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Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering
International Journal of Refractory Metals & Hard Materials 25 (2007) 144–152 www.elsevier.com/locate/ijrmhm
Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering D. Sivaprahasam *, S.B. Chandrasekar, R. Sundaresan International Advanced Research Centre for Powder Metallurgy and New Materials (ARC-I), Balapur P.O., RCI Road, Hyderabad 500 005, India Received 22 February 2006; accepted 29 March 2006
Abstract Cemented carbide powders based on WC–12Co with a grain size of 40–250 nm were generated by high-energy ball milling. The powders with different extents of VC and (VC + Cr3C2) addition were consolidated to full density by spark plasma sintering (SPS). The density, microstructure, grain size and fracture toughness KIc of the SPS consolidated samples were measured and compared with samples made by liquid phase sintering. Dense samples with a pore rating
1. Introduction Based on the benefits in mechanical properties, WC–Co cemented carbide with ultra fine/nanograin structure has been a subject of considerable research over the last two decades [1–5]. Several studies have been carried out on various aspects involved in manufacturing of such ultra fine/ nanograined cemented carbides and their superior mechanical properties such as high hardness, high temperature strength and elastic modulus [6–10]. While these mechanical properties depend on many factors such as alloy composition, purity, homogeneity of the structure and particle size of the initial powder, for a given alloy composition it is the microstructure (WC grain size, mean free *
Corresponding author. Tel.: +91 40 24441075x334; fax: +91 40 24442699. E-mail address: [email protected] (D. Sivaprahasam). 0263-4368/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2006.03.008
path of cobalt and the contiguity of WC particles) that decides the mechanical properties. The final microstructure, in turn, depends on the type of sintering process adopted and the sintering parameters such as temperature, time, pressure and atmosphere. Conventionally, WC–Co cemented carbides are fabricated by compaction and liquid phase sintering carried out well above the eutectic temperature of WC–Co (1280– 1310 C). The liquid phase in sintering also assists grain growth by providing a rapid diffusion path. In the case of ultra fine and nanocrystalline WC–Co system, the grain growth is further enhanced by the high surface free energy. Addition of refractory carbides like VC, Nb3C2 and Cr3C2 controls the grain growth during sintering either by modifying the WC–Co interface or by providing resistance to diffusion. However, the results so far indicate a final WC grain size in ultra fine range (200–300 nm). Alternate sintering processes such as microwave sintering [11], electric discharge
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compaction (EDC) [12], plasma pressure consolidation (P2C) [13], high frequency induction heated sintering (HFIHS) [14] and spark plasma sintering (SPS) [15–19] are being explored to overcome the problem of grain growth. Since these processes have the advantage of high heating rate with shorter sintering time, at comparatively lower sintering temperature, the final sintered grain size is expected to be much lower than in conventional sintering. A study on WC–12Co powder (added with 0.88Cr3C2 0.4VC) powder of 250 nm initial WC grain size showed the average grain size of 300 nm after spark plasma sintering [15]. A spray conversion synthesized (SCP) WC–10Co of 100 nm average WC particle size without any inhibitor addition fabricated by SPS at 1000 C had led to a sintered grain size of 300 nm [16]. Combining the benefit of both grain growth inhibitor addition and rapid sintering it may be possible to restrict the grain growth further. But reports on studies carried out in this direction are limited [15,17–19]. In the present study a nanograined WC–Co added with different amounts of various inhibitors like VC and VC + Cr3C2 were consolidated to full density by SPS. The effect of SPS processing parameters and inhibitor addition on density, microstructure and mechanical properties were investigated and compared with the samples consolidated by conventional compaction and liquid phase sintering. 2. Experimental 2.1. Powder preparation The nanocrystalline WC–12Co powder used in this study was generated by high-energy ball milling of commercial graded powder with average WC grain size of 3 lm. Mechanical milling was carried out in a Fritsch Pulverisette 5 planetary ball mill in methanol under argon cover, using WC–6Co milling vial and media for 12–48 h. The milling was carried out at a ball to powder weight ratio of 15:1 and two different speeds (250 and 150 rpm). The mean particle size (by Fischer sub-sieve analyzer), crystallite size (from X-ray diffraction peak width), composition (by wet chemical analysis) and carbon content (by Leco C-S analyzer) of the WC–Co powder were measured both in the as received powder and after milling. The carbide inhibitors VC and Cr3C2 were both obtained from AlfaAeser Chemicals and had particle sizes of 44 lm (325 mesh). Specified quantities of VC or VC + Cr3C2 were added to WC–Co powder before milling. 2.2. Sintering Spark plasma sintering was carried out in the equipment Dr. Sinter 1050 supplied by Sumitomo Coal Mining Co., Japan, under a vacuum of 4 Pa in a cylindrical graphite die with an inner diameter of 20 mm. The experiments were done in the temperature range 1000–1100 C for 3–10 min under a pressure of 50 MPa. The heating rate for all SPS experiments was maintained at around 150 C/min, and
