J. An. Cern Soc..87[1212297-20002004) ournal Spark-Plasma Sintering of Silicon Carbide Whiskers(SiCw) Reinforced Nanocrystalline alumina Guo-Dong Zhan, *Joshua D. Kuntz,* Ren-Guan Duan, and Amiya K Mukherjee* f Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 The combined effect of rapid sintering by spark-plasma have been used for processing of nanocrystalline y-Al,O,. High ring(SPS) technique and mechanical milling of y-AL2O pressure sintering(HPS)"-- has also been used for the sinterin illing (HEBM on the of nanocrystalline y-alumina powders. Among these techniques, microstructural development and mechanical properties of HPS seems to be the only way one can obtain fully dense stalline alumina matrix composites ened by 20 nanocrystalline y-AL, O, at the present time, but it is limited to vol% silicon carbide whiskers was investigated. SiCW/y-AL,O small and simple sample shape due to the dimensional constraints nopowders processed by hEBM can be successfully consol of the high-pressure requirement. A new processing technique idated to full density by sPs at a temperature as low as 1125C park plasma sintering(SPS; also referred to as electric-field and still retain a near-nanocrystalline matrix grain size(118 assisted sintering), that has much better control of the microstruc- nm). However, to densify the same nanopowder mixture to full ture and properties of materials than PAS, has been used for the density without the benefit of HEBM procedure, the require present study temperature for sintering was higher than 1200oC, where one The research on nanocrystalline ceramics shows that they are encountered excessive grain growth. X-ray diffraction (XRD not inherently tougher than their microcrystalline counterparts. For and scanning electron microscopy(SEM) results indicated that instance, the fracture toughness of fully dense nanocrystalline a-AL,O, of the starting powder but rather induced possible lower than some coarse-grained pure alumina. Therefore, re- residual stress that enhances the densification at lower tem- search on processing fully dense bulk ceramic nanocomposites that peratures. The SiCwHEBMy-AL,O3 nanocomposite with gra retain nanocrystalline matrix grain size(<100 nm)and improved ize of 118 nm has attractive mechanical properties, i. e, fracture toughness as well remains a challenging problem. It is Vickers hardness of 26.1 GPa and fracture toughness of 6.2 well-known that the incorporation of strong, small-diameter whis MPam/ ers into a ceramic matrix can improve the fracture toughness of the resulting composites. For example, the fracture toughness of nina is increased to 10 MPa m/ with the ad L. Introduction vol% SiCw 3-7 The mechanisms responsible for such whisker include crack deflection and both whisker brid ANOCERAMICs can exhibit superior properties over their coarse lycrystalline counterparts because of their nanoscaled grain whisker pullout within a zone immediately behind the crack tip. size and large volume fraction of grain boundaries. Densification However, all the research was concerned with the microcrystalline of bulk nanocrystalline ceramics, however, encounters obstacles in alumina matrix composites. In the present study, we report tl controlling grain growth while trying to achieve processing, microstructure, and mechanical properties of SiC, eI challenging task of fabricating nanocrystalline ceramics does not reinforced nanocrystalline alumina matrix composites by SPs depend on one's ability to obtain a alline powder but rather on one's ability to manipulate that na wder into I. Experimental Procedure size. It has been pointed out that grain coarsening becoms. dense ceramic compo ith a nanocrystal Advanced Refractory Technologies, Inc, NY, supplied silicon particularly severe in nanocrystalline materials when densities are carbide whiskers used in the present study. Most of the SiCw have more than 90% of theoretical density. The difficulties in obtain- a diameter 0.1-3 um and aspect ratios of 5-100 ing high-density alumina nanocrystalline ceramics may be related The gas condensation synthesized y-Al2O3 with an average to the fact that the sintering temperatures during pressureless or particle size of 32 nm was obtained from Nanophase Technologies low-pressure sintering are higher than the y- to a-AlO3 Corp(Darien, IL). In the present study, we select 20 vol% SiC mation temperature(-12000C). Therefore, sinteinogA as a toughening phase in the nanocomposites. This is an optimun that require lower sintering temperatures and shorter ontent for coarse-grained alumina matrix composite..Ther the ideal choice. Fast densification techniques, such as microwave are two routes to prepare the starting powders. The first is that sintering and