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Effect of second phase on mechanical properties and toughening of Al2O3 based ceramic composites
Composite~ Engineering, Vol. 5, No. 11~1I, pp. 1275 1286, 1995 Copyright© 1995ElsevierScienceLtd Printed in Great Britain. All rightsreserved 0961-9526/95$950 + .00
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0961-9526(95) 00069-0
EFFECT OF SECOND PHASE ON MECHANICAL P R O P E R T I E S A N D T O U G H E N I N G O F A1203 B A S E D CERAMIC COMPOSITES Byung-Koog Jang, a'b Manabu Enoki,b Teruo Kishib and Hee-Kap Oh" "Ssangyong Research Center, Ssangyong Cement Industrial Co., Ltd., P.O. Box 12, Yuseong, DaeJeon 305-345, South Korea ~'Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, Japan
(Received 12 January 1995;final version accepted 19 April 1995) Abstract--This work considers the fabrication of A1203 - based composites with three different types of microstructure: nano composites with the nano-dispersed second phase, hybrid composites with both micro- and nano-sized dispersed second phase, and elongated composites with needle-like in situ dispersed second phases. Methods for improving the mechanical properties of A1203 ceramics were investigated using Al203/5 vol% SiC composites fabricated by hot-pressing. Very fine SiC particles were dispersed uniformly in an A1203 matrix. However, larger SiC particulates existed in grain-boundaries of alumina. The flexural strength was inversely proportional to the square root of the matrix grain size. TEM observation indicated that propagating cracks were deflected by the dispersed SiC particulates. High density A1203/SiC/YAG hybrid composites having an equiaxed second phase were fabricated in the temperature range from 1000 to 1800°C using SiC and Y203 powders as additives. YAG (yttrium aluminum garnet, Y3A 1sO iz) was formed as the second phase from the reaction between A1203 and Y203 above 1400°C. Also, AI203/LaAIlIOI8 (lanthanum-fl-alumina) composites, having an elongated second phase, were successfully fabricated using La203 powder as additives. Microstructural observation of the hot-pressed samples were done by SEM + TEM; the planes were analyzed by XRD. Mechanical properties such as the flexural strength and the fracture toughness of the composites were investigated and exceeded the mechanical properties of the monolithic AI203. Additionally, the composites having elongated grains showed higher toughness, due to grain bridging, than the composites having an equiaxed second phase. I. INTRODUCTION
Generally, A1203 ceramics have excellent properties such as high hardness, low electrical conductivity, good chemical stability (Bowen, 1984). Many studies have focused on particulatedispersed A1203 composites in order to improve both room and high temperature mechanical properties (Harmer et al., 1992; Evans, 1990). The brittle nature of ceramics, including A1203, has prompted many investigations to explore a variety of approaches to enhance their mechanical properties with respect to strength, fracture toughness and high temperature performance. The mechanical properties of ceramics can be improved by the incorporation of various reinforcing second phases which control the microstructure (e.g. suppression of grain growth, control of grain morphology) and improve toughening (through deflection, microcracking, grain bridging). In particulate reinforced composites, the mechanical properties can be improved through controlling the microstructure of the matrix (Green, 1982). For alumina-based composites, it can be expected that the morphology of A1203 grains influences the mechanical properties, thus, the microstructural control of composites is very important. In the present work, microstructurally controlled composites, composed of either equiaxed grains or elongated grains as the second phase in a ceramic-matrix was demonstrated. This work discusses the processing and fabrication of the composites by simultaneous consolidation of micro- and nano-particulates of equiaxed grains as well as elongated grains dispersed in the matrix. Evaluation of microstructural characteristics, mechanical properties and fracture behavior of the Al203-based composites are compared to monolithic A1203. 2. EXPERIMENTAL
2.1. Fabrication of composites
The AIzO3 powder used had an average particle size of 0.2 gm, and the fl-SiC powder used as the reinforcing material had a particle size averaging 0.3 gm, with the total particle 1275
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distribution in the range under 1 lam. For fabrication of A1203/SiC nano-composites, the composite studied consisted of A1203 powder with 5 vol% SiC. For fabrication of A1203/SiC/YAG hybrid composites, A1203, 0.3 ~tm r-SiC and 99.9% pure Y203 were used. A model composite composition was considered as a-A1203 as the matrix, 5 vol% r-sic, 20 vol% YAG phase as dispersed phase. For fabrication of A1203/LaAIt lOis composites, A1203 and La203 were used. The LaAl11018 phase precipitated as the second phase from the reaction of A1203 and La203. The studied composition range for composites was 0-20 vol% of LaAlt ~O~s as a second phase. These compositions were homogenized by ball milling in methanol, using a polyethylene jar and A1203 balls for 24 h. After ball milling the slurry was dried. The mixed powders were uniaxially hot pressed at temperatures between 1000 and 1800°C; for 2 h under 30 MPa pressure in Ar atmosphere. 2.2. Evaluation of properties The hot-pressed specimens were cut to nominally 3 mm by 3 mm by 40 mm test bar blades and polished with 3 lam size diamond pastes to obtain a mirror surface. Specimen densities were determined by the Archimedes immersion technique using room-temperature water. The crystalline phases in the hot-pressed samples were identified by X-ray diffraction (XRD). For evaluating the grain structure, thermal etching was performed at 1500°C in an Ar atmosphere. For microstructural analysis, the polished and thermally etched surfaces of specimens were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM samples were prepared using conventional thinning procedures including dimple grinding and finally argon ion milling. Flexural strength was measured using the four-point bend test. Fracture toughness was determined using the single edge precracked beam (SEPB) method. In order to observe the crack behaviour, a Vickers diamond indentation was used to introduce a controlled crack on the polished surfaces. The crack propagation behavior was investigated by SEM and TEM. 3. R E S U L T S A N D D I S C U S S I O N