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the natural cooling rate down to 800 C was 100–150 C/ min. Temperature control was by PID control based on K-type thermocouple positioned close to the sample in the die. Shrinkage was monitored from the dilatometer provided with the SPS machine. For conventional consolidation the nanopowder was mixed with 2–4% paraffin wax dissolved in heptane, granulated and pressed to 10 mm diameter compacts. Pressing was carried out at 2–3 tons/cm2 and the green density varied in the range 53–58%. Dewaxing of green compacts was carried out in a separate cycle in flowing hydrogen. Sintering was carried out under positive pressure of H2. The sintering cycle adopted included the following steps: heating to 900 C at 10 C/min, holding for 30 min, heating to 1300 C at 5 C/ min followed by heating to final sintering temperature (1400 and 1450 C) at the rate of 20 C/min. The holding time at the sintering temperature was 30–45 min. 2.3. Characterization of sintered WC–Co The density of the sintered samples was measured by Archimedes method. The porosity of the sintered WC–Co cemented carbides was rated by comparing the microstructure with ISO4505 standards using optical microscope. The phases in the sintered samples were characterized by X-ray diffraction (XRD) from a Bruker’s diffractometer (AXS Model No. D8 Advance System). The WC grain sizes, and other major microstructural parameters like contiguity and cobalt mean free path were measured in the microstructure of the sintered samples by linear intercept method on a 5K· and 10K· scanning electron micrograph on the back scattered image using the equation given in Golovchan et al. [20]. The values reported are by averaging measurements made on five micrographs taken from different locations in the cross section of the samples. Following the standard procedure used the hardness of the sintered WC–Co was measured by Vicker’s hardness testers using 30 kg load, as given in ISO3878. The values reported are the arithmetic mean of 5–10 readings from each sample. The fracture toughness KIc of the samples was computed by the equation below [10,13]: K Ic ¼ 0:016ðE=H Þ1=2 P =C 3=2 MN m3=2
ð1Þ
where E is the Young’s modulus of the alloy, H is the hardness (N/mm2), P is the load (N) and C is the crack length from the center of the indent to tip of the crack. Young’s modulus of the sintered samples was measured by nanoindentation technique using MTS NanoIndentation XP machine. The value used (E = 540 GPa) is an average of 25 readings. 3. Results and discussion 3.1. Nanocrystalline WC–Co powder The mean particle size, WC crystallite size, carbon and oxygen content of both unmilled and milled powders are
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given in Table 1. The grain size obtained after different milling conditions are given in Table 2. The grain sizes reported are measured by X-ray diffraction method using Scherrer equation with appropriate care taken for instrumental broadening and strain effect. A few milled powder samples were also evaluated for grain size using TEM. The milling parameters were optimized based on the grain size and chemistry of the WC–Co powder. All the powders milled at 250 rpm showed cobalt content that differed significantly from the initial powder composition because of the ball erosion. Milling at 150 rpm avoids such erosion and the change in cobalt content. Powders milled 24 h at 150 rpm resulted in WC grain size of 38 nm as measured from XRD peak width. Further increase in the milling time (up to 48 h) gave little further reduction in grain size. The TEM image of the powder obtained after 24 h milling shows WC grains of around 40 nm though a few bigger grains of size up to 250 nm are also observed. The XRD analysis of the powder showed only WC and FCC cobalt phase. 3.2. Densification behaviour Fig. 1 shows the variation of shrinkage rate with temperature during heating to SPS temperature. Significant shrinkage started at 800–850 C and a maximum shrinkage rate was observed in the range 950–1000 C, irrespective of the final sintering temperature. With the high heating rate (150 C/min) employed in SPS, the actual heating rate value in the sintering runs may vary slightly. But the trend of shrinkage rates is definite and indicates that maximum shrinkage rate is attained before the isothermal hold temperature in these experiments.
Table 1 Characteristics of WC–Co powder Powder property
Before milling
After milling
Mean particle size (lm) FSSS WC grain size (nm)
28 3 lm
0.42 40–250 (TEM) 38 (XRD)
Elements Cobalt, wt.% Total carbon, wt.% Free carbon, wt.% Oxygen, ppm
12.79 5.38 <0.01 1000
12.68 5.32–5.36 – 2250
Fig. 1. Variation of shrinkage rate with temperature during spark plasma sintering of nanocrystalline WC–Co.
The density of the samples without inhibitor addition, fabricated by both spark plasma sintering and liquid phase sintering at different temperatures and time are given in Fig. 2. The density increases with both temperature and hold time. The normally specified density (>14.5 g/cm3) and pore rating (A02B00) for this alloy was obtained in samples spark plasma sintered at 1100 C for 10 min at 50 MPa pressure. To get similar densification and pore rating, a sintering time of 30 min was required at 1450 C in liquid phase sintering. Though the density obtained in 1050 C/10 min SPS sample was also near theoretical (14.46 g/cm3) considerable number of pores of size less than 0.5 lm (outside ISO4505 standards specifications) was observed in the microstructure. SPS at 1100 C brought the porosity level to within acceptable micro pore level. Addition of grain growth inhibitors VC and VC + Cr3C2 led to lower sintered density in both SPS and liquid phase sintering (see Tables 3a and 3b). In 1% VC added sample the decrease in density was to the extent of 0.3–0.55 g/cm3 in SPS and 0.05–0.1 g/cm3 in liquid
Table 2 Variation of WC grain size with milling conditions Milling time
12 24 32 48
WC grain size, nm 250 rpm
150 rpm
30 18 12 10
– 38 35 32
Fig. 2. Variation of density of WC–Co compacts processed by both SPS and liquid phase sintering at different temperature and time.
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Table 3a Microstructure parameters in spark plasma sintered material Material
SPS temperature, C
SPS time, min
Density (% theor.)
WC grain size, lm
Mean Co intercept length, lm
WC grain contiguity
WC–12Co
1000 1050 1100 1100 1100 1100 1100
10 10 10 10 10 10 10
97.93 99.35 99.89 97.66 95.94 90.08 96.64
– 0.70 0.80 0.55 0.47 – 0.49
– 0.29 0.25 0.22 0.21 – 0.21
0.40 0.42 0.49 0.52 – 0.48
WC–12Co–0.5VC WC–12Co–1.0VC WC–12Co–1.5VC WC–12Co–0.75VC–0.25Cr3C2
(0.07) (0.04) (0.10) (0.06) (0.04)
(0.05) (0.06) (0.04) (0.09) (0.06)
(0.10) (0.14) (0.15) (0.01) (0.09)
3
100% density corresponds to 14.55 g/cm . WC grain size, cobalt layer thickness and WC grain contiguity measured as mean linear intercept. Values within the brackets indicate the standard deviation.
Table 3b Microstructure parameters in liquid phase sintered material Material
Sintering temperature, C
Sintering time, min
Density (% theor.)
