March 1997 the lifetime at950°and60°℃C. As shown in Fig.l, the compos Under high applied stress(appl 2 400 MPa), samples failed ites survived 500 h at 950" and 600C at stress levels of <350 during the static-fatigue tests. Any major microstructural MPa; whereas at stresses >400 MPa, the lifetime range was changes in the composites should have occurred during the 1-100 h, regardless of the testing temperatures. However, dif- fatigue tests rather than because of exposure to high tempera ferent retained room-temperature strengths were obtained in the ture after failure, because(i) the typical lifetime under stresses samples surviving the 500 h tests. Compared to the as-received of 400 and 450 MPa is >5 h, and (i) the specimens w re cooled es, samples that had been static-fatigue tested at 950C to room temperature within 20 min after failure. In samples that for 500 h under an applied stress of 350 MPa failed at a similar were fatigued at 950 C under a stress of 450 MPa, fiber pullout a result of tests at 950C with an applied stress Appl 5 0.6oo, RT in the corner regions. No substantial interfacial reaction was of as-received materials ). On the other hand, an-20% degrada observed in the top 0 ply at depths of >50 um from the tensile samples that had been fatigued at 600C for 500 h under an side surface. In contrast, ie n within 150 um from the external tion in the retained room-temperature strength was measured in surface. In the first 90 ply near the tensile surface, reaction only occurred in the regio ed stress of 350 MPa amples that failed at 600C under a No degradation in the room-temperature flexural strength stress of 450 MPa, no fiber pullout was observed in the entire d in the o°and90° was observed in the sample that was simply annealed in air at plies, and the fiber surface had an appearance that was similar 600C for 500 h without applied stress. The stress-displacement curve and the appearance of the fracture surface of the annealed to that of the sample that had been fatigued under a stress of e were similar to that of the sample that was fatigued at 350 MPa 950%C. This is consistent with the previous results from aging The chemistry of the fiber surfaces in the brittle -failure zone experiments at 550 C, 5 showing that the mechanical properties (in a sample that had been fatigued at 600"C) was analyzed of this composite system are not affected by annealing at inter- mediate temperatures when no external stress is applied to surface were oxygen-bonded silicon, carbon-bonded silicon, the material boron, carbon, nitrogen, and oxygen. The boron nitrogen ratio was -1. and the concentration variance from one area to (2) Microstructural Observations another was in the range of 5-13 at. % Thus, the residual fiber (A Fracture Surface--SEM and SAM Studies: The frac coatings(BN and SiC)and silicon oxides are on the fiber ture surface of composites that were fatigued at 950 and 600C surface. No area was found that had boron, silicon, and oxygen differed in appearance. The sample that was fractured at room but no nitrogen, indicating that no borosilicate glass was formed temperature after exposure to Appl values of s350 MPa at and that BN oxidized via volatilization C exhibited extensive fiber pullout across the entire frac (B) Interfacial Microstructure and Chemistry--TEM Studies ure surface. Reaction at the fiber surface was only observed Reaction occurred in the bn coating layer and at the BN/fiber the first 90 ply on the tensile side and very close to the exposed fiber ends at the external side surfaces. Away from the external interface in samples that were static fatigued at 600C, as shown Figs. 4(A)and (B). The sample was under an applied stress fairly smooth and apparently no interfacial reaction occurred In Fig. 4(A), the reaction has occurred in the region between t room mperature after exposure to oal values of 350 in the light-contrast layer beneath the Sic layer and a dark- MPa at 600C, no fiber pullout was observed in the region on the tensile side(Fig. 2(B). This affected region contrast reaction product growing on the surface of the fiber extended from the tensile surface to the third ply in depth The compositions of these two reaction regions were analyzed and 1-2 mm inward from the side surfaces. There Figure 5(A) is the PEELS spectrum taken from area I in fore, the affected zone that developed during the fatigue test at Fig. 4(A), indicating that the light-contrast area is the BN 600C was at least one order of magnitude larger than that coating layer; Fig. 5(B)is from area 2 in Fig. 4(A), showing no occurred at the fiber/matrix interface and glassy reaction prod- peaks are from the carbon coating that has been depositedomp which developed at 950C. In the affected zone, reaction trace of either boron or nitrogen. (In both spectra, the carbe Ict(s)formed in the 0 and 90 plies, as shown in Figs. 3(A) the TEM foil. In Fig. 4(B), large voids and amorphous liga- and (B) ments have formed in the area between the Sic overlayer and Tensile Surface (A) (B) Fig. 2. Fracture surface of samples static fatigued under a stress of 350 MPa for 500 h and then fractured at room temperature(A)no interfacial eaction was observed in the interior of the composite tested at 950C; (B)a brittle-failure zone with no fiber pullout was observed in the sampleMarch 1997 Environmental Effects on BN-Coated SiC-Fiber-Reinforced Glass-Ceramic Composites 611 the lifetime at 950 and 600C. As shown in Fig. 1, the compos- Under high applied stress (appl
