Materials Letters 260 (2020) 126971
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Preparation of ultrafine cemented carbides with uniform structure and high properties by microwave sintering Zhiwei Zhao ⇑, Guoguo Zhang, Shun Wang, Xiaomiao Zhao, Chunlong Guan College of Materials Science and Engineering, Henan University of Technology, Zhengzhou 450001, China
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Article history: Received 15 October 2019 Received in revised form 3 November 2019 Accepted 6 November 2019 Available online 7 November 2019 Keywords: Metals and alloys Microwave sintering Carbon Nanotubes Microstructure Properties
a b s t r a c t Ultrafine cemented carbides were prepared by microwave sintering using nano-WC and nano-Co powders as raw materials, multi-walled carbon nanotubes (MWCNTs) and graphene nanosheets (GNSs) as reinforcing materials. Effects of reinforcing materials on the microstructure and mechanical properties of cemented carbides were studied. After adding 0.2 wt% MWCNTs, the grain size of WC is relatively uniform, and the average grain size is about 500 nm. The Vickers hardness (1812) of the sample increases by 4.4%. After adding 0.2 wt% GNSs, the grain size of WC was not effectively inhibited and the Vickers hardness of the alloy was not improved. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Cemented carbides are widely used for cutting tools, drilling tools and wear-resistant tools because of their excellent tensile strength, fracture toughness, thermal shock resistance and hardness . The mechanical properties of cemented carbides can be greatly improved by reducing the grain size of WC or adding appropriate reinforcing materials . The grain size of WC can be inhibited by adding grain inhibitors or using low-temperature rapid sintering . As a reinforcing material, CNTs have become one of the representative materials since their discovery . MWCNTs possess a mean fracture strength >100 GPa and an exceptionally high Young’s modulus in the terapascal (TPa) range, bearing a solidity much higher than any commercial fiber. Besides, MWCNTs have high microwave absorbance characteristics at specific frequencies, because they couple strongly with both the electric and magnetic components of the applied external field . GNSs are twodimensional materials with some excellent physical properties, such as very high strength (~130 GPa) and high Young’s modulus (~1.0 TPa) at room temperature . Furthermore, microwave heating has a number of potential advantages. It generates first heat within the material and then heats the entire volume. This heating mechanism is advantageous due to uniform, rapid, and volumetric heating, etc . Nowadays, there are many studies on effects of ⇑ Corresponding author. E-mail address: [email protected]
(Z. Zhao). https://doi.org/10.1016/j.matlet.2019.126971 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
grain inhibitors and sintering methods on ultrafine cemented carbides, but few studies on effects of MWCNTs and GNSs on the alloys under microwave heating conditions. In this study, cemented carbides were prepared by microwave rapid sintering using nano-WC and nano-Co powders as raw materials, MWCNTs and GNSs as reinforcing materials. Effects of MWCNTs and GNSs on the microstructure and mechanical properties of cemented carbides were studied. 2. Experimental WC (<200 nm) and Co (<100 nm) powders were used as raw materials. MWCNTs (diameter < 8 nm, length 0.5–2 lm) and GNSs (diameter 1–3 lm, thickness 1–5 nm) were used as reinforcing materials. Firstly, MWCNTs and GNSs were dispersed by ultrasonic wave. Then, 89.8 wt% WC, 10 wt% Co and 0.2 wt% MWCNTs (GNSs) were put into a ball milling pot, and the milling medium was absolute alcohol and a small amount of surface modifying agent. After being milled for 24 h, the mixture was dried at 80 °C for 24 h. The mixture was made into cylinders by using a single cylinder hydraulic press and a cold isostatic press. Finally, the compacts were sintered in a multimode 2.45 GHz RWS microwave furnace in argon gas atmosphere to prepare cemented carbides. The structure of the powder was examined via X-ray diffraction (X0 Pert Powder, Philips, Netherlands). The microstructure and the surface elemental composition of the samples were characterized by Zeiss ULTRA 55 scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) instrument.
Z. Zhao et al. / Materials Letters 260 (2020) 126971
Fig. 1. XRD patterns of different raw materials: (a) WC-Co; (b) WC-Co-0.2 MWCNTs; (c) WC-Co-0.2 GNSs.
Vicker hardness measurements were carried out using a FM-700 Vickers hardness tester (Toshiba Teli Corporation).
