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TiN composites sintered by hot pressing and spark plasma sintering
Materials Research Bulletin 48 (2013) 1927–1933
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Microstructures and properties of Si3N4/TiN composites sintered by hot pressing and spark plasma sintering Manyuan Zhou a,*, Jie Zhong b, Juan Zhao a, Don Rodrigo a, Yi-Bing Cheng a a b
Department of Materials Engineering, Monash University, Australia Wuhan National laboratory for optoelectronics, Wuhan, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 August 2012 Received in revised form 29 December 2012 Accepted 23 January 2013 Available online 8 February 2013
Microstructures and mechanical properties of the Si3N4/TiN composites sintered by hot pressing (HP) and spark plasma sintering (SPS) were studied comparatively. Compared with the non-conductive monophase Si3N4 ceramic which has lower density and a–b transformation ratio prepared by SPS than by HP, the conductive Si3N4/TiN composite sample, however, has higher density and obviously higher a– b transformation ratio sintered by SPS than by HP, which means that the presence of TiN clearly improves the densification and a–b phase transformation of silicon nitride in SPS compared in HP. The microstructure analysis shows that the SPSed Si3N4/TiN composites had lower porosity and aspect ratio than the monophase Si3N4. Both the hardness and fracture toughness of the Si3N4/TiN composite with 50%TiN sintered by SPS are higher than by HP, but the hardness and fracture toughness of the nonconductive monophase Si3N4 ceramics do not have a significant difference either sintered by SPS or HP. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nitrides A. Composites D. Mechanical properties
1. Introduction Silicon nitride ceramics have been developed and attracted much attention as structural materials for high-temperature applications, primarily due to their good mechanical and chemical properties, and also their reliability at room and high temperatures [1,2]. They have great potential for industrial uses as engineering components and cutting-tool applications. For the good physical properties [3–5] and electrical conductivity of TiN, it has been widely used in Si3N4 based composites as promising materials for cutting tools [6,7] and making electrical discharged machining (EDM) possible [8,9]. Spark plasma sintering is a newly developed sintering technique, which extraordinarily enhances the sinterability of most of the materials and made them be rapidly sintered at a relatively lower temperature and short time [10–12], thus extends the possibilities for developing new advanced materials and improving their properties. The essential difference between the SPS and traditional sintering methods, such as Hot Pressing, is the heating mode, because the materials sintered by HP are heated exclusively by thermal conduction from the container. The toughness and hardness of silicon nitride have been reported to be enhanced with the incorporation of TiN particles [13–15]. It also has been reported that conductive TiN/
* Corresponding author. Tel.: +61 3 9905 3213, fax: +61 3 9905 4940. E-mail address: [email protected] (M. Zhou). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.01.045
Si3N4 [16–19,15,20] and TiN/SiAlON [21,22] composites were successfully fabricated by SPS. The microstructural development of Si3N4 and TiN was not described in detail previously, especially in the presence of a pulse current through the sintering compact during a sintering cycle. The relationship between microstructure and performance, like mechanical properties and electrical conductivity, of these Si3N4 ceramics and Si3N4/TiN composites are discussed in the present study. 2. Experimental procedure The starting powders in this study are commercial Si3N4 (containing 90.2% a-phase and 9.8% b-phase, HC Starck, grade M11, grain size 0.6 mm) with the sintering additives of 5 wt% Y2O3, 3 wt% Al2O3 and 2 wt% AlN. The TiN powder (Aldrich Chemical, 99%, grain size less than 10 mm) is used as conductive phase. The composites are designed as 0, 30 and 50 wt% conductive phases in the non-conductive matrix of silicon nitride. The powders are mixed by ball milling for 48 h. In SPS, the samples were first heated to 600 8C in 4 min and then heated to designed temperature (1300–1600 8C) at 100 8C/min in a flowing high-purity nitrogen atmosphere and held for 10 min under a uniaxial pressure of 20 MPa in a SPS machine (Dr. Sinter 1050, Sumitomo Coal Mining, Kawasaki, Japan). Hot pressing was performed in a Thermal Technology Group 1400 Laboratory Hot Press under a flowing high-purity nitrogen atmosphere at a heating rate of 20 8C/min under a uniaxial pressure of 20 MPa with 60 min of holding time at designed temperature.
