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two-step spark plasma sintering
Materials Science & Engineering A 561 (2013) 445–451
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In-situ elongated b-Si3N4 grains toughened WC composites prepared by one/two-step spark plasma sintering Donghai Zheng, Xiaoqiang Li n, Yuanyuan Li, Shengguan Qu, Chao Yang National Engineering Research Center of Near-net-shape Forming Technology for Metallic Materials, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
abstract
Article history: Received 4 September 2012 Received in revised form 15 October 2012 Accepted 15 October 2012 Available online 23 October 2012
The WC–Si3N4 composites toughened by in-situ elongated b-Si3N4 grains were prepared by spark plasma sintering. By exploiting the difference in kinetics between WC grain-growth and b-Si3N4 graingrowth through sintering at an elaborate temperature or two-step sintering, composites with microstructure of in-situ elongated b-Si3N4 grains were grown sufficiently in the fine-grained WC matrix without fast-grain-growth being obtained. The relation of the microstructure and mechanical properties of the composites, as well as the toughening mechanisms, are investigated. Comparing with the fracture toughness of 6.69 MPa m1/2 for the pure WC, that of the obtained WC–Si3N4 specimen reaches 10.94 MPa m1/2. & 2012 Elsevier B.V. All rights reserved.
Keywords: Tungsten carbide Composites Sintering Toughening Mechanical characterization
1. Introduction WC–Co cemented carbides have been widely used as cutting tools and wear-resistant components due to a singular combination of their properties including high hardness, moderate toughness and excellent wear resistance. Considering that the metallic binders in cemented carbides are deleterious on hardness and inferior to WC in corrosion and elevated temperature applications [1], WC-based materials with non-metal reinforcements have been fabricated through some newly developing sintering techniques [2–7], the toughness of which could be elevated to 6 MPa m1/2 from 4 MPa m1/2 for pure WC by particle toughening (VC, Mo2C, Al2O3, etc.) and/or phase-transformation toughening (ZrO2). However, few researches have been reported about using ceramic whiskers as a toughening agent for the binderless WC material. b-Si3N4 whisker is one of the most common ceramic whiskers, which has been used to reinforce the ceramics with low fracture toughness [8,9]. Especially for the self-reinforced Si3N4, large elongated b-Si3N4 grains grown in-situ in a fine matrix act as reinforcing grains [10–17]. Compared with particle toughening, whisker toughening is expected to be more effective due to a combination of toughening mechanisms including crack-bridging, crack-deflection and whisker-pullout. Note also that in-situ growth of whiskers gets rid of the problems associated with fabricating adscititious-whisker-reinforced ceramic composites
n
Corresponding author. Tel./fax: þ 86 20 87112111. E-mail address: [email protected] (X. Li).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.10.059
including the high cost of whiskers, potential human health hazards in their handling, and processing difficulties such as deagglomeration, mixing, and settling [10,18]. Moreover, Si3N4 materials are expected to possess excellent wear resistance [19], which is also one of the reasons for developing WC–Si3N4 composites. During the past decade, a two-step sintering technique developed by Chen and Wang [20–22], which consists of sintering at a high temperature for a short period and subsequent sintering at a low conventional sintering temperature, has been successful for sintering of nanostructured ceramics through solid-state sintering [20–23] or liquid-state sintering [24,25]. The separation of grainboundary diffusion from grain growth in the second-step sintering was achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration [20]. In other respects, for a two-phase material containing compositions with different kinetics of grain growth, a microstructure of one phase grown sufficiently in a ‘‘frozen’’ fine-grained matrix is likely to be obtained by sintering at an elaborate temperature or twostep sintering. In this study, to toughen binderless WC, elongated b-Si3N4 grains are introduced into the matrix by in-situ growing. To the best of our knowledge, this composite system has not been investigated to date. We are attempting to increase the volume fraction of the elongated b-Si3N4 grains within a finegrained WC matrix through one/two-step spark plasma sintering (SPS), and also to study the densification behavior and the relation of the microstructure and mechanical properties of the composites.
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2. Experimental procedure
2.2. Density and mechanical property measurement
2.1. Processing
Based on the Archimedes principle, sintered density was measured using water. The hardness (HV10) was evaluated on a Vickers hardness tester (430SVA, Wilson Wolpert Co. Ltd., China) with a load of 10 kg. The fracture toughness (KIc) was calculated based on the radial crack length produced by Vickers (HV10) indentation, according to Anstis formula [26]. The reported values are the average of the data obtained from five indentation tests. The elastic properties of the bulk samples were determined by non-destructive test via pulse-echo overlap ultrasonic technique using ultrasonic detector.
The starting powders were WC (0.8–1 mm, purity499.9%, Golden Egret Special Alloy Co. Ltd., China), Si3N4 ( 1 mm,495% a-phase, Xuzhou Jiechuang New Material Technology Co. Ltd., China), Y2O3 (5–10 mm, purity499.9%, Sinopharm Chemical Reagent Co. Ltd., China) and Al2O3 ( 1 mm, purity499.9%, Beijing Mountain Technical Development Center, China). WC– 10 wt%Si3N4 (93Si3N4 þ6Y2O3 þ1Al2O3, wt%) powder mixtures were wet mixed on a planetary ball mill (QM-3SP2, Nanjing NanDa Instrument Plant, China) in ethanol for 30 h using cemented carbide milling balls (ball-to-powder weight ratio was 3:1) and cemented carbide vials (250 mL). To minimize the potential Co contamination from the milling balls or vials, milling was conducted in a low energy mode, in which the milling process was paused every 30 min for staying 18 min, subsequently restarted reversely at a constant rotation speed of 180 r/min, and finally stopped after 60 cycles. Furthermore, drying and sieving the milled powders was performed in order to get rid of agglomerates, which may lead to poor sinterability. The pure WC or obtained WC–Si3N4 powders were poured into a cylindrical graphite die with an inner diameter of |20 mm and an outer diameter of |50 mm. Then sintering was conducted on a Dr. Sinter Model SPS-825 Spark Plasma Sintering System (Sumitomo Coal Mining Co. Ltd., Japan) by SPS in vacuum ( r 6 Pa) under an applied pressure of 30 MPa. Three different heating schedules were adopted as shown in Table 1. In accustomed sintering mode, the powder compacts were sintered at various temperatures from 14501 to 1600 1C for 30 min. For zero-time sintering, the specimens were immediately cooled just after the sintering temperature varying from 15501 to 1800 1C was reached. In two-step sintering, the powder compacts were heated to 1700 1C and then immediately cooled to 1450–1600 1C with a holding time of 30 min. Within the sintering period, the heating rate was 100 1C/min, and the cooling rate was 50 1C/min. To investigate the difference between the densification behavior of the pure WC and WC–Si3N4 composites, they were sintered at 1750 1C for dwelling 5 min. In the sintering, graphite papers were used to separate the powders from the graphite die or punch, and the die was surrounded with a 10 mm thick porous carbon felt insulation to minimize the radiation heat loss. An infrared pyrometer ( Z 570 1C) was focused at the bottom of a central core hole in the die wall and with 7.5 mm away from the inner wall.
