L.U.J.T. Ogbuji/ Journal of the European Ceramic Society 23(2003)613-617 is to expose that sheet to extensive attack on its broad fibers. The SylramicTM/BN SiC materials include one surface(instead of its edge as in"normal"pest). That made with fibers that had been covered with oxide- accelerated mode of pest is the main thrust of this based sizing and another with alcohol-based sizing In paper. contrast. all Hi-Nicalon TM fibers had been covered with SiC/BN/ SiC processing occurs in several steps, 4 some alcohol-based g. Scanning electron microscopy of which can degrade interphase integrity: (a)coverin SEM)and Auger electron spectroscopy(AES) of as the fibers with protective"sizing;(b)weaving or braid received samples showed substantial carbon between the ing the fiber tows into a preform; (c)desizing;( d)coating fiber and its bn coating in composites containing either fibers with chemical-vapor-infiltrated BN (cvi-BN) inter- Hi-Nicalon M fibers or oxide-sized SylramicTM fibers phase;(e) protecting the bn by chemical-vapor infiltra- The carbon was in essentially continuous layers in both tion of Sic cladding(cvi-SiC);(f impregnating the cases, albeit skeletal on the oxide-sized SylramicTM preform with a slurry, including carbon to be converted fibers to SiC; and (g) melt-infiltration of Si to form the Sic The samples were made by Honeywell Advanced matrix (mi-SiC). Table 2 outlines the ways in which Composites, Inc, in the approximate processing steps these steps can influence interphase integrity and hence summarized above. Work in our laboratories have impact pest behavior. Some recent remedies aim at shown that a simple and direct way to assess the pest improving the Bn quality: by crystallizing the bn (to behavior of these materials is to expose tensile bar spe densify it), 5 doping it with silicon, or reinforcing the cimens in the 0.3 Mach flame of an atmospheric-pres- normal BN with an adjoining layer of high-temperature sure burner rig(APBR)at 800C, determining their bn generated in-situ. 6 However, it is apparent that residual tensile strength and strain-to-fracture after control of extrinsic factors is still needed to protect the exposure, and analyzing the tensile fracture surfaces by BN layer whether or not its intrinsic properties are micrography and spectrometry. A 100-h APBR expo- ate for scr 2. Materials and methods 3. Results and discussion The four varieties of SiC/BN SiC studied, which are The only SiC/BN SiC varieties that did not undergo of the utmost interest in aerospace applications, are severe(extrinsic) pest in the burner rig(Table 3)are those described in Table 3, along with their respective severity of that featured (stoichiometric) SylramicM fibers with pesting as evidenced in strength loss. They include two PVA sizing; all composites made with(carbon-rich) made with Hi-NicalonTM and Hi-Nicalon(S)TM fibers that Nicalon fibers (i.e. Hi-NicalonTM or Hi-Nicalon(s) ontain 40 and 5% excess carbon, respectively, and as well as those made with PEO-sized SylramicTM fibers wo varieties made with the stoichiometric SylramicM sizing on the fibers exhibited severe pesting in our tests Possible interphase effects of composite processing Process Possible effects Choice of fiber Excess carbon in the fiber can migrate to the fiber surface ome sizing materials can char to carbon on the fiber 23456 izings can leave significant carbon residue on fiber. Interphase deposition Deposition conditions will determine interphase quality cvi-SiC deposition Tecnique can leave residue on interphase. Matrix infiltration High temperatures can cause carbon precipitation on fibers Table 3 Sic/BN/SiC materials tested after 100 h at 800C in the burner rig Fiber/sizing Carbon rength loss Comments under bn in APBr Hi-Nicalon/alcohol-based sizing Continuous 60% fibers became bonded together by SiO2 Hi-Nicalon(S)/alcohol-based sizing Continuous fibers became bonded together by SiO2 Sylramicoxide-based sizing Quasi-continuous -53% b of 405 MPa aaa fibers became bonded together by Sio Sylramic/alcohol-based sizing None None of BN/cvi-SiC interface arbon-rich in some cases(C/Si ratio 1.4) A SiO2 layer grew at that interface during BR exposure.is to expose that sheet to extensive attack on its broad surface (insteadof its edge as in ‘‘normal’’ pest). That acceleratedmode