Am. Ceron Soc87|212297-230(204 Journal 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-plasma have been used for processing of nanocrystalline y-AL,O3. High sintering(SPS)technique and mechanical milling of y-Al2O3 pressure sintering(HPS)-has also been used for the sintering nanopowder via high-energy ball milling(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 nanocrystalline alumina matrix composites toughened by 20 nanocrystalline y-Al,O, at the present time, but it is limited to ol% silicon carbide whiskers was investigated. SiCyy-AL,O3 nanopowders processed by HEBM can be successfully consol of the high-pressure requirement, A new processing technique, dated to full density by SPS at a temperature as low as 1125C spark 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 required present study." temperature for sintering was higher than 1200 C, 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 HEBM did not lead to the transformation of y-Al O, to alumina with mean grain size of 152 nm is 3.0 MPam".That is c-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 tures. The SiCwHEBMy-Al, O, nanocomposite with grai retain nanocrystalline matrix grain size(<100 nm)and improved size 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 kers into a ceramic matrix can improve the fracture toughness of the resulting composites. For example, the fracture toughness of alumina is increased to -10 MPam 2 with the addition of 20 vol% SiCw. The mechanisms responsible for such whisker toughening include crack deflection and both whisker bridging and ANOCERAMICS can exhibit superior properties over their coarser whisker pullout within a zone immediately behind the crack tip controlling grain growth while trying to achieve sintering. The processing, microstructure, and mechanical properties of SiC challenging task of fabricating nanocrystalline ceramics does not reinforced nanocrystalline alumina matrix composites by SPS depend on one's ability to obtain a nanocrystalline powder per se, but rather on one's ability to manipulate that nanopowder into I. Experimental Procedure dense ceramic composite with a nanocrystalline(<100 nm) grain size.It has been pointed out that grain coarsening becomes 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-Al,O, 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-Al,O, transfor Corp. (Darien, IL). In the present study, we select 20 vol% SiC mation temperature(-1200C). Therefore, sintering processes as a toughening phase in the nanocomposites. This is an optimum that require lower sintering temperatures and shorter duration are content for coarse-grained alumina matrix composite, . There 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-AL, O hance sintering and reduce the time available for grain growth, nanopowder in ethanol using zirconia ball media (20 vol% SiCwy-Al2O3). The other is that the as-received y-AL,O,nano- powder was first mechanically milled for 24 h using HEBM in a Spex 8000 mixer mill in a wC vial with I wt% polyvinyl alcoho w.G. Fahrenholtz-contributing editor (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 Manuscript No 10174 Received May It (N&. GPDAAD19-00-1-0185)from the at 350C, for the removal of the PVA. Then the high-energy U.S. Army Research Office with Dr. William mullins as the program manager ball-milled y-Al,O, nanopowder was mixed with 20 vol% SiCw in ethanol using zirconia ball media(20 vol% SICWHEBMY-AL-O3 Author to whom correspondence should be addressed. e-mail: akmukherjeeer The advanced densification technique used in the present study is SPS. It is a comparatively low-pressure sintering method based 2297
