Availableonlineatwww.sciencedirect.com SCIENCE DIRECT MATERIALS ELSEVIER Materials Letters 57(2003)1670-1674 www.elsevier.com/locate/matlet Thermal shock behavior of SiC whisker reinforced Si3N4/BN fibrous monolithic ceramics ShuQin Li", Yong Huang, Yong Ming Luo, ChangAn Wang, Cui we State key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Enginering. TSinghua Universiy. Received 25 June 2001; received in revised form 20 June 2002: accepted 30 June 2002 Abstract Sic whisker reinforced Si3 N,/BN fibrous monolithic ceramics were fabricated by in situ synthesizing. The specimens were heated up to the test temperature and then quenched in a water bath of 25C. The therma resistance parameters were calculated according to the elastic modulus, Poisson ratio, thermal expansion coefficient and work of fracture. The thermal shock resistance critical temperature, AT, was 700C, which was improved more than SiC whisker reinforced Si3N4 ceramic BN soft cell boundaries had an effect on the thermal shock behavior of the si3 N4/Bn fibrous monolithic ceramics. The microstructures of the composites were observed by SEM o 2002 Elsevier Science B.V. All rights reserved. Keywords: Fibrous monolithic ceramics: Thermal shock resistance: Microstructure 1. Introduction years, laminated and fibrous monolithic ceramics have been developed to improve the toughness of ceramics Silicon nitride-based ceramics possess excellent through design of the macro- and microstructure of high temperature strength, oxidation resistance, good composites [3-7]. Therefore, the fracture toughness thermal shock resistance, low density and low coef- was improved greatly due to the change of the ficient of thermal expansion [1, 2]. These properties structure make them ideal candidate materials for high-temper- An important property of the ceramics for high ature applications, such as aerospace structural parts temperature applications is the thermal shock resist and turbine engines. However, the nature of brittleness ance. Sic whisker reinforcement in Si3 N4 matrix is a and thereby the lack of damage tolerance is one of the well-established method of enhancing the thermal most crucial problems in their applications. In recent shock resistance and the addition of bn in silicon nitride ceramic increases a/Ea [8] which improves the thermal shock resistance. In this paper, the effects composition structure on thermal shock resistance of Si3N4/Bn fibrous monolithic ceramic were investi E-mailaddress:Isq97(@mails.tsinghua.edu.cn(S.Li). gated systematically 0167-577X/02/S- see front matter c 2002 Elsevier Science B v. All rights reserved P:s0167-577X(02)01049-2
Thermal shock behavior of SiC whisker reinforced Si3N4/BN fibrous monolithic ceramics ShuQin Li *, Yong Huang, YongMing Luo, ChangAn Wang, CuiWei Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 25 June 2001; received in revised form 20 June 2002; accepted 30 June 2002 Abstract SiC whisker reinforced Si3N4/BN fibrous monolithic ceramics were fabricated by in situ synthesizing. The specimens were heated up to the test temperature and then quenched in a water bath of 25 jC. The thermal shock resistance parameters were calculated according to the elastic modulus, Poisson ratio, thermal expansion coefficient, strength and work of fracture. The thermal shock resistance critical temperature, DT, was 700 jC, which was improved more than SiC whisker reinforced Si3N4 ceramic. BN soft cell boundaries had an effect on the thermal shock behavior of the Si3N4/BN fibrous monolithic ceramics. The microstructures of the composites were observed by SEM. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fibrous monolithic ceramics; Thermal shock resistance; Microstructure 1. Introduction Silicon nitride-based ceramics possess excellent high temperature strength, oxidation resistance, good thermal shock resistance, low density and low coefficient of thermal expansion [1,2]. These properties make them ideal candidate materials for high-temperature applications, such as aerospace structural parts and turbine engines. However, the nature of brittleness and thereby the lack of damage tolerance is one of the most crucial problems in their applications. In recent years, laminated and fibrous monolithic ceramics have been developed to improve the toughness of ceramics through design of the macro- and microstructure of composites [3 –7]. Therefore, the fracture toughness was improved greatly due to the change of the structure. An important property of the ceramics for hightemperature applications is the thermal shock resistance. SiC whisker reinforcement in Si3N4 matrix is a well-established method of enhancing the thermal shock resistance and the addition of BN in silicon nitride ceramic increases r/Ea [8] which improves the thermal shock resistance. In this paper, the effects of composition structure on thermal shock resistance of Si3N4/BN fibrous monolithic ceramic were investigated systematically. 