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Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation
Advanced Powder Technology 24 (2013) 697–702
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation Nuray Karakusß a,⇑, A. Osman Kurt a, Cihangir Duran b, Cem Öztürk c, H. Özkan Toplan a a
Sakarya University, Engineering Faculty, Dept. of Metallurgy and Materials Engineering, 54187 Sakarya, Turkiye Gebze Institute of Technology, Dept. of Materials Science and Engineering, 41400, Gebze-Kocaeli, Turkey c MSE Technology Ltd., Sultan Orhan Mah. Hasköy Industrial Site, 7 Blok No. 6, Gebze-Kocaeli, Turkiye b
a r t i c l e
i n f o
Article history: Received 7 June 2012 Received in revised form 3 December 2012 Accepted 26 December 2012 Available online 11 January 2013 Keywords: Sintering Tape casting Si3N4 Densification Cold isostatic press
a b s t r a c t In this study, the sintering behaviour of silicon nitride (Si3N4) powders (having in situ form sintering aids/ self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised a-phase Si3N4 powders was studied, and the results were compared with a commercial powder. The a-Si3N4 powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of b-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si3N4 powders produced by CRN from silica initially containing 5 wt.% Y2O3. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Silicon nitride (Si3N4)-based structural ceramics have been explored since the late 1960s, primarily for use in high temperature and structural applications such as heat engines [1]. Taking into account their unique combination of properties, Si3N4 and related materials have become the most thoroughly characterised nonoxide ceramics, with a wide range of applications including heat exchangers, turbine and automotive engine components [2], valves and cam roller followers for gasoline and diesel engines [1,3]. The widespread use of Si3N4 ceramics has been limited by their low mechanical stability, their difficulty in machining, and the high manufacturing costs of their components [4], especially for fabricating parts with complex shapes. The manufacturing cost of such ceramics might be reduced using low-cost (but high-grade) starting powders such as powders from the carbothermal reduction and nitridation (CRN) process [5,6] and easy and economical production techniques, like pressureless sintering [7]. In addition, the colloidal processing has been reported as a cost-effective route to prepare ceramics of complicated shape with improved stability [8]. Various colloidal methods have been attempted to prepare
⇑ Corresponding author. E-mail address: [email protected] (N. Karakusß).
bulk Si3N4 samples, including tape casting [9], slip casting [10] and gel casting [1]. Silicon nitride is characterised by covalent bonds and very low atomic self-diffusion. Thus, liquid-forming sintering aids, such as MgO, Y2O3, Al2O3 and rare earth oxides added singularly or in combination, must be used to achieve the theoretical density [8–11]. Liquid-phase sintering of Si3N4 is conventionally considered to be a multi-stage process that includes (i) particle rearrangement aided by the lubricant action of the liquid, (ii) dissolution and crystallization of very fine grains and (iii) dissolution and re-crystallization of coarse grains [12,13]. Thus, the sintering behaviour of Si3N4, which includes densification, a-to-b phase transformation, and grain growth, is influenced substantially by the amount and chemistry of the liquid phase [12]. The most common sintering methods used to consolidate Si3N4-based ceramics are as follows: (i) reaction bonding (RBSN) [14], (ii) hot pressing (HPSN), (iii) hot isostatic pressing (HIPSN) [15], (iv) sintering (SSN) [16], (v) gas pressure sintering (GPSN) [17] and (vi) sintering reaction bonding (SRBSN) [18]. Regardless of the areas of its application, the main concern in the sintering of Si3N4 is to achieve the desired densities using an economical method. Pressureless sintering (SSN), in this respect, is a favourable method to be chosen because it is an easy and a suitable technique for continuous mass production. However, it is rather difficult to achieve high densities using this method without using very high amount of sintering aids, which has the drawback of creating a glassy intergranular phase [19,20].
0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.12.004
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Nevertheless, it is possible to achieve very high densities and good strength with SSN using a moderate amount of sintering aid when the correct powders and preparation methods are used [6,21,22]. It is equally important to have affordable starting powders that possess the desired properties. Currently, it is possible to produce inexpensive Si3N4 powders with certain desired properties using the carbothermal reduction and nitridation (CRN) process. The method is flexible; therefore, it can be adjusted to produce powders which incorporate sintering aids [23]. It is very common to see studies in the literature about the production of Si3N4 powders using the CRN process, though it is very rare to find any information on their sintering behaviour. Therefore, in this paper, the sintering of Si3N4 powders produced by the CRN process is explored.
SiO2 + MgO + C (SN-M)
CRN process 1450 oC – 3 h
SiO2 + Y2O3 + C (SN-Y)
Residual C burning 900 oC – 1 h
CRN process 1475 oC – 3 h
SiO2 + Li2O +Y2O3 + C (SN-LY)
Powder product (Si3N4) Shaping – CIP – Tape Cast
Characterization
Pressureless sintering 1650 – 1750 oC (0.5 – 2h) Fig. 1. Schematic representation of the Si3N4 powder production (by CRN), powder shaping and pressureless sintering processes.
