1674 S. Li et al./ Materials Letters 57(2003)1670-1674 boundaries, as well as significant delamination crack 4. Conclusions ing and sliding, occurs [13]. Fig. 2 shows the crack propagation in Si3 N4/BN fibrous monolithic ceramic Si3N/bn fibrous monolithic ceramics were fabri- In Si3N4/BN fibrous monolithic ceramic, the cated by in situ synthesizing. The thermal shock glassy phase flows to the Bn cell boundary and less behavior was evaluated by water quench method. In glass phase is present in the silicon nitride grains comparison with SiC whisker reinforced Si3 N4 ce- within the cells of fibrous monoliths as compared to ramics, the material showed the excellent thermal silicon nitride grains in a monolithic specimen [9, 13]. shock behavior due to the high WoF resulting from Tanaka et al. [15] reported that the fracture toughness he crack deflection and microcracks on the bn cell of a silicon nitride without sintering aids is about 3 boundary. The resistance against crack initiation of the MPa m, but the fracture toughness of a typica composites was ly better than that of sit silicon nitride with a sintering aid glass is approx- whisker reinforced silicon nitride. but their resistance imately 6 MPa m. Thus, it seems that the sample against crack propagation was much higher than that with less glass phase is more likely to have a lower of Sic whisker reinforced silicon nitride thermal shock resistance. According to analysis of Tanaka et al. [15], the lower value of the silicon nitride without sintering aids may arise from either Acknowledgements the difference in morphology of the grains from the highly elongated ones or the transgranular fractur This work has been supported by National Science mode. In this work, the morphology of Si,N4 grai oundation of China(NSF) of the fibrous monolithic ceramic is studied(as shown in Fig. 3). The results reveal that Si3 N4 grains are highly elongated. Moreover, the fracture mode is References similar to that of the general silicon nitride. So the purified Si3N4 grains boundaries do not affect the [1G. Ziegler, J. Heinrich, G. Wotting, J Mater. Sci. 22(1987) thermal shock resistance of the fibrous monolithic 2]JJ. Mecholsky Jr, Ceram. Bull. 68(1989)1083 ceramic greatly. 3]S Baskaran, S.D. Nunn, D. PoPoVic, J.W. Halloran, J.Am. Ceran.Soc.76(1993)2209 3. 4. Microstructure observations and analysis [4]S. Baskaran, J.w. Halloran, J Am Ceram Soc. 76(1993)2217. 5]SBaskaran, J.w. Halloran, J Am Ceram Soc. 77(1994)1249. A study on the morphology of the fracture surfaces [6S. Baskaran, J.W. Halloran, J. Am. Ceram Soc. 77(1994)1256. R W. Trice, J.W. Halloran, J Am Ceram. Soc. 82( 1999)2943 is observed by SEM. Fig. 4 shows a typical fractur 8] E.H. Lutz, M.V. Swain, J. Am. Ceram Soc. 75(1992)67 surface of thermally shocked material from 825 to 25 9]H Guo, Y Huang, C. Wang, J Mater. Sci. 34(1999)2455 C. The fracture surface of a normal specimen (i.e, o]1.-S. Kim, Mater Res. Bull. 33(1998)1069 specimen did not undergo a thermal shock, just [Il] J. Nakajima, in: R. C. Brandt (Ed), Fracture Mechanics of fractured at room temperature)is shown in Fig. 5 [12] D. P.H. Hasselman, J Am Ceram Soc. 52(1969)600 for comparison. It can be seen at the bonding [13 D. Kovar, B H. King, R w. Trice, J W. Halloran, J. Am. Ce- between Si3 N4 and bn was strengthened for normal ram.Soc.80(1997)2471. specimens(Fig. 5), but that of the thermally shocked [14]S. Mrozwski, Mechanical strength, thermal expansion, and composite was weakened, as shown in Fig. 4. The structure of cokes and carbon. proceeding of the lst and 2nd stresses are caused by mismatched thermal expan- Conference on Carbon, Waverly Press, Baltimore, MD, 1956 ons between bN cell boundary and Si3 N4 fiber at [15]1. Tanaka, G. Pezztti, T. Okamoto, Y. Miyamoto, J. Am. Ce- abrupt thermal shock.boundaries, as well as significant delamination crack￾ing and sliding, occurs [13]. Fig. 2 shows