2.3. Microstructure observation and characterization The microstructure of polished surfaces was examined by high-resolution scanning electron microscopy (SEM, Nova Nano 430, FEI, USA). The average grain size of the WC matrix was obtained by measuring over 300 grains using the Image-ProPlus software [27]. According to the method provided by Pigeon and Varma [28], the wt fraction of b-phase in Si3N4 was calculated based on the ratio of the a(200)/b(200) peak heights (PH), the data of which were collected at 0.021 step 1 between 25 and 291 of 2y (time constant 2 s) with subtraction of the contribution due to background noise by an X-ray diffractometer (XRD, D8 Advance, Bruker Co., Germany) using Cu Ka radiation
a=b phase ratio ¼ 4:9381 ðPHÞ þ 0:1144 ðPHÞ2 þ 0:08106 ðPHÞ3
wt fraction b ¼ 1= 1 þ a=b
ð1Þ ð2Þ
3. Results and discussion 3.1. Densification behavior During SPS, the displacement of the lower punch which reflects the densification process of the sample was automatically stored by a record system. By comparing the shrinkage displacement and shrinkage rate of the pure WC powders with WC–Si3N4 composite, the influence of Si3N4 (6Y1A) addition on the densification behavior of WC-based material is studied. Fig. 1 shows the densification curves of pure WC and WC–Si3N4 composites heated up to 1750 1C for a dwell time of 5 min. For all of these tests, before the real densification of the powders starts, the displacement increases opposite to the shrinkage direction due to the thermal expansion of the compacted sample and/or the punches. As the shrinkage rate turns to be positive, the powders
Table 1 Sintering schedules adopted in this experiment. Sintering methods
Specimens
Sintering conditions (temperature/holding time, 1C/min)
Accustomed sintering
A1 A2 A3 A4
1450/30 1500/30 1550/30 1600/30
Zero-time sintering
Z1 Z2 Z3 Z4 Z5 Z6
1550/0 1600/0 1650/0 1700/0 1750/0 1800/0
Two-step sintering
T1 T2 T3 T4
1700/0-1450/30 1700/0-1500/30 1700/0-1550/30 1700/0-1600/30
Fig. 1. Densification behavior of WC and WC–Si3N4 composites SPSed at 1750 1C for 5 min.
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to be densified, which lasts till the shrinkage rate is down to zero again. For both of the pure WC and WC–Si3N4 composites, the densification process starts at about 1130 1C. Afterwards, the pure WC undergoes a solid-state sintering with gentle shrinkage rates, and the densification process lasts until holding at 1750 1C for more than 2 min. By contrast, the WC–Si3N4 composite obtains much faster densification rates under the same sintering condition especially when the sintering temperature is above 1400 1C, which mainly rests with a small amount of liquid formed by the ternary eutectic melting (SiO2–Y2O3–Al2O3) at about 1371 1C [29]. The presence of the liquid phase assists the rearrangement of the ceramic particles, and also accelerates the mass diffusion during sintering. Finally, the densification process ends at about 1640 1C for the WC–Si3N4 composite, which has been greatly ahead compared with the pure WC. Considering the difference between the actual sample temperature and the measured temperature (The measured point is 7.5 mm away from the sample surface in the present research.) Tiwari et al. reported that low thermal conductivity materials assisted in producing large temperature gradient [30]. In this study, the WC–Si3N4 material is expected to possess lower thermal conductivity compared with the pure WC ( 32 W/m K) due to the low thermal conductivity of Si3N4 ( 2 W/m K), therefore, larger temperature gradient can be obtained during SPSing the WC–Si3N4 composite. With the same actual sample temperature, a little lower measured temperature will be observed for the WC–Si3N4 composite compared with the pure WC. And that also provides a deduction in the sintering temperature for the WC–Si3N4 composite. 3.2. Microstructural analysis During different sintering modes, our attention is focused on some concurrent activities involving densification, a to b-Si3N4 phase transformation, WC grain growth and b-Si3N4 grain growth. In the light of the amount of elongated b-Si3N4 grains grown within the materials, the sintered specimens are subjectively divided into five grades: none, few, moderate, much and plenty. The SEM micrographs of the typical specimens for each grade are illustrated in Fig. 2, and the microstructural characters of all specimens are listed in Table 2 and partially plotted in Fig. 3. Thereinto, the wt fraction (b) of b-phase in Si3N4 was calculated from the XRD data in Fig. 4 by the MDI Jade software [31]. The zero-time sintered specimens can achieve near full density (density Z11.14 g/cm3) when the sintering temperature reached 1600 1C, but the wt fraction b is only 11.73%. It can be concluded that the a–b-Si3N4 transformation process is far behind the densification process. It was reported that the phase transformation of Si3N4 from a to b went through a solution-precipitation process [32,33]. The transformation occurs in a liquid phase generated from the ternary eutectic melting of the SiO2–Y2O3– Al2O3 system in this study. During sintering, a higher temperature or longer holding time is beneficial to the a–b-Si3N4 transformation, just as the wt fraction b increases from 4.07% for 1550 1C/ 0 min to 89.88% for 1800 1C/0 min, or to 34.65% for 1550 1C/ 30 min. Elevating the sintering temperature is more effective for accelerating the transformation process due to the higher diffusion rate and lower viscosity of the liquid mentioned above. But on the other hand, the WC grain-growth is also speeded up while the sintering temperature is above 1700 1C (seen in Fig. 3, specimens Z5 and Z6), suggesting WC grain fast-growth is active. For suppressing the WC grain-growth, lower sintering temperatures are preferred [20]. When the composites are sintered at 1450– 1600 1C for holding 30 min respectively, the average WC grain size for each specimen is less than 1.10 mm. Deservedly, the densification will not complete when the sintering temperature