2. Experimental details Silicon nitride powders having in situ formed self-sintering additives from carbothermal reduction nitridation (hereafter called ‘‘powders from CRN’’) were prepared at 1450 °C (using MgO addition) and at 1475 °C (using Y2O3 and Li2O–Y2O3 addition) for 3 h. Information about the initial powders used in this study was previously given in detail [23,24]. In this paper, the sintering performance of the powders from CRN is reported and compared with a commercial powder. Three as-synthesised high a-phase Si3N4 powders from the CRN process and another three compositions from a commercial powder (a-Si3N4 from Alfa Aesar – 42949#) were prepared for sintering. Table 1 gives the identification code for each powder with its production conditions in the CRN process and powder composition, which are used for preparing reference samples from the commercial powder. Fig. 1 shows a schematic presentation of the Si3N4 powder production, powder shaping and sintering stages. The dashed line indicates the scope and content of this paper. The powders used as additives were MgO, Y2O3 and Li2O. MgO and Y2O3 (99.9% pure) were purchased from Alfa Aesar. The calculated amount of Li2O was obtained by adding Li2CO3, which was 98.5% pure and purchased from Merck. Nitrogen gas (99.99% in purity) was used throughout the pressureless sintering processes. The morphology of the home-made initial powders from the CRN process are given in Fig. 2. The crystallinity and phase constituents of a typical high a-phase Si3N4 powder with Y2O3 addition (picture in Fig. 2b) can be observed from the XRD pattern given in Fig. 3. This powder was synthesised via the CRN process at 1475 °C in 3 h. Specifications for the initial powders (those in Fig. 2) were given in Table 2. Si3N4 powders, either having in situ formed sintering additives from the CRN process or pre-mixed additives in ball-milling before sintering, were shaped using a cold isostatic press (CIP) from Stansted Fluid Power and tape casting (TC) from MSE Teknoloji Ltd. (MSETC-200 C). For CIP treatment, the reference samples with specific compositions (i.e., rSN-M, rSN-Y and rSN-LY) were mixed in a PE
container with alumina balls for 12 h. The initial powders from the CRN process (i.e., SN-M, SN-Y and SN-LY) were pressed without milling. First, all samples were uniaxially pressed in £10 mm die to 3 mm thickness at 8 kg mm2 before CIPing. CIPing was performed using water as the pressure medium in two steps with 12 kg mm2 and 20 kg mm2. The CIP was held for 1 min at maximum pressure. For shaping in TCing, the powders and vinyl-based TC-S1 Binder Solution (MSE Teknoloji Ltd., Kocaeli-Türkiye) were mixed in the ball-milling apparatus for homogenisation. The TCing speed was 10 cm/s, and 200 lm was used as the desired thickness. Twenty layers from each composition were pressed at 4 MPa. The samples were re-sized by cutting before they were sintered. Binder removal took place at 550 °C for 2 h. The cold isostatic pressed (CIPed) and tape casted (TCed) green compacts (after binding removal) were further processed by pressureless sintering in a high temperature graphite furnace (Gero LHTG 100-200/20-1G) under a powder bed consisting of a mixture of 50% (in weight) BN, 45% b-Si3N4 and 5% of either a MgO, a Y2O3 or a Li2O–Y2O3 additive. Sintering was performed using N2 gas flow at 1650–1750 °C for 0.5–2 h. The density of the specimens after sintering was measured by the Archimedes’ principle. The identifications of the crystalline phases in the powders and the a to b phase transformation after sintering in the final products were performed by X-ray diffraction (XRD) with Cu Ka (k: 1.54056) radiation using a RIGAKU D/Max 2200 diffractometer. 3. Results and discussion 3.1. Densification The main scope of this study is to investigate the sintering behaviour of Si3N4 powders produced using the CRN process. In this context, only the density and a to b phase transformations were explored. Because the powder production is on a laboratory-scale, the amount of powders produced in one batch limits
Table 1 The codes and production parameters (in CRN) for the initial powders, the sintered products and the reference compositions (from commercial powders). Code of powders from CRN process
Raw materials and production condition of a-Si3N4 in CRN process
SN-M SN-Y SN-LY
SiO2 + 5 wt.% MgO at 1450 °C – 3 h – 1 l/min N2 SiO2 + 5 wt.% Y2O3 at 1475 °C – 3 h – 1 l/min N2 SiO2 + 5 wt.% Li2O + 5 wt.% Y2O3 at 1475 °C – 3 h – 1 l/min N2
Code of powders from commercial powder
Composition
rSN-M rSN-Y rSN-LY
a-Si3N4 (Alfa Aesar) + 5 wt.% MgO a-Si3N4 (Alfa Aesar) + 5 wt.% Y2O3 a-Si3N4 (Alfa Aesar) + 5 wt.% Li2O + 5 wt.% Y2O3
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Fig. 2. SEM images of initial powders from the CRN process; (a) SN-M, (b) SN-Y and (c) SN-LY. (All scale bars show 5 lm.)
Table 2 The specifications of powders used in sintering. Code
BET (m2 g1)
Amountd of a-Si3N4 (%)b
Secondary phase(s)
SN-M SN-Y SN-LY
4.73 3.89 1.12
86 100a 82
Reference powderc
6.48
86
b-Si3N4, Mg2SiO4 b-Si3N4, Y5Si3O12N b-Si3N4, LiYO2, Si2N2O b-Si3N4
a A very small amount of b-Si3N4 was observed with XRD though it was not possible to calculate due to absence of (2 1 0) peak at 36.1° (2h). b The amount of a phase of total Si3N4 content is given. c From Alfa Aesar – 42949#. d Calculated using Gazzara and Messier‘s equation [25].