journal Spark-Plasma Sintering of Silicon Carbide Whiskers (SiC^) 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 93616 The combined effect of rapid sintering by spark-plasmasintering (SPS) tecbnique and mechanical milling of 7-AI2OJ nanopowder via high-energy ball milling (HEBM) on tbe microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide wbiskers was investigated. SiC«y7-Al2O3 nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1 HS^C and still retain a near-iianocrystalline 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 exce.ssive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of Y-AUO^ to a-AUO, of the starting powder but ratber induced possible residual stress that enhances the densification at lower temperatures. The SiC^/HEBMY-AIjO, 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 I. Introduction N ANOCER.^Mlcs Can exhibit superior propedies over their coarser polycrystalline counterparts because ot" (heir nanoscaled grain size and large volume fraction of grain boundaries. Densitication of bulk nanocrysialline ceramics, bowever, encounters obstacles in controlling grain growth while trying to achieve .sintering.'"'^ The challenging task of Tabricating nanocrystalline ceramics does not depend on one's ability to obtain a nanocrysialline powder per se. but rather on one's ability to manipulate that nanopowder into n dense ceramic composite with a nanocrystalline ('stalline counterparts. For instance, the fracture toughness of fully dense nanocrystalline alumina with mean grain size of I.'i2 nm is 3.0 MPa-ni"". That is lower than some coarse-grained pure alumina." Therefore, research on processing fully dense bulk ceramic nanocomposites that retain nanocrystalline matrix grain size (<l()0 nm) and improved fracture toughness as well remains a challenging problem. It is well-known thai the incorporation of strong, small-diameler 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 MPa*m""^ with Ihc addition of 20 vol^7c SiC^..'"' '^ The mechanisms responsible for such whisker toughening include crack deflection and both whisker bridging and whisker pullout within a zone immediately behind ihe crack tip,"' However, all the research was concerned wiih the microcrystalHne alumina matrix composites. In Ihe present sludy, we report the processing, microstructure. and mechanical properties of SiC^^- reinforced nanocrystalline alumina matrix composites by SPS. II. Experimental Procedure Advanced Refractory Technologies, Inc.. NY. supplied silicon carbide wbiskers used in tbe present study. Most of the SiC^^ have a diameter 0.1-3 [im and aspect ratios of 5-100. The gas condensation synthesized 7-AI;O, with an average particle size of 32 nm was obtained from Nanophasc Technologies Corp. (Darien. IL). In the present study, we select 20 vo]% S\C^ as a toughening phase in ihe nanocomposites. This is an optimum content for coarse-grained alumina matrix composite.'"^"' There are two routes to prepare ihe starting powders. The first is that SiC^ at 20 vol% were mixed for 24 h with the as-received 7-AI2O3 nanopowder in ethanol using zirconia ball media (20 vol% SiC^y-Y-AIjO^). The other is that the as-received -y-AUO, nanopowder was first mechanically milled for 24 h using HEBM in a Spex SOOO mixer mill in a WC vial wiih 1 wt% polyvinyl alcohol (PVA. J. T. Baker, a division of Mallinckrodt Baker, Inc., Phillipsburg. NJt, a dry milling ageni. to prevent severe powder agglomeration. Milling was followed by a vacuum heat treatment at 35()°C. for the removal of the PVA. Then the high-energy ball-milled -y-AUO, nanopowder was mixed with 20 vol% SiC^^, in ethanol using /irconia bail media (20 \o\'/r SiC^^/HEBM^-AUO-,), The advanced densitication technique used in ihe present sludy is SPS. It is a comparatively low-pressure sintering method based 2297
Communications of the American Ceramic Sociery Vol. 87. No. 12 Table L. Physical and Mechanical Properties of 20 vol SiC AL,O, Nanocomposites Produced by Spark- Plasma Sintering and ome Reference Materials Grain size Relative density Material Hardness (GPa) Pam/ References Pure Alo SPS1150/3 349 100 20.3±0.25 3.30±0.14 Present stu 20 vol% SiCwy-AL,O sPS12003 900 .2±0.50 664±0.12 Present stu 20 vol% SiCw SPS12503 1000 23.1±0.36 7.10=0.38 Present study 20 vol% SiC.HEBMy-Al,O 120±0.3 8.66±0.8 Present study 20 vol% SICW/,O SPS112 26.1±0.33 6.17*0.81 Present study 20 vol% SiCwHEBMY-Al,O, SPS11503 26.4±0.29 600±0.72 Present study 20 vol% SICwAL,O HP850 4000 100 10 13-16 33 vol% SICwAL,O ~5000 100 Theoretical density for 20 vol% SiC/Al,O, is 3.83 g/cm: HEBM, high-energy ball