguang Cai a,b , Bo Zhao a,b a
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate School of the Chinese Academy of Sciences, Beijing, PR China Received 13 January 2007; accepted 21 February 2007 Available online 24 February 2007
Abstract A novel cemented carbides (W0.5 Al0.5 )C0.8 –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 superior mechanical properties compared to WC–Co. The density, operating cost of the novel material were much lower than WC–Co. There is almost no -phase in the (W0.5 Al0.5 )C0.8 –Co cemented carbides system although the carbon deficient get the value of 20%, and successfully got the nanostructured rounded (W0.5 Al0.5 )C0.8 particles. © 2007 Elsevier B.V. All rights reserved. Keywords: Novel cemented carbide; (W0.5 Al0.5 )C0.8 –Co; Hot-pressing sintering; Mechanical properties
1. Introduction Tungsten carbide–cobalt composite hard materials (WC–Co) are widely used for a variety of machining, cutting, drilling, and other applications. 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 nonoxide compounds, such as, Ti(C, N), MoC, Cr2 C3, and VC [1–3], 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, Ni, etc. to improve the corrosion resistance and/or mechanical strength.[4–7] Nowadays, some researcher begin to study dissolving metals into WC system, such as molybdenum. Aluminum is more ductile and lighter
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than tungsten, so dissolving aluminum into the lattice of WC to form a solid solution is expected to reduce its high density. However, to date no work has been done on the solid solution of aluminum in 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 other equilibrium methods. In addition, aluminum is inexpensive compared to tungsten, thus operating cost of ternary carbide Al–W–C are surely less than that of WC. But nonequilibrium processes , such as sputter-deposition[10–12] and mechanical alloying (MA) [13–16] technique can solve this problem. In our previous paper [16,17], we reported that (W1−x Alx )Cy (x = 0.1–0.86, y = 0.5–1.0) powder could be synthesized by mechanical alloying and solid state reaction. Because (W1−x Alx )Cy (x = 0.1–0.86, y = 0.5–1.0) has the same hexagonal structure as WC , so we choose cobalt as the binder which is a good binder for WC sintering. One aim of this work is to fabricate the W0.5 Al0.5 C0.8 –Co alloy bulk bodies. Hot-pressing (HP) as a common technique is a suitable method to facilitate the sintering of (W0.5 Al0.5 )C0.8 –Co hard alloy. Additionally, the mechanical
Z. Qiao et al. / Journal of Alloys and Compounds 456 (2008) 514–517
properties and the microstructures of (W0.5 Al0.5 )C0.8 –Co bulk bodies were also discussed. 2. Experimental details Elemental powders of tungsten (0.98 m, 99.8% purity), aluminium (3.59 m, 99.5% purity), cobalt (4.5 m, 99.7%) and carbon (<30 m, 99% purity) were used as raw materials. The (W0.5 Al0.5 C0.8 powders were prepared by mechanical alloying and solid state reaction.[16,17] The (W0.5 Al0.5 )C0.8 –Co powders with different cobalt contents (8.5, 11.7, 14.9 vol%; 8.7, 11.9, 15.2 wt%) were used for this study. All the raw materials of (W0.5 Al0.5 )C0.8 –Co powders had carbon deficient, because it is reported that deficient in carbides has a profound influence on the physical properties of materials.[18,19] The (W0.5 Al0.5 )C0.8 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 (1350–1450 ◦ C) with a heating rate of about 150 ◦ C min−1 ; (b) kept the sintering temperature for the desired duration (15–20 min); (c) cooled down from the sintering temperature to 600 ◦ C at about 400 ◦ C min−1 , 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 grinded and polished. The specimens were investigated by X-ray diffraction (XRD), environment scanning electron microscopy (ESEM). XRD analyses were performed on a ˚ Rigaku D/max-IIB X-ray diffractometer with Cu K␣ radiation (λ = 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). 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 micro-hardness 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−1 ; the gap length of bending test was 30 mm, bending specimens (4 mm × 3 mm × 40 mm) were cut from the hot-pressed alloy bulk bodies. All the reported data were the average of at least three test results.
Fig. 1. The XRD pattern of (W0.5 Al0.5 )C0.8 –11.7 vol% Co bulk alloy obtained at 1400 ◦ C and 40 MPa for 20 min.
3.2. Microstructures and mechanical properties Table 1 shows the experimental density, relative density of (W0.5 Al0.5 )C0.8 –Co bulk specimens with different cobalt contents. The theoretical density of (W0.5 Al0.5 )C0.8 is only 8.665 g cm−3 , it is much lower than the normal hard material WC (15.63 g cm−3 ). The measured (W0.5 Al0.5 )C0.8 density is more consistent with a substitutional solid solution than a interstitial solid solution and also consistent with the XRD result that the (W0.5 Al0.5 )C0.8 compound still has the WC-type after the HP-sintering. The relative density of the specimens reached over 98%. It indicates that cobalt is a good binder for
3. Results and discussion 3.1. X-ray diffractometry and thermal stability Fig. 1 shows XRD pattern of (W0.5 Al0.5 )C0.8 –11.7 vol% Co bulk alloy obtained at 1450 ◦ C and 40 MPa for 20 min. The peaks of (W0.5 Al0.5 )C0.8 phase are still stable and clear. That suggested that the novel solid solution (W0.5 Al0.5 )C0.8 has excellent thermal stability even up to 1450 ◦ C during hot-pressing sintering by reason of finding no peaks of Al and/or aluminous compounds in XRD pattern. Only cobalt peaks were found in Fig. 1, and no other compounds of cobalt were formed during the sintering process. From Fig. 1 it was also concluded that the (W0.5 Al0.5 )C0.8 –11.7 vol% Co hard alloy was not obviously contaminated by oxygen and/or other elements during ball milling and hot-pressing.