Fig. 3. Crystalline phases of the initial a-Si3N4 powder (SN-Y) from the CRN process (pattern taken from sample in Fig. 2b).
how extensively the bulk properties can be studied. Nevertheless, the results given here are sufficient to show the powders‘ usability. The results for the densification of the sintered samples are given in Fig. 4; dark- and light-coloured lines are the samples shaped with cold isostatic press (CIP) and tape cast (TC), respectively. Graphs a, b and c in Fig. 4 are of the densities for sintered samples prepared from Si3N4 powders produced by the CRN process, and Fig. 4a0 –c0 shows the densities for sintered samples prepared from the reference powder. There is no clear effect of the shaping process on densification across all of the samples tested at different temperatures and times, except the Sample SN-Y (in Fig. 4b). SNY contains Y2O3, which was added during the CRN process. It is clear from Fig. 4b that TCed samples yielded higher sintered densi-
ties at all temperatures (i.e., 1650 °C, 1700 °C, 1750 °C) and sintering times (0.5, 1, 2 h) that were tested compared to samples shaped with CIPing (Fig. 4b0 ). In fact, this powder, SN-Y (specifications given in Table 2), exhibited the highest densities, as indicated in Fig. 4b and Table 3, compared to all samples tested in this study. Table 3 provides a summary of the highest density values from Fig. 4. The arrows on the table are shown to allow the reader to follow the density change. By looking at the table, it is clear that the highest density (3.20 g cm3) was obtained after pressureless sintering (at 1700 °C for 0.5 h) of SN-Y powder produced by the CRN process. To evaluate the sintering results, every system (home-made and commercial Si3N4 powders with sintering additives) should be considered individually. Using a MgO additive as a sintering aid, for example, high-sintered densities were obtained from those samples sintered at 1650 °C (Fig. 4a and a0 ). Under a constant sintering time, an increase in temperature (in general) lowered the density values. This is true for both systems (SN-M and rSN-M) as observed in Fig. 4a and a0 . Similar results were obtained when the sintering
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Table 3 Sintering conditions (temperature and time) for the highest densities and their relevance to the shaping method (‘‘CIP’’ for cold isostatic pressing and ‘‘TC’’ for tape casting). Highest density values (g cm3)
Specimen codes
‘‘CIP’’
a
‘‘TC’’
Sintering conditions (°C–h)
SN-M SN-LY SN-Y
"
2.80 2.71 2.55
;
2.66 2.94 3.20a
1650–0.5 1650–0.5 1650–2.0
rSN-M rSN-LY rSN-Y
;
2.84 2.87 2.90
"
2.97 2.86 2.79
1650–1.0 1650–2.0 1650–1.0
Tape casted sample of SN-Y sintered at 1700 °C for 0.5 h.
time was increased at constant temperature. Although the experiments were performed under N2 gas flow, the decline in density with time and temperature might be considered normal because, in the pressureless sintering method, the increase in temperature and/or time results in an increase in the instability of the system, resulting dissociation of the unstable phases. This normally happens for Si3N4 at the temperatures above 1750 °C. However, according to MgO–SiO2 and Y2O3–SiO2 binary phase diagrams
Fig. 5. Amounts of b-phase after sintering for samples given in Table 3.
[26,27], the eutectic phases form as low as temperatures of 1543 °C and 1660 °C, respectively. SiO2 comes from the surface oxide of Si3N4 particles. During the sintering process of these materials in our work at 1650 °C, 1700 °C and 1750 °C, the liquid phases form and their viscosity decreases with an increase in temperature. Due to the existence of unstable phases and the decrease in
(a’)
(a)
3
Density (g/cm3)
Density (g/cm3)
3 2.5 2 1.5
2.5 2 1.5 1
1 0
0.5
1 1.5 Sintering Time (h)
2
2.5
3
3
2.5
2.5
Density (g/cm3)
Density (g/cm3)
0.5
1 1.5 Sintering Time (h)
2
2.5
0
0.5
1 1.5 Sintering Time (h)
2
2.5
0
0.5
1 1.5 Sintering Time (h)
2
2.5
(b’)
(b)
2 1.5
2 1.5
1
1
0
0.5
1 1.5 Sintering Time (h)
2
2.5
(c’)
(c) 3
3
2.5
2.5
Density (g/cm3)
3) % Density (g/cm
0
2 1.5
2 1.5 1
1 0
0.5
1 1.5 Sintering Time (h)
2
2.5
Fig. 4. Densities versus sintering time and temperature for samples (a) SN-M, (a0 ) rSN-M, (b) SN-Y, (b0 ) rSN-Y, (c) SN-LY and (c0 ) rSN-LY.
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exhibited increased density with increasing time. It should be noted, however, that the changes in density for those samples are very small in a given range of testing conditions.
3.2. Phase transition
Fig. 6. Amounts of b-phase after sintering of SN-Y (TCed sample).
