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Mechanical properties and erosion resistance of ceria nano-particle-doped ultrafine WC–12Co composite prepared by spark plasma sintering
Wear 301 (2013) 406–414
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Mechanical properties and erosion resistance of ceria nano-particle-doped ultrafine WC–12Co composite prepared by spark plasma sintering Xiaoguang Sun a,b, You Wang a,n, D.Y. Li a,b,n a b
Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2V4
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2012 Received in revised form 30 January 2013 Accepted 31 January 2013 Available online 8 February 2013
Ultra-fine grained WC–Co composites possess higher hardness or strength and wear resistance, compared to their coarse counterparts. However, abnormal grain growth of ultra-fine WC particles often occurs during the traditional pressureless liquid sintering, which substantially impairs the mechanical properties and erosion resistance of the material. Thus, controlling the abnormal grain growth becomes one of the key issues in fabricating ultra-fine grained cemented carbides. In this study, ultra-fine grained WC–Co composites containing different amounts of ceria nano-particles were prepared by spark plasma sintering and effects of the nano-ceria on mechanical properties and erosion behavior of the WC–Co composites were studied. The results demonstrated that trace nano-ceria addition effectively suppressed the abnormal grain growth of WC, leading to uniform and fine microstructures. Such ultra-fine grained WC–Co composites have both improved hardness and fracture toughness, resulting in enhanced resistance to high-speed solid-particle erosion. However, when the added nano-ceria was more than 0.1 wt%, Co pools started to form, which lowered the material density, hardness and toughness and consequently the erosion resistance. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ultrafine WC–Co Nano-ceria Mechanical properties Erosion resistance
1. Introduction WC–Co composites have been widely used for machining, cutting, mining and drilling tools, due to their high hardness and excellent wear resistance [1]. When the WC grain size is reduced to submicron or nanometer scales, the WC–Co composites possess improved mechanical properties and wear resistance, compared to their coarse-grained counterparts [2]. Thus, ultrafine-grained and nanostructured WC–Co composites have attracted great interest in the field of high-performance hard materials [3]. However, abnormal grain growth of ultrafine WC particles in WC–Co composite occurs easily during the traditional pressureless liquid sintering, which substantially deteriorates their performance. Thus, controlling abnormal grain growth has become one of the key issues in fabricating ultrafine WC–Co composites [4]. The grain growth can be inhibited to a certain extent by using special sintering technologies with accelerated heating rate, increased densification rate, decreased sintering temperature and short holding time [5], such as microwave sintering [6], rapid hot pressing sintering [7], and spark plasma sintering (SPS) or
n
Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (D.Y. Li). 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.113
pulse electric current sintering (PECS) [8]. In particular, SPS is a novel consolidation process that combines fast joule heating, established by flowing a pulsed electric current through the die/ punch/powder compact set-up under an applied pressure. The fast densification process is reported to be benefited from grain boundary diffusion, surface diffusion, volume diffusion, and plastic deformation [9]. During past few years, SPS process was successfully used to synthesize ultra-fine/nano-WC–Co composites [10–12]. Although many studies are focused on the development of sintering techniques, one of the basic and successful ways for controlling the WC grain growth is the addition of small amounts of WC grain growth inhibitors, such as VC, Cr3C2, NbC or Mo2C, to the starting powder mixture, typically less than 1.0 wt% of a metallic carbide [13]. Vanadium carbide (VC) and chromium carbide (Cr3C2) are the most effective grain growth inhibitors due to their high solubility and mobility in the cobalt phase at lower temperatures [14,15]. For all transition metal carbides, however, the mechanism for grain growth inhibition has been related to the slowing down of the solution/re-precipitation reactions at the WC–Co interfaces [16]. These carbides are effective in preventing anomalous grain growth but the drawback is that they may cause the product to be brittle, especially for vanadium carbide [15]. Rare earths and their oxides are effective inhibitors in WC–Co composites [17–19], which may increase the wetting power of Co to WC grains, control the formation of Z
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phase in cemented carbides, raise the bending strength and wearresistance, and also increase the corrosion resistance of cemented carbides [20]. The previous studies [19] on traditional rare earth as additives, have shown that the rare earths improve mechanical properties of WC–Co and optimal performance is observed when the content of added rare earths or their oxides such as CeO2 is more than 1.0 wt% [19–21]. The high melting point of ceria (2673 K) makes it in a solid state during sintering of cemented carbides even using the conventional liquid sintering method in the temperature range of 1723 K. CeO2 particles cannot diffuse effectively during the sintering process, so that the positive effect of rare earths would be minimized when their primary particle size is at or above micron scale. Further, the segregation of rareearth oxide particles at boundary would weaken their positive effect and could even be detrimental. The unstable performance due to the non-uniform distribution of rare-earth oxide particles in cemented carbide and the high cost limited their industrial applications. Our previous study showed that nano-CeO2 is much more effective in cemented carbides and the mechanical properties of WC–Co composites are optimized with only 0.1 wt% nanoCeO2 [3]. To our knowledge, no studies on the WC–Co composites with nano-CeO2 by other researchers have been reported in the literature. In this study, ultra-fine grained WC–Co composites containing different amounts of CeO2 nano-particles were prepared by spark plasma sintering and effects of the nano-CeO2 on mechanical properties and erosion behavior of the WC–Co composites were studied.
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were put into cylindrical graphite dies with an inner diameter of 30 mm. The temperature was monitored using an optical pyrometer aimed at a non-through hole of 0.5 mm in diameter and 2 mm in depth on the graphite die. The heating rate was 100 K/ min, and a pressure of 50 MPa was applied during sintering. The sintering temperature was set at 1473 K with a holding time of 5 min. During the consolidation process, the dimensional changes of powders were monitored by measuring the position of the lower ram, which acted as an electrode to produce high pulsed current within the powder compact. 2.2. Characterization The phase constitution of the composites was determined using a Rikagu X-ray diffractometer with Cu Ka radiation. A field Table 1 Compositions of WC–12Co composites for this study. No.
WC
Co (wt%)
CeO2 (wt%)
1 2 3 4 5
Balance Balance Balance Balance Balance
12 12 12 12 12
0 0.05 0.1 0.3 0.6
2. Experimental 2.1. Materials preparation Ultrafine WC powder ( Z99.5%, 200 nm, free carbon 0.11 wt%, shown in Fig. 1a) and Co powder ( Z99.6%, 60 nm) were used as raw materials. A small quantity (0.05–0.6 wt%) of nano-CeO2 powder ( 10 nm, shown in Fig. 1b) was added to WC–12Co composite powder. The compositions of samples with different amounts of nano-CeO2 are shown in Table 1. The ultrafine WC–12Co composite powders were wet ball-milled for 0.5 h using ethanol as a medium in a high vibration ball milling machine. The ball material was tungsten and the volume ratio of the ball to the material was 5:1. The powders were dried at 333 K in an oven for 10 h. The mixed powders were then sintered using an SPS apparatus (SPS1050, Sumitomo Coal Mining Co., Ltd., Tokyo, Japan). The schematic of SPS apparatus was shown in Fig. 2 [22].The powders
Fig. 2. Schematic illustration of SPS apparatus [19].