milling: HP, hot- on high-temperature plasma(spark plasma)momentarily gener. grains with dendritic protrusions surrounded by continuous pore ated in the gaps between powder materials by electrical discharge channels. The resultant vermicular microstructure requires much during on-off DC switching in a Dr. Sinter 1050 spark plasma higher sintering temperatures to achieve full density. It is very sintering system (Sumitomo Coal Mining Co, Ltd, Japan). SPS interesting to note that the so-called vermicular microstructure did throug h the action of a rapid heating ate presure application, aind sht ws the pocessing coditions for 2o volce SiC JA, o 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 600 C in 2 min and then ramped to the desired sintering temper- 1150 C for 3 min to get full density. The average grain size of the atures for 3 min at a heating rate of 500 C/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 graphite die. the 152-nm grain size alumina(3.03 MPam)that was consoli The final densities of the sintered compacts were determined by dated by high-pressure sintering. Moreover, the present results the Archimedes' method with deionized water as the immersion also highlight the necessity of mechanical milling via HEBM fo medium. The theoretical densities of the specimens were calcu- the starting alumina powder to obtain a nanocrystalline grain size lated according to the rule of mixtures(3. 21 g/mfor the Sic For the composites, using the as-received nanopowders. without whisker and 3.98 g/mfor alumina). The microstructural obser 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 600,000X. Grain sizes were calculated sintering temperature can be decreased to 1125C to obtain 99. 8% from high-resolution SEM images by lineal analysis. The phases TD composite when the as-received y-Al,O, 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-uum diamond paste. Indentation grain growth. The mean grain sizes are almost I um when the tests were performed on a Wilson Tukon hardness tester with sintering temperatures are 1200 C or higher for the composites diamond Vickers indenter. The indentation parameters for fracture without HEBM. For the SiCw/,O, nanocomposite, toughness(Kic) 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. s increased to 95%TD, suggesting that HEBM is beneficial for the was used for the calculation of the toughness K1c=0.01 (a)Before high-energy ball-milling where E. Hy. P, and c represent Youngs modulus, Vickers hardness, the applied indentation load, and the half-length of the radial crack, respectively. Il. Results and Discussion In comparison to conventional processing methods, SPS offers ery attractive way to obtain nanocrystalline ceramics and (b) After high-energy ball-milling 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 heating rate of 200C/min when all the other parameters are the same.Therefore, a heating rate of 500 C/min was used in the undergoes the following polymorphic transformations: y-8=0 20 50 -a. The phase transformation to a-Al,O, occurs by nucleation and growth, where the reconstructive transformation from e-to 20() a-AL,O, is accompanied by a reduction of the specific volume. A low 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-Al,O, and(b) after high-energy ball milling
2298 Communications of the American Ceramic Society Vol. 87, No. 12 Table I. Physical and Mechanical Properties of 20 vol% SiC^yAljO, Nanoconiposites Produced by Spark-Plasma Sintering and Sume Reference Materials Sjmeririg conditions rCVmiii) Gram si/e (nm) Relatiio dcnsiiy' (MPa-m"-l Pure AI.O, 20 vol% Si' 20 vol% Sii 20 vol% Si' 20 vol% Si' 20 vol% Sii 20 vol% Si' 33 vol% SPS 1150/3 SPS 1200/3 SPS 1250/3 SPS 1100/3 SPS 1125/3 SPS 1150/3 HP 1850 _ 349 900 1000 97 118 146 -4000 -5000 100 99.8 100 94.5 99.8 HX) 100 100 20.3 ± 0.25 24.2 ± 0.50 23.1 ±0.36 12.0 ± 0.32 26.1 ±0.33 26.4 ± 0.29 — _ 3.30 ±0.14 6.64 ±0.12 7.10 ± 0.38 8.66 ± 0.80 6.17 ±0.81 6.00 ± 0.72 - 1 0 —6 Present study Present study Present study Present study Present study Present study 13-16 17 'ThtoretJcul density for 20 vol'3' SiC^^VAKO^, is 3.S3 g/cin'; HEBM. high-energy