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 297Spark-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￾sintering (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 consol￾idated 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 tem￾peratures. 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 MPa
m1/2. 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 obtain￾ing 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 transfor￾mation 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 en￾hance sintering and reduce the time available for grain growth, have been used for processing of nanocrystalline -Al2O3. High￾pressure 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-field￾assisted sintering), that has much better control of the microstruc￾ture 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 MPa
m1/2. That is lower than some coarse-grained pure alumina.11 Therefore, re￾search 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 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
m1/2 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 SiCw￾reinforced 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 nano￾powder 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)