S Guo, Y. Kagawa/Journal of the European Ceramic Society 22(2002)2349-2356 with test temperature. Assuming that the fiber stress is uniform and the fiber failure is non-interactive. the ten- sile strength of the composites, aTS, is approximatel given by f iou Noncatastrophic Fracture WLF OH (3b) 02}B where obu is the tensile strength of fiber bundle, fi is the 口1200K◆1400K volume fraction of longitudinal fiber bundle (L f/2) ofu is in situ fiber strength, m is the Weibull modulus of fiber, e is the base of the natural logarithm and r is the Fig. 8. Correlation of tensile strength of the composites normalized by The predicted values are listed in Table 2 to compare the predicted values to in situ fiber strength and interface shear stress with the measured ones. For the Hi-Nicalon/BN/SiC, (BF: brittle fracture, solid symbols: Hi-Nicalon/BN/SiC, dotted sym. the predicted values nearly coincide with the experi- mental results for all the test temperatures. The con- sistency indicates that the tensile strength is governed by the fiber bundle strength and ultimate fracture strength fiber properties and that the full potential of the fibers is is lower than that predicted by fiber bundle model utilized in the composite. This means that the matrix Therefore, although the Nicalon/ C/SiC exhibits a non cracking reached a full-saturated state prior to failure, linear/noncatastrophic behavior in tension, this compo- showing a noncatastrophic fracture mode(Figs. 3, 6 site failed prematurely as compared to the Hi-Nicalon/ and 7) that belongs to a global load sharing(GLS) BN/ SiC(Figs. 2 and 3) condition resulting from the lower interface shear stress Fig. 8 shows correlation of tensile strength and frac- (Table 3). On the contrary, for the Nicalon/C/SiC, the ture behavior of the both composites to fiber strength predicted values are higher than the measured values; in and interface shear stress. It is found that the curve particular, at 1200 K this discrepancy is considerably could be divided into two characteristic regimes: (1) noticeable. Although the fibrous fracture surfaces are of/Ti> 100, tensile strength of the composite coincides observed [Fig. 7(a) and(b)], there is only a modest with the values predicted by fiber strength with a non- amount of fiber pulled-out in the composite, especially catastrophic fracture under GLs condition, and (ii) at 1200 K where the brittle fracture surface is found In f/Ti< 100, tensile strengths of the composite are lower addition, there are regions in all the bundles in which than the predicted values; typically, as fu/Ti is higher the fiber-fracture locations are essentially coplanar with (typical ofu/Ti> 30), the tensile strength of composite another, indicating a strong correlation between breaks. 80% of predicted value(Table 2) with a weakest link This strong correlation probably results from local failure (WLF) mode; in particular as ofu/Ti is lower strong fiber/matrix bonding. Previous studies revealed (typical of/Ti< 10), the tensile strength is only 30% of that the fiber/matrix debonding in CVI-processed Nica- the predicted value(Table 2) with a brittle fracture lonBN/C/SiC is controlled by the thin silica layer fashion. This suggests that the full potential of the fibers resulting from the changes that have occurred near the is utilized only when the ratio of the interface shear surface of the metastable Nicalon SiC fiber during pro- stress to the fiber strength is below a critical value. In the cessing 28.29 Although in the present study the Nicalon/ present work, the critical ratio is approximately 1/100 C/SiC was processed by using non-CVI process but PIP process, this thin silica layer is excepted to be present at the interface, in particular, for the composite sp ecimens 5. Conclusions tested above 700 K oxidation of the C-coating pro- moted formation of silica layer resulting from oxidation of both the matrix and fiber. The interface gener- 1. For the Nicalon/C/SiC, Youngs modulus, ten ated by the silica layer might be the weakest link in the sile strength and strain to failure remained nearly interfacial phase sequence. This silica layer formed dur- constant up to 800 K and significantly then ing processing or/and oxidation generally is dis- decreased at 1200 K. A noncatastrophic fracture continuities and may be regarded as surface flaws which was observed at and below 800K. while a brittle locally weaken the fibers. 