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is too low, e.g. specimens A1, A2; and the a–b-Si3N4 transformation progresses slowly at the processing temperature. For specimens A3 and A4, the transformation is quickened up without noticeable WC-grain growth (seen in Fig. 3), which suggests that the transformation is active but the WC-grain fast-growth is suppressed at the processing temperatures of 1550–1600 1C. Generally in the self-reinforced silicon nitride ceramics, the enhancement of fracture toughness by in-situ elongated b-Si3N4 grains depends on the volume fraction and the aspect ratio of the elongated b-Si3N4 grains, growth of which is dominated by diffusion-controlled Ostwald ripening [34]. The nucleation and growth rate of b-Si3N4 grains are a function of the rate of dissolution of a-Si3N4 particles, solubility of a-Si3N4 into the liquid, supersaturation of Si and N in the liquid, interfacial tension and wetting between the Si3N4 grains and liquid, and viscosity of the liquid. Comparing the microstructure of specimen Z4 (Fig. 2(c)) with that of Z6 (Fig. 2 (h)), it is clear that the anisotropic b-Si3N4 grain growth progresses rapidly within the narrow temperature interval between 1700 and 1800 1C. And plenty of well-developed elongated b-Si3N4 grains are obtained in specimen Z6. Similar b-Si3N4 grain fast-growth has been observed in fabricating silicon nitride ceramics with interlocking microstructures by SPS [35], in which a mechanism of ‘‘dynamic ripening’’ was assumed. For the ‘‘dynamic ripening’’, owing to a fast heating rate applied, the chemical composition of the coexisting liquid phase deviates from that set by thermodynamic equilibrium. When a critical temperature is reached, the dissolution of small Si3N4 grains is promoted so as to produce a momentarily supersaturated liquid, which will greatly accelerate the a to b-Si3N4 phase transformation and the growth of b-Si3N4 grains. Shen et al. thought that the use of fast heating rate was essential for establishing such dynamic process [35]. In this study, a heating rate of 100 1C/min is used and so the ‘‘dynamic ripening’’ is assumed to occur for explaining the fast transformation of Si3N4 from a to b and the b-Si3N4 grain fast-growth happening at 1700– 1800 1C. It is also notable that the growth of WC grains is speeded up when the sintering temperature exceeds 1700 1C. It may be suggesting that grain-growth mechanism of Ostwald ripening for WC is involved when the liquid phase appears during sintering and turns dynamically above 1700 1C, resulting in WC-grain fastgrowth. For one-step sintering (zero-time sintering and accustomed sintering), specimen Z6 after sintering at 1800 1C for 0 min shows a plenty of elongated b-Si3N4 grains in the WC matrix with average WC-grain size of 1.42 mm. However, the specimen A4 after sintering at 1600 1C for 30 min obtains the ‘‘much’’ grade of elongated b-Si3N4 grains with average WC-grain size of 1.10 mm. So, it is concluded that the b-Si3N4 grains growth can be separated from the WC grain growth by treating at a proper low temperature. This suggests that WC grain fast-growth may involve an activation process that has higher activation energy than Ostwald ripening for b-Si3N4 grains; thus, it is active at higher temperatures but is suppressed at lower temperatures. Exploiting the difference in kinetics between WC grain-growth and b-Si3N4 grain-growth, a two-step sintering method is also expected to be used for fabricating WC–Si3N4 composites with a certain number of elongated b-Si3N4 grains distributing within a fine-grained WC matrix. In this study, the first sintering step, heating to 1700 1C, is to obtain a full density and an extent of a to b-Si3N4 transformation without notable WC grain growth (Similar to specimen Z4). The extent of transformation provides b-Si3N4 grain seeds for the subsequent sintering step, during which the transformation and b-Si3N4 grain-growth are keeping active while the WC-grain fast-growth is suppressed (seen for specimens A1–A4). From a comparison of two-step sintered specimens and the corresponding one-step sintered specimens (e.g. T1 and
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Fig. 2. SEM micrographs of the typical specimens for each grade of elongated b-Si3N4 grains content. (a) Z1: none, (b) Z2: few, (c) Z4: few, (d) Z5: moderate, (e) A3: moderate, (f) A4: much, (g) T3: much, (h) Z6: plenty and (i) T4: plenty.
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Table 2 Microstructural characters of the specimens. Specimen
Density (g/cm3)
WC grain size (mm)
Grade
b (%)
A1 A2 A3 A4 Z1 Z2 Z3 Z4 Z5 Z6 T1 T2 T3 T4
10.54 11.06 11.15 11.14 10.92 11.17 11.19 11.21 11.22 11.22 11.22 11.18 11.20 11.20
0.82 0.88 0.96 1.10 0.80 0.90 0.88 0.92 1.22 1.42 1.08 1.09 1.15 1.26
few few moderate much none few few few moderate plenty much much much plenty
9.38 8.17 34.65 76.85 4.07 11.73 11.52 15.14 48.87 89.88 28.17 34.85 56.95 100
Fig. 3. Average WC-grain size and the wt fraction of b-phase in Si3N4 for the sintered WC–Si3N4 specimens.