plasma-activated sintering(PAS)- that can en- SiCw at 20 vol% were mixed for 24 h with the as-received y-Al2O3 hance sintering and reduce the time available for grain growt nanopowder in ethanol using zirconia ball media(20 vol% iCw/y-Al,O3). The other is that the as-received y-Al2O3 nane powder was first mechanically milled for 24 h using HEBM in a Spex 8000 mixer mill in a wC vial with I wt%o polyvinyl alcohol w. G. Fahrenholtz-contributing editor (PVA, J. T. Baker, a division of Mallinckrodt Baker, Inc. Phillipsburg, N), a dry milling agent, to prevent severe powder agglomeration Milling was followed by a vacuum heat treatment at 350C, for the removal of the PVA. Then the high-energy 3C的m器1和 the ball-milled Y-AL2O3 nanopowder was mixed with20v01%sCim hanol using zirconia ball media(20 vol% SiCwHEBMY-AL2O3). whom correspondence should be addressed. e-mail: akmukherjeeG@ The advanced densification technique used in the presen is SPS. It is a comparatively low-pressure sintering method bas 297
Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline Alumina Guo-Dong Zhan,* Joshua D. Kuntz,* Ren-Guan Duan,* and Amiya K. Mukherjee* ,† Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 The combined effect of rapid sintering by spark-plasmasintering (SPS) technique and mechanical milling of -Al2O3 nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiCw/-Al2O3 nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of -Al2O3 to -Al2O3 of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiCw/HEBM-Al2O3 nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPam1/2. I. Introduction NANOCERAMICS can exhibit superior properties over their coarser polycrystalline counterparts because of their nanoscaled grain size and large volume fraction of grain boundaries. Densification of bulk nanocrystalline ceramics, however, encounters obstacles in controlling grain growth while trying to achieve sintering.1–12 The challenging task of fabricating nanocrystalline ceramics does not depend on one’s ability to obtain a nanocrystalline powder per se, but rather on one’s ability to manipulate that nanopowder into a dense ceramic composite with a nanocrystalline (100 nm) grain size.4 It has been pointed out that grain coarsening becomes particularly severe in nanocrystalline materials when densities are more than 90% of theoretical density.11 The difficulties in obtaining high-density alumina nanocrystalline ceramics may be related to the fact that the sintering temperatures during pressureless or low-pressure sintering are higher than the - to -Al2O3 transformation temperature (1200°C). Therefore, sintering processes that require lower sintering temperatures and shorter duration are the ideal choice. Fast densification techniques, such as microwave sintering6 and plasma-activated sintering (PAS)7–9 that can enhance sintering and reduce the time available for grain growth, have been used for processing of nanocrystalline -Al2O3. Highpressure sintering (HPS)10 –12 has also been used for the sintering of nanocrystalline -alumina powders. Among these techniques, HPS seems to be the only way one can obtain fully dense nanocrystalline -Al2O3 at the present time, but it is limited to small and simple sample shape due to the dimensional constraints of the high-pressure requirement. A new processing technique, spark plasma sintering (SPS; also referred to as electric-fieldassisted sintering), that has much better control of the microstructure and properties of materials than PAS, has been used for the present study.9 The research on nanocrystalline ceramics shows that they are not inherently tougher than their microcrystalline counterparts. For instance, the fracture toughness of fully dense nanocrystalline alumina with mean grain size of 152 nm is 3.0 MPam1/2. That is lower than some coarse-grained pure alumina.11 Therefore, research on processing fully dense bulk ceramic nanocomposites that retain nanocrystalline matrix grain size (100 nm) and improved fracture toughness as well remains a challenging problem. It is well-known that the incorporation of strong, small-diameter whiskers into a ceramic matrix can improve the fracture toughness of the resulting composites. For example, the fracture toughness of alumina is increased to 10 MPam1/2 with the addition of 20 vol% SiCw. 13–17 The mechanisms responsible for such whisker toughening include crack deflection and both whisker bridging and whisker pullout within a zone immediately behind the crack tip.16 However, all the research was concerned with the microcrystalline alumina matrix composites. In the present study, we report the processing, microstructure, and mechanical properties of SiCwreinforced nanocrystalline alumina matrix composites by