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A new heating route of spark plasma sintering and its effect on alumina ceramic densification
Materials Science & Engineering A 559 (2013) 462–466
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
A new heating route of spark plasma sintering and its effect on alumina ceramic densification Limeng Liu a,b,n, Zhaoping Hou a, Baoyou Zhang a, Feng Ye a, Zhiguo Zhang b, Yu Zhou a a b
School of Materials Science and Engineering, Harbin Institute of Technology, Xidazhi Street 92, Nangang District, Harbin, Heilongjiang 150001, China Department of Physics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2012 Received in revised form 23 August 2012 Accepted 23 August 2012 Available online 11 September 2012
Alumina ceramics were prepared by spark plasma sintering (SPS). Effects of manipulating the SPS power output on densification, microstructure and flexural strength of the obtained alumina ceramics were investigated. The results showed that use of a few discrete high amplitude units as the heating power instead of the low continuous conventional power output for heating significantly improved compaction of the alumina particles in the early stage of densification, hence more homogeneous microstructures in the consolidated materials. The better flexural strength of the resultant alumina ceramics indicated the higher efficiency of the SPS heating power rearrangement. & 2012 Elsevier B.V. All rights reserved.
Keywords: Alumina Densification Spark plasma sintering
1. Introduction Spark plasma sintering has become a popular technique for ceramic preparation, ever since its emergence in the early 1990s [1,2]. Its prominent feature is to pass a direct current (DC) through the small graphite die that contains the ceramic powders. Very rapid heating and cooling rates, up to 1000 1C and 500 1C/ min respectively, are obtainable, due to the rapid Joule heating, low thermal capacity, and high thermal conductivity of the graphite die. Although the sintering mechanisms which are thought unique to SPS, such as a discharge plasma in the powder compact, is still open to severe debate [2–5], high efficiency in densifying various materials by SPS is recognized [6,7]. By SPS, various powders can be consolidated at a temperature usually 100–300 1C lower than by hot pressing (HP), and in a much shortened time: a typical SPS cycle takes less than 30 min, relative to hours for HP, indicating the high efficiency of SPS. High efficiency in densifying materials by SPS is attributable to the rapid heating rate [7,9,10]. For rapid heating, reactions at lower temperatures can be circumvented and reactions can thus be put off to a higher temperature [8]. Therefore, coarsening in correlation with surface mass transportation, which usually takes place at low temperatures, is suppressed to facilitate densification of the powders at a higher temperature. Although observations on unique sintering behaviors, phase transformation, and n Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Xidazhi Street 92, Nangang District, Harbin, Heilongjiang 150001, China. Tel.: þ86 451 86413921; fax: þ 86 451 86413922. E-mail address: [email protected] (L. Liu).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.126
microstructure developments in SPS are not fully understood yet [2–5], a particular interest in SPS research is to produce and subsequently verify the existence of electric discharge, and try to create electric discharges for powder consolidations. The DC heating current of the SPS is of a highly pulsed nature, i.e. the DC consists of a sequence of electric pulses and then the absence of a few. For example, for a pulse pattern of 12-on and 2-off heating pattern which is conventionally used, 12 pulses are applied, immediately followed by a short duration of two pulses where the current is not applied [2]. Munir and Quach [2] and Hulbert et al. [3] have found that the voltage for a typical SPS sintering at a low temperature (225–650 1C) was usually less than 3 V, too low to produce electric discharges between the consolidating particles. This argument suggests that in a so-called SPS process, there actually lacks the participant of the necessary electric discharge, let along the electric discharge plasma. Fortunately, higher sintering voltage seemed necessary to possibly activate an electric discharge and thus a spark plasma to enhance sintering. Successful trials based on Hulbert et al. [3] analysis should include the use of a much higher heating rate [9,11,12] or lowering the on–off ratios of the pulsed DC, both are helpful in increasing the electric energy level released by a individual DC pulse [13,14]. In a conventional SPS process, output of a higher SPS voltage encounters enormous Joule heat, which will cause temperature control problems, especially for a low temperature sintering routine. In this work, the SPS heating power output was discretized, so that the energy output was distributed into a few units. The energy level of each unit was up to 12–14 kW each lasting for less than 20 s. For comparison, the energy output level
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in a conventional SPS heating up for the same targeted temperature at the same nominal heating rate was several folds lower. In this study, the heating power output rearrangement was realized by modifying a part of the SPS machine. The original programmable SPS controller was replaced with a new one that can more swiftly adjust the power output in coordination with the temperature measurement. Effects of the heating rearrangement on densification and microstructure evolution of a powder compact were demonstrated by the sintering of a model alumina material. We believe this SPS heating rearrangement route should also be applicable to other material systems to obtain better densities, microstructure homogeneities, and mechanical performances.