3.1. AI2Os/SiC nano-composites 3.1.1. Characterization ofmicrostructure. Figure 1 shows the average grain size of the hot-pressed monolithic A1203 and A1203/5 vol% SiC composites. As shown in this figure, the average A1203 grain size in the matrix of the A1203/5 vol% SiC composites was smaller than that in the monolithic A1203. The grain size of monolithic A1203 increased markedly with an increase in the hot-pressing temperature, whereas that of the A1203 matrix in the composites
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Effect of second phase on mechanical properties
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increased little under the same conditions. Monolithic A1203 exhibited abnormal grain growth above 1600°C. Grain growth in A1203 ceramics is known to occur with increase in sintering temperature, but significant grain growth in the A1203/5 vol% SiC composites did not begin until 1700°C, and little further grain growth was observed at 1800°C. These results suggest that a second phase consisting of SiC particulates effectively suppressed grain growth at a high temperature, resulting in increased homogeneity by inhibiting the abnormal grain growth that can occur in pure A1203 during sintering. Figure 2 shows the relationship between AI,O3 grain size and the reciprocal value of the maximum size of SiC grains that could be dispersed within the A1203 grain, as a result of TEM observation. The maximum size of SiC is approximately one-tenth of the A1203 grain size; the larger the A1203 grain, the larger the SiC particulates that can be dispersed within the A1203 grains. The maximum size of the SiC particles that can be present within the A1203 grains increases with hot-pressing temperature. If the SiC particulates are larger than one tenth the size of the A1203 matrix grains, the SiC will be dispersed at the grain boundary. 3.1.2. Mechanical properties. Figure 3 represents the relationship between flexural strength and flaw size in the interior of the materials. The flaw sizes measured for fracture 5O
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surface by fractography of the tested samples. The flaw sizes of the nano-composites were lower than those of monolithic A1203, and strength decreased with increasing flaw size, regardless of material. Control of grain-size distribution apparently affected the critical flaw size in the matrix. Accordingly, the decrease in flaw size of the nano-composites was less than one half that of monolithic A1203, SiC particulates in composites can serve as grain-growth inhibitors, resulting in a stabilizing effect that produces a fine, uniform microstructure and leads to improved strength. Figure 4 shows the cumulative frequency of deflection angles of hot-pressed samples which were hot-pressed at 1600°C. Propagation of a crack was introduced by Vickers indentation of thermally etched specimens. The extent of deflection angles could be measured from the crack propagation along the crack paths. Propagating cracks tended to deflect along the grains and significantly more deflection was exhibited by these composites compared to monolithic A1203. For monolithic A1203, the overall path of crack propagation was comparitively straight, with limited crack deflections. Mostly intergranular crack propagation was observed in the small grains, whereas intragranular crack propagation occurred in the large grains. In contrast, grains of the A1203 matrix in A1203/5 vol% SiC composites were smaller than those of monolithic AlzO3 and mainly intergranular crack propagation along the A1203 grain boundaries was observed. The cumulative frequency of deflection angles for the composites was larger than for monolithic A1203, and resulted in improved fracture toughness, that is, 3.1-3.9 MPa/m. The deflection behavior results seem to indicate the existence of a toughening mechanism in the A1203/5 vol% SiC composites. Additionally, the increase in toughness of the studied composites should be affected by the residual stresses developed in the alumina and SiC. These residual stresses may deflect a propagating crack at SiC particles, leading to a change in the fracture mode and resulting in an increase of fracture toughness. Levin et al. have demonstrated that large compressive micro-stresses (approaching 2 GPa) can exist in the SiC particles in A1203 with 10 vol% sub-micron SiC (Levin et al., 1994). In Fig. 4, the different results for the deflection behavior are related to microstructural properties such as decreased average grain size and incorporation of the nano-sized second phase. 3.2. AI203/SiC/YAG hybrid composites
3.2.1. Fabrication and sintering behavior. The relative density of hot pressed monolithic A1203 and A1203/SiC/Y203 materials as a function of the hot-pressing temperature are given in Fig. 5. The relative density of the monolithic AlzO3 hot-pressed at 1000°C is about 65% and that i
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Fig. 4. Cumulative frequency distributions of measured deflection angles of main crack by Vickers indentation.
