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Microwave sintering of β-SiAlON–ZrO2 composites
Materials and Design 31 (2010) 3641–3646
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microwave sintering of b-SiAlON–ZrO2 composites Sreekumar Chockalingam *, Henry Kodi Traver New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA
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Article history: Received 3 December 2009 Accepted 20 February 2010 Available online 26 February 2010 Keywords: Microwave processing Microstructure SiAlON ZrO2 Hardness
a b s t r a c t Densification, phase transformation, microstructure evolution and hardness of microwave sintered b-SiAlON–ZrO2 composites were investigated and compared with conventionally sintered samples. Sintering trials were performed by a high vacuum capable 2.45 GHz microwave furnace without decomposition. Microwave sintered samples showed better densification behavior than conventional sintered samples. The higher density observed in the case of microwave sintered samples was attributed to volumetric fast heating. X-ray diffraction results of conventionally sintered samples showed b-SiAlON, tetragonal ZrO2 and ZrN phases, while, ZrO2 reacted with nitrogen and completely transformed to ZrN in the case of microwave sintered samples. The aspect ratios of microwave sintered b-SiAlON grains were higher than conventional sintered samples whereas, hardness remained lower. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction An alloy of silicon nitride, b-SiAlON is one of the most promising high temperature structural materials due to its excellent thermal and mechanical properties [1]. The mechanical properties of b-SiAlON is better than that of Si3N4 and Al2O3 due to its superior bond strength caused by the co-ordination of Al with O as AlO4 instead of AlO6 found in Al2O3 [2]. However, their low fracture toughness has impeded their widespread applications. One of the methods to improve the fracture toughness of ceramic materials is to disperse stabilized ZrO2 particles in the SiAlON matrix either by taking the advantages of stress-induced martensitic transformation from tetragonal to monoclinic phase which would absorb the fracture energy or by microcrack toughening of ZrO2 particles [3]. Stabilized zirconia has already been used as a toughening agent in Si3N4 matrix to improve their mechanical properties [4]. Claussen and Jahn reported that addition of 20 vol.% unstabilized ZrO2 into Si3N4 matrix improved the fracture toughness through microcracking toughening mechanism [5]. They also indicated the formation of Si2ON2. Terao et al. found that dispersion of 20 wt.% of 2.5 mol.% Y2O3 stabilized ZrO2 in Si3N4 was advantageous to increase the room-temperature fracture toughness without degradation of hardness [6]. Recently, there were many reports in the literature about the use of microwave energy to sinter ceramic materials [7–13]. The advantages of using microwave energy to sinter ceramic materials compared to conventional techniques are reduced sintering time, temperature and capability of producing unique microstructure * Corresponding author. Tel.: +91 9910801320. E-mail address: [email protected] (S. Chockalingam). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.02.042
that could not be achieved by other conventional methods [7]. Lossy ceramic materials like SiC couple with microwaves very efficiently at room temperature. Low loss ceramic materials such as Al2O3 and SiO2 seldom couple with microwaves at room temperature [7]. However, these materials begin to couple with microwaves above a critical temperature due to the increase of their dielectric properties. For example Y2O3 stabilized ZrO2 begins to couple 2.45 GHz microwaves in the temperature range of 250– 400 °C [14]. The coupling of microwaves at low temperature with a low loss ceramic could be improved by either using high loss SiC based external susceptors or adding lossy conductive or magnetic materials in the form of secondary phases [8]. The lossy secondary phase couples microwave power more efficiently than the low loss matrix and hence, could be heated selectively and rapidly at ambient temperature [8]. Silicon nitride, in its pure form, is a poor microwave absorber due to its low dielectric loss at room temperature [15]. However, heating of Si3N4 takes place above 1300 °C by selective coupling of microwaves to the grain boundary glassy phases [15]. Jones et al. sintered Si3N4 using microwave energy at a lower temperature than that required for standard method and achieved full density [9]. Janney et al. lowered the sintering temperature of 8 mol.% Y2O3 stabilized ZrO2 from 1375 °C to 1200 °C using 2.45 GHz microwave energy [13]. Although Si3N4–ZrO2 [4–6,16,17] and SiAlON–ZrO2 [3,18] composites have been previously studied by many researchers, no study has been reported in the literature about the use of microwave energy to sinter b-SiAlON–ZrO2 composites. The addition of ZrO2 not only improves the oxidation and corrosion resistance of grain boundary phase but also improves the coupling of microwaves [19]. Therefore, the purpose of the present work was to prepare b-SiAlON–ZrO2 composites from a-Si3N4, AlN, Al2O3 and Y2O3