Fig. 1. SEM image of WC powder (a) and TEM image of nano-ceria powder (b).
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K IC ¼ 0:15 ðHV 30 =SlÞ1=2
ð1Þ
where HV30 is the Vickers hardness (N/mm2) with a load of 30 kg, and Sl is the total length of cracks (mm) initiated from the corners of the indentation. For accuracy, the specimen was polished to near-mirror finish and checked for face parallelism before testing. Erosion testing is conducted using an air-jet sand blasting test apparatus, shown in Fig. 3. The tester is operated by feeding the eroding particles from a vibrating hopper into a stream of gas. The particles are blown out from the nozzle at a constant speed, controlled by a manometer, to erode the specimen surface. All tests were performed in air at the room temperature and the erodent velocity was 75 m/s. The erodent flow angles relative to the sample surface were set at 301 and 901, respectively, in order to investigate the responses of the material to relatively lowangle and high-angle solid-particle impingements. AFS 50/70 sand (U.S. Silica Company, USA) was used as the erodent particle. The sample mass was measured before and after the tests using a balance with a precision of 0.1 mg. The erosion rate was calculated by dividing the volume loss of a specimen (mm3) by the total weight of abrasive particles that impacted the specimen (g). Each presented value is an average of at least four measurements. The erosion resistance was represented as the reciprocal of the erosion rate with a unit of g/mm3.
3. Results and discussion 3.1. Densification behavior During the SPS sintering, the main densification shrinkage represented by the linear dimensional change of powder aggregate to the close-packed arrangement took place during heating, involving particle sliding, rotation, grain boundary migration and pore shrinkage [23,24]. Although the shrinkage during SPS is the cumulative shrinkage of the sample and the graphite molds and steel electrodes, the contribution of graphite molds and steel electrodes can be ignored as we intend to compare the relative changes in shrinkage for different samples, since all sample were sintered under the same condition. The recorded shrinkage reflects the densification degree. Variations in shrinkage of WC– 12Co composites with different contents of nano-ceria are shown in Fig. 4. The shrinkage changed with the addition of nano-ceria, which increased with nano-ceria and reached a maximum value at 0.1 wt% ceria. The shrinkage then decreased with further increasing nano-ceria, implying that the densification became worse at higher wt% ceria. The relative densities of the WC–12Co composites with different contents of nano-ceria are presented in Fig. 4, which shows a trend similar to that of the shrinkage. With an increase in the 6.0
100 Shrinkage Relative density
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emission gun scanning electron microscope (SEM, HITACHI S-4700) equipped with X-ray energy dispersive spectroscopy (EDS) was employed to characterize the microstructure of the composites. The grain size of WC and the mean free path in cobalt were measured using the linear intercept method with image analysis software of Image-Pro Plus (Version 6.0, Media Cybernetics, Inc., USA). The density of composites was determined using the Archimedes method with an analytical balance. Hardness (HV) and fracture toughness (KIC) were evaluated at ambient temperature. The hardness was determined by a Vickers indenter with an indent load of 30 kg. At least 10 indents were tested for each composite to obtain a reliable average value. The fracture toughness (KIC) was evaluated based on the crack length measured from the corner of the indentation made by Vicker’s indentation under a load of 30 kg, known as the Palmqvist indentation toughness, could be calculated using the following equation [12]:
Shrinkage (mm)
408
96
0.6
Fig. 4. Variations of the shrinkage and relative density of consolidated WC–12Co composites with different contents of nano-ceria.
•
•
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• •
• 5
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4 3 2 1
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Fig. 3. Schematic illustration of the erosion testing apparatus.
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Fig. 5. XRD patterns of WC–12Co composites with different contents of ceria.
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Fig. 6. SEM images of WC–12Co composites with different contents of nano-ceria: (a) 0 wt%, (b) 0.05 wt%, (c) 0.1 wt%, (d) 0.3 wt%, (e) 0.6 wt%.
content of nano-ceria, the density increased and reached the maximum at 0.1 wt%, and then decreased as more nano-ceria was added. The proper content of nano-ceria helps promote the densification. The reason for decreased density could be ascribed to the possibility that the excess solid ceria nano-particles in the liquid Co might have a negative effect on the flowability and special accessibility of the liquid. However, more studies are needed in order to fully understand the phenomenon. 3.2. Microstructure Fig. 5 illustrates XRD patterns of the WC–12Co composites with different contents of ceria, which show diffraction peaks of WC and Co phases. A little ceria was detected in the sample with 0.6 wt% ceria.
SEM images and WC grain size distributions of the WC–12Co composites with different contents of nano-ceria are shown in Figs. 6 and 7, respectively. One may see that without the nanoceria, abnormal WC grains (as indicated by the arrows in Fig. 6a) can be found in the WC–12Co composite. The average WC grain size is 340 nm. Both the number of the abnormal WC grain and average grain size decrease with the addition of nano-ceria. The WC grain size distribution of the ceria-free sample is much wider. As the addition of nano-ceria increases to 0.1%, there are no visible abnormal WC grains (Fig. 6c). The average WC grain size is decreased to 220 nm, 35% smaller than that without nano-ceria, and the size distribution is much narrower, implying that its microstructure is more homogeneous. Although further increasing of nano-ceria have no obvious negative effect on the WC grain refinement, it may not be beneficial for the microstructure. As
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Relative Frequency (%)
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WC grain size (nm)
Fig. 7. WC grain size distributions in the starting powder (a) and sintered WC–12Co composites with different contents of nano-ceria: (b) 0 wt%, (c) 0.05 wt%, (d) 0.1 wt%, (e) 0.3 wt%, (f) 0.6 wt%. (The red and olive curves in fig7b stand for two peaks of WC grain size distribution, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Reactive defects at a ceria surface. Coordinated Ce4 þ ions in a diametric surface vacancy cluster are reduced to reactive Ce3 þ when the trimer of oxygen ions (surface dimmer plus a subsurface oxygen) is removed [21].
shown in Fig. 6d, ‘‘cobalt pools’’ (as indicated by the arrows) appear in the sample with 0.3 wt% nano-ceria and the volume fraction of ‘‘cobalt pools’’ increases. They distribute more
unevenly when the added nano-ceria is increased to 0.6 wt% (Fig. 6e). It seems that 0.1 wt% nano-ceria resulted in the finest and most uniform microstructure. Excessive addition of the rare earth oxide led to the generation of cobalt pool, which may negatively affect the integrity and thus properties of the WC–Co composites. It is known that the SPS process has the advantage of a high heating rate with a shorter sintering time, and it can be carried out at a relatively lower sintering temperature, so the final grain size of samples is expected to be much smaller than that of samples made using conventional sintering process. The WC grain growth was further suppressed after adding trace nano-ceria, more effective than well-known conventional inhibitors, such as VC and Cr3C2. WC grain growth occurs by Ostwald ripening with dissolution of smaller WC grains and re-precipitation on larger grains in the binder phase Co [4]. The rare-earth segregating at WC/Co interface could help suppress the growth of WC grains. It has been reported that the rare earth can easily accumulate along the WC/Co interfaces [25] and the adsorbed of nano-ceria on certain crystal planes would not only decrease the difference in surface energy among various crystallographic planes, but also prevent the plane with maximum surface energy from further growth [26]. This adsorption of nano-ceria can
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Element
Wt%
At%
CK
14.12
63.70
CeL
02.03
00.78
CoK
17.28
15.89
WL
66.58
19.63
Fig. 9. SEM image and EDS pattern of WC–12Co composite with 0.6 wt% ceria.