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Effect of second phase on mechanical properties
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of the A1203 hot-pressed at 1200°C is 95% with no existence of open pores. Additionally, highly dense A1203 of -> 99% in theory could be obtained beyond 1400°C. On the other hand, for the A1203/SiC/Y203 system, the relative densities of composites hot-pressed below 1400°C were relatively low compared to monolithic A1203. However, considerable densification occurred between 1400 and 1600°C. It seemed to be due to the formation of YAG phase illustrated by the XRD results in Fig. 6. These hot-pressed bodies reached nearly full density above 1600°C. This suggests that sintering of these composites should be carried out at a higher temperature in order to obtain full density. This can be attributed to the very poor sinterability of SiC particulates. X-ray diffraction results for hot-pressed A1203/SiC/Y203 system, are presented in Fig. 6. From XRD analysis, A1203, SiC and Y203 phases were observed in the specimen hot-pressed at 1200°C in Fig. 6(a), whereas the Y203 phase disappeared above 1400°C, and instead YAG phase, resulting from the reaction between Y203 and A1203, was detected and no evidence of any other phase was found. The solid-state reaction for formation of YAG phase can be described by 5 A 1 2 0 3 + 3 Y 2 0 3 --+ 2 Y 3 A 1 5 0 1 2 . It is obvious that the YAG phase was successfully formed from the reaction between A1203 and Y203 above 1400°C shown in Fig. 6(b). The phase diagrams for the Y203-A1203 system indicate a good solubility of yttria in
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A1203 (Levin et al., 1964). Figure 7 shows the TEM observation of A1203/SiC/YAG composites hot-pressed at 1600°C. As shown in the TEM figure, the microstructural results were consistent with three phases, that is, the grains of bright contrast in the center are A1203, while grains with dark contrast are the YAG phase located at A1203 grain boundaries, and nano-sized SiC particulates are homogeneously dispersed in intragranular or intergranular sites in both A1203 matrix and YAG grains or grain boundaries, respectively. Therefore, a hybridizedmicrostructural material, composed of micro-sized YAG and nano-sized SiC as a second phase, is confirmed. 3.2.2. Characterization of properties. Figure 8 shows the average grain size of the monolithic A1203 and the A1203/SiC/YAG composites both hot-pressed at 1600°C. The grain size of monolithic A1203 increased markedly with increasing hot-pressing time, whereas that of the A1203 in the A1203/SiC/Y203 system was comparatively restrained under the same hot-pressing condition. For monolithic A1203, abnormal grain growth was observed above 10 h of hot-pressing time at 1600°C. On the other hand, the significant grain growth of A1203 for A1203/SiC/YAG composites does not rise even at 10 h of hot-pressing time. In addition, the distribution of A1203 grain size in AIzO3/SiC/YAG composites is narrower than those of monolithic A1203. Such microstructural features are expected to affect the mechanical properties. Figure 9 shows the relationship between flexural strength and matrix grain size of the hot-pressed bodies. It appears that the flexural strength was proportional to the inverse of the square root of the matrix grain sizes, as described by the Hall-Petch equation (Butcher et al., 1969). Flexural strengths of the composites are higher than those of the monolithic A1203. The reason for improvement of flexural strength in composites is attributed to a reduction in critical flaw size due to suppression of grain growth by the distribution of nano-sized SiC particulates and micro-sized YAG particulates. The significant decrease of critical flaw size in composites is probably due to suppression of moving grain boundaries of A1203 grains. It has previously been reported that the strength of A1203 decreases with increasing grain size (Chantikul et al., 1990). Figure 10 shows the fracture toughness as a function of hot-pressing temperature using SEPB method. Fracture toughness of A1203/SiC/YAG composites increased with hot-pressing temperature and reached an approximately maximum value at 1600°C. The overall fracture toughness of composites is higher than that of monolithic A1203. Toughening effect of crack deflection by reinforcing SiC phases in A1:O3/SiC/YAG composites existed. Propagating cracks were deflected at nano-sized SiC and micro-sized YAG particulates, which results in a complicated crack propagation path and leads to a higher fracture energy. Thus, this crack deflection may be easily generated at the dispersed second phases in matrix. It was reported that fracture toughness improves due to toughening by crack deflection at second phases in particulate-dispersed composites (Faber and Evans, 1983). This behavior of crack deflection is expected to improve the fracture toughness in the present material. 3.3. AI203/LaAl H018 composites with elongated microstructure 3.3.1. Fabrication and XRD analysis of composites. Figure 11 shows the XRD analysis of A1203/LaAllIOI8 composites hot-pressed at various temperatures. In the case of samples hot-pressed at 1400°C, LaA103 was detected as a second phase, whereas in samples hot-pressed above 1500°C, LaAll ~O18was observed, resulting from the reaction between A1203 and La203. The formation of LaAltlOi8 phase can be described by 11A1203 + La203 ~ 2LaAlliO18. Figure 12 shows the grain structure of thermally etched surfaces of A1203/20 vol% LaAlliOt8 composites as compared with monolithic A1203. The A1203 grains are equiaxed, however, the LaAI~ iO~8 second phase of the composite shows an elongated structure described as needle-like. This means the LaAII~O~8 phase has the hexagonal structure occurred grain growth of c axis orientation. In addition, A1203 grains remained small in comparison with monolithic A1203. It can be explained that the grain growth of A1203 was restrained due to the existence of the LaAIl~O18 second phase. The relative density is higher despite the 1400°C hot-pressing temperature, and increases with increasing hot-pressing temperature. Composites with elongated second phases make it difficult to sinter to full density (Chou and Green, 1992; Nischik et al., 1991). From studied density results, LaAlliO~8 second phase seems to have good sinterability despite having elongated microstructure.
Effect of second phase on mechanical properties
g. 7, TEM micrograph ofA1203/5 vol% SIC~20vol% YAG composites: A = AI20~, S = SiC and Y = YAG.
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Fig. 12. SEM micrograph of AI203,/20 vol% LaAl~O~s composites.
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Effect of second phase on mechanical properties
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Fig. 9. Bending strength vs grain size of matrix in various hot-pressed materials.
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3.3.2. Mechanical properties. Figure 13 shows the relationship of strength and preciptated LaA1~~O ~8 amounts for A1203/LaAI~ lO~8 composites. The strength of the composite increases with increasing LaAI~iO18 amounts up to 10 vol%. Overall strength of the composite hot-pressed at 1700°C is lower than that at 1600°C. This is due to the grain growth of the A1203 matrix and/or second phase, that is, the aspect ratio of LaAll~018 phase increases with increased hot-pressing temperature. This is probably because the higher aspect ratio of the LaAI~IO~8 phase may cause a larger critical flaw size. Generally, the strength of brittle ceramics is closely related to the critical defect size; higher strengths result from smaller critical flaws (Padture and Chan, 1992; Russo et aL, 1991). Figure 14 shows the relationship of fracture toughness and 1000 ~
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Fig. 13. Influence of LaAlllOis content on strength, as a function of hot-pressing temperature, for A1203/LaAltiOls composites.