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with varying amounts of 3 mol.% Y2O3 stabilized ZrO2 powders through 2.45 GHz microwave energy and investigate densification behavior, phase transformation, microstructure development and hardness. 2. Experimental Starting powder mixtures were prepared by using a-Si3N4 powder (E10 grade, UBE Industries Ltd., Yamaguchi, Japan), Al2O3 (Aldrich Chemical Company Inc., Milwaukee, WI, 99.99%) AlN (Grade B, H.C. Starck, Germany) and 3 mol.% Y2O3 stabilized ZrO2 (Tosoh Corporation, Tokyo, Japan). The powder composition for the formation of b-SiAlON was 89.5 wt.% Si3N4, 6.1 wt.% Al2O3, 2.4 wt.% AlN and 2 wt.% Y2O3. Four batches of composite powders were prepared by mixing 0 wt.%, 5 wt.%, 15 wt.% and 25 wt.% of 3 mol.% Y2O3 stabilized ZrO2 powder with b-SiAlON composition. The powders were ball milled in 2-propanol for 12 h using high wear resistant ZrO2 milling media (YTZ Grinding Media, Tosoh Corporation, Tokyo, Japan). The dried powders were ground and sieved through a 38 lm mesh. Five grams of the mixed powders were diepressed at 30 MPa and cold isostatically pressed at 200 MPa into cylindrical green pellets of diameter 23 mm and thickness of 5 mm. Fig. 1a shows the original photograph of the microwave furnace used to sinter b-SiAlON–ZrO2. The schematic of microwave refractory set up is shown in Fig. 1b. Microwave sintering was carried out in a 2.45 GHz, industrial microwave furnace with variable output power of maximum 3 kW. A diffusion pump was fitted to the microwave cavity to obtain high vacuum of the order of 105 Torr. The microwave refractory box was fabricated with high purity alumina fiber board rated for 1800 °C. The pressed b-SiAlON–ZrO2 compacts were placed in a boron nitride (BN) crucible filled with a protective BN powder bed mixed with course SiC powder. The SiC course powder acts as an external susceptor to heat the sample at room temperature. Microwave sintering was performed at 1700 °C under flowing nitrogen. The heating rate was controlled to 25 °C/min by adjusting the input microwave power with the help of computer. The temperature of refractory box was continuously monitored with the help of grounded thermocouples as shown in the inset of the Fig. 1a. Measurement of the surface temperature of the samples were carried out through a hole made through the alumina refractory box and the BN crucible using a computer controlled two-color pyrometer. The temperature of the outside surface of the alumina refractory box was measured with the help of thermocouple. Care was taken to ground the thermocouple to avoid arcing during microwave processing. Conventional sintering trials were performed at 1800 °C under flowing nitrogen using graphite heating element furnace with a heating/cooling rate of 10 °C/min for 1 h.
(a)
The densities of the samples were determined using Archimedes principle with de-ionized water. The microstructures were evaluated from diamond polished and acid etched samples using FEG 200 (FEI Company, Hillsboro, OR) environmental SEM (ESEM) with a field emission gun (FEG) operating at 10 kV. Phase analysis was carried by X-ray diffraction (Philips, XRG 3100 X-ray generator). The generator was set to 40 kV and 20 mA utilizing Cu Ka radiation. X-ray diffraction pattern were analyzed using a software package JADE 7 (Materials Data, Livermore, CA). The same specimens were used to measure Vickers hardness using 30 kg load. 3. Results and discussion It is very critical to select a suitable low dielectric loss refractory material for sintering ceramics at high temperatures (1700– 1800 °C) using microwave energy. Refractory used in low temperature microwave processing fails at high temperature because the dielectric properties of materials changes with temperature, particularly, the dielectric properties of refractory material such as alumina increases with increasing temperature. The dielectric loss of the alumina fiber board at 1450 °C was 0.025 at 2.45 GHz which ensures minimum absorption of microwave energy. The cylindrical shaped insulation box was made in four pieces instead of one single piece to prevent cracking. One of the major issues of using alumina fiber board as a thermal insulation for sintering silicon nitride based ceramics is the reaction of Al2O3 with SiO2 present as a coating in silicon nitride above 1700 °C. At high temperatures (above 1700 °C) SiO2 evaporates and form eutectic composition with alumina and cause melting of the insulation box. In order to avoid this problem the samples were enclosed in a boron nitride (BN) crucible filled with a mixture of BN powder and course silicon carbide (SiC) powder. The microwave transparent BN crucible was then enclosed in an alumina insulation box. The BN powder bed prevents the decomposition of Si3N4 and SiC powder acts as susceptor of microwaves at room temperature and transfer the heat to the sample. When the sample’s temperature reaches a critical temperature, it begins to couple with microwaves itself and heat more efficiently. Fig. 2 shows the heating profiles of microwave and conventional sintering. Conventional sintering trials were performed for 1 h at 1800 °C in a graphite heating element furnace, whereas, microwave sintering for 15 min at the sintering temperature. A heating rate of 25 °C/min. was maintained by adjusting the microwave input power manually. The heating rate of conventional sintering was 10 °C/min. Radiation optical pyrometer was used to measure the temperature during microwave heating. The pyrometer was focused to the boron nitride crucible and measured the surface temperature of the crucible which is obviously lower than the actual
(b) Alumina Insulation BN Crucible Temperature Measurement SiAlON-ZrO2 sample BN powder+ SiC susceptor
Fig. 1. Microwave sintering set up; (a) photograph of 2.45 GHz microwave furnace with diffusion pump and the inset shows the temperature measurement set up; (b) alumina fiber board and boron nitride high temperature refractory arrangement.