2. Experimental A commercial a-Al2O3 powder (Taimicron TM-DS, Taimei Chemical Co., Ltd., Tokyo, Japan) with high purity 499.99% and an average particle size of 0.2 mm was used as the only starting material. The alumina powder was used directly, with no further powder processing prior to sintering. The SPS equipment was a Dr. Sinter 1080 (Sumitomo Coal Mining Co. Ltd., Tokyo, Japan). A typical pulsing pattern of 12-pulse-on and 2-pulse-off was applied. According to the manufacturer’s design and Hulbert et al. [3] verification, each pulse was 3.3 ms in duration. The graphite die in use had a dimension of 40 mm height, 20 mm and 50 mm inner and outer diameter, respectively. Batches of Al2O3 powder, about 3 g each, were poured into the die and previously cold pressed under 10 MPa for 1 min. When the pressure was decreased to 5 MPa, which was a typical starting point for SPS in order to secure good contact between the die setup and the electrodes, the packing density of the alumina powder reached 40%. The sintering temperature was monitored by a pyrometer, focusing on the outer surface of the die. With regards to temperature measurement, although there are arguments for a higher temperature (up to 150–300 1C higher) [8] at the sample local, there is also evidence to support the opposite [15]. Before sintering, the SPS chamber was evacuated to o6 Pa. Four sintering schedules (S1–S4) were designed to compare the effects of the SPS power output management on densification and microstructure evolution of the alumina ceramics. Schedule S1 and S2 were conventional SPS processes. At first stage in S1 and S2 (Fig. 1(a)), the temperature automatically rose to 600 1C in 2 min, and the mechanical pressure increased from 5 to 40 MPa in 4 min. Then a fixed heating rate of 50 1C/min was used to reach the preset 1050 1C for S1 and 1300 1C for S2. After holding for 5 min at the preset 1050 1C for S1 and 1300 1C for S2, the mechanical pressure was released and the SPS power was shut off to allow cooling. As can be seen from Fig. 1(a), at beginning of the sintering process, the heating power output increased automatically to 4.7 kW in the first 2 min, which was necessary to activate the pyrometer, because the pyrometer has a lowest limit of temperature measurement of 570 1C. Catching up to the measured temperature always produces overshootings: in case of S1 up to 667 1C instead of the preset 600 1C by the end of the first 2 min was detected, Fig. 1(a). In the following stages, the power output increased from 0.2 to 2.1 kW with temperature increase from 600 1C to 1050 1C. For the holding at 1050 1C, a reasonably lower heating power of 1.4 kW was sufficient. Unlike S1 and S2, the heating power outputs were rearranged in S3 and S4, Fig. 1(c), in order to increase the energy in an individual current pulse. Similar to S1 and S2, the temperature rose to 600 1C at the first stage in S3 and S4, and the mechanical uniaxial pressure increased from 5 to 40 MPa simultaneously. The SPS power for the following step of heating up was premeditatedly divided into a few units, each with a much higher energy concentration, Fig. 3(c) for S3.
Fig. 1. The heating power and temperature control in the conventional S1 (a) and rearranged S3 (c) spark plasma sintering process. Densification curves (b) as a function of the sintering temperature in conventional SPS differentiates four densifying stages. A few units with high energy concentration were used in S3 in order to produce better densification and microstructure homogeneity in the consolidated materials.
As shown in Fig. 3(c), each energy unit for the heating up was allocated to a fixed time of 60 s which consisted of a very short period of power-on and a subsequent power-off. During power-on, the power output reached a max up to 12–14 kW rapidly in about 20 s, followed by 40 s of power off to allow intermediate cooling. Only the power for the heating up stage was rearranged, for the holding stage the power output was not premeditatedly rearranged, in hope to achieve an isothermal holding: that is, when the holding temperature was reached, the SPS power rearrangement was released. However, due to slight temperature overshootings at the beginning of the holding, the SPS machine had to automatically adjust the power output to even the temperature wave, Fig. 1(c). This meant the power waving in the holding stage was not designed, neither was it desired. An even output of the heating power for holding was preferred, but unlikely to realize in this case due to the
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in dimension and a 10.0 mm span. Three specimens were tested for each condition. The average values and the calculated error ranges, though not unquestionable because of the small database, were reported.
3. Results 3.1. Densifying process reflected by SPS ram displacement
Fig. 2. XRD patterns of the alumina ceramics sintered by the rearranged spark plasma sintering. The 1300 1C material (a) exhibits narrower diffractions lines than the 1050 1C material (b).
In the conventional SPS processes S1 and S2, densification of the Al2O3 powder compact was reflected by the ram displacement as a function of the temperature. In the rearranged schedules S3 and S4, the heating–cooling cycles produced correlated ram displacement strokes in the densification curves (not shown here). Therefore, the sintering stages were undistinguishable in the ram displacement curves. The typical densifying behavior of the Al2O3 powder compact was demonstrated by the sintering curves shown in Fig. 1(b). Despite the temperature maximum of 667 1C as a transient temperature overflow by the end of the first 2 min, four stages were identified and were marked by I–IV on the ram displacement curve in Fig. 1(b). Stage I took place from 25 1C to 720 1C. The ram travel estimated a relative density increase from 40% to 69%, which was ascribable to particle rearrangement for increasing the mechanical pressure and the rapid temperature increase from 25 1C to 700 1C in stage I. A gas release into the SPS chamber was also noticeable. This gas release caused the gas pressure to increase, from the initial o6 Pa to a maximum of 18 Pa. Stage II took place at about 720–890 1C. In this second stage of sintering and hereafter, fixed heating rate of 50 1C/min was used. No sintering shrinkage was presented in stage II. The thermal expansion of the powder-and-die setup produced the negative slope in the ram displacement curve. At the beginning of stage III, a noticeable change in the curve slope indicated initiation of sintering. However, profound densification did not happen till 960 1C. Further densification caused the isothermal holding to last at the preset 1050 1C [16]. During the last stage IV of sintering, the pressure was released, and the SPS power was shut off. The concomitant cooling produced the additional ram travel in Fig. 1(b). 3.2. Densities
Fig. 3. Fractographs of the alumina ceramics sintered by the conventional (a,c) and rearranged (b,d) spark plasma sintering at (a,b) 1050 1C and 1300 1C. Better densities and microstructural homogeneities were obtained by the rearranged SPS. The arrows showed the difference in residual pore concentrations between the different materials.