0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0167-577X(02)01049-2 * Corresponding author. E-mail address: lsq97@mails.tsinghua.edu.cn (S. Li). www.elsevier.com/locate/matlet Materials Letters 57 (2003) 1670 – 1674
S Li et al. Materials Letters 57(2003)1670-1674 1671 2. Experimental methods 3. Results and discussion 2. 1. Raw materials and fabrication 3.1. Determination of the thermal shock resistance Si3N4(Founder High-Tech ceramic, China) pow- ders with 8 wt. Y2O3 (purity >99.9%), 4 wt% A thermal shock weakens the fracture strength of Al2O3(99.9%)were ball milled with 20 wt. Sic the material significantly, because of the cracks whisker (TWS-400, Tokai Carbon Japan)in ethanol formed by the thermal stress [10]. The presence of for 24 h to achieve a homogenous mixture. The the thermal stress is the primary reason for decrease in mixture was mixed with organic binders and then the strength. The tensile stress on the surface by the produced green filaments using an extrusion pr thermal shock is given b ess. The green filaments were subsequently coated (1) dewaxing, the green body was hot pressed in a where o is the tensile stress on the surface by the graphite resistance furnace under N2 at 1820Cfor thermal shock, a is the thermal expansion coefficient, 1.5 h and under pressure 22 MPa. a detailed E is the elastic modulus, p is the poisson ratio andAT description of the fabrication process can be found equals To-T which is the temperature difference in Ref. 9]. The SiC whisker reinforced Si3N4 In general. the gth of the ceramics remains ceramic was fabricated by hot pressing in the same constant until the temperature difference reaches a critical value(ATs). Therefore, AT is often used to characterize the thermal shock behavior of ceramics 2.2. Thermal shock test The fracture initiation and crack propagation resist ance are two design principles used to express thermal Thermal shock experiments were performed by shock resistance For fracture initiation resistance. the measuring the retained bending strength after capacity is described by [ll], quenching specimens from successively higher tem 4 3 X 36 mm'rectangular bars, then polished with R=oB(I-D) peratures in water. The test specimens were cut into (2) diamond pastes down to 3.5 um. After that, the where aB is the room-temperature bending strength and maintained at that temperature for 10 min to under tensile stress eliminate any temperature gradient within them. The Higher R represents greater resistance to the ini- samples dropped parallel to their long axes into the tiation of fracture during rapid quenching and during water. and the time is 0.3 s from the furnace to the a steep temperatur quench bath. For each condition, seven specimens gradient. According to this equation, to obtain an were tested. The retained strength of the thermally improved thermal shock resistance, it is necessary to shocked composites was measured at room temper have higher strength but lower Poisson ratio, thermal ature and a crosshead speed of 0.5 mm/min by three- expansion coefficient, and elastic modulus point bending using an Instron universal testing After cracks are initiated. the resistance of crack machine. The work of fracture (woF) of the propagation is very important. Such a parameter, Si3N4/Bn fibrous monolithic ceramic and the si defined by Hasselman [12], is shown below four-point bending test with lower and upper spans R=-WoE whisker reinforced Si3 N4 ceramic was measured in a G(1-) of 30 and 10 mm, respectively. The bending tests were performed at a crosshead speed of 0.05 mm/ where wo is the work of fracture. The resistance to min using specimens of nominal dimensions 4 x3x crack propagation therefore increases as the fracture 36 mm energy, elastic modulus, and Poisson ratio increase
