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Relationship between hardness and fracture toughness in WC–FeAl composites fabricated by pulse current sintering technique
Int. Journal of Refractory Metals and Hard Materials 42 (2014) 42–46
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Relationship between hardness and fracture toughness in WC–FeAl composites fabricated by pulse current sintering technique R. Furushima ⁎, K. Katou, S. Nakao, Z.M. Sun, K. Shimojima, H. Hosokawa, A. Matsumoto Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
a r t i c l e
i n f o
Article history: Received 10 July 2013 Accepted 10 October 2013 Keywords: Tungsten carbide Pulse current sintering technique Iron aluminide Vickers hardness Fracture toughness Hard metal tools
a b s t r a c t WC–FeAl composites having notable mechanical properties as hard metal materials are successfully fabricated by using pulse current sintering technique. The relationship between hardness HV and fracture toughness KIC of the composites is compared with those of WC–Co materials reported. The comparison suggests successful development of WC–FeAl composites with the characteristics almost equal to WC–Co materials currently used for the hard metal tools. Microstructure of the sintered WC–FeAl is uniform without any grain growth. In the KIC–HV plot, WC–FeAl composites locate at the same position as that of WC–Co materials. The total mechanical property of WC–FeAl is comparable to WC–Co. The developed WC–FeAl is a very promising candidate to replace WC–Co materials. Fully controlled microstructure is crucial. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Bonded tungsten carbide (WC) is one of the key materials in industry owing to its superb hardness, low friction coefficient, good electrical conductivity and high oxidation resistance [1]. It has been used extensively for cutting tools, drilling and mining equipment, which require high fracture toughness and high hardness simultaneously. Metallic cobalt (Co) is commonly used as binder to compensate the brittle characteristic of bonded WC [2,3], but it has some problems. The poor corrosion resistance of Co at moderate temperature reduces the oxidation resistance of bonded WC [1,4]. Limited resource and environmental hazard are problems of Co also. A need for a new binder material is high. Iron aluminide (FeAl) is a promising candidate for the binder material of WC. It consists of only common metals (iron and aluminum) and is environmentally benign. Bonded WC with this material has higher oxidation resistance than WC–Co [5], and is very attractive for application at high temperatures around 700 °C. Schneibel et al., however, reported poor sintering characteristic and significant grain growth in WC bonded with FeAl. The maximum HV was approximately 800 kgf mm−2 for WC–FeAl composites prepared by sintering WC powder and Fe40 at.%Al pre-alloyed powder together in vacuum condition at 1723 K [6]. Pulse current sintering is a promising technique for the densification of materials with poor sintering characteristics. Successful fabrication of hard WC–FeAl composites (more than 1500 kgf mm−2 in HV) has been
⁎ Corresponding author. Tel.: +81 52 736 7603; fax: +81 52 736 7127. E-mail address: [email protected] (R. Furushima). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.10.008
reported with this technique [7]. Their fundamental mechanical properties such as KIC and transverse rupture strength (TRS) must be examined to apply them for cutting tools. Wear characteristics of tools depend on HV and chipping frequency of cutting edge is relevant to KIC. TRS has a similar significance to KIC (this parameter is equivalent to KIC for a given critical crack size). A trade-off relationship is present between HV and KIC in a ceramics/ metal composite [8-18], i.e., HV increases but KIC decreases with increasing fraction of binder (metallic) contents. Clearly, the structural control is needed to improve the net mechanical properties of the composite. High freedom of microstructure control is expected with the pulse current sintering technique, and is very attractive to improve the net mechanical properties of the composite. The objective of this study is to examine the effectiveness of the pulse current sintering technique on the improvement of mechanical properties of WC–FeAl composite. 2. Experimental procedure Three kinds of raw WC powders are used in this work: WC-25 (d50 = 2.3 μm; Japan New Metals Co. Ltd. Japan), WC-F (d50 = 0.73 μm; Japan New Metals Co. Ltd. Japan) and WC-02NR (d50 = 0.12 μm; A.L.M.T. Corp., Japan). They are mixed with FeAl pre-alloyed powder (KYORIX, Fe0.6Al0.4, d50 = 5.6 μm; KCM Corporation Co., Ltd., Japan) and ethanol (100 ml) in a ball mill (the rotation speed 100 rpm). The milling pot is made of stainless-steel and the volume is 420 ml. The milling media are cemented carbide balls (ϕ: 9.35 mm). Fig. 1 shows SEM micrographs of raw powders (three kinds of WC powders and FeAl powder). The volume fraction of FeAl to WC–FeAl (VFeAl) ranges from
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Fig. 1. SEM micrographs of various raw powders. (a) WC-02NR (d50 = 0.20 μm); (b) WC-F (d50 = 0.75 μm); (c) WC-25 (d50 = 2.5 μm); (d) Fe0.6Al0.4 (d50 = 5.6 μm).
