Effects of AGG on fracture toughness of tungsten carbide

Materials Science and Engineering A 445–446 (2007) 587–592

Effects of AGG on fracture toughness of tungsten carbide Tao Li a,b , Qingfa Li b , J.Y.H. Fuh a,∗ , Poh Ching Yu a,b , L. Lu a , C.C. Wu c a

Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore b Singapore Institute of Manufacturing Technology (SIMTech), 71 Nanyang Drive, Singapore 639798, Singapore c Department of Mechanical Engineering, Southern Taiwan University of Technology, Tainan, Taiwan Received 1 June 2006; received in revised form 11 September 2006; accepted 27 September 2006

Abstract The cobalt-bonded tungsten carbide has been regarded as a specific success in improving the fracture toughness of ceramics. However, the metal binder phase lowers the hardness and corrosion resistance of the composite and limits its applications. In this paper, the possibility of in situ-toughening tungsten carbide, making use of abnormal grain growth of WC grains to reinforce WC–Co composites, is investigated. The volume fraction of abnormally large grains in the samples is controlled by tailoring compositions and sintering conditions. The fracture toughness of the samples is evaluated using the indentation technique. The experiment results show that the fracture toughness of the sample could be improved by introducing a certain volume fraction of abnormal grains, but at some expense of the hardness. The highest value of fracture toughness, 7.34 MPa m0.5 , is achieved for WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 ◦ C for 4 h. The main mechanisms for the improved toughness are cutting elongated grains, crack bridging and crack deflection, which could resist crack growth in the sample. Therefore, it is possible to in situ-toughen tungsten carbide using the abnormal grain growth of WC. © 2006 Elsevier B.V. All rights reserved. Keywords: Tungsten carbide; Fracture toughness; Abnormal grain growth; In situ-toughening mechanism

1. Introduction Tungsten carbide, which was discovered in 1898, has long been well known for its exceptional hardness and wear/corrosion resistance. However, like the brittle nature of ceramics, the fracture toughness of tungsten carbide is very low. Therefore, it was only after 1920s with the advent of cemented tungsten carbide, did the tungsten carbide begin to be used as cutting tools and wear resistance parts gradually. The cemented tungsten carbide (usually WC–Co), which consists of tungsten carbide grains embedded in a metal binder phase, exhibits high hardness to combat wear and sufficient toughness to withstand interrupted cuts or vibration occurring during the machining process. The cobalt-bonded tungsten carbide is regarded as a specific success in improving the fracture toughness of ceramics [1]. On the other hand, the metal binder phase lowers the hardness and corrosion resistance of the composite and limits its applications. Other approaches to improve the fracture toughness of tungsten carbide seem to be of necessity.

∗

Corresponding author. Tel.: +65 65166690; fax: +65 67791459. E-mail address: [email protected] (J.Y.H. Fuh).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.076

Ceramics can be reinforced by introducing foreign substances in the form of particles, whiskers, platelets, or fibers, as in the fabrication of ceramic matrix composites (CMCs). But the addition of foreign materials decreases the sinterability of ceramics [2]. Recently, several reports have been published on in situtoughened (or self reinforced) silicon carbide [2–6] and silicon nitride [7,8]. Rather than foreign substances addition, the reinforcing agents in these in situ-toughened composites are formed during the fabrication process, making use of abnormal grain growth (AGG) of some silicon carbide or silicon nitride grains. As shown in Fig. 1(a) [2], the reinforcements in in situtoughened SiC composites are abnormal platelet-like SiC grains, which are obtained by the ␤ to ␣ phase transformation or controlled SiC grain growth during sintering or annealing process. While in in situ-toughened Si3 N4 , the reinforcing agents are large rod-like ␤-Si3 N4 grains as shown in Fig. 1(b) [7], which are grown from the ␤-Si3 N4 seeds incorporated into the ␣-Si3 N4 powders. The presence of these abnormal grains can absorb extra energy through deflecting or bridging propagating cracks, resulting in improved toughness. Although the mechanism may not be the same, the phenomenon of AGG was also often observed in the liquid phase sintering of WC–Co and some substantially large platelet-like

