Journal J, Am Ceran,So,81142-46(2004 Spark-Plasma-Sintering Consolidation of SiC-Whisker-Reinforced Mullite Composites Zhengren Huang, Zhijian Shen, * fF Luwei Lin, Mats Nygren, t and Dongliang Jiang Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People's Republic of China Spark plasma sintering was applied to consolidate a series of provide the user with a cost-competitive production process for SiC-whisker-reinforced mullite-matrix composites containing whisker-reinforced composites up to 30 vol% of whiskers, Bodies with relative densities higher The aim of the present work was to evaluate the efficiency of than 99% were obtained at 1400oC, using a holding time of 3 the SPS technique in consolidating SiCw reinforced mullite-based min and a uniaxial pressure of 50 MPa, The analysis of ramics. Corresponding composites were also prepared by the densification kinetics revealed that a shrinkage rate of about onventional HP process for comparison. The mechanical prop 10-3s-I was obtained even with whisker load, showing ties, e.g., hardness(H, ), fracture toughness(KId), and bending that the spark plasma sintering process is a rapid technique for strength(o ) of spark-plasma-sintered and hot-presssed specimens fabricating whisker composites. The very rapid densification ith densities greater than 99% of TD have been studied and at occurs during spark plasma sintering allows little time for orrelated with recorded microstructural features growth of the matrix grains. This, in turn, leads to improved mechanical properties. I. Experimental Procedur (1) Precursor Preparation The mullite powder was prepared via a wet-chemistry pr CARBIDE whisker(SiCw)reinforced oxide-based cerami with a 3: 2 molar ratio of Al, O,/SiO,. Its specific sites are a family of established engineering materials determined by N, adsorption(BET method)was for load-bearing and abrasive wear-resistant applications. The corresponded to an equivalent spherical particle addition of SiCw. on the one hand, significantly improves the nm. Scanning electron microscope (SEM) studies reveale strength and the damage tolerance of the oxide-based ceramics but, these particles actually consisted of several agglomerated on the other hand, such an addition makes densification more crystallites having an average diameter around 5-10 nm. The difficult. By using pressureless sintering(PLS), composites with p-Sic whiskers(American Matrix Co. had an average length of dispersion of Sic whiskers and mullite powder was achieved by densification for composites with higher whisker loads. Thus, adjusting the ph of the slurry to 10.0-11.0 and using an alumina composites of this type are typically prepared by batch hot alkoxide as a dispersant. The slurry pressing(HP) despite the high cost opposites with SiC whisker contents of 10, 20, and In this article we describe a comparatively new consolidation were prepared in this fashion chnique, spark plasma sintering (SPS), that resembles the ssIng (2) Consolidation owder(green body)is loaded in a die and a uniaxial pressure The dried powder mixtures were consolidated under is applied during sintering. However, instead of using an an SPS apparatus(Dr. Sinter 2050, Sumitomo Coal external heating source, a pulsed direct current is allowed to Ltd, Japan), using cylindrical graphite dies with inner diameters of pass through the electrically conducting pressure die and, in 30 mm. The samples were sintered at 1300 and 1400C. times of 3-20 min w die also acts as a heating source and the sample is heated from MPa was applied at room temperature and held constant during the both outside and inside. The use of a pulsed direct current also entire sintering cycle. The temperature was raised to 600C within exposes the sample to an electric field during sintering, and it 3 min. above 600C it was monitored and regulated by an optical has been suggested that this field will promote mass transp and thus contribute to an improved sintering behavior. A unique pyrometer focused on the surface of the die, at ng rate of feature of SPs is that it enables very rapid consolidation at 100 min". The setup allows a cooling rate of >350%C min" comparatively low temperatures, which, in conjunction with furnished with a dilatometer for recording the linear shrinkage of the comparatively simple construction of the sintering chamber, the sample as functions of temperature and/or time The same powder mixtures were also consolidated by a con- ventional HP technique at temperatures between 1550 and 1700C, with a holding time of 40 min and a pressure of 30 MPa J.J. Petrovic-contributing editor he density of the compacted specin ceived October 13, 2002: approved Sept 改 Archimedes'technique. The already defined TD of composite was calculated by applying the rule of mixture and employing respectively. Hardness (H,) and fracture toughness(KIs)mea- surements were performed by the Vickers diamond indentation FAuthor to whom comespondence should be addressed. e-mail address method at room temperature on specimens polished by standard shen(inorg. sus diamond ing techniques down to l um finish. The results