0.15 to 0.45. The weight ratio of powder/ball is 2:15. The milled suspension (approximately 9 vol.%) is dried in a rotary evaporator at 55 °C under reduced pressure. The dried WC–FeAl powder is packed into a graphite die with a cross section of 5 mm in width and 30 mm in length, and consolidated under a pressure of 40 MPa in a vacuum (10 Pa) using a pulse current sintering equipment (Dr. Sintering Series SPS-515S, Sumitomo Heavy Industries Techno-Fort Ltd., Japan). The average heating rate is 60 °C/min and the maximum temperature ranges from 1140 to 1170 °C. The holding time is 3 min for all samples. The sintered WC–FeAl is machined into specimens of the dimension 2 × 4 × 30mm with a planar grinding machine (grading wheel of 240 mesh roughness). After the density measurements by the Archimedean method, the specimens were edge chamfered. Threepoint bending test is used to determine the transverse rupture strength (TRS) on the specimens of half size (2 × 4 × 15 mm) with the span of 10 mm. The load is applied by a universal testing machine (Autograph AGS-G 1 kN, Shimadzu Corp. Kyoto, Japan) with the crosshead speed of 0.5 mm/min. A buffing machine is used to polish the surface of the specimen after bending tests with 6 and 1 μm diamond slurries. Vickers hardness (HV) is measured on the polished surfaces following the ASTM B294 standard with a Vickers hardness tester (HV114, Mitutoyo Corporation, Japan) at the load of 30 kgf for 15 s. Five indentations are made for each sample to measure the length of indentation diagonals. The fracture toughness KIC is calculated with the Palmqvist model proposed by Niihara et al. [18] K IC ¼ 0:0089
E HV
0:4
P aðC−aÞ0:5
ð1Þ
where E, P, a and C denote Young's modulus, applied load, half-length of indentation diagonal and crack, respectively. Eq. (1) is applicable for C/a b 2.5. Young's modulus E is obtained from velocity
measurements for longitudinal and shear waves in the specimen by ultrasonic pulse echo method using the following equation, 2
E ¼ VS ρ
3V L 2 −4V L 2 V L 2 −V S 2
ð2Þ
where, VL, Vs and ρ denote velocities of longitudinal and shear waves and apparent density of the specimen, respectively. Finally, the microstructure is observed on the polished surface with a scanning electron microscope (ERA-8900FE, ELIONIX INC., Japan). An energy-dispersive X-ray spectrometer (EDS) is used for mapping elements such as Fe and Al. X-ray diffraction (XRD) analysis (X'pertMPD, PANalytical, Netherlands) is used to identify the phases in sintered composite (Cu-Kα, diffraction angle 20°–90°). 3. Results and discussions 3.1. Microstructure and density of WC–FeAl composite Fig. 2 shows SEM micrographs of sintered WC–FeAl composites. The dark and bright features correspond to FeAl and WC, respectively. Grain growth of WC and FeAl not observed for any sample examined. The pulse current sintering is effective in densifying the WC–FeAl composites without the grain growth. Fig. 3 shows EDS maps of a sintered WC–FeAl composite made from the WC-F (d50 = 0.75 μm) powder with VFeAl = 0.25. The black features in the SEM image (Fig. 3(a)) correspond to Fe and Al or Al and O elements from the maps (Fig. 3(b)–(d)). The locations match well for Al and O, and Fe is distributed in small spaces enclosed by WC grains. These results suggest that a part of FeAl powder is oxidized in milling or drying process. Table 1 shows density data for various WC–FeAl composites. The relative density is calculated from the ratio of true density of the WC–FeAl powder to apparent density of the sintered composite. The
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Fig. 2. SEM micrographs of sintered WC–FeAl composites. (a) WC-F (d50 = 0.75 μm), VFeAl = 0.25; (b) WC-02NR (d50 = 0.20 μm), VFeAl = 0.25; (c)W-CF (d50 = 0.75 μm), VFeAl = 0.35.