temperature overshooting and the automatic adjustment of the SPS machine. The obtained samples (named as materials S1, S2, S3 and S4 after the sintering schedules) were sectioned, ground, and polished to 1 mm finish. Densities were measured by the Archimedes method. Relative densities were calculated as percentage fractions of the measured densities to the theoretical (3.987 g/cm3, JCPDS 46-1212). Average values of the densities and statistical errors of five measurements for each material were reported. Phase composition was checked by X-ray diffraction (XRD, Rigaku D/Max 2200VPC, Japan). Fracture surfaces were observed by scanning electron microscopy (SEM, Quanta 200, FEI Co., Oregon, USA). Grain sizes were measured on the fractographs by the Image Pro-Plus software (Image Pro-Plus 5.0.1, Media Cybernetics, Silver Spring, MD). More than 100 grains were measured for each material. The averaged grain diameters and the largest deviations from the averages considered as the statistical errors were reported. Flexure strength was tested by the three-point bending method, using specimens of 2.0 mm 1.0 mm 15.0 mm
The Archimedes measurement results are listed in Table 1. It exhibited that holding at 1050 1C for 5 min by a conventional SPS, i.e. S1, was unable to produce a dense alumina ceramic. The relative density of the S1 material was 83.5%, with a large concentration of open porosity, up to 13.1%. When the isothermal holding temperature was increased to 1300 1C, the S2 material reached a relative density of 98.6%, and 0.1% open porosity was detected. Compared to S1 and S2, densification was significantly improved by using the rearranged sintering schedules. The S3 material obtained a relative density of 96.3%, by holding at 1050 1C for 5 min. Full density was reached in S4 after holding at 1300 1C for 5 min. 3.3. Microstructural homogeneity Selected XRD spectra are shown in Fig. 2. Only a-Al2O3 was detected in different materials. The X-ray diffraction peaks had identical positions, indicating identical chemical compositions. Slight broadening of the X-ray lines was presented in S3 (also in S1) in comparison with S4 and S2, indicating smaller grain sizes in the low-temperature sintered materials (1050 1C vs. 1300 1C). Calculation by Scherer equation indicated that the S1 and S3 materials which were sintered at 1050 1C consisted of smaller crystals with an average diameter of 0.4–0.7 mm, whereas
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Table 1 Density, phase composition, and flexural strength of the sintered alumina ceramics. Materials and the applied sintering route
Total power output Densification (kJ) r (g/cm3) Relative density (%)
S1 by 1050 1C S2 by 1300 1C S3 by rearranged 1050 1C S4 by rearranged 1300 1C
1694 2419 1871
3.31 70.03 83.5 7 0.7 3.93 70.08 98.6 7 2.0 3.84 70.04 96.3 7 1.0
2552
3.96 70.04 99.3 7 1.0
1.5–1.9 mm diameter was present in other materials which were sintered at 1300 1C. The crystal sizes were also measured by the SEM micrographs, using an Image Pro-Plus software compact. The intergranular fractures dominating the fractures as shown by the fractographs in Fig. 3, promised precise grain size measurements. The average grain diameters and the statistical errors are listed in Table 1. The grain size in S1 and S3 (sintered at 1050 1C) was 0.5 70.1 mm and 0.6 70.1 mm respectively. After sintering at 1300 1C, 2.471.2 mm and 1.9 70.7 mm as the grain size value for S2 and S4 was presented respectively. All these data for crystal sizes were in rough consistency with the XRD experiments. As shown in Table 1, the much smaller error range for the grain sizes in S4 illustrated a narrower grain size distribution in S4 material than that in S2. The fractographs in Fig. 3 also reveal a more homogeneous microstructure in S4, while regions consisting of large and small grains were presented in S2 material. The rearranged sintering schedules should have sufficiently enhanced the particle packing of the alumina particles, hence better densification, to result in a uniform microstructure [16–18]. On the contrary, Fig. 3(a) demonstrates a less dense particle packing in the S1 material. Voids marked by the square and a denser neighboring area marked by the character A indicated inhomogeneous packing of the alumina powder. This kind of inhomogeneity could lead to inhomogeneous sintering and hence to an inhomogeneous microstructure. Such microstructural inhomogeneities were also observed in material S2, in comparison with S4, as marked by the circle and character B in Fig. 3(c). 3.4. Mechanical strength The improvement in densification and the microstructural homogeneities improved the flexural strength of the alumina ceramics. The flexural strength of the as-sintered alumina ceramics is listed in Table 1. The S1 and S2 material had a flexural strength of 192 MPa and 335 MPa respectively, compared to 501 MPa and 459 MPa for S3 and S4. The higher mechanical properties of the S3 and S4 material were explained by the denser and more uniform microstructures. The large microstructure inhomogeneities (especially large voids) as observed in the S1 and S2 materials could be flaws that initiate early fracture, thus explaining the lower strength of S1 and S2, relative to S3 and S4. The S3 materials sintered at a lower temperature of 1050 1C consisted of fine grains, giving even higher strength, despite the few percents of higher porosity.
4. Discussions 4.1. General observations Spark plasma sintering has attracted intense research interests because of the high efficiency in preparing various materials.
Phase composition
Grain diameter (mm)
Flexural strength (MPa)
13.1 0.1 0.3
Corundum Corundum Corundum
0.57 0.1 2.47 1.2 0.67 0.1
192 7 27 335 7 29 501 7 41
0
Corundum
1.97 0.7
459 7 39
Open porosity (%)
Considering the Joule heating of the current, generally, in a conventional SPS process, a very small power output as shown in Fig. 1(a) was enough to increase the temperature up to 1300 1C for alumina sintering. Interestingly, it seemed that better densification could be achieved if the heating power was rearranged into discrete units instead of the continuous low amplitude power output. As shown in the rearranged sintering schedule of S3, the heating power was discretized into six units of high amplitude up to 12–14 kW each, but each unit lasting for only less than 20 s, with intervals of 40 s power off. The densities listed in Table 1 show that the S3 material had a relative density of 96.3% after holding at 1050 1C for 5 min, compared to 83.5% for the S1 material, after sintering at the same temperature for 5 min but by a conventional SPS heating schedule. In addition, SEM observations showed improved microstructure homogeneity by the rearranged SPS, as shown both by the microstructure comparison in Fig. 3 and by the grain size statistics listed in Table 1. 4.2. Effects of the total power output It is usually taken for granted that a higher sintering temperature was the most possible reason for the enhanced densification of the S3 and S4 materials, in comparison with S1 and S2. Indeed, temperature overshootings (the few temperature maximum in the holding stage in Fig. 1(c)) were presented in the holding of S3 and S4 at the targeted temperature of 1050 1C and 1300 1C, respectively. Fig. 1(c) exhibits that temperature first reached the maximum of 1104 1C and then gradually decreased to 1056 1C after a few steps of adjustments. Although higher temperature may partly explain the higher densifications for S3 and S4, the large differences in densification and microstructure evolution, especially between S1 and S3, seek other possible explanations. The total power output for each case was calculated and carefully compared. As seen from the results listed in Table 1, the higher total power outputs for the rearranged heating routes of S3 and S4 were in consistency with their higher densifications. However, no simple linear relationship between the densities and the total power output was found, which may suggest additional unknown densification mechanisms in association with the higher DC, as suggested by Raj et al. [5]. 4.3. Effects of possibly enhanced powder packing The good effects of the heating rearrangement were also probably due to the improved powder packing. Based on the SEM observations, local particle packing difference should be present in the powder compact in the early stage of sintering (see the voids and the neighboring denser regions in the S1 material, but a more homogeneous microstructure in S3). It is well-known that such a local packing difference would result in inhomogeneous densification at higher temperatures, hence
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worsened densities and microstructure homogeneities in the consolidated compacts [17,18]. Evidently, in the S2 material after sintering at a higher temperature of 1300 1C, an inhomogeneous microstructure was formed. Large and small grain segregations present in the microstructure of the S2 material were very similar to the other alumina ceramics which were spark plasma sintered in the literature [19]. Optimizing application of the pressure and the heating rate to improve early-stage powder compaction was used in the literature to improve microstructure uniformity [19]. As an analog, the microstructure observations and the density measurements in this study may thus suggest improved powder compaction as one of the good effects of the heating process rearrangement in producing a better microstructural homogeneity in the model alumina ceramics.