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temperature of the sample. Direct access of the surface of the sample through a hole drilled through the BN crucible was not possible in our case due to the excessive lose of heat due to the radiation from the sample. The measured peak sintering temperature was 1700 °C at the crucible surface and the actual temperature at the centre of the sample could be most likely 1800 °C. The actual temperature was estimated by applying a correction of 100 °C based on our previous numerical calculation of temperature distribution within a silicon nitride sample enclosed in an identical BN crucible [19]. Heat distribution was modeled based on the temperature dependent dielectric properties measured at 2.45 GHz using cavity perturbation technique [19]. The peak sintering temperature was further confirmed by measuring the weight loss before and after sintering. Both sintering techniques exhibited an almost identical weight loss (2–2.5%) indicating that microwave peak sintering temperature was close to conventional sintering temperature which was 1800 °C. Liquid phase sintering in Si3N4 based system takes place in three overlapped stages: (1) particle rearrangement, (2) solution precipitation and (3) Ostwald ripening [20]. Neither a-Si3N4 nor ZrO2 powder absorb 2.45 GHz microwaves at room temperature due to their low dielectric losses, however, Y2O3 stabilized ZrO2 starts to couple with microwaves in the temperature range 250– 400 °C [14,15]. As the temperature increases the dielectric loss of ZrO2 increases rapidly and starts to absorb microwave energy and generate heat within the material more efficiently. In Si3N4– ZrN–SiO2–ZrO2 system, ZrO2 is stable with Si3N4 below 1600 °C and above this temperature starts to decompose to SiO and N2 according to the following reaction [21].
4Si3 N4 þ 6ZrO2 ¢ 12SiO þ 6ZrN þ 5N2
ð1Þ
On the contrary, liquid is formed at 1700 °C in Al2O3–AlON– ZrN–ZrO2 and AlN–AlON–ZrN–ZrO2 system due to higher thermal stability of Al2O3 under nitrogen atmosphere than SiO2 [21]. The reaction between AlN and ZrO2 at 1700 °C can be written as [21].
8AlN þ 6ZrO2 ¢ 6ZrN þ 4Al2 O3 þ N2
ð2Þ
The reactions (1) and (2) suggests that during reaction of aSi3N4, AlN, Al2O3 and ZrO2 an oxide melt could have been formed at 1700 °C, causing super saturation of Al and N triggering the precipitation of b-SiAlON. It is also possible that the liquid formed at a lower temperature. The ionic bonds in ZrO2 break easily during liquid formation and lower the viscosity of glassy phase [22]. Moreover, nitrogen, as an additional component, further lowers the eutectic temperature and enhances the diffusion [23]. Fig. 3 shows the densities of the microwave and conventional sintered samples as a function of ZrO2 content. In both cases, the density increased with increasing ZrO2 content due to higher amount of ZrO2, as expected. One can also clearly see that the density of microwave sintered samples was higher than that of the conventionally sintered
Fig. 3. Bulk densities of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content.
samples. The enhanced densification observed in the case of microwave sintered samples could be attributed to reverse thermal gradient, a well known phenomena occurs due to volumetric heating nature of microwaves [24]. Centre of the sample is at higher temperature than that of the surface. The difference in temperature causes reverse porosity gradient which in turn accelerate the densification to obtain a higher final density compared to conventionally sintered samples [25]. Moreover, the high heating rate of microwaves could also be influenced the particle rearrangement, and solution precipitation processes to enhance the densification process. It has been previously reported that fast heating rate improved the densification rate of Al2O3 [26]. Fig. 4 shows the XRD results of microwave and conventional sintered b-SiAlON–ZrO2 samples. The phases identified were b-SiAlON and cubic ZrN in the case of microwave sintered samples, whereas, tetragonal ZrO2 along with cubic ZrN and b-SiAlON were identified in the case of conventional sintered samples, containing 15 wt.% and 25 wt.% ZrO2. The X-ray patterns also revealed complete phase transformation from a-Si3N4 to b-SiAlON in both cases. However, the intensity of the peaks of conventional sintered samples were higher than that of microwave sintered samples, indicating that 60 min of holding time could have an effect in the crystallization of b-SiAlON compared to 15 min in the case of microwave sintered samples. Diffusion kinetics of nitrogen concentration in ZrO2 can vary significantly depending on the matrix, its solubility, and processing conditions [27–29]. Complete conversion of ZrO2 to ZrN in the case of microwave sintered samples could be due to the continuous dissolution of N2 through the open pores available at the surface of the sample. There is also possibility of development of high temperature within the ZrO2 grains due to high dielectric loss of ZrO2 compared to Si3N4 above 800 °C where nitrogen diffusion is high enough for the complete conversion of ZrO2 to ZrN. Similar observation was reported by Tiegs et al. in the case of nitridation of silicon to form reaction bonded silicon nitride using microwave volumetric heating [30]. There were also reports of retaining the transformable tetragonal ZrO2
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(a)
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Fig. 4. XRD results of b-SiAlON–ZrO2 composites as a function of ZrO2 content; (a) microwave sintered samples; (b) conventional sintered samples. The symbol ‘‘b” indicates b-SiAlON.