J. Am. Cemm. Sor.. 87 I I1 42-46 (2W) journal Spark-Plasma-Sintering Consolidation of Sic-Whisker-Reinforced Mullite Composites Zhengren Huang, Zhijian Shen,**'.* Luwei Lin, Mats Nygren,' and Dongliang Jiang Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People's Republic of China Spark plasma sintering was applied to consolidate a series of Sic-whisker-reinforced mullite-matrix composites containing up to 30 vol% of whiskers. Bodies with relative densities higher than 99% were obtained at 14OO"C, using a holding time of 3 min and a uniaxial pressure of 50 MPa. The analysis of densification kinetics revealed that a shrinkage rate of about lO-'s-' was obtained even with a high whisker load, showing that the spark plasma sintering process is a rapid technique for fabricating whisker composites. The very rapid densification that occurs during spark plasma sintering allows little time for growth of the matrix grains. This, in turn, leads to improved mechanical properties. I. Introduction LICON CARBIDE whisker (Sic,) reinforced oxide-based ceramic S composites are a family of established engineering materials for load-bearing and abrasive wear-resistant applications. The addition of Sic,, on the one hand, significantly improves the strength and the damage tolerance of the oxide-based ceramics but, on the other hand, such an addition makes densification more difficult. By using pressureless sintering (PLS), composites with up to 10 vol% SIC, can be densified up to -95% of their theoretical density (TD).'., but it is difficult to reach this degree of densification for composites with higher whisker loads. Thus, composites of this type are typically prepared by batch hot pressing (HP) despite the high cost. In this article we describe a comparatively new consolidation technique, spark plasma sintering (SPS), that resembles the hot-pressing process in several aspects; i.e., the precursor powder (green body) is loaded in a die and a uniaxial pressure is applied during sintering. However, instead of using an external heating source, a pulsed direct current is allowed to pass through the electrically conducting pressure die and, in appropriate cases, also through the sample. Consequently, the die also acts as a heating source and the sample is heated from both outside and inside. The use of a pulsed direct current also exposes the sample to an electric field during sintering, and it has been suggested that this field will promote mass transport and thus contribute to an improved sintering behavior. A unique feature of SPS is that it enables very rapid consolidation at comparatively low temperat~res,~.~ which, in conjunction with the comparatively simple construction of the sintering chamber, J. J. Petrovic-ontributing editor Manuscript No. 186603. Received October 13.2002: approved September 2,2003. Supported by the Swedish Research Council through Grant No. 621-2002-4299, the Swedish National Board for Industrial and Technical Development (NUTEK), and the*Ministry of Science and Technology of China. Member. American Ceramic Society. 'Department of Inorganic Chemistry, Stockholm University, S- 106 91 Stockholm, Sweden. 'Author to whom correspondence should be addressed. e-mail address: shen@inorg.su.se. provide the user with a cost-competitive production process for whisker-reinforced composites. The aim of the present work was to evaluate the efficiency of the SPS technique in consolidating Sic,-reinforced mullite-based ceramics. Corresponding composites were also prepared by the conventional HP process for comparison. The mechanical properties, e.g., hardness (HJ, fracture toughness (Klc), and bending strength (af), of spark-plasma-sintered and hot-pressed specimens with densities greater than 99% of TD have been studied and correlated with recorded microstructural features. 