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Pressureless spark plasma sintering of alumina micro-channel part produced by micro powder injection molding
Available online at www.sciencedirect.com
Scripta Materialia 64 (2011) 237–240 www.elsevier.com/locate/scriptamat
Pressureless spark plasma sintering of alumina micro-channel part produced by micro powder injection molding Junhu Meng,a,⇑ Ngiap Hiang Loh,a Bee Yen Tay,b Shu Beng Tor,a,c Gang Fu,a,c Khiam Aik Khora and Ligen Yua a
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore c Singapore-MIT Alliance (SMA), N3.2-01-36, 65 Nanyang Drive, Singapore 637460, Singapore Received 2 August 2010; revised 11 October 2010; accepted 12 October 2010 Available online 18 October 2010
Alumina micro-channel parts were produced by micro powder injection molding. The debound part was rapidly densified by pressuresless spark plasma sintering. Rapid densification of the micro-channel part proved to be feasible. Good shape retention and high densification were achieved for the final micro-channel part. The microstructural analysis revealed that fine-grained microstructures were obtained at the sintering temperatures of 1250–1300 °C. The nanoindentation tests showed that the nanohardness and Young’s modulus were dependent on the porosity and grain size. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramics; Micro powder injection molding; Spark plasma sintering; Microstructure
Powder injection molding (PIM) is an established near net-shape process for manufacturing metal and ceramic parts. PIM combines the merits of plastic injection molding and powder metallurgy processes, such as suitability for mass production, part shape complexity, low cost and applicability to many materials. Adapted from PIM, micro powder injection molding (micro PIM) has been applied to produce metal and ceramic micro-components. The merits of PIM are also applicable to micro PIM, which makes it more competitive as compared with other micro-manufacturing technologies. With the increasing demand for miniaturized devices and systems, there is great market potential for micro-components in various fields of application, such as micro-system technology, micro-mechanics, microfluidics, micro-sensors and medical technology [1,2]. Similar to conventional PIM, micro PIM has four main processing steps, namely: mixing, injection molding, debinding and sintering [3]. For each processing step, the requirements for micro PIM are stricter and more challenging than those for PIM. To meet the high requirements of micro PIM parts, studies have been con-
⇑ Corresponding yahoo.com.cn
author. Tel.: +65 67906123; e-mail: jhmeng521@
ducted on the development of feedstock, modification of micro injection molding technologies, and improvement of the debinding and sintering methods. As for micro PIM, dimensional accuracy of the sintered parts is a crucial concern [4]. Replication accuracy has to be improved for high-precision micro PIM parts. Shrinkage, surface finish and mechanical properties have all shown a dependence on the grain size [5–7]. Hence, controlling the grain size for micro PIM parts is attracting particular attention. Due to the dominance of grain growth over densification at low temperatures range, a rapid heating rate is required to minimize the undesirable grain growth [8]. Spark plasma sintering (SPS) is an advanced sintering technology for rapid densification of metal or ceramic powder parts at low temperatures. The outstanding advantage of SPS is that high densification can be achieved in just a few minutes under mechanical pressure. It thus has wide applications in the preparation of nanocrystalline materials, translucent ceramic and porous materials. Due to the rapid heating rate, pressureless SPS has also been used to produce carbon–silicon carbide nanowire composites and alumina [9,10]. However, there seems to be no published work on the production of micro-components by micro PIM and pressureless SPS. Alumina micro-channel parts are used in many microsystems such as micro-fluidic devices, micro-reactors,
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.10.016
238
J. Meng et al. / Scripta Materialia 64 (2011) 237–240
micro-transducers and micro-optics. The surface quality and mechanical properties of the micro-channel part are dependent on its microstructure. In the work reported here, in order to obtain a fine and uniform microstructure, alumina micro-channel parts were produced by micro PIM and densified at a rapid heating rate using pressureless SPS. The nanohardness and Young’s modulus of the micro-channel part were characterized by a nanoindentation test. High-purity a-alumina powder (grade TM-DAR from Taimei) with a mean particle size of 170 nm was used. A multi-component binder consisting of low-density polyethylene, ethylene vinyl acetate, paraffin wax and stearic acid was used. To prepare the feedstock for injection molding, the powder and binder components were mixed in a double-Z Linden mixer at 130 °C for 