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Fig. 10. Hardness and fracture toughness of WC–12Co composites with different contents of nano-ceria.
also exert a dragging effect on the further migration and combination of WC grain boundaries [27]. Thus, the nano-ceria addition could be very effective in pinning the grain growth of WC particles and facilitating the densification process. The formation of the cobalt pool may be attributed to the following factor. With a decrease in particle size, the surface of ceria nano-particles has more oxygen vacancies, as shown in Fig. 8, which makes ceria nano-particle surface more reactive [28]. The reactive ceria nano-particles preferably distribute at the interface between the liquid Co phase and WC to reduce the interfacial energy during sintering. This could improve the bonding between WC particles and also reduce the probability for WC particles to join towards abnormal growth. With the increase in the content of ceria, nano-particles dissolved in the Co liquid phase, proved by EDS composition analysis (see Fig. 9). The excessive ceria nano-particles may reduce the flowability of Co and thus affect the distribution of WC particles, leading to the formation of cobalt pools during the sintering process. 3.3. Mechanical properties Fig. 10 shows the effects of the nano-ceria addition on the hardness and fracture toughness of the sintered WC–Co
composites. As shown, WC–12Co composite without nano-ceria has its hardness equal to 1870 kg/mm2. Adding nano-ceria increases the hardness and the composite with 0.1 wt% nanoceria has the maximum hardness of 1998 kg/mm2, which is 7% higher than that of the composite without nano-ceria. When nano-ceria content is higher than 0.3%, the hardness starts to decrease. The toughness shows a similar trend. The smaller WC grain size of composites with nano-ceria should be an important factor affecting the mechanical properties. Densification also plays a role. When the ceria content is high than 0.3 wt%, the grain size does not show obvious growth but the relative density decreases. This phenomenon is related to the formation of cobalt pools, which result in deficient WC in the cobalt pool and consequently affect the mechanical properties negatively. The fracture toughness of WC–12Co composites is 12.5 MPa m1/2, which increases to 14.3–14.4 MPa m1/2, 13.4% higher than that without nano-ceria, as 0.1–0.3 wt% nano-ceria is added. The toughness then decreases when more nano-ceria is added. The improvement in toughness may be attributed to the fact that the finer WC grains deflect cracks more frequently, thus consuming more energy when cracks propagate, as shown in Fig. 11. However, when the addition of nano-ceria is excessive, cobalt pools form, which may facilitate the crack propagation, thus resulting in decreased fracture toughness. Therefore, it is important to control the nano-ceria in a certain range in order to obtain a proper balance between hardness and toughness. Similar to the Hall–Petch relationship between hardness and grain size [29], Hall–Petch fitting for the hardness and WC grain size is plotted in Fig. 12, which reasonably follows the Hall–Petch relationship, except the data point of the composites with 0.6 wt% ceria (surrounded by blue circle). This deviation could result from the inhomogeneous distribution of WC in this specific composite with Co pools as shown in Fig. 6(e). Bear in mind that theoretically the Hall–Petch relationship works for homogeneous materials. 3.4. Erosion behavior Effects of added nano-ceria on erosion rates of WC–12Co composites at impingement angles of 301 and 901 were investigated and results of the corrosion tests are shown in Fig. 13. As illustrated, the erosion rates for 301-impingement and 901impingement tests are close with similar trends. The erosion rate decreases with the addition of nano-ceria considerably and
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Fig. 11. SEM images of an indent on WC–12Co composites þ 0.1 wt% nano-ceria with cracks.emitted from its four corners (a), and an enlarged crack (b) shown in (a).
2000
Hardness (HV30)
1980 1960 1940 1920 1900 1880 1860 1.6
1.8
2.0
2.2
2.4
Inverse sqrt of WC Grain size (µm-0.5) Fig. 12. Hall–petch fitting for WC–12Co composites with different contents of nano-ceria. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
reaches the minimum at 0.1 wt% ceria, about 33% lower than that without nano-ceria. However, as more nano-ceria is added, the erosion rate becomes poor again. The variations in the erosion rate with respect to the nano-ceria content well correspond to the changes in hardness and toughness versus the nano-ceria content as illustrated in Fig. 10. Fig. 13 shows that the erosion rate is higher at 301 than 901, indicating that maximum erosion rate did not occur at 901. This is a common phenomenon for metallic materials and metal-matrix composites. For ceramics or brittle materials, 901 solid-particle impingement generally causes a larger erosion rate than lowerangle impingement. In order to obtain further information on the erosionmicrostructure relationship and clarify mechanisms behind, we analyzed individual effects of WC grain size, binder mean free path, hardness and fracture toughness on the erosion resistance of WC–12Co composites with different contents of nano-ceria. The variations in the erosion resistance with these parameters are shown in Fig. 14, which demonstrate clear relationships between the erosion resistance and these parameters (except the data points of the composite with 0.6 wt% ceria). As shown, the erosion resistance increased with decreasing WC grain size and binder mean free path in the ultrafine WC–12Co composites (Fig. 14a and b). Such phenomena are explainable. The decreases in grain size and the mean free path of the softer binder phase (Co)
Fig. 13. Erosion rate of WC–12Co composites with different contents of nanoceria.
introduce more barriers to dislocation movement and thus raise the hardness. On the other hand, the fine hard-phase grains have a higher resistance to impact than coarser grains with the same volume fraction, since the impact energy can be dissipated easily by the surrounding binder phase when the hard grains are small and crack propagation can be blocked with a higher frequency by high-density fine hard WC grains and thus proceeds in a zigzag path with more energy consumed. These render the material tougher and also elevate its load-bearing capability, leading to a higher resistance to solid-particle erosion. The erosion resistance of the ultrafine WC–12Co composites increased with increases in both hardness and fracture toughness (Fig. 14c and d). In the traditional WC–Co composites with coarser WC grains, hardness is generally inversely correlated with the toughness. When the WC grain size was decreased to submicron or nanometer, a combination of high hardness and high toughness can be obtained. The mechanism responsible for the increases in both hardness and toughness has been discussed in the previous paragraph. This combination is certainly beneficial to the resistance to erosion, particularly to the high-speed solid-particle erosion. It should be indicated that the behavior of the composite with 0.6 wt% ceria however deviates from the trends shown in Fig. 14. The deviation should be attributed to its poor microstructure
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integrity with inhomogeneous distribution of WC grains and especially the cobalt pools, which promote cracking (e.g., nucleation and propagate in the Co pools) and make the removal of WC grains or particles easier.
