Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S Journal of the European Ceramic Society 23(2003)613-617 www.elsevier.com/locate/j Pest-resistance in SiC/BN/SiC composites Linus U.j.T. ogbuji* QSS Inc, NASA Glenn Research Center, Cleveland, Ohio, US.A Received 4 October 2001; received in revised form I July 2002; accepted 14 July 2002 Abstract State-of-the-art non-oxide ceramic-matrix composites(consisting of Sic fibers, cvi-BN interphase coating, and mi-SiC matrix) exhibit excellent mechanical properties at room temperature, as well as above 1000C (where oxidation easily seals flaws with silica); but they are prone to pest degradation at intermediate temperatures in an oxidizing environment, and especially so in the fast, moist flame of a jet engine. Two modes of pest may be distinguished in these composites. The more severe promoted by extraneous factors, like a layer of elemental carbon underlying the Bn interphase and undermining dation resistance. It is shown that, when care is taken to exclude such a carbon layer, SiC/BN SiC composite can 100-h exposure in a burner rig without noticeable loss of strength or strain to fracture. C 2003 Elsevier Science Ltd. All rights reserved Keywords: BN interphase: Composites: Oxidation: Pest; SiC/SIC 1. Introduction CMC is a SylramicM/BN/mi-SiC composite. The sali ent points of its development under a succession of service in advanced engines has been known for paper: programs are highlighted in a companion The potential of ceramic-matrix composites(CMCs) decades, but realization of that promise has been fru- While significant progress has been made on the Cmc strated by a slow developmental curve. Most designs of matrix and fibers, development of the CMC interphase advanced turbines for aircraft engines or land-based (material at the fiber/matrix interface) has been slow, power generators assume significantly higher operating though interphase problems were evident from the start temperatures than in current engines, both for gains in Initially, carbon was used as an interphase in SiC/SiC thermodynamic efficiency and for mandated reductions composites due to its excellent compliance, but that in exhaust emissions. Hence, it is assumed that hot sec- advantage was offset by its oxidative volatility, which tions of advanced turbines (especially combustor liners, rendered carbon unsuitable at intermediate tempera- nozzles, and vanes) will be made of ceramic-matrix tures. Above 500C oxidative loss of interphase carbon composites(CMCs). CMC development has been led by becomes catastrophic only above 1000 C is protection progress in the fiber and matrix constituents For non- achieved as Sio from matrix oxidation becomes sub- oxide CMCs, reaction-bonded silicon nitride(rBsn) stantial enough to seal off ingress of oxidants. Boron and chemical-vapor-infiltrated silicon carbide(cvi-SiC) nitride, the currently preferred interphase, also oxidizes proved unsuitable as matrix materials and melt-infiltrated almost as readily as carbon in the same intermediate silicon carbide(mi-SiC) became the matrix of choice; temperature range, so that SiC/BN/ SiC composites can similarly, "fat""fibers(e.g. SCS-0 and SCS-6)have been pest as severely as Sic/C/SiC composites.Still,BN superseded by thin fibers, with preference shifting succes- remains the interphase material of choice because its sively from NicalonM to Hi-NicalonTM and to Sylra- shortcomings seem, in principle, remediable. However micM varieties of increasingly refined chemistry and the remedies have been slow in coming, and pesting microstructure. The current state-of-the-art non-oxide remains a major obstacle to SiC/BN/SiC utilization *Tel:+1-216-433-6463;fax:+1-216-43-554 also involves selective attack of the interphase by ambi ent oxidants. However, whereas pesting of Sic/C/SiC 0955-2219/03/S. see front matter C 2003 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(02)00268-6Pest-resistance in SiC/BN/SiC composites Linus U.J.T. Ogbuji* QSS Inc., NASA Glenn Research Center, Cleveland, Ohio, USA Received4 October 2001; receivedin revisedform 1 July 2002; accepted14 July 2002 Abstract State-of-the-art non-oxide ceramic-matrix composites (consisting of SiC fibers, cvi-BN interphase coating, and mi-SiC matrix) exhibit excellent mechanical properties at room temperature, as well as above
1000 C (where oxidation easily seals flaws with silica); but they are prone to pest degradation at intermediate temperatures in an oxidizing environment, and especially so in the fast, moist flame of a jet engine. Two modes of pest may be distinguished in these composites. The more severe type of pest is promoted by extraneous factors, like a layer of elemental carbon underlying the BN interphase and undermining its intrinsic oxi￾dation resistance. It is shown that, when care is taken to exclude such a carbon layer, SiC/BN/SiC composite can easily survive a 100-h exposure in a burner rig without noticeable loss of strength or strain to fracture. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: BN interphase; Composites; Oxidation; Pest; SiC/SiC 1. Introduction The potential of ceramic-matrix composites (CMCs) for service in advanced engines has been known for decades, but realization of that promise has been fru￾strated by a slow developmental curve. Most designs of advanced turbines for aircraft engines or land-based power generators assume significantly higher operating temperatures than in current engines, both for gains in thermodynamic efficiency and for mandated reductions in exhaust emissions. Hence, it is assumedthat hot sec￾tions of advanced turbines (especially combustor liners, nozzles, andvanes) will be made of ceramic-matrix composites (CMCs). CMC development has been led by progress in the fiber andmatrix constituents. For non￾oxide CMCs, reaction-bonded silicon nitride (RBSN) andchemical-vapor-infiltratedsilicon carbide (cvi-SiC) provedunsuitable as matrix materials andmelt-infiltrated silicon carbide (mi-SiC) became the matrix of choice; similarly, ‘‘fat’’ fibers (e.g. SCS-0 andSCS-6) have been superseded by thin fibers, with preference shifting succes￾sively from NicalonTM to Hi-NicalonTM andto Sylra￾micTM varieties of increasingly refinedchemistry and microstructure. The current state-of-the-art non-oxide CMC is a SylramicTM/BN/mi-SiC composite. The sali￾ent points of its development under a succession of NASA programs are highlightedin a companion paper.1 While significant progress has been made on the CMC matrix andfibers, development of the CMC interphase (material at the fiber/matrix interface) has been slow, though interphase problems were evident from the start. Initially, carbon was usedas an interphase in SiC/SiC composites due to its excellent compliance, but that advantage was offset by its oxidative volatility, which rendered carbon unsuitable at intermediate tempera￾tures. Above 500 C oxidative loss of interphase carbon becomes catastrophic;2 only above 1000 C is protection achievedas SiO2 from matrix oxidation becomes sub￾stantial enough to seal off ingress of oxidants. Boron nitride, the currently preferred interphase, also oxidizes almost as readily as carbon in the same intermediate temperature range, so that SiC/BN/SiC composites can pest as severely as SiC/C/SiC composites.3 Still, BN remains the interphase material of choice because its shortcomings seem, in principle, remediable. However, the remedies have been slow in coming, and pesting remains a major obstacle to SiC/BN/SiC utilization. As in a SiC/C/SiC composite, pesting of SiC/BN/SiC also involves selective attack of the interphase by ambi￾ent oxidants. However, whereas pesting of SiC/C/SiC 0955-2219/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00268-6 Journal of the European Ceramic Society 23 (2003) 613–617 www.elsevier.com/locate/jeurceramsoc * Tel.: +1-216-433-6463; fax: +1-216-433-5544. E-mail address: thomas-ogbuji@grc.nasa.gov (L.U.J.T. Ogbuji).