SPS. II. Experimental Procedure Advanced Refractory Technologies, Inc., NY, supplied silicon carbide whiskers used in the present study. Most of the SiCw have a diameter 0.1–3 m and aspect ratios of 5–100. The gas condensation synthesized -Al2O3 with an average particle size of 32 nm was obtained from Nanophase Technologies Corp. (Darien, IL). In the present study, we select 20 vol% SiCw as a toughening phase in the nanocomposites. This is an optimum content for coarse-grained alumina matrix composite.15,16 There are two routes to prepare the starting powders. The first is that SiCw at 20 vol% were mixed for 24 h with the as-received -Al2O3 nanopowder in ethanol using zirconia ball media (20 vol% SiCw/-Al2O3). The other is that the as-received -Al2O3 nanopowder was first mechanically milled for 24 h using HEBM in a Spex 8000 mixer mill in a WC vial with 1 wt% polyvinyl alcohol (PVA, J. T. Baker, a division of Mallinckrodt Baker, Inc., Phillipsburg, NJ), a dry milling agent, to prevent severe powder agglomeration. Milling was followed by a vacuum heat treatment at 350°C, for the removal of the PVA. Then the high-energy ball-milled -Al2O3 nanopowder was mixed with 20 vol% SiCw in ethanol using zirconia ball media (20 vol% SiCw/HEBM-Al2O3). The advanced densification technique used in the present study is SPS. It is a comparatively low-pressure sintering method based W. G. Fahrenholtz—contributing editor Manuscript No. 10174. Received May 1, 2003; approved June 4, 2004. This investigation was supported by a grant (No. G-DAAD 19-00-1-0185) from the U.S. Army Research Office with Dr. William Mullins as the Program Manager. *Member, American Ceramic Society. † Author to whom correspondence should be addressed. e-mail: akmukherjee@ ucdavis.edu. 2297 journal J. Am. Ceram. Soc., 87 [12] 2297–2300 (2004)
2298 Communications of the American Ceramic Societ Vol 87. No. 12 Table L. Physical and Mechanical Properties of 20 vol% SiCWAL,O3 Nanocomposites Produced by Spark-Plasma Sintering and Some reference materials conditions raIn size Relative density Material Hardness( GPa) Pure alo SPS1150/3 20.3±0.25 3.30 +0.14 Present study 20 vol% SiC/y-AL,O 1000 100 7.10 + 0.38 Present stud 0 vol% SICwHEBMY-ALO SPS1LO0 12.0±0.32 8.66±0.80 Present stud 0 vol% SICwHEBMY-AL,O3 sPS1125/3 6.1±0.33 6.17 +0.81 Present stud SPS1150/3 64±0.29 20 vol% SICwAL,O 6.00 + 0.72 Present study HP1850 13-16 100 17 Theoretical density for 20 vol% SiCw/Al2O3 is 3.83 g/cm; HEBM, high-energy ball milling: HP, hot-pressing. on high-temperature plasma(spa momentarily gener- grains with dendritic protrusions surrounded by continuous pore ated in the gaps between powder electrical discharge channels. The resultant vermicular microstructure requires much sintering system(Sumitomo Coal Mining Co, Ltd, Japan). SPs interesting to note that the so-called vermicular microstructure did could rapidly consolidate powders to near theoretical density not show up in our nanocomposites produced by SPS. Table I through the action of a rapid heating rate, pressure application, and shows the processing conditions for 20 vol% SICw/, nano- powder surface cleaning Sintering was conducted in vacuo. After composites by SPS at different sintering temperatures for 3 min. applying the given pressure(63 MPa), samples were heated to The pure alumina nanopowder could be consolidated by SPs at 600C in 2 min and then ramped to the desired sintering temper 1 150C for 3 min to get full density. The average grain size of th atures for 3 min at a heating rate of 500C/min. The temperature pure alumina is 349 nm. Mechanical properties in terms of Vickers was monitored with an optical pyrometer that was focused on the hardness and toughness for this pure alumina are 20.3 GPa and 3.3 non-through"hole (0.5-mm diameter and 2-mm depth) in the MPam respectively. This value is a little higher than that for aplite die the 152-nm grain size alumina(3.03 MPam 2)that was consol The final densities of the sintered compacts were determined by dated by high-pressure sintering. Moreover, the present result the Archimedes' method with deionized water as the immersion also highlight the necessity of mechanical milling via HEBM for The theoretical densities of th mens were calcu- the starting alumina powder to obtain a nanocrystalline grain size ccording to the rule of mixtures (3.21 g/cm for the Sic For the composites, using the as-received nanopowders, without bser- high-energy ball milling, only 80% of theoretical density(TD) vation was conducted using a FEl XL30-SFEG high-resolution could be achieved at 1150C. To achieve a full density, the scanning electron microscope with a resolution better than 2 nm sintering temperature had to be increased to 1250C. However, the and magnification over 600000X. Grain sizes were calculated tering temperature can be decreased to 1125C to obtain 99.8%0 rom high-resolution SEM images by lineal analysis. The phase TD composite when the as-received y-Al2O3 nanopowder was present were determined by X-ray diffraction(XRD)using CuKo processed by HEBM. Higher sintering temperatures can be used to radiation. Bulk specimens were sectioned and mounted in epoxy get fully dense materials, but this leads to significant amounts of and then polished though 0. 