4. Conclusions Ultra-fine grained WC–Co composites containing different amounts of ceria nano-particles were prepared by spark plasma sintering and effects of the added nano-ceria on mechanical properties and erosion behavior of the WC–Co composites were studied. It was demonstrated that the trace nano-ceria addition effectively suppressed the abnormal grain growth of WC, leading to the uniform and fine microstructures. Such ultra-fine grained WC–Co composites exhibit both improved hardness and fracture toughness, resulting in enhanced resistance to high speed solidparticle erosion. However, when the added nano-ceria was more than 0.1 wt%, Co pools started to form, which lowered the material density, hardness and toughness and consequently the erosion resistance.
Acknowledgments This work was supported by the Program of Excellent Team at Harbin Institute of Technology and the Natural Science and Engineering Research Council of Canada (NSERC, CRD). The authors would like to thank Dr. Yanping Wang for her eager help on the erosion test. The first author gratefully acknowledges the financial support from the Chinese Scholarship Council (CSC).
References [1] R. Koc, S.K. Kodambaka, Tungsten carbide (WC) synthesis from novel precursors, Journal of the European Ceramic Society 20 (2000) 1859–1869. [2] C. Allen, M. Sheen, J. Williams, V.A. Pugsley, The wear of ultrafine WC–Co hard metals, Wear 250 (2001) 604–610. [3] X. Sun, Y. Wang, F. Peng, Z. Pan, L. Wang, Optimization of processing parameters for WC–11Co cemented carbide doped with nano-crystalline CeO2, Journal of Materials Engineering and Performance 22, 2013, 112–117. [4] L. Sun, T.E. Yang, C.C. Jia, J. Xiong, VC, Cr3C2 doped ultrafine WC–Co cemented carbides prepared by spark plasma sintering, International Journal of Refractory Metals & Hard Materials 29 (2011) 147–152. [5] F. Zhang, J. Shen, J. Sun, The effect of phosphorus additions on densification, grain growth and properties of nanocrystalline WC–Co composites, Journal of Alloys and Compounds 385 (2004) 96–103. [6] E. Breval, J.P. Cheng, D.K. Agrawal, P. Gigl, M. Dennis, R. Roy, A.J. Papworth, Comparison between microwave and conventional sintering of WC/Co composites, Materials Science and Engineering A 391 (2005) 285–295. [7] H.C. Kim, D.Y. Oh, I.J. Shon, Sintering of nanophase WC–15 vol%Co hard metals by rapid sintering process, International Journal of Refractory Metals & Hard Materials 22 (2004) 197–203. [8] D. Sivaprahasam, S.B. Chandrasekar, R. Sundaresan, Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering, International Journal of Refractory Metals and Hard Materials 25 (2007) 144–152. [9] M. Omori, Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS), Materials Science and Engineering A 287 (2000) 183–188. [10] C.B. Wei, X.Y. Song, J. Fu, X.M. Liu, Y. Gao, H.B. Wang, S.X. Zhao, Microstructure and properties of ultrafine cemented carbides—differences in spark plasma sintering and sinter-HIP, Materials Science and Engineering A 552 (2012) 427–433. [11] X.Y. Song, X.M. Liu, J.X. Zhang, S.X. Zhao, Temperature distribution and neck formation of WC–Co combined particles during spark plasma sintering, Materials Science and Engineering A 488 (2008) 1–7. [12] S.I. Cha, S.H. Hong, B.K. Kim, Spark plasma sintering behavior of nanocrystalline WC–10Co cemented carbide powders, Materials Science and Engineering A 351 (2003) 31–38. [13] B. Wittmann, W.-D. Schubert, B. Lux, WC grain growth and grain growth inhibition in nickel and iron binder hardmetals, International Journal of Refractory Metals and Hard Materials 20 (2002) 51–60.
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[14] A.G.P. Da Silva, C.P. De Souza, U.U. Gomes, F.F.P. Medeiros, C. Ciaravino, M. Roubin, A low temperature synthesized NbC as grain growth inhibitor for WC–Co composites, Materials Science and Engineering A 293 (2000) 242–246. [15] C.W. Morton, D.J. Wills, K. Stjernberg, The temperature ranges for maximum effectiveness of grain growth inhibitors in WC–Co alloys, International Journal of Refractory Metals and Hard Materials 23 (2005) 287–293. [16] W.D. Schubert, A. Bock, B. Lux, General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production, International Journal of Refractory Metals and Hard Materials 13 (1995) 281–296. [17] C. Lin, Action of new rare earth additions in cemented carbide, Journal of Materials Science and Technology 9 (1993) 177–184. [18] J. Xiong, J.G. Yang, X.H. Guo, Application of rare earth elements in cemented carbide inserts, drawing dies and mining tools, Materials Science and Engineering A 209 (1996) 287–293. [19] L. Zhang, S. Chen, W.D. Schubert, B.Y. Huang, Microstructures and properties of WC–20Co–1Y(2)O(3) cemented carbide by hot-press and liquid phase sintering, Journal of Central South University of Technology 11 (2004) 119–123. [20] Q. Pan, Effects of rare earth oxides on the properties of WC–Co cemented carbide, Rare Metal Materials And Engineering 22 (1993) 35–41. [21] S. Liu, Z.-L. Huang, G. Liu, G.-B. Yang, Preparing nano-crystalline rare earth doped WC/Co powder by high energy ball milling, International Journal of Refractory Metals and Hard Materials 24 (2006) 461–464.
[22] J. Zhao, T. Holland, C. Unuvar, Z.A. Munir, Sparking plasma sintering of nanometric tungsten carbide, International Journal of Refractory Metals and Hard Materials 27 (2009) 130–139. [23] R. Marder, R. Chaim, G. Chevallier, C. Estournes, Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering, Materials Science and Engineering A 528 (2011) 7200–7206. [24] R. Chaim, M. Levin, A. Shlayer, C. Estournes, Sintering and densification of nanocrystalline ceramic oxide powders: a review, Advances in Applied Ceramics 107 (2008) 159–169. [25] C.M.L. Wu, D.Q. Yu, C.M.T. Law, L. Wang, Properties of lead-free solder alloys with rare earth element additions, Materials Science and Engineering Reports 44 (2004) 1–44. [26] Y.S. Tian, C.Z. Chen, L.X. Chen, Q.H. Huo, Effect of RE oxides on the microstructure of the coatings fabricated on titanium alloys by laser alloying technique, Scripta Materialia 54 (2006) 847–852. [27] D. Gu, Y. Shen, L. Zhao, J. Xiao, P. Wu, Y. Zhu, Effect of rare earth oxide addition on microstructures of ultra-fine WC–Co particulate reinforced Cu matrix composites prepared by direct laser sintering, Materials Science and Engineering A 445–446 (2007) 316–322. [28] C.T. Campbell, C.H.F. Peden, Oxygen vacancies and catalysis on ceria surfaces, Science 309 (2005) 713–714. [29] S.I. Cha, K.H. Lee, H.J. Ryu, S.H. Hong, Analytical modeling to calculate the hardness of ultra-fine WC–Co cemented carbides, Materials Science and Engineering A 489 (2008) 234–244.
