Microstructure and properties of ultrafine cemented carbides—Differences in spark plasma sintering and sinter-HIP

Microstructure and properties of ultrafine cemented carbides—Differences in spark plasma sintering and sinter-HIP

Materials Science and Engineering A 552 (2012) 427–433 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering A journa...

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Materials Science and Engineering A 552 (2012) 427–433

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea

Microstructure and properties of ultrafine cemented carbides—Differences in spark plasma sintering and sinter-HIP C.B. Wei, X.Y. Song ∗ , J. Fu, X.M. Liu, Y. Gao, H.B. Wang, S.X. Zhao College of Materials Science and Engineering, Key Lab of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100124, China

a r t i c l e

i n f o

Article history: Received 23 November 2011 Received in revised form 16 March 2012 Accepted 22 May 2012 Available online 1 June 2012 Keywords: Tungsten cemented carbides Spark plasma sintering Sinter-HIP Orientation relationship Mechanical properties

a b s t r a c t The spark plasma sintering (SPS) and hot isostatic pressing (sinter-HIP) were taken as the representative methods of rapid sintering and liquid-state sintering technologies, respectively, to fabricate the WC–Co cemented carbides. The microstructures and properties of the bulk materials prepared by the two techniques were characterized and compared systematically. It was demonstrated that the big difference in mechanical properties of the cemented carbides prepared by SPS and sinter-HIP depends on the configuration of WC and Co phases and the WC/Co orientation relationship, which result from the intrinsic features of the sintering technologies. The mechanisms for the excellent properties obtained in the cemented carbides prepared by sinter-HIP were proposed, based on which the favorable microstructures and optimized processing methods concerning liquid-state sintering may be developed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Owing to the cooperation of the hard WC phase and the soft Co binder phase, the cemented tungsten carbides have unique good combination of high hardness and transverse rupture strength (TRS) and moderate fracture toughness [1]. It was proposed that significant improvement in the mechanical properties could be achieved in the cemented carbides with finer grain sizes. With the increase of the mechanical properties, the finegrained cemented carbides find increasingly wider applications in industries, such as tips of cutting and drilling tools, extrusion and pressing dies and wear-resistant surfaces in many types of machines [2–4]. As reported in the literature, the cemented carbides were fabricated mainly by two kinds of technologies, one is the liquid-state sintering, such as vacuum sintering (VS) and hot isostatic pressing (sinter-HIP) [5,6]; the other is the recently developed rapid sintering, such as microwave sintering (MS) [7], ultrahigh pressure rapid hot consolidation (UPRC) [8] and spark plasma sintering (SPS) [9]. The VS and sinter-HIP techniques have advantages of low cost and large-scaled production and have been widely used for sintering cemented carbides in industries. They have disadvantage in controlling the grain growth due to the slow heating rate and

∗ Corresponding author. Tel.: +86 10 67392311; fax: +86 10 67392311. E-mail address: [email protected] (X.Y. Song). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.05.065

high sintering temperature, and thus have big difficulty to obtain the ultrafine and nanocrystalline cemented carbides. Rapid sintering methods are favorable to obtain the ultrafine grain structure of cemented carbides, owing to their advantages of high heating rate, relatively lower sintering temperature, and very short holding time. However, the WC–Co bulk materials prepared by the rapid sintering methods generally have lower mechanical properties as compared with those prepared by the liquid-state sintering methods, especially with respect to the fracture toughness and TRS [2,10]. Currently, the rapid sintering methods are mainly used in laboratory studies, it is still a big challenge for them to attain a large-scale production. In contrast to a number of publications in which the liquid-state and rapid sintering methods were compared in the respect of material properties, studies on the mechanisms for the difference, particularly on the essential difference in the microstructural characteristics and its effect on the mechanical properties of the bulk material, have been rarely reported in the literature. In the present study, we choose two kinds of techniques, SPS and sinter-HIP, as the representatives of the rapid sintering methods and liquid-state, respectively, to fabricate the WC–Co cemented carbides. The microstructures and various properties of the bulk materials prepared by the two techniques are characterized for comparison. The aim is to disclose the mechanisms for the differences in the microstructural characteristics and mechanical properties of the WC–Co bulk materials prepared by liquid-state and rapid sintering methods.

