M.B. Ruggles-Wrenn et al /Composites Science and Technology 66(2006)2089-2099 10 mm 10 mm Fig. ll. Fracture surfaces of the N610/A specimens tested at 900C at creep stress levels of:(a)73 MPa and(b)80 MPa. fracture surface associated with catastrophic failure. These Views of the coated fiber composite microstructure at dramatically different fracture surfaces reflect the very dif- different scales are shown in Figs. 12 and 13. Fig. 12(a) ferent creep lives of the two specimens. The N610/A spec- shows the random failure of the fibers in the 0o plies and imen suffered brittle failure after 5.5 h. In the case of the resulting"brushes "or fiber pullout. A region of extensive N610/M/A specimen, the monazite coating effectively pre- fiber pullout with pullout of both individual fibers and fiber vented fiber/matrix bonding, allowing fiber pullout, and bundles is shown in Fig. 12(b). It is seen that the locations consequently extended the creep life beyond 145 h. While of the fiber breaks within an individual tow, and conse- the fracture surface of the N610/A specimen tested at quently the lengths of fiber pullout exhibit a broad distribu- 73 MPa shows some pullout of fiber tows, the pullout of tion. a considerable amount of individual fiber pullout is individual fibers, such as seen for the coated fiber compos- seen in Fig. 12(b), where individual fibers are clearly dis- te, is not observed. Fast fracture failure surfaces of the cernable, which demonstrates that the monazite coating N10/A CMC are nearly planar and similar in appearance has prevented the fibers from sintering together at the to those produced in creep elevated test temperatures. As seen in Fig. 12(c), fracture I(c) 100pm ply and the nearly planar fracture of the 90 plyfracture surface associated with catastrophic failure. These dramatically different fracture surfaces reflect the very dif￾ferent creep lives of the two specimens. The N610/A spec￾imen suffered brittle failure after 5.5 h. In the case of the N610/M/A specimen, the monazite coating effectively pre￾vented fiber/matrix bonding, allowing fiber pullout, and consequently extended the creep life beyond 145 h. While the fracture surface of the N610/A specimen tested at 73 MPa shows some pullout of fiber tows, the pullout of individual fibers, such as seen for the coated fiber compos￾ite, is not observed. Fast fracture failure surfaces of the N610/A CMC are nearly planar and similar in appearance to those produced in creep. Views of the coated fiber composite microstructure at different scales are shown in Figs. 12 and 13. Fig. 12(a) shows the random failure of the fibers in the 0 plies and resulting ‘‘brushes’’ or fiber pullout. A region of extensive fiber pullout with pullout of both individual fibers and fiber bundles is shown in Fig. 12(b). It is seen that the locations of the fiber breaks within an individual tow, and conse￾quently the lengths of fiber pullout exhibit a broad distribu￾tion. A considerable amount of individual fiber pullout is seen in Fig. 12(b), where individual fibers are clearly dis￾cernable, which demonstrates that the monazite coating has prevented the fibers from sintering together at the elevated test temperatures. As seen in Fig. 12(c), fracture Fig. 11. Fracture surfaces of the N610/A specimens tested at 900 C at creep stress levels of: (a) 73 MPa and (b) 80 MPa. Fig. 12. Fracture surfaces of a Nextel 610/Monazite/Alumina specimen tested in creep at 900 C (creep stress = 120 MPa, time to rupture = 432,175 s) showing: (a) extensive fiber pullout, (b) individual fiber pullout, (c) matrix holes left by fiber pullout and (d) region of coordinated fiber fracture in the 0 ply and the nearly planar fracture of the 90 ply. 2096 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 66 (2006) 2089–2099