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Characterizations of WC–10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide
Materials Characterization 57 (2006) 358 – 370
Characterizations of WC–10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide X.L. Shi ⁎, G.Q. Shao, X.L. Duan, Z. Xiong, H. Yang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122Luoshi Road, Wuhan 430070, P.R. China Received 4 January 2006; accepted 28 February 2006
Abstract Ultrafine WC–Co cemented carbides, combining high hardness and high toughness, are expected to find broad applications. In this study, WC–10Co–0.4VC–0.4Cr3C2 (wt.%) nanocomposite powders, whose average grain size was about 30 nm, were fabricated by spray pyrolysis-continuous reduction and carbonization technology. The as-prepared nanocomposite powders were characterized and analyzed by chemical methods, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis and atomic force microscopy (AFM). Furthermore, “sinterhip” was used in the sintering process, by which ultrafine WC–10Co cemented carbides with an average grain size of 240 nm were prepared. The material exhibited high Rockwell A hardness of HRA 92.8, Vickers hardness HV1 1918, and transverse rapture strength (TRS) of 3780 MPa. The homogeneously dispersed grain growth inhibitors such as VC, Cr3C2 in nanocomposite powder and the special nonmetal–metal nanocomposite structure of WC–10Co nanocomposite powder played very important roles in obtaining ultrafine WC–10Co cemented carbide with the desired properties and microstructure. There was an abundance of triple junctions in the ultrafine WC–10Co cemented carbide; these triple junctions endowed the sintered specimen with high mechanical properties. © 2006 Elsevier Inc. All rights reserved. Keywords: WC–10Co; Nanocomposite powders; Atomic force microscope (AFM); Sinterhip; Ultrafine cemented carbide
1. Introduction The current trend in the hardmetals industry to finer and finer-grained alloys has put high demands on the manufacturing process, for both powders and alloys [1– 3]. These applications include use as abrasives, cutting tools, saw blade tips, milling cutters, dies, inserts in valve stems, sand blast nozzles and other wear-resistant components owing to the high hardness and wearresistant properties. The driving force behind this development was the considerably improved perfor⁎ Corresponding author. Tel./fax: +86 27 87216912. E-mail address: [email protected] (X.L. Shi). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.02.013
mance of these ultrafine hardmetals that in many applications clearly exceeded the performance of hardmetals with WC-grains greater than 1 μm [4–9]. As a result of an extremely fine microstructure, nanostructured materials have long been recognized to have remarkable and technologically attractive properties. Ultrafine grains can endow materials with improved hardness and strength, as anticipated from the wellknown empirical Hall–Petch relationship; moreover, the increased volume fraction of grain boundaries may enhance the toughness and ductility of the materials. This turns out to be one of the driving forces for the fast growing technologies for synthesizing nanophase or ultrafine WC–Co composite powders [10].
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Table 1 Composition of the WC–10Co nanocomposite powders Total carbon content (wt.%)
Free carbon content (wt.%)
Oxygen content (wt.%)
Surface area (m2 g− 1)
Co content (wt.%)
VC/Cr3C2 content (wt.%)
5.48
0.16
0.20
11.7800
10.05
0.40/0.40
Many methods have been developed to synthesize nanophase or ultrafine WC–Co powder, such as mechanical alloying [11,12], mechanochemical processing [13], two-step processing [14], and co-precipitation methods [15]. However, there are some limitations to these methods. For example, impurities may be introduced in the mechanical alloying and other approaches. WC–Co nanocomposite powders have been fabricated by spray pyrolysis-continuous reduction and carbonization technology [16]. The WC particle sizes can be reduced below 20 nm, making possible the manufacture of ultrafine or even nanocrystalline WC– Co cemented carbide with excellent properties. These techniques can assure that nanocrystalline grain growth inhibitors such as VC, Cr3C2 disperse uniformly in
WC–10Co nanocomposite powders, and permit the use of complex starting solutions that contains two or more precursor compounds. The present study was focused on characterizing WC–10Co nanocomposite powders prepared by spray pyrolysis-continuous reduction and carbonization technology. Furthermore, “sinterhip” was used in the subsequent sintering process to produce ultrafine WC– 10Co cemented carbide with high mechanical properties, fine and homogeneous microstructure.
Fig. 1. SEM photographs of the WC–10Co nanocomposite powders.
Fig. 2. TEM photographs of the WC–10Co nanocomposite powders.