S3 and S4 materials. The gas pressure increase in the early stage of sintering, with a corresponding large power output by the end of the first 2 min, was also a possible indicator for the existence of a spark plasma. This was because the high power may have overcome the voltage critic as suggested in Ref. [3], to produce an electric discharge according to the calculations in Ref. [14], thus resulting in particle surface cleaning, and consequently a gas release. Last, in addition to the power output manipulation by S3 and S4, other methods, such as application of a high heating rate of 400 1C/min or a low DC pulse on–off ratio [7,9,11,13,14] may also be the right way to possibly overcome the voltage critic for electric discharges, thus resulting in enhanced densification of ceramics.
4.4. Temperature control
5. Conclusions
The original idea of the heating rearrangement for better microstructural homogeneity was based on the densifying features of the powder compacts. According to the densification reflected by the ram displacement in Fig. 1(b), see the previous results and discussions, the sintering process of the alumina powder started at about 900 1C. This meant large amount of sintering necks formed at 900 1C and above. Because the rigid skeleton resulting from the sintering necks would hinder particle rearrangement, the heating management must be placed at a temperature range below 900 1C. For the rearranged heating schedules S3 and S4, the heating power was interrupted into individual units; each was given a high energy density. However, the short operating time of an individual unit in combination with the rapid cooling capacity of the powder-and-die setup secured the temperature limit. The temperature control curve shown in Fig. 1(c) evidenced the success in curbing the temperatures o900 1C, by the rearranged heating schedules S3 and S4. At the same time, transient heating and cooling cycles resulted from individual units. These rapid heating and cooling cycles may help relax inter-particle friction, hence higher packing efficiency for better densification [20].
To sum up, a new route consisting of a few units with high energy concentration instead of continuous low-power heating was developed for the spark plasma sintering. The good effects of this new SPS route were demonstrated by densification of model alumina ceramics. Inhomogeneous densification was avoided by enhancing particle packing before the early sintering necks formation, thus producing dense alumina ceramics with uniform microstructures at a low temperature of 1050 1C.
4.5. Possible electric discharge Fig. 1(b) exhibits a significant increase in gas pressure in the SPS chamber, with the initial heating up from 25 1C to 600 1C. This kind of pressure increase was traditionally explained by deadsorption of the adsorbed O2, CO2, and moisture. However, with regards to the surface cleansing effect by an electric discharge [1], it was also possibly due to electric discharges, if electric discharges could be coordinately created. Unfortunately for a conventional SPS process, electric discharge was not observed in the powder compacts during a conventional SPS process [3]. Hulbert et al. [3] thought that the voltage for a low temperature SPS ( o650 1C) was not high enough to produce electric discharges. In this study, the SPS heating power was concentrated into several units, so that the maximum of power outputs of an individual unit in both S3 and S4 reached 14 kW (up to 6.89 V, 2054A, Fig. 1(c)), the existence of electric discharges may be possible. Therefore, unique sintering effects of SPS, i.e. a spark plasma, can be included to explain the enhanced sintering of the
Acknowledgments This work was financially supported by ‘‘the Fundamental Research Funds for the Central Universities’’ under Grant no. HIT. NSRIF 2010111, and ‘‘National Natural Science Foundation of China’’ under Grant no. NSFC 51021002. References [1] M. Omori, Mater. Sci. Eng. A 287 (2000) 183–188. [2] Z.A. Munir, D.V. Quach, J. Am. Ceram. Soc. 94 (1) (2011) 1–19. [3] D.M. Hulbert, A. Anders, J. Andersson, E.J. Lavernia, A.K. Mukherjee, Scr. Mater. 60 (2009) 835–838. [4] J. Langer, M.J. Hoffmann, O. Guillonw, J. Am. Ceram. Soc. 94 (1) (2011) 24–31. [5] R. Raj, M. Cologna, J. Francis, J. Am. Ceram. Soc. 94 (7) (2011) 1941–1965. [6] M. Nygren, Z. Shen, Solid State Sci. 5 (2003) 125–131. [7] G. Zhan, J. Kuntz, J. Wan, J. Garay, A.K. Mukherjee, J. Am. Ceram. Soc. 86 (1) (2002) 200–202. [8] Z. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 417 (5) (2002) 266–269. [9] Y. Aman, V. Garnier, E. Djurado, J. Am. Ceram. Soc. 94 (9) (2011) 2825–2833. [10] K.M. Reddy, N. Kumar, B. Basu, Scr. Mater. 63 (2010) 585–588. [11] Y. Zhou, K. Hirao, M. Toriyama, H. Tanaka, J. Am. Ceram. Soc. 83 (3) (2000) 654–656. [12] D. Jiang, D.M. Hulbert, U. Tamburini, T. Ng, A.K. Mukherjee, J. Am. Ceram. Soc. 91 (1) (2008) 151–154. [13] J. Santanach, C. Estourne s, A. Weibel, G. Chevallier, V. Bley, C. Laurent, A. Peigney, J. Eur. Ceram. Soc. 31 (13) (2011) 2247–2254. [14] M. Belmonte, M.I. Osendi, P. Miranzo, Scr. Mater. 65 (3) (2011) 273–276. [15] K. Vanmeensel, A. Laptev, J. Hennicke, J. Vleugels, O.V. Biest., Acta Mater. 53 (2005) 4379–4388. [16] D. Chakravarty, S. Bysakh, K. Muraleedharan, T. Rao, R. Sundaresan, J. Am. Ceram. Soc. 91 (1) (2008) 203–208. [17] M.N. Rahaman, Ceramic Processing and Sintering, second edition, Marcel Dekker Inc, New York, 2003. [18] D. Jayaseelan, S. Ueno, T. Ohji, S. Kanzaki, J. Am. Ceram. Soc. 87 (1) (2004) 159–161. [19] S. Grasso, C. Hu, G. Maizza, B. Kim, Y. Sakka, J. Am. Ceram. Soc. 94 (5) (2011) 1405–1409. [20] L. Liu, F. Ye, Y. Zhou, Mater. Sci. Eng. A 528 (2011) 4710–4714.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