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precipitated LaAll iOi8 amounts for A1203/LaAI1 iO18 composites. Fracture toughness increased with increasing hot-pressing temperature, as well as LaAlllO~8 volume. The improving tendency in fracture toughness corresponds with the increased aspect ratio of the LaAI~1018 phase. It can be explained that an elongated structure, having a larger aspect ratio, is necessary to obtain a higher fracture toughness (Becher, 1991). Thus, the formation of LaAlj1018 phase effectively enhanced the fracture toughness of the studied composites. It can be considered that the enhanced toughness is related to crack bridging due to elongated LaAl11018 phase. In order to investigate the toughening mechanism, crack path was investigated using the SEM. Figure l 5 shows the crack, which was introduced by a Vickers indentor on thermally etched specimens. Cracks show deflection and crack bridging by elongated LaAll iO18 second phase. At a higher volume fraction of LaA111018, such crack bridging was frequently observed. The toughening effect due to crack bridging of the second phase is similar to that observed in A1203/SiC whisker composites (Campbell et al., 1990; Becher et al., 1988). It can be considered that when a propagating crack undergoes crack deflection or crack bridging at second phases, improvement of toughness is achieved through the diminished stress intensity at crack tip, i.e. reduction of the crack driving force. Thus, improvement of toughness for ceramic composites can be expected from this toughening mechanism. 4. CONCLUSIONS
Fabrication of particulate-reinforced composites, and the influence of the morphology of the reinforcing phase on the mechanical properties and toughening mechanism have been investigated. Based on the present work, the following conclusions can be drawn. (1) High-density A1203-based composites were successfully obtained by dispersing nano-sized SiC particulates intragranularly within the A1203 matrix grains and larger particulates at the grain boudaries. Grain growth within the A1203 matrix grains of the AI203/SiC composites was effectively restrained by this method, apparently because the SiC particulates inhibited the movement of the grain boundaries. Strength and fracture toughness were effectively increased by the dispersion of nano-sized particulates within the composites. Strength improvement was attributed to the reduction of critical flaw size resulting from suppressed grain growth, that is, to grain boundary pinning. (2) A1203/SiC/YAG hybrid composites having equiaxed structure consisting of micro-sized YAG particulates and nano-sized SiC particulates were successfully obtained. YAG (yttrium aluminum garnet) as a second phase was formed by the reaction between alumina and yttria in the A1203/SiC/Y203 composition. The grain growth of A1203 matrix grains in the A 1 2 0 3 / S i C / Y A G hybrid composites was effectively restrained. The mechanical properties, such as flexural strength and fracture toughness of A1203/SiC/YAG hybrid composites are higher than those of monolithic A1203. The improvements of mechanical properties are due to a fine grained, uniform microstructure and a toughening mechanism providing crack deflection at nano-sized SiC and micro-sized YAG particulates.
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(3) AlzO3/LaAII~OI8 composites with an elongated reinforcing phase were successfully fabricated. LaAll iO18 (lanthanum-fl-alumina) particulates were formed by the reaction b e t w e e n A1203 and La203 w h e n hot-pressing above 1500°C. Strength and toughness o f composites increased with increasing fraction o f the second phase. Fracture toughness increased with the increased aspect ratio o f the second phase. Grain bridging and/or crack deflection at the elongated second phases was observed to contribute to the increased composite toughness relative to monolithic A1203. REFERENCES Becher, P. F. (1991). Microstructural design of toughened ceramics. J. Am. Ceram. Soe. 74(2), 255-269. Becher, P. F., Hsueh, C. H., Angelini, P. and Tiegs, T. N. (1988). Toughening behavior in whisker-reinforced ceramic matrix composites. J. Am. Ceram. Soc. 71(12), 1050-1061. Bowen, H. K. (1984). Ceramics as engineering materials: Structure-property-processing. Mater. Res. Soc. Syrup. Proc. 24, l-I 1. Butcher, J. H., Grozier, J. D. and Enrietto, J. F. (1969). Strength and toughness of hot-rolled ferrite-pearlite. In Fracture, Vol. 4, pp. 253-54. Academic Press, New York. Campbell, K. P., Riihle, M., Dalgleish, B. J. and Evans. A. G. (1990). Processing and mechanical properties of SiC-whisker-A1203-matrixcomposites, d. Am. Ceram. Soc. 73(3), 521-530. Chantikul, P., Bennison, S. J. and Lawn, B. R. (1990). Role of grain size in the strength and R-curve properties of alumina, d. Am. Ceram. Soc. 73(8), 2419-2427. Chou, Y. S. and Green, D. J. (1992). Silicon carbide platelet/alumina composites: I. Effect of forming technique on platelet orientation, d. Am. Ceram. Soc. 75(12), 3346-3352. Evans, A. G. (1990). Perspective on the development of high-toughness ceramics. J. Am. Ceram. Soc. 73(2), 187-206. Faber, K T. and Evans, A. G. (1983). Crack deflection processes-lI. Experiment. Acta Metall. 31(4), 577-584. Green, D. J. (1982). Critical microstructures for microcracking in AI203-ZrO 2 composites, d. Am. Ceram. Soc. 65(12), 61(~614. Harmer, M. P., Chan, H. M. and Miller, G. A. (1992). Unique opportunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Soc. 75(7), 1715-1728. Levin, E. M., Robbins, C. R. and McMurdie, H. F. (1964). Phase Diagrams for Ceramists (Edited by M. K. Reser), Fig. 311. American Ceramic Society, Columbus, OH. Levin, I., Kaplan, W. D. and Brandon, D. G. (1994). Microstructure developments in pressureless-sintered fl-SiC materials with A1, B and C additions. Acta Metall. Mater. 42(4), 1147-1154. Nischik, C., Siebold, M. M., Travitzky, N. A. and Claussen, N. (1991). Effect of processing on mechanical properties of platelet-reinforced mullite composites, d. Am. Ceram. Soc. 74(10), 2464-2468. Padture, N. and Chan, H. M. (1992). Improved flaw tolerance in alumina-I vol% anorthite via crystallization of the intergranular glass. J. Am. Ceram. Soc. 75(7), 1870-1875. Russo, C. J., Harmer, M. P., Chan, H. M. and Miller, G. A. (1991). Design of a laminated ceramic composite for improved strength and toughness. 93rd Annual Meeting o f the American Ceramic Society, Cincinnati.