2. Experimental WC–10Co (wt.%) nanocomposite powders produced by spray pyrolysis-continuous reduction and
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carbonization technology were used for this study. The powder was ball-milled in acetone for 48 h. After milling it was dried at 90 °C in a vacuum oven. The
green compacts were consolidated by sinterhip at 1320 °C for 60 min with an Ar pressure of 5.5 MPa. In order to improve the carbon content, some carbon
Fig. 3. AFM images of the WC–10Co nanocomposite powder (a) height image; (b) 3D image; (c) section analysis; (d) average grain size.
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361
Fig. 3 (continued).
powders were placed beside the specimens during this operation. The composition of the nanocomposite powder was analyzed by chemical methods. The
shape and particle size of starting WC–10Co nanocomposite powders were characterized by a JSM5610LV scanning electron microscope (SEM) operated
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at 15 kV, a JEM-2010 transmission electron microscope (TEM) operated at 200 kV and an atomic force microscope (AFM) (Digital Instruments NanoScope?,
VEECO company, USA)(tapping mode). The particle size was also characterized by a BET analyzer. Sintered specimens had the dimensions 20 × 6.5 ×
Fig. 4. AFM images of the WC–10Co nanocomposite powders (a) height image; (b) 3D image; (c) section analysis.
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363
Fig. 4 (continued).
5.25 mm. The density was determined according to ISO3369-1975. The Vickers hardness (HV) (@1 kg), transverse rupture strength (TRS), and Rockwell A hardness (HRA) of the sintered specimens were measured. The saturation magnetization was measured by using a saturation induction measuring system, and the coercivity force was measured by a FörsterKoerzimat 1.095 system. The specimens were sawed into specimens with a size of 5.25 × 6.5 × 2.25 mm by a diamond saw. The fractured surfaces of the specimens were polished mechanically with emery papers down to 1200 grade, and then with 0.05 μm wet polishing diamond pastes. The polished samples were etched in a Murakami solution [K3Fe(CN)6 10 g, NaOH 10 g, and distilled water 100 g] for 20 s. 3. Results and discussion The composition of the nanocomposite powders is listed in Table 1. Carbon content was controlled with a concentration higher than the theoretical value, taking into account the oxygen concentration. The specific surface area of the WC–10Co nanocomposite powders was 11.7800 m2·g− 1 and the equivalent mean particle size was about 30 nm. Fig. 1 shows the SEM micrographs of the WC–10Co nanocomposite pow-
ders prior to being ball-milled. The powder is seen to consist of big agglomerated particles of 1–5 μm in diameter. The big nanocomposite powders consisted of small agglomerated particles, and the average small agglomerated grain size was about 120 nm. Spray pyrolysis-continuous reduction and carbonization technology can ensure that nanocrystalline grain growth inhibitors such as VC, Cr3C2 are dispersed uniformly in the WC–10Co nanocomposite powders. It also provides the ability to deal with a complex starting solution that contains two or more precursor compounds; the concentration of the inhibitors was shown in Table 1. Fig. 2 shows the TEM micrographs of the WC–10Co nanocomposite powders prior to the ball-milling. It confirms that the size of the WC grains is 30 nm, and the grain size distribution of the nanocomposite powder is homogeneous. The grain shape could be classified as irregular. The WC grains are homogeneously dispersed in the cobalt phase, which is very important to obtaining an ultrafine WC–10Co cemented carbide with good mechanical properties, and a fine and homogeneous microstructure. Fig. 3 presents the AFM images of WC–10Co nanocomposite powders prior to the ball-milling. These images clearly show that the nanocrystalline
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powder consisted at this stage of a series of agglomerated particles. The average grain size was about 35 nm, in good agreement with the results of the BET analyzer analysis.
Fig. 4 presents the AFM images of WC–10Co nanocomposite powders ball-milled in acetone for 48 h. As seen in Fig. 4, the WC particles were homogeneously dispersed within the Co phase in the WC–10Co
Fig. 5. AFM images of the WC–10Co nanocomposite powders (a) phase image; (b) 3D image; (c) section analysis.