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noticeable WC-grain growth does not occur during the second step of sintering at 1450–1600 1C for 30 min. For specimen T4 after heated to 1700 1C and treated at 1600 1C for 30 min (shown in Fig. 2 (i)), the a–b-Si3N4 transformation has nearly completed, as well as obtains plenty of elongated b-Si3N4 grains which are entangled and linked together. And the average size of WC-grain in specimen T4 does not increase rapidly, being 1.26 mm. 3.3. Mechanical properties The hardness and fracture toughness of the sintered pure WC and WC–Si3N4 specimens sintered in different conditions are plotted in Fig. 5. As the Vickers indentation technique may not be viable for the specimens without full density, the fracture toughness data of specimens Z1, A1 and A2 (density o11.14 g/ cm3) are not involved. For intuitively correlating the hardness with the microstructure, the average WC-grain sizes of the WC– Si3N4 specimens are also plotted in Fig. 5. Combining the data of density listed in Table 2, it can be seen that the hardness of the composites mainly depends on the densification degree and WCgrain size of the as-sintered materials. As seen in Fig. 5, for the WC–Si3N4 specimens with near full density (except Z1, A1), the hardness is inversely correlated with the average WC-grain size. Therefore, full densification and suppression of WC grain-growth are believed to be important parameters for fabricating binderless WC-based materials maintaining high hardness. As expected, the fracture toughness of the composites is closely connected to the ‘‘grade’’ of elongated b-Si3N4 grains. Well developed elongated bSi3N4 grains lead to high fracture toughness. Compared with the sintered pure WC with a fracture toughness of 6.69 MPa m1/2, the WC–Si3N4 specimens with ‘‘much’’ grade of elongated b-Si3N4 grains possess a fracture toughness higher than 8.87 MPa m1/2, while those with ‘‘plenty’’ grade show the highest fracture toughness of 10.94 MPa m1/2. From the observation of the indentation cracks in the composites, toughening mechanisms of elongated Si3N4 grain-pullout and crack-bridging by elongated Si3N4 grain are found, as shown in Fig. 6. This result is consistent with the findings reported by Tajima et al., e.g. that the toughening contribution from crack-bridging strongly increased with an increase in volume fraction of elongated grains [37], which is denoted as the grade of elongated b-Si3N4 grains in this paper. With plenty of elongated b-Si3N4 grains and average WC grain size of 1.42 mm, specimen Z6 possesses a hardness of 16.88 GPa and fracture toughness of 9.54 MPa m1/2. By suppressing the WC grain fast-growth and keeping the b-Si3N4 grain-growth active, specimen A4 shows a higher hardness of 18.32 GPa and fracture toughness of 9.46 MPa m1/2, while those of the two-step sintered specimens T1–T4 are in the range of 17.65–18.69 GPa and 8.87– 10.94 MPa m1/2 respectively.
4. Conclusions The WC–Si3N4 composites toughened by in-situ elongated
b-Si3N4 grains have been successfully prepared by spark plasma Fig. 4. XRD patterns for a-Si3N4 (200) and b-Si3N4 (200) crystal planes: (a) accustomedly sintered specimens; (b) zero-time sintered specimens; and (c) two-step sintered specimens.
Z4, A1), the former exhibits more sufficient development of elongated b-Si3N4 grains in the matrix. That was reported that during a–b-Si3N4 phase transformation, it would be easier for the new b-phase to precipitate on the original b-Si3N4 grains than to nucleate new b-nuclei [36], which eventually caused selective grain growth as proposed previously. On the other hand,
sintering. The addition of Si3N4 (6Y1A) accelerates the densification process of the WC materials, and improves the fracture toughness from 6.69 MPa m1/2 for the pure WC to 10.94 MPa m1/2 for the WC–Si3N4 specimen with the highest fracture toughness. By exploiting the difference in kinetics between WC graingrowth and b-Si3N4 grain-growth through sintering at an elaborate temperature or two-step sintering, composites with microstructure of in-situ elongated b-Si3N4 grains grown sufficiently in the fine-grained WC matrix without fast-grain-growth are obtained.
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Fig. 5. Hardness and fracture toughness of the sintered WC and WC–Si3N4 specimens, coupled with average WC-grain size of the WC–Si3N4 specimens.
sintered at 1600 1C for 30 min possesses average WC grain size of 1.10 mm, hardness of 18.32 GPa and fracture toughness of 9.46 MPa m1/2, while those of the WC–Si3N4 specimen after heated to 1700 1C and treated at 1600 1C for 30 min are 1.26 mm, 17.65 GPa and 10.94 MPa m1/2 respectively. The major toughening mechanisms are found to be elongated Si3N4 grain-pullout and crackbridging by elongated Si3N4 grain.
crack Si3N4 Si3N4 WC
Acknowledgment
WC
Si3N4
2 μm
This topic of research was financed by the National Nature Science Foundation (no. 51174095), the Fundamental Research Funds for the Central Universities (no. 2012ZG0006), the Program for New Century Excellent Talents in University (no. NCET-10– 0364), the Key Technology R&D Program of the Chinese Ministry of Education (no. 62501036011) and the National Basic Research Program of China (no. 2010CB635104). References
crack
Si3N4 WC
1 μm Fig. 6. SEM images of indentation cracks observed in the sintered WC-Si3N4 composites: (a) elongated Si3N4 grain-pullout; and (b) crack-bridging by elongated Si3N4 grain.