Fig. 5 gives the amount of the b-phase (of the total Si3N4 content) after sintering. The first two columns in each part of the figure are samples consisting of the initial powders from the CRN process. The third and fourth columns are the samples prepared from the reference powder. For the samples that were sintered, all but one composition (i.e., the tape-casted sample of rSN-LY, the first column on the far right of Fig. 5) demonstrated only a partial conversion to b-Si3N4. Samples containing yttrium oxide additives (i.e., SN-Y and rSN-Y in middle column on Fig. 5) exhibited very low b-phase conversion compared to the Si3N4 powders with MgO or Li2O–Y2O3 additives. As can be seen in Fig. 6, a full conversion to b-phase Si3N4 was possible by increasing the holding time during sintering from 0.5 to 2 h. With an increase in the sintering time, an unidentified crystalline secondary phase, observed in shorter sintering periods, disappeared completely. On the contrary, extending the sintering period from 0.5 h to 2 h lowers the density from 3.20 g cm3 to 3.02 g cm3, respectively (Fig. 4b). Sintering of SN-Y at 1700 °C for 2 h under N2 flow may have resulted in the disappearance of the metastable crystalline Y5Si3O12N phase by means of dissociation, which may be the reason for the decrease in density. The Y5Si3O12N was the only crystalline secondary phase present in the initial SN-Y powder (Fig. 3).
3.3. SEM analysis Fig. 7. SEM analysis of SN-Y after sintering at 1700 °C for 0.5 h (TCed sample).
viscosity, then the densities of samples reduced noticeably. The processing conditions of our sintering system may have also helped to this in one way (the presence PO2) or another, since all the samples regardless of whether they are home-made or as-received one had similar behaviour. Therefore, the decrease in viscosity of formed liquid phases may have helped the instability of the system resulting in a decline of the final densities. Nevertheless, since sintering studies were performed under identical conditions for samples from commercial powders and powders produced by CRN method, the results are to be used for comparison. For this reason, our findings are in accord with the literature [28]. A few of the results of the series in this study, however, did not obey this assumption, such as the samples sintered at 1750 °C (from SN-Y). Samples at 1650 °C from the rSN-LY system
Due to the scope of this study and the limited amount of powder synthesised, the results from the SEM analyses were limited to a selected number of samples. Fig. 7 shows the SEM images (using back-scattering electron detection) for the tape-casted Si3N4 sample (SN-Y) after sintering at 1700 °C for 0.5 h. This was the sample with the highest density, though its microstructure did not demonstrate complete conversion to the b-Si3N4 phase. The aspect ratio of the rod-shaped b-Si3N4 grains, with a cross-section of approximately 400–600 nm, are in the range of 3–20, and there are a small amount of pores present in the sample (Fig. 8a). The a-grains cannot be clearly distinguished in the image. An intergranular glassy phase was relatively well distributed, though it was unable to fully wet the b-grains. Some samples that were shaped using the colloidal process (tape casting) at low temperatures resulted in higher sintered densities compared to samples produced from commercial powder (Fig. 8a compared to b).
Fig. 8. SEM images of samples after sintering; (a) SN-Y and (b) rSN-Y.
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4. Conclusion This study investigated the sintering behaviour of a-phase silicon nitride powders containing in situ-formed self-sintering additives produced by carbothermal reduction nitridation (CRN). Powders produced using the CRN process and commercial powders were pressureless-sintered at 1650–1750 °C for up to 2 h. Based on the findings from this study, the following conclusions can be made: The CRN process is a promising alternative to the commercial ceramic powder production methods, especially for synthesising sinterable a- and/or a/b-phase Si3N4. The premixture of SiO2 with an oxide additive (i.e., 5% MgO, 5% Y2O3 or 5% Li2O– 5% Y2O3) resulted in powders (a-Si3N4 with small amount of secondary phases) that were appropriate for sintering. a-Si3N4 powders generated by CRN using Y2O3, MgO, or Li2O– Y2O3 as an additive gave identical or better density and b-phase conversion after sintering compared to samples from the commercial powder. Powders that initially contained a yttrium-based secondaryphase oxide additive after CRN processing were highly sinterable in pressureless liquid-phase-sintering at all temperatures and sintering times used in this study. CRN-produced powders containing the yttrium additive exhibited better sintering density and b-phase conversion compared to samples produced with the reference powder under the same testing conditions. The highest density of 3.20 g cm3 was obtained from sintering (1700 °C for 0.5 h) the a-Si3N4 powder that was produced from the CRN process. The highest density value using the commercial Si3N4 powder was only 2.97 g cm3 (1650 °C for 1 h).
Acknowledgements The authors wish to acknowledge funding from DPT (State Planning Organisation of Turkiye – Grant No. DPT-2003K120970). We also thank EGE Kimya A.S ß . for their help in providing raw materials. References [1] M.H.B. Bernal, B. Matovic, Dense and near net shape fabrication of Si3N4 ceramics, Mater. Sci. Eng. A Struct. Mater. 500 (2009) 130–149. [2] A. Okada, Automotive and industrial applications of structural ceramics in Japan, J. Eur. Ceram. Soc 28 (2008) 1097–1104. [3] A. Rosenflanz, Silicon nitride and sialon ceramics, Curr. Opin. Solid State Mater. Sci. 4 (1999) 453–459. [4] M.H.B. Bernal, B. Matovic, Mechanical properties of silicon nitride-based ceramics and its use in structural applications at high temperatures, Mater. Sci. Eng. A 527 (2010) 1314–1338.