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365
Fig. 5 (continued).
nanocomposite powders. There were a lot of weak or strong agglomerations in the nanocomposite powders prior to the ball-milling, but the extent to which this was true was much less after ball-milling. This was important with respect to improving the density of the green and sintered specimens. The average grain size after ball-milling was about 30 nm. In order to obtain the best mechanical properties, the ideal structure of cermets should consist of the fine crystalline ceramic phase dispersed evenly in the metallic phase, with the ceramic grains covered by a continuous film of the metallic phase [15]. Fig. 5 shows the AFM images of the WC–10Co nanocomposite powders. Fig. 5 (a) (phase image) agrees with the high resolution transmission electron microscope (HREM) image; according to Fig. 5 (c) (section analysis of the phase image), the average distance between the two adjacent peaks was 2.053 nm which is approximately ten times the 0.2046 nm lattice spacing β-Co(111). Only one phase was visible in the AFM phase image. According to these results, the WC– 10Co nanocomposite powders prepared by the spray pyrolysis-continuous reduction and carbonization process should have special nonmetal–metal nanocomposite structure; i.e., the single WC crystal was covered
by the Co film. The WC–10Co nanocomposite powders prepared in this manner should permit the preparation of sintered specimens with high mechanical properties, and a fine and homogeneous microstructure. Fig. 6 presents the SEM micrographs of the fractured surface of the sintered specimens prepared after being etched by the Murakami solution for 20 s. The average grain size was about 240 nm, and the grain size distribution was homogeneous. As expected from previous studies, steps were present along the WC/Co interfaces in this cemented carbide. Although the Co phase was partially etched, the steps were still noticeable along the WC grain boundaries (Fig. 6 (b)). During the sintering process, small WC grains dissolved due to their higher dissolution potential and reprecipitated after diffusion through the binder of the coarser WC grains. The intensity of this dissolution–reprecipitation process and, therefore the WC growth rate, was increased by the high carbon content and effectively decreased by the presence of the VC, Cr3C2, etc. grain growth inhibitors. The grain growth inhibitors were soluble in the Co binder and segregate at the WC/Co interfaces or as separate particles during sintering. The additives altered the
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Fig. 6. SEM micrographs of the fractured surface of the sintered specimens after being etched for 30 s.
WC/Co interfaces by forming mixed crystals or new phases, reducing the interface energy and therefore the driving force for grain growth. During the densification, WC grain growth was not restricted to liquidphase sintering but also occurred to a remarkable extent as the result of solid-state sintering. This was one of the reasons for the addition of the grain growth inhibitors as early and as uniformly as possible to the WC–Co mixture or to the WC powder, or to cocarburize the WC with the additives. Table 2 shows the properties of the ultrafine WC– Co cemented carbide materials. The specimens prepared by the sinterhip process have high mechanical properties; this is largely attributed to the sintering pressure and temperature during the sinterhip process. The saturated magnetization and density are useful in determining the cobalt and carbon content. The coercive force and the grain size are also a function of the cobalt and carbon content. When the sintering process proceeded from the sintering to the pressing stage, more and more WC was dissolved in the Co binder
phase whereas less and less free carbon content was retained in the specimens. The grain growth and mechanical properties changed accordingly. With solid solution grain growth inhibitors and an improved carbon balance, nearly full theoretical-density specimens were achieved. Fig. 7 shows the AFM images of the fractured surface of the sintered specimens after being polished mechanically without etching. It was difficult to distinguish between the Co phase and the WC phase. A high density of pores, that were the result of pulled-out WC grains, were present after the cutting operation (Fig. 7 (c)). The AFM images of the fractured surface of the sintered specimens after mechanical polishing and etching are shown in Fig. 8. Because the Co phase was partially etched by the Murakami solution, it was easy to distinguish the WC grains. A lot of triple junctions were noted in the sintered specimens. The triple junctions in ultrafine WC–Co cemented carbide were more abundant than those seen in the conventional coarse-grain WC–Co cemented carbide. During the bending of this material, the triple junctions moved and led to the softening of interfaces, this led to an increase in the ductility/toughness of the ultrafine WC–Co material, and improved the macroscopic transverse rupture strength (TRS). As shown in Fig. 9, the minimum WC grain size in the cemented carbide was about 80 nm, the maximum was about 300 nm, and the mean grain size was about 240 nm. Although the Co phase had been etched partially by the Murakami solution, it was easy to determine and find that the mean free path of the Co phase was about 80 nm. It is believed that the ultrafine WC–Co cemented carbides fractured in a brittle manner due to the propagation of intercrystalline cracking. The origin of the fractures appeared to be the small quantity of
Table 2 Properties of ultrafine WC–10Co cemented carbide materials Properties
Value
Measured density (g cm− 3) Density (%TD a) Average grain size (nm) TRS (MPa) Rockwell A hardness (HRA) Vickers hardness (HV1) Saturated magnetization (%) Coercive force (kA m−1) Porosity
14.34 99.48 240 3780 92.8 1918 92 28.8 A02B02C00
a
TD, theoretical density (Calculated value).