2. Experimental methods 2.1. Raw materials and fabrication Si3N4 (Founder High-Tech ceramic, China) powders with 8 wt.% Y2O3 (purity >99.9%), 4 wt.% Al2O3 (>99.9%) were ball milled with 20 wt.% SiC whisker (TWS-400, Tokai Carbon Japan) in ethanol for 24 h to achieve a homogenous mixture. The mixture was mixed with organic binders and then produced green filaments using an extrusion process. The green filaments were subsequently coated with slurry of 25 wt.% BN and 75 wt.% Al2O3, dried and parallel packed into a graphite die. After dewaxing, the green body was hot pressed in a graphite resistance furnace under N2 at 1820 jC for 1.5 h and under pressure 22 MPa. A detailed description of the fabrication process can be found in Ref. [9]. The SiC whisker reinforced Si3N4 ceramic was fabricated by hot pressing in the same condition. 2.2. Thermal shock test Thermal shock experiments were performed by measuring the retained bending strength after quenching specimens from successively higher temperatures in water. The test specimens were cut into 4 3 36 mm3 rectangular bars, then polished with diamond pastes down to 3.5 Am. After that, the specimens were heated up to the testing temperature and maintained at that temperature for 10 min to eliminate any temperature gradient within them. The samples dropped parallel to their long axes into the water, and the time is 0.3 s from the furnace to the quench bath. For each condition, seven specimens were tested. The retained strength of the thermally shocked composites was measured at room temperature and a crosshead speed of 0.5 mm/min by threepoint bending using an Instron universal testing machine. The work of fracture (WOF) of the Si3N4/BN fibrous monolithic ceramic and the SiC whisker reinforced Si3N4 ceramic was measured in a four-point bending test with lower and upper spans of 30 and 10 mm, respectively. The bending tests were performed at a crosshead speed of 0.05 mm/ min using specimens of nominal dimensions 4 3 36 mm3 . 3. Results and discussion 3.1. Determination of the thermal shock resistance parameters A thermal shock weakens the fracture strength of the material significantly, because of the cracks formed by the thermal stress [10]. The presence of the thermal stress is the primary reason for decrease in the strength. The tensile stress on the surface by the thermal shock is given by rf ¼ aE 1 t ðT0 TÞ ð1Þ where rf is the tensile stress on the surface by the thermal shock, a is the thermal expansion coefficient, E is the elastic modulus, t is the Poisson ratio, and DT equals T0 T which is the temperature difference. In general, the strength of the ceramics remains constant until the temperature difference reaches a critical value (DTc). Therefore, DTc is often used to characterize the thermal shock behavior of ceramics. The fracture initiation and crack propagation resistance are two design principles used to express thermal shock resistance. For fracture initiation resistance, the capacity is described by [11], RI ¼ rBð1 tÞ aE ð2Þ where rB is the room-temperature bending strength under tensile stress. Higher RI represents greater resistance to the initiation of fracture during rapid quenching and during steady-state heat flow down a steep temperature gradient. According to this equation, to obtain an improved thermal shock resistance, it is necessary to have higher strength but lower Poisson ratio, thermal expansion coefficient, and elastic modulus. After cracks are initiated, the resistance of crack propagation is very important. Such a parameter, as defined by Hasselman [12], is shown below: RII ¼ w0E r2 Bð1 tÞ ð3Þ where w0 is the work of fracture. The resistance to crack propagation therefore increases as the fracture energy, elastic modulus, and Poisson ratio increase S. Li et al. / Materials Letters 57 (2003) 1670–1674 1671