2 h. The feedstock was then granulated into the small pellets to facilitate the injection molding. The granulated feedstock was injection molded into the micro-channel parts on a Battenfeld 250 CDC horizontal injection molding machine. A silicon mold insert with 11 micro-channels was used to mold the alumina micro-channel part. The dimensions of each microchannel as measured by a PLl confocal profiler were a width of 200 lm and a depth of 135 lm. The length was 4 mm. The silicon mold insert was fabricated by deep reactive ion etching, as described in Ref. [11].The silicon mold insert was mounted onto the circular cavity of the movable mold. This produced the micro-channel part, which consisted of a circular disc, a base, microchannels and micro-structures. Figure 1a shows the schematic of the micro-channel part. The molded micro-channel part was thermally debound to remove the binders in a Lindberg tube furnace with a gas mixture consisting of 95% argon and 5% hydrogen at a suitable debinding profile. A suitable debinding profile enabled to remove progressively the binder components over a wide temperature range and prevent the formation of debinding defects, such as blister, cracking
Figure 1. (a) Schematic of the micro-channel part and (b) photograph of the sintered micro-channel part.
and slumping. After debinding, pressureless SPS was conducted in vacuum using an SPS machine (SPS-1050, Sumitomo). The sintering temperatures were varied from 1150 to 1350 °C in intervals of 50 °C. A heating rate of 100 °C min 1 and holding time at that temperature of 5 min were used. When a higher heating rate of 200 °C min 1 was used, high densification was not obtained with a short sintering time under pressureless conditions. The dimensions of the graphite die were: 25 mm inner diameter and 50 mm outer diameter. The dimensions of the two (upper and lower) graphite punches were: 30 mm diameter and 15 mm height. Minimal contact pressure between the die and the punches was maintained to ensure that the pulsed DC current could heat up the graphite die. The debound part was placed on the lower punch and was free of pressure as the diameter of the punch was larger than the outer diameter of the die. A pulse duration of 3.3 ms and a pulse sequence of 12 pulses “ON” followed by 2 pulses “OFF” were used in the study. The densities of the micro-channel parts were determined using Archimedes’s principle. The theoretical density (TD) of alumina is 3.98 g cm 3. The dimensions of the molded and sintered micro-channels and microstructures were measured by a PLl confocal profiler. The measurements were repeated in six different locations on the micro-channels and micro-structures. The micro-channel parts were sectioned and then mounted in epoxy for the polishing of the cross-sections. The polished crosssections of the micro-channel parts were subjected to nanoindentation tests and microstructure analysis. The nanohardness and Young’s modulus were measured on an MTS Nano Indenter XP system using the continuous stiffness measurement mode. The measurement conditions were: load depth limit 1200 nm; strain rate target: 0.05 s 1; harmonic displacement target: 2 nm; frequency target: 45 Hz. The results from the last unload were taken. At least 18 indentations were made for the circular disc and base and 12 were made for the micro-structures. The polished cross-sections of the micro-channel parts were thermally etched in air below the actual sintering temperatures for 15–30 min. The etched cross-sections were observed using a field emission scanning electron microscope (LEO 1550). The grain sizes were determined by the linear intercept method (ASTM E112–96), with a correction factor of 1.56. Figure 1b shows the photograph of the sintered microchannel part, which consisted of a circular disc, microchannels and micro-structures. The micro-channel part had good shape retention and was replicated well from the silicon mold insert. Macro-defects, such as warpage and blister, were not visible in the final sintered microchannel part. For micro PIM, defects are not rectified by subsequent processing steps. Figure 2a shows the average dimensions of the micro-channels and micro-structures after molding and sintering. The width and height of the micro-structures after molding and sintering showed similar trends to each other. The dimension of the silicon mold insert was used as a reference. Compared with the silicon mold insert, the width and height of the molded micro-structures increased slightly. The reason for this phenomenon was that the volumetric expansion due to the decrease in cavity pressure was more significant than the volumetric shrinkage
J. Meng et al. / Scripta Materialia 64 (2011) 237–240 16
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(c) Figure 2. Dimensions of (a) the micro-channels and micro-structures and (b) the circular disc after molding and sintering, and (c) the relative densities and grain sizes of the micro-channel parts for different sintering temperatures.