WC grain size, lm
Mean Co intercept length, lm
WC grain contiguity
WC–12Co
1400 1450 1450 1450 1450 1100
30 30 45 45 45 10
98.62 99.79 99.86 99.79 99.51 90.08
0.78 0.86 1.06 0.75 0.62 –
0.34 0.32 0.26 0.27 0.21 –
0.46 0.39 0.42 0.47 0.52 –
WC–12Co–0.5VC WC–12Co–1.0VC WC–12Co–1.5VC
(0.13) (0.14) (0.09) (0.18) (0.09)
(0.10) (0.07) (0.11) (0.16) (0.12)
(0.14) (0.11) (0.08) (0.14) (0.11)
100% density corresponds to 14.55 g/cm3. WC grain size, cobalt layer thickness and WC grain contiguity measured as mean linear intercept. Values within the brackets indicate the standard deviation.
phase sintering. In liquid phase sintering however, complete densification was attained on extending the sintering to 45 min. Increasing the VC addition further (more than 1%) decreases the density significantly in SPS. 3.3. Phases, microstructure and grain size XRD investigation of the WC–Co samples spark plasma sintered to full density reveals that oxygen level in the powder higher than 0.5 wt.% led to Co3W3C (‘‘eta’’) phase formation during spark plasma sintering. By passivation of the milled powders by slow exposure to atmosphere, and by minimum exposure to open air it was possible to reduce oxygen pick up by the active powder, but the oxygen content was still about 2250 ppm. Addition of 0.1% carbon in the powder during milling eliminated the eta phase. However, it was seen that even a small excess in carbon addition resulted in free carbon peak in XRD. The microstructure of WC–Co spark plasma sintered at temperatures 1000 and 1100 C are given in Fig. 3a and b. Samples sintered at 1000 C showed incomplete sintering (density of 14.25 g/cm3) with rounded WC grains and cobalt ‘‘lakes’’ along with macro- and micro-pores. WC grains became faceted and the cobalt distribution got more uniform as the sintering temperature was raised to 1100 C. In comparison Fig. 4 shows the microstructure of WC–Co liquid phase sintered at 1450 C/30 min. The microstructure obtained with SPS at 1100 C/10 min and with liquid phase sintering at 1450 C/30 min appear similar. Tables 3a and 3b give the microstructural parameters WC grain
size, mean cobalt intercept length and contiguity measured on samples densified by SPS and liquid phase sintering under different temperature/time conditions. Full density and acceptable level of microporosity in WC–Co without any additives was obtained in both the methods of sintering, but at different temperatures and hold times. Density obtained in liquid phase sintering at 1450 C for 30 min and 45 min gave values (99.79% and 99.86%, respectively) comparable to that obtained by SPS at 1100 C/10 min (99.89%). The grain size obtained in the 1100 C/10 min SPS sample was finer than both (Tables 3a and 3b). At closely comparable density value (SPS 1100 C/10 min and liquid phase sintering 1450 C/ 45 min) the grain size in liquid phase sintered sample was 32% coarser. A grain size of 0.80 lm as in the SPS 1100 C/10 sample was in fact comparable to that in sample liquid phase sintered at 1400 C, which had not sintered fully. The mean intercept length of Co in this SPS sample was less than that in liquid phase sintered samples, indicating good coverage of the inter-particle surfaces in the fine grain structure. Under the conditions of applied pressure and temperature in SPS, cobalt in the solid state is seen to flow into the inter-particle crevices fully, much as liquid phase spreads on the particles by surface tension. This is supported by the observation that the contiguity is comparable in the two: where the particles are already in close contact, in the absence of a crevice/surface, cobalt may not penetrate either as liquid or as solid flow. Fig. 5 shows WC grain size distribution in the two cases (SPS 1100 C/10 min and liquid phase sintering 1450 C/
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Fig. 5. Grain size distribution, WC–12Co: (a) spark plasma sintered at 1100 C for 10 min and (b) liquid phase sintered at 1450 C for 30 min. Fig. 3. SEM micrograph (back scattered mode) of WC–12Co spark plasma sintered at (a) 1000 C for 10 min and (b) 1100 C for 10 min.
Fig. 4. SEM micrograph (back scattered mode) of WC–12Co liquid phases sintered at 1450 C for 30 min.
30 min). Both show reasonably continuous distribution, but SPS shows a narrower distribution with the near absence of grains over 2 lm.
Adding inhibitor VC decreases the WC grain growth in both in SPS and in liquid phase sintering. The parameters WC grain size, mean cobalt intercept length and contiguity in these systems are given in Tables 3a and 3b. Figs. 6 and 7 show the microstructures of WC–Co SPS at 1100 C for 10 min and liquid phase sintered at 1450 C for 45 min, respectively with different amount of VC addition. The extent of decrease in grain size in both cases is comparable: With 1 wt.% VC addition WC grain size decreased from 0.80 to 0.47 lm (41.3%) in SPS processing whereas in liquid phase sintering the grain size decreased from 1.06 to 0.62 lm (41.5%). The grain size of WC–Co with 1.5 VC was not quantifiable because of the low sintered density of the samples. Replacing part of VC with Cr3C2 did not show any noticeable difference in the microstructural parameters, although there was in improvement in the density. WC grain growth in liquid phase sintering of WC–Co occurs by Ostwald ripening with dissolution of smaller WC grain and reprecipitation on larger grains in liquid Co. This growth is predominantly interface controlled [21], and addition of refractory carbides such as VC prob-
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Fig. 6. SEM micrograph of WC–12Co with (a) 0.5VC (b) 1VC; SPS at 1100 C for 10 min.
Fig. 7. SEM micrograph of WC–12Co with (a) 0.5VC (b) 1VC, liquid phase sintered at 1450 C for 45 min.
ably alters the interface energy and interferes with the dissolution–reprecipitation steps. However, this whole process is not restricted to liquid phase sintering alone but occurs to a remarkable extent during solid state sintering also [6,22]. This implies that significant extent of diffusion of WC occurs in cobalt in the solid state. In this case any change in the interfacial energy by the addition of inhibitors may not have any role to play. It is likely that the presence of dispersions on the WC–Co interface interferes with the dissolution and/or precipitation of WC in Co.
published relating hardness and microstructure in straight WC–Co cemented carbides [23–26]. The most comprehensive among these [23,26] give the hardness of the cemented carbide HCC as
3.4. Mechanical properties Fig. 8a and b gives the effects of VC addition on hardness and toughness of samples spark plasma sintered or liquid phase sintered under different conditions. The hardness values of spark plasma sintered samples in all cases are higher than those of liquid phase sintered samples (Table 4). This is evidently a result of microstructural parameters, mainly the finer grain size. There have been several papers
H CC ¼ k 1 H WC V WC C þ k 2 H Co ð1 V WC CÞ
ð2Þ
where H refers to hardness, V, to volume fraction and the subscripts WC and Co refer to the phases and C is the contiguity. HWC and HCo are related with the corresponding grain size/mean free path of the corresponding phases. In the equation by Lee and Gurland [23] the values of k1 and k2 are both 1, while in that by Xu and Agren, [26] k1 is 1.205 and k2, 0.9. The values of hardness based on Eq. (2) are shown in Table 5. It appears that Lee and Gurland model is eminently more suited for hardness below 15 GPa. The good correlation indicates that there is no major factor other than microstructural indicators that dictate hardness in straight WC–Co cemented carbides. Hence, the higher hardness exhibited in SPS carbides can be attributed entirely to microstructure refinement provided by the process.