phase in SiAlON matrix [3,18]. Gain and Lewis [3] retained the transformable tetragonal ZrO2 when they hot pressed ZrO2–SiAlON composites at 1700 °C with applying a pressure of 22 MPa. In order to retain the transformable tetragonal ZrO2 phase in SiAlON matrix, they first formed a solid solution of ZrO2 and Y2O3 by pre-reacting commercially available tetragonal zirconia polycrystals (TZP) powder with 3 mol.% Y2O3 then added varying volume fractions (10– 30 vol.%) of pre-alloyed ZrO2 with a-Si3N4 and 2 wt.% a-Al2O3 as sintering aid. Cheng and Drennan [18] also reported the formation of transformable tetragonal ZrO2 phase when they added ZrO2 powder to O0 –SiAlON. The composition selected was 25 wt.% TZ3Y ZrO2, 6 wt.% Sm2O3, 7 wt.% Al2O3, 24 wt.% SiO2 and 38 wt.% Si3N4 to represent a overall composition of 25 wt.% ZrO2 and 75 wt.% O0 –SiAlON. They argued that the oxygen rich inter-granular glassy phase prevented the zirconia from nitridation and maintained transformable tetragonal ZrO2 phase in O0 –SiAlON matrix. Fig. 5 shows a comparison of microstructures of microwave and conventional sintered samples. All the samples show a characteristic bimodal microstructure. The aspect ratios of microwave sintered b-SiAlON grains were higher than conventional sintered samples as shown in Fig. 6. It is interesting to note that enhanced grain growth occurred in longitudinal direction (c-axis of b-SiAlON) compared to thickness (a-axis of b-SiAlON). The anisotropic growth of b-SiAlON could be explained based on the roughening transition mechanism [31]. The atomically rough interface along the c-axis facilitates normal grain growth, while atomically flat interface along a-axis rate limited due to bond continuity [31]. The results indicate that microwaves contributing to the grain growth of b-SiAlON. Similar observation has previously been reported by Hirota et al. in millimeter wave (28 GHz) sintering of Si3N4 with sesquioxides sintering additives [32]. The enhanced grain growth along the c-axis could also be related to non-thermal interaction of microwaves on mass transfer [33]. However the claims of non-thermal ‘‘microwave effect” is still the subject of debate and a through kinetic study needs to be performed to arrive a conclusion. Fig. 7 shows a comparison of Vicker’s hardness of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content. The hardness of conventional sintered samples was better than microwave sintered samples. This may be due to the decreased grain size of conventional sintered samples compared to microwave sintered samples as revealed by the comparison of the aspect ratios of SiAlON grains in Fig. 6 [34]. An alternative explanation is the differences in the surface morphology of microwave and conventional sintered samples where the indentations were performed. Obviously the surface of the microwave sintered samples was not as dense as the conventional sin-
C 0 wt% ZrO2 C-
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Fig. 5. SEM microstructures of microwave and conventional sintered b-SiAlON– ZrO2 composites as a function of ZrO2 content.
tered samples due to reverse density gradient of microwave sintered samples [34]. One can also argue that isolated porosity could have been responsible for the marginal reduction in hardness value. It is also interesting to note that both microwave and con-
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of b-SiAlON grains were found to be higher in microwave sintered samples. The anisotropic grain growth is most likely caused by enhanced mass transport due to microwave field effect. Presence of tetragonal ZrO2 was identified in the case of conventional sintered samples whereas ZrO2 completely transformed to ZrN in microwave sintered samples. The hardness of microwave sintered samples was found to be lower than conventional sintered samples due to reverse porosity gradient.
References
Fig. 6. A comparison of aspect ratios of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content.
Fig. 7. Vicker’s hardness of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content.
