Journal of the American Ceramic Society--Zawada et al Vol. 86. No 6 200a1100 日1100c 150 091031021031041051051 Time(s) Fig. 15. Creep rupture stress level versus time to failure for Nextel 610/ AS tested at1000°andl1o°C 20 Fig. 14(b)is only 25 000 s. When plotted on the same scale 1000C tests, the traces appear to be almost vertical, with MPa test running only -7 h Such short lives at 1100oC indicate that this oxide/oxide CMC should be used only 1000C under static loading Larger creep strain and longer creep life were observed at lower applied stresses at both temperatures. At steady state, it was sumed that the stress in all phases equilibrated; therefore, the matrix and fiber creep rates should be the same. In most CMCs, matrix damage dominates creep rupture lifetimes. At high temper- ature, oxidization of the carbon or BN fiber-matrix interphase and the SiC fiber through matrix cracks controls the rupture process and rupture time. 6.47 Matrix damage already exists in this oxide/ oxide and oxidation is not a concern. The influence of the cracked oint. Obviously, the fiber axial stresses were far higher than other nated by creer ure of Nextel 610 fibers. The of Fig. 16. (a) High-resolution SEM micrograph of a Nextel 610 fiber failed individual Nextel 610 fibers was believed to be due to boundary under creep rupture, Fracture surface of the fiber is characterized by cavity coalescence, but this was often after strains as high as 30% tergranular crack growth. (b) High-resolution SEM micrograph of a accumulated, Postfailure analysis indicated that intergranular ailed creep rupture. Fracture surface clearly shows that matrix remains bonded to the tibers failure was the predominant characteristic failure mode in the ruptured specimens (Fig. 16(a)). The mechanism controlling steady-state creep of Nextel 610 fibers was suggested to be interface-reaction-controlled diffusion creep with fine intergranu inherently better creep resistance of the N720 fiber. Creep strain lar crack formation. -o The lack of a noticeable tertiary creep versus time traces were similar to the nolo/As traces recorded in region suggested that there was a spontaneous linkage of creep. this investigation nucleated cracks that occurred abruptly just before failure. Cracks continued to nucleate throughout the creep process until a critical crack density was reached, which caused spontaneous linkage and failure The 75 MPa test at 1000%C reached run-out and was tested at There are several distinct behaviors of porous-matrix oxide/ room temperature for residual strength. Even though the specimen oxide CMCs that are different from traditional CMCs with a had experienced almost I% creep strain, there was no decrease in fiber-matrix interphase. In general, SiC fibers are stronger than tensile strength. It was interesting to observe so much creep strain are in the range of 200-250 MPa, whereas Nicalon-containing d yet measure no decrease in strength. This suggested that CMCs typically range from 200 to 350 MPa for 8HSW cross-ply th-governing flaws did not enlarge during the initial or composites. Even with linear stress versus strain behavior intermediate stages of creep, Observations of the fracture surfaces xide/oxide CMCs have shown good fracture toughness and are vealed that most of the exposed fibers on the fracture surface had relatively notch insensitive during fast fracture, 20,22-51.52The natrix remaining on the fibers(Fig. 16(b)), which indicated that id would have been shed to the matrix. a denser matrix would Dom-temperature fatigue performance is very similar to man improve the creep resistance of N61O/AS other CMCs, with run-out a high tote limit level of-155 MPa There is little creep data in the literature for 8HSW oxide/oxide significantly higher than that measured for most other CMCs composites containing Nextel 610 fibers(N610). Creep rupture with similar fiber volume fractions. A horizontal o/N plot suggests tests were conducted on a 8HSw N720/AS composite at that there is no progressive fatigue damage developing. No 1100 C. This composite demonstrated a run-out of 100 h at a SiC-fiber CMCs perform as well in fatigue at 1000C once the stress level of 150 MPa. Such results clearly documented the matrix is cracked. This is a very important observation, because