ball milling; HP. on high-temperature plasma (spark plasma) momenuirily generated in the gaps between powder materials by electrical discharge during tin-oft' DC .switching in a Dr. Sinter 1050 spark plasma sinlering 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. Alter applying the given pre.ssure (63 MPa), samples were heated to 60()°C in 2 min and then ramped to the desired sintering temperatures for 3 min at a heating rate of 500°Cymin. TTie temperature was monitored with an optical pyrometer thai was focused on the "non-through" hole (0.5-niin diameter and 2-mm depth) in the graphite die. The tinal densities ot" the sintered eompacts were determined hy the Archimedes' method wilh deionized water as the immersion medium. The theoretical densities of the specimens were calculated according to the rule of mixtures {3.21 g/cm"* for the SiC whisker and 3.98 g/cm' 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 magnillcation over 600,(H)0X. Grain sizes were calculated from high-resolution SEM images by (inoal analysis. The phases present were determined by X-ray diffraction (XRD) using CuKa radiation. Bulk specimens were sectioned and mounted in epoxy and then polished though 0.25-|j.m diamond paste. Indentation tests were performed on a Wilson Tukon hardness tester with a diamond Vickers indenter. The indentation parameters for fracture toughness (A'n-) measurements were a 2.5-kg load with a dwell time of 15 s. The following equation, proposed by Anstis er «/..'" was used for the calculation of the toughness: where E. H^.. P. and c represent Young's modulus. Vickers hardness, the applied indentation load, and the half-length of the radial crack, respectively. III. Kesult.s 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 lo dense nanocrystalline grain size than a heating rate of 200T/min when ail 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. 7-AI2O3 undergoes the following polymorphic transformations: 7 —* 5 —» 8 —* a. The phase transformation to «-AUO, occurs by nucleation and growth, where the reconstructive transformation from H- to «-AUO, is accompanied by a reduction of the specific volume. A low intrinsic nucleation density results in larger spacing between nucleation events and the formation ot" micrometer-scale a-Al20, grains with dendritic protrusions surrounded by continuous pore channels.-" 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 1 shows the processing conditions for 20 vol% SiC^^AUO, nanocomposites by SPS at different sintering temperatures for 3 min. The pure alumina nanopowder could be consolidated by SPS at 1150*^0 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 MPa-m"~. respectively. This value is a little higher than that for the 152-nm grain size alumina i3.()3 MPa*m""^) that was consolidated by high-pressure sintering." Moreover, the present results also highlight the necessity of mechanical milling via HHBM for the starting alumina powder to obtain a nanocrystalline grain size. For the composites, using the as-received nanopowders. without high-energy ball miiiing. only 80% of theoretical density (TD) could be achieved at II50°C. To achieve a full density, the sintering temperature had to be increased to 125O''C. However, the sintering temperature can be decreased to 1125°C to obtain 99.8% TD composite when the as received 7'AliO, 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 I |jLin when the sintering temperatures are HWC or higher for the composites without HEBM. For the SiC^HEBM7-Al,O, nanocomposite, processed by HEBM and at llOCC for 3 min. the density is increased to 95% TD. suggesting that HEBM is beneficial for the (a) Before high-energy ball-milling (b) After high-energy ball-milling 2QC) Fig, 1. XRD profiles of the as-received -y-AljOj nanopowders (a) before and (b) after high-energy ball milling