Fig. 3. EDS maps of a sintered WC–FeAl composite made from the WC-F (d50 = 0.75 μm) powder with VFeAl = 0.25. (a) SEM image; (b) Fe (c) Al (d)O.
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Table 1 Density data for various WC–FeAl composites. Sample
Theoretical density/g cm−3
Powder density/g cm−3
Bulk density/g cm−3
WCF–15 vol.%FeAl WCF–25 vol.%FeAl WCF–35 vol.%FeAl WCF–45 vol.%FeAl WC25–25 vol.%FeAl WC02NR–25 vol.%FeA1
14.1 13.1 12.2 11.2 13.1 13.1
13.7 12.9 12.2 10.9 12.7 12.7
13.6 12.7 11.9 10.9 12.6 12.6
true density of each powder is slightly lower than the theoretical value. Oxidation of FeAl powder before firing is responsible to this low true density. The formation of α-Al2O3 reduces the true density of WC–FeAl powder. Almost full densification is achieved without significant grain growth. The relative density exceeds more than 98% for all samples. Fig. 4 shows an XRD pattern of a sintered WC–FeAl composite, which is used for the EDS mapping discussed above. Logarithmic scale is used in the ordinate axis for easy identification of minor components such as α-Al2O3. All discriminable peaks correspond to those of WC or Al0.4Fe0.6. No peak belonging to α-Al2O3 is detected indicating that the sizes of α-Al2O3 grains fall below the detection capability of the XRD apparatus. 3.2. Variation of mechanical properties with WC particle size and volume fraction of FeAl Fig. 5 shows the dependence of Vickers hardness HV on the particle size of WC and VFeAl. The hardness decreases with increasing mean particle size of WC in Fig. 5(a) and with increasing in VFeAl in Fig. 5(b).
Fig. 6. Dependence of fracture toughness on the WC particle size and VFeAl. (a) Dependence on WC mean particle size (VFeAl: 0.25); (b) dependence on VFeAl (WC mean particle size: 0.75 μm).
The former result is understood by considering the Hall–Petch equation [19,20] and the relationship between yield stress and hardness [21]. −0:5
H ¼ H0 þ kd
ð3Þ
where, H and d are the hardness and the average grain size of the material, respectively, and Η0 and k are constants. Eq. (3) is applicable to the ceramic/metal composites. The result in Fig. 5(b) can be easily explained; FeAl has much lower hardness than WC. Fig. 6 shows the dependence of fracture toughness on the WC particle size and VFeAl. The hardness increases with increasing mean particle size of WC and with increasing VFeAl. Recalling the trade-off relationship between the HV and KIC, these results are reasonable. The strong dependence of the hardness on VFeAl shows that the FeAl binder phases absorb the energy of crack propagation markedly. Fig. 7 shows the dependence of TRS values on WC particle size and VFeAl. The TRS value has almost a linear relationship with the WC mean particle size and VFeAl. This result suggests interesting characteristics of cracks. The parameter TRS involves the critical crack size in addition to the resistance against crack propagation like KIC. The similar tendency for TRS and KIC suggests almost constant sizes of critical crack for all samples. 3.3. Relationship between Vickers hardness and fracture toughness
Fig. 4. XRD pattern of a sintered WC–FeAl composite (WC mean particle size: 0.75 μm, VFeAl: 0.25).
Fig. 5. Dependence of Vickers hardness HV on the particle size of WC and VFeAl. (a) dependence on WC mean particle size (V FeAl: 0.25); (b) dependence on V FeAl (WC mean particle size: 0.75 μm).
Fig. 8 compares the KIC–HV plots for WC–FeAl of this study and those reported for WC–Co [8–14]. The trade-off relationships are noted between HV and KIC for both WC–Co and WC–FeAl systems. The data of WC–FeAl are located at almost the same position as those
Fig. 7. Dependence of TRS values on WC particle size and VFeAl. (a) Dependence on WC mean particle size (VFeAl: 0.25); (b) dependence on VFeAl (WC mean particle size: 0.75 μm).