11. Experimental Procedure (1) Precursor Preparation The mullite powder was prepared via a wet-chemistry process, with a 3:2 molar ratio of Al,0,/Si0,.5 Its specific surface area as determined by N, adsorption (BET method) was 28 m2/g, which corresponded to an equivalent spherical particle diameter of 68 nm. Scanning electron microscope (SEM) studies revealed that these particles actually consisted of several agglomerated mullite crystallites having an average diameter around 5-10 nm. The p-Sic whiskers (American Matrix Co.) had an average length of about 20-50 pm and a diameter of 0.5-1.0 pm. A homogeneous dispersion of SIC whiskers and mullite powder was achieved by adjusting the pH of the slurry to 10.0-1 1.0 and using an alumina alkoxide as a dispersant. The slurry was ball-milled for 24 h. Composites with Sic whisker contents of 10, 20, and 30 vol% were prepared in this fashion. (2) Consolidation The dried powder mixtures were consolidated under vacuum in an SPS apparatus (Dr. Sinter 2050, Sumitomo Coal Mining Co. Ltd., Japan), using cylindrical graphite dies with inner diameters of 20 or 30 mm. The samples were sintered at 1300" and 1400°C. and holding times of 3-20 min were used. A uniaxial pressure of 50 MPa was applied at room temperature and held constant during the entire sintering cycle. The temperature was raised to 600°C within 3 min. Above 600°C it was monitored and regulated by an optical pyrometer focused on the surface of the die, at a heating rate of 1W0C.min-'. The setup allows a cooling rate of >35O"C~nin-~ from the sintering temperature down to 1000°C. The SPS unit is furnished with a dilatometer for recording the linear shrinkage of the sample as functions of temperature and/or time. The same powder mixtures were also consolidated by a conventional HP technique at temperatures between 1550" and 1700"C, with a holding time of 40 min and a pressure of 30 MPa. (3) Characterization The density of the compacted specimens was measured using Archimedes' technique. The already defined TD of composites was calculated by applying the rule of mixture and employing TD values of 3.14 and 3.21 g.cm-3 for mullite and Sic,, respectively. Hardness (H,) and fracture toughness (Klc) measurements were performed by the Vickers diamond indentation method at room temperature on specimens polished by standard diamond polishing techniques down to 1 p,m finish. The results 42
January 2004 SPS Consolidation of Sic-Whisker-Reinforced Mullite Composites 43 were evaluated by means of equations given by Evans et al. 6 We are aware that this technique is developed for monophasic samples implying that the obtained Kis data are most probably lower than the true fracture toughness, Anyhow, here the K data are used only in a comparative manner. Three-point bending strength(o) measurements were conducted using 4 mm x 3 mm X 23 mm bars with beveled corners, using a 20 microstructure was investigated by previously defined sEM. to btain the best contrast, the specimens were thermally etched for 30 min in argon at 1300-1350.C. The reported grain sizes were determined with a linear-intercept method. The sizes of more than 50 grains were determined with an image analysis program(Image tool, UTHSCSA) I. Results and Discussion () Densification Monophasic mullite was consolidated at 1300 and 1400C b ith holding times of 0, 3, and 20 min. Relative densities of 99.5% were obtained at 1300C with a holding time of 3 min (Fig. 1(a)). Prolonging the holding time to 20 min and/or increasing the sintering te the density. SEM studies of polished surfaces showed submi- crometer isolated pores(Fig. 2). The formation of such isolated pores appears to be due to the agglomeration in the precursor 1400 1300 by sps at 300o g and 140 c cbr, ts:ing thholding me f 20 mip and powder. Once formed, the pores are difficult to remove, because of the speed of the densification process. The pure mullite ceramics retained a fine-grained microstructure at 1300C, consisting of more or less equiaxed grains with an average size of -0 4 um, even after sintering for 20 min. However, samples sintered at 1400C for 20 min contained well-faceted grains, Holding time( min) and a large po of the grains had developed an elongated morphology with average width and length of -0.6 and 2-3 The composite powder mixtures were consolidated at 1400C in the SPs, using holding times of 3 and 20 min. The correlation between obtained density and holding time for different Sic. contents is shown in Fig. 1(b). Using a holding time of 3 min, we obtained a relative density of >99%, independent of siC content By prolonging the holding time to 20 min, the density increased slightly to.3-99.6% depending on the Sic content. The densities of the composites prepared by conven- tional HP were all around 98%(Fig. I(b)). High-density composites were formed at lower temperatures and shorter -qSPs1400°/20mn holding times by the sPs process compared with conventional HP1650°c/40min SPS at 1400.C for 20 min with SiCw contents from 0 to 30 vol% were examined(Fig 3), revealing the following: (i)TI growth of mullite is suppressed by addition of sic average grain size of mullite in a pure mullite sample under this condition is -0.6 um(width), which was reduced to SiC content( vol%) 0.52, 0.44, and 0. 