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Wear journal homepage: www.elsevier.com/locate/wear
Mechanical properties and erosion resistance of ceria nano-particle-doped ultrafine WC–12Co composite prepared by spark plasma sintering Xiaoguang Sun a,b, You Wang a,n, D.Y. Li a,b,n a b
Department of Materials Science, Harbin Institute of Technology, Harbin 150001, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2V4
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2012 Received in revised form 30 January 2013 Accepted 31 January 2013 Available online 8 February 2013
Ultra-fine grained WC–Co composites possess higher hardness or strength and wear resistance, compared to their coarse counterparts. However, abnormal grain growth of ultra-fine WC particles often occurs during the traditional pressureless liquid sintering, which substantially impairs the mechanical properties and erosion resistance of the material. Thus, controlling the abnormal grain growth becomes one of the key issues in fabricating ultra-fine grained cemented carbides. In this study, ultra-fine grained WC–Co composites containing different amounts of ceria nano-particles were prepared by spark plasma sintering and effects of the nano-ceria on mechanical properties and erosion behavior of the WC–Co composites were studied. The results demonstrated that trace nano-ceria addition effectively suppressed the abnormal grain growth of WC, leading to uniform and fine microstructures. Such ultra-fine grained WC–Co composites have both improved hardness and fracture toughness, resulting in enhanced resistance to high-speed solid-particle erosion. However, when the added nano-ceria was more than 0.1 wt%, Co pools started to form, which lowered the material density, hardness and toughness and consequently the erosion resistance. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ultrafine WC–Co Nano-ceria Mechanical properties Erosion resistance
1. Introduction WC–Co composites have been widely used for machining, cutting, mining and drilling tools, due to their high hardness and excellent wear resistance [1]. When the WC grain size is reduced to submicron or nanometer scales, the WC–Co composites possess improved mechanical properties and wear resistance, compared to their coarse-grained counterparts [2]. Thus, ultrafine-grained and nanostructured WC–Co composites have attracted great interest in the field of high-performance hard materials [3]. However, abnormal grain growth of ultrafine WC particles in WC–Co composite occurs easily during the traditional pressureless liquid sintering, which substantially deteriorates their performance. Thus, controlling abnormal grain growth has become one of the key issues in fabricating ultrafine WC–Co composites [4]. The grain growth can be inhibited to a certain extent by using special sintering technologies with accelerated heating rate, increased densification rate, decreased sintering temperature and short holding time [5], such as microwave sintering [6], rapid hot pressing sintering [7], and spark plasma sintering (SPS) or
n
Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (D.Y. Li). 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.113
pulse electric current sintering (PECS) [8]. In particular, SPS is a novel consolidation process that combines fast joule heating, established by flowing a pulsed electric current through the die/ punch/powder compact set-up under an applied pressure. The fast densification process is reported to be benefited from grain boundary diffusion, surface diffusion, volume diffusion, and plastic deformation [9]. During past few years, SPS process was successfully used to synthesize ultra-fine/nano-WC–Co composites [10–12]. Although many studies are focused on the development of sintering techniques, one of the basic and successful ways for controlling the WC grain growth is the addition of small amounts of WC grain growth inhibitors, such as VC, Cr3C2, NbC or Mo2C, to the starting powder mixture, typically less than 1.0 wt% of a metallic carbide [13]. Vanadium carbide (VC) and chromium carbide (Cr3C2) are the most effective grain growth inhibitors due to their high solubility and mobility in the cobalt phase at lower temperatures [14,15]. For all transition metal carbides, however, the mechanism for grain growth inhibition has been related to the slowing down of the solution/re-precipitation reactions at the WC–Co interfaces [16]. These carbides are effective in preventing anomalous grain growth but the drawback is that they may cause the product to be brittle, especially for vanadium carbide [15]. Rare earths and their oxides are effective inhibitors in WC–Co composites [17–19], which may increase the wetting power of Co to WC grains, control the formation of Z
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phase in cemented carbides, raise the bending strength and wearresistance, and also increase the corrosion resistance of cemented carbides [20]. The previous studies [19] on traditional rare earth as additives, have shown that the rare earths improve mechanical properties of WC–Co and optimal performance is observed when the content of added rare earths or their oxides such as CeO2 is more than 1.0 wt% [19–21]. The high melting point of ceria (2673 K) makes it in a solid state during sintering of cemented carbides even using the conventional liquid sintering method in the temperature range of 1723 K. CeO2 particles cannot diffuse effectively during the sintering process, so that the positive effect of rare earths would be minimized when their primary particle size is at or above micron scale. Further, the segregation of rareearth oxide particles at boundary would weaken their positive effect and could even be detrimental. The unstable performance due to the non-uniform distribution of rare-earth oxide particles in cemented carbide and the high cost limited their industrial applications. Our previous study showed that nano-CeO2 is much more effective in cemented carbides and the mechanical properties of WC–Co composites are optimized with only 0.1 wt% nanoCeO2 [3]. To our knowledge, no studies on the WC–Co composites with nano-CeO2 by other researchers have been reported in the literature. In this study, ultra-fine grained WC–Co composites containing different amounts of CeO2 nano-particles were prepared by spark plasma sintering and effects of the nano-CeO2 on mechanical properties and erosion behavior of the WC–Co composites were studied.
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were put into cylindrical graphite dies with an inner diameter of 30 mm. The temperature was monitored using an optical pyrometer aimed at a non-through hole of 0.5 mm in diameter and 2 mm in depth on the graphite die. The heating rate was 100 K/ min, and a pressure of 50 MPa was applied during sintering. The sintering temperature was set at 1473 K with a holding time of 5 min. During the consolidation process, the dimensional changes of powders were monitored by measuring the position of the lower ram, which acted as an electrode to produce high pulsed current within the powder compact. 2.2. Characterization The phase constitution of the composites was determined using a Rikagu X-ray diffractometer with Cu Ka radiation. A field Table 1 Compositions of WC–12Co composites for this study. No.
WC
Co (wt%)
CeO2 (wt%)
1 2 3 4 5
Balance Balance Balance Balance Balance
12 12 12 12 12
0 0.05 0.1 0.3 0.6
2. Experimental 2.1. Materials preparation Ultrafine WC powder ( Z99.5%, 200 nm, free carbon 0.11 wt%, shown in Fig. 1a) and Co powder ( Z99.6%, 60 nm) were used as raw materials. A small quantity (0.05–0.6 wt%) of nano-CeO2 powder ( 10 nm, shown in Fig. 1b) was added to WC–12Co composite powder. The compositions of samples with different amounts of nano-CeO2 are shown in Table 1. The ultrafine WC–12Co composite powders were wet ball-milled for 0.5 h using ethanol as a medium in a high vibration ball milling machine. The ball material was tungsten and the volume ratio of the ball to the material was 5:1. The powders were dried at 333 K in an oven for 10 h. The mixed powders were then sintered using an SPS apparatus (SPS1050, Sumitomo Coal Mining Co., Ltd., Tokyo, Japan). The schematic of SPS apparatus was shown in Fig. 2 [22].The powders
Fig. 2. Schematic illustration of SPS apparatus [19].