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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Density of the cleaned samples was determined by the Archimedes method using water as the medium. The four-probe method and a micro-ohm meter were used for the electrical resistivity measurement. The microstructural characterization was carried out using a JEOL 7001F scanning electron microscope. Standard X-ray diffraction scans for phase identification were done using a Philips PW 1140/90 X-ray Diffractometer with a Cu Ka radiation (l = 1.542 A˚) for the range of 2u angles from 108 to 808 at a scan rate of 28/min with a step size of 0.028. The b/(a + b) ratio of the phases in sintered samples was determined by XRD peak-area method of Kall [23]:
b ða þ bÞ
¼ 100
Ib ð1 0 1Þ Ib ð1 0 1Þ þ KIa ð1 0 2Þ
(1)
where Ia ð1 0 2Þ and Ib ð1 0 1Þ are the peak integrated intensities (peak areas) of the a(1 0 2) and b(1 0 1) reflections for the a and b phases, respectively. K is the normalizing parameter, which was taken to be 1.652. Hardness and fracture toughness of samples were determined using a Vickers hardness indentation tester. A weight of 10 kg was applied to the surface of the sample via the Vickers diamond indenter for 10 s. 3. Results and discussion In this study, Si3N4 with different mass fracture of TiN (0%, 30% and 50%) were sintered comparatively by SPS and HP at the similar conditions. The electrical conductivity data of the samples are listed in Table 1. According to the result, 100% Si3N4 sample is absolutely non-conductive and 70% Si3N4–30% TiN sample is moderately conductive. On the other hand, 50% Si3N4–50% TiN sample is almost as conductive as the graphite, which means that the large current may come through the sample and the graphite die at the same time during the SPS process. The electrical conductivity data of the samples at the temperature higher than 500 8C are not given here due to the experiment limitation. Because electrical conductivity of Si3N4 increases with the temperature from about 1015 (V cm)1 at room temperature to about 107 (V cm)1 at 1500 8C [24], the electrical conductivity of the Si3N4/TiN composite at the sintering temperatures (1300– 1600 8C) will be higher than that at room temperature so that the large current may pass through the sample more than the graphite die during the SPS process. 3.1. Densification The relative densities of Si3N4 with 0%, 30% and 50% TiN respectively sintered at different temperatures by HP and SPS are shown in Table 2. We can see that, at 1600 8C, Si3N4 based composites sintered whether by HP or SPS are almost fully densified. The results also indicate that the relative density of Si3N4/TiN composite is lower than that of monophase silicon
nitride in HP at higher temperatures (1500 8C and 1600 8C), which means the presence of TiN hinders the densification of silicon nitride. However, in SPS, the relative density of electrically conductive Si3N4/TiN composite (50% Si3N4 + 50% TiN) is clearly higher than non-conductive monophase silicon nitride after sintering at all temperatures. In addition, the sample with 30% TiN, which is moderately conductive, also has higher relative density than the one without any TiN. It means the densification of silicon nitride is improved obviously with the increase of the electrical conductivity of materials. The relative densities of Si3N4 with 0%, 30% and 50% TiN sintered by HP and SPS are also compared and shown in Fig. 1. The relative densities of the samples with 0% and 30% TiN are obviously higher in HP than in SPS because of much longer holding time in HP (60 min) than in SPS (10 min). However, the sample with 50%TiN, which is electrically conductive, has higher density in SPS than in HP at all temperatures except 1300 8C, which means that the densification of the electrically conductive materials is promoted obviously in SPS due to the current passing through the sample during the SPS process. The shrinkage curves for Si3N4 based composites sintered by SPS at 1600 8C are shown in Fig. 2. It should be noted that the dimensional changes of the samples are not directly comparable due to the differences in the particle size and the amount of the raw powder used in sample preparations. However, the temperature at which the shrinkage due to sintering exceeds the thermal expansion of the assembly (the lowest point on the displacement curve) can be easily identified for each composition. The results show that this happened at a temperature of the die wall (measured by the pyrometer) about 120 8C lower for the electrically conducting specimen with 50% TiN added to Si3N4 than for the non-conducting Si3N4 specimen without any addition of TiN. Similarly, the corresponding temperature was about 70 8C lower for the moderately conducting specimen with 30% TiN added to Si3N4, than for the non-conducting monophase Si3N4 specimen. In the absence of any beneficial chemical effect of TiN on the densification, as shown by the densification data for hot pressed composites, the commencement of sintering of Si3N4/TiN composites in SPS at apparently a lower temperature than for monophase Si3N4 can be attributed to a physical effect arising from the presence of conductive TiN, which is the temperature difference between the die and the core of the sample. For the non-conductive monophase Si3N4 sample, the core of the sample is at a lower temperature than that measured on the surface of the die, whereas, for the conductive composites, which can be self-heated to some extent by Joule heating, the temperature of the core of the sample can be higher than that measured on the surface of the die. This is also known as the overshooting effect which is a very important behavior during SPS process. Previous studies have presented some evidence to support this overshooting effect [27–31]. The
Table 1 Electrical resistivity of samples and graphite die (unit: V cm). Temperature (8C)
Material
25 a
Graphite die 100% Si3N4b 70% Si3N4 + 30% TiNc 50% Si3N4 + 50% TiN TiNd a b c d
3
0.1 10 Approx. 1014 >2 105 4.6 103 20 10 106
100
200
300
400
500
– – – 4.2 103 –
– – – 3.9 103 –
– – – 3.4 103 –
– – >2 105 3.0 103 –
– 2 1013 – – –
According to the data from the graphite die manufacturer. http://www.siliconfareast.com/sio2si3n4.htm [25]. This value is the upper limit of the instrument used. The resistivity of this composition should be less than that for 100% silicon nitride. Ref. [26].
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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Table 2 The densities of Si3N4 with different contents of TiN sintered by HP and SPS. Temperature (8C)
100% Si3N4 + 0% TiN
1300 1400 1500 1600
70% Si3N4 + 30% TiN
HP
SPS
HP
SPS
HP
SPS
62.4 0.1 68.8 0.1 97.0 0.1 100 0.1
57.8 0.1 64.6 0.1 84.7 0.1 98.6 0.1
64.5 0.1 71.9 0.1 95.0 0.1 99.2 0.1
59.0 0.1 71.4 0.1 89.2 0.1 99.6 0.1
62.6 0.1 71.0 0.1 88.5 0.1 96.9 0.1
61.5 0.1 72.8 0.1 91.3 0.1 100 0.1
temperature difference between the measured temperature (on the surface of the die) and the actual temperature (on the surface of the sample) is about 140 8C and 170 8C for nonconducting silicon nitride and conducting tungsten carbide [28]. According to Wang Yucheng’s research, the temperature difference between the measured temperature and the actual temperature (the center of the sample) for conducting material (TiB2 and BN) can be even up to 345 8C [27].