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Fig. 7. AFM images of the fractured surface of the sintered specimens after being polished mechanically (a) height image; (b) phase image; (c) 3D image.
micropores and the extraordinary coarse WC grains [11]. The whole fracture course occurred along the interface of the WC and Co phases. A cleavage crack was found in the WC, (large black arrow in Fig. 8 (a, c)). The fracture strength of the ultrafine WC–Co cemented carbide depended on the extent of reinforcement and adhesion between the WC and Co interfaces. However, the segregation of impurity elements to these interfaces, as well as the precipitation of brittle second phases at the interface, impaired the strength of the interface, and consequently increased the tendency for intercrystalline
cracking. This suggests the necessity for exercising reasonable control of the concentrations of the grain inhibitor. 4. Conclusions WC–10Co–0.4VC–0.4Cr3C2 (wt.%) nanocomposite powders were produced by spray pyrolysiscontinuous reduction and carbonization technology. The average grain size of WC in the nanocomposite powders was about 30 nm. The WC–10Co
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Fig. 8. AFM images of the ultrafine WC–10Co cemented carbide after being etched (a) height image; (b) phase image; (c) 3D image.
nanocomposite powders had a special nonmetal–metal nanocomposite structure, and the individual WC crystals were covered by a Co film. The use of this preparation technique ensured that nanocrystalline grain growth inhibitors such as VC, Cr3C2 were uniformly dispersed in the WC–10Co nanocomposite powders; moreover, it was able to deal with the complex starting solution that contained a variety of precursor compounds. The homogeneously dispersed grain growth inhibitors such as VC, Cr3C2 in the nanocomposite WC–10Co powders played an essen-
tial role in obtaining the ultrafine WC–10Co cemented carbide having good mechanical properties, and a finescale homogeneous microstructure. The WC–10Co nanocomposite powders could be sintered by the sinterhip process to 99.48% of the theoretical density of the cemented carbide having an average grain size of about 240 nm. The resulting ultrafine WC–Co cemented carbide combined high strength (TRS 3780 MPa) and high hardness (HRA 92.8, HV1 1918). Control of the carbon and oxygen concentrations was very important in ensuring that the
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Fig. 9. AFM images of the ultrafine WC–10Co cemented carbide after being etched (a) height image; (b) phase image; (c) 3D image.
WC–10Co cemented carbide would have the desired mechanical and microstructural characteristics.
(105123). The authors would like to thank Mr. Wang Tian-guo and Ms. Zhou Fu-rong for the appropriate assistance in the experiment.
Acknowledgments References This work was supported by the Chinese Nature Science Foundation (50502026), Key Project for Science and Technology Development of Wuhan City (20041003068-04), Science Foundation of Wuhan University of Technology (xjj2005166), and the Key Project for the Science and Technology Research of the Chinese Ministry of Education
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[3] Agrawal Dinesh K. Microwave processing of ceramics. Curr Opin Solid State Mater Sci 1998;3(5):480–5. [4] Petersson A, Agren J. Constitutive behaviour of WC–Co materials with different grain size sintered under load. Acta Mater 2004;52(7):1847–58. [5] Kim S, Han SH, Park JK, Kim HE. Variation of WC grain shape with carbon content in the WC–Co alloys during liquid-phase sintering. Scr Mater 2003;48(5):635–9. [6] Gille G, Szesny B, Dreyer K, van den Berg H, Schmidt J, Gestrich T, et al. Submicron and ultrafine grained hardmetals for microdrills and metal cutting inserts. Int J Refract Met Hard Mater 2002;20(1):3–22. [7] Shao GQ, Duan XL, Xie JR, Yu XH, Zhang WF, Yuan RZ. Sintering of nanocrystalline WC–Co composite powder. Rev Adv Mater Sci 2003;5(4):281–6. [8] Upadhyaya GS. Materials science of cemented carbides—an overview. Mater Des 2001;22(6):483–9. [9] Carroll DF. Sintering and microstructural development in WC/ Co-based alloys made with superfine WC powder. Int J Refract Met Hard Mater 1999;17(1–3):123–32. [10] Mi S, Courtney TH. Synthesis of WC and WC–Co cermets by mechanical alloying and subsequent hot isostatic pressing. Scr Mater 1997;38(1):171–6.
