Fabrication and mechanical properties of nanosized SiC particulate reinforced yttria stabilized zirconia composites

NanoSt~ctured Materials, Voi. 9, pp. 497-500,1997 Blaevier Scitm~ Ltd © 1997 #.at MetaUurgica Inc. Printed h~the USA. All rights reserved 0965-9773/97 $17.00 + .00

Pergamon

PII S0965-9773(97)00109-8

FABRICATION AND MECHANICAL PROPERTIES OF N A N O S I Z E D SiC P A R T I C U L A T E R E I N F O R C E D Y T T R I A STABILIZED Z1RCONIA COMPOSITES N. Bamba, Y.H. Choa and K. Niihara The Institute of Scientific and Industrial Research, Osaka University 8-1, Mihogaoka, Ibaraki, Osaka 567, Japan Abstract-- Nanocomposite technology was applied to 8 tool% yttriafully-stabilized zirconia (8YSZ) for improving its mechanical properties. 8YSZ and nanosized SiC powders with particle size of 40 to 100 nm were homogeneously mixed in ethanol using a ball milling technique. After milling, the mixed powders were sintered by hot-pressing routes. Transmission electron microscopy (TEM) observations revealed that nanosized SiC particles were homogeneously dispersed within the 8YSZ matrix grains and at grain boundaries. The fracture strength was improved two to three times compared with the monolithic 8YSZ. Maximum strength of 750 MPa was obtained for the 8YSZ/2Ovol.% SiC nanocomposite. This strength improvement may be due to fine and homogeneous microstructure and to the compressive internal stresses caused by thermal mismatch between the 8YSZ and SiC particulates. © 1997 Acta Metallurgica Inc. INTRODUCTION Yttria stabilized zirconia (YSZ) having cubic structure is a solid electrolyte with a high oxygen ionic conductivity and it is widely used for oxygen sensors, heating elements, and fuel cells. In particular, 8YSZ has been recently recognized as a promising material for use as solid oxide fuel cells (SOFC) due to its high oxygen ionic conductivity and its chemical/structural stability (1). In these applications, 8YSZ requires not only high conductivity but also better mechanical, chemical and electrical stability in the service environments. However, 8YSZ is not a strong material. Therefore, it is necessary to improve its strength and toughness in order to widen its application. There are two major directions to strengthen and toughen ceramics: one is to use phase transformation of ZrO2 from tetragonal to monoclinic induced by mechanical stresses, and the other is to use composite routes using whiskers or platelets at reinforcements(2,3). Recent studies have shown that ceramic composites having nanosized particulate dispersion show excellent mechanical properties such as hardness, Young's modulus, fracture strength and toughness, even at high temperatures (4,5). In this study, the nanocomposite techniques were applied to improve the mechanical properties of YSZ. Nanosize SiC particles were used as a dispersion phase because SiC has much lower thermal expansion coefficient than YSZ and higher Young's modulus and strength. The main purpose of the present work is to evaluate the roles of nanosized SiC in microstructure evolution and mechanical property improvement of nanocomposites fabricated by hot-pressing technique. 497

498

N BAM~, YH CHOAANDK N.HARA

EXPERIMENTAL PROCEDURE 8YSZ with 20 nm of average grain size (Sumitomo Osaka Cement Co., Ltd.) and l-SiC with 150 nm (Mitsui touatsu Ltd.) were used as the matrix and dispersion, respectively. 8YSZ and 13SiC powders were mixed in EtOH for 48 h by ZrO2 balls, where I3-SiC content was 0, 5, 10 and 20 vol.% with 8YSZ. The mixed powders were dried in a rotating vacuum drier, and then dry ballmilled for 12 h and sieved under 150 tim. The powders were hot-pressed at 1300-1900°C for 1 h under 30 MPa of pressure in At. The sintered specimens were cut and ground into rectangular bar specimens (4 x 3 x 36 mm) and polished using diamond paste with 0.5 lain for the evaluation of microstructure and mechanical properties. Densities of specimens were measured by the Archimedes method using toluene. Fracture toughness was evaluated from the indentation fracture method with 49 N-load at room temperature. Fracture strength was measured by the 3point bending test (span: 30 ram, crosshead speed: 0.5 mm/min). The phase of each specimen was identified by X-ray diffraction (XRD) analysis. The microstructurewas observed by scanning electron microscopy (SEM) and transmission electron microscopy(TEM). For comparison, 8YSZ and nanocomposites were thermally etched to estimate their respective grain size and distribution using SEM. RESULTS Figure 1 shows the variation of relative density with sintering temperature for 8YSZ and for 8YSZ/SiC nanocomposites. For 8YSZ, high density (>99%) was obtained at 1400°C, whereas 8YSZ/5voI% SiC nanocomposite hot-pressed at 1400°C showed much lower density than 8YSZ. The sintering temperature to obtain full density (>99%) increased with increasing SiC content: for instance, 1600°C and 1800°C wererespectively neededfor 5 vol% and 20 vol% SiC nanocompsoites. X-ray analysis revealed that 8YSZ/SiC nanocomposites hot-pressed from 1400°C to 1900°C were composed only of 8YSZ and I]-SiC and were free from any reaction phases.