December 2004 Communications of the American Ceramic Society 2299 densification process. The mean grain size for the alumina matrix shear stresses during the milling. Both Panchula et al. and in the 20 vol% SiCwHEBMY-Al, O, nanocomposite by SPS at investigators in the present work used a similar setup for HEBM 1125C is as small as 1 18 nm. These results suggest that the However, in our case, the charge ratio(ball-to-powder ratio) was combined 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 com 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 he 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. During the HEBM Table I also shows the mechanical properties in terms of process, the impact energy (locally high pressures and tempera- Vickers hardness and fracture toughness for these 20 vol% tures)can result in small regions of the y-Al,O, powder trans- SiCwY-Al,O, and 20 vol% SiCwHEBMY-ALO, composites. It forming into c-Al,O, 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 a 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 transformation 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-Al-O, nano- it decreases to 23. 1 GPa when grain size is -I um. The superior powder. This broadening of the peak is not related to the hardness in the nanocrystalline nanocomposites is likely related to refinement of the starting powder as this can be ruled out since our the nanocrystalline matrix grain size, SEM observations indicated that there is no obvious difference in terms of the particle size(Fig. 2). This is consistent with another port that no refinement of the primary particle size took IV. Summary luring milling. The effect of HEBM on the phase transformation and 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% SiCwAL,O, 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% SiCwHEBMY-AL,O, nanocomposite with grain size of 118 nm has superior mechanical properties with a fracture 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 phase transformation that takes place during sintering. Reference P C Panda, J, Wang, and R. Raj. "Sinter-Forging Characteristics of Fine-Grained Zirconia, "J. Anm. Ceran. Soc. 71[12)C-507-C-509(1988) R. S. Averback. H. J. Hofler, H. Hahn, and J, C. Logas. "Sintering and Grain rowth in Nanocrystalline Ceramics," Nanostruct. Mater. 1. 173-78(1992) ' M. I. Mayo and D. C, Hague. "Porosity-Grain Gro onship in the Sintering of Nanocrystalline Ceramics, Nanostruct. Mater. 3. 43-53(1993). M. J. Mayo, "Superplasticity of Nanostructured Ceramics: es and (a) tation Behavior of Materials Having Ultrafine Microstructures. Edited by M. Nastasi, D. M. Parkin, and H. Gleiter, Kluwer Academic Publishers. Dordrecht, Netherlands, 1993. PM. J. Mayo, "Nanocrystalline Ceramics for Structural Applications: Processing and Properties": pp. 361-85 in Nanostructured Materials, Edited by G M. Chow and N. 1. Noskov ver Academic Publishers, Dordrecht, Netherlands, 1998 eU Freim, J. McKittrick, I. Katz, and K Scickafus, "Microwave Sintering of "s. H. Risbud. S. H. Shan. A. K. Mukherjee, M. 3. Kim. j 85(1994) and R. A. Holl R S. Mishra, J. Schneider, J. F. Shackelford, and A, K Mukherjee, "Plas Activated Sintering of Nanocrystalline y-Al,O, Nanestruct. Mater. 5 15] 525-44 R. S. Mishra, A. K. Mukherjee, K. Yamazaki, and K Shoda."Effect of Tio oping on Rapid Densification of Alumina by Plasma Activated Sintering. "J. s.1115144-48(1996) R.S. Mishra, C, E. Leshier, and A. K. Mukherjee. "High Pressure Consolidation Nano-Nano Alumina Composites": pp. 173-79 in Synthesis and Processing of 100nm Nanocrystalline Powder. Edited by D, L. Bourell. TMS, Warrendale, PA, 1996. IR. S. Mishra, C. E. Lesher, and A. K. Mukherjee, "High Pressure Sintering of J.Am. Ceran soc.,79l12989-92(1996) 2s.-C. Liao, Y.J. Chen, B. H, Kear, and W. E. Mayo. "High Pressure/Lot Temperature Sintering of Nanocrystalline Alumina, Nanostruct. Mater, 10 SP F Becher and G. C. Wei Alumina. / Am Ceram Soc, 6 Fig. 2. HRSEM micrographs of the as-received y-Al, O, nanopowders (a) G. C. Wei and P F Becher before and (b) after high-energy ball milling. ics,Anm. Ceram. Soc. Bull. 64 [21298-304(1985)