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toughness, other two compositions with certain amount of VC addition were selected as comparison: WC (0.7 ␮m)–0.5 wt% Co–0.25 wt% VC and WC (0.7 ␮m)–0.5 wt% Co–0.5 wt% VC. 2.2. Sample preparation Commercial WC powders (average particle size of 0.7 ␮m) were used as the bulk phase in the composites. The commercially available Co powders (average particle size of 1 ␮m) were used as the binder phase and vanadium carbide (VC) as the grain growth inhibitor. The powders of an amount of 50 ± 1 g with compositions mentioned above were mixed for 24 h in a Bioengineering Inversina© shaker mixer. WC balls were used as the stirring agents. The mixed powders were then uniaxially pressed into green compacts with a pressure about 300 MPa using WC die and punch. The green compacts were then sintered in vacuum (P < 0.02 bar) at 2000 ◦ C for different time durations. The sintering process was carried out in an IVI furnace with graphite heating elements. 2.3. Property characterization

Fig. 1. Pictures of in situ-toughened (or self reinforced) silicon carbide and silicon nitride. (a) Platelet-like SiC grains in in situ-toughened silicon carbide. (b) Rod-like ␤-Si3 N4 grains in in situ-toughened silicon nitride.

WC grains appear in these samples [9–12]. Therefore, it is possible to produce in situ-toughened tungsten carbide by using of abnormally grown WC grains to reinforce WC–Co composites. To achieve this target, the size, content and distribution of abnormal WC grains must be tailored carefully. It has been reported in our work that the Co concentration has an obvious effect on the AGG of WC and a range of Co concentrations may exist for the large amount of AGG [13]. Apart from the compositions, the AGG of WC is also affected by the sintering temperature and sintering time. In this work, the influence of AGG on fracture toughness of WC composite is investigated by regulating composition and sintering time. 2. Experimental procedure 2.1. Composition selection A large number of abnormal grains were observed in the sintered WC (0.7 ␮m)–0.5 wt% Co and the distribution of these abnormal grains was very uniform in the whole sample. So the composition of WC (0.7 ␮m)–0.5 wt% Co is selected as the sample to investigate the effect of AGG on the fracture toughness of WC–Co composite. It is also well known that VC can be acted as grain growth inhibitor in liquid phase sintering of WC–Co [14]. To study the effect of abnormally large grains on the fracture

The actual density of the sintered specimens was measured in water according to the Archimedes method and the theoretical density is calculated using the rule of mixtures, relative density being defined as the ratio of actual density to theoretical density. The specimens were then grounded and polished down to 1 ␮m. The Vickers hardness (Hv ) and fracture toughness (Kc ) of the samples can be determined from the indentation technique as follows [15]: Hv =

1.854P d2

(1)

in which P is the applied load and d is the average length of diagonals, and  1/2  3/2 E P Kc = ξv (2) Hv c Here, ξ v , E, and c represent a materials-independent constant for Vickers-produced radial crack, Young’s modulus and the half length of the radial crack, respectively. In this study, ξ v = 0.016 and E is also calculated using the rule of mixtures. The indentation was produced using a Mitutoyo hardness tester with a load force of 2 kg for 10 s. The reported values of hardness and fracture toughness were the compilation of the average data obtained from five sets of indentation test. The microstructures of the etched [HNO3 (65%) + H2 O2 (30%) at 60 ◦ C] specimens were characterized using scanning electron microscopy (SEM). 3. Results and discussion 3.1. Microstructure of samples The monocarbide, WC, has a simple hexagonal crystal structure with two atoms per unit cell. Because of the different spacing of the tungsten and carbon planes in [1 0 1¯ 0] direction, there are

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Fig. 2. Shapes of WC crystals in pores and at polished surface: (a) WC crystals in pores; (b) WC crystals at polished surface.