Relave density (%theorecal density)
HP 100
SPS
(a)
90 80 70 60 50 1300
50% Si3N4 + 50% TiN
1400
1500
1600
3.2. Phase transformation The b-ratios (b/(a + b)) of Si3N4 with different percentage of TiN sintered by HP and SPS are listed in Table 3. The b-ratio here is used to describe the extent of a–b phase transformation of silicon nitride. We can see that, in SPS, the b-ratio of the conductive sample (50% Si3N4 + 50% TiN) is obviously higher than that of the non-conductive one (100% Si3N4), especially in the liquid phase sintering stage (1500–1600 8C). In addition, the samples with 30% TiN also have higher b-ratios than those with 0% TiN, which means that the a–b transformation of silicon nitride is promoted with the increase of conductivity. That is also because the rapid Joule heating effect caused by the large pulse current passing through conductive samples leads to the temperature difference between the actual temperature at the center of the conductive samples and the nominal temperature measured on the out surface of the graphite die, which is in agreement with the shrinkage curve of the samples (Fig. 2). On the contrary, in HP, the b-ratio of the conductive Si3N4/TiN composites is lower than that of the nonconductive monophase Si3N4 because the presence of TiN suppresses the elongation of b-Si3N4 grains and hence the a–b transformation of Si3N4.
Temperature ( C)
Relave density (%theorecal density)
HP 100
SPS
(b)
90 80 70 60 50 1300
1400
1500
1600
Temperature ( C)
Relave density (%theorecal density)
HP 100
SPS
(c)
90 80 70 60 50 1300
1400
1500
1600
Temperature ( C) Fig. 1. Relative density of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
Fig. 2. Shrinkage curve of Si3N4/TiN composites SPSed at 1600 8C.
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M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
Table 3 Comparison of b-ratio of Si3N4 with different mass fracture of TiN sintered by HP and SPS. Temperature (8C)
100% Si3N4 + 0% TiN
1300 1400 1500 1600
70% Si3N4 + 30% TiN
HP
SPS
HP
SPS
HP
SPS
10.1 0.5 12.8 0.5 32.3 0.8 73.9 0.8
10.6 0.3 11.0 0.3 12.6 0.3 70.7 1.8
10.1 0.6 12.2 0.6 23.3 0.8 72.0 0.6
10.9 0.5 11.0 1.2 13.0 0.9 76.8 0.7
9.7 0.2 9.8 0.9 18.0 1.1 70.8 1.5
10.6 0.8 13.0 1.2 26.4 2.1 81.8 1.8
The b-ratios of Si3N4 with 0%, 30% and 50% TiN sintered by HP and SPS are also compared and shown in Fig. 3. The results indicate that, for the samples with 0% and 30% TiN, the b-ratios are higher in HP than in SPS, except the 70% Si3N4 + 30% TiN sample sintered at 1600 8C, because the holding time in HP is longer than in SPS and the solution–diffusion–precipitation involved in the a–b phase transformation of silicon nitride is a time-consuming process.
Relave density (%theorecal density)
HP 80 (a) 70 60 50 40 30 20 10 0 1300
SPS
1400
Relave density (%theorecal density)
HP
1500
1600
SPS
(b)
80 60 40 20 0 1300
1400
1500
1600
Temperature ( C)
Relave density (%theorecal density)
HP 100
SPS
(c)
80
as stated in the last paragraph; while in SPS, the introduction of the conductive phase, TiN, assists the a–b phase transformation. Combining these two points, it can be said that the introduction of conductive phase of TiN promotes the a–b phase transformation of silicon nitride clearly due to the Joule heating effect of the large current during the SPS process compared with no current passing through samples in HP.
The microstructures of Si3N4/TiN composites sintered by SPS compared with HP at 1600 8C are shown in Fig. 4 (the dark grains are Si3N4 and the light grains are TiN). The results show that there is minor difference in the porosity between the monophase Si3N4 and Si3N4/TiN composites, that is the porosity of Si3N4/TiN composites (Fig. 4b and c) is a little bit lower than monophase Si3N4 (Fig. 4a) in SPS, which means that the conductive phase of TiN promotes the densification of Si3N4 because of the Joule heating effect. However, the porosity of Si3N4/TiN composites (Fig. 4e and f) is a little bit higher than that of monophase Si3N4 (Fig. 4d) in HP because the presence of TiN hinders the densification of silicon nitride. This result is also in agreement with the density data in Section 3.1. The average grain size and aspect ratio of the Si3N4/TiN composites sintered by HP and SPS at 1600 8C are listed in Table 4. From the results of Fig. 4 and Table 4, it can be seen clearly that there are fewer elongated b-Si3N4 grains and the aspect ratio is obviously smaller for Si3N4/TiN composites (Fig. 4b, c, e, and f) than monophase Si3N4 (Fig. 4a and d) either sintered by SPS or HP, because the presence of TiN suppressed the b-Si3N4 grains’ elongation. There is no much difference between the average grain sizes of the monophase Si3N4 samples sintered by SPS (Fig. 4a) and by HP (Fig. 4d). However, the average grain sizes of the Si3N4/TiN composite samples sintered by SPS (Fig. 4b and c) are bigger than those sintered by HP (Fig. 4e and f), which is because of the difference between the actual temperature at the center of conductive Si3N4/TiN composite samples and the nominal temperature measured on the surface of the die, due to the Joule heating effect of the conductive samples, promotes the b-Si3N4 grain growth in SPS compared with in HP. 3.4. Mechanical properties
60 40 20 0 1300
However, for the electrically conductive sample with 50%TiN, the
b-ratio is higher in SPS than in HP at all temperatures. In HP, the existence of TiN grains suppresses the a–b phase transformation
3.3. Microstructure
Temperature ( C)
100
50% Si3N4 + 50% TiN
1400
1500
1600
Temperature ( C) Fig. 3. b-Ratio of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
The hardness and fracture toughness of monophase Si3N4 and Si3N4/TiN composites sintered by HP and SPS are shown in Figs. 5 and 6, respectively. The results show that the hardness of the nonconductive sample with 0% TiN and moderate conductive sample with 30% TiN sintered by SPS are lower than sintered by HP. However, the hardness of the electrically conductive sample with 50% TiN sintered by SPS is higher than sintered by HP, which means that the hardness of the conductive composite material is improved due to electrical current passing through during the SPS process. It should be pointed out that the hardness of
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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Fig. 4. SEM Micrographs of Si3N4/TiN composites sintered at 1600 8C (a) 100% Si3N4 + 0% TiN by SPS, (b) 70% Si3N4 + 30% TiN by SPS, (c) 50% Si3N4 + 50% TiN by SPS, (d) 100% Si3N4 + 0% TiN by HP, (e) 70% Si3N4 + 30% TiN by HP, and (f) 50% Si3N4 + 50% TiN by HP.