Fig. 1. SEM image of WC powder (a) and TEM image of nano-ceria powder (b).
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K IC ¼ 0:15 ðHV 30 =SlÞ1=2
ð1Þ
where HV30 is the Vickers hardness (N/mm2) with a load of 30 kg, and Sl is the total length of cracks (mm) initiated from the corners of the indentation. For accuracy, the specimen was polished to near-mirror finish and checked for face parallelism before testing. Erosion testing is conducted using an air-jet sand blasting test apparatus, shown in Fig. 3. The tester is operated by feeding the eroding particles from a vibrating hopper into a stream of gas. The particles are blown out from the nozzle at a constant speed, controlled by a manometer, to erode the specimen surface. All tests were performed in air at the room temperature and the erodent velocity was 75 m/s. The erodent flow angles relative to the sample surface were set at 301 and 901, respectively, in order to investigate the responses of the material to relatively lowangle and high-angle solid-particle impingements. AFS 50/70 sand (U.S. Silica Company, USA) was used as the erodent particle. The sample mass was measured before and after the tests using a balance with a precision of 0.1 mg. The erosion rate was calculated by dividing the volume loss of a specimen (mm3) by the total weight of abrasive particles that impacted the specimen (g). Each presented value is an average of at least four measurements. The erosion resistance was represented as the reciprocal of the erosion rate with a unit of g/mm3.
3. Results and discussion 3.1. Densification behavior During the SPS sintering, the main densification shrinkage represented by the linear dimensional change of powder aggregate to the close-packed arrangement took place during heating, involving particle sliding, rotation, grain boundary migration and pore shrinkage [23,24]. Although the shrinkage during SPS is the cumulative shrinkage of the sample and the graphite molds and steel electrodes, the contribution of graphite molds and steel electrodes can be ignored as we intend to compare the relative changes in shrinkage for different samples, since all sample were sintered under the same condition. The recorded shrinkage reflects the densification degree. Variations in shrinkage of WC– 12Co composites with different contents of nano-ceria are shown in Fig. 4. The shrinkage changed with the addition of nano-ceria, which increased with nano-ceria and reached a maximum value at 0.1 wt% ceria. The shrinkage then decreased with further increasing nano-ceria, implying that the densification became worse at higher wt% ceria. The relative densities of the WC–12Co composites with different contents of nano-ceria are presented in Fig. 4, which shows a trend similar to that of the shrinkage. With an increase in the 6.0
100 Shrinkage Relative density
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97
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emission gun scanning electron microscope (SEM, HITACHI S-4700) equipped with X-ray energy dispersive spectroscopy (EDS) was employed to characterize the microstructure of the composites. The grain size of WC and the mean free path in cobalt were measured using the linear intercept method with image analysis software of Image-Pro Plus (Version 6.0, Media Cybernetics, Inc., USA). The density of composites was determined using the Archimedes method with an analytical balance. Hardness (HV) and fracture toughness (KIC) were evaluated at ambient temperature. The hardness was determined by a Vickers indenter with an indent load of 30 kg. At least 10 indents were tested for each composite to obtain a reliable average value. The fracture toughness (KIC) was evaluated based on the crack length measured from the corner of the indentation made by Vicker’s indentation under a load of 30 kg, known as the Palmqvist indentation toughness, could be calculated using the following equation [12]:
Shrinkage (mm)
408
96
0.6
Fig. 4. Variations of the shrinkage and relative density of consolidated WC–12Co composites with different contents of nano-ceria.
•
•
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•
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♥ -CeO2
• •
• 5
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•
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• ♦
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20
Fig. 3. Schematic illustration of the erosion testing apparatus.
40
60 2theta (deg.)
80
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Fig. 5. XRD patterns of WC–12Co composites with different contents of ceria.
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Fig. 6. SEM images of WC–12Co composites with different contents of nano-ceria: (a) 0 wt%, (b) 0.05 wt%, (c) 0.1 wt%, (d) 0.3 wt%, (e) 0.6 wt%.
content of nano-ceria, the density increased and reached the maximum at 0.1 wt%, and then decreased as more nano-ceria was added. The proper content of nano-ceria helps promote the densification. The reason for decreased density could be ascribed to the possibility that the excess solid ceria nano-particles in the liquid Co might have a negative effect on the flowability and special accessibility of the liquid. However, more studies are needed in order to fully understand the phenomenon. 3.2. Microstructure Fig. 5 illustrates XRD patterns of the WC–12Co composites with different contents of ceria, which show diffraction peaks of WC and Co phases. A little ceria was detected in the sample with 0.6 wt% ceria.
SEM images and WC grain size distributions of the WC–12Co composites with different contents of nano-ceria are shown in Figs. 6 and 7, respectively. One may see that without the nanoceria, abnormal WC grains (as indicated by the arrows in Fig. 6a) can be found in the WC–12Co composite. The average WC grain size is 340 nm. Both the number of the abnormal WC grain and average grain size decrease with the addition of nano-ceria. The WC grain size distribution of the ceria-free sample is much wider. As the addition of nano-ceria increases to 0.1%, there are no visible abnormal WC grains (Fig. 6c). The average WC grain size is decreased to 220 nm, 35% smaller than that without nano-ceria, and the size distribution is much narrower, implying that its microstructure is more homogeneous. Although further increasing of nano-ceria have no obvious negative effect on the WC grain refinement, it may not be beneficial for the microstructure. As
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Relative Frequency (%)
410
WC grain size (nm)
Fig. 7. WC grain size distributions in the starting powder (a) and sintered WC–12Co composites with different contents of nano-ceria: (b) 0 wt%, (c) 0.05 wt%, (d) 0.1 wt%, (e) 0.3 wt%, (f) 0.6 wt%. (The red and olive curves in fig7b stand for two peaks of WC grain size distribution, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Reactive defects at a ceria surface. Coordinated Ce4 þ ions in a diametric surface vacancy cluster are reduced to reactive Ce3 þ when the trimer of oxygen ions (surface dimmer plus a subsurface oxygen) is removed [21].
shown in Fig. 6d, ‘‘cobalt pools’’ (as indicated by the arrows) appear in the sample with 0.3 wt% nano-ceria and the volume fraction of ‘‘cobalt pools’’ increases. They distribute more
unevenly when the added nano-ceria is increased to 0.6 wt% (Fig. 6e). It seems that 0.1 wt% nano-ceria resulted in the finest and most uniform microstructure. Excessive addition of the rare earth oxide led to the generation of cobalt pool, which may negatively affect the integrity and thus properties of the WC–Co composites. It is known that the SPS process has the advantage of a high heating rate with a shorter sintering time, and it can be carried out at a relatively lower sintering temperature, so the final grain size of samples is expected to be much smaller than that of samples made using conventional sintering process. The WC grain growth was further suppressed after adding trace nano-ceria, more effective than well-known conventional inhibitors, such as VC and Cr3C2. WC grain growth occurs by Ostwald ripening with dissolution of smaller WC grains and re-precipitation on larger grains in the binder phase Co [4]. The rare-earth segregating at WC/Co interface could help suppress the growth of WC grains. It has been reported that the rare earth can easily accumulate along the WC/Co interfaces [25] and the adsorbed of nano-ceria on certain crystal planes would not only decrease the difference in surface energy among various crystallographic planes, but also prevent the plane with maximum surface energy from further growth [26]. This adsorption of nano-ceria can
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Element
Wt%
At%
CK
14.12
63.70
CeL
02.03
00.78
CoK
17.28
15.89
WL
66.58
19.63
Fig. 9. SEM image and EDS pattern of WC–12Co composite with 0.6 wt% ceria.
