January 2004 Effect of a BN Interphase That Debonds betwveen the Interphase and the Matrit in SiC/SiC Composites 0.601201.802.40 0.601.201.8024 601.201.80240 101703009460k11.3mm×10.0kSE(L)02212001 500um Fig. 10. SEM micrograph and EDS spectra for an oxide layer on the outside of the BN, the BN interphase, and the SiC fiber surface of an outside-debonding SYL-iBN SIC/SiC composite fracture surface after 815 C burner-rig exposure and tensile testing at room temperature tress states would be expected to be quite complex. Nevertheless existence of a"gap between the BN and the CVI-SiC, i.e,,an the interphase and interfaces between the fibers and matrix should already debonded interface before testing be subjected to residual tensile and shear stress. This could create the scenario where, if the strength of those interfaces were weak enough, the interface could debond during cooling of the compos- ite or result in a stress state ahead of an approaching crack that could lead to preferential debonding of the BN-CVI SiC interface MI SiC/SiC composites with BN interphases that exhibited rather than the fiber-BN interface In this regard, since MI systems interface debonding and sliding at the BN-CVI SiC interface will inherently have residual compression in the matrix, they may showed significantly higher strain capabilities and intermediate be an ideal composite system to enable outside debonding mperature stress-rupture life over conventional composites that Ithough outside-debonding type of behavior has been observed in exhibit interface debonding and sliding at the BN-fiber interface SiC/BN/CVI SiC minicomposites with tailored BN interfaces. Higher strain to failure was attributed to lower interfacial shear Regarding the observance of a weaker BN-matrix interface for stress at the BN-CVI SiC interface. Improved intermediate outside-debonding CMCs, the presence of carbon either as a thin mperature properties were attributed to the protection from the layer outside of the BN or in an enriched form appears to be the oxidizing environment due to the adherence of the BN layer to the most likely factor, even though the detection of carbon enrichment fiber surface which is not the situation for the inside-debonding is not compelling. One other possible explanation is that differ- composites. Thus the environment does not have direct access to low-temperature-deposited BN. BN shrinkage. the formation of a oxidative process of strongly bonding fibers to nearest-neighbor been observed for fiber/BN/CVI SiC preforms that have been served after 100 h burner- rig exposure, which typically occurs heat-treated to higher temperatures. Nevertheless, oxidation that when carbon exists at the fiber/BN interface occurs between the BN and the CVI-SiC matrix during burner-rig The cause of outside debonding was believed to be due to(1)a xposure clearly implies either the presence of a C layer, as was weaker BN/CVI-SiC interface than BN/fiber i the case for the earlier-mentioned composite systems where a thin caused by the presence of C at the BN/CVI-SiC it C layer existed between the fiber and the BN, 522- or the residual tensile/shear stress at the BN-CVI SiC