monophase silicon nitride and the Si3N4/TiN composite with 30% TiN samples sintered by SPS at 1500 8C is obviously lower than by HP because their much lower density (Fig. 1a and b). It also can be seen that the hardness of Si3N4/TiN composites is lower than that of monophase Si3N4 either sintered by HP or SPS at all temperatures because of the introduction of soft phase, TiN. The fracture toughness of both the monophase Si3N4 and Si3N4/TiN
composites sintered by SPS is higher than sintered by HP at all temperatures. It also can be found that, with the presence of TiN, the Si3N4/TiN composites have obviously higher fracture toughness than monophase Si3N4, especially the conductive sample with 50% TiN. The hardness and fracture toughness data show that the introduction of TiN in Si3N4/TiN composite sample with 50% TiN increases the fracture toughness obviously (Fig. 6a and c) without
Table 4 Average grain size and aspect ratio of the Si3N4/TiN composites sintered by SPS and HP at 1600 8C. 100% a-Si3N4 + 0% TiN
Grain size (mm) Aspect ratio
70% a-Si3N4 + 30% TiN
50% a-Si3N4 + 50% TiN
HP
SPS
HP
SPS
HP
SPS
0.3 7
0.3 9
0.35 4
0.5 3
0.35 3
0.6 2
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
1932
100%Si3N4+0%TiN
2000
(a)
HP
SPS
1500 1000 500 0 1500
1600
1700
Temperature( C)
Fracture toughness(MPa.m1/2)
Vicker Hardness (Kg/mm2)
100%Si3N4+0%TiN 10
(a)
6 4 2 0 1500
1600
HP
SPS
1500 1000 500 0 1500
1600
1700
Temperature( C)
10
(b)
SPS
(c)
1500 1000 500 0 1500
1600
SPS
6 4 2 0 1500
1600
1700
Temperature( C)
50%Si3N4+50%TiN
1700
Temperature( C) Fig. 5. Vicker hardness of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
decreasing the hardness of the Si3N4 (Fig. 5a and c). The mechanical properties of Si3N4/TiN composites sintered by SPS are improved compared with by HP because of the Joule heating effect generated by the electrical current passing through the conductive samples which is the only difference between the SPS and HP process. 4. Conclusions The microstructures and mechanical properties of Si3N4/TiN composites sintered by HP and SPS are compared in the present study. The introduction of TiN into silicon nitride increased the electrical conductivity of the composite materials and consequently affected their sintering behavior, microstructure and properties in SPS compared with in HP. In HP, the densification and a–b transformation of silicon nitride are suppressed by the presence of TiN. However, in SPS, the densification and a–b transformation of the Si3N4/TiN composites are obviously improved by the increase of electrical conductivity due to the introduction of conductive phase of TiN. The shrinkage curves
Fracture toughness(MPa.m1/2)
Vicker Hardness(Kg/mm2)
HP
HP
8
50%Si3N4+50%TiN 2000
1700
Temperature( C)
70%Si3N4+30%TiN Fracture toughness(MPa.m1/2)
Vicker Hardness(Kg/mm2)
(b)
SPS
8
70%Si3N4+30%TiN 2000
HP
10
(c)
HP
SPS
8 6 4 2 0 1500
1600
1700
Temperature( C) Fig. 6. Fracture toughness of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
showed that the densification for the conductive Si3N4/TiN composites started at an about 120 8C lower temperature than that for the non-conductive monophase Si3N4 because the actual temperature of the conductive samples is higher than the measured temperature. From the microstructural analysis, the aspect ratio of Si3N4/TiN composites is less than that of monophase Si3N4 both in HP and SPS because the presence of TiN suppresses the elongation of the b-Si3N4 grains. The porosity of Si3N4/TiN composites is higher than that of monophase Si3N4 in HP, but in SPS, the result is just the reverse. The result of the mechanical property tests shows that the introduction of TiN in Si3N4/TiN composite sample with 50% TiN increases the fracture toughness obviously without decreasing the hardness of the Si3N4. Acknowledgements M. Zhou acknowledges the Monash scholarship. The Authors acknowledge the use of facilities within the Monash Centre for Electron Microscopy.