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Fig. 10. Hardness and fracture toughness of WC–12Co composites with different contents of nano-ceria.
also exert a dragging effect on the further migration and combination of WC grain boundaries [27]. Thus, the nano-ceria addition could be very effective in pinning the grain growth of WC particles and facilitating the densification process. The formation of the cobalt pool may be attributed to the following factor. With a decrease in particle size, the surface of ceria nano-particles has more oxygen vacancies, as shown in Fig. 8, which makes ceria nano-particle surface more reactive [28]. The reactive ceria nano-particles preferably distribute at the interface between the liquid Co phase and WC to reduce the interfacial energy during sintering. This could improve the bonding between WC particles and also reduce the probability for WC particles to join towards abnormal growth. With the increase in the content of ceria, nano-particles dissolved in the Co liquid phase, proved by EDS composition analysis (see Fig. 9). The excessive ceria nano-particles may reduce the flowability of Co and thus affect the distribution of WC particles, leading to the formation of cobalt pools during the sintering process. 3.3. Mechanical properties Fig. 10 shows the effects of the nano-ceria addition on the hardness and fracture toughness of the sintered WC–Co
composites. As shown, WC–12Co composite without nano-ceria has its hardness equal to 1870 kg/mm2. Adding nano-ceria increases the hardness and the composite with 0.1 wt% nanoceria has the maximum hardness of 1998 kg/mm2, which is 7% higher than that of the composite without nano-ceria. When nano-ceria content is higher than 0.3%, the hardness starts to decrease. The toughness shows a similar trend. The smaller WC grain size of composites with nano-ceria should be an important factor affecting the mechanical properties. Densification also plays a role. When the ceria content is high than 0.3 wt%, the grain size does not show obvious growth but the relative density decreases. This phenomenon is related to the formation of cobalt pools, which result in deficient WC in the cobalt pool and consequently affect the mechanical properties negatively. The fracture toughness of WC–12Co composites is 12.5 MPa m1/2, which increases to 14.3–14.4 MPa m1/2, 13.4% higher than that without nano-ceria, as 0.1–0.3 wt% nano-ceria is added. The toughness then decreases when more nano-ceria is added. The improvement in toughness may be attributed to the fact that the finer WC grains deflect cracks more frequently, thus consuming more energy when cracks propagate, as shown in Fig. 11. However, when the addition of nano-ceria is excessive, cobalt pools form, which may facilitate the crack propagation, thus resulting in decreased fracture toughness. Therefore, it is important to control the nano-ceria in a certain range in order to obtain a proper balance between hardness and toughness. Similar to the Hall–Petch relationship between hardness and grain size [29], Hall–Petch fitting for the hardness and WC grain size is plotted in Fig. 12, which reasonably follows the Hall–Petch relationship, except the data point of the composites with 0.6 wt% ceria (surrounded by blue circle). This deviation could result from the inhomogeneous distribution of WC in this specific composite with Co pools as shown in Fig. 6(e). Bear in mind that theoretically the Hall–Petch relationship works for homogeneous materials. 3.4. Erosion behavior Effects of added nano-ceria on erosion rates of WC–12Co composites at impingement angles of 301 and 901 were investigated and results of the corrosion tests are shown in Fig. 13. As illustrated, the erosion rates for 301-impingement and 901impingement tests are close with similar trends. The erosion rate decreases with the addition of nano-ceria considerably and
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Fig. 11. SEM images of an indent on WC–12Co composites þ 0.1 wt% nano-ceria with cracks.emitted from its four corners (a), and an enlarged crack (b) shown in (a).
2000
Hardness (HV30)
1980 1960 1940 1920 1900 1880 1860 1.6
1.8
2.0
2.2
2.4
Inverse sqrt of WC Grain size (µm-0.5) Fig. 12. Hall–petch fitting for WC–12Co composites with different contents of nano-ceria. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
reaches the minimum at 0.1 wt% ceria, about 33% lower than that without nano-ceria. However, as more nano-ceria is added, the erosion rate becomes poor again. The variations in the erosion rate with respect to the nano-ceria content well correspond to the changes in hardness and toughness versus the nano-ceria content as illustrated in Fig. 10. Fig. 13 shows that the erosion rate is higher at 301 than 901, indicating that maximum erosion rate did not occur at 901. This is a common phenomenon for metallic materials and metal-matrix composites. For ceramics or brittle materials, 901 solid-particle impingement generally causes a larger erosion rate than lowerangle impingement. In order to obtain further information on the erosionmicrostructure relationship and clarify mechanisms behind, we analyzed individual effects of WC grain size, binder mean free path, hardness and fracture toughness on the erosion resistance of WC–12Co composites with different contents of nano-ceria. The variations in the erosion resistance with these parameters are shown in Fig. 14, which demonstrate clear relationships between the erosion resistance and these parameters (except the data points of the composite with 0.6 wt% ceria). As shown, the erosion resistance increased with decreasing WC grain size and binder mean free path in the ultrafine WC–12Co composites (Fig. 14a and b). Such phenomena are explainable. The decreases in grain size and the mean free path of the softer binder phase (Co)
Fig. 13. Erosion rate of WC–12Co composites with different contents of nanoceria.
introduce more barriers to dislocation movement and thus raise the hardness. On the other hand, the fine hard-phase grains have a higher resistance to impact than coarser grains with the same volume fraction, since the impact energy can be dissipated easily by the surrounding binder phase when the hard grains are small and crack propagation can be blocked with a higher frequency by high-density fine hard WC grains and thus proceeds in a zigzag path with more energy consumed. These render the material tougher and also elevate its load-bearing capability, leading to a higher resistance to solid-particle erosion. The erosion resistance of the ultrafine WC–12Co composites increased with increases in both hardness and fracture toughness (Fig. 14c and d). In the traditional WC–Co composites with coarser WC grains, hardness is generally inversely correlated with the toughness. When the WC grain size was decreased to submicron or nanometer, a combination of high hardness and high toughness can be obtained. The mechanism responsible for the increases in both hardness and toughness has been discussed in the previous paragraph. This combination is certainly beneficial to the resistance to erosion, particularly to the high-speed solid-particle erosion. It should be indicated that the behavior of the composite with 0.6 wt% ceria however deviates from the trends shown in Fig. 14. The deviation should be attributed to its poor microstructure
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integrity with inhomogeneous distribution of WC grains and especially the cobalt pools, which promote cracking (e.g., nucleation and propagate in the Co pools) and make the removal of WC grains or particles easier.