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation Nuray Karakusß a,⇑, A. Osman Kurt a, Cihangir Duran b, Cem Öztürk c, H. Özkan Toplan a a
Sakarya University, Engineering Faculty, Dept. of Metallurgy and Materials Engineering, 54187 Sakarya, Turkiye Gebze Institute of Technology, Dept. of Materials Science and Engineering, 41400, Gebze-Kocaeli, Turkey c MSE Technology Ltd., Sultan Orhan Mah. Hasköy Industrial Site, 7 Blok No. 6, Gebze-Kocaeli, Turkiye b
a r t i c l e
i n f o
Article history: Received 7 June 2012 Received in revised form 3 December 2012 Accepted 26 December 2012 Available online 11 January 2013 Keywords: Sintering Tape casting Si3N4 Densification Cold isostatic press
a b s t r a c t In this study, the sintering behaviour of silicon nitride (Si3N4) powders (having in situ form sintering aids/ self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised a-phase Si3N4 powders was studied, and the results were compared with a commercial powder. The a-Si3N4 powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sintering, the density and the amount of b-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20 g cm3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si3N4 powders produced by CRN from silica initially containing 5 wt.% Y2O3. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Silicon nitride (Si3N4)-based structural ceramics have been explored since the late 1960s, primarily for use in high temperature and structural applications such as heat engines [1]. Taking into account their unique combination of properties, Si3N4 and related materials have become the most thoroughly characterised nonoxide ceramics, with a wide range of applications including heat exchangers, turbine and automotive engine components [2], valves and cam roller followers for gasoline and diesel engines [1,3]. The widespread use of Si3N4 ceramics has been limited by their low mechanical stability, their difficulty in machining, and the high manufacturing costs of their components [4], especially for fabricating parts with complex shapes. The manufacturing cost of such ceramics might be reduced using low-cost (but high-grade) starting powders such as powders from the carbothermal reduction and nitridation (CRN) process [5,6] and easy and economical production techniques, like pressureless sintering [7]. In addition, the colloidal processing has been reported as a cost-effective route to prepare ceramics of complicated shape with improved stability [8]. Various colloidal methods have been attempted to prepare
⇑ Corresponding author. E-mail address: [email protected] (N. Karakusß).
bulk Si3N4 samples, including tape casting [9], slip casting [10] and gel casting [1]. Silicon nitride is characterised by covalent bonds and very low atomic self-diffusion. Thus, liquid-forming sintering aids, such as MgO, Y2O3, Al2O3 and rare earth oxides added singularly or in combination, must be used to achieve the theoretical density [8–11]. Liquid-phase sintering of Si3N4 is conventionally considered to be a multi-stage process that includes (i) particle rearrangement aided by the lubricant action of the liquid, (ii) dissolution and crystallization of very fine grains and (iii) dissolution and re-crystallization of coarse grains [12,13]. Thus, the sintering behaviour of Si3N4, which includes densification, a-to-b phase transformation, and grain growth, is influenced substantially by the amount and chemistry of the liquid phase [12]. The most common sintering methods used to consolidate Si3N4-based ceramics are as follows: (i) reaction bonding (RBSN) [14], (ii) hot pressing (HPSN), (iii) hot isostatic pressing (HIPSN) [15], (iv) sintering (SSN) [16], (v) gas pressure sintering (GPSN) [17] and (vi) sintering reaction bonding (SRBSN) [18]. Regardless of the areas of its application, the main concern in the sintering of Si3N4 is to achieve the desired densities using an economical method. Pressureless sintering (SSN), in this respect, is a favourable method to be chosen because it is an easy and a suitable technique for continuous mass production. However, it is rather difficult to achieve high densities using this method without using very high amount of sintering aids, which has the drawback of creating a glassy intergranular phase [19,20].
0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.12.004
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Nevertheless, it is possible to achieve very high densities and good strength with SSN using a moderate amount of sintering aid when the correct powders and preparation methods are used [6,21,22]. It is equally important to have affordable starting powders that possess the desired properties. Currently, it is possible to produce inexpensive Si3N4 powders with certain desired properties using the carbothermal reduction and nitridation (CRN) process. The method is flexible; therefore, it can be adjusted to produce powders which incorporate sintering aids [23]. It is very common to see studies in the literature about the production of Si3N4 powders using the CRN process, though it is very rare to find any information on their sintering behaviour. Therefore, in this paper, the sintering of Si3N4 powders produced by the CRN process is explored.
SiO2 + MgO + C (SN-M)
CRN process 1450 oC – 3 h
SiO2 + Y2O3 + C (SN-Y)
Residual C burning 900 oC – 1 h
CRN process 1475 oC – 3 h
SiO2 + Li2O +Y2O3 + C (SN-LY)
Powder product (Si3N4) Shaping – CIP – Tape Cast
Characterization
Pressureless sintering 1650 – 1750 oC (0.5 – 2h) Fig. 1. Schematic representation of the Si3N4 powder production (by CRN), powder shaping and pressureless sintering processes.