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Fig. 8. Variation of (a) hardness (HV30) and (b) fracture toughness (KIc) with VC addition.
Addition of VC increases the hardness both in SPS and liquid phase sintered samples. While Eq. (2) is limited to carbides without any additions, it can be surmised that the resulting improved hardness is due to the effect of grain growth inhibition both in SPS and liquid phase sintering.
Sample
Lee and Gurland Model (in GPa)
Xu and Argon model (in GPa)
SPS at 1050 C for 10 min SPS at 1100 C for 10 min Liquid phase sintered at 1450 C for 30 min
14.40 14.75 13.77
15.34 15.68 14.61
Toughness is also seen to improve with VC addition both in SPS and liquid phase sintering (Fig. 8b). Partial substitution of VC by Cr3C2 further improves the toughness in SPS. The increase in KIc is higher in liquid phase sintered samples than in SPS samples. Fracture in coarse grained (d > 2 lm) cemented carbide is reported to occur by cleavage, while in fine grained material fracture occurs by interfacial decohesion [27]. In liquid phase sintering, with complete wetting of the carbide surface by liquid cobalt in sintering, the WC–Co interface is likely to be more cohesive than is the case with SPS where cobalt in the plastic state is pushed into the gaps between carbide particles by the applied pressure. The trend of fracture toughness readings (Table 4) may give some further insight into the fracture mechanism. A correlation between KIc and density is shown in Fig. 9. With increasing VC content in SPS samples, fracture toughness increases with decreasing density. With fracture occurring through interfacial decohesion rather than cleavage, pores in the cobalt layer are likely to provide crack arrestors, which can contribute to improved toughness. Density changes with VC addition in liquid phase sintered samples are considerably less marked. However, there is significant improvement in toughness with VC addition over that of straight WC–Co. In the hardness range reported here, Schubert et al. [28] found no significant change in toughness with VC addition, but suggested that at higher hardness values (>20 GPa) with nanograin precursors VC addition may contribute to higher toughness. In the present study, it is shown that with nanograin pre-
Table 4 Hardness and fracture toughness of samples spark plasma sintered or liquid phase sintered under different conditions Material
Process
Sintering temperature C
Sintering time, min
WC–12Co
SPS
1000 1050 1100 1400 1450 1450 1100 1450 1100 1450 1100 1100
10 10 10 30 30 45 10 45 10 45 10 10
Liquid phase sintering
WC–12Co–0.5VC WC–12Co–1VC WC–12Co–1.5VC WC–12Co–0.75VC–0.25Cr3C2
SPS Liquid phase sintering SPS Liquid phase sintering SPS SPS
Hardness (HV30)
HV0.5
1472 1512 1450 1381 1386 1370 1490 1445 1570 1486 1587 1590
1557 1613 1579 1498 1473 1465 1600 1484 1608 1514 1624 1626
Fracture toughness, MPa m1/2 10.0 9.55 10.90 17.30 13.53 14.04 10.86 15.70 11.42 15.60 11.43 12.1
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This could be directly correlated with the finer WC grain size and cobalt distribution. Addition of these carbides also improved the fracture toughness in both SPS and liquid phase sintering. Since the fracture occurs in fine grained materials predominantly in the cobalt layer and the interface, increased porosity in the cobalt layer caused by the addition of VC probably provides crack arrest for improved toughness. Acknowledgements
Fig. 9. Variation of fracture toughness with density.
cursors improved toughness results even at much lower hardness ranges (that is higher cobalt content). 4. Conclusions Nanocrystalline powder of WC–Co was generated by mechanical milling of conventional powder with a grain size of 3 lm. The milling process was optimized to realize an average grain size of 38 nm. Consolidation of the milled nanocrystalline powder was studied by spark plasma sintering as well as by liquid phase sintering. Acceptable density level (>14.5 g/cm3) and pore rating (
XRD analyses of the samples reported here have been done by Dr. G.V.N. Rao, ARC-I, which is gratefully acknowledged. The authors also acknowledge help from Dr. G. Ravichandra, ARC-I with nanoindentation for elastic modulus measurements. Dr. Vaidehi Joshi, DMRL (Defence Metallurgical Research Laboratory), Hyderabad provided support with SEM. References [1] McCandlish LE, Kear BH, Kim BK. Processing and properties of nanostructured WC–Co. NanoStruct Mater 1992;1(2):119–24. [2] Kear BH, McCandlish LE. Chemical processing and properties of nanostructured WC–Co materials. NanoStruct Mater 1993;3:19–30. [3] Seegopaul P, McCandlish LE, Shinneman FM. Production capability and powder processing methods for nanostructured WC–Co powder. Int J Refract Mater Hard Mater 1997;15:133–8. [4] Corroll DF. Sintering and microstructural development in WC/Co based alloys made with super fine WC powder. Int J Refract Mater Hard Mater 1997;17:123–32. [5] Sherif El-Eskandarany M, Mahday A, Ahmed HA, Amer AH. Synthesis and characterizations of ball milled nanocrystalline WC and nanocomposite WC–Co powders and subsequent consolidation. J Alloys Comp 2000;312:315–25. [6] Schubert WD, Bock A, Lux B. General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production. Int J Refract Mater Hard Mater 1995;13:281–96. [7] Shao G, Duan X, Xie J, Yu X, Zhang W, et al. Rev Adv Mater Sci 2003;5:281–6. [8] Kim BK, Ha GH, Lee DW. Sintering and microstructure of nanophase WC/Co hardmetals. J Mater Process Tech 1997;63:317–21. [9] Jia K, Fischer TE, Gallois B. Microstructure hardness and toughness of nanostructured and conventional WC–Co composites. Nanostruct Mater 1998;10:875–91. [10] Cha SI, Hong SH, Ha GH, Kim BK. Mechanical properties of WC– 10Co cemented carbides sintered from nanocrystalline spray conversion processed powder. Int J Ref Mater Hard Mater 2001;19: 397–403. [11] Agarwal D, Cheng J, Seegopaul P, Gao L. Grain growth control in microwave sintering of ultrafine WC–Co composite powder compacts. Powder Metall 2000;43:15–6. [12] Wu XY, Zhang W, Wang W, Yang F, Min JY, et al. Ultrafine WC– 10Co cemented carbides fabricated by electric discharge compaction. J Mater Res 2004;19:2240–4. [13] Sampath A, Stiglich JJ, Sudarshan JS, Singh R, Choi KS. Fast consolidation of WC–Co. Powder Metall 2002;45:25–7. [14] Kim HC, Oh DY, Shon IJ. Synthesis of WC and dense WC– xvol.%Co hard materials by high-frequency induction heated combustion method. Int J Ref Mater Hard Mater 2004;22:197–203. [15] Moriguchi H, Tsuduki K, Ikegaya A. Ultrafine grained cemented carbides sintered by pulse current process. Powder Metall 2000;43:17–9.