4. Conclusions Ultra-fine grained WC–Co composites containing different amounts of ceria nano-particles were prepared by spark plasma sintering and effects of the added nano-ceria on mechanical properties and erosion behavior of the WC–Co composites were studied. It was demonstrated that the trace nano-ceria addition effectively suppressed the abnormal grain growth of WC, leading to the uniform and fine microstructures. Such ultra-fine grained WC–Co composites exhibit both improved hardness and fracture toughness, resulting in enhanced resistance to high speed solidparticle erosion. However, when the added nano-ceria was more than 0.1 wt%, Co pools started to form, which lowered the material density, hardness and toughness and consequently the erosion resistance.
Acknowledgments This work was supported by the Program of Excellent Team at Harbin Institute of Technology and the Natural Science and Engineering Research Council of Canada (NSERC, CRD). The authors would like to thank Dr. Yanping Wang for her eager help on the erosion test. The first author gratefully acknowledges the financial support from the Chinese Scholarship Council (CSC).
References [1] R. Koc, S.K. Kodambaka, Tungsten carbide (WC) synthesis from novel precursors, Journal of the European Ceramic Society 20 (2000) 1859–1869. [2] C. Allen, M. Sheen, J. Williams, V.A. Pugsley, The wear of ultrafine WC–Co hard metals, Wear 250 (2001) 604–610. [3] X. Sun, Y. Wang, F. Peng, Z. Pan, L. Wang, Optimization of processing parameters for WC–11Co cemented carbide doped with nano-crystalline CeO2, Journal of Materials Engineering and Performance 22, 2013, 112–117. [4] L. Sun, T.E. Yang, C.C. Jia, J. Xiong, VC, Cr3C2 doped ultrafine WC–Co cemented carbides prepared by spark plasma sintering, International Journal of Refractory Metals & Hard Materials 29 (2011) 147–152. [5] F. Zhang, J. Shen, J. Sun, The effect of phosphorus additions on densification, grain growth and properties of nanocrystalline WC–Co composites, Journal of Alloys and Compounds 385 (2004) 96–103. [6] E. Breval, J.P. Cheng, D.K. Agrawal, P. Gigl, M. Dennis, R. Roy, A.J. Papworth, Comparison between microwave and conventional sintering of WC/Co composites, Materials Science and Engineering A 391 (2005) 285–295. [7] H.C. Kim, D.Y. Oh, I.J. Shon, Sintering of nanophase WC–15 vol%Co hard metals by rapid sintering process, International Journal of Refractory Metals & Hard Materials 22 (2004) 197–203. [8] D. Sivaprahasam, S.B. Chandrasekar, R. Sundaresan, Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering, International Journal of Refractory Metals and Hard Materials 25 (2007) 144–152. [9] M. Omori, Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS), Materials Science and Engineering A 287 (2000) 183–188. [10] C.B. Wei, X.Y. Song, J. Fu, X.M. Liu, Y. Gao, H.B. Wang, S.X. Zhao, Microstructure and properties of ultrafine cemented carbides—differences in spark plasma sintering and sinter-HIP, Materials Science and Engineering A 552 (2012) 427–433. [11] X.Y. Song, X.M. Liu, J.X. Zhang, S.X. Zhao, Temperature distribution and neck formation of WC–Co combined particles during spark plasma sintering, Materials Science and Engineering A 488 (2008) 1–7. [12] S.I. Cha, S.H. Hong, B.K. Kim, Spark plasma sintering behavior of nanocrystalline WC–10Co cemented carbide powders, Materials Science and Engineering A 351 (2003) 31–38. [13] B. Wittmann, W.-D. Schubert, B. Lux, WC grain growth and grain growth inhibition in nickel and iron binder hardmetals, International Journal of Refractory Metals and Hard Materials 20 (2002) 51–60.
414
X. Sun et al. / Wear 301 (2013) 406–414
[14] A.G.P. Da Silva, C.P. De Souza, U.U. Gomes, F.F.P. Medeiros, C. Ciaravino, M. Roubin, A low temperature synthesized NbC as grain growth inhibitor for WC–Co composites, Materials Science and Engineering A 293 (2000) 242–246. [15] C.W. Morton, D.J. Wills, K. Stjernberg, The temperature ranges for maximum effectiveness of grain growth inhibitors in WC–Co alloys, International Journal of Refractory Metals and Hard Materials 23 (2005) 287–293. [16] W.D. Schubert, A. Bock, B. Lux, General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production, International Journal of Refractory Metals and Hard Materials 13 (1995) 281–296. [17] C. Lin, Action of new rare earth additions in cemented carbide, Journal of Materials Science and Technology 9 (1993) 177–184. [18] J. Xiong, J.G. Yang, X.H. Guo, Application of rare earth elements in cemented carbide inserts, drawing dies and mining tools, Materials Science and Engineering A 209 (1996) 287–293. [19] L. Zhang, S. Chen, W.D. Schubert, B.Y. Huang, Microstructures and properties of WC–20Co–1Y(2)O(3) cemented carbide by hot-press and liquid phase sintering, Journal of Central South University of Technology 11 (2004) 119–123. [20] Q. Pan, Effects of rare earth oxides on the properties of WC–Co cemented carbide, Rare Metal Materials And Engineering 22 (1993) 35–41. [21] S. Liu, Z.-L. Huang, G. Liu, G.-B. Yang, Preparing nano-crystalline rare earth doped WC/Co powder by high energy ball milling, International Journal of Refractory Metals and Hard Materials 24 (2006) 461–464.
[22] J. Zhao, T. Holland, C. Unuvar, Z.A. Munir, Sparking plasma sintering of nanometric tungsten carbide, International Journal of Refractory Metals and Hard Materials 27 (2009) 130–139. [23] R. Marder, R. Chaim, G. Chevallier, C. Estournes, Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering, Materials Science and Engineering A 528 (2011) 7200–7206. [24] R. Chaim, M. Levin, A. Shlayer, C. Estournes, Sintering and densification of nanocrystalline ceramic oxide powders: a review, Advances in Applied Ceramics 107 (2008) 159–169. [25] C.M.L. Wu, D.Q. Yu, C.M.T. Law, L. Wang, Properties of lead-free solder alloys with rare earth element additions, Materials Science and Engineering Reports 44 (2004) 1–44. [26] Y.S. Tian, C.Z. Chen, L.X. Chen, Q.H. Huo, Effect of RE oxides on the microstructure of the coatings fabricated on titanium alloys by laser alloying technique, Scripta Materialia 54 (2006) 847–852. [27] D. Gu, Y. Shen, L. Zhao, J. Xiao, P. Wu, Y. Zhu, Effect of rare earth oxide addition on microstructures of ultra-fine WC–Co particulate reinforced Cu matrix composites prepared by direct laser sintering, Materials Science and Engineering A 445–446 (2007) 316–322. [28] C.T. Campbell, C.H.F. Peden, Oxygen vacancies and catalysis on ceria surfaces, Science 309 (2005) 713–714. [29] S.I. Cha, K.H. Lee, H.J. Ryu, S.H. Hong, Analytical modeling to calculate the hardness of ultra-fine WC–Co cemented carbides, Materials Science and Engineering A 489 (2008) 234–244.