Compared with the WC–Si3N4 specimen sintered at 1800 1C for 0 min with average WC grain size of 1.42 mm, hardness of 16.88 GPa and fracture toughness of 9.54 MPa m1/2, the WC–Si3N4 specimen
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
In-situ elongated b-Si3N4 grains toughened WC composites prepared by one/two-step spark plasma sintering Donghai Zheng, Xiaoqiang Li n, Yuanyuan Li, Shengguan Qu, Chao Yang National Engineering Research Center of Near-net-shape Forming Technology for Metallic Materials, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
abstract
Article history: Received 4 September 2012 Received in revised form 15 October 2012 Accepted 15 October 2012 Available online 23 October 2012
The WC–Si3N4 composites toughened by in-situ elongated b-Si3N4 grains were prepared by spark plasma sintering. By exploiting the difference in kinetics between WC grain-growth and b-Si3N4 graingrowth through sintering at an elaborate temperature or two-step sintering, composites with microstructure of in-situ elongated b-Si3N4 grains were grown sufficiently in the fine-grained WC matrix without fast-grain-growth being obtained. The relation of the microstructure and mechanical properties of the composites, as well as the toughening mechanisms, are investigated. Comparing with the fracture toughness of 6.69 MPa m1/2 for the pure WC, that of the obtained WC–Si3N4 specimen reaches 10.94 MPa m1/2. & 2012 Elsevier B.V. All rights reserved.
Keywords: Tungsten carbide Composites Sintering Toughening Mechanical characterization
1. Introduction WC–Co cemented carbides have been widely used as cutting tools and wear-resistant components due to a singular combination of their properties including high hardness, moderate toughness and excellent wear resistance. Considering that the metallic binders in cemented carbides are deleterious on hardness and inferior to WC in corrosion and elevated temperature applications [1], WC-based materials with non-metal reinforcements have been fabricated through some newly developing sintering techniques [2–7], the toughness of which could be elevated to 6 MPa m1/2 from 4 MPa m1/2 for pure WC by particle toughening (VC, Mo2C, Al2O3, etc.) and/or phase-transformation toughening (ZrO2). However, few researches have been reported about using ceramic whiskers as a toughening agent for the binderless WC material. b-Si3N4 whisker is one of the most common ceramic whiskers, which has been used to reinforce the ceramics with low fracture toughness [8,9]. Especially for the self-reinforced Si3N4, large elongated b-Si3N4 grains grown in-situ in a fine matrix act as reinforcing grains [10–17]. Compared with particle toughening, whisker toughening is expected to be more effective due to a combination of toughening mechanisms including crack-bridging, crack-deflection and whisker-pullout. Note also that in-situ growth of whiskers gets rid of the problems associated with fabricating adscititious-whisker-reinforced ceramic composites
n
Corresponding author. Tel./fax: þ 86 20 87112111. E-mail address: [email protected] (X. Li).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.10.059
including the high cost of whiskers, potential human health hazards in their handling, and processing difficulties such as deagglomeration, mixing, and settling [10,18]. Moreover, Si3N4 materials are expected to possess excellent wear resistance [19], which is also one of the reasons for developing WC–Si3N4 composites. During the past decade, a two-step sintering technique developed by Chen and Wang [20–22], which consists of sintering at a high temperature for a short period and subsequent sintering at a low conventional sintering temperature, has been successful for sintering of nanostructured ceramics through solid-state sintering [20–23] or liquid-state sintering [24,25]. The separation of grainboundary diffusion from grain growth in the second-step sintering was achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration [20]. In other respects, for a two-phase material containing compositions with different kinetics of grain growth, a microstructure of one phase grown sufficiently in a ‘‘frozen’’ fine-grained matrix is likely to be obtained by sintering at an elaborate temperature or twostep sintering. In this study, to toughen binderless WC, elongated b-Si3N4 grains are introduced into the matrix by in-situ growing. To the best of our knowledge, this composite system has not been investigated to date. We are attempting to increase the volume fraction of the elongated b-Si3N4 grains within a finegrained WC matrix through one/two-step spark plasma sintering (SPS), and also to study the densification behavior and the relation of the microstructure and mechanical properties of the composites.
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2. Experimental procedure
2.2. Density and mechanical property measurement
2.1. Processing
Based on the Archimedes principle, sintered density was measured using water. The hardness (HV10) was evaluated on a Vickers hardness tester (430SVA, Wilson Wolpert Co. Ltd., China) with a load of 10 kg. The fracture toughness (KIc) was calculated based on the radial crack length produced by Vickers (HV10) indentation, according to Anstis formula [26]. The reported values are the average of the data obtained from five indentation tests. The elastic properties of the bulk samples were determined by non-destructive test via pulse-echo overlap ultrasonic technique using ultrasonic detector.
The starting powders were WC (0.8–1 mm, purity499.9%, Golden Egret Special Alloy Co. Ltd., China), Si3N4 ( 1 mm,495% a-phase, Xuzhou Jiechuang New Material Technology Co. Ltd., China), Y2O3 (5–10 mm, purity499.9%, Sinopharm Chemical Reagent Co. Ltd., China) and Al2O3 ( 1 mm, purity499.9%, Beijing Mountain Technical Development Center, China). WC– 10 wt%Si3N4 (93Si3N4 þ6Y2O3 þ1Al2O3, wt%) powder mixtures were wet mixed on a planetary ball mill (QM-3SP2, Nanjing NanDa Instrument Plant, China) in ethanol for 30 h using cemented carbide milling balls (ball-to-powder weight ratio was 3:1) and cemented carbide vials (250 mL). To minimize the potential Co contamination from the milling balls or vials, milling was conducted in a low energy mode, in which the milling process was paused every 30 min for staying 18 min, subsequently restarted reversely at a constant rotation speed of 180 r/min, and finally stopped after 60 cycles. Furthermore, drying and sieving the milled powders was performed in order to get rid of agglomerates, which may lead to poor sinterability. The pure WC or obtained WC–Si3N4 powders were poured into a cylindrical graphite die with an inner diameter of |20 mm and an outer diameter of |50 mm. Then sintering was conducted on a Dr. Sinter Model SPS-825 Spark Plasma Sintering System (Sumitomo Coal Mining Co. Ltd., Japan) by SPS in vacuum ( r 6 Pa) under an applied pressure of 30 MPa. Three different heating schedules were adopted as shown in Table 1. In accustomed sintering mode, the powder compacts were sintered at various temperatures from 14501 to 1600 1C for 30 min. For zero-time sintering, the specimens were immediately cooled just after the sintering temperature varying from 15501 to 1800 1C was reached. In two-step sintering, the powder compacts were heated to 1700 1C and then immediately cooled to 1450–1600 1C with a holding time of 30 min. Within the sintering period, the heating rate was 100 1C/min, and the cooling rate was 50 1C/min. To investigate the difference between the densification behavior of the pure WC and WC–Si3N4 composites, they were sintered at 1750 1C for dwelling 5 min. In the sintering, graphite papers were used to separate the powders from the graphite die or punch, and the die was surrounded with a 10 mm thick porous carbon felt insulation to minimize the radiation heat loss. An infrared pyrometer ( Z 570 1C) was focused at the bottom of a central core hole in the die wall and with 7.5 mm away from the inner wall.