A new heating route of spark plasma sintering and its effect on alumina ceramic densification Limeng Liu a,b,n, Zhaoping Hou a, Baoyou Zhang a, Feng Ye a, Zhiguo Zhang b, Yu Zhou a a b
School of Materials Science and Engineering, Harbin Institute of Technology, Xidazhi Street 92, Nangang District, Harbin, Heilongjiang 150001, China Department of Physics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2012 Received in revised form 23 August 2012 Accepted 23 August 2012 Available online 11 September 2012
Alumina ceramics were prepared by spark plasma sintering (SPS). Effects of manipulating the SPS power output on densification, microstructure and flexural strength of the obtained alumina ceramics were investigated. The results showed that use of a few discrete high amplitude units as the heating power instead of the low continuous conventional power output for heating significantly improved compaction of the alumina particles in the early stage of densification, hence more homogeneous microstructures in the consolidated materials. The better flexural strength of the resultant alumina ceramics indicated the higher efficiency of the SPS heating power rearrangement. & 2012 Elsevier B.V. All rights reserved.
Keywords: Alumina Densification Spark plasma sintering
1. Introduction Spark plasma sintering has become a popular technique for ceramic preparation, ever since its emergence in the early 1990s [1,2]. Its prominent feature is to pass a direct current (DC) through the small graphite die that contains the ceramic powders. Very rapid heating and cooling rates, up to 1000 1C and 500 1C/ min respectively, are obtainable, due to the rapid Joule heating, low thermal capacity, and high thermal conductivity of the graphite die. Although the sintering mechanisms which are thought unique to SPS, such as a discharge plasma in the powder compact, is still open to severe debate [2–5], high efficiency in densifying various materials by SPS is recognized [6,7]. By SPS, various powders can be consolidated at a temperature usually 100–300 1C lower than by hot pressing (HP), and in a much shortened time: a typical SPS cycle takes less than 30 min, relative to hours for HP, indicating the high efficiency of SPS. High efficiency in densifying materials by SPS is attributable to the rapid heating rate [7,9,10]. For rapid heating, reactions at lower temperatures can be circumvented and reactions can thus be put off to a higher temperature [8]. Therefore, coarsening in correlation with surface mass transportation, which usually takes place at low temperatures, is suppressed to facilitate densification of the powders at a higher temperature. Although observations on unique sintering behaviors, phase transformation, and n Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Xidazhi Street 92, Nangang District, Harbin, Heilongjiang 150001, China. Tel.: þ86 451 86413921; fax: þ 86 451 86413922. E-mail address: [email protected] (L. Liu).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.126
microstructure developments in SPS are not fully understood yet [2–5], a particular interest in SPS research is to produce and subsequently verify the existence of electric discharge, and try to create electric discharges for powder consolidations. The DC heating current of the SPS is of a highly pulsed nature, i.e. the DC consists of a sequence of electric pulses and then the absence of a few. For example, for a pulse pattern of 12-on and 2-off heating pattern which is conventionally used, 12 pulses are applied, immediately followed by a short duration of two pulses where the current is not applied [2]. Munir and Quach [2] and Hulbert et al. [3] have found that the voltage for a typical SPS sintering at a low temperature (225–650 1C) was usually less than 3 V, too low to produce electric discharges between the consolidating particles. This argument suggests that in a so-called SPS process, there actually lacks the participant of the necessary electric discharge, let along the electric discharge plasma. Fortunately, higher sintering voltage seemed necessary to possibly activate an electric discharge and thus a spark plasma to enhance sintering. Successful trials based on Hulbert et al. [3] analysis should include the use of a much higher heating rate [9,11,12] or lowering the on–off ratios of the pulsed DC, both are helpful in increasing the electric energy level released by a individual DC pulse [13,14]. In a conventional SPS process, output of a higher SPS voltage encounters enormous Joule heat, which will cause temperature control problems, especially for a low temperature sintering routine. In this work, the SPS heating power output was discretized, so that the energy output was distributed into a few units. The energy level of each unit was up to 12–14 kW each lasting for less than 20 s. For comparison, the energy output level
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in a conventional SPS heating up for the same targeted temperature at the same nominal heating rate was several folds lower. In this study, the heating power output rearrangement was realized by modifying a part of the SPS machine. The original programmable SPS controller was replaced with a new one that can more swiftly adjust the power output in coordination with the temperature measurement. Effects of the heating rearrangement on densification and microstructure evolution of a powder compact were demonstrated by the sintering of a model alumina material. We believe this SPS heating rearrangement route should also be applicable to other material systems to obtain better densities, microstructure homogeneities, and mechanical performances.