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2. Experimental 2.1. Preparation of WC–Co composite powder and bulk materials The WC–10 wt.%Co bulk is the target material in the present study. Firstly, the WC–Co composite powder was synthesized by a rapid route of in situ reduction and carbonization reactions of metallic oxides and carbon [11,12]. Then the synthesized composite powder was sintered by SPS and sinter-HIP, respectively, to prepare the WC–Co bulk materials. The detailed procedures are described as follows. The blue tungsten oxide (WO2.9 ) powder with 99.5% purity, cobalt (II, III) oxide (Co3 O4 ) powder with 98.5% purity and carbon black powder with 99.5% purity were used as the raw materials. The 0.2 wt.% vanadium carbide (VC) and 0.8 wt.% chromium carbide(Cr3 C2 ) were added to the raw powders as the grain growth inhibitor. The raw powders were mixed and ball milled for 20 h in a planetary mill (Planetary Milling 20 L) using pure ethanol as the liquid medium. Subsequently, the as-milled powder mixture was put into the vacuum furnace and heated up to 1000 ◦ C with a heating rate of 15 ◦ C/min and held for 3 h at this temperature to perform the in situ reduction and carbonization reactions. The synthesized WC–Co composite powder was divided into two portions. One was put into the graphite die and consolidated by the SPS technique. The processing parameters were taken from our previous studies [9,13], with a sintering temperature of 1160 ◦ C, a heating rate of 100 ◦ C/min, a constant pressure of 60 MPa and a holding time of 10 min. The other portion was mixed with 1.5 wt.% polyethylene glycol (PEG 1500), which was used as the forming binder, and compressed to form a number of compacts. The parameters for the sinter-HIP process were optimized with reference to the experimental reports in the literature [10,12]. The green compacts were dewaxed in vacuum at 400 ◦ C for 60 min. Then they were densified by the sinter-HIP technique with a heating rate of 8 ◦ C/min and sintering at 1420 ◦ C for 60 min, during this period there was an isothermal holding for 30 min which was performed in the Ar atmosphere with a gas pressure of 2 MPa.

2.2. Characterizations of composite powder and bulk materials The carbon content in the synthesized WC–Co composite powder was evaluated by the standard combustion analysis (LECO CS200). The composition of the composite powder was examined by the chemical analyses. The phases in the sintered bulk specimens were detected by X-ray diffraction (XRD) with Cu K␣ radiation. The morphology of the composite powder and the grain structure of the sintered bulk were observed by the field-emission scanning electron microscope (SEM). The WC grain sizes were measured with the linear intercept method performed on a series of SEM images. The microstructure details were studied by the transmission electron microscopy (TEM) and high resolution TEM (HRTEM), which were carried out with the JEOL JEM-3010 operated at 300 kV. The density of the sintered bulk specimens was measured by the Archimedes method. The hardness was measured by the Vickers hardness tester with a load of 30 kg according to the ISO-3878 standard. The fracture toughness was determined based on the measurements on the length of cracks generated by the Vickers indentation and calculations with the equation KIC = 0.0028 (Hv P/L)1/2 [14], where Hv is the indentation hardness, P is the indentation load, and L is the total crack length. The TRS was measured on the specimens with dimensions of 20 mm × 6.5 mm × 5.25 mm according to the standard of ISO 3327:2009. At least five specimens were prepared for the measurements of each property.

Fig. 1. SEM image of the morphology and the statistical particle size distribution of the in situ synthesized WC–Co composite powder. Table 1 Composition analysis of the WC–10 wt.%Co composite powder (wt.%). W

Co

Total carbon

Free carbon

Oxygen

83.76

9.93

5.61

0.19

0.19

3. Results and discussions 3.1. Submicron WC–Co composite powder The morphology of the in situ synthesized WC–Co composite powder is shown in Fig. 1. It is observed that the powder particles are well dispersed and have a homogeneous size distribution mainly in a narrow range of 150–250 nm (see inset in Fig. 1). The mean particle size of the composite powder is about 190 nm. It indicates that the WC–Co composite powder with the submicron-scale particle size was obtained by the in situ reduction and carbonization reactions of the tungsten oxide, cobalt oxide and carbon. The composition analysis of the elemental contents in the WC–10 wt.%Co composite powder is shown in Table 1. The atomic ratio of W and Co is close to the stoichiometric WC:Co = 9:1. The total carbon content in the composite powder is 5.61 wt.%, which is a little higher than the theoretical stoichiometry of 5.52 wt.%. The contents of the free carbon and the oxygen are at a low level. Due to the co-existence of the free carbon and oxygen in the composite powder, in the subsequent sintering process, it is possible to get rid of the impurity through the reaction of carbon and oxygen. Therefore, the existence of the free carbon at an appropriate content in the composite powder is favorable to obtain the pure phase constitution in the final cemented carbides bulk. 3.2. Phase constitution and microstructure of sintered WC–Co bulk Fig. 2 shows the XRD analysis of the phase constitution in the sintered WC–Co bulk specimens prepared by SPS and sinter-HIP, respectively. It is found that the SPSed bulk specimen contains the ␩ (Co6 W6 C) phase in addition to the main phases of WC and Co. In contrast, in the specimen prepared by sinter-HIP, pure phase constitution of only WC and Co phases is achieved, no impurity phases such as ␩ phase and W2 C are observed.