2.3. Microstructure observation and characterization The microstructure of polished surfaces was examined by high-resolution scanning electron microscopy (SEM, Nova Nano 430, FEI, USA). The average grain size of the WC matrix was obtained by measuring over 300 grains using the Image-ProPlus software [27]. According to the method provided by Pigeon and Varma [28], the wt fraction of b-phase in Si3N4 was calculated based on the ratio of the a(200)/b(200) peak heights (PH), the data of which were collected at 0.021 step 1 between 25 and 291 of 2y (time constant 2 s) with subtraction of the contribution due to background noise by an X-ray diffractometer (XRD, D8 Advance, Bruker Co., Germany) using Cu Ka radiation
a=b phase ratio ¼ 4:9381 ðPHÞ þ 0:1144 ðPHÞ2 þ 0:08106 ðPHÞ3
wt fraction b ¼ 1= 1 þ a=b
ð1Þ ð2Þ
3. Results and discussion 3.1. Densification behavior During SPS, the displacement of the lower punch which reflects the densification process of the sample was automatically stored by a record system. By comparing the shrinkage displacement and shrinkage rate of the pure WC powders with WC–Si3N4 composite, the influence of Si3N4 (6Y1A) addition on the densification behavior of WC-based material is studied. Fig. 1 shows the densification curves of pure WC and WC–Si3N4 composites heated up to 1750 1C for a dwell time of 5 min. For all of these tests, before the real densification of the powders starts, the displacement increases opposite to the shrinkage direction due to the thermal expansion of the compacted sample and/or the punches. As the shrinkage rate turns to be positive, the powders
Table 1 Sintering schedules adopted in this experiment. Sintering methods
Specimens
Sintering conditions (temperature/holding time, 1C/min)
Accustomed sintering
A1 A2 A3 A4
1450/30 1500/30 1550/30 1600/30
Zero-time sintering
Z1 Z2 Z3 Z4 Z5 Z6
1550/0 1600/0 1650/0 1700/0 1750/0 1800/0
Two-step sintering
T1 T2 T3 T4
1700/0-1450/30 1700/0-1500/30 1700/0-1550/30 1700/0-1600/30
Fig. 1. Densification behavior of WC and WC–Si3N4 composites SPSed at 1750 1C for 5 min.
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to be densified, which lasts till the shrinkage rate is down to zero again. For both of the pure WC and WC–Si3N4 composites, the densification process starts at about 1130 1C. Afterwards, the pure WC undergoes a solid-state sintering with gentle shrinkage rates, and the densification process lasts until holding at 1750 1C for more than 2 min. By contrast, the WC–Si3N4 composite obtains much faster densification rates under the same sintering condition especially when the sintering temperature is above 1400 1C, which mainly rests with a small amount of liquid formed by the ternary eutectic melting (SiO2–Y2O3–Al2O3) at about 1371 1C [29]. The presence of the liquid phase assists the rearrangement of the ceramic particles, and also accelerates the mass diffusion during sintering. Finally, the densification process ends at about 1640 1C for the WC–Si3N4 composite, which has been greatly ahead compared with the pure WC. Considering the difference between the actual sample temperature and the measured temperature (The measured point is 7.5 mm away from the sample surface in the present research.) Tiwari et al. reported that low thermal conductivity materials assisted in producing large temperature gradient [30]. In this study, the WC–Si3N4 material is expected to possess lower thermal conductivity compared with the pure WC ( 32 W/m K) due to the low thermal conductivity of Si3N4 ( 2 W/m K), therefore, larger temperature gradient can be obtained during SPSing the WC–Si3N4 composite. With the same actual sample temperature, a little lower measured temperature will be observed for the WC–Si3N4 composite compared with the pure WC. And that also provides a deduction in the sintering temperature for the WC–Si3N4 composite. 3.2. Microstructural analysis During different sintering modes, our attention is focused on some concurrent activities involving densification, a to b-Si3N4 phase transformation, WC grain growth and b-Si3N4 grain growth. In the light of the amount of elongated b-Si3N4 grains grown within the materials, the sintered specimens are subjectively divided into five grades: none, few, moderate, much and plenty. The SEM micrographs of the typical specimens for each grade are illustrated in Fig. 2, and the microstructural characters of all specimens are listed in Table 2 and partially plotted in Fig. 3. Thereinto, the wt fraction (b) of b-phase in Si3N4 was calculated from the XRD data in Fig. 4 by the MDI Jade software [31]. The zero-time sintered specimens can achieve near full density (density Z11.14 g/cm3) when the sintering temperature reached 1600 1C, but the wt fraction b is only 11.73%. It can be concluded that the a–b-Si3N4 transformation process is far behind the densification process. It was reported that the phase transformation of Si3N4 from a to b went through a solution-precipitation process [32,33]. The transformation occurs in a liquid phase generated from the ternary eutectic melting of the SiO2–Y2O3– Al2O3 system in this study. During sintering, a higher temperature or longer holding time is beneficial to the a–b-Si3N4 transformation, just as the wt fraction b increases from 4.07% for 1550 1C/ 0 min to 89.88% for 1800 1C/0 min, or to 34.65% for 1550 1C/ 30 min. Elevating the sintering temperature is more effective for accelerating the transformation process due to the higher diffusion rate and lower viscosity of the liquid mentioned above. But on the other hand, the WC grain-growth is also speeded up while the sintering temperature is above 1700 1C (seen in Fig. 3, specimens Z5 and Z6), suggesting WC grain fast-growth is active. For suppressing the WC grain-growth, lower sintering temperatures are preferred [20]. When the composites are sintered at 1450– 1600 1C for holding 30 min respectively, the average WC grain size for each specimen is less than 1.10 mm. Deservedly, the densification will not complete when the sintering temperature