2. Experimental A commercial a-Al2O3 powder (Taimicron TM-DS, Taimei Chemical Co., Ltd., Tokyo, Japan) with high purity 499.99% and an average particle size of 0.2 mm was used as the only starting material. The alumina powder was used directly, with no further powder processing prior to sintering. The SPS equipment was a Dr. Sinter 1080 (Sumitomo Coal Mining Co. Ltd., Tokyo, Japan). A typical pulsing pattern of 12-pulse-on and 2-pulse-off was applied. According to the manufacturer’s design and Hulbert et al. [3] verification, each pulse was 3.3 ms in duration. The graphite die in use had a dimension of 40 mm height, 20 mm and 50 mm inner and outer diameter, respectively. Batches of Al2O3 powder, about 3 g each, were poured into the die and previously cold pressed under 10 MPa for 1 min. When the pressure was decreased to 5 MPa, which was a typical starting point for SPS in order to secure good contact between the die setup and the electrodes, the packing density of the alumina powder reached 40%. The sintering temperature was monitored by a pyrometer, focusing on the outer surface of the die. With regards to temperature measurement, although there are arguments for a higher temperature (up to 150–300 1C higher) [8] at the sample local, there is also evidence to support the opposite [15]. Before sintering, the SPS chamber was evacuated to o6 Pa. Four sintering schedules (S1–S4) were designed to compare the effects of the SPS power output management on densification and microstructure evolution of the alumina ceramics. Schedule S1 and S2 were conventional SPS processes. At first stage in S1 and S2 (Fig. 1(a)), the temperature automatically rose to 600 1C in 2 min, and the mechanical pressure increased from 5 to 40 MPa in 4 min. Then a fixed heating rate of 50 1C/min was used to reach the preset 1050 1C for S1 and 1300 1C for S2. After holding for 5 min at the preset 1050 1C for S1 and 1300 1C for S2, the mechanical pressure was released and the SPS power was shut off to allow cooling. As can be seen from Fig. 1(a), at beginning of the sintering process, the heating power output increased automatically to 4.7 kW in the first 2 min, which was necessary to activate the pyrometer, because the pyrometer has a lowest limit of temperature measurement of 570 1C. Catching up to the measured temperature always produces overshootings: in case of S1 up to 667 1C instead of the preset 600 1C by the end of the first 2 min was detected, Fig. 1(a). In the following stages, the power output increased from 0.2 to 2.1 kW with temperature increase from 600 1C to 1050 1C. For the holding at 1050 1C, a reasonably lower heating power of 1.4 kW was sufficient. Unlike S1 and S2, the heating power outputs were rearranged in S3 and S4, Fig. 1(c), in order to increase the energy in an individual current pulse. Similar to S1 and S2, the temperature rose to 600 1C at the first stage in S3 and S4, and the mechanical uniaxial pressure increased from 5 to 40 MPa simultaneously. The SPS power for the following step of heating up was premeditatedly divided into a few units, each with a much higher energy concentration, Fig. 3(c) for S3.
Fig. 1. The heating power and temperature control in the conventional S1 (a) and rearranged S3 (c) spark plasma sintering process. Densification curves (b) as a function of the sintering temperature in conventional SPS differentiates four densifying stages. A few units with high energy concentration were used in S3 in order to produce better densification and microstructure homogeneity in the consolidated materials.
As shown in Fig. 3(c), each energy unit for the heating up was allocated to a fixed time of 60 s which consisted of a very short period of power-on and a subsequent power-off. During power-on, the power output reached a max up to 12–14 kW rapidly in about 20 s, followed by 40 s of power off to allow intermediate cooling. Only the power for the heating up stage was rearranged, for the holding stage the power output was not premeditatedly rearranged, in hope to achieve an isothermal holding: that is, when the holding temperature was reached, the SPS power rearrangement was released. However, due to slight temperature overshootings at the beginning of the holding, the SPS machine had to automatically adjust the power output to even the temperature wave, Fig. 1(c). This meant the power waving in the holding stage was not designed, neither was it desired. An even output of the heating power for holding was preferred, but unlikely to realize in this case due to the
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in dimension and a 10.0 mm span. Three specimens were tested for each condition. The average values and the calculated error ranges, though not unquestionable because of the small database, were reported.
3. Results 3.1. Densifying process reflected by SPS ram displacement
Fig. 2. XRD patterns of the alumina ceramics sintered by the rearranged spark plasma sintering. The 1300 1C material (a) exhibits narrower diffractions lines than the 1050 1C material (b).
In the conventional SPS processes S1 and S2, densification of the Al2O3 powder compact was reflected by the ram displacement as a function of the temperature. In the rearranged schedules S3 and S4, the heating–cooling cycles produced correlated ram displacement strokes in the densification curves (not shown here). Therefore, the sintering stages were undistinguishable in the ram displacement curves. The typical densifying behavior of the Al2O3 powder compact was demonstrated by the sintering curves shown in Fig. 1(b). Despite the temperature maximum of 667 1C as a transient temperature overflow by the end of the first 2 min, four stages were identified and were marked by I–IV on the ram displacement curve in Fig. 1(b). Stage I took place from 25 1C to 720 1C. The ram travel estimated a relative density increase from 40% to 69%, which was ascribable to particle rearrangement for increasing the mechanical pressure and the rapid temperature increase from 25 1C to 700 1C in stage I. A gas release into the SPS chamber was also noticeable. This gas release caused the gas pressure to increase, from the initial o6 Pa to a maximum of 18 Pa. Stage II took place at about 720–890 1C. In this second stage of sintering and hereafter, fixed heating rate of 50 1C/min was used. No sintering shrinkage was presented in stage II. The thermal expansion of the powder-and-die setup produced the negative slope in the ram displacement curve. At the beginning of stage III, a noticeable change in the curve slope indicated initiation of sintering. However, profound densification did not happen till 960 1C. Further densification caused the isothermal holding to last at the preset 1050 1C [16]. During the last stage IV of sintering, the pressure was released, and the SPS power was shut off. The concomitant cooling produced the additional ram travel in Fig. 1(b). 3.2. Densities
Fig. 3. Fractographs of the alumina ceramics sintered by the conventional (a,c) and rearranged (b,d) spark plasma sintering at (a,b) 1050 1C and 1300 1C. Better densities and microstructural homogeneities were obtained by the rearranged SPS. The arrows showed the difference in residual pore concentrations between the different materials.