ventional sintered samples did not show any trend in hardness value with increasing the weight percent of ZrO2. The value of the hardness increased with the addition of 5 wt.% ZrO2, thereafter it decreased when the amount of ZrO2 was increased to 15 wt.%. Again a marginal improvement was observed when amount of ZrO2 was increased to 25 wt.%. The hardness of Si3N4 based materials is not only related to their microstructural parameters such as grain size, and porosity but also the grain boundary phase [35]. The other important parameters that could affect hardness are the amount and type of the secondary phase, whether they are in crystalline or amorphous state. The presence of impurities could also affect the hardness value [35]. The oscillating nature of hardness value with increasing ZrO2 in the present case may be most likely associated with non uniform distribution of ZrO2 in SiAlON matrix or the local chemical changes occurred in grain boundary phase due to reaction of ZrO2 with Y2O3, Al2O3 and/or AlN during sintering. There is also possibility of formation of b-SiAlON solid solution. One needs to perform a through TEM based microstructural analysis to find whether ZrO2 particulates is present in the grain boundary or embedded in the large SiAlON grains as evident in our previous TEM studies of microwave sintered ZrO2–Si3N4 composites [36]. 4. Conclusions Densification, phase formation, microstructural evolution and hardness of microwave sintered b-SiAlON–ZrO2 composites were studied and the results were compared with conventional sintered samples. Microwave sintered samples exhibited higher density than conventional sintered samples. The improved densification behavior is attributed to volumetric fast heating. The aspect ratios
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microwave sintering of b-SiAlON–ZrO2 composites Sreekumar Chockalingam *, Henry Kodi Traver New York State College of Ceramics, Alfred University, Alfred, NY 14802, USA
a r t i c l e
i n f o
Article history: Received 3 December 2009 Accepted 20 February 2010 Available online 26 February 2010 Keywords: Microwave processing Microstructure SiAlON ZrO2 Hardness
a b s t r a c t Densification, phase transformation, microstructure evolution and hardness of microwave sintered b-SiAlON–ZrO2 composites were investigated and compared with conventionally sintered samples. Sintering trials were performed by a high vacuum capable 2.45 GHz microwave furnace without decomposition. Microwave sintered samples showed better densification behavior than conventional sintered samples. The higher density observed in the case of microwave sintered samples was attributed to volumetric fast heating. X-ray diffraction results of conventionally sintered samples showed b-SiAlON, tetragonal ZrO2 and ZrN phases, while, ZrO2 reacted with nitrogen and completely transformed to ZrN in the case of microwave sintered samples. The aspect ratios of microwave sintered b-SiAlON grains were higher than conventional sintered samples whereas, hardness remained lower. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction An alloy of silicon nitride, b-SiAlON is one of the most promising high temperature structural materials due to its excellent thermal and mechanical properties [1]. The mechanical properties of b-SiAlON is better than that of Si3N4 and Al2O3 due to its superior bond strength caused by the co-ordination of Al with O as AlO4 instead of AlO6 found in Al2O3 [2]. However, their low fracture toughness has impeded their widespread applications. One of the methods to improve the fracture toughness of ceramic materials is to disperse stabilized ZrO2 particles in the SiAlON matrix either by taking the advantages of stress-induced martensitic transformation from tetragonal to monoclinic phase which would absorb the fracture energy or by microcrack toughening of ZrO2 particles [3]. Stabilized zirconia has already been used as a toughening agent in Si3N4 matrix to improve their mechanical properties [4]. Claussen and Jahn reported that addition of 20 vol.% unstabilized ZrO2 into Si3N4 matrix improved the fracture toughness through microcracking toughening mechanism [5]. They also indicated the formation of Si2ON2. Terao et al. found that dispersion of 20 wt.% of 2.5 mol.% Y2O3 stabilized ZrO2 in Si3N4 was advantageous to increase the room-temperature fracture toughness without degradation of hardness [6]. Recently, there were many reports in the literature about the use of microwave energy to sinter ceramic materials [7–13]. The advantages of using microwave energy to sinter ceramic materials compared to conventional techniques are reduced sintering time, temperature and capability of producing unique microstructure * Corresponding author. Tel.: +91 9910801320. E-mail address: [email protected] (S. Chockalingam). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.02.042
that could not be achieved by other conventional methods [7]. Lossy ceramic materials like SiC couple with microwaves very efficiently at room temperature. Low loss ceramic materials such as Al2O3 and SiO2 seldom couple with microwaves at room temperature [7]. However, these materials begin to couple with microwaves above a critical temperature due to the increase of their dielectric properties. For example Y2O3 stabilized ZrO2 begins to couple 2.45 GHz microwaves in the temperature range of 250– 400 °C [14]. The coupling of microwaves at low temperature with a low loss ceramic could be improved by either using high loss SiC based external susceptors or adding lossy conductive or magnetic materials in the form of secondary phases [8]. The lossy secondary phase couples microwave power more efficiently than the low loss matrix and hence, could be heated selectively and rapidly at ambient temperature [8]. Silicon nitride, in its pure form, is a poor microwave absorber due to its low dielectric loss at room temperature [15]. However, heating of Si3N4 takes place above 1300 °C by selective coupling of microwaves to the grain boundary glassy phases [15]. Jones et al. sintered Si3N4 using microwave energy at a lower temperature than that required for standard method and achieved full density [9]. Janney et al. lowered the sintering temperature of 8 mol.% Y2O3 stabilized ZrO2 from 1375 °C to 1200 °C using 2.45 GHz microwave energy [13]. Although Si3N4–ZrO2 [4–6,16,17] and SiAlON–ZrO2 [3,18] composites have been previously studied by many researchers, no study has been reported in the literature about the use of microwave energy to sinter b-SiAlON–ZrO2 composites. The addition of ZrO2 not only improves the oxidation and corrosion resistance of grain boundary phase but also improves the coupling of microwaves [19]. Therefore, the purpose of the present work was to prepare b-SiAlON–ZrO2 composites from a-Si3N4, AlN, Al2O3 and Y2O3