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is too low, e.g. specimens A1, A2; and the a–b-Si3N4 transformation progresses slowly at the processing temperature. For specimens A3 and A4, the transformation is quickened up without noticeable WC-grain growth (seen in Fig. 3), which suggests that the transformation is active but the WC-grain fast-growth is suppressed at the processing temperatures of 1550–1600 1C. Generally in the self-reinforced silicon nitride ceramics, the enhancement of fracture toughness by in-situ elongated b-Si3N4 grains depends on the volume fraction and the aspect ratio of the elongated b-Si3N4 grains, growth of which is dominated by diffusion-controlled Ostwald ripening [34]. The nucleation and growth rate of b-Si3N4 grains are a function of the rate of dissolution of a-Si3N4 particles, solubility of a-Si3N4 into the liquid, supersaturation of Si and N in the liquid, interfacial tension and wetting between the Si3N4 grains and liquid, and viscosity of the liquid. Comparing the microstructure of specimen Z4 (Fig. 2(c)) with that of Z6 (Fig. 2 (h)), it is clear that the anisotropic b-Si3N4 grain growth progresses rapidly within the narrow temperature interval between 1700 and 1800 1C. And plenty of well-developed elongated b-Si3N4 grains are obtained in specimen Z6. Similar b-Si3N4 grain fast-growth has been observed in fabricating silicon nitride ceramics with interlocking microstructures by SPS [35], in which a mechanism of ‘‘dynamic ripening’’ was assumed. For the ‘‘dynamic ripening’’, owing to a fast heating rate applied, the chemical composition of the coexisting liquid phase deviates from that set by thermodynamic equilibrium. When a critical temperature is reached, the dissolution of small Si3N4 grains is promoted so as to produce a momentarily supersaturated liquid, which will greatly accelerate the a to b-Si3N4 phase transformation and the growth of b-Si3N4 grains. Shen et al. thought that the use of fast heating rate was essential for establishing such dynamic process [35]. In this study, a heating rate of 100 1C/min is used and so the ‘‘dynamic ripening’’ is assumed to occur for explaining the fast transformation of Si3N4 from a to b and the b-Si3N4 grain fast-growth happening at 1700– 1800 1C. It is also notable that the growth of WC grains is speeded up when the sintering temperature exceeds 1700 1C. It may be suggesting that grain-growth mechanism of Ostwald ripening for WC is involved when the liquid phase appears during sintering and turns dynamically above 1700 1C, resulting in WC-grain fastgrowth. For one-step sintering (zero-time sintering and accustomed sintering), specimen Z6 after sintering at 1800 1C for 0 min shows a plenty of elongated b-Si3N4 grains in the WC matrix with average WC-grain size of 1.42 mm. However, the specimen A4 after sintering at 1600 1C for 30 min obtains the ‘‘much’’ grade of elongated b-Si3N4 grains with average WC-grain size of 1.10 mm. So, it is concluded that the b-Si3N4 grains growth can be separated from the WC grain growth by treating at a proper low temperature. This suggests that WC grain fast-growth may involve an activation process that has higher activation energy than Ostwald ripening for b-Si3N4 grains; thus, it is active at higher temperatures but is suppressed at lower temperatures. Exploiting the difference in kinetics between WC grain-growth and b-Si3N4 grain-growth, a two-step sintering method is also expected to be used for fabricating WC–Si3N4 composites with a certain number of elongated b-Si3N4 grains distributing within a fine-grained WC matrix. In this study, the first sintering step, heating to 1700 1C, is to obtain a full density and an extent of a to b-Si3N4 transformation without notable WC grain growth (Similar to specimen Z4). The extent of transformation provides b-Si3N4 grain seeds for the subsequent sintering step, during which the transformation and b-Si3N4 grain-growth are keeping active while the WC-grain fast-growth is suppressed (seen for specimens A1–A4). From a comparison of two-step sintered specimens and the corresponding one-step sintered specimens (e.g. T1 and
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Fig. 2. SEM micrographs of the typical specimens for each grade of elongated b-Si3N4 grains content. (a) Z1: none, (b) Z2: few, (c) Z4: few, (d) Z5: moderate, (e) A3: moderate, (f) A4: much, (g) T3: much, (h) Z6: plenty and (i) T4: plenty.
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Table 2 Microstructural characters of the specimens. Specimen
Density (g/cm3)
WC grain size (mm)
Grade
b (%)
A1 A2 A3 A4 Z1 Z2 Z3 Z4 Z5 Z6 T1 T2 T3 T4
10.54 11.06 11.15 11.14 10.92 11.17 11.19 11.21 11.22 11.22 11.22 11.18 11.20 11.20
0.82 0.88 0.96 1.10 0.80 0.90 0.88 0.92 1.22 1.42 1.08 1.09 1.15 1.26
few few moderate much none few few few moderate plenty much much much plenty
9.38 8.17 34.65 76.85 4.07 11.73 11.52 15.14 48.87 89.88 28.17 34.85 56.95 100
Fig. 3. Average WC-grain size and the wt fraction of b-phase in Si3N4 for the sintered WC–Si3N4 specimens.
