December 2004 Communications of the American Ceramic Society 2299 densification rix shear stresses during the milling. 23 Both Panchula et aL. and in the 20 vol% SiCwHEBMY-AL2O3 nanocomposite by SPS at investigators in the present work used a similar setup for HEBM 1125C is as small as 118 nm. These results suggest that the However, in our case, the charge ratio(ball-to-powder ratio)was ombined effect of rapid heating rate by SPS and powder prepa- 1.8, whereas Panchula et al. applied a charge ratio as high as 5. It ration by heBm can result in truly nanocrystalline matrix cor is entirely possible that, in the hEBM processing of powders posites. HEBM is known to enhance the solid-state phase trans- Panchula et al succeeded in imparting far more internal energy to formation. In fact, mechanical attrition(high-energy ball milling the powders which allowed them to overcome the phase transfor as a method has been widely used for preparation of nanocrystal mation barrier, leading to the transformation of y- to a-alumina line metallic materials. This method has also been applied to during ball milling preparation of some ceramic nanopowders. 2. During the HEBM Table I also shows the mechanical pi process, the impact energy (locally high pressures and tempera Vickers hardness and fracture toughness hese 20 vol%o tures)can result in small regions of the y-Al,, powder trans- SiCw/y-Al2O3 and 20 vol% SiCwHEBMY-Al2O, composites. It forming into a-Al2O3 and in a decrease of the crystallite size can be seen that a significant improvement in fracture toughness during milling. For instance, Panchula and Ying found that the was achieved compared with pure nanocrystalline alumina. The phase transformation from y to o took place during the milling achieved increase in toughness for these nanocomposites is com- process. Our present results are quite different from their obser- parable to that for coarse-grained alumina matrix composites vation. As shown in Fig. 1, XRD did not indicate any phase ghened by SiC whisker. It is also very interesting to note ransformation occurring during the 24-h HEBM period even that a dramatic improvement in Vickers hardness was achieved in though it is longer than the reported minimal time for the complete the nanocomposites when the grain size is reduced to 100 nm. The transformation, 10 h. However, it is very interesting to note that hardness goes up 26. 1 GPa when the grain size is 118 nm, while very broad y peaks are observed in the HEBM y-Al2O3 nano- it decreases to 23. 1 GPa when grain size is "l um. The superior ler. This broadening of the peak is not related to the ement of the starting powder as this can be ruled out since the nanocrystalline matrix grain size /1 posites is likely related to ardness in the nanocrystalline nanocomp SeM observations indicated that there is no obvious difference in terms of the particle size(Fig. 2). This is consistent with another report that no refinement of the primary particle size took place Summary during milling. The effect of HEBM on the phase transformation nd microstructural development can be postulated as follows The combination of spark-plasma-sintering and high-energy High-energy ball milling can lead to higher green density of the ball milling has been demonstrated to be an effective method for compacts due to pore collapse from the high compressive and obtaining fully dense nanocomposites with a nanocrystalline alu mina matrix. A fully dense 20 vol% SiCwALO3 nanocomposite with 118-nm matrix grain size was successfully consolidated by SPS at 1125C for 3 min. a significant increase in terms of hardness and toughness has been achieved in the dense nanocom- posite. The 20 vol% SiCwHEBMY-AL,O, nanocomposite with grain size of 118 nm has superior mechanical properties wi acture toughness of 6.2 MPam/ and Vickers hardness of 26.1 GPa. HEBM does not lead to the phase transformation during milling but induces possible residual strain that is beneficial to the ohase transformation that takes place during sintering References P C Panda, J. Wang, and R. Raj.""Sinter-Forging Characteristics of Fine-Grained -R.S. Averback. H. J. Hofler, H. Hahn, and J C. Logas, ""Sintering and Grain 100nm Growth in Nanocryst mics, Nanostruct. Mater, 1. 