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R. Furushima et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 42–46 Table 2 Charpy impact values and Young's moduli for Co and FeAl (Fe40 at.%Al). Binder phase
Charpy impact value (J/cm2)
Young's modulus (GPa)
Co Fe40 at.%Al
8.5–10 [22] 3–5 [23]
210 190
mechanical property of WC–FeAl composites fabricated in this work is comparable to WC–Co materials. Fully controlled microstructure is crucially important to replace WC–Co materials. Acknowledgment The authors are graceful to Prof. K. Uematsu for variable comments on this paper. References Fig. 8. KIC–HV plots for WC–Co and WC–FeAl.
of WC–Co. Clearly, the total mechanical property of the developed WC– FeAl is comparable to that of WC–Co. Past study suggests that WC–Co has superior total mechanical property to the WC–FeAl composites fabricated with conventional sintering techniques. This conclusion may be only apparent. Fracture toughness of composite KBIC is strongly affected by that of binder phase KBIC, and is calculated with the following equation, B B B 0:5 K IC ¼ E G
ð4Þ
where EB and GB denote Young's modulus and toughness of binder phase, respectively. Table 2 shows the Charpy impact values and the Young's moduli for Co and FeAl (Fe40 at.%Al) [22,23]. Both Charpy impact value and Young's modulus are higher in WC–Co than that in WC–FeAl. These results suggest that WC composite bonded with FeAl should have lower toughness GB than WC–Co. Surprisingly, KIC–HV plots for WC–FeAl and WC–Co are almost the same in Fig. 8. The low fracture toughness of FeAl apparently has little effect on the KIC–HV plot. The pulse current sintering technique appears to compensate the difference of mechanical characteristics between WC–FeAl and WC–Co. The origin of the compensation is ascribed to the capability of the equipment for fabricating dense sintered compacts with little grain growth. It is concluded that microstructure control is very important for fabricating alternative materials of WC–Co which has high total mechanical properties. 4. Conclusions The relationship between the Vickers hardness HV and fracture toughness KIC is investigated in WC–FeAl composites fabricated by pulse current sintering technique. The obtained composites have the Vickers hardness ranging from 1100 to 2100 kgf mm−2 and fracture toughness from 8 to 15 MPa m0.5. The KIC–HV plot of the WC–FeAl composites is compared with the plots of WC–Co materials previously reported. In the KIC–HV plot, WC–FeAl composites locate at the same position as that of WC–Co materials. This result indicates that the total
[1] El-Eskandarany MS, Omori M, Ishikuro M, Konno TJ, Takada K, Sumiyama K, et al. Synthesis of full-density nanocrystalline tungsten carbide by reduction of tungstic oxide at room temperature. Metall Mater Trans 1996;A27:4210–3. [2] Deorsola FA, Vallauri D, Ortigoza-Villalba GA, Benedetti BD. Densification of ultrafine WC–12Co cermets by pressure assisted fast electric sintering. Int J Refract Met Hard Mater 2010;28:254–9. [3] Kim HC, Oh DY, Shon IJ. Sintering of nanophase WC–15vol.%Co hard metals by rapid sintering process. Int J Refract Met Hard Mater 2004;22:197–203. [4] Imasato S, Kei T, Tetsunori K, Shigeya S. Properties of ultra-fine grain binderless cemented carbide ‘RCCFN’. Int J Refract Met Hard Mater 1995;13:305–12. [5] Matsumoto A, Kobayashi K, Nishio T, Ozaki K. Based WC composites prepared by combustion synthesis. 16th International Plansee Seminar, 2; 2005. p. 287–94. [6] Schneibel JH, Carmichael CA, Specht ED, Subramanian R. Liquid-phase sintered iron aluminide–ceramic composites. Intermetallics 1997;5:61–7. [7] Matsumoto A, Kato K, Shimojima K, Hosokawa H, Takada M, Hiramatsu T. Preparation and mechanical property for WC–25vol%FeAl using wet milling process. J Jpn Soc Powder Powder Metall 2010;57:764–8. [8] Pickens JR, Gurland J. The fracture toughness of WC/Co alloys measured on singleedge notched beam specimens precracked by electron discharge machining. Mater Sci Eng 1978;33:135–42. [9] Laugier MT. Validation of the Palmqvist indentation approach to toughness determination in WC/Co composites. Ceram Int 1989;15:121–5. [10] O'Quigley DGF, Luyckx S, James MN. New results on the relationship between hardness and fracture toughness of WC/Co hardmetal. Mater Sci Eng A 1996;209:228–30. [11] Schubert WD, Neumeister H, Kinger G, Lux B. Hardness to toughness relationship of fine-grained WC/Co hardmetals. Int J Refract Met Hard Mater 1998;16:133–42. [12] Fang ZZ. Correlation of transverse rupture strength of WC/Co with hardness. Int J Refract Met Hard Mater 2005;23:119–27. [13] Sivaprahasam D, Chandrasekar SB, Sundaresan R. Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering. Int J Refract Met Hard Mater 2007;25:144–52. [14] Huang SG, Li L, Vanmeensel K, Van der Biest O, Vleugels J. VC, Cr3C2 and NbC doped WC/Co cemented carbides prepared by pulsed electric current sintering. Int J Refract Met Hard Mater 2007;25:417–22. [15] Roebuck B, Coles W. Mechanical test discriminability for WC hardmetals. Int J Refract Met Hard Mater 1992;11:127–36. [16] Bhaumik SK, Upadhyaya GS, Vaidya ML. Microstructure and mechanical properties of WC–TiN–Co and WC–TiN–Mo2C–Co(Ni) cemented carbides. Ceram Int 1992;18:327–36. [17] Jia K, Fischer TE, Gallois B. Microstructure, hardness and toughness of nanostructured and conventional WC/Co composites. Nanostruct Mater 1998;10:875–91. [18] Niihara K, Morena R, Hasselman DPH. Evaluation of KIC of brittle solids by the indentation method with low crack-to-indent ratios. J Mater Sci Lett 1982;1:13–6. [19] Hall HO. The deformation and ageing of mild steel: III Discussion of results. Proc Phys Soc 1951;B 64:747–53. [20] Petch NJ. The cleavage of polycrystals. J Iron Steel Inst 1953;174:25–8. [21] Tabor D. The hardness of metals. London: Clarendon Press; 1951. [22] Karimpoor AA, Aust KT, Erb U. Charpy impact energy of nanocrystalline and polycrystalline cobalt. Scr Mater 2007;56:201–4. [23] Deevi SC, Sikka VK. Nickel and iron aluminides: an overview on properties, processing, and applications. Intermetallics 1996;4:357–75.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Relationship between hardness and fracture toughness in WC–FeAl composites fabricated by pulse current sintering technique R. Furushima ⁎, K. Katou, S. Nakao, Z.M. Sun, K. Shimojima, H. Hosokawa, A. Matsumoto Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
a r t i c l e
i n f o
Article history: Received 10 July 2013 Accepted 10 October 2013 Keywords: Tungsten carbide Pulse current sintering technique Iron aluminide Vickers hardness Fracture toughness Hard metal tools
a b s t r a c t WC–FeAl composites having notable mechanical properties as hard metal materials are successfully fabricated by using pulse current sintering technique. The relationship between hardness HV and fracture toughness KIC of the composites is compared with those of WC–Co materials reported. The comparison suggests successful development of WC–FeAl composites with the characteristics almost equal to WC–Co materials currently used for the hard metal tools. Microstructure of the sintered WC–FeAl is uniform without any grain growth. In the KIC–HV plot, WC–FeAl composites locate at the same position as that of WC–Co materials. The total mechanical property of WC–FeAl is comparable to WC–Co. The developed WC–FeAl is a very promising candidate to replace WC–Co materials. Fully controlled microstructure is crucial. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Bonded tungsten carbide (WC) is one of the key materials in industry owing to its superb hardness, low friction coefficient, good electrical conductivity and high oxidation resistance [1]. It has been used extensively for cutting tools, drilling and mining equipment, which require high fracture toughness and high hardness simultaneously. Metallic cobalt (Co) is commonly used as binder to compensate the brittle characteristic of bonded WC [2,3], but it has some problems. The poor corrosion resistance of Co at moderate temperature reduces the oxidation resistance of bonded WC [1,4]. Limited resource and environmental hazard are problems of Co also. A need for a new binder material is high. Iron aluminide (FeAl) is a promising candidate for the binder material of WC. It consists of only common metals (iron and aluminum) and is environmentally benign. Bonded WC with this material has higher oxidation resistance than WC–Co [5], and is very attractive for application at high temperatures around 700 °C. Schneibel et al., however, reported poor sintering characteristic and significant grain growth in WC bonded with FeAl. The maximum HV was approximately 800 kgf mm−2 for WC–FeAl composites prepared by sintering WC powder and Fe40 at.%Al pre-alloyed powder together in vacuum condition at 1723 K [6]. Pulse current sintering is a promising technique for the densification of materials with poor sintering characteristics. Successful fabrication of hard WC–FeAl composites (more than 1500 kgf mm−2 in HV) has been