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Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering D. Sivaprahasam *, S.B. Chandrasekar, R. Sundaresan International Advanced Research Centre for Powder Metallurgy and New Materials (ARC-I), Balapur P.O., RCI Road, Hyderabad 500 005, India Received 22 February 2006; accepted 29 March 2006
Abstract Cemented carbide powders based on WC–12Co with a grain size of 40–250 nm were generated by high-energy ball milling. The powders with different extents of VC and (VC + Cr3C2) addition were consolidated to full density by spark plasma sintering (SPS). The density, microstructure, grain size and fracture toughness KIc of the SPS consolidated samples were measured and compared with samples made by liquid phase sintering. Dense samples with a pore rating
1. Introduction Based on the benefits in mechanical properties, WC–Co cemented carbide with ultra fine/nanograin structure has been a subject of considerable research over the last two decades [1–5]. Several studies have been carried out on various aspects involved in manufacturing of such ultra fine/ nanograined cemented carbides and their superior mechanical properties such as high hardness, high temperature strength and elastic modulus [6–10]. While these mechanical properties depend on many factors such as alloy composition, purity, homogeneity of the structure and particle size of the initial powder, for a given alloy composition it is the microstructure (WC grain size, mean free *
Corresponding author. Tel.: +91 40 24441075x334; fax: +91 40 24442699. E-mail address: [email protected] (D. Sivaprahasam). 0263-4368/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2006.03.008
path of cobalt and the contiguity of WC particles) that decides the mechanical properties. The final microstructure, in turn, depends on the type of sintering process adopted and the sintering parameters such as temperature, time, pressure and atmosphere. Conventionally, WC–Co cemented carbides are fabricated by compaction and liquid phase sintering carried out well above the eutectic temperature of WC–Co (1280– 1310 C). The liquid phase in sintering also assists grain growth by providing a rapid diffusion path. In the case of ultra fine and nanocrystalline WC–Co system, the grain growth is further enhanced by the high surface free energy. Addition of refractory carbides like VC, Nb3C2 and Cr3C2 controls the grain growth during sintering either by modifying the WC–Co interface or by providing resistance to diffusion. However, the results so far indicate a final WC grain size in ultra fine range (200–300 nm). Alternate sintering processes such as microwave sintering [11], electric discharge
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compaction (EDC) [12], plasma pressure consolidation (P2C) [13], high frequency induction heated sintering (HFIHS) [14] and spark plasma sintering (SPS) [15–19] are being explored to overcome the problem of grain growth. Since these processes have the advantage of high heating rate with shorter sintering time, at comparatively lower sintering temperature, the final sintered grain size is expected to be much lower than in conventional sintering. A study on WC–12Co powder (added with 0.88Cr3C2 0.4VC) powder of 250 nm initial WC grain size showed the average grain size of 300 nm after spark plasma sintering [15]. A spray conversion synthesized (SCP) WC–10Co of 100 nm average WC particle size without any inhibitor addition fabricated by SPS at 1000 C had led to a sintered grain size of 300 nm [16]. Combining the benefit of both grain growth inhibitor addition and rapid sintering it may be possible to restrict the grain growth further. But reports on studies carried out in this direction are limited [15,17–19]. In the present study a nanograined WC–Co added with different amounts of various inhibitors like VC and VC + Cr3C2 were consolidated to full density by SPS. The effect of SPS processing parameters and inhibitor addition on density, microstructure and mechanical properties were investigated and compared with the samples consolidated by conventional compaction and liquid phase sintering. 2. Experimental 2.1. Powder preparation The nanocrystalline WC–12Co powder used in this study was generated by high-energy ball milling of commercial graded powder with average WC grain size of 3 lm. Mechanical milling was carried out in a Fritsch Pulverisette 5 planetary ball mill in methanol under argon cover, using WC–6Co milling vial and media for 12–48 h. The milling was carried out at a ball to powder weight ratio of 15:1 and two different speeds (250 and 150 rpm). The mean particle size (by Fischer sub-sieve analyzer), crystallite size (from X-ray diffraction peak width), composition (by wet chemical analysis) and carbon content (by Leco C-S analyzer) of the WC–Co powder were measured both in the as received powder and after milling. The carbide inhibitors VC and Cr3C2 were both obtained from AlfaAeser Chemicals and had particle sizes of 44 lm (325 mesh). Specified quantities of VC or VC + Cr3C2 were added to WC–Co powder before milling. 2.2. Sintering Spark plasma sintering was carried out in the equipment Dr. Sinter 1050 supplied by Sumitomo Coal Mining Co., Japan, under a vacuum of 4 Pa in a cylindrical graphite die with an inner diameter of 20 mm. The experiments were done in the temperature range 1000–1100 C for 3–10 min under a pressure of 50 MPa. The heating rate for all SPS experiments was maintained at around 150 C/min, and