Fig. 2. The microhardnesses of the (W0.5 Al0.5 )C0.8 –Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy.
Table 1 The experimental density, relative density and the electronic conductivity of (W0.5 Al0.5 )C0.8 –Co bulk specimens with different cobalt contents Specimen specification
Theoretical density (g cm−3 )
Experimental density (g cm−3 )
Relative density (%)
(W0.5 Al0.5 )C0.8 –8.5 vol% Co (W0.5 Al0.5 )C0.8 –11.7 vol% Co (W0.5 Al0.5 )C0.8 –14.9 vol% Co
8.682 8.688 8.694
8.552 8.566 8.607
98.5 98.6 99.0
Z. Qiao et al. / Journal of Alloys and Compounds 456 (2008) 514–517
Fig. 3. The bending strength of (W0.5 Al0.5 )C0.8 –Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy.
(W0.5 Al0.5 )C0.8 and hot-pressing is a suitable technology for sintering (W0.5 Al0.5 )C0.8 . With the various cobalt contents, the hardness of (W0.5 Al0.5 )C0.8 –Co hard alloy are 20.83, 17.37, 15.07 GPa, while the cobalt contents are 8.5, 11.7, 14.9 vol%. Fig. 2 shows the microhardnesses of the (W0.5 Al0.5 )C0.8 –Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy. It shows that the microhardnesses of (W0.5 Al0.5 )C0.8 –Co are much higher than that of WC–Co (8.5, 11.7, 14.9 vol%; 5, 7, 9 wt%) hard alloy with same cobalt contents by volume. Fig. 3 shows the bending strength of (W0.5 Al0.5 )C0.8 –Co bulk specimens sintered with different cobalt contents compared with WC–Co hard alloy. As the cobalt contents increased from 8.5 to 14.9 vol%, the bending strength increased from 1281 to 1824 MPa. The bending strengths of
(W0.5 Al0.5 )C0.8 –Co bulk bodies are near to the WC–Co hard alloy (1300–1800 MPa) with the same cobalt content by volume. It can be concluded that the aluminum dissolving and the carbon deficient can remarkably enhance the hardness of the (W0.5 Al0.5 )C0.8 –Co system, and did not decrease the strength of the WC system. The microstructures of various specimens were observed by environment scanning electron microscope (ESEM) as shown in Fig. 4. Fig. 4(A, C) are from a fractured specimen with cobalt content 8.5 vol%, Fig. 4(B, D) are from a fractured specimen with cobalt content 14.9 vol%. From Fig. 4, it can find that the (W0.5 Al0.5 )C0.8 particle size in the sintered dense specimens was less than 500 nm, about 300 nm and (W0.5 Al0.5 )C0.8 particles near pores appear rounded. This result shows that (W0.5 Al0.5 )C0.8 –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. So, in order to control the grain growth in nanostructured WC–Co composites, one of the keys is to add some inhibitors, such as Cr2 C3, and VC, but the second-phase additive will decrease the strength of the hard alloy. In (W1−x Alx )C system, Al was dissolved into W lattice, there is no inhibitor addition, but it can easily obtain nanoscale grain size in the fully sintered bulk alloy. From Fig. 4, the shape of the (W0.5 Al0.5 )C0.8 grains are rounded other than prismatic grains, the rounded grains do not contain the sharp edge, which form local tensile stress concentrations on loading of the composite. So, the rounded (W0.5 Al0.5 )C0.8 grains can lead to improved toughness and reduce the sensitivity to crack. In addition, from Fig. 1 and Fig. 4, it should be noted that there is almost no -phase in the (W0.5 Al0.5 )C0.8 –Co cemented carbides system which is a complex compound of WC and Co. This is a very interesting phenomenon different from WC–Co cemented carbides system. It is well known that deficient in Vb
Fig. 4. A and C are from a fractured specimen with cobalt content 8.5 vol%; B and D are from a fractured specimen with cobalt content 14.9 vol%.
Z. Qiao et al. / Journal of Alloys and Compounds 456 (2008) 514–517
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 -phase when WC–Co cemented carbides deficient in carbon and the -phase can decrease the mechanical strength of WC–Co cemented carbides. We can conclude that the aluminum dissolving influenced the properties of WC system very much, and prevented the W–C–Co to form (W3 Co3 C) phase. This novel property will give us more choice to obtain the new materials. 4. Conclusions A novel cemented carbides materials (W0.5 Al0.5 )C0.8 –Co with different cobalt contents were prepared by mechanical alloying and hot-press. The new material has excellent thermal stability, superior mechanical properties. The aluminum dissolving can overcome many shortcomings of the normal WC–Co cemented carbides. It is easier to process nanosintering compared with WC–Co, and the shape of the (W0.5 Al0.5 )C0.8 particles are rounded in the (W0.5 Al0.5 )C0.8 –Co composite. The most interesting phenomenon is that no -phase will be formed during (W0.5 Al0.5 )C0.8 –Co sintering although the carbon deficient get the value of 20%. This novel property can give us more choice to design and gain new materials what we need. And it also has superior mechanical properties, higher hardness and toughness, lower density compared to the normal hard alloy (WC). Considering its superior mechanical properties, lower density, and the simple producing technology (W0.5 Al0.5 )C0.8 –Co hard alloys are expected to be a new cemented carbides with lower operating costs and can replace the standard materials for cutting tools, wear parts, electrode materials, etc. Acknowledgments This work was supported by the National Natural Science Foundation of China with program: 50371080, and was also sup-
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