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Microstructures and properties of Si3N4/TiN composites sintered by hot pressing and spark plasma sintering Manyuan Zhou a,*, Jie Zhong b, Juan Zhao a, Don Rodrigo a, Yi-Bing Cheng a a b
Department of Materials Engineering, Monash University, Australia Wuhan National laboratory for optoelectronics, Wuhan, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 August 2012 Received in revised form 29 December 2012 Accepted 23 January 2013 Available online 8 February 2013
Microstructures and mechanical properties of the Si3N4/TiN composites sintered by hot pressing (HP) and spark plasma sintering (SPS) were studied comparatively. Compared with the non-conductive monophase Si3N4 ceramic which has lower density and a–b transformation ratio prepared by SPS than by HP, the conductive Si3N4/TiN composite sample, however, has higher density and obviously higher a– b transformation ratio sintered by SPS than by HP, which means that the presence of TiN clearly improves the densification and a–b phase transformation of silicon nitride in SPS compared in HP. The microstructure analysis shows that the SPSed Si3N4/TiN composites had lower porosity and aspect ratio than the monophase Si3N4. Both the hardness and fracture toughness of the Si3N4/TiN composite with 50%TiN sintered by SPS are higher than by HP, but the hardness and fracture toughness of the nonconductive monophase Si3N4 ceramics do not have a significant difference either sintered by SPS or HP. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nitrides A. Composites D. Mechanical properties
1. Introduction Silicon nitride ceramics have been developed and attracted much attention as structural materials for high-temperature applications, primarily due to their good mechanical and chemical properties, and also their reliability at room and high temperatures [1,2]. They have great potential for industrial uses as engineering components and cutting-tool applications. For the good physical properties [3–5] and electrical conductivity of TiN, it has been widely used in Si3N4 based composites as promising materials for cutting tools [6,7] and making electrical discharged machining (EDM) possible [8,9]. Spark plasma sintering is a newly developed sintering technique, which extraordinarily enhances the sinterability of most of the materials and made them be rapidly sintered at a relatively lower temperature and short time [10–12], thus extends the possibilities for developing new advanced materials and improving their properties. The essential difference between the SPS and traditional sintering methods, such as Hot Pressing, is the heating mode, because the materials sintered by HP are heated exclusively by thermal conduction from the container. The toughness and hardness of silicon nitride have been reported to be enhanced with the incorporation of TiN particles [13–15]. It also has been reported that conductive TiN/
* Corresponding author. Tel.: +61 3 9905 3213, fax: +61 3 9905 4940. E-mail address: [email protected] (M. Zhou). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.01.045
Si3N4 [16–19,15,20] and TiN/SiAlON [21,22] composites were successfully fabricated by SPS. The microstructural development of Si3N4 and TiN was not described in detail previously, especially in the presence of a pulse current through the sintering compact during a sintering cycle. The relationship between microstructure and performance, like mechanical properties and electrical conductivity, of these Si3N4 ceramics and Si3N4/TiN composites are discussed in the present study. 2. Experimental procedure The starting powders in this study are commercial Si3N4 (containing 90.2% a-phase and 9.8% b-phase, HC Starck, grade M11, grain size 0.6 mm) with the sintering additives of 5 wt% Y2O3, 3 wt% Al2O3 and 2 wt% AlN. The TiN powder (Aldrich Chemical, 99%, grain size less than 10 mm) is used as conductive phase. The composites are designed as 0, 30 and 50 wt% conductive phases in the non-conductive matrix of silicon nitride. The powders are mixed by ball milling for 48 h. In SPS, the samples were first heated to 600 8C in 4 min and then heated to designed temperature (1300–1600 8C) at 100 8C/min in a flowing high-purity nitrogen atmosphere and held for 10 min under a uniaxial pressure of 20 MPa in a SPS machine (Dr. Sinter 1050, Sumitomo Coal Mining, Kawasaki, Japan). Hot pressing was performed in a Thermal Technology Group 1400 Laboratory Hot Press under a flowing high-purity nitrogen atmosphere at a heating rate of 20 8C/min under a uniaxial pressure of 20 MPa with 60 min of holding time at designed temperature.