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noticeable WC-grain growth does not occur during the second step of sintering at 1450–1600 1C for 30 min. For specimen T4 after heated to 1700 1C and treated at 1600 1C for 30 min (shown in Fig. 2 (i)), the a–b-Si3N4 transformation has nearly completed, as well as obtains plenty of elongated b-Si3N4 grains which are entangled and linked together. And the average size of WC-grain in specimen T4 does not increase rapidly, being 1.26 mm. 3.3. Mechanical properties The hardness and fracture toughness of the sintered pure WC and WC–Si3N4 specimens sintered in different conditions are plotted in Fig. 5. As the Vickers indentation technique may not be viable for the specimens without full density, the fracture toughness data of specimens Z1, A1 and A2 (density o11.14 g/ cm3) are not involved. For intuitively correlating the hardness with the microstructure, the average WC-grain sizes of the WC– Si3N4 specimens are also plotted in Fig. 5. Combining the data of density listed in Table 2, it can be seen that the hardness of the composites mainly depends on the densification degree and WCgrain size of the as-sintered materials. As seen in Fig. 5, for the WC–Si3N4 specimens with near full density (except Z1, A1), the hardness is inversely correlated with the average WC-grain size. Therefore, full densification and suppression of WC grain-growth are believed to be important parameters for fabricating binderless WC-based materials maintaining high hardness. As expected, the fracture toughness of the composites is closely connected to the ‘‘grade’’ of elongated b-Si3N4 grains. Well developed elongated bSi3N4 grains lead to high fracture toughness. Compared with the sintered pure WC with a fracture toughness of 6.69 MPa m1/2, the WC–Si3N4 specimens with ‘‘much’’ grade of elongated b-Si3N4 grains possess a fracture toughness higher than 8.87 MPa m1/2, while those with ‘‘plenty’’ grade show the highest fracture toughness of 10.94 MPa m1/2. From the observation of the indentation cracks in the composites, toughening mechanisms of elongated Si3N4 grain-pullout and crack-bridging by elongated Si3N4 grain are found, as shown in Fig. 6. This result is consistent with the findings reported by Tajima et al., e.g. that the toughening contribution from crack-bridging strongly increased with an increase in volume fraction of elongated grains [37], which is denoted as the grade of elongated b-Si3N4 grains in this paper. With plenty of elongated b-Si3N4 grains and average WC grain size of 1.42 mm, specimen Z6 possesses a hardness of 16.88 GPa and fracture toughness of 9.54 MPa m1/2. By suppressing the WC grain fast-growth and keeping the b-Si3N4 grain-growth active, specimen A4 shows a higher hardness of 18.32 GPa and fracture toughness of 9.46 MPa m1/2, while those of the two-step sintered specimens T1–T4 are in the range of 17.65–18.69 GPa and 8.87– 10.94 MPa m1/2 respectively.
4. Conclusions The WC–Si3N4 composites toughened by in-situ elongated
b-Si3N4 grains have been successfully prepared by spark plasma Fig. 4. XRD patterns for a-Si3N4 (200) and b-Si3N4 (200) crystal planes: (a) accustomedly sintered specimens; (b) zero-time sintered specimens; and (c) two-step sintered specimens.
Z4, A1), the former exhibits more sufficient development of elongated b-Si3N4 grains in the matrix. That was reported that during a–b-Si3N4 phase transformation, it would be easier for the new b-phase to precipitate on the original b-Si3N4 grains than to nucleate new b-nuclei [36], which eventually caused selective grain growth as proposed previously. On the other hand,
sintering. The addition of Si3N4 (6Y1A) accelerates the densification process of the WC materials, and improves the fracture toughness from 6.69 MPa m1/2 for the pure WC to 10.94 MPa m1/2 for the WC–Si3N4 specimen with the highest fracture toughness. By exploiting the difference in kinetics between WC graingrowth and b-Si3N4 grain-growth through sintering at an elaborate temperature or two-step sintering, composites with microstructure of in-situ elongated b-Si3N4 grains grown sufficiently in the fine-grained WC matrix without fast-grain-growth are obtained.
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Fig. 5. Hardness and fracture toughness of the sintered WC and WC–Si3N4 specimens, coupled with average WC-grain size of the WC–Si3N4 specimens.
sintered at 1600 1C for 30 min possesses average WC grain size of 1.10 mm, hardness of 18.32 GPa and fracture toughness of 9.46 MPa m1/2, while those of the WC–Si3N4 specimen after heated to 1700 1C and treated at 1600 1C for 30 min are 1.26 mm, 17.65 GPa and 10.94 MPa m1/2 respectively. The major toughening mechanisms are found to be elongated Si3N4 grain-pullout and crackbridging by elongated Si3N4 grain.
crack Si3N4 Si3N4 WC
Acknowledgment
WC
Si3N4
2 μm
This topic of research was financed by the National Nature Science Foundation (no. 51174095), the Fundamental Research Funds for the Central Universities (no. 2012ZG0006), the Program for New Century Excellent Talents in University (no. NCET-10– 0364), the Key Technology R&D Program of the Chinese Ministry of Education (no. 62501036011) and the National Basic Research Program of China (no. 2010CB635104). References
crack
Si3N4 WC
1 μm Fig. 6. SEM images of indentation cracks observed in the sintered WC-Si3N4 composites: (a) elongated Si3N4 grain-pullout; and (b) crack-bridging by elongated Si3N4 grain.
Compared with the WC–Si3N4 specimen sintered at 1800 1C for 0 min with average WC grain size of 1.42 mm, hardness of 16.88 GPa and fracture toughness of 9.54 MPa m1/2, the WC–Si3N4 specimen
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