⁎ Corresponding author. Tel.: +81 52 736 7603; fax: +81 52 736 7127. E-mail address: [email protected] (R. Furushima). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.10.008
reported with this technique [7]. Their fundamental mechanical properties such as KIC and transverse rupture strength (TRS) must be examined to apply them for cutting tools. Wear characteristics of tools depend on HV and chipping frequency of cutting edge is relevant to KIC. TRS has a similar significance to KIC (this parameter is equivalent to KIC for a given critical crack size). A trade-off relationship is present between HV and KIC in a ceramics/ metal composite [8-18], i.e., HV increases but KIC decreases with increasing fraction of binder (metallic) contents. Clearly, the structural control is needed to improve the net mechanical properties of the composite. High freedom of microstructure control is expected with the pulse current sintering technique, and is very attractive to improve the net mechanical properties of the composite. The objective of this study is to examine the effectiveness of the pulse current sintering technique on the improvement of mechanical properties of WC–FeAl composite. 2. Experimental procedure Three kinds of raw WC powders are used in this work: WC-25 (d50 = 2.3 μm; Japan New Metals Co. Ltd. Japan), WC-F (d50 = 0.73 μm; Japan New Metals Co. Ltd. Japan) and WC-02NR (d50 = 0.12 μm; A.L.M.T. Corp., Japan). They are mixed with FeAl pre-alloyed powder (KYORIX, Fe0.6Al0.4, d50 = 5.6 μm; KCM Corporation Co., Ltd., Japan) and ethanol (100 ml) in a ball mill (the rotation speed 100 rpm). The milling pot is made of stainless-steel and the volume is 420 ml. The milling media are cemented carbide balls (ϕ: 9.35 mm). Fig. 1 shows SEM micrographs of raw powders (three kinds of WC powders and FeAl powder). The volume fraction of FeAl to WC–FeAl (VFeAl) ranges from
R. Furushima et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 42–46
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Fig. 1. SEM micrographs of various raw powders. (a) WC-02NR (d50 = 0.20 μm); (b) WC-F (d50 = 0.75 μm); (c) WC-25 (d50 = 2.5 μm); (d) Fe0.6Al0.4 (d50 = 5.6 μm).
0.15 to 0.45. The weight ratio of powder/ball is 2:15. The milled suspension (approximately 9 vol.%) is dried in a rotary evaporator at 55 °C under reduced pressure. The dried WC–FeAl powder is packed into a graphite die with a cross section of 5 mm in width and 30 mm in length, and consolidated under a pressure of 40 MPa in a vacuum (10 Pa) using a pulse current sintering equipment (Dr. Sintering Series SPS-515S, Sumitomo Heavy Industries Techno-Fort Ltd., Japan). The average heating rate is 60 °C/min and the maximum temperature ranges from 1140 to 1170 °C. The holding time is 3 min for all samples. The sintered WC–FeAl is machined into specimens of the dimension 2 × 4 × 30mm with a planar grinding machine (grading wheel of 240 mesh roughness). After the density measurements by the Archimedean method, the specimens were edge chamfered. Threepoint bending test is used to determine the transverse rupture strength (TRS) on the specimens of half size (2 × 4 × 15 mm) with the span of 10 mm. The load is applied by a universal testing machine (Autograph AGS-G 1 kN, Shimadzu Corp. Kyoto, Japan) with the crosshead speed of 0.5 mm/min. A buffing machine is used to polish the surface of the specimen after bending tests with 6 and 1 μm diamond slurries. Vickers hardness (HV) is measured on the polished surfaces following the ASTM B294 standard with a Vickers hardness tester (HV114, Mitutoyo Corporation, Japan) at the load of 30 kgf for 15 s. Five indentations are made for each sample to measure the length of indentation diagonals. The fracture toughness KIC is calculated with the Palmqvist model proposed by Niihara et al. [18] K IC ¼ 0:0089