from cooling [12]. An apparent decrease in the width and height of the micro-structures occurred at the sintering temperature of 1150 °C. For sintering temperatures of 1250 °C and above, the width and height of the micro-structure were relatively similar. In the case of the molded micro-channel, the width was smaller than that of the silicon mold insert. The width of the micro-channel showed a tendency similar to that of the micro-structure with increasing sintering temperature. As shown in Figure 2b, the macroscopic dimensions, namely the diameter and thickness of the circular disc, also showed a similar trend. Figure 2c shows the relative densities and the grain sizes (at the base) of the micro-channel parts for different sintering temperatures. Up to 1250 °C, the relative densities increased significantly, namely from 76% TD at 1150 °C to 97% TD at 1250 °C. The micro-channel parts above 1300 °C were fully densified and relative densities of more than 99% TD were obtained. As shown in Figure 2c, the densification of the micro-channel part was accompanied by grain growth. The grain size increased with increasing sintering temperature. At 1150 °C, there was no obvious grain growth and the grain size was about 0.17 lm, which was close to the original powder size. The grain size increased gradually in the temperature range 1200–1300 °C. The grain sizes at 1200, 1250 and 1300 °C were 0.30, 0.64, and 0.81 lm, respectively. However, significant grain growth occurred at 1350 °C. The grain size at 1350 °C was 1.65 lm, about 10 times larger than the original powder size of 0.17 lm. Grain coarsening and densification occur simultaneously during sintering. There is a high-sintering temperature region where bulk diffusion is enhanced relative to surface diffusion and hence densification is favored over grain coarsening [13,14]. At the rapid heating rate, the micro-channel part quickly passed through the low-temperature range and proceeded directly to the higher temperature range, which improved densification
while suppressing significant grain coarsening, especially at the temperatures of 1250–1300 °C. Figure 3 shows field emission scanning electron microscopy (FESEM) micrographs of the micro-channel
Figure 3. FESEM micrographs of the micro-channel part for different sintering temperatures: (a) 1150 °C, (b) 1200 °C, (c) 1250 °C, (d) 1300 °C, (e) 1350 °C.
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J. Meng et al. / Scripta Materialia 64 (2011) 237–240 30
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similar trend for the Young’s modulus was also observed in this study, as shown in Figure 4b. In the low-densification micro-channel parts, the Young’s modulus was dependent on the degree of densification. From 1150 to 1250 °C, the improved densification led to a significant increase in the Young’s modulus, which contributed to the fast neck growth [16,17]. However, the Young’s modulus of the coarse-grained alumina at 1350 °C was smaller compared with the fine-grained alumina at 1300 °C, as the fine-grained alumina was more dense and defect-free than the coarse-grained alumina [18]. In summary, alumina micro-channel parts were successfully produced by micro PIM. After rapid densification by pressureless SPS, the micro-channel part had good shape retention and micro-mechanical properties (nanohardness and Young’s modulus). High densification of the micro-channel part was achieved by inhibiting grain coarsening. The sintering of the alumina microchannel part by pressureless SPS proved to be feasible and thus this method may be applied to the densification of other micro-components.
o
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(b) Figure 4. (a) Nanohardness and (b) Young’s modulus of the microchannel part for different sintering temperatures.
