Composites: Part A 39(2008)1829-1837 Contents lists available at ScienceDirect Composites: Part A ELSEVIER journalhomepagewww.elsevier.com/locate/compositesa Effects of steam environment on compressive creep behavior of Nextelm720 Alumina ceramic composite at 1200C M B. Ruggles-Wrenn N.R. Szymczak Department of Aeronautics and Astronautics, Air Force institute of Technology wright-Patterson Air Force Base, OH 45433-7765, US ARTICLE IN F O ABSTRACT Article history The compressive creep behavior of an oxide-oxide ceramic-matrix composite( MC) was investigated at Received 18 January 2008 1200C in laboratory air and in steam. The composite consis porous alumina matrix reinforced with Received in revised form 27 August 2008 ccepted 5 September 2008 laminated, woven mullite/ alumina(Nextel 720)fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance. The compressive stress-strain behavior was investi- gated and the compressive properties measured. The influence of the loading rate on compressive stress- strain response and on compressive properties was also explored. A change in loading rate by four orders A: Ceramic-matrix composites(CMCs) or magnitude had a profound effect on compressive properties in air and especially in steam Compres- ve creep behavior was examined for creep stresses in the -40 to-100 MPa range Minimum creep rate was reached in all tests In air, compressive creep strain magnitudes remained <0.4% and compressi reep strain rates approached -35 x 10-75-1. Creep run- out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes In steam, compressive creep strains approached -1.6%, and compressive creep strain rat es,-19x 10-2s-1 In steam, maximum time to rupture was only 3.9 h. Composite microstruc- ture, as well as damage and failure mechanisms were investigated. Published by Elsevier Ltd. 1 Introduction stable composites motivated the development of CMCs based on environmentally stable oxide constituents [6-11- Advances in power generation systems for aircraft engines The main advantage of CMCs over monolithic ceramics is their nd-based turbines, rockets, and, most recently, hypersonic mis- superior toughness, tolerance to the presence of cracks and defects, siles and flight vehicles have raised the demand for structural and non-catastrophic mode of failure. It is widely accepted that in materials that have superior long-term mechanical properties order to avoid brittle fracture behavior in CMCs and improve the and retained properties under high-temperature, high pressure, damage tolerance, a weak fiber/matrix interface is needed, which and varying environmental factors, such as moisture [1]. Cera- serves to deflect matrix cracks and to allow subsequent fiber pull- mic-matrix composites, capable of maintaining excellent strength out[12-14. It has been demonstrated that similar crack-deflecting and fracture toughness at high-temperatures are prime candidate behavior can also be achieved by means of a finely distributed materials for such applications. Additionally, lower densities of porosity in the matrix instead of a separate interface between ma- CMCs and their higher use temperatures, together with a reduced trix and fibers 15. This microstructural design philosophy implic need for cooling air, allow for improved high-temperature perfor- itly accepts the strong fiber/matrix interface. The concept has been ance when compared to conventional nickel-based superalloys successfully demonstrated for oxide-oxide composites 6,9, 11 [2]. Advanced aerospace turbine engines will likely incorporate fi- 16, 17]. Resulting oxide/oxide CMCs exhibit damage tolerance com- ber-reinforced CMCs in critical components, such as combustor bined with inherent oxidation resistance. An extensive review of walls [3-5]. Because these applications require exposure to oxidiz- the mechanisms and mechanical properties of porous-matrix CMCs ing environments, the thermodynamic stability and oxidation is given in [18, 19 resistance of CMCs are vital issues. The need for environmentally Porous-matrix oxide/oxide CMCs exhibit several behavio trends that are distinctly different from those exhibited by tradi- tional non-oxide Cmcs with a fiber-matrix interface For the non- oxide CmCs, fatigue is significantly more damaging than creep. al policy Contrastingly, Zawada et al. [20] examined the high-temperature Go Osition of the United States Air Force, Department of Defense or the Us mechanical behavior of a porous-matrix Nextel610/Aluminosili Corresponding author. Tel : +1 937 255 3636x4641: fax: +1 937 656 7053 cate composite Results revealed excellent fatigue performance at mail address: marina. ruggles-wrenn@afit.edu(M B. Ruggles-wrennl 