of pest is the main thrust of this paper. SiC/BN/SiC processing occurs in several steps,14 some of which can degrade interphase integrity: (a) covering the fibers with protective ‘‘sizing’’; (b) weaving or braid￾ing the fiber tows into a preform; (c) desizing; (d) coating fibers with chemical-vapor-infiltratedBN (cvi-BN) inter￾phase; (e) protecting the BN by chemical-vapor infiltra￾tion of SiC cladding (cvi-SiC); (f) impregnating the preform with a slurry, including carbon to be converted to SiC; and(g) melt-infiltration of Si to form the SiC matrix (mi-SiC). Table 2 outlines the ways in which these steps can influence interphase integrity andhence impact pest behavior. Some recent remedies aim at improving the BN quality: by crystallizing the BN (to densify it),15 doping it with silicon,8 or reinforcing the normal BN with an adjoining layer of high-temperature BN generatedin-situ.16 However, it is apparent that control of extrinsic factors is still needed to protect the BN layer whether or not its intrinsic properties are improved. 2. Materials and methods The four varieties of SiC/BN/SiC studied, which are of the utmost interest in aerospace applications, are described in Table 3, along with their respective severity of pesting as evidenced in strength loss. They include two made with Hi-NicalonTM andHi-Nicalon(S)TM fibers that contain
40 and
5% excess carbon, respectively, and two varieties made with the stoichiometric SylramicTM fibers. The SylramicTM/BN/SiC materials include one made with fibers that had been covered with oxide￾basedsizing andanother with alcohol-basedsizing. In contrast, all Hi-NicalonTM fibers hadbeen coveredwith alcohol-basedsizing. Scanning electron microscopy (SEM) andAuger electron spectroscopy (AES) of as￾receivedsamples showedsubstantial carbon between the fiber andits BN coating in composites containing either Hi-NicalonTM fibers or oxide-sized SylramicTM fibers. The carbon was in essentially continuous layers in both cases, albeit skeletal on the oxide-sized SylramicTM fibers. The samples were made by Honeywell Advanced Composites, Inc., in the approximate processing steps summarizedabove. Work in our laboratories have shown that a simple anddirect way to assess the pest behavior of these materials is to expose tensile bar spe￾cimens in the 0.3 Mach flame of an atmospheric-pres￾sure burner rig (APBR) at 800 C, determining their residual tensile strength and strain-to-fracture after exposure, andanalyzing the tensile fracture surfaces by micrography andspectrometry. A 100-h APBR expo￾sure has been foundadequate for screening purposes. 3. Results and discussion The only SiC/BN/SiC varieties that did not undergo severe (extrinsic) pest in the burner rig (Table 3) are those that featured(stoichiometric) SylramicTM fibers with PVA sizing; all composites made with (carbon-rich) Nicalon fibers (i.e. Hi-NicalonTM or Hi-Nicalon(S)TM) as well as those made with PEO-sized SylramicTM fibers sizing on the fibers exhibitedsevere pesting in our tests. Table 2 Possible interphase effects of composite processing Step Process Possible effects 1 Choice of fiber Excess carbon in the fiber can migrate to the fiber surface. 2 Choice of sizing Some sizing materials can char to carbon on the fiber. 3 De-sizing Sizings can leave significant carbon residue on fiber. 4 Interphase deposition Deposition conditions will determine interphase quality. 5 cvi-SiC deposition Tecnique can leave residue on interphase. 6 Matrix infiltration High temperatures can cause carbon precipitation on fibers. Table 3 SiC/BN/SiC materials testedafter 100 h at 800 C in the burner rig Fiber/sizing Carbon under BN Strength loss in APBR Comments Hi-Nicalon/alcohol-basedsizing Continuous 60% of 380 MPa After BR exposure adjacent fibers became bonded together by SiO2. Hi-Nicalon(S)/alcohol-basedsizing Continuous 56% of 395 MPa After BR exposure adjacent fibers became bonded together by SiO2. Sylramic/oxide-based sizing Quasi-continuous 53% of 405 MPa After BR exposure adjacent fibers became bonded together by SiO2. Sylramic/alcohol-basedsizing None None of 420 MPa BN/cvi-SiC interface was carbon-rich in some cases (C/Si ratio
1.4). A SiO2 layer grew at that interface during BR exposure. L.U.J.T. Ogbuji / Journal of the European Ceramic Society 23 (2003) 613–617 615