two sets of three equivalent (1 0 1¯ 0) planes rather than six equivalent (1 0 1¯ 0) planes [16]. This is the reason why WC crystals assume the typical shapes shown in Fig. 2(a), which correspond to the ditrigonal–bipyramidal class of the crystal system. While in the two-dimensional images of polished surface, most of the WC crystals assume elongated square shapes (Fig. 2(b)) due to the intersections of the crystals with the cross-section plane. The microstructure of the samples, which have been etched for 12 min to reveal the abnormal grains clearly, are shown in Figs. 3–5. As can be seen clearly from these figures, the abnormal grain growth of WC is greatly affected by the composition and sintering time. For the sample of WC–0.5 wt% Co without VC addition, a significantly high density of the abnormally large grains is observed even sintered at 2000 ◦ C for only 1 min. Extending sintering time to 2 or 4 h leads to the increase of the volume fraction of abnormal grains. However, the size of these large grains is limited on account of the impingement of grains with each other. Another feature worthy of note is that quite a few pores appear in the sample, especially beside the large grains. Two factors may contribute to this phenomenon. First is the impingement of the large grains making the elimination of pores difficult; second is the long etching time, which will dissolve some small grains and leave pores there.

Fig. 3. SEM images of WC–0.5 wt% Co with different sintering time: (a) 2000 ◦ C for 1 min; (b) 2000 ◦ C for 2 h; (c) 2000 ◦ C for 4 h.

The phenomenon of abnormal grain growth is retarded dramatically, as shown in Figs. 4 and 5, by addition of VC in the samples. In WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 ◦ C for 1 min, only very few grains larger than l0 ␮m are detected. The number and size of abnormal grains become much larger if the sintering time is extended to 2 or 4 h. It seems that the amount of 0.25 wt% VC addition is not enough to inhibit the abnormal grain growth completely.

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Fig. 4. SEM images of WC–0.5 wt% Co–0.25 wt% VC with different sintering time: (a) 2000 ◦ C for 1 min; (b) 2000 ◦ C for 2 h; (c) 2000 ◦ C for 4 h.

However, the inhibiting effect is far more obvious with 0.5 wt% VC addition in that almost no grains larger than l0 ␮m are observed even the sample is sintered at 2000 ◦ C for 4 h, as shown in Fig. 5(c). The impingement of large grains in the samples of WC–0.5 wt% Co may account for its density decrease with extending sintering time, which is indicated in Fig. 6. As seen from Fig. 6, the density of WC–0.5 wt% Co–0.25 wt% VC and WC–0.5 wt% Co–0.5 wt% VC increases

Fig. 5. SEM images of WC–0.5 wt% Co–0.5 wt% VC with different sintering time: (a) 2000 ◦ C for 1 min; (b) 2000 ◦ C for 2 h; (c) 2000 ◦ C for 4 h.

with extending sintering time from 1 min to 4 h, resulting from the grain growth, pores elimination and sample shrinkage during sintering process. More than 99% of relative density can be achieved for WC–0.5 wt% Co–0.5 wt% VC sintered at 2000 ◦ C for 4 h. On the other hand, the density of WC–0.5 wt% Co decreases with extending sintering time. The relative density of WC–0.5 wt% Co sintered at 2000 ◦ C for 1 min is about 98.9%, but decreases to about 95.6% if the sintering time is prolonged to 4 h. This may be due to the impingement of the large grains in

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Fig. 6. Density variations of different samples with sintering time.

Fig. 7. Indentation produced by Vickers hardness test.

the sample, which makes the shrinkage of the sample difficult. At the same time, the evaporation of materials from the sample cannot be ignored, leading to the density decrease. 3.2. Fracture toughness and hardness of samples A square-base diamond pyramid is used as the indenter in the Vickers hardness test. The image of indentation on the polished surface of sample WC–0.5 wt% Co–0.5 wt% VC is shown in Fig. 7, from which Vickers hardness and fracture toughness can be calculated using Eqs. (1) and (2). Fracture toughness and hardness of the samples are given in Figs. 8 and 9, as functions of compositions (the amount of VC addition) and sintering durations (2000 ◦ C for 2 and

Fig. 8. Fracture toughness variations with compositions and sintering durations.