173-78(1992). M. J. Mayo and D. C. Hague, "Porosity-Grain Growth Relationship in the of Nanostructured Cera Mechanical Properties and Deformation Behavior of Materials Having Ultrafine Microstructures. Edited by M. Nastasi, D. M. Parkin, and H. Gleiter. Kluwer Academic Publishers, Dordrecht, Netherlands,, I rocessing N. I.Noskoes; pp.361-85 in Nanostructured Materials.Edited by GMChow and cova. Kluwer Academic Publishers. Dordrecht. bJ. Freim. J. McKittrick. J. Katz, and K. Scickafus, "Microwave Sintering of Y-Al2O3, Nanostruct. Mater., 4[4]371-85(1994). S. H. Risbud, S.H. Shan, A K. Mukherjee, M. J. Kim, J.S. Bow, and R. A. Holl, tructure in Aluminum Oxide by Very Rapid Sintering at oeC,"J. Mater..Res,1012]237-39(1995) R. S. Mishra J. Schneider. J. F. Shackelford, and A. K. Mu “ Plasma Activated Sintering of Nanocrystalline Y-Al2O3. " Nanostruct. Mater, 5 [51 525-44 (1995) R.S. Mishra, A K. Mukherjee, K. Yamazaki, and K. Shoda, "Effect of TiO Doping on Rapid Densification of Alumina by Plasma Activated Sintering. "JMater. Res,11[51144-48(1996) IRSMishra, C.E. Leshier, and A.KMukherjee,"High Nano-Nano Alumina Composites"; pp. 173-79 in Synthesis and Processing of 100nm anocrystalline Powder. Edited by D. L. Bourell. TMS, Warrendale, PA, 1996 IR. S. Mishra, C.E. Lesher and A K. Mukh High Pressure Sintering of Nanocrystalline y-Al2O3. J Am Ceram Soc., 79 [11] 2989-92(1996). 12s.-C. Liao, YJ. Chen, B. H. Kear, and W. E. Mayo, "High Pressure/Low Sintering of Nanocrystalline Alumina, Nanostruct. Mater, 10 16 P. F. Becher and G. C. Wei, "Toughening Behavior in SiC-Whisker-Reinforced Alumina. "J. Am. Ceran. Soc., 67 [12]C-267-C-269(1984) Fig. 2. HRSEM micrographs of the as-received y-Al,O, nanopowders(a) I+G. C. Wei and P. F. Becher, "Development of SiC-Whisker-Reinforced Ceram- before and(b) after high-energy ball milling ics. "Am. Ceram. Soc. Bull. 64 298-304(1985densification process. The mean grain size for the alumina matrix in the 20 vol% SiCw/HEBM-Al2O3 nanocomposite by SPS at 1125°C is as small as 118 nm. These results suggest that the combined effect of rapid heating rate by SPS and powder prepa￾ration by HEBM can result in truly nanocrystalline matrix com￾posites. HEBM is known to enhance the solid-state phase trans￾formation. In fact, mechanical attrition (high-energy ball milling) as a method has been widely used for preparation of nanocrystal￾line metallic materials.21 This method has also been applied to preparation of some ceramic nanopowders.22,23 During the HEBM process, the impact energy (locally high pressures and tempera￾tures) can result in small regions of the -Al2O3 powder trans￾forming into -Al2O3 and in a decrease of the crystallite size during milling. For instance, Panchula and Ying23 found that the phase transformation from to  took place during the milling process. Our present results are quite different from their obser￾vation. As shown in Fig. 1, XRD did not indicate any phase transformation occurring during the 24-h HEBM period even though it is longer than the reported minimal time for the complete transformation, 10 h.23 However, it is very interesting to note that very broad peaks are observed in the HEBM -Al2O3 nano￾powder. This broadening of the peak is not related to the refinement of the starting powder as this can be ruled out since our SEM observations indicated that there is no obvious difference in terms of the particle size (Fig. 2). This is consistent with another report that no refinement of the primary particle size took place during milling.22 The effect of HEBM on the phase transformation and microstructural development can be postulated as follows. High-energy ball milling can lead to higher green density of the compacts due to pore collapse from the high compressive and shear stresses during the milling.23 Both Panchula et al. and investigators in the present work used a similar setup for HEBM. However, in our case, the charge ratio (ball-to-powder ratio) was 1.8, whereas Panchula et al. applied a charge ratio as high as 5. It is entirely possible that, in the HEBM processing of powders, Panchula et al. succeeded in imparting far more internal energy to the powders which allowed them to overcome the phase transfor￾mation barrier, leading to the transformation of - to -alumina during ball milling. Table I also shows the