part (at the base) for different sintering temperatures. The etched surfaces revealed that the microstructures for different temperatures consisted mainly of equiaxed alumina grains. At 1150 °C, pores between the alumina grains were visible, which indicated that the microchannel part had low densification. Upon increasing the temperature to 1200 °C, the microstructures become denser than at 1150 °C and fewer tiny pores were found. When the sintering temperature was greater than 1250 °C, dense microstructures were achieved without the presence of pores. In particular, abnormal grain growth occurred at 1350 °C and the grain size was 1.65 lm. Figure 4 shows the nanohardness and Young’s modulus of the micro-channel part for different sintering temperatures. It can be seen that the nanohardness and Young’s modulus showed similar dependence on sintering temperature. Below 1250 °C, the nanohardness and Young’s modulus increased with the sintering temperature, although the nanohardnesses were very similar at 1250 and 1300 °C. At higher temperature of 1350 °C, the nanohardness and Young’s modulus decreased. The variations in nanohardness and Young’s modulus were strongly affected by the microstructure (including porosity and grain size). As mentioned above, low densification of the micro-channel parts was obtained below 1250 °C. Pores between the grains caused a reduction in the nanohardness. For the dense micro-channel part, the grain size was a crucial factor in determining the nanohardness. Compared with at 1300 °C, the larger grain size at 1350 °C led to a reduction in the nanohardness, in agreement with the Hall–Petch relation. This phenomenon is normally attributed to the reduced free path for dislocations as the grain size decreases [15]. A
The financial support of research grant RGM 6/07 from The Nanyang Technological University is gratefully acknowledged. The authors would also like to thank Dr Li Xiaocheng for assistance on the FESEM micrograph. [1] F. Petzoldt, Powder Injection Moulding Int. 2 (2008) 37– 42. [2] B.Y. Tay, L. Liu, N.H. Loh, S.B. Tor, Y. Murakoshi, R. Maeda, Microsyst. Technol. 11 (2005) 210–213. [3] G. Fu, N.H. Loh, S.B. Tor, Y. Murakoshi, R. Maeda, Mater. Des. 25 (2004) 729–733. [4] V. Piotter, M. Beck, K. Plewa, H.J. Ritzhaupt-Kleissl, A. Ruh, J. Hausselt, Met. Powder Rep. 64 (2009) 35–37. [5] S.G. Li, G. Fu, I. Reading, S.B. Tor, N.H. Loh, P. Chaturvedi, S.F. Yoon, K. Youcef-Toumi, Appl. Phys. A 89 (2007) 721–728. [6] B.Y. Tay, L. Liu, N.H. Loh, S.B. Tor, Y. Murakoshi, R. Maeda, Mater. Sci. Eng. A 396 (2005) 311–319. [7] L. Liu, N.H. Loh, B.Y. Tay, S.B. Tor, Y. Murakoshi, R. Maeda, Scr. Mater. 55 (2006) 1103–1106. [8] J.H. She, T. Ohji, J. Mater. Sci. Lett. 21 (2002) 1405–1406. [9] W.L. Bradbury, E.A. Olevsky, Scr. Mater. 63 (2010) 77– 80. [10] D. Salamon, Z. Shen, Mater. Sci. Eng., A 475 (2008) 105– 107. [11] J. Meng, N.H. Loh, G. Fu, S.B. Tor, B.Y. Tay, J. Alloys Compd. 496 (2010) 293–299. [12] G. Fu, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, 2008. [13] R. Mazumder, D. Chakravarty, D. Bhattacharya, A. Sen, Mater. Res. Bull. 44 (2009) 555–559. [14] J. Zhang, F. Meng, R.I. Todd, Z. Fu, Scr. Mater. 62 (2010) 658–661. [15] J. Wang, L.L. Shaw, J. Am. Ceram. Soc. 93 (2010) 601– 604. [16] D. Hardy, D.J. Green, J. Eur. Ceram. Soc. 15 (1995) 769– 775. [17] T. Ostrowski, J. Ro¨del, J. Am. Ceram. Soc. 82 (1999) 3080–3086. [18] S. Guicciardi, T. Shimozono, G. Pezzotti, Adv. Eng. Mater. 8 (2006) 994–997.