E HV
0:4
P aðC−aÞ0:5
ð1Þ
where E, P, a and C denote Young's modulus, applied load, half-length of indentation diagonal and crack, respectively. Eq. (1) is applicable for C/a b 2.5. Young's modulus E is obtained from velocity
measurements for longitudinal and shear waves in the specimen by ultrasonic pulse echo method using the following equation, 2
E ¼ VS ρ
3V L 2 −4V L 2 V L 2 −V S 2
ð2Þ
where, VL, Vs and ρ denote velocities of longitudinal and shear waves and apparent density of the specimen, respectively. Finally, the microstructure is observed on the polished surface with a scanning electron microscope (ERA-8900FE, ELIONIX INC., Japan). An energy-dispersive X-ray spectrometer (EDS) is used for mapping elements such as Fe and Al. X-ray diffraction (XRD) analysis (X'pertMPD, PANalytical, Netherlands) is used to identify the phases in sintered composite (Cu-Kα, diffraction angle 20°–90°). 3. Results and discussions 3.1. Microstructure and density of WC–FeAl composite Fig. 2 shows SEM micrographs of sintered WC–FeAl composites. The dark and bright features correspond to FeAl and WC, respectively. Grain growth of WC and FeAl not observed for any sample examined. The pulse current sintering is effective in densifying the WC–FeAl composites without the grain growth. Fig. 3 shows EDS maps of a sintered WC–FeAl composite made from the WC-F (d50 = 0.75 μm) powder with VFeAl = 0.25. The black features in the SEM image (Fig. 3(a)) correspond to Fe and Al or Al and O elements from the maps (Fig. 3(b)–(d)). The locations match well for Al and O, and Fe is distributed in small spaces enclosed by WC grains. These results suggest that a part of FeAl powder is oxidized in milling or drying process. Table 1 shows density data for various WC–FeAl composites. The relative density is calculated from the ratio of true density of the WC–FeAl powder to apparent density of the sintered composite. The
44
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Fig. 2. SEM micrographs of sintered WC–FeAl composites. (a) WC-F (d50 = 0.75 μm), VFeAl = 0.25; (b) WC-02NR (d50 = 0.20 μm), VFeAl = 0.25; (c)W-CF (d50 = 0.75 μm), VFeAl = 0.35.
Fig. 3. EDS maps of a sintered WC–FeAl composite made from the WC-F (d50 = 0.75 μm) powder with VFeAl = 0.25. (a) SEM image; (b) Fe (c) Al (d)O.
R. Furushima et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 42–46
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Table 1 Density data for various WC–FeAl composites. Sample
Theoretical density/g cm−3
Powder density/g cm−3
Bulk density/g cm−3
WCF–15 vol.%FeAl WCF–25 vol.%FeAl WCF–35 vol.%FeAl WCF–45 vol.%FeAl WC25–25 vol.%FeAl WC02NR–25 vol.%FeA1
14.1 13.1 12.2 11.2 13.1 13.1
13.7 12.9 12.2 10.9 12.7 12.7
13.6 12.7 11.9 10.9 12.6 12.6
true density of each powder is slightly lower than the theoretical value. Oxidation of FeAl powder before firing is responsible to this low true density. The formation of α-Al2O3 reduces the true density of WC–FeAl powder. Almost full densification is achieved without significant grain growth. The relative density exceeds more than 98% for all samples. Fig. 4 shows an XRD pattern of a sintered WC–FeAl composite, which is used for the EDS mapping discussed above. Logarithmic scale is used in the ordinate axis for easy identification of minor components such as α-Al2O3. All discriminable peaks correspond to those of WC or Al0.4Fe0.6. No peak belonging to α-Al2O3 is detected indicating that the sizes of α-Al2O3 grains fall below the detection capability of the XRD apparatus. 3.2. Variation of mechanical properties with WC particle size and volume fraction of FeAl Fig. 5 shows the dependence of Vickers hardness HV on the particle size of WC and VFeAl. The hardness decreases with increasing mean particle size of WC in Fig. 5(a) and with increasing in VFeAl in Fig. 5(b).
Fig. 6. Dependence of fracture toughness on the WC particle size and VFeAl. (a) Dependence on WC mean particle size (VFeAl: 0.25); (b) dependence on VFeAl (WC mean particle size: 0.75 μm).
