612 Journal of the American Ceramic Sociery-Sun et al Vol. 80. No. 3 (A) (B matrix er Interfacial reaction occurred in the sample fatigued at 600C for 500 h under an applied stress of 350 MPa(A) in the top 0 ply, glassy product(s)formed at the fiber/matrix interface(arrow) and no fiber pullout was observed; (B)in the first 90 ply, viscous glass phase(s) on the fiber surfaces) the Nicalon fiber, where the bn coating layer has been oxi- initiated at the tensile surface and propagated inward, deflected dized. The areas shown in Figs. 4(A)and(B)are two different at the BN/fiber interface, and left the 0 fibers bridging the interfacial regions along the same fiber, with the area in rack surfaces in the crack wake; whereas in the first 90 ply, Fig. 4(B) closer to the external surface. Considering the fact cracks initiated at the external side surface and propagated that interfacial reactions proceed along the fibers from the along the BN/fiber interface At 950C, the BN coating layer in a SiC/BN system remained intact and the crack opening at the the composite, Fig 4(A) shows an early stage of the reaction, BN/fiber interface was quickly sealed by the oxidation products reaction was initiated at the BN/fiber interface and proceeded For the 0 bridging fibers in the top ply, fiber oxidation occurred into the BN layer. Again, neither boron nor nitrogen w only in a very short zone that had been exposed to the matrix detected in the amorphous ligaments. Electron-diffraction pa crack, and the fiber strength might not degrade significantl tern and EDS analyses indicated that this reaction product was Thus, cracks in the matrix in the top ply were unlikely to grow amorphous silica. These observations are consistent with the with the fibers bridging the crack surfaces in the crack wake SAM results described above This is evidenced by the fact that, in the samples that were tested at 950C (500 h and 350 MPa), the brittle zone IV. Discussion limited within a depth of 50 um from the tensile surface Similarly, in the first 90 ply, the crack opening at the bN/fibe The rimental results presented above indicate that, for interface was sealed and the affected zone did not extend the Sic/bn dual-coated Nicalon-fiber-reinforced BMAS glass- eyond 100 um. However, at lower temperatures(600%), the ceramic composites, stress-induced degradation in an oxidizing opening at the BN/fiber interface could not be fully sealed by environment occurred more readily at intermediate tempera- the reaction products, because of the lower oxidation rates of tures than at high temperatures. Because of the high stress level the SiC fibers than those at high temperatures. Oxidation stud used in the present study(appl value of -60%-70% of oorr) es of SiC fibers indicated that the thickness of the Sio, film matrix cracking was induced under the applied load, especially that formed on the fiber was proportional to the oxidation rate near the tensile surface, where the applied tensile stress was constant, which was a factor of -3 larger at 950C than that at maximum. Previous studies have revealed that interfacial 600.. In addition, the bn coating layer on the SiC fibe debonding in this composite system occurred mostly at the oxidized via volatilization at 600.. Therefore, the cracks at the BN/fiber interface. o Therefore, in the top ply (0), the crack BN/fiber interface were likely to remain open and function as a 150nm 200nm void-forming SiC SiC BN ea area 2 fiber reaction product Fig. 4. Interfacial microstructures in a sample static fatigued at 600C under a stress of 450 MPa for 32 h before failure(A)an earlier stage and612 Journal of the American Ceramic Society— Sun et al. Vol. 80, No. 3 (A) (B) Fig. 3. Interfacial reaction occurred in the sample fatigued at 600C for 500 h under an applied stress of 350 MPa ((A) in the top 0 ply, glassy reaction product(s) formed at the fiber/matrix interface (arrow) and no fiber pullout was observed; (B) in the first 90 ply, viscous glass phase(s) formed on the fiber surfaces). the Nicalon fiber, where the BN coating layer has been oxi- initiated at the tensile surface and propagated inward, deflected dized. The areas shown in Figs. 4(A) and (B) are two different at the BN/fiber interface, and left the 0 fibers bridging the interfacial regions along the same fiber, with the area in crack surfaces in the crack wake; whereas in the first 90 ply, Fig. 4(B) closer to the external surface. Considering the fact cracks initiated at the external side surface and propagated that interfacial reactions proceed along the fibers from the along the BN/fiber interface. At 950C, the BN coating layer in exposed fiber ends at the external surface toward the interior of a SiC/BN system remained intact and the crack opening at the the composite, Fig. 4(A) shows an early stage of the reaction, BN/fiber interface was quickly sealed by the oxidation products whereas Fig. 4(B) shows a more-advanced stage. Therefore, the of the SiC fiber. The reactions did not extend along the fibers. reaction was initiated at the BN/fiber interface and proceeded For the 0 bridging fibers in the top ply, fiber oxidation occurred into the BN layer. Again, neither boron nor nitrogen was only in a very short zone that had been exposed to the matrix detected in the amorphous ligaments. Electron-diffraction pat- crack, and the fiber strength might not degrade significantly. tern and EDS analyses indicated that this reaction product was Thus, cracks in the matrix in the top ply were unlikely to grow amorphous silica. These observations are consistent with the with the fibers bridging the crack surfaces in the crack wake. SAM results described above. This is evidenced by the fact that, in the samples that were tested at 950C (500 h and 350 MPa), the brittle zone was limited within a depth of 50 m from the tensile surface. IV. Discussion Similarly, in the first 90 ply, the crack opening at the BN/fiber The experimental results presented above indicate that, for interface was sealed and the affected zone did not extend the SiC/BN dual-coated Nicalon-fiber-reinforced BMAS glass- beyond 100 m. However, at lower temperatures (600C), the ceramic composites, stress-induced degradation in an oxidizing opening at the BN/fiber interface could not be fully sealed by environment occurred more readily at intermediate tempera- the reaction products, because of the lower oxidation rates of tures than at high temperatures. Because of the high stress level the SiC fibers than those at high temperatures. Oxidation stud￾used in the present study (appl value of
60%–70% of 0,RT), ies of SiC fibers indicated that the thickness of the SiO2 film matrix cracking was induced under the applied load, especially that formed on the fiber was proportional to the oxidation rate near the tensile surface, where the applied tensile stress was constant, which was a factor of
3 larger at 950C than that at 600C.17,18 maximum. Previous studies have revealed that interfacial In addition, the BN coating layer on the SiC fiber debonding in this composite system occurred mostly at the oxidized via volatilization at 600C. Therefore, the cracks at the BN/fiber interface. BN/fiber interface were likely to remain open and function as a 8,10 Therefore, in the top ply (0), the crack (A) (B) Fig. 4. Interfacial microstructures in a sample static fatigued at 600C under a stress of 450 MPa for 32 h before failure ((A) an earlier stage and (B) a more-advanced stage of the reaction); the two regions in these figures were along a same fiber, with the region in Fig. 4(B) closer to the external surface. Therefore, these figures indicate that oxidation initiated at the BN/fiber interface and developed into the BN coating layer.