ournal JAm. Geren so,82112134-500(1999 Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites and Its Effect on Fiber Strength Jose Perez-Rigueiro, Jose Antonio Celemin, and Javier LLorca Departamento de Ciencia de Materiales, Universidad Politecnica de Madrid, EtS de Ingenieros de Caminos, 28040 Madrid, Spain Pilar herrero Instituto de ciencia de materiales deMadrid,CSIC,Cantoblanco,28049Madrid,Spain Microstructural changes at the interface were analyzed in degrading the fiber properties, and the oxidation resista two Nicalon-fiber ceramic-matrix composites with a dual BN is significantly better than that of carbon. 10 In addition, BN BN/SiC coating on the fibers after thermal exposure at dif. coatings exhibit a hexagonal layered s n coating outer trating cture with turbostratic which pre ferent environments(air and argon). The outer SiC coating motes fiber/matrix decohesion, whereas the acted as a barrier to oxygen, which penetrated into the protects the Bn from chemical attack during matrix infiltration omposite via pipeline diffusion along the BN/fiber inter- and limits the access of oxygen to the interface faces. Oxygen penetration led to the formation of an Sic These duplex BN/SiC duce. but do not eliminate layer by oxidation of the fiber surfaces. The in situ fiber oxidation problems at high temperature. For instance, the bI strength at different temperatures, as determined from the Nicalon-fiber interface can be oxidized at 1200C, leading to radIus of th e mIrror on on the fiber fracture surface the nucleation and growth of defects on the fiber surface, an ndicated that this SiO, layer severely degraded the fiber effect that significantly reduces the in situ fiber strength and strength. Oxidation was highly dependent on the nature of thus, the overall composite strength and toughness. 3 Other the BN/fiber interface. The presence of a thin carbon-rich investigations 4, s also present evidence of oxidation at the nterlayer, which burned out rapidly at high temperature fiber/BN interface at lower temperatures(6000-850oC)in com- favored the entry of oxygen and accelerated oxidation of posites with duplex BN/SiC coatings, although the effect of he fibers oxidation on the fiber and interface properties has not been measured. Determining the suitability of duplex BN/SIC coat . Introduction ings for high-temperature applications requires a better knowl- edge of the factors that control chemical stability at the inter F BER-REINFORCED ceramics with a weak fiber/matrix inter face and of the effect of oxidation on the fiber properties. The face exhibit damage-tolerant, tough behavior. This weal present investigation is aimed at analyzing these questions in nterface appears spontaneously in the form of a thin carbon- two fiber-reinforced ceramics in the temperature range 800 rich layer when polymer-derived Si-C-O fibers are incorp ceramic matrices at high temperature or in- troduced by coating the fibers with one or several carbon layers IL. Materials rior to matrix infiltration. 3, 4 However, these composites often experience severe embrittlement when they are tested at high Two different fiber-reinforced ceramics(Lanxide Corp temperature in oxidizing atmospheres. -7 Under such condi newark, DE) tions, the carbonaceous coating of burned-out carbon is re were reinforced with 37 vol% of ceramic-grade Nicalon(Nip- placed by an amorphous silicate, formed from the reaction pon Carbon Co., Tokyo, Japan) SiC fibers. The fiber preform between the oxidized fiber surface and the matrix and which was manufactured by stacking several layers of bidirectional bonds the fiber strongly to the matrix. -lc 0o-90o)Nicalon 8 harness satin-weave fabric. The fibers in Such results have spurred the development of new coatings the lay-up were coated by CVD with a thin layer of BN(-100- vith relatively low interfacial shear strength and good oxida- 300 nm)and afterward with a thicker layer of SiC (3 um) tion resistance at high temperature(21100C). Very promising onto the bn. The Al,O matrix in the first composite was results have been obtained to date using dual BN/SiC fiber infiltrated using a direct metal-oxidatiss jumin\ s jd th ocess. The preform coatings in Al,O, /Nicalon and barium magnesium alumino- was brought into contact with molten silicate(BMAS)glass-ceramic/Nicalon/2 composites, which -1000C. The aluminum reacted with the oxygen to maintain their ambient-temperature mechanical properties ul matrix of porous Al,O3, which grew into the preform, and the to 1200@C. Duplex BN/SiC layers can be applied sequentially residual aluminum was removed afterward from the al, O,ma- onto the fibers by chemical vapor deposition(CVD)without where 1 The composite was received in the form of rectangu x. More details of the processing route are found else- lar plates of 3 mm nominal thickness. The porosity of the composites was -8% R. Naslain--contributing editor The matrix in the second cor te was introduced user polymer infiltration and pyrolysis. The precursor thermoset mer(polyureasilazane) was infiltrated into the fiber pre- form using a vacuum bagging approach. It then was pyrolyzed in an argon atmosphere at temperatures <1200C, leading to Member, American Ceramic Society. Mdrl de Madrid, and NATO an Si-C-N amorphous matrix. The infiltration-pyrolysis cycle was repeated several times until the required density was 3494Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites and Its Effect on Fiber Strength Jose´ Pe´rez-Rigueiro, Jose´ Antonio Celemín, and Javier LLorca* Departamento de