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 millon high-temperature plasma (spark plasma) momentarily gener￾ated 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 temper￾atures 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 calcu￾lated according to the rule of mixtures (3.21 g/cm3 for the SiC whisker and 3.98 g/cm3 for alumina). The microstructural obser￾vation 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 nano￾composites 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 MPa
m1/2, respectively. This value is a little higher than that for the 152-nm grain size alumina (3.03 MPa
m1/2) that was consoli￾dated 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 (MPa
m1/2) 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