2. Experimental details Silicon nitride powders having in situ formed self-sintering additives from carbothermal reduction nitridation (hereafter called ‘‘powders from CRN’’) were prepared at 1450 °C (using MgO addition) and at 1475 °C (using Y2O3 and Li2O–Y2O3 addition) for 3 h. Information about the initial powders used in this study was previously given in detail [23,24]. In this paper, the sintering performance of the powders from CRN is reported and compared with a commercial powder. Three as-synthesised high a-phase Si3N4 powders from the CRN process and another three compositions from a commercial powder (a-Si3N4 from Alfa Aesar – 42949#) were prepared for sintering. Table 1 gives the identification code for each powder with its production conditions in the CRN process and powder composition, which are used for preparing reference samples from the commercial powder. Fig. 1 shows a schematic presentation of the Si3N4 powder production, powder shaping and sintering stages. The dashed line indicates the scope and content of this paper. The powders used as additives were MgO, Y2O3 and Li2O. MgO and Y2O3 (99.9% pure) were purchased from Alfa Aesar. The calculated amount of Li2O was obtained by adding Li2CO3, which was 98.5% pure and purchased from Merck. Nitrogen gas (99.99% in purity) was used throughout the pressureless sintering processes. The morphology of the home-made initial powders from the CRN process are given in Fig. 2. The crystallinity and phase constituents of a typical high a-phase Si3N4 powder with Y2O3 addition (picture in Fig. 2b) can be observed from the XRD pattern given in Fig. 3. This powder was synthesised via the CRN process at 1475 °C in 3 h. Specifications for the initial powders (those in Fig. 2) were given in Table 2. Si3N4 powders, either having in situ formed sintering additives from the CRN process or pre-mixed additives in ball-milling before sintering, were shaped using a cold isostatic press (CIP) from Stansted Fluid Power and tape casting (TC) from MSE Teknoloji Ltd. (MSETC-200 C). For CIP treatment, the reference samples with specific compositions (i.e., rSN-M, rSN-Y and rSN-LY) were mixed in a PE
container with alumina balls for 12 h. The initial powders from the CRN process (i.e., SN-M, SN-Y and SN-LY) were pressed without milling. First, all samples were uniaxially pressed in £10 mm die to 3 mm thickness at 8 kg mm2 before CIPing. CIPing was performed using water as the pressure medium in two steps with 12 kg mm2 and 20 kg mm2. The CIP was held for 1 min at maximum pressure. For shaping in TCing, the powders and vinyl-based TC-S1 Binder Solution (MSE Teknoloji Ltd., Kocaeli-Türkiye) were mixed in the ball-milling apparatus for homogenisation. The TCing speed was 10 cm/s, and 200 lm was used as the desired thickness. Twenty layers from each composition were pressed at 4 MPa. The samples were re-sized by cutting before they were sintered. Binder removal took place at 550 °C for 2 h. The cold isostatic pressed (CIPed) and tape casted (TCed) green compacts (after binding removal) were further processed by pressureless sintering in a high temperature graphite furnace (Gero LHTG 100-200/20-1G) under a powder bed consisting of a mixture of 50% (in weight) BN, 45% b-Si3N4 and 5% of either a MgO, a Y2O3 or a Li2O–Y2O3 additive. Sintering was performed using N2 gas flow at 1650–1750 °C for 0.5–2 h. The density of the specimens after sintering was measured by the Archimedes’ principle. The identifications of the crystalline phases in the powders and the a to b phase transformation after sintering in the final products were performed by X-ray diffraction (XRD) with Cu Ka (k: 1.54056) radiation using a RIGAKU D/Max 2200 diffractometer. 3. Results and discussion 3.1. Densification The main scope of this study is to investigate the sintering behaviour of Si3N4 powders produced using the CRN process. In this context, only the density and a to b phase transformations were explored. Because the powder production is on a laboratory-scale, the amount of powders produced in one batch limits
Table 1 The codes and production parameters (in CRN) for the initial powders, the sintered products and the reference compositions (from commercial powders). Code of powders from CRN process
Raw materials and production condition of a-Si3N4 in CRN process
SN-M SN-Y SN-LY
SiO2 + 5 wt.% MgO at 1450 °C – 3 h – 1 l/min N2 SiO2 + 5 wt.% Y2O3 at 1475 °C – 3 h – 1 l/min N2 SiO2 + 5 wt.% Li2O + 5 wt.% Y2O3 at 1475 °C – 3 h – 1 l/min N2
Code of powders from commercial powder
Composition
rSN-M rSN-Y rSN-LY
a-Si3N4 (Alfa Aesar) + 5 wt.% MgO a-Si3N4 (Alfa Aesar) + 5 wt.% Y2O3 a-Si3N4 (Alfa Aesar) + 5 wt.% Li2O + 5 wt.% Y2O3
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Fig. 2. SEM images of initial powders from the CRN process; (a) SN-M, (b) SN-Y and (c) SN-LY. (All scale bars show 5 lm.)
Table 2 The specifications of powders used in sintering. Code
BET (m2 g1)
Amountd of a-Si3N4 (%)b
Secondary phase(s)
SN-M SN-Y SN-LY
4.73 3.89 1.12
86 100a 82
Reference powderc
6.48
86
b-Si3N4, Mg2SiO4 b-Si3N4, Y5Si3O12N b-Si3N4, LiYO2, Si2N2O b-Si3N4
a A very small amount of b-Si3N4 was observed with XRD though it was not possible to calculate due to absence of (2 1 0) peak at 36.1° (2h). b The amount of a phase of total Si3N4 content is given. c From Alfa Aesar – 42949#. d Calculated using Gazzara and Messier‘s equation [25].