Scripta Materialia 64 (2011) 237–240 www.elsevier.com/locate/scriptamat
Pressureless spark plasma sintering of alumina micro-channel part produced by micro powder injection molding Junhu Meng,a,⇑ Ngiap Hiang Loh,a Bee Yen Tay,b Shu Beng Tor,a,c Gang Fu,a,c Khiam Aik Khora and Ligen Yua a
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore c Singapore-MIT Alliance (SMA), N3.2-01-36, 65 Nanyang Drive, Singapore 637460, Singapore Received 2 August 2010; revised 11 October 2010; accepted 12 October 2010 Available online 18 October 2010
Alumina micro-channel parts were produced by micro powder injection molding. The debound part was rapidly densified by pressuresless spark plasma sintering. Rapid densification of the micro-channel part proved to be feasible. Good shape retention and high densification were achieved for the final micro-channel part. The microstructural analysis revealed that fine-grained microstructures were obtained at the sintering temperatures of 1250–1300 °C. The nanoindentation tests showed that the nanohardness and Young’s modulus were dependent on the porosity and grain size. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramics; Micro powder injection molding; Spark plasma sintering; Microstructure
Powder injection molding (PIM) is an established near net-shape process for manufacturing metal and ceramic parts. PIM combines the merits of plastic injection molding and powder metallurgy processes, such as suitability for mass production, part shape complexity, low cost and applicability to many materials. Adapted from PIM, micro powder injection molding (micro PIM) has been applied to produce metal and ceramic micro-components. The merits of PIM are also applicable to micro PIM, which makes it more competitive as compared with other micro-manufacturing technologies. With the increasing demand for miniaturized devices and systems, there is great market potential for micro-components in various fields of application, such as micro-system technology, micro-mechanics, microfluidics, micro-sensors and medical technology [1,2]. Similar to conventional PIM, micro PIM has four main processing steps, namely: mixing, injection molding, debinding and sintering [3]. For each processing step, the requirements for micro PIM are stricter and more challenging than those for PIM. To meet the high requirements of micro PIM parts, studies have been con-
⇑ Corresponding yahoo.com.cn
author. Tel.: +65 67906123; e-mail: jhmeng521@
ducted on the development of feedstock, modification of micro injection molding technologies, and improvement of the debinding and sintering methods. As for micro PIM, dimensional accuracy of the sintered parts is a crucial concern [4]. Replication accuracy has to be improved for high-precision micro PIM parts. Shrinkage, surface finish and mechanical properties have all shown a dependence on the grain size [5–7]. Hence, controlling the grain size for micro PIM parts is attracting particular attention. Due to the dominance of grain growth over densification at low temperatures range, a rapid heating rate is required to minimize the undesirable grain growth [8]. Spark plasma sintering (SPS) is an advanced sintering technology for rapid densification of metal or ceramic powder parts at low temperatures. The outstanding advantage of SPS is that high densification can be achieved in just a few minutes under mechanical pressure. It thus has wide applications in the preparation of nanocrystalline materials, translucent ceramic and porous materials. Due to the rapid heating rate, pressureless SPS has also been used to produce carbon–silicon carbide nanowire composites and alumina [9,10]. However, there seems to be no published work on the production of micro-components by micro PIM and pressureless SPS. Alumina micro-channel parts are used in many microsystems such as micro-fluidic devices, micro-reactors,
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.10.016
238
J. Meng et al. / Scripta Materialia 64 (2011) 237–240
micro-transducers and micro-optics. The surface quality and mechanical properties of the micro-channel part are dependent on its microstructure. In the work reported here, in order to obtain a fine and uniform microstructure, alumina micro-channel parts were produced by micro PIM and densified at a rapid heating rate using pressureless SPS. The nanohardness and Young’s modulus of the micro-channel part were characterized by a nanoindentation test. High-purity a-alumina powder (grade TM-DAR from Taimei) with a mean particle size of 170 nm was used. A multi-component binder consisting of low-density polyethylene, ethylene vinyl acetate, paraffin wax and stearic acid was used. To prepare the feedstock for injection molding, the powder and binder components were mixed in a double-Z Linden mixer at 130 °C for 2 h. The feedstock was then granulated into the small pellets to facilitate the injection molding. The granulated feedstock was injection molded into the micro-channel parts on a Battenfeld 250 CDC horizontal injection molding machine. A silicon mold insert with 11 micro-channels was used to mold the alumina micro-channel part. The dimensions of each microchannel as measured by a PLl confocal profiler were a width of 200 lm and a depth of 135 lm. The length was 4 mm. The silicon mold insert was fabricated by deep reactive ion etching, as described in Ref. [11].The silicon mold insert was mounted onto the circular cavity of the movable mold. This produced the micro-channel part, which consisted of a circular disc, a base, microchannels and micro-structures. Figure 1a shows the schematic of the micro-channel part. The molded micro-channel part was thermally debound to remove the binders in a Lindberg tube furnace with a gas mixture consisting of 95% argon and 5% hydrogen at a suitable debinding profile. A suitable debinding profile enabled to remove progressively the binder components over a wide temperature range and prevent the formation of debinding defects, such as blister, cracking
Figure 1. (a) Schematic of the micro-channel part and (b) photograph of the sintered micro-channel part.