[11] Sun JF, Zhang FM, Shen J. Characterizations of ball-milled nanocrystalline WC–Co composite powders and subsequently rapid hot pressing sintering cermets. Mater Lett 2003;57 (21):3140–8. [12] Ma XM, Gang JI. Nanostructured WC–Co prepared by mechanical alloying. J Alloys Compd 1996;245(1–2):L30–2. [13] Ha GH, Kim BK. Synthesis of ultrafine WC/Co powder by mechanochemical process. Powder Metall 2002;45(2):29–32. [14] Fu L, Cao LH, Fan YS. Two-step synthesis of nanostructured tungsten carbide–cobalt powders. Scr Mater 2001;44 (7):1061–8. [15] Zhang ZY, Wahlberg S, Wang MS, Muhammed M. Processing of nanostructured WC–Co powders from precursor obtained by co-precipitation. Nanostruct Mater 1999;12(1–4):163–6. [16] Shao GQ, Duan XL, Wu BL, Xie JR, Wei MK, Yuan RZ. In: Singh JP, Bansal NP, Ustundag E, editors. Continuous Reduction Carburisation Mechanism of Precursor-Derived Nanocrystalline WC–Co, Advances in Ceramic Matrix Composites VI. Ohio, USA: The American Ceramic Society; 2000. p. 207–17.
Characterizations of WC–10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide X.L. Shi ⁎, G.Q. Shao, X.L. Duan, Z. Xiong, H. Yang State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122Luoshi Road, Wuhan 430070, P.R. China Received 4 January 2006; accepted 28 February 2006
Abstract Ultrafine WC–Co cemented carbides, combining high hardness and high toughness, are expected to find broad applications. In this study, WC–10Co–0.4VC–0.4Cr3C2 (wt.%) nanocomposite powders, whose average grain size was about 30 nm, were fabricated by spray pyrolysis-continuous reduction and carbonization technology. The as-prepared nanocomposite powders were characterized and analyzed by chemical methods, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis and atomic force microscopy (AFM). Furthermore, “sinterhip” was used in the sintering process, by which ultrafine WC–10Co cemented carbides with an average grain size of 240 nm were prepared. The material exhibited high Rockwell A hardness of HRA 92.8, Vickers hardness HV1 1918, and transverse rapture strength (TRS) of 3780 MPa. The homogeneously dispersed grain growth inhibitors such as VC, Cr3C2 in nanocomposite powder and the special nonmetal–metal nanocomposite structure of WC–10Co nanocomposite powder played very important roles in obtaining ultrafine WC–10Co cemented carbide with the desired properties and microstructure. There was an abundance of triple junctions in the ultrafine WC–10Co cemented carbide; these triple junctions endowed the sintered specimen with high mechanical properties. © 2006 Elsevier Inc. All rights reserved. Keywords: WC–10Co; Nanocomposite powders; Atomic force microscope (AFM); Sinterhip; Ultrafine cemented carbide
1. Introduction The current trend in the hardmetals industry to finer and finer-grained alloys has put high demands on the manufacturing process, for both powders and alloys [1– 3]. These applications include use as abrasives, cutting tools, saw blade tips, milling cutters, dies, inserts in valve stems, sand blast nozzles and other wear-resistant components owing to the high hardness and wearresistant properties. The driving force behind this development was the considerably improved perfor⁎ Corresponding author. Tel./fax: +86 27 87216912. E-mail address: [email protected] (X.L. Shi). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.02.013
mance of these ultrafine hardmetals that in many applications clearly exceeded the performance of hardmetals with WC-grains greater than 1 μm [4–9]. As a result of an extremely fine microstructure, nanostructured materials have long been recognized to have remarkable and technologically attractive properties. Ultrafine grains can endow materials with improved hardness and strength, as anticipated from the wellknown empirical Hall–Petch relationship; moreover, the increased volume fraction of grain boundaries may enhance the toughness and ductility of the materials. This turns out to be one of the driving forces for the fast growing technologies for synthesizing nanophase or ultrafine WC–Co composite powders [10].