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Fig. 9. Hardness variations with compositions and sintering durations.

4 h). As expected, WC–0.5 wt% Co–0.5 wt% VC sintered for 2 h has the lowest fracture toughness of about 5.7 MPa m0.5 , corresponding to very few large grains in the sample. This sample also has a very high hardness, namely 20 GPa shown in Fig. 9. Reducing VC amount will increase the volume fraction of abnormally large grains, demonstrated by Figs. 3–5, and also increase the fracture toughness of the sample, demonstrated by the lighter line in Fig. 8. The fracture toughness of WC–0.5 wt% Co–0.25 wt% VC and WC–0.5 wt% Co sintered at 2000 ◦ C for 2 h is 6.84 MPa m0.5 and 6.93 MPa m0.5 , respectively. Unfortunately, the hardness of these samples is decreasing from about 20 to about 16 GPa, because of the increasing fraction of abnormal grains. Prolonging the sintering time from 2 to 4 h increases the fracture toughness of WC–0.5 wt% Co–0.5 wt% VC to about 6.11 MPa m0.5 , which may be the result of whole grain coarsening in the process. The maximum value of fracture toughness, 7.34 MPa m0.5 , is achieved for WC–0.5 wt% Co–0.25 wt% VC sintered for 4 h. A lower than that of sample with 0.5 wt% VC addition, but moderate hardness (18.5 GPa) is obtained for this sample. The relatively higher volume fraction of abnormally large grains and higher density of this sample are the main reasons for the highest fracture toughness and moderate hardness. The highest volume fraction of abnormal grains is observed in WC–0.5 wt% Co sintered for 4 h as shown in Fig. 3. However, the fracture toughness and hardness of this sample unexpectedly both drop significantly. The possible reason is that the highest density of abnormal grains decreases the hardness while the lowest relative density (only 95.6%) caused by the impingement of those large grains decreases the fracture toughness. Too many pores in the sample make it weaker. Therefore, it can be concluded that the abnormal grains in tungsten carbide samples have considerable effects on the fracture toughness. In the condition of without reducing the density of the sample, it seems that increasing the volume fraction of abnormal grain in the sintering process can toughen the sample, probably leading to in situ-toughened tungsten carbide, but at the expense of the hardness. To analyze the toughening mechanism, the crack paths, introduced by indentation test, are investigated. Just like in in situtoughened Si3 N4 and SiC [2–8], cutting elongated grains, crack bridging and crack deflection are also observed in the sample of WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 ◦ C for 4 h, as shown in Fig. 10, the presence of which can resist crack growth in the sample and consume more energy for separa-

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Fig. 10. Crack paths in WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 ◦ C for 4 h: (a) cutting elongated grains; (b) crack bridging; (c) crack deflection; (d) crack deflection and cutting elongated grains.

tion of the fracture surface, macroscopically termed as increased toughness. 4. Summary The possibility of in situ-toughening tungsten carbide, making use of abnormal grain growth of WC grains to reinforce WC–Co composites, is investigated in this work. Because the abnormal grain growth of WC is strongly affected by the compositions of the sample and sintering conditions, a considerable volume fraction of abnormally large grains can be introduced by tailoring compositions and sintering conditions. The fracture toughness of the samples is evaluated using indentation technique. The results show that the fracture toughness of the sample could be increased by introducing a certain volume fraction of abnormal grains, but at the expense of the hardness. The highest value of fracture toughness, 7.34 MPa m0.5 , is achieved for WC–0.5 wt% Co–0.25 wt% VC sintered at 2000 ◦ C for 4 h and the hardness of this sample, 18.5 GPa, is moderate. Cutting elongated grains, crack bridging and crack deflection may be the main mechanisms for improved fracture toughness. However, the quantitative relationship between the fracture toughness and the volume fraction of abnormal grains need to be investigated further.

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