and slumping. After debinding, pressureless SPS was conducted in vacuum using an SPS machine (SPS-1050, Sumitomo). The sintering temperatures were varied from 1150 to 1350 °C in intervals of 50 °C. A heating rate of 100 °C min 1 and holding time at that temperature of 5 min were used. When a higher heating rate of 200 °C min 1 was used, high densification was not obtained with a short sintering time under pressureless conditions. The dimensions of the graphite die were: 25 mm inner diameter and 50 mm outer diameter. The dimensions of the two (upper and lower) graphite punches were: 30 mm diameter and 15 mm height. Minimal contact pressure between the die and the punches was maintained to ensure that the pulsed DC current could heat up the graphite die. The debound part was placed on the lower punch and was free of pressure as the diameter of the punch was larger than the outer diameter of the die. A pulse duration of 3.3 ms and a pulse sequence of 12 pulses “ON” followed by 2 pulses “OFF” were used in the study. The densities of the micro-channel parts were determined using Archimedes’s principle. The theoretical density (TD) of alumina is 3.98 g cm 3. The dimensions of the molded and sintered micro-channels and microstructures were measured by a PLl confocal profiler. The measurements were repeated in six different locations on the micro-channels and micro-structures. The micro-channel parts were sectioned and then mounted in epoxy for the polishing of the cross-sections. The polished crosssections of the micro-channel parts were subjected to nanoindentation tests and microstructure analysis. The nanohardness and Young’s modulus were measured on an MTS Nano Indenter XP system using the continuous stiffness measurement mode. The measurement conditions were: load depth limit 1200 nm; strain rate target: 0.05 s 1; harmonic displacement target: 2 nm; frequency target: 45 Hz. The results from the last unload were taken. At least 18 indentations were made for the circular disc and base and 12 were made for the micro-structures. The polished cross-sections of the micro-channel parts were thermally etched in air below the actual sintering temperatures for 15–30 min. The etched cross-sections were observed using a field emission scanning electron microscope (LEO 1550). The grain sizes were determined by the linear intercept method (ASTM E112–96), with a correction factor of 1.56. Figure 1b shows the photograph of the sintered microchannel part, which consisted of a circular disc, microchannels and micro-structures. The micro-channel part had good shape retention and was replicated well from the silicon mold insert. Macro-defects, such as warpage and blister, were not visible in the final sintered microchannel part. For micro PIM, defects are not rectified by subsequent processing steps. Figure 2a shows the average dimensions of the micro-channels and micro-structures after molding and sintering. The width and height of the micro-structures after molding and sintering showed similar trends to each other. The dimension of the silicon mold insert was used as a reference. Compared with the silicon mold insert, the width and height of the molded micro-structures increased slightly. The reason for this phenomenon was that the volumetric expansion due to the decrease in cavity pressure was more significant than the volumetric shrinkage
J. Meng et al. / Scripta Materialia 64 (2011) 237–240 16
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(c) Figure 2. Dimensions of (a) the micro-channels and micro-structures and (b) the circular disc after molding and sintering, and (c) the relative densities and grain sizes of the micro-channel parts for different sintering temperatures.