25-Hum diamond paste. Indentation grain growth. The mean grain sizes are almost 1 um when the tests were performed on a Wilson Tukon hardness tester with a sintering temperatures are 1200oC or higher for the composites diamond Vickers indenter. The indentation parameters for fracture without HEBM. For the SiCwHEBMy-AL2O3 nanocomposi toughness (Kid) measurements were a 2.5-kg load with a dwell processed by HEBM and at 1100C for 3 min, the density is time of 15 s. The following equation, proposed by Anstis et aL, 8 increased to 95% TD, suggesting that heBm is beneficial for the was used for the calculation of the toughness 0.016 where E, Hy, P, and c represent Youngs modulus, Vickers hardness, the applied indentation load, and the half-length of the II Results and discussion In comparison to conventional processing methods, SPS offers a very attractive way to obtain nanocrystalline ceramics and After high-energy ball-mi ling nanocomposites. Moreover, it was found the heating rate is a critical factor for obtaining fully dense nanocrystalline composites by SPS. For example, we have found in our previous study that a heating rate of 500%C/min is much more effective in consolidating nanocrystalline powder to dense nanocrystalline grain size than a heating rate of 200%C/min when all the other parameters are the same. Therefore, a heating rate of 500 C/min was used in the present study. Depending on the sintering temperatures, Y-AL2O3 a. The phase transformation to a-AL,O, occurs by nucleation and growth, where the reconstructive transformation from B-to 20() a-AL,O, is accompanied by a reduction of the specific volume. A w intrinsic nucleation density results in larger spacing between Fig. 1. XRD profiles of the as-received y-Al,O, nanopowders(a) before nucleation events and the formation of micrometer-scale a-AlO and() after high-energy ball mill
on high-temperature plasma (spark plasma) momentarily generated in the gaps between powder materials by electrical discharge during on– off DC switching in a Dr. Sinter 1050 spark plasma sintering system (Sumitomo Coal Mining Co., Ltd., Japan). SPS could rapidly consolidate powders to near theoretical density through the action of a rapid heating rate, pressure application, and powder surface cleaning. Sintering was conducted in vacuo. After applying the given pressure (63 MPa), samples were heated to 600°C in 2 min and then ramped to the desired sintering temperatures for 3 min at a heating rate of 500°C/min. The temperature was monitored with an optical pyrometer that was focused on the “non-through” hole (0.5-mm diameter and 2-mm depth) in the graphite die. The final densities of the sintered compacts were determined by the Archimedes’ method with deionized water as the immersion medium. The theoretical densities of the specimens were calculated according to the rule of mixtures (3.21 g/cm3 for the SiC whisker and 3.98 g/cm3 for alumina). The microstructural observation was conducted using a FEI XL30-SFEG high-resolution scanning electron microscope with a resolution better than 2 nm and magnification over 600,000. Grain sizes were calculated from high-resolution SEM images by lineal analysis. The phases present were determined by X-ray diffraction (XRD) using CuK radiation. Bulk specimens were sectioned and mounted in epoxy and then polished though 0.25-m diamond paste. Indentation tests were performed on a Wilson Tukon hardness tester with a diamond Vickers indenter. The indentation parameters for fracture toughness (KIC) measurements were a 2.5-kg load with a dwell time of 15 s. The following equation, proposed by Anstis et al., 18 was used for the calculation of the toughness: KIC 0.016 E H 1/ 2 P c3/ 2 (1) where E, Hv, P, and c represent Young’s modulus, Vickers hardness, the applied indentation load, and the half-length of the radial crack, respectively. III. Results and Discussion In comparison to conventional processing methods, SPS offers a very attractive way to obtain nanocrystalline ceramics and nanocomposites. Moreover, it was found the heating rate is a critical factor for obtaining fully dense nanocrystalline composites by SPS. For example, we have found in our previous study that a heating rate of 500°C/min