January 2004 Effect of a BN Interphase That Debonds between the interphase and the Matrix in SiC/SiC Composites 111 0.601201.802,40 0.601.201.80240 0.601.201.80240 101703009460kV11.3mmx10.0kSE(L)02/21/2001 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 YL-iBN SiC/SiC composite fracture surface after 815oC burner- rig exposure and tensile testing at room temperatu stress states would be expected to be quite complex. Nevertheles 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 creat the scenario where, if the strength of those interfaces were weak enough, the interface could debond during cooling of the compos V. Conclusion It in a stress state ahead of ial debonding of the BN-CVI SiC interface MI SiC/SiC composites with bn interp rather than the fiber-BN interface In this regard, since MI systems interface debonding and sliding at the BN-CVI SiC interface ill 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. temperature stress-rupture life over conventional composites that although 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 temperature 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 not compelling. One other possible explanation is that differ- composites. Thus the environment does not have direct access to nces in processing conditions led to more shrinkage of the the fibers, which prohibits or stalls the rapid strength-degrading w-temperature-deposited BN. BN shrinkage, the formation of oxidative process of strongly bonding fibers to I-neighbor gap between the bn and the CVI SiC, and outside debonding have fibers. In addition, no degradation in retained th was ob- een 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 the when carbon exists at the fiber/BN occurs between the bn and the CVI-SiC matrix during burner- rig The cause of outside debonding w to(1)a exposure clearly implies either the presence of a C layer, as was eaker BN/CVI-SiC interface than he case for the earlier-mentioned composite systems where a thin caused by the presence of C at the BN/CVI-SiC C layer existed between the fiber and the BN or the residual tensile/shear stress at the bn-cvi sic interface cJanuary 2004 Effect of a BN Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites 111 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 SiClSiC composite fracture surface after 8 15°C burner-rig exposure and tensile testing at room temperature. stress states would be expected to be quite complex. Nevertheless, the interphase and interfaces between the fibers and matrix, should 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 rather than the fiber-BN interface. In this regard, since MI systems will inherently have residual compression in the matrix, they may be an ideal composite system to enable outside debonding, although outside-debonding type of behavior has been observed in SiC/BN/CVI Sic minicomposites with tailored BN interfaces2’ Regarding the observance of a weaker BN-matrix interface for outside-debonding CMCs, the presence of carbon either as a thin layer outside of the BN or in an enriched form appears to be the most likely factor, even though the detection of carbon enrichment is not compelling. One other possible explanation is that differ￾ences in processing conditions led to more shrinkage of the low-temperature-deposited BN. BN shrinkage, the formation of a gap between the BN and the CVI Sic, and outside debonding have been observed for fiber/BN/CVI Sic preforms that have been heat-treated to higher temperature^.^^ Nevertheless, oxidation that occurs between the BN and the CVI-SIC matrix during burner-rig exposure clearly implies either the presence of a C layer, as was the case for the earlier-mentioned composite systems where a thin C layer existed between the fiber and the BN,’”22-23 or the existence of a “gap” between the BN and the CVI-SIC, i.e., an already debonded interface before testing. V. Conclusion MI SiC/SiC composites with BN interphases that exhibited interface debonding and sliding at the BN-CVI Sic interface showed significantly higher strain capabilities and intermediate￾temperature stress-rupture life over conventional composites that exhibit interface debonding and sliding at the BN-fiber interface. Higher strain to failure was attributed to lower interfacial shear stress at the BN-CVI Sic interface. Improved intermediate￾temperature properties were attributed to the protection from the oxidizing environment due to the adherence of the BN layer to the fiber surface, which is not the situation for the inside-debonding composites. Thus the environment does not have direct access to the fibers, which prohibits or stalls the rapid strength-degrading oxidative process of strongly bonding fibers to nearest-neighbor fibers. In addition, no degradation in retained strength was ob￾served after 100 h burner-rig exposure, which typically occurs when carbon exists at the fiber/BN interface. The cause of outside debonding was believed to be due to (1) a weaker BN/CVI-Sic interface than BN/fiber interface, perhaps caused by the presence of C at the BNICVI-Sic interface, and (2) residual tensilekhear stress at the BN-CVI Sic interface that was