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0961-9526(95) 00069-0
EFFECT OF SECOND PHASE ON MECHANICAL P R O P E R T I E S A N D T O U G H E N I N G O F A1203 B A S E D CERAMIC COMPOSITES Byung-Koog Jang, a'b Manabu Enoki,b Teruo Kishib and Hee-Kap Oh" "Ssangyong Research Center, Ssangyong Cement Industrial Co., Ltd., P.O. Box 12, Yuseong, DaeJeon 305-345, South Korea ~'Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, Japan
(Received 12 January 1995;final version accepted 19 April 1995) Abstract--This work considers the fabrication of A1203 - based composites with three different types of microstructure: nano composites with the nano-dispersed second phase, hybrid composites with both micro- and nano-sized dispersed second phase, and elongated composites with needle-like in situ dispersed second phases. Methods for improving the mechanical properties of A1203 ceramics were investigated using Al203/5 vol% SiC composites fabricated by hot-pressing. Very fine SiC particles were dispersed uniformly in an A1203 matrix. However, larger SiC particulates existed in grain-boundaries of alumina. The flexural strength was inversely proportional to the square root of the matrix grain size. TEM observation indicated that propagating cracks were deflected by the dispersed SiC particulates. High density A1203/SiC/YAG hybrid composites having an equiaxed second phase were fabricated in the temperature range from 1000 to 1800°C using SiC and Y203 powders as additives. YAG (yttrium aluminum garnet, Y3A 1sO iz) was formed as the second phase from the reaction between A1203 and Y203 above 1400°C. Also, AI203/LaAIlIOI8 (lanthanum-fl-alumina) composites, having an elongated second phase, were successfully fabricated using La203 powder as additives. Microstructural observation of the hot-pressed samples were done by SEM + TEM; the planes were analyzed by XRD. Mechanical properties such as the flexural strength and the fracture toughness of the composites were investigated and exceeded the mechanical properties of the monolithic AI203. Additionally, the composites having elongated grains showed higher toughness, due to grain bridging, than the composites having an equiaxed second phase. I. INTRODUCTION
Generally, A1203 ceramics have excellent properties such as high hardness, low electrical conductivity, good chemical stability (Bowen, 1984). Many studies have focused on particulatedispersed A1203 composites in order to improve both room and high temperature mechanical properties (Harmer et al., 1992; Evans, 1990). The brittle nature of ceramics, including A1203, has prompted many investigations to explore a variety of approaches to enhance their mechanical properties with respect to strength, fracture toughness and high temperature performance. The mechanical properties of ceramics can be improved by the incorporation of various reinforcing second phases which control the microstructure (e.g. suppression of grain growth, control of grain morphology) and improve toughening (through deflection, microcracking, grain bridging). In particulate reinforced composites, the mechanical properties can be improved through controlling the microstructure of the matrix (Green, 1982). For alumina-based composites, it can be expected that the morphology of A1203 grains influences the mechanical properties, thus, the microstructural control of composites is very important. In the present work, microstructurally controlled composites, composed of either equiaxed grains or elongated grains as the second phase in a ceramic-matrix was demonstrated. This work discusses the processing and fabrication of the composites by simultaneous consolidation of micro- and nano-particulates of equiaxed grains as well as elongated grains dispersed in the matrix. Evaluation of microstructural characteristics, mechanical properties and fracture behavior of the Al203-based composites are compared to monolithic A1203. 2. EXPERIMENTAL
2.1. Fabrication of composites
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distribution in the range under 1 lam. For fabrication of A1203/SiC nano-composites, the composite studied consisted of A1203 powder with 5 vol% SiC. For fabrication of A1203/SiC/YAG hybrid composites, A1203, 0.3 ~tm r-SiC and 99.9% pure Y203 were used. A model composite composition was considered as a-A1203 as the matrix, 5 vol% r-sic, 20 vol% YAG phase as dispersed phase. For fabrication of A1203/LaAIt lOis composites, A1203 and La203 were used. The LaAl11018 phase precipitated as the second phase from the reaction of A1203 and La203. The studied composition range for composites was 0-20 vol% of LaAlt ~O~s as a second phase. These compositions were homogenized by ball milling in methanol, using a polyethylene jar and A1203 balls for 24 h. After ball milling the slurry was dried. The mixed powders were uniaxially hot pressed at temperatures between 1000 and 1800°C; for 2 h under 30 MPa pressure in Ar atmosphere. 2.2. Evaluation of properties The hot-pressed specimens were cut to nominally 3 mm by 3 mm by 40 mm test bar blades and polished with 3 lam size diamond pastes to obtain a mirror surface. Specimen densities were determined by the Archimedes immersion technique using room-temperature water. The crystalline phases in the hot-pressed samples were identified by X-ray diffraction (XRD). For evaluating the grain structure, thermal etching was performed at 1500°C in an Ar atmosphere. For microstructural analysis, the polished and thermally etched surfaces of specimens were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM samples were prepared using conventional thinning procedures including dimple grinding and finally argon ion milling. Flexural strength was measured using the four-point bend test. Fracture toughness was determined using the single edge precracked beam (SEPB) method. In order to observe the crack behaviour, a Vickers diamond indentation was used to introduce a controlled crack on the polished surfaces. The crack propagation behavior was investigated by SEM and TEM. 3. R E S U L T S A N D D I S C U S S I O N