1672 S Li et al./Materials Letters 57(2003)1670-1674 and strength decreases. It is noted from Eqs. (2)and (a) ()that strength, elastic modulus and Poisson ratio have an adverse effect on crack initiation and crack propagation. However, if the work of fracture of materials is significantly increased, that is, if the ratio of Ewo/ab increases even for a high value of GB and a low value of e, then maximizing both resistances can be achieved The Rand Rare calculated by Eqs. (2)and(3), respectively; the results are shown in Table 1. It can be seen that the composite specimens exhibited slightly higher R' values than that of Sic whisker reinforced T(°C) Si3N4. However, the rvalues of the composite speci- (b) mens were more than five times as ther value of sic concluded that the resistance against crack initiation of 3 80 whisker reinforced Si3N4. From the results, it can be the materials prepared is slightly better than that of Sic whisker reinforced Si3 N4, but the resistance of materi als against crack propagation is much higher than the Sic whisker reinforced Si3N4 3. 2. Thermal shock resistance of the Si, N,BN fibrou aIcs Fig. 1(a) and(b) shows the relationship of the retained bending strength of the quenched specimens and the quenching temperature. The results reveal that Fig. I. The retained as a function of the the bending sti ength of the Si3 N/BN fibrous mono- difference of thermal 如 (a) SigN//bn fibrous monolithic lithic ceramic degrades abruptly above 700C, so the ceramics,(b)SiC whisker reinforced Si3N4 CI thermal shock critical temperature, ATc is 700C which exceeded 150C as compared with the Sic materials. The WoF of the Si3 N,/BN fibrous mono- whisker reinforced Si3 N4 ceramic. The result coin- lithic ceramic is above 4000 J/m", which is higher cides with the above analysis greatly than Sic whisker reinforced Si3 N4 ceramic whose WOF is only 780 J/m". The high WOF leads to 3.3. Effect of Bn cell boundary on thermal shock he excellent thermal shock resistance of the material Si3N/bn fibrous monolithic ceramic sintered tem- perature is 1820C. bn cell boundary will not be It is well known that there are many factors which sintered at this temperature and is a soft interlayer. In affect the thermal shock resistance, but according to the basal plane, the coefficient(CET)of BN is slightly the preceding analysis, the high work of fracture is negative through 800 C, about -29x10/C. important for high thermal shock resistance of the Perpendicular to the basal plane, the CEt is very large Thermal shock resistance parameters and room-temperature properties of Si N,/BN fibrous monolithic ceramics and SiC whisker reinforced SinA ceramics x(×10-5)E(GPa)wo(Jm2)R2(×10-3)R(×10-3) Si3N//BN fibrous monolithic ceramics 689 0.2834 4000 61
and strength decreases. It is noted from Eqs. (2) and (3) that strength, elastic modulus and Poisson ratio have an adverse effect on crack initiation and crack propagation. However, if the work of fracture of materials is significantly increased, that is, if the ratio of Ew0/rB 2 increases even for a high value of rB and a low value of E, then maximizing both resistances can be achieved. The RI and RII are calculated by Eqs. (2) and (3), respectively; the results are shown in Table 1. It can be seen that the composite specimens exhibited slightly higher RI values than that of SiC whisker reinforced Si3N4. However, the RII values of the composite specimens were more than five times as the RII value of SiC whisker reinforced Si3N4. From the results, it can be concluded that the resistance against crack initiation of the materials prepared is slightly better than that of SiC whisker reinforced Si3N4, but the resistance of materials against crack propagation is much higher than the SiC whisker reinforced Si3N4. 3.2. Thermal shock resistance of the Si3N4/BN fibrous monolithic ceramics Fig. 1(a) and (b) shows the relationship of the retained bending strength of the quenched specimens and the quenching temperature. The results reveal that the bending strength of the Si3N4/BN fibrous monolithic ceramic degrades abruptly above 700 jC, so the thermal shock critical temperature, DTc is 700 jC which exceeded 150 jC as compared with the SiC whisker reinforced Si3N4 ceramic. The result coincides with the above analysis. 