41 um with the addition of 10, 20, and 30 vol% of SiCw, respectively. (ii) The distribution of SiC whis kers in the composites is homogeneous. (iii) There are no ig. 1. Relative densities of mullite ceramics (a)and Sic-whisker. obvious whisker agglomerates inforced mullite composites(b) plotted versus holding time and whisker The normalized shrinkage and prepared by SPS are plotted versus temperature in Fig. 4. Pure
January 2004 were evaluated by means of equations given by Evans et aL6 We are aware that this technique is developed for monophasic samples implying that the obtained K,, data are most probably lower than the true fracture toughness. Anyhow, here the K,, data are used only in a comparative manner. Three-point bending strength (uf) measurements were conducted using 4 mm X 3 mm X 23 mm bars with beveled corners, using a 20 mm span under a crosshead speed of 0.5 mm/min. The microstructure was investigated by previously defined SEM. To obtain the best contrast, the specimens were thermally etched for 30 min in argon at 1300-1350°C. The reported grain sizes were determined with a linear-intercept method.' The sizes of more than 50 grains were determined with an image analysis program (Image tool, UTHSCSA). SPS Consolidation of Sic- Whisker-Reinforced Mullite Composites 111. Results and Discussion (I) Densification Monophasic mullite was consolidated at 1300" and 1400°C with holding times of 0, 3, and 20 min. Relative densities of -99.5% were obtained at 1300°C with a holding time of 3 min (Fig. l(a)). Prolonging the holding time to 20 rnin and/or increasing the sintering temperature to 1400°C did not improve the density. SEM studies of polished surfaces showed submicrometer isolated pores (Fig. 2). The formation of such isolated pores appears to be due to the agglomeration in the precursor 1400°C o 1300°C s_" 0 10 Holding time (min) (a) 20 100 - 99 9 c Q 'CI 98 .- J 2 - - m 97 96 9 \ \ B- -V SPS 1400 %/20 rnin D - Q SPS 1400 "(213 min Q- 4 HP 1650 OW40 rnin 0 10 20 30 Sic, content ( vol% ) (b) Fig. 1. Relative densities of mullite ceramics (a) and Sic-whiskerreinforced mullite composites (b) plotted versus holding time and whisker content, respectively. 43 Fig. 2. Scanning electron micrographs of the mullite ceramics prepared by SPS at 1300" (a) and 1400°C (b), using a holding time of 20 min and a pressure of 50 MPa. powder. Once formed, the pores are difficult to remove, because of the speed of the densification process. The pure mullite ceramics retained a fine-grained microstructure at 1 300"C, consisting of more or less equiaxed grains with an average size of -0.4 Fm, even after sintering for 20 min. However, samples sintered at 1400°C for 20 min contained well-faceted grains, and a large portion of the grains had developed an elongated morphology with average width and length of -0.6 and 2-3 pm, respectively. The composite powder mixtures were consolidated at 1400°C in the SPS, using holding times of 3 and 20 min. The correlation between obtained density and holding time for different Sic, contents is shown in Fig. I(b). Using a holding time of 3 rnin, we obtained a relative density of >99%, independent of Sic, content. By prolonging the holding time to 20 min, the density increased slightly to -99.3-99.6% depending on the SIC, content. The densities of the composites prepared by conventional HP were all around 98% (Fig. l(b)). High-density composites were formed at lower temperatures and shorter holding times by the SPS process compared with conventional HP. Fracture surfaces in SiC,/mullite composites compacted by SPS at 1400°C for 20 min with Sic, contents from 0 to 30 vol% were examined (Fig. 3), revealing the following: (i) The grain growth of mullite is suppressed by addition of Sic,. The average grain size of mullite in a pure mullite sample prepared under this condition is -0.6 pm (width), which was reduced to 0.52, 0.44, and 0.41 pm with the addition of 10, 20, and 30 vol% of Sic,, respectively. (ii) The distribution of Sic whiskers in the composites is homogeneous. (iii) There are no obvious whisker agglomerates. The normalized shrinkage and shrinkage rate of the samples prepared by SPS are plotted versus temperature in Fig. 4. Pure