Fig. 3. Crystalline phases of the initial a-Si3N4 powder (SN-Y) from the CRN process (pattern taken from sample in Fig. 2b).
how extensively the bulk properties can be studied. Nevertheless, the results given here are sufficient to show the powders‘ usability. The results for the densification of the sintered samples are given in Fig. 4; dark- and light-coloured lines are the samples shaped with cold isostatic press (CIP) and tape cast (TC), respectively. Graphs a, b and c in Fig. 4 are of the densities for sintered samples prepared from Si3N4 powders produced by the CRN process, and Fig. 4a0 –c0 shows the densities for sintered samples prepared from the reference powder. There is no clear effect of the shaping process on densification across all of the samples tested at different temperatures and times, except the Sample SN-Y (in Fig. 4b). SNY contains Y2O3, which was added during the CRN process. It is clear from Fig. 4b that TCed samples yielded higher sintered densi-
ties at all temperatures (i.e., 1650 °C, 1700 °C, 1750 °C) and sintering times (0.5, 1, 2 h) that were tested compared to samples shaped with CIPing (Fig. 4b0 ). In fact, this powder, SN-Y (specifications given in Table 2), exhibited the highest densities, as indicated in Fig. 4b and Table 3, compared to all samples tested in this study. Table 3 provides a summary of the highest density values from Fig. 4. The arrows on the table are shown to allow the reader to follow the density change. By looking at the table, it is clear that the highest density (3.20 g cm3) was obtained after pressureless sintering (at 1700 °C for 0.5 h) of SN-Y powder produced by the CRN process. To evaluate the sintering results, every system (home-made and commercial Si3N4 powders with sintering additives) should be considered individually. Using a MgO additive as a sintering aid, for example, high-sintered densities were obtained from those samples sintered at 1650 °C (Fig. 4a and a0 ). Under a constant sintering time, an increase in temperature (in general) lowered the density values. This is true for both systems (SN-M and rSN-M) as observed in Fig. 4a and a0 . Similar results were obtained when the sintering
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Table 3 Sintering conditions (temperature and time) for the highest densities and their relevance to the shaping method (‘‘CIP’’ for cold isostatic pressing and ‘‘TC’’ for tape casting). Highest density values (g cm3)
Specimen codes
‘‘CIP’’
a
‘‘TC’’
Sintering conditions (°C–h)
SN-M SN-LY SN-Y
"
2.80 2.71 2.55
;
2.66 2.94 3.20a
1650–0.5 1650–0.5 1650–2.0
rSN-M rSN-LY rSN-Y
;
2.84 2.87 2.90
"
2.97 2.86 2.79
1650–1.0 1650–2.0 1650–1.0
Tape casted sample of SN-Y sintered at 1700 °C for 0.5 h.
time was increased at constant temperature. Although the experiments were performed under N2 gas flow, the decline in density with time and temperature might be considered normal because, in the pressureless sintering method, the increase in temperature and/or time results in an increase in the instability of the system, resulting dissociation of the unstable phases. This normally happens for Si3N4 at the temperatures above 1750 °C. However, according to MgO–SiO2 and Y2O3–SiO2 binary phase diagrams
Fig. 5. Amounts of b-phase after sintering for samples given in Table 3.
[26,27], the eutectic phases form as low as temperatures of 1543 °C and 1660 °C, respectively. SiO2 comes from the surface oxide of Si3N4 particles. During the sintering process of these materials in our work at 1650 °C, 1700 °C and 1750 °C, the liquid phases form and their viscosity decreases with an increase in temperature. Due to the existence of unstable phases and the decrease in
(a’)
(a)
3
Density (g/cm3)
Density (g/cm3)
3 2.5 2 1.5
2.5 2 1.5 1
1 0
0.5
1 1.5 Sintering Time (h)
2
2.5
3
3
2.5
2.5
Density (g/cm3)
Density (g/cm3)
0.5
1 1.5 Sintering Time (h)
2
2.5
0
0.5
1 1.5 Sintering Time (h)
2
2.5
0
0.5
1 1.5 Sintering Time (h)
2
2.5
(b’)
(b)
2 1.5
2 1.5
1
1
0
0.5
1 1.5 Sintering Time (h)
2
2.5
(c’)
(c) 3
3
2.5
2.5
Density (g/cm3)
3) % Density (g/cm
0
2 1.5
2 1.5 1
1 0
0.5
1 1.5 Sintering Time (h)
2
2.5
Fig. 4. Densities versus sintering time and temperature for samples (a) SN-M, (a0 ) rSN-M, (b) SN-Y, (b0 ) rSN-Y, (c) SN-LY and (c0 ) rSN-LY.
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exhibited increased density with increasing time. It should be noted, however, that the changes in density for those samples are very small in a given range of testing conditions.
3.2. Phase transition
Fig. 6. Amounts of b-phase after sintering of SN-Y (TCed sample).