3.1. AI2Os/SiC nano-composites 3.1.1. Characterization ofmicrostructure. Figure 1 shows the average grain size of the hot-pressed monolithic A1203 and A1203/5 vol% SiC composites. As shown in this figure, the average A1203 grain size in the matrix of the A1203/5 vol% SiC composites was smaller than that in the monolithic A1203. The grain size of monolithic A1203 increased markedly with an increase in the hot-pressing temperature, whereas that of the A1203 matrix in the composites
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increased little under the same conditions. Monolithic A1203 exhibited abnormal grain growth above 1600°C. Grain growth in A1203 ceramics is known to occur with increase in sintering temperature, but significant grain growth in the A1203/5 vol% SiC composites did not begin until 1700°C, and little further grain growth was observed at 1800°C. These results suggest that a second phase consisting of SiC particulates effectively suppressed grain growth at a high temperature, resulting in increased homogeneity by inhibiting the abnormal grain growth that can occur in pure A1203 during sintering. Figure 2 shows the relationship between AI,O3 grain size and the reciprocal value of the maximum size of SiC grains that could be dispersed within the A1203 grain, as a result of TEM observation. The maximum size of SiC is approximately one-tenth of the A1203 grain size; the larger the A1203 grain, the larger the SiC particulates that can be dispersed within the A1203 grains. The maximum size of the SiC particles that can be present within the A1203 grains increases with hot-pressing temperature. If the SiC particulates are larger than one tenth the size of the A1203 matrix grains, the SiC will be dispersed at the grain boundary. 3.1.2. Mechanical properties. Figure 3 represents the relationship between flexural strength and flaw size in the interior of the materials. The flaw sizes measured for fracture 5O
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surface by fractography of the tested samples. The flaw sizes of the nano-composites were lower than those of monolithic A1203, and strength decreased with increasing flaw size, regardless of material. Control of grain-size distribution apparently affected the critical flaw size in the matrix. Accordingly, the decrease in flaw size of the nano-composites was less than one half that of monolithic A1203, SiC particulates in composites can serve as grain-growth inhibitors, resulting in a stabilizing effect that produces a fine, uniform microstructure and leads to improved strength. Figure 4 shows the cumulative frequency of deflection angles of hot-pressed samples which were hot-pressed at 1600°C. Propagation of a crack was introduced by Vickers indentation of thermally etched specimens. The extent of deflection angles could be measured from the crack propagation along the crack paths. Propagating cracks tended to deflect along the grains and significantly more deflection was exhibited by these composites compared to monolithic A1203. For monolithic A1203, the overall path of crack propagation was comparitively straight, with limited crack deflections. Mostly intergranular crack propagation was observed in the small grains, whereas intragranular crack propagation occurred in the large grains. In contrast, grains of the A1203 matrix in A1203/5 vol% SiC composites were smaller than those of monolithic AlzO3 and mainly intergranular crack propagation along the A1203 grain boundaries was observed. The cumulative frequency of deflection angles for the composites was larger than for monolithic A1203, and resulted in improved fracture toughness, that is, 3.1-3.9 MPa/m. The deflection behavior results seem to indicate the existence of a toughening mechanism in the A1203/5 vol% SiC composites. Additionally, the increase in toughness of the studied composites should be affected by the residual stresses developed in the alumina and SiC. These residual stresses may deflect a propagating crack at SiC particles, leading to a change in the fracture mode and resulting in an increase of fracture toughness. Levin et al. have demonstrated that large compressive micro-stresses (approaching 2 GPa) can exist in the SiC particles in A1203 with 10 vol% sub-micron SiC (Levin et al., 1994). In Fig. 4, the different results for the deflection behavior are related to microstructural properties such as decreased average grain size and incorporation of the nano-sized second phase. 3.2. AI203/SiC/YAG hybrid composites
3.2.1. Fabrication and sintering behavior. The relative density of hot pressed monolithic A1203 and A1203/SiC/Y203 materials as a function of the hot-pressing temperature are given in Fig. 5. The relative density of the monolithic AlzO3 hot-pressed at 1000°C is about 65% and that i