Journal of the American Ceramic Society-Huang et aL. Vol. 87. No. I b d whiskers prepared by SPS at 1400C, using a holding time of 20 min and a pressure of 5o mPosites containing 0(a), 10(b), 20(c), and 30(d)vol% SiC Fig. 3. Scanning electron micrographs of fracture surfaces of whisker-reinforced mullite mullite exhibits its maximum shrinkage rate at-1215C. For (2) Mechanical Properties comparison, this temperature is slightly higher than that required to The indentation hardness(Hv) and the fracture toughness densify sub-micrometer-sized Al, powder(-.C). The (K,s) of the spark-plasma-sintered composites were measured observed maximum shrinkage rate value respectively, on polished surfaces normal and parallel to the also higher than that of alumina(0.003s )under identic pressing direction (referred to as the normal plane and the sintering conditions.For the mullite composites the densification parallel plane below). The bending strength (o )was measured onset temperature was shifted toward lower temperatures by the on bars having their long axial normal to the pressing direction addition of SiCw, and the maximum shrinkage rate was reduced The H, values increased slightly with increasing SiC whisker ith increasing SiCw content. The shrinkage rate for pure mullite content, from 12.8 GPa for mullite ceramics(99.5% TD)to 13.8 was double that of the sample containing 30 vol% SiC whiskers. GPa for composites containing 30 vol%SiCw(99.2% TD),with Even with 30 vol% SiCw, this shrinkage rate is still of the same no noticeable difference between normal and parallel planes he crack extension on the alumina powder, and 1 or 2 orders of magnitude higher than that pic, with pronounced elongation perpendicular to the pressing bserved in conventional pressureless sintering of alumina. 0 The fact that the SPs process yields better densification at lower almost isotropic, however, increasing from 2.1 MPam.9s direction. The Kts value measured on a normal plane was mperatures than the HP process may be attributed to more matrix mullite to 4.5 MPa'm in the composite containing 30 efficient heat transfer to the sample and to the use of the pulsed vol% SiCw, revealing a clear tendency of toughness increase direct current. The pulsed direct current exposes the samples to a with whisker addition Fig. 5). The bending strength,g pulsed electric field, and it has been suggested that this field increased almost linearly with increasing content of SiC whis- promotes grain-boundary diffusion and grain-boundary migra kers, from 267 MPa for mullite to 566 MPa for the composite tion. The lowering of the densification onset temperature by the containing 30 vol% SiCw(Fig. 5) addition of SiCw is somewhat surprising. It is most likely related to the presence of increasing amounts of amorphous silica and sintered composites may be attributed to the strong tendency for other impurities that may initially form a small amount of whiskers to align preferentially normal to the pressing direc lica-rich liquid with a low eutectic temperature Densification of tion, as noted in conventional hot-pressed composites too. This SiCw -containing samples may also be increased because of en- in turn suggests that whisker bridging is one possible toughen hancement of the discharge effect by addition of SiCw, since it ng mechanism. Inspection of the microstructures depicted in ncreases the number of conducting spots in the sample Indepen Fig. 3 reveals that the whiskers are well preserved, and the le densification process used, increasing additions of SiC. presence of cavities originating from distinct whisker the difficulty of densificatic ing and pulled-out whiskers shows that the bonding pathways in the matrix are hindered by the added SiCw and the mullite matrix in spark-plasma-sintered tes is weak relative to the strength of the matrix or the whisker
44 Journal of the American Ceramic Society-Huang et al. Vol. 87. No. 1 Fig. 3. Scanning electron micrographs of fracture surfaces of whisker-reinforced mullite composites containing 0 (a), 10 (b), 20 (c), and 30 (d) vol% SIC whiskers prepared by SPS at 1400”C, using a holding time of 20 min and a pressure of 50 MPa. mullite exhibits its maximum shrinkage rate at -1215°C. For comparison, this temperature is slightly higher than that required to densify sub-micrometer-sized A1,0, powder (- 1140°C). The observed maximum shrinkage rate value (0.007 s-’) for mullite is also higher than that of alumina (0.003 s-I) under identical sintering conditions.’ For the mullite composites the densification onset temperature was shifted toward lower temperatures by the addition of Sic,, and the maximum shrinkage rate was reduced with increasing Sic, content. The shrinkage rate for pure mullite was double that of the sample containing 30 vol% Sic whiskers. Even with 30 vol% Sic,, this shrinkage rate is still of the same order of magnitude as that reported for sub-micrometer-sized alumina powder, and 1 or 2 orders of magnitude higher than that observed in conventional pressureless sintering of The fact that the SPS process yields better densification at lower temperatures than the HP process may be attributed to more efficient heat transfer to the sample and to the use of the pulsed direct current. The pulsed direct current exposes the samples to a pulsed electric field, and it has been suggested that this field promotes grain-boundary diffusion and grain-boundary migrati~n.~’’ The lowering of the densification onset temperature by the addition of Sic, is somewhat surprising. It is most likely related to the presence of increasing amounts of amorphous silica and other impurities that may initially form a small amount of silica-rich liquid with a low eutectic temperature. DensiFication of Sic,-containing samples may also be increased because of enhancement of the discharge effect by addition of Sic,, since it increases the number of conducting spots in the sample. Independent of the densification process used, increasing additions of Sic, increase the difficulty of densification, most probably because the diffusion pathways in the matrix are hindered by the added whiskers. (2) Mechanical Properties The indentation hardness (H,) and the fracture toughness (K,,) of the spark-plasma-sintered composites were measured, respectively, on polished surfaces normal and parallel to the pressing direction (referred to as the normal plane and the parallel plane below). The bending strength (a,) was measured on bars having their long axial normal to the pressing direction. The H, values increased slightly with increasing Sic whisker content, from 12.8 GPa for mullite ceramics (99.5% TD) to 13.8 GPa for composites containing 30 vol% Sic, (99.2% TD), with no noticeable difference between normal and parallel planes. The crack extension on the parallel planes was highly anisotropic, with pronounced elongation perpendicular to the pressing direction. The K,, value measured on a normal plane was almost isotropic, however, increasing from 2.1 MPa.m”2 in matrix mullite to 4.5 MPa.m’’2 in the composite containing 30 vol% Sic,, revealing a clear tendency of toughness increase with whisker addition (Fig. 5). The bending strength, uf, increased almost linearly with increasing content of SIC whiskers, from 267 MPa for mullite to 566 MPa for the composite containing 30 vol% Sic, (Fig. 5). The observed anisotropy of crack extension in spark-plasmasintered composites may be attributed to the strong tendency for whiskers to align preferentially normal to the pressing direction, as noted in conventional hot-pressed composites too. This in turn suggests that whisker bridging is one possible toughening mechanism.’ ’ Inspection of the microstructures depicted in Fig. 3 reveals that the whiskers are well preserved, and the presence of cavities originating from distinct whisker debonding and pulled-out whiskers shows that the bonding between Sic, and the mullite matrix in spark-plasma-sintered composites is weak relative to the strength of the matrix or the whisker
January 2004 SPS Consolidation of Sic-Whisker-Reinforced Mullite Composites e10=-30m%sc Sic whisker content(vol % Temperature(°c) (a) re toughness versus SiC whisker pared by SPS at 1400C for 3 min 0008 entional hot pressing at 1650C for 40 min under a pressure respectively. 0006 10vo% 一日 Mullite the hot-pressed ones. (ii) The grain size of the matrix is smaller in spark-plasma-sintered samples than in hot-pressed ones 0004 T. Concluding Remarks (1) Mullite ceramics with a relative density exceeding 99.5%o and having a sub-micrometer-sized microstructure have beer prepared by the SPs process at 1300.C, using a pressure of 50 MPa and a holding time of 3 min. Increasing the holding time from 3 to 20 min did not result in any appreciable grain growth However, samples sintered at 1400oC for 20 min exhibited 1500 well-faceted grains, and a large portion of the grains had developed Temperature('C) an elongated morphology with average width and length of 0.6 and 2-3 um, respectively. The addition of siC whiskers sup- ressed the grain growth of mullite, so that the sub-micrometer-sized grains even after being spark-plasma- Fig. 4. Normalized shrinkage (a) and shrinkage rate(b)curves of sintered at 1400 C for 