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 61and 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 speci￾mens 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 materi￾als 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 mono￾lithic 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 coin￾cides 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 mono￾lithic 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 tem￾perature 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