478 C Li et al. Materials Letters 57(2003)3473-3478 BN interlayers and Si3 N4 matrix layers in MYA-LCs ceramics Laminated Si3//BN ceramics with MgO than those in LYA-LCs, and there also are fewer Y2O3-Al2O3 as sintering aid possesses good bending defects in BN interlayer of MYA-LCs than those in strength at room temperature, but has poor work LYA-LCS. So the structure of MYA-LCs is denser fracture at room temperature, low strength and work than that of LYA-LCs. This also agrees well to the of fracture at elevated temperature. As a comparison, discussion above laminated Si3N4/BN ceramics with La, O3-Y2O3 Fig. 6 shows the different microstructural charac- Al2O3 as sintering aid has lower bending strength at teristics of Si3 N4 matrix layer in MYA-LCs and LYA- room temperature. However, the work of fracture at Cs, respectively. It is very clear to see the elongated room temperature and bending strength and work of and interlocked B-Si3N4 grains in the Si3 N4 matrix fracture at elevated temperature of LYA- LCs are much layers, however, the aspect ratios of B-Si3N4 grains in higher than those of MYA-LCs. In addition, the LYa LYA-LCs(Fig. 6(b) are much higher that those in sintering aid is good for forming of elongated and MYA-LCs(Fig 6(a)) interlocked B-Si3N4 grains and beneficial to the As mentioned above, the Si3 N4 raw material con- mechanical properties of the laminated Si3 N4/BN sists of >92 wt. a-Si3N4, so the B-Si3N4 grains are ceramics. derived from the phase transformation of the a-Si3N4 because of liquid phase sintering mechanism [10, 11 In the beginning of sintering, a-Si3N4 grains dissolve Acknowledgements in the liquid phase and then precipitate as needle like B-Si3N4 grains. The growth of B-Si3N4 grains is This work was supported by the National Science controlled by diffusion of silicon and nitrogen through Foundation of China the liquid phase [121, so change of liquid phase composition, modifying silicon and nitrogen diffusion rates, mostly affect the phase transformation and the References B-Si3N4 grains' growth rate. For MYA-LCs, the viscosity of the sintering aid liquid phase formed at w.J. Clegg. Nature 34(990)7455 high temperature is low because of the existence of Mgo, which results in the good fluidity of the liquid [3] H. G phase. So during hot-pressing, the liquid phase is easy Society 25(1997)36(in Chinese) to migrate into the Bn interlayers under the ambient 0 LHaiyan, S M. Hsu, Journal of the American Ceramic Society pressure and the concentration gradient. Thus, there not enough liquid phase left for B-Si3N4 grains to 5] K Desiderio, M D. Thouless, J.w. Halloran, Joumal of the American Ceramic Society 81(1998)1004 grow fully. However, for LYA-LCs, the viscosity of H. Yong, G. Hai, X. Zhipeng, Joumal of Materials Science the sintering aid liquid phase is much higher than that Letters17(1998)569 of MYA-LCs due to the addition of La O3, so there is [7 P.A. Drew, Progress in Nitrogen Ceramics, 1983, P. 323 enough liquid phase for B-Si3N4 to grow well. Thus Boston the aspect ratio of B-Si3N4 grains in LYA-LCs is muc 8]MJ. Hoffmann, G. Petzow, Tailoring of Mechanical Proper- es of Si,N4 Ceramic, 1994, p. 63, Netherlands. igher than that of MYA-LCs. [9]Y Chen, L.L. Huang, X W. Sun, Joumal of the Chinese Ce- ramic Society 25(1997)184. [10 M. Kitayama, K. Hirao, M. Toryama, Acta Materialia 4. Conclusions (1998)6541 [11 M. Kitayama, K. Hirao, M. Toriyama, Acta Materialia (1998)6551 Sintering aids influence evidently the microstruc- [12) E.L. Riley, Journal of the American Ceramic Society 83 ture and mechanical properties of laminated Si3 N4/BN (2000)250BN interlayers and Si3N4 matrix layers in MYA-LCs than those in LYA-LCs, and there also are fewer defects in BN interlayer of MYA-LCs than those in LYA-LCs. So the structure of MYA-LCs is denser than that of LYA-LCs. This also agrees well to the discussion above. Fig. 6. shows the different microstructural charac￾teristics of Si3N4 matrix layer in MYA-LCs and LYA￾LCs, respectively. It is very clear to see the elongated and interlocked h-Si3N4 grains in the Si3N4 matrix layers, however, the aspect ratios of h-Si3N4 grains in LYA-LCs (Fig. 6(b)) are much higher that those in MYA-LCs (Fig. 6(a)). As mentioned above, the Si3N4 raw material con￾sists of >92 wt.