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900





800

WC



Co



700

Intensity ( a.u.)



η

600 ■

500 400





♦▲







SPS



300 200 100

Sinter-HIP

0 25 30 35 40 45 50 55 60 65 70 75 80 85

2θ ( deg.) Fig. 2. XRD analysis of phase constitutions in the WC–Co bulks sintered by SPS and sinter-HIP.

The formation of ␩ phase in the SPSed bulk is considered to be caused by the feature of the sintering method. In the early stage, due to the rapid heating rate, the reaction of the free carbon and oxygen existing in the composite powder is not sufficient. Moreover, during the solid-state sintering densification of SPS with relatively low sintering temperature and short holding time, the free carbon diffuses slowly within the solid-state sintering material. Consequently, local carbon shortage occurs in the composite powder, leading to the formation of the ␩ phase [15,16]. However, in the process of sinter-HIP with a much slower heating rate and a higher sintering temperature, the free carbon in the composite powder can react with oxygen or the intermediate phase during the liquid-state sintering, in which the atomic diffusion is accelerated. Therefore, a pure phase constitution of WC and Co can be obtained in the bulk specimen prepared by sinter-HIP. The microstructure morphologies of the bulk specimens prepared by SPS and sinter-HIP are shown in Fig. 3. As seen from the SEM image in Fig. 3(a), the SPSed specimen has a finer grain structure than the specimen prepared by sinter-HIP. The WC grain sizes distribute statistically in a range of 0.25–0.45 ␮m (see inset in Fig. 3(a)) and the mean grain size is about 0.35 ␮m. With the energy dispersive X-ray spectroscopy (EDX) analysis, the ␩ phase is distinguished in the microstructure of the WC–Co bulk, as indicated by the arrows in Fig. 3(a). The finding confirms the XRD analysis of the phase constitution in the SPSed specimen (see Fig. 2). In the specimen prepared by sinter-HIP, a large fraction of grains have sizes in a range of 0.3–0.7 ␮m (see inset in Fig. 3(b)), with a mean grain size of about 0.50 ␮m. Apparently, the WC–Co bulk prepared by sinter-HIP has a coarser grain structure than the SPSed bulk.

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The configurations of the WC grains and the Co binder phase are studied by TEM analyses for the bulk specimens prepared by SPS and sinter-HIP, respectively, as shown in Figs. 4 and 5. For the SPSed specimen, it is seen that the WC grains mainly have a multiangular shape Fig. 4(a), which is reserved from the grain structure of the composite powder. However, in the specimen prepared by sinter-HIP, the WC grains typically have prismatic and trigonal shapes (Fig. 5(a), (c)), due to the more sufficient development during the liquid-state sintering. Moreover, in the SPSed specimen, the Co phase distributes massively in the matrix of fine WC grains, as demonstrated by the dark-field images of Co phase in Fig. 4(a). The selected area diffraction pattern (SADP) at the WC/Co phase boundary (marked by the white square) is shown in Fig. 4(b). The indexing indicates that the neighboring WC and Co phases do not show a definite orientation relationship. The microstructure details of the specimen prepared by sinterHIP are shown in Fig. 5. It is observed that the Co phase distributes in a characteristic shape of thin layers or films in the matrix of the WC grains (Fig. 5(a)), which are coarser than those in the SPSed specimen. This is more clearly exhibited by the dark-field image of Co phase in Fig. 5(b). It seems that the Co phase forms a near-network structure among the WC grains. As compared with the SPSed specimen, the contiguity of WC grains in the specimen prepared by sinter-HIP is remarkably reduced, which will be favorable for the fracture toughness and TRS of the cemented carbides. An example of the local configuration of WC grains and Co phase is shown in Fig. 5(c), with the SADPs and the indexing of the respective WC and Co phases, showing that both phases have the hexagonal crystal structure. Further, the HRTEM image for the local enlargement of the configuration of WC and Co phases is shown in Fig. 5(d). It is observed that the WC and Co lattices match well at the WC/Co phase boundary, and obvious lattice distortions are not found. The SADP at the WC/Co phase boundary (marked by the white square in Fig. 5(c)) and the indexing indicate that the neighboring WC and Co phases have distinct orientation relationship, which are [1 1 1]Co ||[0 1 0]WC , (0 1 −1)Co || (1 0 −1)WC. It is considered that the WC/Co orientation relationship forms during the solution–precipitation process [17] that takes place in the liquid-state sintering. The combined characteristics of the Co network distribution and the WC/Co orientation relationship are the favorable microstructure state for the good mechanical properties of the bulk material.

