Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Journal of the European Ceramic Society 29(2009)525-535 www.elsevier.comlocate/jeurceramsoc Mechanical properties of Hi-Nicalon fiber-reinforced celsian composites after high-temperature exposures in air Narottam p bansal Structures and Materials Division, NASA Glenn Research Center. Cleveland OH 44135 USA Received 21 April 2008: received in revised form 13 June 2008; accepted 19 June 2008 Available online 26 July 2008 Abstract BN/SiC-coated Hi-Nicalon fiber-reinforced celsian matrix composites(CMCs) were annealed for 100 h in air at various temperatures to 1200C, followed by flexural strength measurements at room temperature. Values of yield stress and strain, ultimate strength, and composite modulus remain almost unchanged for samples annealed up to 1100C. a thin porous layer formed on the surface of the 1100 C annealed sample and its density decreased from 3.09 to 2.90 g/em. The specimen annealed at 1200C gained 0.43% weight, was severely deformed, and was covered with a porous layer of thick shiny glaze which could be easily peeled off. Some gas bubbles were also present on the surface. This surface layer consisted of elongated crystals of monoclinic celsian and some amorphous phase(s). The fibers in this surface ply of the CMC had broken into small pieces. The fiber-matrix interface strength was characterized through fiber push-in technique. Values of debond stress, ad, and frictional sliding stress, tf, for the as-fabricated CMC were 0.31+0. 14 GPa and 10.4+3. 1 MPa, respectively. These values compared with 0.53+0.47 GPa and 8.33+ 1.72 MPa for the fibers in the interior of the 1200C annealed sample, indicating hardly any change in fiber-matrix interface strength. The effects of thermal aging on microstructure were investigated using scanning electron microscopy. Only the surface ply of the 1200C annealed specimens had degraded from oxidation whereas the bulk interior part of the CMC was unaffected. A mechanism is proposed explaining the various steps involved during the degradation of the CMC on annealing in air at 1200C Published by elsevier Ltd Keywords: Ceramic composites; Mechanical properties; SiC fibers; Barium aluminosilicate: Fiber-matrix interface 1. Introduction hot sections of turbine engines Results for Nicalon and hi- Nicalon fiber-reinforced celsian matrix composites have been Fiber-reinforced ceramic matrix composites(CMCs)are reported earlier. 6-17 During high-temperature use, CMC com- prospective candidate materials for high gh-temperature struct ponents are prone to degradation in their mechanical properties applications in various industries such as aerospace, power due to oxidation. Tensile, flexural, and shear properties, at generation, energy conservation, nuclear, petrochemical, and temperatures up to 1200C in air, have been reported for cel transportation. A number of ceramic and glass-ceramic com- sian matrix composites reinforced with Nicalon as well as posite systems. are being developed in various research Hi-Nicalon2 14.16 fibers. However, no information is available laboratories. Barium aluminosilicate with monoclinic celsian about the influence of long-term high-temperature exposures on phase is one of the most refractory glass-ceramics. It has a the mechanical properties of these CMCs. The primary objective melting point of >1700C, is phase stable to 1600C, and of this study was to investigate the effects of high-temperature is oxidation resistant. Over the last few years, at NASA Glenn annealing in oxidizing environment on the mechanical proper Research Center, celsian matrix composites'-9reinforced with ties and microstructural stability of Hi-Nicalon fiber-reinforced silicon carbide-based fibers have been investigated for use in celsian matrix composites. The room temperature strength of the composites, after annealing in air at various temperatures from 550 to 1200C, was measured in three-point flexure. The Tel:+12164333855 fiber-matrix interface strength was analyzed using a fiber push E-mail address: narottamP bansal(@ nasa.gov. in technique. 