Fig. 5 gives the amount of the b-phase (of the total Si3N4 content) after sintering. The first two columns in each part of the figure are samples consisting of the initial powders from the CRN process. The third and fourth columns are the samples prepared from the reference powder. For the samples that were sintered, all but one composition (i.e., the tape-casted sample of rSN-LY, the first column on the far right of Fig. 5) demonstrated only a partial conversion to b-Si3N4. Samples containing yttrium oxide additives (i.e., SN-Y and rSN-Y in middle column on Fig. 5) exhibited very low b-phase conversion compared to the Si3N4 powders with MgO or Li2O–Y2O3 additives. As can be seen in Fig. 6, a full conversion to b-phase Si3N4 was possible by increasing the holding time during sintering from 0.5 to 2 h. With an increase in the sintering time, an unidentified crystalline secondary phase, observed in shorter sintering periods, disappeared completely. On the contrary, extending the sintering period from 0.5 h to 2 h lowers the density from 3.20 g cm3 to 3.02 g cm3, respectively (Fig. 4b). Sintering of SN-Y at 1700 °C for 2 h under N2 flow may have resulted in the disappearance of the metastable crystalline Y5Si3O12N phase by means of dissociation, which may be the reason for the decrease in density. The Y5Si3O12N was the only crystalline secondary phase present in the initial SN-Y powder (Fig. 3).
3.3. SEM analysis Fig. 7. SEM analysis of SN-Y after sintering at 1700 °C for 0.5 h (TCed sample).
viscosity, then the densities of samples reduced noticeably. The processing conditions of our sintering system may have also helped to this in one way (the presence PO2) or another, since all the samples regardless of whether they are home-made or as-received one had similar behaviour. Therefore, the decrease in viscosity of formed liquid phases may have helped the instability of the system resulting in a decline of the final densities. Nevertheless, since sintering studies were performed under identical conditions for samples from commercial powders and powders produced by CRN method, the results are to be used for comparison. For this reason, our findings are in accord with the literature [28]. A few of the results of the series in this study, however, did not obey this assumption, such as the samples sintered at 1750 °C (from SN-Y). Samples at 1650 °C from the rSN-LY system
Due to the scope of this study and the limited amount of powder synthesised, the results from the SEM analyses were limited to a selected number of samples. Fig. 7 shows the SEM images (using back-scattering electron detection) for the tape-casted Si3N4 sample (SN-Y) after sintering at 1700 °C for 0.5 h. This was the sample with the highest density, though its microstructure did not demonstrate complete conversion to the b-Si3N4 phase. The aspect ratio of the rod-shaped b-Si3N4 grains, with a cross-section of approximately 400–600 nm, are in the range of 3–20, and there are a small amount of pores present in the sample (Fig. 8a). The a-grains cannot be clearly distinguished in the image. An intergranular glassy phase was relatively well distributed, though it was unable to fully wet the b-grains. Some samples that were shaped using the colloidal process (tape casting) at low temperatures resulted in higher sintered densities compared to samples produced from commercial powder (Fig. 8a compared to b).
Fig. 8. SEM images of samples after sintering; (a) SN-Y and (b) rSN-Y.
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4. Conclusion This study investigated the sintering behaviour of a-phase silicon nitride powders containing in situ-formed self-sintering additives produced by carbothermal reduction nitridation (CRN). Powders produced using the CRN process and commercial powders were pressureless-sintered at 1650–1750 °C for up to 2 h. Based on the findings from this study, the following conclusions can be made: The CRN process is a promising alternative to the commercial ceramic powder production methods, especially for synthesising sinterable a- and/or a/b-phase Si3N4. The premixture of SiO2 with an oxide additive (i.e., 5% MgO, 5% Y2O3 or 5% Li2O– 5% Y2O3) resulted in powders (a-Si3N4 with small amount of secondary phases) that were appropriate for sintering. a-Si3N4 powders generated by CRN using Y2O3, MgO, or Li2O– Y2O3 as an additive gave identical or better density and b-phase conversion after sintering compared to samples from the commercial powder. Powders that initially contained a yttrium-based secondaryphase oxide additive after CRN processing were highly sinterable in pressureless liquid-phase-sintering at all temperatures and sintering times used in this study. CRN-produced powders containing the yttrium additive exhibited better sintering density and b-phase conversion compared to samples produced with the reference powder under the same testing conditions. The highest density of 3.20 g cm3 was obtained from sintering (1700 °C for 0.5 h) the a-Si3N4 powder that was produced from the CRN process. The highest density value using the commercial Si3N4 powder was only 2.97 g cm3 (1650 °C for 1 h).
Acknowledgements The authors wish to acknowledge funding from DPT (State Planning Organisation of Turkiye – Grant No. DPT-2003K120970). We also thank EGE Kimya A.S ß . for their help in providing raw materials. References [1] M.H.B. Bernal, B. Matovic, Dense and near net shape fabrication of Si3N4 ceramics, Mater. Sci. Eng. A Struct. Mater. 500 (2009) 130–149. [2] A. Okada, Automotive and industrial applications of structural ceramics in Japan, J. Eur. Ceram. Soc 28 (2008) 1097–1104. [3] A. Rosenflanz, Silicon nitride and sialon ceramics, Curr. Opin. Solid State Mater. Sci. 4 (1999) 453–459. [4] M.H.B. Bernal, B. Matovic, Mechanical properties of silicon nitride-based ceramics and its use in structural applications at high temperatures, Mater. Sci. Eng. A 527 (2010) 1314–1338.
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