20 min es. The data were recorded with durinng rate of 100C-min and a uniaxial pressure of 50 MPa applied (2) The SPS sintering curve revealed that the maximum ng the sPs proces shrinkage rate for mullite was 0.007s at 1215 C at a uniaxial pressure of 50 MPa and a heating rate of 100oC min Although most probably because the short processing time applied omposite containing 30 vol% whiskers still had a remarkably diminishes the detrimental reaction of Sic with the surround high maximum shrinkage rate of 0.003s at 1190oC, indicating ing liquid. Such a reaction would otherwise become more the potential of spark plasma sintering as a rapid processing evere in composites with high whisker load, especially when a longer processing time is required as in the conventional HP (3) The bending strength and fracture toughness of mullite ncreased with increasing addition of Sic whiskers. Moreover, the whisker addition could th with mullite and Sic to form opposites prepared by SPs exhibited better bending strength a eutectic liquid during ng, deteriorating the whisker values than corresponding composites prepared by the HP process properties and increasing the bonding between whisker and sintered samples have higher density than the hot-pressed ones. (ii) The bending strength values of the hot-pressed samples are in The matrix grains are smaller in spark-plasma-sintered samples general observed to be somewhat lower than those of the than in hot-pressed ones park-plasma-sintered samples. The values obtained in the ent study are somewhat higher than previously reported for References M. 1. Osendi. B. A, Bender, and D. Lewis ll, Microstructure and mechanic ing process; thus Samanta et al. reported or increasing Properties of Mullite-Silicon Carbide Composites. "J.Am. Ceram. soc. 72[61 from 186 MPa for mullite to 386 MPa for a composition 0 vol% SiCw, and one of the present aut reported that or increased from 320 MPa for mullite to 420 MPa Technology. "J. Soc. Powder Technol.,Jpn.30[111 790-804(199 for a composition containing 20 vol% SiCw. The fact that park-plasma-sintered samples exhibit higher bending strength values than hot-pressed ones can be ascribed to the Dispersion and Rheology Characterization of Sic whisker in Mullite (i) The spark-plasma-sintered samples have higher de pp.658 roceedings of the 4th international Conference an High
January 2004 SPS Consolidation of Sic- Whisker-Reinforced Mullite Composites 45 - Mullite 50 - Temperature ("C) (a) *-v 20vol% Temperature ("C) (b) Fig. 4. Normalized shrinkage (a) and shrinkage rate (b) curves of whisker-reinforced mullite composites. The data were recorded with a heating rate of IOO"C*min~' and a uniaxial pressure of 50 MPa applied during the SPS process. most probably because the short processing time applied diminishes the detrimental reaction of SIC, with the surrounding liquid. Such a reaction would otherwise become more severe in composites with high whisker load, especially when a longer processing time is required as in the conventional HP process. The increased amount of impurities introduced by whisker addition could then react with mullite and Sic, to form a eutectic liquid during sintering, deteriorating the whisker properties and increasing the bonding between whisker and mullite matrix. The bending strength values of the hot-pressed samples are in general observed to be somewhat lower than those of the spark-plasma-sintered samples. The values obtained in the present study are somewhat higher than previously reported for similar compositions consolidated by the conventional hotpressing process; thus Samanta et ul. '' reported uf increasing from 186 MPa for mullite to 386 MPa for a composition containing 30 vol% Sic,. Tiegs et uI.* reported a uf increase from 200 MPa for mullite to 425 MPa for a composition containing 20 vol% Sic,, and one of the present authorsi3 reported that ut increased from 320 MPa for mullite to 420 MPa for a composition containing 20 vol% Sic,. The fact that spark-plasma-sintered samples exhibit higher bending strength values than hot-pressed ones can be ascribed to the following: (i) The spark-plasma-sintered samples have higher density than m Y s d " 0 10 20 30 Sic whisker content (~01%) Fig. 5. Bending strength and fracture toughness versus Sic whisker content of SiCJmullite composites prepared by SPS at 1400°C for 3 min under a pressure of 50 MPa, and by conventional hot pressing at 1650°C for 40 min under a pressure of 30 MPa, respectively. the hot-pressed ones. (ii) The grain size of the matrix is smaller in spark-plasma-sintered samples than in hot-pressed