D. Wei et al /Ceramics Intemational 32(2006)549-554 55 5 um Fig. 2. SEM micrographs of the etched surface of (a)SNO, (b)SN2, (c) SN4,(d) SN6,(e)SN& and(f)SN10 increasing h-BN content as shown in Fig 3, which is mainly shows the relation of the fracture toughness of h-BN/Si3 N4 due to the low strength of h-BN and weak interface between h- ceramic composites versus h-BN content. As can be seen BN and Si3N4 grains from this figure, the monolithic Si3N4 has high fracture Nevertheless, the bending strength of h-BN/Si3 Na ceramic toughness. However, the fracture toughness of h-BN/Si3 N4 opposites decreases slightly with increasing h-BN content, ceramic composites deceases slightly with increasing h-BN which is due to the reduction of B-Si3N4 grain size. Generally conter of Si3N4 ceramic can be enhanced by Many investigators have demonstrated that the fracture reducing the grain size of B-Si3N4 [13]. According to the toughness of B-Si3N4 ceramic is strongly dependent upon microstructural analysis, the diameter and aspect ratio of B- grain morphology [14-16]. The improved fracture tough Si3N4 grains in composites decrease with increasing h-Bn ness of B-si3N4 ceramic is the result of a reinforcin content. Therefore, the strength has a tendency to be improved phenomenon from the whisker-like B-Si3N4 grains, similar with the reduction of B-Si3N4 grain size. This result reveals to the behavior observed in whisker-reinforced ceramics. It that Siz Na ceramic still has a high bending strength as a small has been demonstrated that the fracture toughness of Si3N4 quantity of h-BN is incorporated into SiNa ceramic matrix. ceramic can be enhanced by increasing the diameter and igh strength ofh-BN/Si3N4 ceramic composites is necessary aspect ratio of elongated B-Si3 N4 grains [1, 17-20). In for its structural application addition, other factors can also affect the fracture toughness such as volume fraction of the bridging grains and the compositions and properties of the boundary phase [21] The effect ofh-BN content on the fracture toughness of h- As mentioned above, the fracture toughness of B-si3N4 BN/Si3N4 ceramic composites was investigated. Fig. 3 ceramic is strongly dependent upon B-Si3 N4 grain morphol ogy. That the monolithic Si3 N4 has high fracture toughness is mainly attributed to large numbers of coarse elongated B- Fracture toughness3日 Si3N4 grains with high aspect ratios, yielding various toughening mechanisms such as grain bridging, crack deflection and pull-out of the elongated B-Si3 N4 grains. As shown in Fig. 4a, many coarse voids due to the pull-out of m0+ the elongated B-Si3N4 grains were observed on the fractured surface of monolithic Si3 N4. However, with increasing h-BN content, the diameter and aspect ratio of elongated B-si3N4 grains decrease, which weakens the toughening mechanisms Fig. 3. Effect of h-BN content on the flexural strength and fracture toughness of h-BN/Si3N4 ceramic composites decreases toughness of h-BN/ N4 composites. with increasing h-BN contentincreasing h-BN content as shown in Fig. 3, which is mainly due to the low strength of h-BN and weak interface between h￾BN and Si3N4 grains. Nevertheless, the bending strength of h-BN/Si3N4 ceramic composites decreases slightly with increasing h-BN content, which is due to the reduction of b-Si3N4 grain size. Generally speaking, the strength of Si3N4 ceramic can be enhanced by reducing the grain size of b-Si3N4 [13]. According to the microstructural analysis, the diameter and aspect ratio of b￾Si3N4 grains in composites decrease with increasing h-BN content. Therefore, the strength has a tendency to be improved with the reduction of b-Si3N4 grain size. This result reveals that Si3N4 ceramic still has a high bending strength as a small quantity of h-BN is incorporated into Si3N4 ceramic matrix. High strength of h-BN/Si3N4 ceramic composites is necessary for its structural application. 3.2.2. Fracture toughness The effect of h-BN content on the fracture toughness of h￾BN/Si3N4 ceramic composites was investigated. Fig. 3 shows the relation of the fracture toughness of h-BN/Si3N4 ceramic composites versus h-BN content. As can be seen from this figure, the monolithic Si3N4 has high fracture toughness. However, the fracture toughness of h-BN/Si3N4 ceramic composites deceases slightly with increasing h-BN content. Many investigators have demonstrated that the fracture toughness of b-Si3N4 ceramic is strongly dependent upon grain morphology [14–16]. The improved fracture tough￾ness of b-Si3N4 ceramic is the result of a reinforcing phenomenon from the whisker-like b-Si3N4 grains, similar to the behavior observed in whisker-reinforced ceramics. It has been demonstrated that the fracture toughness of Si3N4 ceramic can be enhanced by increasing the diameter and aspect ratio of elongated b-Si3N4 grains [1,17–20]. In addition, other factors can also affect the fracture toughness such as volume fraction of the bridging grains and the compositions and properties of the boundary phase [21]. As mentioned above, the fracture toughness of b-Si3N4 ceramic is strongly dependent upon b-Si3N4 grain morphol￾ogy. That the monolithic Si3N4 has high fracture toughness is mainly attributed to large numbers of coarse elongated b￾Si3N4 grains with high aspect ratios, yielding various toughening mechanisms such as grain bridging, crack deflection and pull-out of the elongated b-Si3N4 grains. As shown in Fig. 4a, many coarse voids due to the pull-out of the elongated b-Si3N4 grains were observed on the fractured surface of monolithic Si3N4. However, with increasing h-BN content, the diameter and aspect ratio of elongated b-Si3N4 grains decrease, which weakens the toughening mechanisms of elongated b-Si3N4 grains. Therefore, the fracture toughness of h-BN/Si3N4 ceramic composites decreases with increasing h-BN content. D. Wei et al. / Ceramics International 32 (2006) 549–554 551 Fig. 2. SEM micrographs of the etched surface of (a) SN0, (b) SN2, (c) SN4, (d) SN6, (e) SN8 and (f) SN10. Fig. 3. Effect of h-BN content on the flexural strength and fracture toughness of h-BN/Si3N4 composites