The former result is understood by considering the Hall–Petch equation [19,20] and the relationship between yield stress and hardness [21]. −0:5
H ¼ H0 þ kd
ð3Þ
where, H and d are the hardness and the average grain size of the material, respectively, and Η0 and k are constants. Eq. (3) is applicable to the ceramic/metal composites. The result in Fig. 5(b) can be easily explained; FeAl has much lower hardness than WC. Fig. 6 shows the dependence of fracture toughness on the WC particle size and VFeAl. The hardness increases with increasing mean particle size of WC and with increasing VFeAl. Recalling the trade-off relationship between the HV and KIC, these results are reasonable. The strong dependence of the hardness on VFeAl shows that the FeAl binder phases absorb the energy of crack propagation markedly. Fig. 7 shows the dependence of TRS values on WC particle size and VFeAl. The TRS value has almost a linear relationship with the WC mean particle size and VFeAl. This result suggests interesting characteristics of cracks. The parameter TRS involves the critical crack size in addition to the resistance against crack propagation like KIC. The similar tendency for TRS and KIC suggests almost constant sizes of critical crack for all samples. 3.3. Relationship between Vickers hardness and fracture toughness
Fig. 4. XRD pattern of a sintered WC–FeAl composite (WC mean particle size: 0.75 μm, VFeAl: 0.25).
Fig. 5. Dependence of Vickers hardness HV on the particle size of WC and VFeAl. (a) dependence on WC mean particle size (V FeAl: 0.25); (b) dependence on V FeAl (WC mean particle size: 0.75 μm).
Fig. 8 compares the KIC–HV plots for WC–FeAl of this study and those reported for WC–Co [8–14]. The trade-off relationships are noted between HV and KIC for both WC–Co and WC–FeAl systems. The data of WC–FeAl are located at almost the same position as those
Fig. 7. Dependence of TRS values on WC particle size and VFeAl. (a) Dependence on WC mean particle size (VFeAl: 0.25); (b) dependence on VFeAl (WC mean particle size: 0.75 μm).
46
R. Furushima et al. / Int. Journal of Refractory Metals and Hard Materials 42 (2014) 42–46 Table 2 Charpy impact values and Young's moduli for Co and FeAl (Fe40 at.%Al). Binder phase
Charpy impact value (J/cm2)
Young's modulus (GPa)
Co Fe40 at.%Al
8.5–10 [22] 3–5 [23]
210 190
mechanical property of WC–FeAl composites fabricated in this work is comparable to WC–Co materials. Fully controlled microstructure is crucially important to replace WC–Co materials. Acknowledgment The authors are graceful to Prof. K. Uematsu for variable comments on this paper. References Fig. 8. KIC–HV plots for WC–Co and WC–FeAl.
of WC–Co. Clearly, the total mechanical property of the developed WC– FeAl is comparable to that of WC–Co. Past study suggests that WC–Co has superior total mechanical property to the WC–FeAl composites fabricated with conventional sintering techniques. This conclusion may be only apparent. Fracture toughness of composite KBIC is strongly affected by that of binder phase KBIC, and is calculated with the following equation, B B B 0:5 K IC ¼ E G
ð4Þ
where EB and GB denote Young's modulus and toughness of binder phase, respectively. Table 2 shows the Charpy impact values and the Young's moduli for Co and FeAl (Fe40 at.%Al) [22,23]. Both Charpy impact value and Young's modulus are higher in WC–Co than that in WC–FeAl. These results suggest that WC composite bonded with FeAl should have lower toughness GB than WC–Co. Surprisingly, KIC–HV plots for WC–FeAl and WC–Co are almost the same in Fig. 8. The low fracture toughness of FeAl apparently has little effect on the KIC–HV plot. The pulse current sintering technique appears to compensate the difference of mechanical characteristics between WC–FeAl and WC–Co. The origin of the compensation is ascribed to the capability of the equipment for fabricating dense sintered compacts with little grain growth. It is concluded that microstructure control is very important for fabricating alternative materials of WC–Co which has high total mechanical properties. 4. Conclusions The relationship between the Vickers hardness HV and fracture toughness KIC is investigated in WC–FeAl composites fabricated by pulse current sintering technique. The obtained composites have the Vickers hardness ranging from 1100 to 2100 kgf mm−2 and fracture toughness from 8 to 15 MPa m0.5. The KIC–HV plot of the WC–FeAl composites is compared with the plots of WC–Co materials previously reported. In the KIC–HV plot, WC–FeAl composites locate at the same position as that of WC–Co materials. This result indicates that the total
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