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with varying amounts of 3 mol.% Y2O3 stabilized ZrO2 powders through 2.45 GHz microwave energy and investigate densification behavior, phase transformation, microstructure development and hardness. 2. Experimental Starting powder mixtures were prepared by using a-Si3N4 powder (E10 grade, UBE Industries Ltd., Yamaguchi, Japan), Al2O3 (Aldrich Chemical Company Inc., Milwaukee, WI, 99.99%) AlN (Grade B, H.C. Starck, Germany) and 3 mol.% Y2O3 stabilized ZrO2 (Tosoh Corporation, Tokyo, Japan). The powder composition for the formation of b-SiAlON was 89.5 wt.% Si3N4, 6.1 wt.% Al2O3, 2.4 wt.% AlN and 2 wt.% Y2O3. Four batches of composite powders were prepared by mixing 0 wt.%, 5 wt.%, 15 wt.% and 25 wt.% of 3 mol.% Y2O3 stabilized ZrO2 powder with b-SiAlON composition. The powders were ball milled in 2-propanol for 12 h using high wear resistant ZrO2 milling media (YTZ Grinding Media, Tosoh Corporation, Tokyo, Japan). The dried powders were ground and sieved through a 38 lm mesh. Five grams of the mixed powders were diepressed at 30 MPa and cold isostatically pressed at 200 MPa into cylindrical green pellets of diameter 23 mm and thickness of 5 mm. Fig. 1a shows the original photograph of the microwave furnace used to sinter b-SiAlON–ZrO2. The schematic of microwave refractory set up is shown in Fig. 1b. Microwave sintering was carried out in a 2.45 GHz, industrial microwave furnace with variable output power of maximum 3 kW. A diffusion pump was fitted to the microwave cavity to obtain high vacuum of the order of 105 Torr. The microwave refractory box was fabricated with high purity alumina fiber board rated for 1800 °C. The pressed b-SiAlON–ZrO2 compacts were placed in a boron nitride (BN) crucible filled with a protective BN powder bed mixed with course SiC powder. The SiC course powder acts as an external susceptor to heat the sample at room temperature. Microwave sintering was performed at 1700 °C under flowing nitrogen. The heating rate was controlled to 25 °C/min by adjusting the input microwave power with the help of computer. The temperature of refractory box was continuously monitored with the help of grounded thermocouples as shown in the inset of the Fig. 1a. Measurement of the surface temperature of the samples were carried out through a hole made through the alumina refractory box and the BN crucible using a computer controlled two-color pyrometer. The temperature of the outside surface of the alumina refractory box was measured with the help of thermocouple. Care was taken to ground the thermocouple to avoid arcing during microwave processing. Conventional sintering trials were performed at 1800 °C under flowing nitrogen using graphite heating element furnace with a heating/cooling rate of 10 °C/min for 1 h.
(a)
The densities of the samples were determined using Archimedes principle with de-ionized water. The microstructures were evaluated from diamond polished and acid etched samples using FEG 200 (FEI Company, Hillsboro, OR) environmental SEM (ESEM) with a field emission gun (FEG) operating at 10 kV. Phase analysis was carried by X-ray diffraction (Philips, XRG 3100 X-ray generator). The generator was set to 40 kV and 20 mA utilizing Cu Ka radiation. X-ray diffraction pattern were analyzed using a software package JADE 7 (Materials Data, Livermore, CA). The same specimens were used to measure Vickers hardness using 30 kg load. 3. Results and discussion It is very critical to select a suitable low dielectric loss refractory material for sintering ceramics at high temperatures (1700– 1800 °C) using microwave energy. Refractory used in low temperature microwave processing fails at high temperature because the dielectric properties of materials changes with temperature, particularly, the dielectric properties of refractory material such as alumina increases with increasing temperature. The dielectric loss of the alumina fiber board at 1450 °C was 0.025 at 2.45 GHz which ensures minimum absorption of microwave energy. The cylindrical shaped insulation box was made in four pieces instead of one single piece to prevent cracking. One of the major issues of using alumina fiber board as a thermal insulation for sintering silicon nitride based ceramics is the reaction of Al2O3 with SiO2 present as a coating in silicon nitride above 1700 °C. At high temperatures (above 1700 °C) SiO2 evaporates and form eutectic composition with alumina and cause melting of the insulation box. In order to avoid this problem the samples were enclosed in a boron nitride (BN) crucible filled with a mixture of BN powder and course silicon carbide (SiC) powder. The microwave transparent BN crucible was then enclosed in an alumina insulation box. The BN powder bed prevents the decomposition of Si3N4 and SiC powder acts as susceptor of microwaves at room temperature and transfer the heat to the sample. When the sample’s temperature reaches a critical temperature, it begins to couple with microwaves itself and heat more efficiently. Fig. 2 shows the heating profiles of microwave and conventional sintering. Conventional sintering trials were performed for 1 h at 1800 °C in a graphite heating element furnace, whereas, microwave sintering for 15 min at the sintering temperature. A heating rate of 25 °C/min. was maintained by adjusting the microwave input power manually. The heating rate of conventional sintering was 10 °C/min. Radiation optical pyrometer was used to measure the temperature during microwave heating. The pyrometer was focused to the boron nitride crucible and measured the surface temperature of the crucible which is obviously lower than the actual
(b) Alumina Insulation BN Crucible Temperature Measurement SiAlON-ZrO2 sample BN powder+ SiC susceptor
Fig. 1. Microwave sintering set up; (a) photograph of 2.45 GHz microwave furnace with diffusion pump and the inset shows the temperature measurement set up; (b) alumina fiber board and boron nitride high temperature refractory arrangement.