3.3. Effect of BN cell boundary on thermal shock resistance It is well known that there are many factors which affect the thermal shock resistance, but according to the preceding analysis, the high work of fracture is important for high thermal shock resistance of the materials. The WOF of the Si3N4/BN fibrous monolithic ceramic is above 4000 J/m2 , which is higher greatly than SiC whisker reinforced Si3N4 ceramic whose WOF is only 780 J/m2 . The high WOF leads to the excellent thermal shock resistance of the material. Si3N4/BN fibrous monolithic ceramic sintered temperature is 1820 jC. BN cell boundary will not be sintered at this temperature and is a soft interlayer. In the basal plane, the coefficient (CET) of BN is slightly negative through 800 jC, about 2.9 10 6 /jC. Perpendicular to the basal plane, the CET is very large Table 1 Thermal shock resistance parameters and room-temperature properties of Si3N4/BN fibrous monolithic ceramics and SiC whisker reinforced Si3N4 ceramics rB (MPa) t a ( 10 6 ) E (GPa) w0 (J/m2 ) RI ( 10 3 ) RII ( 10 3 ) Si3N4/BN fibrous monolithic ceramics 689 0.28 3.4 260 4000 561 3040 SiC whisker reinforced Si3N4 ceramics 785 0.26 3.3 315 780 559 538.8 Fig. 1. The retained strength as a function of the temperature difference of thermal shock. (a) Si3N4/BN fibrous monolithic ceramics, (b) SiC whisker reinforced Si3N4 ceramics. 1672 S. Li et al. / Materials Letters 57 (2003) 1670–1674
S Li et al. Materials Letters 57(2003)1670-1674 8314832ek Fig. 2. The crack propagation in Si,N/BN fibrous monolithic Fig. 4. SEM of a typical fracture surface of a quenched specimen from825to25°C direction. The major features of the bn are extensive and positive, about 405X 10/C [14]. As the microcracks between the(0001) basal planes of BN posite is cooled from the hot-pressing temperature platelets. The microcrack structure in the Si,N/BN (1820C), the Bn contracts perpendicular to the basal fibrous monolithic ceramic has been described by plane (i.e, in the [000l] direction), while there is a Kovar et al A similar structure has been reported small expansion within the plane. If the surrounding by Mrozwski [14] in graphite that has a crystalline Si3N4 grains or glassy phase constrain the bn plate structure similar to bn. due to the microcrack struc lets, large tensile stresses are developed perpendicular ture, the intensive dissipation of energy absorbed by to the basal plane upon cooling. This acts to numerous microcracks is helpful to reinforce the the bn platelet into layers along the basal plane thermal damage resistance. Meanwhile, because the BN cell boundary is a weak interlayer and possesses fine-microstructure. crack deflection at the bn cell 811e8228KU49*2NM Fig. 3. SEM of Si3 N4 perpendicular to the hot-pressing direction. Fig. 5. SEM of the fracture surface of a normal specimen
and positive, about 40.5 10 6 /jC [14]. As the composite is cooled from the hot-pressing temperature (1820 jC), the BN contracts perpendicular to the basal plane (i.e., in the [0001] direction), while there is a small expansion within the plane. If the surrounding Si3N4 grains or glassy phase constrain the BN platelets, large tensile stresses are developed perpendicular to the basal plane upon cooling. This acts to separate the BN platelet into layers along the basal plane direction. The major features of the BN are extensive microcracks between the (0001) basal planes of BN platelets. The microcrack structure in the Si3N4/BN fibrous monolithic ceramic has been described by Kovar et al. [13]. A similar structure has been reported by Mrozwski [14] in graphite that has a crystalline structure similar to BN. Due to the microcrack structure, the intensive dissipation of energy absorbed by numerous microcracks is helpful to reinforce the thermal damage resistance. Meanwhile, because the BN cell boundary is a weak interlayer and possesses fine-microstructure, crack deflection at the BN cell Fig. 2. The crack propagation in Si3N4/BN fibrous monolithic ceramic. Fig. 3. SEM of Si3N4 perpendicular to the hot-pressing direction. Fig. 4. SEM of a typical fracture surface of a quenched specimen from 825 to 25 jC. Fig. 5. SEM of the fracture surface of a normal specimen. S. Li et al. / Materials Letters 57 (2003) 1670–1674 1673
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 cracking 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 approximately 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 expansions between BN cell boundary and Si3N4 fiber at abrupt thermal shock. 4. Conclusions Si3N4/BN fibrous monolithic ceramics were fabricated by in situ synthesizing. The thermal shock behavior was evaluated by water quench method. In comparison with SiC whisker reinforced Si3N4 ceramics, 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. Ceram. 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. Ceram. Soc. 72 (1989) 1656. 1674 S. Li et al. / Materials Letters 57 (2003) 1670–1674