3.3. Properties of sintered WC–Co bulk The measured densities of a group of 5 specimens are displayed in Fig. 6, with the comparison between SPS and sinter-HIP. The mean values of densities of the specimens prepared by SPS and

Fig. 3. SEM images and WC grain size distributions of the WC–Co bulk specimens prepared by (a) SPS and (b) sinter-HIP.

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Fig. 4. TEM analyses of microstructure details of SPSed WC–Co bulk: (a) Configuration of WC grains and Co binder phase and (b) SADP and its indexing for the WC/Co phase boundary.

sinter-HIP are 14.54 g/cm3 and 14.50 g/cm3 , respectively. The existence of ␩ phase in the SPSed specimen leads to a higher density of the WC–Co bulk than the specimen prepared by sinter-HIP, which has pure phase constitution of WC and Co in the bulk. Fig. 7 shows the comparison of mechanical properties of the specimens prepared by SPS and sinter-HIP, respectively. As seen in Fig. 7(a) and (b), the SPSed specimens have higher hardness but lower fracture toughness, with mean values of 1707 kg/mm2 and 12.1 MPa m1/2 , respectively. However, the specimens prepared by sinter-HIP show an excellent combination of the mean hardness as 1543 kg/mm2 and the mean fracture toughness as 13.6 MPa m1/2 . The ultrafine grain size in the SPSed specimen results in a higher hardness. However, due to the very fine WC grain size and the massive distribution of the Co binder phase in the matrix, the fracture toughness is reduced. Furthermore, the existence of ␩ phase in the SPSed specimen is harmful to the fracture toughness of the bulk material. A more notable difference is found for the TRS of the specimens prepared by SPS and sinter-HIP, as shown in Fig. 7(c). It is seen that the TRS of the specimens prepared by sinter-HIP is drastically increased with respect to that of the SPSed specimens. The mean TRS of the specimens prepared by sinter-HIP is measured

as 4210 MPa, and the highest value achieves 4400 MPa. The SPSed specimens have a mean TRS of 2337 MPa and a highest value of 2478 MPa. Thus, the TRS of the specimens prepared by sinter-HIP is about 80% higher than that of the SPSed specimens. TRS is a critical mechanical property of the cemented carbides, and it is one of the most frequently used property parameters that characterize the applications of the cemented carbides in industry. The TRS property is sensitive to the porosity, phase constitution and microstructure characteristics. In the condition that a full relative density is obtained for the bulk material, the pure phase constitution of only WC and Co phases and the homogeneous and suitable configuration between WC and Co phases are dominant for the high TRS of the WC–Co bulk. TRS can be considered as a combined performance of the hardness and fracture toughness [2,18]. Generally, while the hardness increases with the decrease of WC grain size, the fracture toughness decreases when the WC grain size is decreased from the micron scale to the ultrafine (submicron) scale. However, this tendency depends strongly on the evolution of the Co distribution with the decrease of WC grain size. It means that at an appropriate grain size level, the optimized combination of hardness and fracture toughness can be achieved.

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Fig. 5. TEM analyses of microstructure details of WC–Co bulk prepared by sinter-HIP: (a) bright-field image of the microstructure, (b) dark-field image for the configuration of WC grains and Co binder phase, (c) local enlargement of the microstructure, with SADP and its indexing showing the orientations of Co and WC respectively, and (d) HRTEM image of the configuration of WC and Co phases, with SADP and its indexing indicating the WC/Co orientation relationship.