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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Density of the cleaned samples was determined by the Archimedes method using water as the medium. The four-probe method and a micro-ohm meter were used for the electrical resistivity measurement. The microstructural characterization was carried out using a JEOL 7001F scanning electron microscope. Standard X-ray diffraction scans for phase identification were done using a Philips PW 1140/90 X-ray Diffractometer with a Cu Ka radiation (l = 1.542 A˚) for the range of 2u angles from 108 to 808 at a scan rate of 28/min with a step size of 0.028. The b/(a + b) ratio of the phases in sintered samples was determined by XRD peak-area method of Kall [23]:
b ða þ bÞ
¼ 100
Ib ð1 0 1Þ Ib ð1 0 1Þ þ KIa ð1 0 2Þ
(1)
where Ia ð1 0 2Þ and Ib ð1 0 1Þ are the peak integrated intensities (peak areas) of the a(1 0 2) and b(1 0 1) reflections for the a and b phases, respectively. K is the normalizing parameter, which was taken to be 1.652. Hardness and fracture toughness of samples were determined using a Vickers hardness indentation tester. A weight of 10 kg was applied to the surface of the sample via the Vickers diamond indenter for 10 s. 3. Results and discussion In this study, Si3N4 with different mass fracture of TiN (0%, 30% and 50%) were sintered comparatively by SPS and HP at the similar conditions. The electrical conductivity data of the samples are listed in Table 1. According to the result, 100% Si3N4 sample is absolutely non-conductive and 70% Si3N4–30% TiN sample is moderately conductive. On the other hand, 50% Si3N4–50% TiN sample is almost as conductive as the graphite, which means that the large current may come through the sample and the graphite die at the same time during the SPS process. The electrical conductivity data of the samples at the temperature higher than 500 8C are not given here due to the experiment limitation. Because electrical conductivity of Si3N4 increases with the temperature from about 1015 (V cm)1 at room temperature to about 107 (V cm)1 at 1500 8C [24], the electrical conductivity of the Si3N4/TiN composite at the sintering temperatures (1300– 1600 8C) will be higher than that at room temperature so that the large current may pass through the sample more than the graphite die during the SPS process. 3.1. Densification The relative densities of Si3N4 with 0%, 30% and 50% TiN respectively sintered at different temperatures by HP and SPS are shown in Table 2. We can see that, at 1600 8C, Si3N4 based composites sintered whether by HP or SPS are almost fully densified. The results also indicate that the relative density of Si3N4/TiN composite is lower than that of monophase silicon
nitride in HP at higher temperatures (1500 8C and 1600 8C), which means the presence of TiN hinders the densification of silicon nitride. However, in SPS, the relative density of electrically conductive Si3N4/TiN composite (50% Si3N4 + 50% TiN) is clearly higher than non-conductive monophase silicon nitride after sintering at all temperatures. In addition, the sample with 30% TiN, which is moderately conductive, also has higher relative density than the one without any TiN. It means the densification of silicon nitride is improved obviously with the increase of the electrical conductivity of materials. The relative densities of Si3N4 with 0%, 30% and 50% TiN sintered by HP and SPS are also compared and shown in Fig. 1. The relative densities of the samples with 0% and 30% TiN are obviously higher in HP than in SPS because of much longer holding time in HP (60 min) than in SPS (10 min). However, the sample with 50%TiN, which is electrically conductive, has higher density in SPS than in HP at all temperatures except 1300 8C, which means that the densification of the electrically conductive materials is promoted obviously in SPS due to the current passing through the sample during the SPS process. The shrinkage curves for Si3N4 based composites sintered by SPS at 1600 8C are shown in Fig. 2. It should be noted that the dimensional changes of the samples are not directly comparable due to the differences in the particle size and the amount of the raw powder used in sample preparations. However, the temperature at which the shrinkage due to sintering exceeds the thermal expansion of the assembly (the lowest point on the displacement curve) can be easily identified for each composition. The results show that this happened at a temperature of the die wall (measured by the pyrometer) about 120 8C lower for the electrically conducting specimen with 50% TiN added to Si3N4 than for the non-conducting Si3N4 specimen without any addition of TiN. Similarly, the corresponding temperature was about 70 8C lower for the moderately conducting specimen with 30% TiN added to Si3N4, than for the non-conducting monophase Si3N4 specimen. In the absence of any beneficial chemical effect of TiN on the densification, as shown by the densification data for hot pressed composites, the commencement of sintering of Si3N4/TiN composites in SPS at apparently a lower temperature than for monophase Si3N4 can be attributed to a physical effect arising from the presence of conductive TiN, which is the temperature difference between the die and the core of the sample. For the non-conductive monophase Si3N4 sample, the core of the sample is at a lower temperature than that measured on the surface of the die, whereas, for the conductive composites, which can be self-heated to some extent by Joule heating, the temperature of the core of the sample can be higher than that measured on the surface of the die. This is also known as the overshooting effect which is a very important behavior during SPS process. Previous studies have presented some evidence to support this overshooting effect [27–31]. The
Table 1 Electrical resistivity of samples and graphite die (unit: V cm). Temperature (8C)
Material
25 a
Graphite die 100% Si3N4b 70% Si3N4 + 30% TiNc 50% Si3N4 + 50% TiN TiNd a b c d
3
0.1 10 Approx. 1014 >2 105 4.6 103 20 10 106
100
200
300
400
500
– – – 4.2 103 –
– – – 3.9 103 –
– – – 3.4 103 –
– – >2 105 3.0 103 –
– 2 1013 – – –
According to the data from the graphite die manufacturer. http://www.siliconfareast.com/sio2si3n4.htm [25]. This value is the upper limit of the instrument used. The resistivity of this composition should be less than that for 100% silicon nitride. Ref. [26].
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Table 2 The densities of Si3N4 with different contents of TiN sintered by HP and SPS. Temperature (8C)
100% Si3N4 + 0% TiN
1300 1400 1500 1600
70% Si3N4 + 30% TiN
HP
SPS
HP
SPS
HP
SPS
62.4 0.1 68.8 0.1 97.0 0.1 100 0.1
57.8 0.1 64.6 0.1 84.7 0.1 98.6 0.1
64.5 0.1 71.9 0.1 95.0 0.1 99.2 0.1
59.0 0.1 71.4 0.1 89.2 0.1 99.6 0.1
62.6 0.1 71.0 0.1 88.5 0.1 96.9 0.1
61.5 0.1 72.8 0.1 91.3 0.1 100 0.1
temperature difference between the measured temperature (on the surface of the die) and the actual temperature (on the surface of the sample) is about 140 8C and 170 8C for nonconducting silicon nitride and conducting tungsten carbide [28]. According to Wang Yucheng’s research, the temperature difference between the measured temperature and the actual temperature (the center of the sample) for conducting material (TiB2 and BN) can be even up to 345 8C [27].