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Table 1 Composition of the WC–10Co nanocomposite powders Total carbon content (wt.%)
Free carbon content (wt.%)
Oxygen content (wt.%)
Surface area (m2 g− 1)
Co content (wt.%)
VC/Cr3C2 content (wt.%)
5.48
0.16
0.20
11.7800
10.05
0.40/0.40
Many methods have been developed to synthesize nanophase or ultrafine WC–Co powder, such as mechanical alloying [11,12], mechanochemical processing [13], two-step processing [14], and co-precipitation methods [15]. However, there are some limitations to these methods. For example, impurities may be introduced in the mechanical alloying and other approaches. WC–Co nanocomposite powders have been fabricated by spray pyrolysis-continuous reduction and carbonization technology [16]. The WC particle sizes can be reduced below 20 nm, making possible the manufacture of ultrafine or even nanocrystalline WC– Co cemented carbide with excellent properties. These techniques can assure that nanocrystalline grain growth inhibitors such as VC, Cr3C2 disperse uniformly in
WC–10Co nanocomposite powders, and permit the use of complex starting solutions that contains two or more precursor compounds. The present study was focused on characterizing WC–10Co nanocomposite powders prepared by spray pyrolysis-continuous reduction and carbonization technology. Furthermore, “sinterhip” was used in the subsequent sintering process to produce ultrafine WC– 10Co cemented carbide with high mechanical properties, fine and homogeneous microstructure.
Fig. 1. SEM photographs of the WC–10Co nanocomposite powders.
Fig. 2. TEM photographs of the WC–10Co nanocomposite powders.
2. Experimental WC–10Co (wt.%) nanocomposite powders produced by spray pyrolysis-continuous reduction and
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carbonization technology were used for this study. The powder was ball-milled in acetone for 48 h. After milling it was dried at 90 °C in a vacuum oven. The
green compacts were consolidated by sinterhip at 1320 °C for 60 min with an Ar pressure of 5.5 MPa. In order to improve the carbon content, some carbon
Fig. 3. AFM images of the WC–10Co nanocomposite powder (a) height image; (b) 3D image; (c) section analysis; (d) average grain size.
X.L. Shi et al. / Materials Characterization 57 (2006) 358–370
361
Fig. 3 (continued).
powders were placed beside the specimens during this operation. The composition of the nanocomposite powder was analyzed by chemical methods. The
shape and particle size of starting WC–10Co nanocomposite powders were characterized by a JSM5610LV scanning electron microscope (SEM) operated
362
X.L. Shi et al. / Materials Characterization 57 (2006) 358–370
at 15 kV, a JEM-2010 transmission electron microscope (TEM) operated at 200 kV and an atomic force microscope (AFM) (Digital Instruments NanoScope?,
VEECO company, USA)(tapping mode). The particle size was also characterized by a BET analyzer. Sintered specimens had the dimensions 20 × 6.5 ×
Fig. 4. AFM images of the WC–10Co nanocomposite powders (a) height image; (b) 3D image; (c) section analysis.
X.L. Shi et al. / Materials Characterization 57 (2006) 358–370
363
Fig. 4 (continued).
5.25 mm. The density was determined according to ISO3369-1975. The Vickers hardness (HV) (@1 kg), transverse rupture strength (TRS), and Rockwell A hardness (HRA) of the sintered specimens were measured. The saturation magnetization was measured by using a saturation induction measuring system, and the coercivity force was measured by a FörsterKoerzimat 1.095 system. The specimens were sawed into specimens with a size of 5.25 × 6.5 × 2.25 mm by a diamond saw. The fractured surfaces of the specimens were polished mechanically with emery papers down to 1200 grade, and then with 0.05 μm wet polishing diamond pastes. The polished samples were etched in a Murakami solution [K3Fe(CN)6 10 g, NaOH 10 g, and distilled water 100 g] for 20 s. 3. Results and discussion The composition of the nanocomposite powders is listed in Table 1. Carbon content was controlled with a concentration higher than the theoretical value, taking into account the oxygen concentration. The specific surface area of the WC–10Co nanocomposite powders was 11.7800 m2·g− 1 and the equivalent mean particle size was about 30 nm. Fig. 1 shows the SEM micrographs of the WC–10Co nanocomposite pow-
ders prior to being ball-milled. The powder is seen to consist of big agglomerated particles of 1–5 μm in diameter. The big nanocomposite powders consisted of small agglomerated particles, and the average small agglomerated grain size was about 120 nm. Spray pyrolysis-continuous reduction and carbonization technology can ensure that nanocrystalline grain growth inhibitors such as VC, Cr3C2 are dispersed uniformly in the WC–10Co nanocomposite powders. It also provides the ability to deal with a complex starting solution that contains two or more precursor compounds; the concentration of the inhibitors was shown in Table 1. Fig. 2 shows the TEM micrographs of the WC–10Co nanocomposite powders prior to the ball-milling. It confirms that the size of the WC grains is 30 nm, and the grain size distribution of the nanocomposite powder is homogeneous. The grain shape could be classified as irregular. The WC grains are homogeneously dispersed in the cobalt phase, which is very important to obtaining an ultrafine WC–10Co cemented carbide with good mechanical properties, and a fine and homogeneous microstructure. Fig. 3 presents the AFM images of WC–10Co nanocomposite powders prior to the ball-milling. These images clearly show that the nanocrystalline
364
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powder consisted at this stage of a series of agglomerated particles. The average grain size was about 35 nm, in good agreement with the results of the BET analyzer analysis.