000C. Conversely, creep lives were short, indicating low creep latter Published by Elsevier Ltd.Effects of steam environment on compressive creep behavior of NextelTM720/Alumina ceramic composite at 1200 C q M.B. Ruggles-Wrenn *, N.R. Szymczak Department of Aeronautics and Astronautics, Air Force Institute of Technology Wright-Patterson Air Force Base, OH 45433-7765, USA article info Article history: Received 18 January 2008 Received in revised form 27 August 2008 Accepted 5 September 2008 Keywords: A: Ceramic-matrix composites (CMCs) B: Creep B: Environmental degradation D: Mechanical testing abstract The compressive creep behavior of an oxide–oxide ceramic-matrix composite (CMC) was investigated at 1200 C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (NextelTM720) fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance. The compressive stress–strain behavior was investi￾gated and the compressive properties measured. The influence of the loading rate on compressive stress– strain response and on compressive properties was also explored. A change in loading rate by four orders or magnitude had a profound effect on compressive properties in air and especially in steam. Compres￾sive creep behavior was examined for creep stresses in the 40 to 100 MPa range. Minimum creep rate was reached in all tests. In air, compressive creep strain magnitudes remained <0.4% and compressive creep strain rates approached 3.5 107 s1 . Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, compressive creep strains approached 1.6%, and compressive creep strain rates, 1.9 102 s 1 . In steam, maximum time to rupture was only 3.9 h. Composite microstruc￾ture, as well as damage and failure mechanisms were investigated. Published by Elsevier Ltd. 1. Introduction Advances in power generation systems for aircraft engines, land-based turbines, rockets, and, most recently, hypersonic mis￾siles and flight vehicles have raised the demand for structural materials that have superior long-term mechanical properties and retained properties under high-temperature, high pressure, and varying environmental factors, such as moisture [1]. Cera￾mic-matrix composites, capable of maintaining excellent strength and fracture toughness at high-temperatures are prime candidate materials for such applications. Additionally, lower densities of CMCs and their higher use temperatures, together with a reduced need for cooling air, allow for improved high-temperature perfor￾mance when compared to conventional nickel-based superalloys [2]. Advanced aerospace turbine engines will likely incorporate fi- ber-reinforced CMCs in critical components, such as combustor walls [3–5]. Because these applications require exposure to oxidiz￾ing environments, the thermodynamic stability and oxidation resistance of CMCs are vital issues. The need for environmentally stable composites motivated the development of CMCs based on environmentally stable oxide constituents [6–11]. The main advantage of CMCs over monolithic ceramics is their superior toughness, tolerance to the presence of cracks and defects, and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pull￾out [12–14]. It has been demonstrated that similar crack-deflecting behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate interface between ma￾trix and fibers [15]. This microstructural design philosophy implic￾itly accepts the strong fiber/matrix interface. The concept has been successfully demonstrated for oxide–oxide composites [6,9,11, 16,17]. Resulting oxide/oxide CMCs exhibit damage tolerance com￾bined with inherent oxidation resistance. An extensive review of the mechanisms and mechanical properties of porous-matrix CMCs is given in [18,19]. Porous-matrix oxide/oxide CMCs exhibit several behavior trends that are distinctly different from those exhibited by tradi￾tional non-oxide CMCs with a fiber-matrix interface. For the non￾oxide CMCs, fatigue is significantly more damaging than creep. Contrastingly, Zawada et al. [20] examined the high-temperature mechanical behavior of a porous-matrix Nextel610/Aluminosili￾cate composite. Results revealed excellent fatigue performance at 1000 C. Conversely, creep lives were short, indicating low creep 1359-835X/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compositesa.2008.09.005 q The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the US Government. * Corresponding author. Tel.: +1 937 255 3636x4641; fax: +1 937 656 7053. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. Ruggles-Wrenn). Composites: Part A 39 (2008) 1829–1837 Contents lists available at ScienceDirect Composites: Part A journal homepage: www.elsevier.com/locate/compositesa