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temperature of the sample. Direct access of the surface of the sample through a hole drilled through the BN crucible was not possible in our case due to the excessive lose of heat due to the radiation from the sample. The measured peak sintering temperature was 1700 °C at the crucible surface and the actual temperature at the centre of the sample could be most likely 1800 °C. The actual temperature was estimated by applying a correction of 100 °C based on our previous numerical calculation of temperature distribution within a silicon nitride sample enclosed in an identical BN crucible [19]. Heat distribution was modeled based on the temperature dependent dielectric properties measured at 2.45 GHz using cavity perturbation technique [19]. The peak sintering temperature was further confirmed by measuring the weight loss before and after sintering. Both sintering techniques exhibited an almost identical weight loss (2–2.5%) indicating that microwave peak sintering temperature was close to conventional sintering temperature which was 1800 °C. Liquid phase sintering in Si3N4 based system takes place in three overlapped stages: (1) particle rearrangement, (2) solution precipitation and (3) Ostwald ripening [20]. Neither a-Si3N4 nor ZrO2 powder absorb 2.45 GHz microwaves at room temperature due to their low dielectric losses, however, Y2O3 stabilized ZrO2 starts to couple with microwaves in the temperature range 250– 400 °C [14,15]. As the temperature increases the dielectric loss of ZrO2 increases rapidly and starts to absorb microwave energy and generate heat within the material more efficiently. In Si3N4– ZrN–SiO2–ZrO2 system, ZrO2 is stable with Si3N4 below 1600 °C and above this temperature starts to decompose to SiO and N2 according to the following reaction [21].
4Si3 N4 þ 6ZrO2 ¢ 12SiO þ 6ZrN þ 5N2
ð1Þ
On the contrary, liquid is formed at 1700 °C in Al2O3–AlON– ZrN–ZrO2 and AlN–AlON–ZrN–ZrO2 system due to higher thermal stability of Al2O3 under nitrogen atmosphere than SiO2 [21]. The reaction between AlN and ZrO2 at 1700 °C can be written as [21].
8AlN þ 6ZrO2 ¢ 6ZrN þ 4Al2 O3 þ N2
ð2Þ
The reactions (1) and (2) suggests that during reaction of aSi3N4, AlN, Al2O3 and ZrO2 an oxide melt could have been formed at 1700 °C, causing super saturation of Al and N triggering the precipitation of b-SiAlON. It is also possible that the liquid formed at a lower temperature. The ionic bonds in ZrO2 break easily during liquid formation and lower the viscosity of glassy phase [22]. Moreover, nitrogen, as an additional component, further lowers the eutectic temperature and enhances the diffusion [23]. Fig. 3 shows the densities of the microwave and conventional sintered samples as a function of ZrO2 content. In both cases, the density increased with increasing ZrO2 content due to higher amount of ZrO2, as expected. One can also clearly see that the density of microwave sintered samples was higher than that of the conventionally sintered
Fig. 3. Bulk densities of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content.
samples. The enhanced densification observed in the case of microwave sintered samples could be attributed to reverse thermal gradient, a well known phenomena occurs due to volumetric heating nature of microwaves [24]. Centre of the sample is at higher temperature than that of the surface. The difference in temperature causes reverse porosity gradient which in turn accelerate the densification to obtain a higher final density compared to conventionally sintered samples [25]. Moreover, the high heating rate of microwaves could also be influenced the particle rearrangement, and solution precipitation processes to enhance the densification process. It has been previously reported that fast heating rate improved the densification rate of Al2O3 [26]. Fig. 4 shows the XRD results of microwave and conventional sintered b-SiAlON–ZrO2 samples. The phases identified were b-SiAlON and cubic ZrN in the case of microwave sintered samples, whereas, tetragonal ZrO2 along with cubic ZrN and b-SiAlON were identified in the case of conventional sintered samples, containing 15 wt.% and 25 wt.% ZrO2. The X-ray patterns also revealed complete phase transformation from a-Si3N4 to b-SiAlON in both cases. However, the intensity of the peaks of conventional sintered samples were higher than that of microwave sintered samples, indicating that 60 min of holding time could have an effect in the crystallization of b-SiAlON compared to 15 min in the case of microwave sintered samples. Diffusion kinetics of nitrogen concentration in ZrO2 can vary significantly depending on the matrix, its solubility, and processing conditions [27–29]. Complete conversion of ZrO2 to ZrN in the case of microwave sintered samples could be due to the continuous dissolution of N2 through the open pores available at the surface of the sample. There is also possibility of development of high temperature within the ZrO2 grains due to high dielectric loss of ZrO2 compared to Si3N4 above 800 °C where nitrogen diffusion is high enough for the complete conversion of ZrO2 to ZrN. Similar observation was reported by Tiegs et al. in the case of nitridation of silicon to form reaction bonded silicon nitride using microwave volumetric heating [30]. There were also reports of retaining the transformable tetragonal ZrO2
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(a)
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Fig. 4. XRD results of b-SiAlON–ZrO2 composites as a function of ZrO2 content; (a) microwave sintered samples; (b) conventional sintered samples. The symbol ‘‘b” indicates b-SiAlON.