temperature overshooting and the automatic adjustment of the SPS machine. The obtained samples (named as materials S1, S2, S3 and S4 after the sintering schedules) were sectioned, ground, and polished to 1 mm finish. Densities were measured by the Archimedes method. Relative densities were calculated as percentage fractions of the measured densities to the theoretical (3.987 g/cm3, JCPDS 46-1212). Average values of the densities and statistical errors of five measurements for each material were reported. Phase composition was checked by X-ray diffraction (XRD, Rigaku D/Max 2200VPC, Japan). Fracture surfaces were observed by scanning electron microscopy (SEM, Quanta 200, FEI Co., Oregon, USA). Grain sizes were measured on the fractographs by the Image Pro-Plus software (Image Pro-Plus 5.0.1, Media Cybernetics, Silver Spring, MD). More than 100 grains were measured for each material. The averaged grain diameters and the largest deviations from the averages considered as the statistical errors were reported. Flexure strength was tested by the three-point bending method, using specimens of 2.0 mm 1.0 mm 15.0 mm
The Archimedes measurement results are listed in Table 1. It exhibited that holding at 1050 1C for 5 min by a conventional SPS, i.e. S1, was unable to produce a dense alumina ceramic. The relative density of the S1 material was 83.5%, with a large concentration of open porosity, up to 13.1%. When the isothermal holding temperature was increased to 1300 1C, the S2 material reached a relative density of 98.6%, and 0.1% open porosity was detected. Compared to S1 and S2, densification was significantly improved by using the rearranged sintering schedules. The S3 material obtained a relative density of 96.3%, by holding at 1050 1C for 5 min. Full density was reached in S4 after holding at 1300 1C for 5 min. 3.3. Microstructural homogeneity Selected XRD spectra are shown in Fig. 2. Only a-Al2O3 was detected in different materials. The X-ray diffraction peaks had identical positions, indicating identical chemical compositions. Slight broadening of the X-ray lines was presented in S3 (also in S1) in comparison with S4 and S2, indicating smaller grain sizes in the low-temperature sintered materials (1050 1C vs. 1300 1C). Calculation by Scherer equation indicated that the S1 and S3 materials which were sintered at 1050 1C consisted of smaller crystals with an average diameter of 0.4–0.7 mm, whereas
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Table 1 Density, phase composition, and flexural strength of the sintered alumina ceramics. Materials and the applied sintering route
Total power output Densification (kJ) r (g/cm3) Relative density (%)
S1 by 1050 1C S2 by 1300 1C S3 by rearranged 1050 1C S4 by rearranged 1300 1C
1694 2419 1871
3.31 70.03 83.5 7 0.7 3.93 70.08 98.6 7 2.0 3.84 70.04 96.3 7 1.0
2552
3.96 70.04 99.3 7 1.0
1.5–1.9 mm diameter was present in other materials which were sintered at 1300 1C. The crystal sizes were also measured by the SEM micrographs, using an Image Pro-Plus software compact. The intergranular fractures dominating the fractures as shown by the fractographs in Fig. 3, promised precise grain size measurements. The average grain diameters and the statistical errors are listed in Table 1. The grain size in S1 and S3 (sintered at 1050 1C) was 0.5 70.1 mm and 0.6 70.1 mm respectively. After sintering at 1300 1C, 2.471.2 mm and 1.9 70.7 mm as the grain size value for S2 and S4 was presented respectively. All these data for crystal sizes were in rough consistency with the XRD experiments. As shown in Table 1, the much smaller error range for the grain sizes in S4 illustrated a narrower grain size distribution in S4 material than that in S2. The fractographs in Fig. 3 also reveal a more homogeneous microstructure in S4, while regions consisting of large and small grains were presented in S2 material. The rearranged sintering schedules should have sufficiently enhanced the particle packing of the alumina particles, hence better densification, to result in a uniform microstructure [16–18]. On the contrary, Fig. 3(a) demonstrates a less dense particle packing in the S1 material. Voids marked by the square and a denser neighboring area marked by the character A indicated inhomogeneous packing of the alumina powder. This kind of inhomogeneity could lead to inhomogeneous sintering and hence to an inhomogeneous microstructure. Such microstructural inhomogeneities were also observed in material S2, in comparison with S4, as marked by the circle and character B in Fig. 3(c). 3.4. Mechanical strength The improvement in densification and the microstructural homogeneities improved the flexural strength of the alumina ceramics. The flexural strength of the as-sintered alumina ceramics is listed in Table 1. The S1 and S2 material had a flexural strength of 192 MPa and 335 MPa respectively, compared to 501 MPa and 459 MPa for S3 and S4. The higher mechanical properties of the S3 and S4 material were explained by the denser and more uniform microstructures. The large microstructure inhomogeneities (especially large voids) as observed in the S1 and S2 materials could be flaws that initiate early fracture, thus explaining the lower strength of S1 and S2, relative to S3 and S4. The S3 materials sintered at a lower temperature of 1050 1C consisted of fine grains, giving even higher strength, despite the few percents of higher porosity.
4. Discussions 4.1. General observations Spark plasma sintering has attracted intense research interests because of the high efficiency in preparing various materials.