Ciencia de Materiales, Universidad Polite´cnica de Madrid, ETS de Ingenieros de Caminos, 28040 Madrid, Spain Pilar Herrero Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Microstructural changes at the interface were analyzed in two Nicalon-fiber ceramic-matrix composites with a dual BN/SiC coating on the fibers after thermal exposure at dif￾ferent temperatures (in the range 800°–1400°C) and in dif￾ferent environments (air and argon). The outer SiC coating acted as a barrier to oxygen, which penetrated into the composite via pipeline diffusion along the BN/fiber inter￾faces. Oxygen penetration led to the formation of an SiO2 layer by oxidation of the fiber surfaces. The in situ fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO2 layer severely degraded the fiber strength. Oxidation was highly dependent on the nature of the BN/fiber interface. The presence of a thin carbon-rich interlayer, which burned out rapidly at high temperature, favored the entry of oxygen and accelerated oxidation of the fibers. I. Introduction FIBER-REINFORCED ceramics with a weak fiber/matrix inter￾face exhibit damage-tolerant, tough behavior. This weak interface appears spontaneously in the form of a thin carbon￾rich layer when polymer-derived Si–C–O fibers are incorpo￾rated into glass-ceramic matrices at high temperature1,2 or in￾troduced by coating the fibers with one or several carbon layers prior to matrix infiltration.3,4 However, these composites often experience severe embrittlement when they are tested at high temperature in oxidizing atmospheres.5–7 Under such condi￾tions, the carbonaceous coating of burned-out carbon is re￾placed by an amorphous silicate, formed from the reaction between the oxidized fiber surface and the matrix, and which bonds the fiber strongly to the matrix.8–10 Such results have spurred the development of new coatings with relatively low interfacial shear strength and good oxida￾tion resistance at high temperature ($1100°C). Very promising results have been obtained to date using dual BN/SiC fiber coatings in Al2O3/Nicalon11 and barium magnesium alumino￾silicate (BMAS) glass-ceramic/Nicalon12 composites, which maintain their ambient-temperature mechanical properties up to 1200°C. Duplex BN/SiC layers can be applied sequentially onto the fibers by chemical vapor deposition (CVD) without degrading the fiber properties, and the oxidation resistance of BN is significantly better than that of carbon.10 In addition, BN coatings exhibit a hexagonal layered structure with turbostratic disorder, analogous to that of carbon-rich coatings, which pro￾motes fiber/matrix decohesion, whereas the SiC outer coating protects the BN from chemical attack during matrix infiltration and limits the access of oxygen to the interface. These duplex BN/SiC coatings reduce, but do not eliminate, oxidation problems at high temperature. For instance, the BN/ Nicalon-fiber interface can be oxidized at 1200°C, leading to the nucleation and growth of defects on the fiber surface, an effect that significantly reduces the in situ fiber strength and, thus, the overall composite strength and toughness.13 Other investigations14,15 also present evidence of oxidation at the fiber/BN interface at lower temperatures (600°–850°C) in com￾posites with duplex BN/SiC coatings, although the effect of oxidation on the fiber and interface properties has not been measured. Determining the suitability of duplex BN/SiC coat￾ings for high-temperature applications requires a better knowl￾edge of the factors that control chemical stability at the inter￾face and of the effect of oxidation on the fiber properties. The present investigation is aimed at analyzing these questions in two fiber-reinforced ceramics in the temperature range 800°– 1400°C. II. Materials Two different fiber-reinforced ceramics (Lanxide Corp., Newark, DE) were used in the present study. Both composites were reinforced with 37 vol% of ceramic-grade Nicalon (Nip￾pon Carbon Co., Tokyo, Japan) SiC fibers. The fiber preform was manufactured by stacking several layers of bidirectional (0°–90°) Nicalon 8 harness satin-weave fabric. The fibers in the lay-up were coated by CVD with a thin layer of BN (∼100– 300 nm) and afterward with a thicker layer of SiC (∼3 mm) onto the BN. The Al2O3 matrix in the first composite was infiltrated using a direct metal-oxidation process. The preform was brought into contact with molten aluminum in air at ∼1000°C. The aluminum reacted with the oxygen to form a matrix of porous Al2O3, which grew into the preform, and the residual aluminum was removed afterward from the Al2O3 ma￾trix. More details of the processing route are found else￾where.11 The composite was received in the form of rectangu￾lar plates of 3 mm nominal thickness. The porosity of the composites was ∼8%. The matrix in the second composite was introduced using polymer infiltration and pyrolysis. The precursor thermoset polymer (polyureasilazane) was infiltrated into the fiber pre￾form using a vacuum bagging approach. It then was pyrolyzed in an argon atmosphere at temperatures <1200°C, leading to an Si–C–N amorphous matrix. The infiltration–pyrolysis cycle was repeated several times until the required density was R. Naslain—contributing editor Manuscript No 189770. Received October 30, 1998; approved June 24, 1999. Supported by CICYT (Spain), Universidad Polite´cnica de Madrid, and NATO through Grants No. MAT 95-787, A9708, and CRG-941033, respectively. *Member, American Ceramic Society. J. Am. Ceram. Soc., 82 [12] 3494–500 (1999) Journal 3494