December 2004 Communications of the American Ceramic Society 2299 densification process. The mean grain size for the alumina matrix in the 20 vol% SiC^/HEBM7-AI.O3 nanocomposite by SPS at I125°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 taily 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 nant)crystaU line metallic materiids."' This method has also been applied to preparation of some ceramic nanopowders.'""' During the HEBM process, the impact energy (locally high pressures and temperatures) can result in small regions of the 7-Al,0, powder transforming into tt-AI->0, and in a decrease of the crystallite size during milling. For instance. Panchula and Ying" found that the phase transformation from 7 to a took place during the milling process. Our present results are quite different from their observation. As shown in Fig. I. 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."^"* However, it is very interesting to note that very broad 7 peaks are observed in the HEBM 7-AUO, nanopowder. This broadening of the peak is not related to the refinement of the staning 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."~ The effect of HEBM on the phase transformation and niicn>structural 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 Fig. 2. HRSEM iniurographs of the as-received "y-.A.liO, nanopowders (a) before and (b) after high-energy ball milling. shear stresses during the milling."'' Both Panchula el 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 I't al. applied a charge ratio as high as 5. It is entirely possible that, in the HEBM processing of powders, Panchula ci al. sticceeded in imp;irting far more internal energy to the powders whicb allowed them to overcome the phase transformation bamer. leading to the transformation of 7- to a-alumina during ball milling. Table I also shows the mechanical properties in terms of Vickers hardness and fracture toughness for these 20 vol% SiC^/7-AUO, and 20 vol% SiC^yHEBM7-Ai,0, 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.'*'"'^ 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 lo 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 —I ^.m. The superior hardness in the nanocrystalline nanoconiposites is likely related to the nanocrystalline matrix grain size.'' rV. 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% SiC^^/Al20, nanocomposite with 1 i8-nm matrix grain size was successfully consolidated by SPS at I125X for 3 min. A significant increase in terms of hardness and toughness has been achieved in the dense nanocomposite. The 20 vol'J'r SiC,^/HEBM7-Al,0, nanocomposite with grain size of 118 nm has superior mechanical properties with a fracture toughness of 6.2 MPa-m""^ 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 transibmiation that takes place during sintering. References 'P.C. Panda. J, Wong, nnd R. Raj."Sinter-ForgingCharacterisdcsof Fine-Graincd Ziri:onia;' / Am- Ceivin. Soc. 71 jl2| C-507-C-50y ) 19881. -R. S, Averhatk. H. J. Hotlfr. H. Hahn. anil J. C. L^gas. "Sintering and Grain (imwlh in Nanocrystalline Ceramics." Namisiruci, Muter.. 1. 173-78 (1992). ' M . J. Mayo and t). C. hlugue, "Poms ity-Gra in Growth Rclaiionship in the Sinlering of Nanocrystalline Ceramics." Nanustrua. Muter., 3. 4.1-5.1 (IW.1). "M. J. Mayo. "Superplasliciiy of Nanostructored Ceramics": pp. ?6l-80 in Mechiinical Properiies (md tJefrniiiiiion Behiiviiir <•) Miiieriiils tiaviiig Utirafine MicriKilriictures. EtiiceJ by M. Nastii.'ii. D. M. Parkin, and H. Gleitcr. Kluwcr Academic Publishers, ticrdrechi. Netherlands. 1993. ' M . J. Mayo. '"Nanocrystalline Ceramics for Slruciiiral Applications: Professing and Properties"; pp. .161-K."! m Noiiosmiviured Mmeriuis. Edited by G. M.Chow und N. 1. Noskova. Kluwer Academic Publishers, Dordrecht. Netherlands. IW8. ••J. Freim. J, McKittrick. J. Katz. and K. Stricfcafus, "Microwiivc Sintering of Nanocry stain ne 7-AliO,," Nunostriul. Muter.. 4 14| 371- IS5 (IW4), '.S, H. Rishud. .S.-H. Shan, A, K, MulLhcrjcc. M. J. Kim. J. S. Bow. and R. A. HoU. "Retention of Nanostructure m Ahnninum Oxide by Very Rapid Sintering at \\5iTCr J. Mater. /?<•,!,. 10 |2] 2yi-^^(\^5). ••R, S. Mishra. J, Schneider. J. F. Shackelford. and A, K, Mukhorjee. "Tlavma Activated Sintering of Nanocrystalline y-.W^Oy" Naiii'Stniri. Mater. 