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the natural cooling rate down to 800 C was 100–150 C/ min. Temperature control was by PID control based on K-type thermocouple positioned close to the sample in the die. Shrinkage was monitored from the dilatometer provided with the SPS machine. For conventional consolidation the nanopowder was mixed with 2–4% paraffin wax dissolved in heptane, granulated and pressed to 10 mm diameter compacts. Pressing was carried out at 2–3 tons/cm2 and the green density varied in the range 53–58%. Dewaxing of green compacts was carried out in a separate cycle in flowing hydrogen. Sintering was carried out under positive pressure of H2. The sintering cycle adopted included the following steps: heating to 900 C at 10 C/min, holding for 30 min, heating to 1300 C at 5 C/ min followed by heating to final sintering temperature (1400 and 1450 C) at the rate of 20 C/min. The holding time at the sintering temperature was 30–45 min. 2.3. Characterization of sintered WC–Co The density of the sintered samples was measured by Archimedes method. The porosity of the sintered WC–Co cemented carbides was rated by comparing the microstructure with ISO4505 standards using optical microscope. The phases in the sintered samples were characterized by X-ray diffraction (XRD) from a Bruker’s diffractometer (AXS Model No. D8 Advance System). The WC grain sizes, and other major microstructural parameters like contiguity and cobalt mean free path were measured in the microstructure of the sintered samples by linear intercept method on a 5K· and 10K· scanning electron micrograph on the back scattered image using the equation given in Golovchan et al. [20]. The values reported are by averaging measurements made on five micrographs taken from different locations in the cross section of the samples. Following the standard procedure used the hardness of the sintered WC–Co was measured by Vicker’s hardness testers using 30 kg load, as given in ISO3878. The values reported are the arithmetic mean of 5–10 readings from each sample. The fracture toughness KIc of the samples was computed by the equation below [10,13]: K Ic ¼ 0:016ðE=H Þ1=2 P =C 3=2 MN m3=2
ð1Þ
where E is the Young’s modulus of the alloy, H is the hardness (N/mm2), P is the load (N) and C is the crack length from the center of the indent to tip of the crack. Young’s modulus of the sintered samples was measured by nanoindentation technique using MTS NanoIndentation XP machine. The value used (E = 540 GPa) is an average of 25 readings. 3. Results and discussion 3.1. Nanocrystalline WC–Co powder The mean particle size, WC crystallite size, carbon and oxygen content of both unmilled and milled powders are
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given in Table 1. The grain size obtained after different milling conditions are given in Table 2. The grain sizes reported are measured by X-ray diffraction method using Scherrer equation with appropriate care taken for instrumental broadening and strain effect. A few milled powder samples were also evaluated for grain size using TEM. The milling parameters were optimized based on the grain size and chemistry of the WC–Co powder. All the powders milled at 250 rpm showed cobalt content that differed significantly from the initial powder composition because of the ball erosion. Milling at 150 rpm avoids such erosion and the change in cobalt content. Powders milled 24 h at 150 rpm resulted in WC grain size of 38 nm as measured from XRD peak width. Further increase in the milling time (up to 48 h) gave little further reduction in grain size. The TEM image of the powder obtained after 24 h milling shows WC grains of around 40 nm though a few bigger grains of size up to 250 nm are also observed. The XRD analysis of the powder showed only WC and FCC cobalt phase. 3.2. Densification behaviour Fig. 1 shows the variation of shrinkage rate with temperature during heating to SPS temperature. Significant shrinkage started at 800–850 C and a maximum shrinkage rate was observed in the range 950–1000 C, irrespective of the final sintering temperature. With the high heating rate (150 C/min) employed in SPS, the actual heating rate value in the sintering runs may vary slightly. But the trend of shrinkage rates is definite and indicates that maximum shrinkage rate is attained before the isothermal hold temperature in these experiments.
Table 1 Characteristics of WC–Co powder Powder property
Before milling
After milling
Mean particle size (lm) FSSS WC grain size (nm)
28 3 lm
0.42 40–250 (TEM) 38 (XRD)
Elements Cobalt, wt.% Total carbon, wt.% Free carbon, wt.% Oxygen, ppm
12.79 5.38 <0.01 1000
12.68 5.32–5.36 – 2250
Fig. 1. Variation of shrinkage rate with temperature during spark plasma sintering of nanocrystalline WC–Co.
The density of the samples without inhibitor addition, fabricated by both spark plasma sintering and liquid phase sintering at different temperatures and time are given in Fig. 2. The density increases with both temperature and hold time. The normally specified density (>14.5 g/cm3) and pore rating (A02B00) for this alloy was obtained in samples spark plasma sintered at 1100 C for 10 min at 50 MPa pressure. To get similar densification and pore rating, a sintering time of 30 min was required at 1450 C in liquid phase sintering. Though the density obtained in 1050 C/10 min SPS sample was also near theoretical (14.46 g/cm3) considerable number of pores of size less than 0.5 lm (outside ISO4505 standards specifications) was observed in the microstructure. SPS at 1100 C brought the porosity level to within acceptable micro pore level. Addition of grain growth inhibitors VC and VC + Cr3C2 led to lower sintered density in both SPS and liquid phase sintering (see Tables 3a and 3b). In 1% VC added sample the decrease in density was to the extent of 0.3–0.55 g/cm3 in SPS and 0.05–0.1 g/cm3 in liquid
Table 2 Variation of WC grain size with milling conditions Milling time
12 24 32 48
WC grain size, nm 250 rpm
150 rpm
30 18 12 10
– 38 35 32
Fig. 2. Variation of density of WC–Co compacts processed by both SPS and liquid phase sintering at different temperature and time.
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Table 3a Microstructure parameters in spark plasma sintered material Material
SPS temperature, C
SPS time, min
Density (% theor.)