5 0955-2219/S-see front matter. Published by Elsevier Ltd. doi: 10.1016/j-jeurceramsoc200806.023Available online at www.sciencedirect.com Journal of the European Ceramic Society 29 (2009) 525–535 Mechanical properties of Hi-Nicalon fiber-reinforced celsian composites after high-temperature exposures in air Narottam P. Bansal ∗ Structures and Materials Division, NASA Glenn Research Center, Cleveland, OH 44135, USA Received 21 April 2008; received in revised form 13 June 2008; accepted 19 June 2008 Available online 26 July 2008 Abstract BN/SiC-coated Hi-Nicalon fiber-reinforced celsian matrix composites (CMCs) were annealed for 100 h in air at various temperatures to 1200 ◦C, followed by flexural strength measurements at room temperature. Values of yield stress and strain, ultimate strength, and composite modulus remain almost unchanged for samples annealed up to 1100 ◦C. A thin porous layer formed on the surface of the 1100 ◦C annealed sample and its density decreased from 3.09 to 2.90 g/cm3. The specimen annealed at 1200 ◦C gained 0.43% weight, was severely deformed, and was covered with a porous layer of thick shiny glaze which could be easily peeled off. Some gas bubbles were also present on the surface. This surface layer consisted of elongated crystals of monoclinic celsian and some amorphous phase(s). The fibers in this surface ply of the CMC had broken into small pieces. The fiber–matrix interface strength was characterized through fiber push-in technique. Values of debond stress, σd, and frictional sliding stress, τf, for the as-fabricated CMC were 0.31 ± 0.14 GPa and 10.4 ± 3.1 MPa, respectively. These values compared with 0.53 ± 0.47 GPa and 8.33 ± 1.72 MPa for the fibers in the interior of the 1200 ◦C annealed sample, indicating hardly any change in fiber–matrix interface strength. The effects of thermal aging on microstructure were investigated using scanning electron microscopy. Only the surface ply of the 1200 ◦C annealed specimens had degraded from oxidation whereas the bulk interior part of the CMC was unaffected. A mechanism is proposed explaining the various steps involved during the degradation of the CMC on annealing in air at 1200 ◦C. Published by Elsevier Ltd. Keywords: Ceramic composites; Mechanical properties; SiC fibers; Barium aluminosilicate; Fiber–matrix interface 1. Introduction Fiber-reinforced ceramic matrix composites (CMCs) are prospective candidate materials for high-temperature structural applications in various industries such as aerospace, power generation, energy conservation, nuclear, petrochemical, and transportation. A number of ceramic and glass–ceramic com￾posite systems1,2 are being developed in various research laboratories. Barium aluminosilicate with monoclinic celsian phase is one of the most refractory glass–ceramics. It has a melting point of >1700 ◦C, is phase stable to ∼1600 ◦C, and is oxidation resistant. Over the last few years, at NASA Glenn Research Center, celsian matrix composites3–9 reinforced with silicon carbide-based fibers have been investigated for use in ∗ Tel.: +1 216 433 3855. E-mail address: narottam.p.bansal@nasa.gov. hot sections of turbine engines. Results for Nicalon and Hi￾Nicalon fiber-reinforced celsian matrix composites have been reported earlier.6–17 During high-temperature use, CMC com￾ponents are prone to degradation in their mechanical properties due to oxidation. Tensile, flexural, and shear properties, at temperatures up to 1200 ◦C in air, have been reported for cel￾sian matrix composites reinforced with Nicalon6 as well as Hi-Nicalon12,14,16 fibers. However, no information is available about the influence of long-term high-temperature exposures on the mechanical properties of these CMCs. The primary objective of this study was to investigate the effects of high-temperature annealing in oxidizing environment on the mechanical proper￾ties and microstructural stability of Hi-Nicalon fiber-reinforced celsian matrix composites. The room temperature strength of the composites, after annealing in air at various temperatures from 550 to 1200 ◦C, was measured in three-point flexure. The fiber–matrix interface strength was analyzed using a fiber push￾in technique.15 0955-2219/$ – see front matter. Published by Elsevier Ltd. doi:10.1016/j.jeurceramsoc.2008.06.023