5 I5| 52S-44 (1995). ''R. S. Mishra, A, K. Mukheiice. K, Yamazaki. and K. Shoda. "Effccl of TiO; Doping on Rapid tJensififalion of Alumina by Plasma Aciivaied Sintering." J. Mater. Res., II [5| 1144-48 11996). '"R. S. Mishra, C. E. Leshier. and A. K. Mukherjee. "High Pressure Consolidation of Nann-Nant) Aluniitia Coniposiies"; pp. 17.V79 in Syi}lhesi.\ ami Procfssinf- of NamcrysmlUne Powder. Edited by D. t,. Bourell. TM.S. Warrendale. PA. 1996. " R . S. Mishra. C. E- Lvsher. and A. K, Mukherjee. "High Pressure Sintering of Nanocryslalline 7-AUO,." J. Am- Ceram. Soc 79 1111 29S9-92 (1996). '-S.-C, Liao. Y.-j" Chen, B. H, Kear. and W. E. Mayo. "High Pressure/Low Temperature Sinlering of Nanocryslaliine Alumina." Nannsirua. Maler-- 10 16| 1063-79 (1998). "P. F. Betrher and G- C. Wei. "Toughening Behavior in SiC-Whisker-Reinforced Alutnina." / Am. Cemm. Soc. 67 [12] C-267-C-2fi9 (1984). '•"G. C. Wei and P, F. Becher. "Developnwnt of SiC-Whisker-Reinforced Ceramics," Am. Ceram. So,:. Bull.. 64 (21 298^304 (1985)
00 Communications of the American Ceramic Sociery Vol. 87. No. 12 P. F. Becher, T. N. Tiegs, J. C. Ogle, and W.H G.D. Zhan, J. Kuntz, J. Wan, J. Garay, and A. K. Mukherjee,"A Novel Processing Route To Develop a Dense Na line Alumina Matrix(<I00 nm) ol. 7, Composites, Impact Statistics, and High-Temperature Phenomena. Edited by m.Soc,861200-202(2003 R. C, Bradt, D, P. H. Hasselman, A. G. Evans, and F. F. Lange, Plenum Press, New York, 1986. P. F. Becher, "Microstructural Design of Toughened Ceramics, "J, Am. Ceram. YA, Powders,". Eur. Ceram Soc. 22, 415 3 2- 200y-Sceded Nanocrystalline C.S. Nordahl and G L Messing, ""S C. C. Koch, "Synthesis of Nanostructured Materials by Mechanical Milling Soc,74[21255-69(19 G. R. Sarmafi-Nour and T. w. Coyle, "Temperature Dependence of Crack Wake H, Ferkel and R J. Hellming, "Effect of Nanopowder Deagglomeration on th ridging Stresses in A SiC- Whisker-Reinforced Alumina, "Acta Mater, 49, 355.3-63 Densities of Nanocrystalline Ceramic Green Bodies and Their Sintering Behavior. Nanastruct. Mater, 11 151617-22(1999). ING. R. Anstis, P, Chantikul, B. R. Lawn, and D. B. Marshall."A Critical M. L Panchula and J. Y. Ying,"Mechanical Synthesis of Nanocrystalline Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct a-Al,O, Seeds for Enhanced Transformation Kinetics, " Nanostruct. Mater, 9LI Crack Measurement. "J Am Ceram. Soc.. 64[91533-38(1981) 161-64(1997)
2300 Communications of the American Ceramic Society Vol. 87, No. 12 ''p. F. Btchcr. T. N. Tiegs. J, C. Ogle, and W. H. Warwick. •Toughening of Ceramics by Whisker Reinforcement": pp. 61-73 in Fracture Mechanics of Ceramics. Vol. 7. Compimies. Impua Staiistks. and High-Temperature Phenomena. Edited by R. C. BruJl. D. P. H, Hasselman, A. G. Evans, and F. F. Langc. Plenum Press. New York, 1986. '"P. F. Becher, "Microstruciural Design of Toughened Ceramics." / ,4m. Ceram. .SW., 74 [21 255-69 (IWh, "G. R, Smrafi-Nour and T, W. Coyte. •Temperuiiire Depentlence of Crack Wake Bridging Stresses in A SiC-Whisker-Reinforced A[\im\aa," Aciii Mater.. A9. 3553-63 (2001). '"G. R. Anstis. P. Chaniikul. B. R. l^awn, and D. B. Marshall. "A Critical Evaluation of Indentation Techniques (or Measuring Fracture Toughness: I, Direci Coick Measuremenu" J. Am. Ceram. Sac. 64 [91 533-38(1981). '"G.-D. Zhiin. J. Kunt^, J. Wrni. J, Garay. and A- K. Mukherjee. "A Novel Processing Rouie To Develop a Dense Nanocr)'stalline Alumina Matrix l<l(Hf nm) Nunocompo^ite Maierial," / Am. Cerum. Soi.. 86 111 2()(l-2()2 (2003), '"C. S. Nordahl and G. L. Messing. '".Sinluring of u - A UO,-Seeded Nanocrystalline 7-AI,0, Powders." / Btr. Ceram. Soc. 22, 415-22 (2(ii)2l. •^'C. C. Koch. "Synthesis of Nanostnictured Materials hy Mechanical Milling: Problems and Opportunities," Nano.iiriui. Maler.. 9 |1-81 13-22 H')^)l). '~H. Ferkel and R. J. Heliming, "Effect of Nanopowder Dcagglomeration on the Densities of Niinofrysialline Ceramic Green Bodies and Their Sintering Behavior," Namstruci. Mater.. II [S] 617-22 (IW9). ^^M. L. Panchula and J. Y. Ying. "Mechanical Synthesis of Nanocrystalline Q-AUO, Seeds lor Enbanced Transfonnation Kinetics." Mi'i(i,vm«7. Mater.. 9 [1-8] 161-64(1997). D
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