Relave density (%theorecal density)
HP 100
SPS
(a)
90 80 70 60 50 1300
50% Si3N4 + 50% TiN
1400
1500
1600
3.2. Phase transformation The b-ratios (b/(a + b)) of Si3N4 with different percentage of TiN sintered by HP and SPS are listed in Table 3. The b-ratio here is used to describe the extent of a–b phase transformation of silicon nitride. We can see that, in SPS, the b-ratio of the conductive sample (50% Si3N4 + 50% TiN) is obviously higher than that of the non-conductive one (100% Si3N4), especially in the liquid phase sintering stage (1500–1600 8C). In addition, the samples with 30% TiN also have higher b-ratios than those with 0% TiN, which means that the a–b transformation of silicon nitride is promoted with the increase of conductivity. That is also because the rapid Joule heating effect caused by the large pulse current passing through conductive samples leads to the temperature difference between the actual temperature at the center of the conductive samples and the nominal temperature measured on the out surface of the graphite die, which is in agreement with the shrinkage curve of the samples (Fig. 2). On the contrary, in HP, the b-ratio of the conductive Si3N4/TiN composites is lower than that of the nonconductive monophase Si3N4 because the presence of TiN suppresses the elongation of b-Si3N4 grains and hence the a–b transformation of Si3N4.
Temperature ( C)
Relave density (%theorecal density)
HP 100
SPS
(b)
90 80 70 60 50 1300
1400
1500
1600
Temperature ( C)
Relave density (%theorecal density)
HP 100
SPS
(c)
90 80 70 60 50 1300
1400
1500
1600
Temperature ( C) Fig. 1. Relative density of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
Fig. 2. Shrinkage curve of Si3N4/TiN composites SPSed at 1600 8C.
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M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
Table 3 Comparison of b-ratio of Si3N4 with different mass fracture of TiN sintered by HP and SPS. Temperature (8C)
100% Si3N4 + 0% TiN
1300 1400 1500 1600
70% Si3N4 + 30% TiN
HP
SPS
HP
SPS
HP
SPS
10.1 0.5 12.8 0.5 32.3 0.8 73.9 0.8
10.6 0.3 11.0 0.3 12.6 0.3 70.7 1.8
10.1 0.6 12.2 0.6 23.3 0.8 72.0 0.6
10.9 0.5 11.0 1.2 13.0 0.9 76.8 0.7
9.7 0.2 9.8 0.9 18.0 1.1 70.8 1.5
10.6 0.8 13.0 1.2 26.4 2.1 81.8 1.8
The b-ratios of Si3N4 with 0%, 30% and 50% TiN sintered by HP and SPS are also compared and shown in Fig. 3. The results indicate that, for the samples with 0% and 30% TiN, the b-ratios are higher in HP than in SPS, except the 70% Si3N4 + 30% TiN sample sintered at 1600 8C, because the holding time in HP is longer than in SPS and the solution–diffusion–precipitation involved in the a–b phase transformation of silicon nitride is a time-consuming process.
Relave density (%theorecal density)
HP 80 (a) 70 60 50 40 30 20 10 0 1300
SPS
1400
Relave density (%theorecal density)
HP
1500
1600
SPS
(b)
80 60 40 20 0 1300
1400
1500
1600
Temperature ( C)
Relave density (%theorecal density)
HP 100
SPS
(c)
80
as stated in the last paragraph; while in SPS, the introduction of the conductive phase, TiN, assists the a–b phase transformation. Combining these two points, it can be said that the introduction of conductive phase of TiN promotes the a–b phase transformation of silicon nitride clearly due to the Joule heating effect of the large current during the SPS process compared with no current passing through samples in HP.
The microstructures of Si3N4/TiN composites sintered by SPS compared with HP at 1600 8C are shown in Fig. 4 (the dark grains are Si3N4 and the light grains are TiN). The results show that there is minor difference in the porosity between the monophase Si3N4 and Si3N4/TiN composites, that is the porosity of Si3N4/TiN composites (Fig. 4b and c) is a little bit lower than monophase Si3N4 (Fig. 4a) in SPS, which means that the conductive phase of TiN promotes the densification of Si3N4 because of the Joule heating effect. However, the porosity of Si3N4/TiN composites (Fig. 4e and f) is a little bit higher than that of monophase Si3N4 (Fig. 4d) in HP because the presence of TiN hinders the densification of silicon nitride. This result is also in agreement with the density data in Section 3.1. The average grain size and aspect ratio of the Si3N4/TiN composites sintered by HP and SPS at 1600 8C are listed in Table 4. From the results of Fig. 4 and Table 4, it can be seen clearly that there are fewer elongated b-Si3N4 grains and the aspect ratio is obviously smaller for Si3N4/TiN composites (Fig. 4b, c, e, and f) than monophase Si3N4 (Fig. 4a and d) either sintered by SPS or HP, because the presence of TiN suppressed the b-Si3N4 grains’ elongation. There is no much difference between the average grain sizes of the monophase Si3N4 samples sintered by SPS (Fig. 4a) and by HP (Fig. 4d). However, the average grain sizes of the Si3N4/TiN composite samples sintered by SPS (Fig. 4b and c) are bigger than those sintered by HP (Fig. 4e and f), which is because of the difference between the actual temperature at the center of conductive Si3N4/TiN composite samples and the nominal temperature measured on the surface of the die, due to the Joule heating effect of the conductive samples, promotes the b-Si3N4 grain growth in SPS compared with in HP. 3.4. Mechanical properties
60 40 20 0 1300
However, for the electrically conductive sample with 50%TiN, the
b-ratio is higher in SPS than in HP at all temperatures. In HP, the existence of TiN grains suppresses the a–b phase transformation
3.3. Microstructure
Temperature ( C)
100
50% Si3N4 + 50% TiN
1400
1500
1600
Temperature ( C) Fig. 3. b-Ratio of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
The hardness and fracture toughness of monophase Si3N4 and Si3N4/TiN composites sintered by HP and SPS are shown in Figs. 5 and 6, respectively. The results show that the hardness of the nonconductive sample with 0% TiN and moderate conductive sample with 30% TiN sintered by SPS are lower than sintered by HP. However, the hardness of the electrically conductive sample with 50% TiN sintered by SPS is higher than sintered by HP, which means that the hardness of the conductive composite material is improved due to electrical current passing through during the SPS process. It should be pointed out that the hardness of
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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Fig. 4. SEM Micrographs of Si3N4/TiN composites sintered at 1600 8C (a) 100% Si3N4 + 0% TiN by SPS, (b) 70% Si3N4 + 30% TiN by SPS, (c) 50% Si3N4 + 50% TiN by SPS, (d) 100% Si3N4 + 0% TiN by HP, (e) 70% Si3N4 + 30% TiN by HP, and (f) 50% Si3N4 + 50% TiN by HP.