Fig. 4 presents the AFM images of WC–10Co nanocomposite powders ball-milled in acetone for 48 h. As seen in Fig. 4, the WC particles were homogeneously dispersed within the Co phase in the WC–10Co
Fig. 5. AFM images of the WC–10Co nanocomposite powders (a) phase image; (b) 3D image; (c) section analysis.
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365
Fig. 5 (continued).
nanocomposite powders. There were a lot of weak or strong agglomerations in the nanocomposite powders prior to the ball-milling, but the extent to which this was true was much less after ball-milling. This was important with respect to improving the density of the green and sintered specimens. The average grain size after ball-milling was about 30 nm. In order to obtain the best mechanical properties, the ideal structure of cermets should consist of the fine crystalline ceramic phase dispersed evenly in the metallic phase, with the ceramic grains covered by a continuous film of the metallic phase [15]. Fig. 5 shows the AFM images of the WC–10Co nanocomposite powders. Fig. 5 (a) (phase image) agrees with the high resolution transmission electron microscope (HREM) image; according to Fig. 5 (c) (section analysis of the phase image), the average distance between the two adjacent peaks was 2.053 nm which is approximately ten times the 0.2046 nm lattice spacing β-Co(111). Only one phase was visible in the AFM phase image. According to these results, the WC– 10Co nanocomposite powders prepared by the spray pyrolysis-continuous reduction and carbonization process should have special nonmetal–metal nanocomposite structure; i.e., the single WC crystal was covered
by the Co film. The WC–10Co nanocomposite powders prepared in this manner should permit the preparation of sintered specimens with high mechanical properties, and a fine and homogeneous microstructure. Fig. 6 presents the SEM micrographs of the fractured surface of the sintered specimens prepared after being etched by the Murakami solution for 20 s. The average grain size was about 240 nm, and the grain size distribution was homogeneous. As expected from previous studies, steps were present along the WC/Co interfaces in this cemented carbide. Although the Co phase was partially etched, the steps were still noticeable along the WC grain boundaries (Fig. 6 (b)). During the sintering process, small WC grains dissolved due to their higher dissolution potential and reprecipitated after diffusion through the binder of the coarser WC grains. The intensity of this dissolution–reprecipitation process and, therefore the WC growth rate, was increased by the high carbon content and effectively decreased by the presence of the VC, Cr3C2, etc. grain growth inhibitors. The grain growth inhibitors were soluble in the Co binder and segregate at the WC/Co interfaces or as separate particles during sintering. The additives altered the
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Fig. 6. SEM micrographs of the fractured surface of the sintered specimens after being etched for 30 s.