ones. IV. Concluding Remarks (1) Mullite ceramics with a relative density exceeding 99.5% and having a sub-micrometer-sized microstructure have been prepared by the SPS process at 1300"C, using a pressure of 50 MPa and a holding time of 3 min. Increasing the holding time from 3 to 20 min did not result in any appreciable grain growth. However, samples sintered at 1400°C for 20 min exhibited well-faceted grains, and a large portion of the grains had developed an elongated morphology with average width and length of -0.6 and 2-3 pm, respectively. The addition of Sic whiskers suppressed the grain growth of mullite, so that the matrix contained sub-micrometer-sized grains even after being spark-plasmasintered at 1400°C for 20 min. (2) The SPS sintering curve revealed that the maximum shrinkage rate for mullite was 0.007 s-' at 1215°C at a uniaxial pressure of 50 MPa and a heating rate of 100°Cmin-i. Although the densification was impaired by the whisker addition, the composite containing 30 vol% whiskers still had a remarkably high maximum shrinkage rate of 0.003 s-' at 1 190°C, indicating the potential of spark plasma sintering as a rapid processing technique for manufacturing whisker composites. (3) The bending strength and fracture toughness of mullite increased with increasing addition of Sic whiskers. Moreover, the composites prepared by SPS exhibited better bending strength values than corresponding composites prepared by the HP process. This can be ascribed to the following: (i) The spark-plasmasintered samples have higher density than the hot-pressed ones. (ii) The matrix grains are smaller in spark-plasma-sintered samples than in hot-pressed ones. References 'M. 1. Osendi. 8. A. Bender, and D. Lewis 111. "Microstructure and Mechanical Properties of Mullite-Silicon Carbide Composites," J. Am. Cerum. sox-.. 72 [6] 1049-54 (1989). 'T. Tiegs, P. Becher, and P. Angelini. "Microstructures and Properties of Silicon Carbide Whisker-Reinforced Mullite Composites," Cerum. Trans., 6, 463-72 (1990). 'M. Tokita, "Trends in Advanced SPS Spark Plasma Sintering System and Technology." J. Soc. Powder Techno/., Jpn., 30 [I 11 790-804 (1993). 4L. Ciao, 2. I. Shen, H. Miyamoto, and M. Nygren, "Superfast Densification of Oxideloxide Ceramic Composites," J. Am. Cerum. Soc., 82 [4] 1061-63 (1999). 'D. J. Jiang, L. W. Ling, 2. R. Huang, J. X. Zhang, 2. J. Shen, and M. Nygren. "Dispersion and Rheology Characterization of Sic Whisker in Mullite Slurries"; pp. 658-63 in Proceedings of the 4th International Conference on High
Journal of the American Ceramic Society-Huang et al. Temperature Ceramic Matrit Composites. Edited by w.KI R. Naslain, and oF J. T. Lin and L, C. De Jonghe, " Initial H Schneider. wiley-VCH Verlag GmbH, Weinheim, Ge of Fast-Fired and MgO-Doped Al,O,, "J. Am Ceram Soc., 80[11] 2891-96(1997) IP. Becher, C. H. Hsueh, P. Angelini, and T. Tiegs, "Toughening Behav m. Ceram.Soc.591-8371-72(1976) Whisker-Reinforced Ceramic Matrix Composites. " J. Am. Ceram, Soc.,71 112 M. I. Mendelson verage Grain Size 050-6l(1988) Ceram. soc..52|8]443-46(196 J. Shen, M. Johnsson, Z. Zhao, and M. Nygren, "Spark Plasma Sintering of Composites, "Ceram. Eng ullite System. Wuhan ity, J, Arm, Ceram. Soc., 72 [13-15(1989 Gongye Daxue Xuebao, 15 [11 11-16(1993)
46 hmrnal of the American Ceramic Society-Huang et al. Vol. 87, No. 1 Temperature Crrumic Matrix Composites. Edited by W. Krenkel. R. Naslain, and H. Schneider. Wiley-VCH Verlag GmhH, Weinheim, Germany, 2001. ‘A. G. Evans and E. A. Charles, “Fracture Toughness Determination by Indentation.” J. Am. Ceram. Soc., 59 [7-81 371-72 (1976). ’M. 1. Mendelson. “Average Grain Size in Polycrystalline Ceramics.” J. Am. CEjrum. Suc.. 52 181 443-46 (1969). HZ. J. Shen. M. Johnsson, Z. Zhao, and M. Nygren, “Spark Plasma Sintering of Alumina,” J. Am. Cerum. Soc.. 85 181 1921-27 (2002). “F. F. Lange. “Powder Processing Science and Technology for lncreased Reliahilicy.” J. Am. Cerum. Soc.. 72 [I] 3-15 (1989). ‘“F. I. T. Lin and L. C. De Jonghe, “Initial Coarsening and MimsbucturaJ Evolution of Fast-Fired and MgO-Doped AI,O,,” J. Am Ceram. Soc.. So [I I] 2891-96 (1997). “P. Becher, C. H. Hsueh, P. Angelini, and T. Tiegs. ‘Toughening Behavior in Whisker-Reinforced Ceramic Matrix Composites,” J. Am. Cerum. Soc.. 71 1121 1050-61 (1988). ”S. C. Samata and S. Musikant. “Sic-Whisker-Reinforced Ceramic Matrix Composites,” Ceram. Eng. Sci. Proc., 6 17-81 663-67 (1985). ‘Z. I. Shen and 2. S. Ding, “Microstructure and Toughening Behavior of Mullite Matrix Multiphase Composites: I, Sic Whisker/Mullite System,” Wuhan Gongye Daxue Xuebao, 15 [l] 11-16 (1993). 0