Compare the specimens prepared by SPS and sinter-HIP, except the phase constitution, the Co distribution plays a significant role in the property of TRS [19]. The high TRS of the specimens prepared by sinter-HIP are attributed to the advantages of the sintering technology, which generates a suitable microstructure in the densified

bulk material. In the sinter-HIP process, the pressure is impacted through the Ar gas at the later stage of sintering densification. The gas pressure is advantageous in respect of the equivalent balance in any directions. As a result, when the gas pressure is applied in the later stage of liquid-state sintering, the flow of the melted Co

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14.58 SPS Sinter-HIP

14.54

3

Density(g/cm )

14.56

14.52 14.50 14.48 14.46 14.44 14.42 14.40

1

2

3

4

5

Specimens Fig. 6. Comparison of densities of WC–Co bulk specimens prepared by SPS and sinter-HIP.

2

Hv30 ( kg/mm )

1800

(a)

SPS Sinter-HIP

1700

1600

1500

1400 1

2

3

4

5

will be accelerated, and the homogeneity of Co distribution will be improved effectively. This facilitates the homogeneous threedimensional distribution of Co phase among WC grains, which forms a Co network in a form of thin layers or films along the WC grain boundaries. Moreover, due to the solution-precipitation mechanism [20] occurred during the liquid-state sintering, the WC/Co orientation relationship forms at the phase boundaries. The good matching of WC and Co lattices at the contact regions reduces the probability of the formation and propagation of the microcracks, thus increases the strength of the WC/Co phase boundaries. As a result, the TRS of the bulk material is enhanced. In particular, as compared with the reports in the literature on the TRS of the WC–10 wt.%Co bulk materials prepared by various methods [2,10,21], the TRS of the present specimens prepared by sinter-HIP has achieved a fairly high level, with all the measured values higher than 4000 MPa. SPS technique is distinctly advantageous for preparing cemented carbides with ultrafine grain sizes and even nanocrystalline structure, due to its intrinsic features of rapid heating rate, low sintering temperature and short holding time. However, in the solid-state sintering process of SPS, the atomic diffusion cannot proceed sufficiently in a short time. Moreover, the Co phase tends to have a massive distribution in the matrix due to its low liquidity. Consequently, the discontinuous Co blocks appear in the microstructure due to the absence of re-distribution. Therefore, the cemented carbides prepared by SPS may have relatively higher hardness owing to the fine grain size, however, may not have high TRS. To the best knowledge of the authors, the highest TRS reported in the literature for the SPSed WC–10 wt.%Co bulk materials is 3100 MPa [22]. In comparison, the cemented carbides prepared by sinter-HIP have remarkably enhanced mechanical properties.

Fracture toughness (MPa⋅m1/2)

Specimens 16 15

4. Conclusions

(b)

SPS Sinter-HIP

14 13 12 11 10 9 8

1

2

3

4

5

Specimens 5000

(c)

SPS Sinter-HIP

4500 4000

TRS( MPa)

In the present study, using SPS and sinter-HIP as the representative methods of the rapid sintering and liquid-state sintering technologies, respectively, we prepared the cemented carbides with the in situ synthesized WC–Co composite powder as the sintering material. The microstructures and various properties of the bulk materials prepared by the two techniques were characterized and compared systematically. The following conclusions are drawn from the investigations:

3500 3000 2500 2000 1500 1

2

3

4

5

Specimens Fig. 7. Comparison of mechanical properties of WC–Co bulk specimens prepared by SPS and sinter-HIP: (a) Vickers hardness, (b) fracture toughness, and (c) TRS.

(1) In the SPSed cemented carbides, the ␩ (Co6 W6 C) phase formed in addition to the main phases of WC and Co. In spite of the ultrafine grain size, the Co phase distributes massively in the matrix due to the insufficient diffusion in the solid-state sintering. In contrast, in the cemented carbides prepared by sinter-HIP, the phase constitution only contains WC and Co. In the bulk material prepared by sinter-HIP, the Co phase distributes homogeneously in a form of network among the WC grains. (2) Distinct WC/Co orientation relationship exists in the cemented carbides prepared by sinter-HIP. However, in the SPSed cemented carbides the orientation relationship between WC and Co phases is very rarely found. The formation of the orientation relationship is a result of solution–precipitation occurred during the liquid-state sintering. (3) As compared with the SPSed cemented carbides, the material prepared by sinter-HIP has a decrease by ∼10% in the hardness, however, a remarkably increased TRS, which has a mean value of 4210 MPa. The excellent mechanical properties are attributed to the network distribution of Co phase and the WC/Co orientation relationship, which result from the features of liquid-state sintering.

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