WC/Co interfaces by forming mixed crystals or new phases, reducing the interface energy and therefore the driving force for grain growth. During the densification, WC grain growth was not restricted to liquidphase sintering but also occurred to a remarkable extent as the result of solid-state sintering. This was one of the reasons for the addition of the grain growth inhibitors as early and as uniformly as possible to the WC–Co mixture or to the WC powder, or to cocarburize the WC with the additives. Table 2 shows the properties of the ultrafine WC– Co cemented carbide materials. The specimens prepared by the sinterhip process have high mechanical properties; this is largely attributed to the sintering pressure and temperature during the sinterhip process. The saturated magnetization and density are useful in determining the cobalt and carbon content. The coercive force and the grain size are also a function of the cobalt and carbon content. When the sintering process proceeded from the sintering to the pressing stage, more and more WC was dissolved in the Co binder
phase whereas less and less free carbon content was retained in the specimens. The grain growth and mechanical properties changed accordingly. With solid solution grain growth inhibitors and an improved carbon balance, nearly full theoretical-density specimens were achieved. Fig. 7 shows the AFM images of the fractured surface of the sintered specimens after being polished mechanically without etching. It was difficult to distinguish between the Co phase and the WC phase. A high density of pores, that were the result of pulled-out WC grains, were present after the cutting operation (Fig. 7 (c)). The AFM images of the fractured surface of the sintered specimens after mechanical polishing and etching are shown in Fig. 8. Because the Co phase was partially etched by the Murakami solution, it was easy to distinguish the WC grains. A lot of triple junctions were noted in the sintered specimens. The triple junctions in ultrafine WC–Co cemented carbide were more abundant than those seen in the conventional coarse-grain WC–Co cemented carbide. During the bending of this material, the triple junctions moved and led to the softening of interfaces, this led to an increase in the ductility/toughness of the ultrafine WC–Co material, and improved the macroscopic transverse rupture strength (TRS). As shown in Fig. 9, the minimum WC grain size in the cemented carbide was about 80 nm, the maximum was about 300 nm, and the mean grain size was about 240 nm. Although the Co phase had been etched partially by the Murakami solution, it was easy to determine and find that the mean free path of the Co phase was about 80 nm. It is believed that the ultrafine WC–Co cemented carbides fractured in a brittle manner due to the propagation of intercrystalline cracking. The origin of the fractures appeared to be the small quantity of
Table 2 Properties of ultrafine WC–10Co cemented carbide materials Properties
Value
Measured density (g cm− 3) Density (%TD a) Average grain size (nm) TRS (MPa) Rockwell A hardness (HRA) Vickers hardness (HV1) Saturated magnetization (%) Coercive force (kA m−1) Porosity
14.34 99.48 240 3780 92.8 1918 92 28.8 A02B02C00
a
TD, theoretical density (Calculated value).
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Fig. 7. AFM images of the fractured surface of the sintered specimens after being polished mechanically (a) height image; (b) phase image; (c) 3D image.
micropores and the extraordinary coarse WC grains [11]. The whole fracture course occurred along the interface of the WC and Co phases. A cleavage crack was found in the WC, (large black arrow in Fig. 8 (a, c)). The fracture strength of the ultrafine WC–Co cemented carbide depended on the extent of reinforcement and adhesion between the WC and Co interfaces. However, the segregation of impurity elements to these interfaces, as well as the precipitation of brittle second phases at the interface, impaired the strength of the interface, and consequently increased the tendency for intercrystalline
cracking. This suggests the necessity for exercising reasonable control of the concentrations of the grain inhibitor. 4. Conclusions WC–10Co–0.4VC–0.4Cr3C2 (wt.%) nanocomposite powders were produced by spray pyrolysiscontinuous reduction and carbonization technology. The average grain size of WC in the nanocomposite powders was about 30 nm. The WC–10Co
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Fig. 8. AFM images of the ultrafine WC–10Co cemented carbide after being etched (a) height image; (b) phase image; (c) 3D image.
nanocomposite powders had a special nonmetal–metal nanocomposite structure, and the individual WC crystals were covered by a Co film. The use of this preparation technique ensured that nanocrystalline grain growth inhibitors such as VC, Cr3C2 were uniformly dispersed in the WC–10Co nanocomposite powders; moreover, it was able to deal with the complex starting solution that contained a variety of precursor compounds. The homogeneously dispersed grain growth inhibitors such as VC, Cr3C2 in the nanocomposite WC–10Co powders played an essen-
tial role in obtaining the ultrafine WC–10Co cemented carbide having good mechanical properties, and a finescale homogeneous microstructure. The WC–10Co nanocomposite powders could be sintered by the sinterhip process to 99.48% of the theoretical density of the cemented carbide having an average grain size of about 240 nm. The resulting ultrafine WC–Co cemented carbide combined high strength (TRS 3780 MPa) and high hardness (HRA 92.8, HV1 1918). Control of the carbon and oxygen concentrations was very important in ensuring that the
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Fig. 9. AFM images of the ultrafine WC–10Co cemented carbide after being etched (a) height image; (b) phase image; (c) 3D image.
WC–10Co cemented carbide would have the desired mechanical and microstructural characteristics.
(105123). The authors would like to thank Mr. Wang Tian-guo and Ms. Zhou Fu-rong for the appropriate assistance in the experiment.
Acknowledgments References This work was supported by the Chinese Nature Science Foundation (50502026), Key Project for Science and Technology Development of Wuhan City (20041003068-04), Science Foundation of Wuhan University of Technology (xjj2005166), and the Key Project for the Science and Technology Research of the Chinese Ministry of Education
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