phase in SiAlON matrix [3,18]. Gain and Lewis [3] retained the transformable tetragonal ZrO2 when they hot pressed ZrO2–SiAlON composites at 1700 °C with applying a pressure of 22 MPa. In order to retain the transformable tetragonal ZrO2 phase in SiAlON matrix, they first formed a solid solution of ZrO2 and Y2O3 by pre-reacting commercially available tetragonal zirconia polycrystals (TZP) powder with 3 mol.% Y2O3 then added varying volume fractions (10– 30 vol.%) of pre-alloyed ZrO2 with a-Si3N4 and 2 wt.% a-Al2O3 as sintering aid. Cheng and Drennan [18] also reported the formation of transformable tetragonal ZrO2 phase when they added ZrO2 powder to O0 –SiAlON. The composition selected was 25 wt.% TZ3Y ZrO2, 6 wt.% Sm2O3, 7 wt.% Al2O3, 24 wt.% SiO2 and 38 wt.% Si3N4 to represent a overall composition of 25 wt.% ZrO2 and 75 wt.% O0 –SiAlON. They argued that the oxygen rich inter-granular glassy phase prevented the zirconia from nitridation and maintained transformable tetragonal ZrO2 phase in O0 –SiAlON matrix. Fig. 5 shows a comparison of microstructures of microwave and conventional sintered samples. All the samples show a characteristic bimodal microstructure. The aspect ratios of microwave sintered b-SiAlON grains were higher than conventional sintered samples as shown in Fig. 6. It is interesting to note that enhanced grain growth occurred in longitudinal direction (c-axis of b-SiAlON) compared to thickness (a-axis of b-SiAlON). The anisotropic growth of b-SiAlON could be explained based on the roughening transition mechanism [31]. The atomically rough interface along the c-axis facilitates normal grain growth, while atomically flat interface along a-axis rate limited due to bond continuity [31]. The results indicate that microwaves contributing to the grain growth of b-SiAlON. Similar observation has previously been reported by Hirota et al. in millimeter wave (28 GHz) sintering of Si3N4 with sesquioxides sintering additives [32]. The enhanced grain growth along the c-axis could also be related to non-thermal interaction of microwaves on mass transfer [33]. However the claims of non-thermal ‘‘microwave effect” is still the subject of debate and a through kinetic study needs to be performed to arrive a conclusion. Fig. 7 shows a comparison of Vicker’s hardness of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content. The hardness of conventional sintered samples was better than microwave sintered samples. This may be due to the decreased grain size of conventional sintered samples compared to microwave sintered samples as revealed by the comparison of the aspect ratios of SiAlON grains in Fig. 6 [34]. An alternative explanation is the differences in the surface morphology of microwave and conventional sintered samples where the indentations were performed. Obviously the surface of the microwave sintered samples was not as dense as the conventional sin-
C 0 wt% ZrO2 C-
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Fig. 5. SEM microstructures of microwave and conventional sintered b-SiAlON– ZrO2 composites as a function of ZrO2 content.
tered samples due to reverse density gradient of microwave sintered samples [34]. One can also argue that isolated porosity could have been responsible for the marginal reduction in hardness value. It is also interesting to note that both microwave and con-
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of b-SiAlON grains were found to be higher in microwave sintered samples. The anisotropic grain growth is most likely caused by enhanced mass transport due to microwave field effect. Presence of tetragonal ZrO2 was identified in the case of conventional sintered samples whereas ZrO2 completely transformed to ZrN in microwave sintered samples. The hardness of microwave sintered samples was found to be lower than conventional sintered samples due to reverse porosity gradient.
References
Fig. 6. A comparison of aspect ratios of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content.
Fig. 7. Vicker’s hardness of microwave and conventional sintered b-SiAlON–ZrO2 composites as a function of ZrO2 content.
ventional sintered samples did not show any trend in hardness value with increasing the weight percent of ZrO2. The value of the hardness increased with the addition of 5 wt.% ZrO2, thereafter it decreased when the amount of ZrO2 was increased to 15 wt.%. Again a marginal improvement was observed when amount of ZrO2 was increased to 25 wt.%. The hardness of Si3N4 based materials is not only related to their microstructural parameters such as grain size, and porosity but also the grain boundary phase [35]. The other important parameters that could affect hardness are the amount and type of the secondary phase, whether they are in crystalline or amorphous state. The presence of impurities could also affect the hardness value [35]. The oscillating nature of hardness value with increasing ZrO2 in the present case may be most likely associated with non uniform distribution of ZrO2 in SiAlON matrix or the local chemical changes occurred in grain boundary phase due to reaction of ZrO2 with Y2O3, Al2O3 and/or AlN during sintering. There is also possibility of formation of b-SiAlON solid solution. One needs to perform a through TEM based microstructural analysis to find whether ZrO2 particulates is present in the grain boundary or embedded in the large SiAlON grains as evident in our previous TEM studies of microwave sintered ZrO2–Si3N4 composites [36]. 4. Conclusions Densification, phase formation, microstructural evolution and hardness of microwave sintered b-SiAlON–ZrO2 composites were studied and the results were compared with conventional sintered samples. Microwave sintered samples exhibited higher density than conventional sintered samples. The improved densification behavior is attributed to volumetric fast heating. The aspect ratios
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