January 2004 Effect of a BN Interphase That Debonds between the interphase and the Matrix in SiC/SiC Composites 1800 Outside Debonding +350 o SYL-IBN (SYL-IBN 1600 L1200 SYL v1000 ”;……… nside Debonding 600f squares a diamonds"SYL as-produced 100 1000 Time to fail, hr Fig. 6. Stress-rupture of inside- and outside- debonding composites with Sylramic (SYL)and Sylramic-iBN(SYL-iBN) fiber reinforcement in air at 815C The data are plotted as stress on the fibers, i. e, composite stress divided by f. The composite stress for an f =0. 2 is plotted on the right axi hand the outside-debonding specimen with sYL-ibn fibers that for (1)Hi-Nicalon due to a carbon rich layer that occurs for MI was precracked did not fail after 330 h compared with the pristine rM due to a carbon-rich layer on and com possible to conclude that precracked outside -debonding specimens processing. 2 and (3)sytramacer com posits s her facie after fiber on the fibers that was not burned off completely before BN first time a precracked SiC/SiC specimen outperformed a nonprec interphase deposition. SYL and SYL-iBN composites with a racked specimen from the same panel re time to sizing that has low char yield are unaffected by burner-rig failure of the nonprecracked specimen was greater than 10 h exposure, Figure 9 compares the burner-rig degradation(or lack Sylramic, SYL-iBN, and HNS outside-debonding composites thereof). Also shown is an example of outside-debonding SYL- were subjected to burner-rig exposure at 815oC for-100 h with no ibN before and after burner-rig exposure No significant strength applied stress, and then were tested at room temperature to degradation has been observed for SYL-iBN and SYL composites determine the retained strength properties. Whereas the ruptu with outside debonding and complete sizing removal. Burner-rig- tests evaluate the durability of composites when cracked. the exposed SYL-iBN and Sylramic composites with outside debond- zero-stress burner- rig experiment has proved to be an effective test ing were often observed to stiffen and fracture at slightly lower to evaluate the ability of an undamaged composite material to ultimate strain(Fig. 9, Sylramic composite not shown). However, withstand severe intermediate- temperature oxidation through the HNS outside- debonding composites were degraded after burner exposed (as-machined)edges of the composite specimen. It has rig exposure due to the presence of a carbon layer on the fiber been found that if carbon exists on the surface of any fiber type the surface HNS outside - debonding composites were also observed to SiC/SiC MI composites will be significantly degraded after stiffen slightly after burmer-rig exposure Stiffening does not occur burner-rig exposure. This type of degradation has been observed Fig. 7. SEM micrograph from the fracture surface of outside-debonding composite after stress-rupture after 100 h at 815.C.January 2004 Effect of a BN Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites 109 1800 2 1600 @ 1400 9) LL 1200 E g c 0 8 1000 9) L 800 600 Outside Debonding I -0. *%. Inside SYL-IEN Debonding -I (SYL-\EN) 1'" 8 SYL circles = SYL-IBN v) 250 8 200 3 R u) + Y t lbO z v P) I squares (L diamonds = SYL as-produced I 10 100 1000 Time to fail, hr Fig. 6. The data are plotted as stress on the fibers, i.e., composite stress divided by5 The composite stress for anf = 0.2 is plotted on the right axis. Stress-rupture of inside- and outside-debonding composites with Sylramic@ (SYL) and Sylramic-iBN (SYL-iBN) fiber reinforcement in air at 815°C. hand, the outside-debonding specimen with SYL-iBN fibers that was precracked did not fail after 330 h compared with the pristine specimen which failed after -190 h. With so few data it is not possible to conclude that precracked outside-debonding specimens are superior to pristine specimens in rupture. However, this is the first time a precracked SiC/SiC specimen outperformed a nonprec￾racked specimen from the same panel at a stress where time to failure of the nonprecracked specimen was greater than 10 h. Sylramic, SYL-iBN, and HNS outside-debonding composites were subjected to burner-rig exposure at 815°C for - 100 h with no applied stress, and then were tested at room temperature to determine the retained strength properties. Whereas the rupture tests evaluate the durability of composites when cracked, the zero-stress burner-rig experiment has proved to be an effective test to evaluate the ability of an undamaged composite material to withstand severe intermediate-temperature oxidation through the exposed (as-machined) edges of the composite specimen. It has been found that if carbon exists on the surface of any fiber type, the SiC/SiC MI composites will be significantly degraded after burner-rig exposure. This type of degradation has been observed for (1) Hi-Nicalon due to a carbon-rich layer that occurs for MI composites after fiber and composite processing," (2) Hi-Nicalon STM due to a carbon-rich layer on the fiber surface after fiber processing,22 and (3) Sylramica composites when a sizing is used on the fibers that was not burned off completely before BN interphase depo~ition.~~ SYL and SYL-iBN composites with a sizing that has low char yield are unaffected by burner-rig exposure. Figure 9 compares the burner-rig degradation (or lack thereof). Also shown is an example of outside-debonding SYL￾iBN before and after burner-rig exposure. No significant strength degradation has been observed for SYL-iBN and SYL composites with outside debonding and complete sizing removal. Burner-rig￾exposed SYL-IBN and Sylramic composites with outside debond￾ing were often observed to stiffen and fracture at slightly lower ultimate strain (Fig. 9, Sylramic composite not shown). However, HNS outside-debonding composites were degraded after burner￾rig exposure due to the presence of a carbon layer on the fiber surface. HNS outside-debonding composites were also observed to stiffen slightly after burner-rig exposure. Stiffening does not occur for inside-debonding composites. Fig. 7. SEM micrograph from the fracture surface of outside-debonding composite after stress-rupture after -100 h at 815°C