the crack propagation in Si3N4/BN fibrous monolithic ceramic. In Si3N4/BN fibrous monolithic ceramic, the glassy phase flows to the BN cell boundary and less glass phase is present in the silicon nitride grains within the cells of fibrous monoliths as compared to silicon nitride grains in a monolithic specimen [9,13]. Tanaka et al. [15] reported that the fracture toughness of a silicon nitride without sintering aids is about 3 MPa m1/2, but the fracture toughness of a typical silicon nitride with a sintering aid glass is approx￾imately 6 MPa m1/2. Thus, it seems that the sample with less glass phase is more likely to have a lower thermal shock resistance. According to analysis of Tanaka et al. [15], the lower value of the silicon nitride without sintering aids may arise from either the difference in morphology of the grains from the highly elongated ones or the transgranular fracture mode. In this work, the morphology of Si3N4 grains of the fibrous monolithic ceramic is studied (as shown in Fig. 3). The results reveal that Si3N4 grains are highly elongated. Moreover, the fracture mode is similar to that of the general silicon nitride. So the purified Si3N4 grains boundaries do not affect the thermal shock resistance of the fibrous monolithic ceramic greatly. 3.4. Microstructure observations and analysis A study on the morphology of the fracture surfaces is observed by SEM. Fig. 4 shows a typical fracture surface of thermally shocked material from 825 to 25 jC. The fracture surface of a normal specimen (i.e., specimen did not undergo a thermal shock, just fractured at room temperature) is shown in Fig. 5. for comparison. It can be seen that the bonding between Si3N4 and BN was strengthened for normal specimens (Fig. 5), but that of the thermally shocked composite was weakened, as shown in Fig. 4. The stresses are caused by mismatched thermal expan￾sions between BN cell boundary and Si3N4 fiber at abrupt thermal shock. 4. Conclusions Si3N4/BN fibrous monolithic ceramics were fabri￾cated by in situ synthesizing. The thermal shock behavior was evaluated by water quench method. In comparison with SiC whisker reinforced Si3N4 ce￾ramics, the material showed the excellent thermal shock behavior due to the high WOF resulting from the crack deflection and microcracks on the BN cell boundary. The resistance against crack initiation of the composites was slightly better than that of SiC whisker reinforced silicon nitride, but their resistance against crack propagation was much higher than that of SiC whisker reinforced silicon nitride. Acknowledgements This work has been supported by National Science Foundation of China (NSF). References [1] G. Ziegler, J. Heinrich, G. Wotting, J. Mater. Sci. 22 (1987) 3041. [2] J.J. Mecholsky Jr., Ceram. Bull. 68 (1989) 1083. [3] S. Baskaran, S.D. Nunn, D. PoPoVic, J.W. Halloran, J. Am. Ceram. Soc. 76 (1993) 2209. [4] S. Baskaran, J.W. Halloran, J. Am. Ceram. Soc. 76 (1993) 2217. [5] S. Baskaran, J.W. Halloran, J. Am. Ceram. Soc. 77 (1994) 1249. [6] S. Baskaran, J.W. Halloran, J. Am. Ceram. Soc. 77 (1994) 1256. [7] R.W. Trice, J.W. Halloran, J. Am. Ceram. Soc. 82 (1999) 2943. [8] E.H. Lutz, M.V. Swain, J. Am. Ceram. Soc. 75 (1992) 67. [9] H. Guo, Y. Huang, C. Wang, J. Mater. Sci. 34 (1999) 2455. [10] I.-S. Kim, Mater. Res. Bull. 33 (1998) 1069. [11] J. Nakajima, in: R.C. Brandt (Ed.), Fracture Mechanics of Ceramics, vol. 2, Plenum, New York, 1974, p. 759. [12] D.P.H. Hasselman, J. Am. Ceram. Soc. 52 (1969) 600. [13] D. Kovar, B.H. King, R.W. Trice, J.W. Halloran, J. Am. Ce￾ram. Soc. 80 (1997) 2471. [14] S. Mrozwski, Mechanical strength, thermal expansion, and structure of cokes and carbon, Proceeding of the 1st and 2nd Conference on Carbon, Waverly Press, Baltimore, MD, 1956, p. 31. [15] I. Tanaka, G. Pezztti, T. Okamoto, Y. Miyamoto, J. Am. Ce￾ram. Soc. 72 (1989) 1656. 1674 S. Li et al. / Materials Letters 57 (2003) 1670–1674