% a-Si3N4, so the h-Si3N4 grains are derived from the phase transformation of the a-Si3N4 because of liquid phase sintering mechanism [10,11]. In the beginning of sintering, a-Si3N4 grains dissolve in the liquid phase and then precipitate as needle like h-Si3N4 grains. The growth of h-Si3N4 grains is controlled by diffusion of silicon and nitrogen through the liquid phase [12], so change of liquid phase composition, modifying silicon and nitrogen diffusion rates, mostly affect the phase transformation and the h-Si3N4 grains’ growth rate. For MYA-LCs, the viscosity of the sintering aid liquid phase formed at high temperature is low because of the existence of MgO, which results in the good fluidity of the liquid phase. So during hot-pressing, the liquid phase is easy to migrate into the BN interlayers under the ambient pressure and the concentration gradient. Thus, there is not enough liquid phase left for h-Si3N4 grains to grow fully. However, for LYA-LCs, the viscosity of the sintering aid liquid phase is much higher than that of MYA-LCs due to the addition of La2O3, so there is enough liquid phase for h-Si3N4 to grow well. Thus, the aspect ratio of h-Si3N4 grains in LYA-LCs is much higher than that of MYA-LCs. 4. Conclusions Sintering aids influence evidently the microstruc￾ture and mechanical properties of laminated Si3N4/BN ceramics. Laminated Si3N4/BN ceramics with MgO – Y2O3 –Al2O3 as sintering aid possesses good bending strength at room temperature, but has poor work of fracture at room temperature, low strength and work of fracture at elevated temperature. As a comparison, laminated Si3N4/BN ceramics with La2O3 –Y2O3 – Al2O3 as sintering aid has lower bending strength at room temperature. However, the work of fracture at room temperature and bending strength and work of fracture at elevated temperature of LYA-LCs are much higher than those of MYA-LCs. In addition, the LYA sintering aid is good for forming of elongated and interlocked h-Si3N4 grains and beneficial to the mechanical properties of the laminated Si3N4/BN ceramics. Acknowledgements This work was supported by the National Science Foundation of China. References [1] W.J. Clegg, Nature 34 (1990) 7455. [2] Y. Huang, C. Li, et al., Materials Review 14 (2000) 8 (in Chinese). [3] H. Guo, Y. Huang, et al., Journal of the Chinese Ceramic Society 25 (1997) 36 (in Chinese). [4] L. Haiyan, S.M. Hsu, Journal of the American Ceramic Society 79 (1996) 2452. [5] K. Desiderio, M.D. Thouless, J.W. Halloran, Journal of the American Ceramic Society 81 (1998) 1004. [6] H. Yong, G. Hai, X. Zhipeng, Journal of Materials Science Letters 17 (1998) 569. [7] P.A. Drew, Progress in Nitrogen Ceramics, 1983, p. 323, Boston. [8] M.J. Hoffmann, G. Petzow, Tailoring of Mechanical Proper￾ties of Si3N4 Ceramic, 1994, p. 63, Netherlands. [9] Y. Chen, L.L. Huang, X.W. Sun, Journal of the Chinese Ce￾ramic Society 25 (1997) 184. [10] M. Kitayama, K. Hirao, M. Toriyama, Acta Materialia 46 (1998) 6541. [11] M. Kitayama, K. Hirao, M. Toriyama, Acta Materialia 46 (1998) 6551. [12] F.L. Riley, Journal of the American Ceramic Society 83 (2000) 250. 3478 C. Li et al. / Materials Letters 57 (2003) 3473–3478