WC grain size, lm
Mean Co intercept length, lm
WC grain contiguity
WC–12Co
1000 1050 1100 1100 1100 1100 1100
10 10 10 10 10 10 10
97.93 99.35 99.89 97.66 95.94 90.08 96.64
– 0.70 0.80 0.55 0.47 – 0.49
– 0.29 0.25 0.22 0.21 – 0.21
0.40 0.42 0.49 0.52 – 0.48
WC–12Co–0.5VC WC–12Co–1.0VC WC–12Co–1.5VC WC–12Co–0.75VC–0.25Cr3C2
(0.07) (0.04) (0.10) (0.06) (0.04)
(0.05) (0.06) (0.04) (0.09) (0.06)
(0.10) (0.14) (0.15) (0.01) (0.09)
3
100% density corresponds to 14.55 g/cm . WC grain size, cobalt layer thickness and WC grain contiguity measured as mean linear intercept. Values within the brackets indicate the standard deviation.
Table 3b Microstructure parameters in liquid phase sintered material Material
Sintering temperature, C
Sintering time, min
Density (% theor.)
WC grain size, lm
Mean Co intercept length, lm
WC grain contiguity
WC–12Co
1400 1450 1450 1450 1450 1100
30 30 45 45 45 10
98.62 99.79 99.86 99.79 99.51 90.08
0.78 0.86 1.06 0.75 0.62 –
0.34 0.32 0.26 0.27 0.21 –
0.46 0.39 0.42 0.47 0.52 –
WC–12Co–0.5VC WC–12Co–1.0VC WC–12Co–1.5VC
(0.13) (0.14) (0.09) (0.18) (0.09)
(0.10) (0.07) (0.11) (0.16) (0.12)
(0.14) (0.11) (0.08) (0.14) (0.11)
100% density corresponds to 14.55 g/cm3. WC grain size, cobalt layer thickness and WC grain contiguity measured as mean linear intercept. Values within the brackets indicate the standard deviation.
phase sintering. In liquid phase sintering however, complete densification was attained on extending the sintering to 45 min. Increasing the VC addition further (more than 1%) decreases the density significantly in SPS. 3.3. Phases, microstructure and grain size XRD investigation of the WC–Co samples spark plasma sintered to full density reveals that oxygen level in the powder higher than 0.5 wt.% led to Co3W3C (‘‘eta’’) phase formation during spark plasma sintering. By passivation of the milled powders by slow exposure to atmosphere, and by minimum exposure to open air it was possible to reduce oxygen pick up by the active powder, but the oxygen content was still about 2250 ppm. Addition of 0.1% carbon in the powder during milling eliminated the eta phase. However, it was seen that even a small excess in carbon addition resulted in free carbon peak in XRD. The microstructure of WC–Co spark plasma sintered at temperatures 1000 and 1100 C are given in Fig. 3a and b. Samples sintered at 1000 C showed incomplete sintering (density of 14.25 g/cm3) with rounded WC grains and cobalt ‘‘lakes’’ along with macro- and micro-pores. WC grains became faceted and the cobalt distribution got more uniform as the sintering temperature was raised to 1100 C. In comparison Fig. 4 shows the microstructure of WC–Co liquid phase sintered at 1450 C/30 min. The microstructure obtained with SPS at 1100 C/10 min and with liquid phase sintering at 1450 C/30 min appear similar. Tables 3a and 3b give the microstructural parameters WC grain
size, mean cobalt intercept length and contiguity measured on samples densified by SPS and liquid phase sintering under different temperature/time conditions. Full density and acceptable level of microporosity in WC–Co without any additives was obtained in both the methods of sintering, but at different temperatures and hold times. Density obtained in liquid phase sintering at 1450 C for 30 min and 45 min gave values (99.79% and 99.86%, respectively) comparable to that obtained by SPS at 1100 C/10 min (99.89%). The grain size obtained in the 1100 C/10 min SPS sample was finer than both (Tables 3a and 3b). At closely comparable density value (SPS 1100 C/10 min and liquid phase sintering 1450 C/ 45 min) the grain size in liquid phase sintered sample was 32% coarser. A grain size of 0.80 lm as in the SPS 1100 C/10 sample was in fact comparable to that in sample liquid phase sintered at 1400 C, which had not sintered fully. The mean intercept length of Co in this SPS sample was less than that in liquid phase sintered samples, indicating good coverage of the inter-particle surfaces in the fine grain structure. Under the conditions of applied pressure and temperature in SPS, cobalt in the solid state is seen to flow into the inter-particle crevices fully, much as liquid phase spreads on the particles by surface tension. This is supported by the observation that the contiguity is comparable in the two: where the particles are already in close contact, in the absence of a crevice/surface, cobalt may not penetrate either as liquid or as solid flow. Fig. 5 shows WC grain size distribution in the two cases (SPS 1100 C/10 min and liquid phase sintering 1450 C/
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Fig. 5. Grain size distribution, WC–12Co: (a) spark plasma sintered at 1100 C for 10 min and (b) liquid phase sintered at 1450 C for 30 min. Fig. 3. SEM micrograph (back scattered mode) of WC–12Co spark plasma sintered at (a) 1000 C for 10 min and (b) 1100 C for 10 min.
Fig. 4. SEM micrograph (back scattered mode) of WC–12Co liquid phases sintered at 1450 C for 30 min.
30 min). Both show reasonably continuous distribution, but SPS shows a narrower distribution with the near absence of grains over 2 lm.
Adding inhibitor VC decreases the WC grain growth in both in SPS and in liquid phase sintering. The parameters WC grain size, mean cobalt intercept length and contiguity in these systems are given in Tables 3a and 3b. Figs. 6 and 7 show the microstructures of WC–Co SPS at 1100 C for 10 min and liquid phase sintered at 1450 C for 45 min, respectively with different amount of VC addition. The extent of decrease in grain size in both cases is comparable: With 1 wt.% VC addition WC grain size decreased from 0.80 to 0.47 lm (41.3%) in SPS processing whereas in liquid phase sintering the grain size decreased from 1.06 to 0.62 lm (41.5%). The grain size of WC–Co with 1.5 VC was not quantifiable because of the low sintered density of the samples. Replacing part of VC with Cr3C2 did not show any noticeable difference in the microstructural parameters, although there was in improvement in the density. WC grain growth in liquid phase sintering of WC–Co occurs by Ostwald ripening with dissolution of smaller WC grain and reprecipitation on larger grains in liquid Co. This growth is predominantly interface controlled [21], and addition of refractory carbides such as VC prob-
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Fig. 6. SEM micrograph of WC–12Co with (a) 0.5VC (b) 1VC; SPS at 1100 C for 10 min.