monophase silicon nitride and the Si3N4/TiN composite with 30% TiN samples sintered by SPS at 1500 8C is obviously lower than by HP because their much lower density (Fig. 1a and b). It also can be seen that the hardness of Si3N4/TiN composites is lower than that of monophase Si3N4 either sintered by HP or SPS at all temperatures because of the introduction of soft phase, TiN. The fracture toughness of both the monophase Si3N4 and Si3N4/TiN
composites sintered by SPS is higher than sintered by HP at all temperatures. It also can be found that, with the presence of TiN, the Si3N4/TiN composites have obviously higher fracture toughness than monophase Si3N4, especially the conductive sample with 50% TiN. The hardness and fracture toughness data show that the introduction of TiN in Si3N4/TiN composite sample with 50% TiN increases the fracture toughness obviously (Fig. 6a and c) without
Table 4 Average grain size and aspect ratio of the Si3N4/TiN composites sintered by SPS and HP at 1600 8C. 100% a-Si3N4 + 0% TiN
Grain size (mm) Aspect ratio
70% a-Si3N4 + 30% TiN
50% a-Si3N4 + 50% TiN
HP
SPS
HP
SPS
HP
SPS
0.3 7
0.3 9
0.35 4
0.5 3
0.35 3
0.6 2
M. Zhou et al. / Materials Research Bulletin 48 (2013) 1927–1933
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100%Si3N4+0%TiN
2000
(a)
HP
SPS
1500 1000 500 0 1500
1600
1700
Temperature( C)
Fracture toughness(MPa.m1/2)
Vicker Hardness (Kg/mm2)
100%Si3N4+0%TiN 10
(a)
6 4 2 0 1500
1600
HP
SPS
1500 1000 500 0 1500
1600
1700
Temperature( C)
10
(b)
SPS
(c)
1500 1000 500 0 1500
1600
SPS
6 4 2 0 1500
1600
1700
Temperature( C)
50%Si3N4+50%TiN
1700
Temperature( C) Fig. 5. Vicker hardness of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
decreasing the hardness of the Si3N4 (Fig. 5a and c). The mechanical properties of Si3N4/TiN composites sintered by SPS are improved compared with by HP because of the Joule heating effect generated by the electrical current passing through the conductive samples which is the only difference between the SPS and HP process. 4. Conclusions The microstructures and mechanical properties of Si3N4/TiN composites sintered by HP and SPS are compared in the present study. The introduction of TiN into silicon nitride increased the electrical conductivity of the composite materials and consequently affected their sintering behavior, microstructure and properties in SPS compared with in HP. In HP, the densification and a–b transformation of silicon nitride are suppressed by the presence of TiN. However, in SPS, the densification and a–b transformation of the Si3N4/TiN composites are obviously improved by the increase of electrical conductivity due to the introduction of conductive phase of TiN. The shrinkage curves
Fracture toughness(MPa.m1/2)
Vicker Hardness(Kg/mm2)
HP
HP
8
50%Si3N4+50%TiN 2000
1700
Temperature( C)
70%Si3N4+30%TiN Fracture toughness(MPa.m1/2)
Vicker Hardness(Kg/mm2)
(b)
SPS
8
70%Si3N4+30%TiN 2000
HP
10
(c)
HP
SPS
8 6 4 2 0 1500
1600
1700
Temperature( C) Fig. 6. Fracture toughness of Si3N4/TiN composites sintered by HP and SPS (a) 100% Si3N4 + 0% TiN, (b) 70% Si3N4 + 30% TiN, and (c) 50% Si3N4 + 50% TiN.
showed that the densification for the conductive Si3N4/TiN composites started at an about 120 8C lower temperature than that for the non-conductive monophase Si3N4 because the actual temperature of the conductive samples is higher than the measured temperature. From the microstructural analysis, the aspect ratio of Si3N4/TiN composites is less than that of monophase Si3N4 both in HP and SPS because the presence of TiN suppresses the elongation of the b-Si3N4 grains. The porosity of Si3N4/TiN composites is higher than that of monophase Si3N4 in HP, but in SPS, the result is just the reverse. The result of the mechanical property tests shows that the introduction of TiN in Si3N4/TiN composite sample with 50% TiN increases the fracture toughness obviously without decreasing the hardness of the Si3N4. Acknowledgements M. Zhou acknowledges the Monash scholarship. The Authors acknowledge the use of facilities within the Monash Centre for Electron Microscopy.
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