28,29 In the case of the weakest fracture fashion similar to monolithic ceramics link, fracture of composite is no longer dominated by occurred at 1200 Kwith test temperature. Assuming that the fiber stress is uniform and the fiber failure is non-interactive, the ten￾sile strength of the composites, TS, is approximately given by27 TS ¼ fLbu ð3aÞ bu ¼ 1 me 1=m fu 1 þ 1 m ð3bÞ where bu is the tensile strength of fiber bundle, fL is the volume fraction of longitudinal fiber bundle (fLf/2), fu is in situ fiber strength, m is the Weibull modulus of fiber, e is the base of the natural logarithm and is the gamma function. The predicted values are listed in Table 2 to compare with the measured ones. For the Hi-Nicalon/BN/SiC, the predicted values nearly coincide with the experi￾mental results for all the test temperatures. The con￾sistency indicates that the tensile strength is governed by fiber properties and that the full potential of the fibers is utilized in the composite. This means that the matrix cracking reached a full-saturated state prior to failure, showing a noncatastrophic fracture mode (Figs. 3, 6 and 7) that belongs to a global load sharing (GLS) condition resulting from the lower interface shear stress (Table 3). On the contrary, for the Nicalon/C/SiC, the predicted values are higher than the measured values; in particular, at 1200 Kthis discrepancy is considerably noticeable. Although the fibrous fracture surfaces are observed [Fig. 7(a) and (b)], there is only a modest amount of fiber pulled-out in the composite, especially at 1200 Kwhere the brittle fracture surface is found. In addition, there are regions in all the bundles in which the fiber-fracture locations are essentially coplanar with another, indicating a strong correlation between breaks. This strong correlation probably results from local strong fiber/matrix bonding. Previous studies revealed that the fiber/matrix debonding in CVI-processed Nica￾lon/BN/C/SiC is controlled by the thin silica layer resulting from the changes that have occurred near the surface of the metastable Nicalon SiC fiber during pro￾cessing.28,29 Although in the present study the Nicalon/ C/SiC was processed by using non-CVI process but PIP process, this thin silica layer is excepted to be present at the interface, in particular, for the composite specimens tested above 700 Koxidation of the C-coating pro￾moted formation of silica layer resulting from oxidation of both the matrix and fiber.22,23 The interface gener￾ated by the silica layer might be the weakest link in the interfacial phase sequence. This silica layer formed dur￾ing processing or/and oxidation generally is dis￾continuities and may be regarded as surface flaws which locally weaken the fibers.28,29 In the case of the weakest link, fracture of composite is no longer dominated by the fiber bundle strength and ultimate fracture strength is lower than that predicted by fiber bundle model. Therefore, although the Nicalon/C/SiC exhibits a non￾linear/noncatastrophic behavior in tension, this compo￾site failed prematurely as compared to the Hi-Nicalon/ BN/SiC (Figs. 2 and 3). Fig. 8 shows correlation of tensile strength and frac￾ture behavior of the both composites to fiber strength and interface shear stress. It is found that the curve could be divided into two characteristic regimes: (i) fu/Ti5100, tensile strength of the composite coincides with the values predicted by fiber strength with a non￾catastrophic fracture under GLS condition, and (ii) fu/Ti<100, tensile strengths of the composite are lower than the predicted values; typically, as fu/Ti is higher (typical fu/Ti530), the tensile strength of composite is 80% of predicted value (Table 2) with a weakest link failure (WLF) mode; in particular as fu/Ti is lower (typical fu/Ti410), the tensile strength is only 30% of the predicted value (Table 2) with a brittle fracture fashion. This suggests that the full potential of the fibers is utilized only when the ratio of the interface shear stress to the fiber strength is below a critical value. In the present work, the critical ratio is approximately 1/100. 5. Conclusions 1. For the Nicalon/C/SiC, Young’s modulus, ten￾sile strength and strain to failure remained nearly constant up to 800 Kand significantly then decreased at 1200 K. A noncatastrophic fracture was observed at and below 800 K, while a brittle fracture fashion similar to monolithic ceramics occurred at 1200 K. Fig. 8. Correlation of tensile strength of the composites normalized by the predicted values to in situ fiber strength and interface shear stress (BF: brittle fracture, solid symbols: Hi-Nicalon/BN/SiC, dotted sym￾bols: Nicalon/C/SiC). S. Guo, Y. Kagawa / Journal of the European Ceramic Society 22 (2002) 2349–2356 2355