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Figure 1. Variation of relative density with sintering temperature for monolithic 8YSZ and 8YSZ/SiC nanocomposites.

Figure 2. Transmission electron microscopy image of 8YSZ/5voI% SiC sintered at 1700°C.

FABRICATION AND MECHANICAL

PROPERTIES OF Y'n'RIA

STABIUZED

ZIRCONIA COMPOSITES

499

A TEM micrograph taken of 8YSZ/5voI% SiC nanocomposite hot-pressed at 1700°C is shown in Figure 2. The SiC particles in the specimen were located both within the 8 YSZ matrix grains and at the grain boundaries. Figure 3 is SEM photographs of thermally-etched (a) 8YSZ sintered at 1400°C, (b) 8YSZ/5voI% SiC, and (c) 8YSZ/20vol% SiC nanocomposites both sintered at 1800°C. It is clearly seen from Figure 3 that the grain size of matrix decreased with the addition of SiC particulates. Figure 4 shows the result of 3 points bending strength measurement. 8YSZ sintered at 1400°C exhibited a maximum strength of 300 MPa, while the fracture strength of 750 MPa was achieved for the 8YSZ/20vol% SiC nanocomposite. Maximum fracture strength of 8YSZ/5vol% SiC and 10vol% SiC were about 550 MPa. Figure 5 shows the fracture toughness, KIC, evaluated by the IF method. K[c is slightly improved by the addition of SiC particulates.

Figure 3. Scanning electron microscopy nnages o[ thermally etched (a) monolithic 8YSZ sintered at 1400°C, (b) 8YSZ/5voI% SiC, and (c) 8YSZ/20voI% SiC nanocomposites sintered at 1800°C. 1000 ~. 800

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Figure 4. Variation of strength with sintering temperature for monolithic 8YSZ and 8YSZ/ SiC nanocomposites.

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Figure 5. Variation of fracture toughness with sintering temperature for monolithic 8YSZ and 8YSZ/SiC nanocomposites.

500

N BAMe~,YH CHOAANOK NI=HARA

DISCUSSION The fracture strength of 8YSZ was remarkably improved from 300 MPa to 750 MPa by incorporating up to 20 vol% SiC particulates. This improvement may be attributed to the following reasons. First is the refinement of the matrix grain size. The mean grain size with 7 p.m for 8YSZ sintered at 1400°C is decreased to 3 tun and 1 gm of mean grain size by the addition of 5voi% and of 20vo1% SiC sintered at 1700°(2 and 1850°C, respectively. Grain growth for 8YSZ rapidly progressed with increasing the sintering temperature, whereas grain growth was significantly inhibited by the addition of fine SiC particulates. The inhibition of grain growth became more effective with increasing SiC content. In addition, it must be considered that the homogeneous grain size distribution was caused by fine SiC additions. The nanocomposites display narrow grain size distributions compared with that of 8YSZ. In fact, the distributions are sharper with increasing the SiC content. From these results, the strength improvement may be due to the fine and homogeneous microstructure for the nanocomposite systems. The other is the increasing KIc by SiC dispersions. The change of KIc with the SiC content is shown in Figure 5, and KIc increased with increasing SiC content. This improvement can be explained by a crack deflection mechanism at the crack tip and the residual stresses caused by thermal mismatch between matrix 8YSZ and the SiC dispersion. SiC exhibits higher Young's modulus and strength than 8YSZ and then the crack cannot propagate through SiC particles, but it can propagate through 8YSZ malrix and it will be deflected by SiC particles. In fact, a number of crack deflections along the cracks produced by indentation increased with increasing the SiC content. Thus, we believe that the fracture toughness of 20vo1% SiC dispersed nanocomposite was improved by 30% compared with that of 8YSZ. However, the observed increase in fracture strength cannot be explained by the improvement of this fracture toughness. It is believed that only the strong increase in strength is mainly due to the presence of nanosized SiC particulates in the nanocomposites. CONCLUSIONS Nanosized SiC particules dispersed in 8YSZ composites were consolidated using hotpressing technique. The fracture strength of 8YSZ was remarkably improved from 300 MPa to 750 MPa by addition of 20vo1% SiC particulates. This increase in strength is mainly attributed to the fine and homogeneous microstructure developed by the addition of fine SiC particulates. Fracture toughness was also improved by the SiC dispersion. REFERENCES

1. 2. 3. 4. 5.

S. Somiya, Zirconia ceramics 1,109, Uchida Rokakuho, Japan, (1983). N. Claussen, K.-L. Weisskopf and M. Riihle, J. Am. Ceram. Soc., 69(3), 288 (1986). X. Miao, W.E. Lee and W.M. Rainforth, Br. Ceram. Trans., 93(3), 119(1994). K. Niihara, J. Ceram. Soc. Jpn., 99, 974 (1991). T. Ohmi, T. Hirano, A. Nakahira and K. Niihara, J. Am. Ceram. Soc., 79(10), 35, (1996).