from cooling [12]. An apparent decrease in the width and height of the micro-structures occurred at the sintering temperature of 1150 °C. For sintering temperatures of 1250 °C and above, the width and height of the micro-structure were relatively similar. In the case of the molded micro-channel, the width was smaller than that of the silicon mold insert. The width of the micro-channel showed a tendency similar to that of the micro-structure with increasing sintering temperature. As shown in Figure 2b, the macroscopic dimensions, namely the diameter and thickness of the circular disc, also showed a similar trend. Figure 2c shows the relative densities and the grain sizes (at the base) of the micro-channel parts for different sintering temperatures. Up to 1250 °C, the relative densities increased significantly, namely from 76% TD at 1150 °C to 97% TD at 1250 °C. The micro-channel parts above 1300 °C were fully densified and relative densities of more than 99% TD were obtained. As shown in Figure 2c, the densification of the micro-channel part was accompanied by grain growth. The grain size increased with increasing sintering temperature. At 1150 °C, there was no obvious grain growth and the grain size was about 0.17 lm, which was close to the original powder size. The grain size increased gradually in the temperature range 1200–1300 °C. The grain sizes at 1200, 1250 and 1300 °C were 0.30, 0.64, and 0.81 lm, respectively. However, significant grain growth occurred at 1350 °C. The grain size at 1350 °C was 1.65 lm, about 10 times larger than the original powder size of 0.17 lm. Grain coarsening and densification occur simultaneously during sintering. There is a high-sintering temperature region where bulk diffusion is enhanced relative to surface diffusion and hence densification is favored over grain coarsening [13,14]. At the rapid heating rate, the micro-channel part quickly passed through the low-temperature range and proceeded directly to the higher temperature range, which improved densification
while suppressing significant grain coarsening, especially at the temperatures of 1250–1300 °C. Figure 3 shows field emission scanning electron microscopy (FESEM) micrographs of the micro-channel
Figure 3. FESEM micrographs of the micro-channel part for different sintering temperatures: (a) 1150 °C, (b) 1200 °C, (c) 1250 °C, (d) 1300 °C, (e) 1350 °C.
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1300
1350
similar trend for the Young’s modulus was also observed in this study, as shown in Figure 4b. In the low-densification micro-channel parts, the Young’s modulus was dependent on the degree of densification. From 1150 to 1250 °C, the improved densification led to a significant increase in the Young’s modulus, which contributed to the fast neck growth [16,17]. However, the Young’s modulus of the coarse-grained alumina at 1350 °C was smaller compared with the fine-grained alumina at 1300 °C, as the fine-grained alumina was more dense and defect-free than the coarse-grained alumina [18]. In summary, alumina micro-channel parts were successfully produced by micro PIM. After rapid densification by pressureless SPS, the micro-channel part had good shape retention and micro-mechanical properties (nanohardness and Young’s modulus). High densification of the micro-channel part was achieved by inhibiting grain coarsening. The sintering of the alumina microchannel part by pressureless SPS proved to be feasible and thus this method may be applied to the densification of other micro-components.
o
Sintering temperature ( C)
(b) Figure 4. (a) Nanohardness and (b) Young’s modulus of the microchannel part for different sintering temperatures.
part (at the base) for different sintering temperatures. The etched surfaces revealed that the microstructures for different temperatures consisted mainly of equiaxed alumina grains. At 1150 °C, pores between the alumina grains were visible, which indicated that the microchannel part had low densification. Upon increasing the temperature to 1200 °C, the microstructures become denser than at 1150 °C and fewer tiny pores were found. When the sintering temperature was greater than 1250 °C, dense microstructures were achieved without the presence of pores. In particular, abnormal grain growth occurred at 1350 °C and the grain size was 1.65 lm. Figure 4 shows the nanohardness and Young’s modulus of the micro-channel part for different sintering temperatures. It can be seen that the nanohardness and Young’s modulus showed similar dependence on sintering temperature. Below 1250 °C, the nanohardness and Young’s modulus increased with the sintering temperature, although the nanohardnesses were very similar at 1250 and 1300 °C. At higher temperature of 1350 °C, the nanohardness and Young’s modulus decreased. The variations in nanohardness and Young’s modulus were strongly affected by the microstructure (including porosity and grain size). As mentioned above, low densification of the micro-channel parts was obtained below 1250 °C. Pores between the grains caused a reduction in the nanohardness. For the dense micro-channel part, the grain size was a crucial factor in determining the nanohardness. Compared with at 1300 °C, the larger grain size at 1350 °C led to a reduction in the nanohardness, in agreement with the Hall–Petch relation. This phenomenon is normally attributed to the reduced free path for dislocations as the grain size decreases [15]. A
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