L.U.J.T. Ogbuji/ Journal of the European Ceramic Society 23(2003)613-617 The origin of the carbon depends on the fiber or coat ing. With carbon-rich fibers the excess carbon builds up between the fiber surface and Bn interphase; with oxide-sized fibers it comes from residual char yield upon de-sizing. Alcohol sizing seems to avoid the latter prob N lem, reflecting either an intrinsic superiority or a more negligible char yield, since fiber makers tend to make the sizing considerably thicker when it is oxide-based Fig 3 shows that in these composites(which have a high SiC fiber fiber fraction, >40 vol %fiber to maximize load bear ing) nearly all fibers are in contact with their neighbors, the number of contacts averaging 3.3 instead of the ideal zero. The higher-magnification image inset in this Fig. 4. Following burner rig exposure, a layer of silica covers the BN figure reveals a carbon sublayer under the bn inter- terphase in a SylramicM/BN/ SiC material that was carbon rich at phase-which probably links up with the carbon layers the BN cvi-SiC interface. on adjacent fibers. Contact of [0] and [90] tows in the third dimension extends the web of carbon throughout 4. Summary and conclusion the composite and leads to a rigid network of silica fol the l:g burn-out of carbon. This occurs most easily in Pest resistance of SiC/BN composites has been the high flame velocity of the burner rig but the effect studied by burner-rig exposure at intermediate tem has been suspected in Hi-NicalonTM/BN/SiC that pes- peratures, followed by tensile tests and microstructural ted during stress-rupture tests in air at intermediate characterization to determine residual strengths and temperatures. pest-related features. All samples reinforced with In some Sylramic/BN/SiC materials that did not Hi-NicalonM and Hi-Nicalon(S) M fibers exhibited the exhibit extrinsic pest, significant carbon enrichment was severe, extrinsic mode of pesting. Those with SylramicTM detected at the BN/cvi-SiC interface(see the bottom of fiber reinforcements resisted extrinsic pesting except in Table 3). After burner rig exposure that carbon-rich cases where the fibers had been sized with oxide-based layer was replaced by silica, as shown in Fig. 4. The sizing. The common characteristic in all cases where silica may afford protection to the bn and so prove extrinsic pesting occurred is that a film of carbon was beneficial to overall durability. However, its actual observed between the bn interphase and fiber, originat effect is yet to be determined ing from excess carbon in the Nicalon-type fibers and Carbon A85530kv127mm×200kSE(L)081 ig. 3. SEM image showing carbon beneath the BN layer in Hi-Nicalon(S)M/BN/SiC.The origin of the carbon depends on the fiber or coat￾ing. With carbon-rich fibers the excess carbon builds up between the fiber surface andBN interphase; with oxide-sized fibers it comes from residual char yield upon de-sizing. Alcohol sizing seems to avoid the latter prob￾lem, reflecting either an intrinsic superiority or a more negligible char yield, since fiber makers tend to make the sizing considerably thicker when it is oxide-based. Fig. 3 shows that in these composites (which have a high fiber fraction, 540 vol.% fiber to maximize loadbear￾ing) nearly all fibers are in contact with their neighbors, the number of contacts averaging 3.3 insteadof the ideal zero. The higher-magnification image inset in this figure reveals a carbon sublayer under the BN inter￾phase—which probably links up with the carbon layers on adjacent fibers. Contact of [0] and[90] tows in the third dimension extends the web of carbon throughout the composite andleads to a rigidnetwork of silica fol￾lowing burn-out of carbon. This occurs most easily in the high flame velocity of the burner rig but the effect has been suspectedin Hi-NicalonTM/BN/SiC that pes￾ted during stress-rupture tests in air at intermediate temperatures.11 In some Sylramic/BN/SiC materials that did not exhibit extrinsic pest, significant carbon enrichment was detected at the BN/cvi-SiC interface (see the bottom of Table 3). After burner rig exposure that carbon-rich layer was replacedby silica, as shown in Fig. 4. The silica may affordprotection to the BN andso prove beneficial to overall durability. However, its actual effect is yet to be determined. 4. Summary and conclusion Pest resistance of SiC/BN/SiC composites has been studied by burner-rig exposure at intermediate tem￾peratures, followedby tensile tests andmicrostructural characterization to determine residual strengths and pest-relatedfeatures. All samples reinforcedwith Hi-NicalonTM andHi-Nicalon(S)TM fibers exhibitedthe severe, extrinsic mode of pesting. Those with SylramicTM fiber reinforcements resistedextrinsic pesting except in cases where the fibers hadbeen sizedwith oxide-based sizing. The common characteristic in all cases where extrinsic pesting occurredis that a film of carbon was observedbetween the BN interphase andfiber, originat￾ing from excess carbon in the Nicalon-type fibers and Fig. 3. SEM image showing carbon beneath the BN layer in Hi-Nicalon(S)TM/BN/SiC. Fig. 4. Following burner rig exposure, a layer of silica covers the BN interphase in a SylramicTM/BN/SiC material that was carbon rich at the BN/cvi-SiC interface. 616 L.U.J.T. Ogbuji / Journal of the European Ceramic Society 23 (2003) 613–617