is much more effective in consolidating nanocrystalline powder to dense nanocrystalline grain size than a heating rate of 200°C/min when all the other parameters are the same.19 Therefore, a heating rate of 500°C/min was used in the present study. Depending on the sintering temperatures, -Al2O3 undergoes the following polymorphic transformations: 3 3 3 . The phase transformation to -Al2O3 occurs by nucleation and growth, where the reconstructive transformation from - to -Al2O3 is accompanied by a reduction of the specific volume. A low intrinsic nucleation density results in larger spacing between nucleation events and the formation of micrometer-scale -Al2O3 grains with dendritic protrusions surrounded by continuous pore channels.20 The resultant vermicular microstructure requires much higher sintering temperatures to achieve full density. It is very interesting to note that the so-called vermicular microstructure did not show up in our nanocomposites produced by SPS. Table I shows the processing conditions for 20 vol% SiCw/Al2O3 nanocomposites by SPS at different sintering temperatures for 3 min. The pure alumina nanopowder could be consolidated by SPS at 1150°C for 3 min to get full density. The average grain size of the pure alumina is 349 nm. Mechanical properties in terms of Vickers hardness and toughness for this pure alumina are 20.3 GPa and 3.3 MPam1/2, respectively. This value is a little higher than that for the 152-nm grain size alumina (3.03 MPam1/2) that was consolidated by high-pressure sintering.11 Moreover, the present results also highlight the necessity of mechanical milling via HEBM for the starting alumina powder to obtain a nanocrystalline grain size. For the composites, using the as-received nanopowders, without high-energy ball milling, only 80% of theoretical density (TD) could be achieved at 1150°C. To achieve a full density, the sintering temperature had to be increased to 1250°C. However, the sintering temperature can be decreased to 1125°C to obtain 99.8% TD composite when the as-received -Al2O3 nanopowder was processed by HEBM. Higher sintering temperatures can be used to get fully dense materials, but this leads to significant amounts of grain growth. The mean grain sizes are almost 1 m when the sintering temperatures are 1200°C or higher for the composites without HEBM. For the SiCw/HEBM-Al2O3 nanocomposite, processed by HEBM and at 1100°C for 3 min, the density is increased to 95% TD, suggesting that HEBM is beneficial for the Fig. 1. XRD profiles of the as-received -Al2O3 nanopowders (a) before and (b) after high-energy ball milling. Table I. Physical and Mechanical Properties of 20 vol% SiCw/Al2O3 Nanocomposites Produced by Spark-Plasma Sintering and Some Reference Materials Material Sintering conditions (°C/min) Grain size (nm) Relative density† (%) Hardness (GPa) Toughness (MPam1/2) References Pure Al2O3 SPS1150/3 349 100 20.3 0.25 3.30 0.14 Present study 20 vol% SiCw/-Al2O3 SPS1200/3 900 99.8 24.2 0.50 6.64 0.12 Present study 20 vol% SiCw/-Al2O3 SPS1250/3 1000 100 23.1 0.36 7.10 0.38 Present study 20 vol% SiCw/HEBM-Al2O3 SPS1100/3 97 94.5 12.0 0.32 8.66 0.80 Present study 20 vol% SiCw/HEBM-Al2O3 SPS1125/3 118 99.8 26.1 0.33 6.17 0.81 Present study 20 vol% SiCw/HEBM-Al2O3 SPS1150/3 146 100 26.4 0.29 6.00 0.72 Present study 20 vol% SiCw/Al2O3 HP1850 4000 100 – 10 13–16 33 vol% SiCw/Al2O3 – 5000 100 – 6 17 † Theoretical density for 20 vol% SiCw/Al2O3 is 3.83 g/cm3 ; HEBM, high-energy ball milling; HP, hot-pressing. 2298 Communications of the American Ceramic Society Vol. 87, No. 12
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(1985
densification 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 preparation by HEBM can result in truly nanocrystalline matrix composites. HEBM is known to enhance the solid-state phase transformation. In fact, mechanical attrition (high-energy ball milling) as a method has been widely used for preparation of nanocrystalline 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 temperatures) can result in small regions of the -Al2O3 powder transforming 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 observation. 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 nanopowder. 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 transformation 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 comparable 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 alumina 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 nanocomposite. The 20 vol% SiCw/HEBM-Al2O3 nanocomposite with grain size of 118 nm has superior mechanical properties with a fracture toughness of 6.2 MPam1/2 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. 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