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of the A1203 hot-pressed at 1200°C is 95% with no existence of open pores. Additionally, highly dense A1203 of -> 99% in theory could be obtained beyond 1400°C. On the other hand, for the A1203/SiC/Y203 system, the relative densities of composites hot-pressed below 1400°C were relatively low compared to monolithic A1203. However, considerable densification occurred between 1400 and 1600°C. It seemed to be due to the formation of YAG phase illustrated by the XRD results in Fig. 6. These hot-pressed bodies reached nearly full density above 1600°C. This suggests that sintering of these composites should be carried out at a higher temperature in order to obtain full density. This can be attributed to the very poor sinterability of SiC particulates. X-ray diffraction results for hot-pressed A1203/SiC/Y203 system, are presented in Fig. 6. From XRD analysis, A1203, SiC and Y203 phases were observed in the specimen hot-pressed at 1200°C in Fig. 6(a), whereas the Y203 phase disappeared above 1400°C, and instead YAG phase, resulting from the reaction between Y203 and A1203, was detected and no evidence of any other phase was found. The solid-state reaction for formation of YAG phase can be described by 5 A 1 2 0 3 + 3 Y 2 0 3 --+ 2 Y 3 A 1 5 0 1 2 . It is obvious that the YAG phase was successfully formed from the reaction between A1203 and Y203 above 1400°C shown in Fig. 6(b). The phase diagrams for the Y203-A1203 system indicate a good solubility of yttria in
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A1203 (Levin et al., 1964). Figure 7 shows the TEM observation of A1203/SiC/YAG composites hot-pressed at 1600°C. As shown in the TEM figure, the microstructural results were consistent with three phases, that is, the grains of bright contrast in the center are A1203, while grains with dark contrast are the YAG phase located at A1203 grain boundaries, and nano-sized SiC particulates are homogeneously dispersed in intragranular or intergranular sites in both A1203 matrix and YAG grains or grain boundaries, respectively. Therefore, a hybridizedmicrostructural material, composed of micro-sized YAG and nano-sized SiC as a second phase, is confirmed. 3.2.2. Characterization of properties. Figure 8 shows the average grain size of the monolithic A1203 and the A1203/SiC/YAG composites both hot-pressed at 1600°C. The grain size of monolithic A1203 increased markedly with increasing hot-pressing time, whereas that of the A1203 in the A1203/SiC/Y203 system was comparatively restrained under the same hot-pressing condition. For monolithic A1203, abnormal grain growth was observed above 10 h of hot-pressing time at 1600°C. On the other hand, the significant grain growth of A1203 for A1203/SiC/YAG composites does not rise even at 10 h of hot-pressing time. In addition, the distribution of A1203 grain size in AIzO3/SiC/YAG composites is narrower than those of monolithic A1203. Such microstructural features are expected to affect the mechanical properties. Figure 9 shows the relationship between flexural strength and matrix grain size of the hot-pressed bodies. It appears that the flexural strength was proportional to the inverse of the square root of the matrix grain sizes, as described by the Hall-Petch equation (Butcher et al., 1969). Flexural strengths of the composites are higher than those of the monolithic A1203. The reason for improvement of flexural strength in composites is attributed to a reduction in critical flaw size due to suppression of grain growth by the distribution of nano-sized SiC particulates and micro-sized YAG particulates. The significant decrease of critical flaw size in composites is probably due to suppression of moving grain boundaries of A1203 grains. It has previously been reported that the strength of A1203 decreases with increasing grain size (Chantikul et al., 1990). Figure 10 shows the fracture toughness as a function of hot-pressing temperature using SEPB method. Fracture toughness of A1203/SiC/YAG composites increased with hot-pressing temperature and reached an approximately maximum value at 1600°C. The overall fracture toughness of composites is higher than that of monolithic A1203. Toughening effect of crack deflection by reinforcing SiC phases in A1:O3/SiC/YAG composites existed. Propagating cracks were deflected at nano-sized SiC and micro-sized YAG particulates, which results in a complicated crack propagation path and leads to a higher fracture energy. Thus, this crack deflection may be easily generated at the dispersed second phases in matrix. It was reported that fracture toughness improves due to toughening by crack deflection at second phases in particulate-dispersed composites (Faber and Evans, 1983). This behavior of crack deflection is expected to improve the fracture toughness in the present material. 3.3. AI203/LaAl H018 composites with elongated microstructure 3.3.1. Fabrication and XRD analysis of composites. Figure 11 shows the XRD analysis of A1203/LaAllIOI8 composites hot-pressed at various temperatures. In the case of samples hot-pressed at 1400°C, LaA103 was detected as a second phase, whereas in samples hot-pressed above 1500°C, LaAll ~O18was observed, resulting from the reaction between A1203 and La203. The formation of LaAltlOi8 phase can be described by 11A1203 + La203 ~ 2LaAlliO18. Figure 12 shows the grain structure of thermally etched surfaces of A1203/20 vol% LaAlliOt8 composites as compared with monolithic A1203. The A1203 grains are equiaxed, however, the LaAI~ iO~8 second phase of the composite shows an elongated structure described as needle-like. This means the LaAII~O~8 phase has the hexagonal structure occurred grain growth of c axis orientation. In addition, A1203 grains remained small in comparison with monolithic A1203. It can be explained that the grain growth of A1203 was restrained due to the existence of the LaAIl~O18 second phase. The relative density is higher despite the 1400°C hot-pressing temperature, and increases with increasing hot-pressing temperature. Composites with elongated second phases make it difficult to sinter to full density (Chou and Green, 1992; Nischik et al., 1991). From studied density results, LaAlliO~8 second phase seems to have good sinterability despite having elongated microstructure.