mechanical properties in terms of Vickers hardness and fracture toughness for these 20 vol% SiCw/-Al2O3 and 20 vol% SiCw/HEBM-Al2O3 composites. It can be seen that a significant improvement in fracture toughness was achieved compared with pure nanocrystalline alumina. The achieved increase in toughness for these nanocomposites is com￾parable to that for coarse-grained alumina matrix composites toughened by SiC whisker.15–17 It is also very interesting to note that a dramatic improvement in Vickers hardness was achieved in the nanocomposites when the grain size is reduced to 100 nm. The hardness goes up 26.1 GPa when the grain size is 118 nm, while it decreases to 23.1 GPa when grain size is
1 m. The superior hardness in the nanocrystalline nanocomposites is likely related to the nanocrystalline matrix grain size.11 IV. Summary The combination of spark-plasma-sintering and high-energy ball milling has been demonstrated to be an effective method for obtaining fully dense nanocomposites with a nanocrystalline alu￾mina matrix. A fully dense 20 vol% SiCw/Al2O3 nanocomposite with 118-nm matrix grain size was successfully consolidated by SPS at 1125°C for 3 min. A significant increase in terms of hardness and toughness has been achieved in the dense nanocom￾posite. The 20 vol% SiCw/HEBM-Al2O3 nanocomposite with grain size of 118 nm has superior mechanical properties with a fracture toughness of 6.2 MPa
m1/2 and Vickers hardness of 26.1 GPa. HEBM does not lead to the phase transformation during milling but induces possible residual strain that is beneficial to the phase transformation that takes place during sintering. References 1 P. C. Panda, J. Wang, and R. Raj, “Sinter-Forging Characteristics of Fine-Grained Zirconia,” J. Am. Ceram. Soc., 71 [12] C-507–C-509 (1988). 2 R. S. Averback, H. J. Hofler, H. Hahn, and J. C. Logas, “Sintering and Grain Growth in Nanocrystalline Ceramics,” Nanostruct. Mater., 1, 173–78 (1992). 3 M. J. Mayo and D. C. Hague, “Porosity-Grain Growth Relationship in the Sintering of Nanocrystalline Ceramics,” Nanostruct. Mater., 3, 43–53 (1993). 4 M. J. Mayo, “Superplasticity of Nanostructured Ceramics”; pp. 361– 80 in Mechanical Properties and Deformation Behavior of Materials Having Ultrafine Microstructures. Edited by M. Nastasi, D. M. Parkin, and H. Gleiter. Kluwer Academic Publishers, Dordrecht, Netherlands, 1993. 5 M. J. Mayo, “Nanocrystalline Ceramics for Structural Applications: Processing and Properties”; pp. 361– 85 in Nanostructured Materials. Edited by G. M. Chow and N. I. Noskova. Kluwer Academic Publishers, Dordrecht, Netherlands, 1998. 6 J. Freim, J. McKittrick, J. Katz, and K. Scickafus, “Microwave Sintering of Nanocrystalline -Al2O3,” Nanostruct. Mater., 4 [4] 371– 85 (1994). 7 S. H. Risbud, S.-H. Shan, A. K. Mukherjee, M. J. Kim, J. S. Bow, and R. A. Holl, “Retention of Nanostructure in Aluminum Oxide by Very Rapid Sintering at 1150°C,” J. Mater. Res., 10 [2] 237–39 (1995). 8 R. S. Mishra, J. Schneider, J. F. Shackelford, and A. K. Mukherjee, “Plasma Activated Sintering of Nanocrystalline -Al2O3,” Nanostruct. Mater., 5 [5] 525– 44 (1995). 9 R. S. Mishra, A. K. Mukherjee, K. Yamazaki, and K. Shoda, “Effect of TiO2 Doping on Rapid Densification of Alumina by Plasma Activated Sintering,” J. Mater. Res., 11 [5] 1144 – 48 (1996). 10R. S. Mishra, C. E. Leshier, and A. K. Mukherjee, “High Pressure Consolidation of Nano–Nano Alumina Composites”; pp. 173–79 in Synthesis and Processing of Nanocrystalline Powder. Edited by D. L. Bourell. TMS, Warrendale, PA, 1996. 11R. S. Mishra, C. E. Lesher, and A. K. Mukherjee, “High Pressure Sintering of Nanocrystalline -Al2O3,” J. Am. Ceram. Soc., 79 [11] 2989 –92 (1996). 12S.-C. Liao, Y.-J. Chen, B. H. Kear, and W. E. Mayo, “High Pressure/Low Temperature Sintering of Nanocrystalline Alumina,” Nanostruct. Mater., 10 [6] 1063–79 (1998). 13P. F. Becher and G. C. Wei, “Toughening Behavior in SiC-Whisker-Reinforced Alumina,” J. Am. Ceram. Soc., 67 [12] C-267–C-269 (1984). 14G. C. Wei and P. F. Becher, “Development of SiC-Whisker-Reinforced Ceram￾ics,” Am. Ceram. Soc. Bull., 64 [2] 298 –304 (1985). Fig. 2. HRSEM micrographs of the as-received -Al2O3 nanopowders (a) before and (b) after high-energy ball milling. December 2004 Communications of the American Ceramic Society 2299