400 MPa), samples failed ites survived 500 h at 950 and 600C at stress levels of 350 during the static-fatigue tests. Any major microstructural MPa; whereas at stresses 400 MPa, the lifetime range was changes in the composites should have occurred during the 1–100 h, regardless of the testing temperatures. However, dif- fatigue tests rather than because of exposure to high tempera￾ferent retained room-temperature strengths were obtained in the ture after failure, because (i) the typical lifetime under stresses samples surviving the 500 h tests. Compared to the as-received of 400 and 450 MPa is 5 h, and (ii) the specimens were cooled samples, samples that had been static-fatigue tested at 950C to room temperature within 20 min after failure. In samples that for 500 h under an applied stress of 350 MPa failed at a similar were fatigued at 950C under a stress of 450 MPa, fiber pullout stress level. Thus, little stress-induced degradation occurred as still occurred in most of the area on the tensile side except a result of tests at 950C with an applied stress appl 0.60,RT in the corner regions. No substantial interfacial reaction was (where 0,RT is the room-temperature ultimate flexural strength observed in the top 0 ply at depths of 50 m from the tensile of as-received materials). On the other hand, an
20% degrada- surface. In the first 90 ply near the tensile surface, reaction tion in the retained room-temperature strength was measured in only occurred in the region within 150 m from the external samples that had been fatigued at 600C for 500 h under an side surface. In contrast, in samples that failed at 600C under a applied stress of 350 MPa. stress of 450 MPa, no fiber pullout was observed in the entire No degradation in the room-temperature flexural strength tensile region. Interfacial reaction occurred in the 0 and 90 was observed in the sample that was simply annealed in air at plies, and the fiber surface had an appearance that was similar 600C for 500 h without applied stress. The stress–displacement to that of the sample that had been fatigued under a stress of curve and the appearance of the fracture surface of the annealed 350 MPa (Figs. 3(A) and (B)). sample were similar to that of the sample that was fatigued at The chemistry of the fiber surfaces in the brittle-failure zone 950C. This is consistent with the previous results from aging (in a sample that had been fatigued at 600C) was analyzed experiments at 550C,15 showing that the mechanical properties using SAM. The elements that were detected on the fiber of this composite system are not affected by annealing at inter￾mediate temperatures when no external stress is applied to surface were oxygen-bonded silicon, carbon-bonded silicon, boron, carbon, nitrogen, and oxygen. The boron:nitrogen ratio the material. was
1, and the concentration variance from one area to (2) Microstructural Observations another was in the range of 5–13 at.%. Thus, the residual fiber (A) Fracture Surface—SEM and SAM Studies: The frac- coatings (BN and SiC) and silicon oxides are on the fiber ture surface of composites that were fatigued at 950 and 600C surface. No area was found that had boron, silicon, and oxygen differed in appearance. The sample that was fractured at room but no nitrogen, indicating that no borosilicate glass was formed temperature after exposure to appl values of 350 MPa at and that BN oxidized via volatilization. 950C exhibited extensive fiber pullout across the entire frac- (B) Interfacial Microstructure and Chemistry—TEM Studies: ture surface. Reaction at the fiber surface was only observed in Reaction occurred in the BN coating layer and at the BN/fiber the first 90 ply on the tensile side and very close to the exposed interface in samples that were static fatigued at 600C, as shown fiber ends at the external side surfaces. Away from the external in Figs. 4(A) and (B). The sample was under an applied stress side surfaces, typically beyond
100 m, the fiber surface was of 450 MPa and exposed to air at 600C for 32 h before failure. fairly smooth and apparently no interfacial reaction occurred In Fig. 4(A), the reaction has occurred in the region between (Fig. 2(A)). On the other hand, in samples that were fractured the Nicalon fiber and the SiC overcoat layer, with voids forming at room temperature after exposure to appl values of 350 in the light-contrast layer beneath the SiC layer and a dark- MPa at 600C, no fiber pullout was observed in the corner contrast reaction product growing on the surface of the fiber. region on the tensile side (Fig. 2(B)). This affected region The compositions of these two reaction regions were analyzed. extended from the tensile surface to the third ply in depth Figure 5(A) is the PEELS spectrum taken from area 1 in (
500 m) and 1–2 mm inward from the side surfaces. There￾fore, the affected zone that developed during the fatigue test at Fig. 4(A), indicating that the light-contrast area is the BN 600C was at least one order of magnitude larger than that coating layer; Fig. 5(B) is from area 2 in Fig. 4(A), showing no which developed at 950C. In the affected zone, reaction trace of either boron or nitrogen. (In both spectra, the carbon occurred at the fiber/matrix interface and glassy reaction prod- peaks are from the carbon coating that has been deposited on uct(s) formed in the 0 and 90 plies, as shown in Figs. 3(A) the TEM foil.) In Fig. 4(B), large voids and amorphous liga￾and (B). ments have formed in the area between the SiC overlayer and (A) (B) Fig. 2. Fracture surface of samples static fatigued under a stress of 350 MPa for 500 h and then fractured at room temperature ((A) no interfacial reaction was observed in the interior of the composite tested at 950C; (B) a brittle-failure zone with no fiber pullout was observed in the sample tested at 600C).