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 2297journal 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-plasma￾sintering (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 consol￾idated 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 tem￾peratures. 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 (<l()0 nm) grain si/e."* Ii has been pointed out that grain coarsening becomes particularly severe in nanocrystalline materials when densities are more than 9U9r of iheoreiical density.'' The difficulties in obtain￾ing high-density alumina nanocrystallinc ceramics may be related to Ihe fact that ihe sintering temperatures during pressureless or low-pressure sintering are blgber than the y- to a-AUO, transfor￾mation temperature (~1200"C). Therefore, sintering processes tbat require lower sintering temperatures and shorter duration are [be ideal choice. Fasi densification techniques, sucb as microwave sintering*' and plasma-activated sintering (PAS)^"'^ thai can en￾hance sintering and reduce the time available for grain growth. W. G. Fahrenhottz—coniributin^ editor Miinuscrip[ No. 10174. Received May 1. 2003: approved June 4, 2(W4. Tliis invcsligaiion was supported by a granHNo, G-DAAD 19-(X)-1-0185) rrom the U.S. Arniy Research OITice wiih Dr. William Mullins as the Program Manager. 'Member. American Ceramic Swiety. 'Auihur to whiim correspondence should be addressed, e-mail; akmukherjee® ucdavis.edu. have been used for processing of nanocrystalline y-AUOj. High￾pressure sintering (HPS)'""'" has also been used for the sintering of nanocrystalline -y-alumina powders. Among these techniques, HPS seems to be the only way one can obtain fully dense nanocrystalline 7-AU0-, 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 eleciric-t1eid￾assisted sintering), that bas much betler control of tbe microstruc￾ture and properties of materials than PAS. bas been used for the present study.'' The research on nanocrystalline ceramics shows tbat they are not inherently tougher ihan their microcr>'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, re￾search 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 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 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, nano￾powder 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