3. Results and discussion Fig. 1 shows XRD patterns of different raw materials. As shown in Fig. 1, the phase composition of raw materials is basically unchanged after adding 0.2 wt% MWCNTs (GNSs), mainly consisting of WC (JCPDS 65-4539) and Co (JCPDS 15-0806). However, the intensity of diffraction peaks of WC and Co decreases slightly. There are no diffraction peaks of MWCNTs and GNSs in the figure, mainly due to the small amount of additions (<1.0 wt%). Fig. 2 shows the SEM and EDS images of different raw materials. WC exists in an irregular shape with a size of about 200 nm, and Co
exists as spherical particles, and the average particle size is about 100 nm. MWCNTs are evenly distributed among particles (Fig. 2 (b)), which indicates that MWCNTs after surface treatment and ultrasonic dispersion have good dispersibility. After ball milling, MWCNTs were further dispersed by the mixing and crushing action of the ball mill . Full mixing of the materials reduces the possibility of agglomeration of MWCNTs, makes them easily distributed between the crystal grains, is conducive to inhibiting the growth of WC grains in sintering process, and realizes the enhancement of MWCNTs through fiber extraction and bridging. GNSs exist in the form of large sheets surrounded by nano-WC and nano-Co particles (Fig. 2(c)). All specimens are mainly composed of W, C, and Co elements (Fig. 2(d)–(f)). Fig. 3 shows typical backscattered electron (BSE) micrographs of the compacts sintered by microwave heating at 1300 °C for 20 min. As shown in Fig. 3(a), a large number of WC grains grow abnormally, and the average grain size is about 1 lm. This is mainly because the interface of WC grains increases with the increase of temperature in the solid phase sintering stage, and binds to large grains with the increase of sintering temperature. Besides, in the liquid phase sintering stage, some active WC particles are dissolved in the liquid phase, then precipitate on some WC surfaces, and gradually grow into large WC particles as sintering progresses . After adding MWCNTs, the grain size of WC is relatively uniform, and the average grain size is about 500 nm (Fig. 3(b)), indicating that MWCNTs can effectively inhibit the grain growth of WC. Similar phenomena have been observed in the literatures [10,11]. Besides, the abnormal growth of WC grains can be effectively inhibited by microwave sintering. After adding GNSs, the grain size of WC was not significantly inhibited, and the average grain size was 0.5–1 lm (Fig. 3(c)). Fig. 4 shows Vickers hardness of different specimens sintered at 1300 °C for 20 min. Vickers hardness of alloy without MWCNTs and GNSs is 1736. After adding MWCNTs, the cracks of alloy become narrower and the Vickers hardness value increases to 1812, which increases by 4.4%. Because Vickers hardness is
Fig. 2. SEM and EDS images of (a) and (d) WC-Co; (b) and (e) WC-Co-0.2 MWCNTs; (c) and (f) WC-Co-0.2 GNSs.
Z. Zhao et al. / Materials Letters 260 (2020) 126971
Fig. 3. SEM (BSE) micrographs of the compacts sintered at 1300 °C for 20 min: (a) WC-Co; (b) WC-Co-0.2 MWCNTs; and (c) WC-Co-0.2 GNSs. (A) WC/WC grain boundary (GB) completely wetted by the Co-rich melt; (B) WC/WC GB incompletely wetted by the melt.
proportional to fracture toughness (according to the Anstis equation) , the specimen under this condition also has high fracture toughness. This is mainly because the addition of MWCNTs can increase the bonding strength between grains. It will consume a large amount of energy when MWCNTs are pulled out during the crack of the alloy. And cracks are preferentially developed from the place without MWCNTs, which will avoid the expansion of crack line and thus improve the toughness . The toughening mechanism of CNTs has been previously studied and involves deflection of cracks around CNTs and their pull-out from the matrices . In addition, the addition of MWCNTs can inhibit the
growth of WC grains to a certain extent and optimize the microstructure of cemented carbide (Fig. 3(b)), thus helping to improve the mechanical properties of the alloy. After adding GNSs, the Vickers hardness of the sample is 1685, which decreases by 2.9%. One of the reasons is that GNSs with large specific surface area leads to high agglomeration in the matrix. The graphene agglomerations will act as crack-initiation sites during compressive loading and consequently deteriorate the hardness and fracture toughness of alloy. Another reason for the low mechanical properties of samples (1) and (3) is that the growing WC grains do not adjust with each other resulting in the formation of the ‘‘pseudo-skeleton”, and most of WC/WC GBs are not completely wetted by the melt (Fig. 3(a) and (c)). Another reason for the high mechanical properties of sample (2) is that most of WC/WC GBs are completely wetted by the Co-rich melt with a contact angle of zero (Fig. 3(b)). Similar phenomena have been observed in the references [15–17].
Fig. 4. Vickers hardness of different specimens sintered at 1300 °C for 20 min: (1) WC-Co; (2) WC-Co-0.2 MWCNTs; and (3) WC-Co-0.2 GNSs.
Ultrafine cemented carbides were prepared by microwave sintering using nano-WC and nano-Co powders as raw materials, MWCNTs and GNSs as reinforcing materials. MWCNTs can effectively inhibit WC grain growth and improve mechanical properties of cemented carbides. The grain size of WC in cemented carbide containing 0.2 wt% MWCNTs is uniform, and the average grain size is about 500 nm. After adding MWCNTs, the Vickers hardness of the sample increases to 1812. This method provides a fast way to prepare high performance cemented carbides. The combination of microwave sintering and reinforcing materials provides a new method for the preparation of other alloys.
Z. Zhao et al. / Materials Letters 260 (2020) 126971
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The research was supported by the Natural Science Foundation of China (51304063) and the Key Scientific and Technological Research Projects in Henan Province (182102210389), China. References  V.K. Sarin, Comprehensive Hard Materials, Elsevier, Oxford, UK, 2014.
               
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