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3.3.2. Mechanical properties. Figure 13 shows the relationship of strength and preciptated LaA1~~O ~8 amounts for A1203/LaAI~ lO~8 composites. The strength of the composite increases with increasing LaAI~iO18 amounts up to 10 vol%. Overall strength of the composite hot-pressed at 1700°C is lower than that at 1600°C. This is due to the grain growth of the A1203 matrix and/or second phase, that is, the aspect ratio of LaAll~018 phase increases with increased hot-pressing temperature. This is probably because the higher aspect ratio of the LaAI~IO~8 phase may cause a larger critical flaw size. Generally, the strength of brittle ceramics is closely related to the critical defect size; higher strengths result from smaller critical flaws (Padture and Chan, 1992; Russo et aL, 1991). Figure 14 shows the relationship of fracture toughness and 1000 ~
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precipitated LaAll iOi8 amounts for A1203/LaAI1 iO18 composites. Fracture toughness increased with increasing hot-pressing temperature, as well as LaAlllO~8 volume. The improving tendency in fracture toughness corresponds with the increased aspect ratio of the LaAI~1018 phase. It can be explained that an elongated structure, having a larger aspect ratio, is necessary to obtain a higher fracture toughness (Becher, 1991). Thus, the formation of LaAlj1018 phase effectively enhanced the fracture toughness of the studied composites. It can be considered that the enhanced toughness is related to crack bridging due to elongated LaAl11018 phase. In order to investigate the toughening mechanism, crack path was investigated using the SEM. Figure l 5 shows the crack, which was introduced by a Vickers indentor on thermally etched specimens. Cracks show deflection and crack bridging by elongated LaAll iO18 second phase. At a higher volume fraction of LaA111018, such crack bridging was frequently observed. The toughening effect due to crack bridging of the second phase is similar to that observed in A1203/SiC whisker composites (Campbell et al., 1990; Becher et al., 1988). It can be considered that when a propagating crack undergoes crack deflection or crack bridging at second phases, improvement of toughness is achieved through the diminished stress intensity at crack tip, i.e. reduction of the crack driving force. Thus, improvement of toughness for ceramic composites can be expected from this toughening mechanism. 4. CONCLUSIONS
Fabrication of particulate-reinforced composites, and the influence of the morphology of the reinforcing phase on the mechanical properties and toughening mechanism have been investigated. Based on the present work, the following conclusions can be drawn. (1) High-density A1203-based composites were successfully obtained by dispersing nano-sized SiC particulates intragranularly within the A1203 matrix grains and larger particulates at the grain boudaries. Grain growth within the A1203 matrix grains of the AI203/SiC composites was effectively restrained by this method, apparently because the SiC particulates inhibited the movement of the grain boundaries. Strength and fracture toughness were effectively increased by the dispersion of nano-sized particulates within the composites. Strength improvement was attributed to the reduction of critical flaw size resulting from suppressed grain growth, that is, to grain boundary pinning. (2) A1203/SiC/YAG hybrid composites having equiaxed structure consisting of micro-sized YAG particulates and nano-sized SiC particulates were successfully obtained. YAG (yttrium aluminum garnet) as a second phase was formed by the reaction between alumina and yttria in the A1203/SiC/Y203 composition. The grain growth of A1203 matrix grains in the A 1 2 0 3 / S i C / Y A G hybrid composites was effectively restrained. The mechanical properties, such as flexural strength and fracture toughness of A1203/SiC/YAG hybrid composites are higher than those of monolithic A1203. The improvements of mechanical properties are due to a fine grained, uniform microstructure and a toughening mechanism providing crack deflection at nano-sized SiC and micro-sized YAG particulates.
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(3) AlzO3/LaAII~OI8 composites with an elongated reinforcing phase were successfully fabricated. LaAll iO18 (lanthanum-fl-alumina) particulates were formed by the reaction b e t w e e n A1203 and La203 w h e n hot-pressing above 1500°C. Strength and toughness o f composites increased with increasing fraction o f the second phase. Fracture toughness increased with the increased aspect ratio o f the second phase. Grain bridging and/or crack deflection at the elongated second phases was observed to contribute to the increased composite toughness relative to monolithic A1203. REFERENCES Becher, P. F. (1991). Microstructural design of toughened ceramics. J. Am. Ceram. Soe. 74(2), 255-269. Becher, P. F., Hsueh, C. H., Angelini, P. and Tiegs, T. N. (1988). Toughening behavior in whisker-reinforced ceramic matrix composites. J. Am. Ceram. Soc. 71(12), 1050-1061. Bowen, H. K. (1984). Ceramics as engineering materials: Structure-property-processing. Mater. Res. Soc. Syrup. Proc. 24, l-I 1. Butcher, J. H., Grozier, J. D. and Enrietto, J. F. (1969). Strength and toughness of hot-rolled ferrite-pearlite. In Fracture, Vol. 4, pp. 253-54. Academic Press, New York. Campbell, K. P., Riihle, M., Dalgleish, B. J. and Evans. A. G. (1990). Processing and mechanical properties of SiC-whisker-A1203-matrixcomposites, d. Am. Ceram. Soc. 73(3), 521-530. Chantikul, P., Bennison, S. J. and Lawn, B. R. (1990). Role of grain size in the strength and R-curve properties of alumina, d. Am. Ceram. Soc. 73(8), 2419-2427. Chou, Y. S. and Green, D. J. (1992). Silicon carbide platelet/alumina composites: I. Effect of forming technique on platelet orientation, d. Am. Ceram. Soc. 75(12), 3346-3352. Evans, A. G. (1990). Perspective on the development of high-toughness ceramics. J. Am. Ceram. Soc. 73(2), 187-206. Faber, K T. and Evans, A. G. (1983). Crack deflection processes-lI. Experiment. Acta Metall. 31(4), 577-584. Green, D. J. (1982). Critical microstructures for microcracking in AI203-ZrO 2 composites, d. Am. Ceram. Soc. 65(12), 61(~614. Harmer, M. P., Chan, H. M. and Miller, G. A. (1992). Unique opportunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Soc. 75(7), 1715-1728. Levin, E. M., Robbins, C. R. and McMurdie, H. F. (1964). Phase Diagrams for Ceramists (Edited by M. K. Reser), Fig. 311. American Ceramic Society, Columbus, OH. Levin, I., Kaplan, W. D. and Brandon, D. G. (1994). Microstructure developments in pressureless-sintered fl-SiC materials with A1, B and C additions. Acta Metall. Mater. 42(4), 1147-1154. Nischik, C., Siebold, M. M., Travitzky, N. A. and Claussen, N. (1991). Effect of processing on mechanical properties of platelet-reinforced mullite composites, d. Am. Ceram. Soc. 74(10), 2464-2468. Padture, N. and Chan, H. M. (1992). Improved flaw tolerance in alumina-I vol% anorthite via crystallization of the intergranular glass. J. Am. Ceram. Soc. 75(7), 1870-1875. Russo, C. J., Harmer, M. P., Chan, H. M. and Miller, G. A. (1991). Design of a laminated ceramic composite for improved strength and toughness. 93rd Annual Meeting o f the American Ceramic Society, Cincinnati.