Fig. 7. SEM micrograph of WC–12Co with (a) 0.5VC (b) 1VC, liquid phase sintered at 1450 C for 45 min.
ably alters the interface energy and interferes with the dissolution–reprecipitation steps. However, this whole process is not restricted to liquid phase sintering alone but occurs to a remarkable extent during solid state sintering also [6,22]. This implies that significant extent of diffusion of WC occurs in cobalt in the solid state. In this case any change in the interfacial energy by the addition of inhibitors may not have any role to play. It is likely that the presence of dispersions on the WC–Co interface interferes with the dissolution and/or precipitation of WC in Co.
published relating hardness and microstructure in straight WC–Co cemented carbides [23–26]. The most comprehensive among these [23,26] give the hardness of the cemented carbide HCC as
3.4. Mechanical properties Fig. 8a and b gives the effects of VC addition on hardness and toughness of samples spark plasma sintered or liquid phase sintered under different conditions. The hardness values of spark plasma sintered samples in all cases are higher than those of liquid phase sintered samples (Table 4). This is evidently a result of microstructural parameters, mainly the finer grain size. There have been several papers
H CC ¼ k 1 H WC V WC C þ k 2 H Co ð1 V WC CÞ
ð2Þ
where H refers to hardness, V, to volume fraction and the subscripts WC and Co refer to the phases and C is the contiguity. HWC and HCo are related with the corresponding grain size/mean free path of the corresponding phases. In the equation by Lee and Gurland [23] the values of k1 and k2 are both 1, while in that by Xu and Agren, [26] k1 is 1.205 and k2, 0.9. The values of hardness based on Eq. (2) are shown in Table 5. It appears that Lee and Gurland model is eminently more suited for hardness below 15 GPa. The good correlation indicates that there is no major factor other than microstructural indicators that dictate hardness in straight WC–Co cemented carbides. Hence, the higher hardness exhibited in SPS carbides can be attributed entirely to microstructure refinement provided by the process.
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Fig. 8. Variation of (a) hardness (HV30) and (b) fracture toughness (KIc) with VC addition.
Addition of VC increases the hardness both in SPS and liquid phase sintered samples. While Eq. (2) is limited to carbides without any additions, it can be surmised that the resulting improved hardness is due to the effect of grain growth inhibition both in SPS and liquid phase sintering.
Sample
Lee and Gurland Model (in GPa)
Xu and Argon model (in GPa)
SPS at 1050 C for 10 min SPS at 1100 C for 10 min Liquid phase sintered at 1450 C for 30 min
14.40 14.75 13.77
15.34 15.68 14.61
Toughness is also seen to improve with VC addition both in SPS and liquid phase sintering (Fig. 8b). Partial substitution of VC by Cr3C2 further improves the toughness in SPS. The increase in KIc is higher in liquid phase sintered samples than in SPS samples. Fracture in coarse grained (d > 2 lm) cemented carbide is reported to occur by cleavage, while in fine grained material fracture occurs by interfacial decohesion [27]. In liquid phase sintering, with complete wetting of the carbide surface by liquid cobalt in sintering, the WC–Co interface is likely to be more cohesive than is the case with SPS where cobalt in the plastic state is pushed into the gaps between carbide particles by the applied pressure. The trend of fracture toughness readings (Table 4) may give some further insight into the fracture mechanism. A correlation between KIc and density is shown in Fig. 9. With increasing VC content in SPS samples, fracture toughness increases with decreasing density. With fracture occurring through interfacial decohesion rather than cleavage, pores in the cobalt layer are likely to provide crack arrestors, which can contribute to improved toughness. Density changes with VC addition in liquid phase sintered samples are considerably less marked. However, there is significant improvement in toughness with VC addition over that of straight WC–Co. In the hardness range reported here, Schubert et al. [28] found no significant change in toughness with VC addition, but suggested that at higher hardness values (>20 GPa) with nanograin precursors VC addition may contribute to higher toughness. In the present study, it is shown that with nanograin pre-
Table 4 Hardness and fracture toughness of samples spark plasma sintered or liquid phase sintered under different conditions Material
Process
Sintering temperature C
Sintering time, min
WC–12Co
SPS
1000 1050 1100 1400 1450 1450 1100 1450 1100 1450 1100 1100
10 10 10 30 30 45 10 45 10 45 10 10
Liquid phase sintering
WC–12Co–0.5VC WC–12Co–1VC WC–12Co–1.5VC WC–12Co–0.75VC–0.25Cr3C2
SPS Liquid phase sintering SPS Liquid phase sintering SPS SPS
Hardness (HV30)
HV0.5
1472 1512 1450 1381 1386 1370 1490 1445 1570 1486 1587 1590
1557 1613 1579 1498 1473 1465 1600 1484 1608 1514 1624 1626
Fracture toughness, MPa m1/2 10.0 9.55 10.90 17.30 13.53 14.04 10.86 15.70 11.42 15.60 11.43 12.1
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This could be directly correlated with the finer WC grain size and cobalt distribution. Addition of these carbides also improved the fracture toughness in both SPS and liquid phase sintering. Since the fracture occurs in fine grained materials predominantly in the cobalt layer and the interface, increased porosity in the cobalt layer caused by the addition of VC probably provides crack arrest for improved toughness. Acknowledgements
Fig. 9. Variation of fracture toughness with density.
cursors improved toughness results even at much lower hardness ranges (that is higher cobalt content). 4. Conclusions Nanocrystalline powder of WC–Co was generated by mechanical milling of conventional powder with a grain size of 3 lm. The milling process was optimized to realize an average grain size of 38 nm. Consolidation of the milled nanocrystalline powder was studied by spark plasma sintering as well as by liquid phase sintering. Acceptable density level (>14.5 g/cm3) and pore rating (
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