Phase composition
Grain diameter (mm)
Flexural strength (MPa)
13.1 0.1 0.3
Corundum Corundum Corundum
0.57 0.1 2.47 1.2 0.67 0.1
192 7 27 335 7 29 501 7 41
0
Corundum
1.97 0.7
459 7 39
Open porosity (%)
Considering the Joule heating of the current, generally, in a conventional SPS process, a very small power output as shown in Fig. 1(a) was enough to increase the temperature up to 1300 1C for alumina sintering. Interestingly, it seemed that better densification could be achieved if the heating power was rearranged into discrete units instead of the continuous low amplitude power output. As shown in the rearranged sintering schedule of S3, the heating power was discretized into six units of high amplitude up to 12–14 kW each, but each unit lasting for only less than 20 s, with intervals of 40 s power off. The densities listed in Table 1 show that the S3 material had a relative density of 96.3% after holding at 1050 1C for 5 min, compared to 83.5% for the S1 material, after sintering at the same temperature for 5 min but by a conventional SPS heating schedule. In addition, SEM observations showed improved microstructure homogeneity by the rearranged SPS, as shown both by the microstructure comparison in Fig. 3 and by the grain size statistics listed in Table 1. 4.2. Effects of the total power output It is usually taken for granted that a higher sintering temperature was the most possible reason for the enhanced densification of the S3 and S4 materials, in comparison with S1 and S2. Indeed, temperature overshootings (the few temperature maximum in the holding stage in Fig. 1(c)) were presented in the holding of S3 and S4 at the targeted temperature of 1050 1C and 1300 1C, respectively. Fig. 1(c) exhibits that temperature first reached the maximum of 1104 1C and then gradually decreased to 1056 1C after a few steps of adjustments. Although higher temperature may partly explain the higher densifications for S3 and S4, the large differences in densification and microstructure evolution, especially between S1 and S3, seek other possible explanations. The total power output for each case was calculated and carefully compared. As seen from the results listed in Table 1, the higher total power outputs for the rearranged heating routes of S3 and S4 were in consistency with their higher densifications. However, no simple linear relationship between the densities and the total power output was found, which may suggest additional unknown densification mechanisms in association with the higher DC, as suggested by Raj et al. [5]. 4.3. Effects of possibly enhanced powder packing The good effects of the heating rearrangement were also probably due to the improved powder packing. Based on the SEM observations, local particle packing difference should be present in the powder compact in the early stage of sintering (see the voids and the neighboring denser regions in the S1 material, but a more homogeneous microstructure in S3). It is well-known that such a local packing difference would result in inhomogeneous densification at higher temperatures, hence
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worsened densities and microstructure homogeneities in the consolidated compacts [17,18]. Evidently, in the S2 material after sintering at a higher temperature of 1300 1C, an inhomogeneous microstructure was formed. Large and small grain segregations present in the microstructure of the S2 material were very similar to the other alumina ceramics which were spark plasma sintered in the literature [19]. Optimizing application of the pressure and the heating rate to improve early-stage powder compaction was used in the literature to improve microstructure uniformity [19]. As an analog, the microstructure observations and the density measurements in this study may thus suggest improved powder compaction as one of the good effects of the heating process rearrangement in producing a better microstructural homogeneity in the model alumina ceramics.
S3 and S4 materials. The gas pressure increase in the early stage of sintering, with a corresponding large power output by the end of the first 2 min, was also a possible indicator for the existence of a spark plasma. This was because the high power may have overcome the voltage critic as suggested in Ref. [3], to produce an electric discharge according to the calculations in Ref. [14], thus resulting in particle surface cleaning, and consequently a gas release. Last, in addition to the power output manipulation by S3 and S4, other methods, such as application of a high heating rate of 400 1C/min or a low DC pulse on–off ratio [7,9,11,13,14] may also be the right way to possibly overcome the voltage critic for electric discharges, thus resulting in enhanced densification of ceramics.
4.4. Temperature control
5. Conclusions
The original idea of the heating rearrangement for better microstructural homogeneity was based on the densifying features of the powder compacts. According to the densification reflected by the ram displacement in Fig. 1(b), see the previous results and discussions, the sintering process of the alumina powder started at about 900 1C. This meant large amount of sintering necks formed at 900 1C and above. Because the rigid skeleton resulting from the sintering necks would hinder particle rearrangement, the heating management must be placed at a temperature range below 900 1C. For the rearranged heating schedules S3 and S4, the heating power was interrupted into individual units; each was given a high energy density. However, the short operating time of an individual unit in combination with the rapid cooling capacity of the powder-and-die setup secured the temperature limit. The temperature control curve shown in Fig. 1(c) evidenced the success in curbing the temperatures o900 1C, by the rearranged heating schedules S3 and S4. At the same time, transient heating and cooling cycles resulted from individual units. These rapid heating and cooling cycles may help relax inter-particle friction, hence higher packing efficiency for better densification [20].
To sum up, a new route consisting of a few units with high energy concentration instead of continuous low-power heating was developed for the spark plasma sintering. The good effects of this new SPS route were demonstrated by densification of model alumina ceramics. Inhomogeneous densification was avoided by enhancing particle packing before the early sintering necks formation, thus producing dense alumina ceramics with uniform microstructures at a low temperature of 1050 1C.
4.5. Possible electric discharge Fig. 1(b) exhibits a significant increase in gas pressure in the SPS chamber, with the initial heating up from 25 1C to 600 1C. This kind of pressure increase was traditionally explained by deadsorption of the adsorbed O2, CO2, and moisture. However, with regards to the surface cleansing effect by an electric discharge [1], it was also possibly due to electric discharges, if electric discharges could be coordinately created. Unfortunately for a conventional SPS process, electric discharge was not observed in the powder compacts during a conventional SPS process [3]. Hulbert et al. [3] thought that the voltage for a low temperature SPS ( o650 1C) was not high enough to produce electric discharges. In this study, the SPS heating power was concentrated into several units, so that the maximum of power outputs of an individual unit in both S3 and S4 reached 14 kW (up to 6.89 V, 2054A, Fig. 1(c)), the existence of electric discharges may be possible. Therefore, unique sintering effects of SPS, i.e. a spark plasma, can be included to explain the enhanced sintering of the
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