Composites Science and Technology 68(2008)2260-2266 Contents lists available at Science Direct Composites Science and Technology ELSEVIER journalhomepagewww.elsevier.com/locate/compscitech Creep behavior in interlaminar shear of NextelmM720/ alumina ceramic composite at elevated temperature in air and in steam" M.B. Ruggles-Wrenn, P D. Laffey Department of Aeronautics and Astronautics, Air Force institute of Technology, wright-Patterson Air Force Base, OH 45433-7765, United States ARTICLE INFO A BSTRACT The creep behavior in interlaminar shear of an oxide-oxide ceramic matrix composite(CMc) eceived 14 March 2008 ated at 1200C in laboratory air and in steam using double-notch shear test specimens. The ceived in revised form 4 April 2008 Accepted 9 April 2008 consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina(Ne Available online 15 April 2008 fibers, has no interface between the fiber and matrix, and relies on the porous matrix for fla The interlaminar shear properties were measured. The creep behavior was examined for interlamir near stresses in the 4-6.5 MPa range. Primary and secondary creep regimes were observed in all tests A Ceramic-matrix composites(CMCs onducted in air. In steam, the composite exhibited primary, secondary and tertiary creep In air, creep run-out defined as 100 h at creep stress was achieved in all tests. In the presence of steam, creep perfor- mance deteriorated rapidly and run-out was achieved only at 4 MPa (50% of the interlaminar shear B High-temperature properties strength at 1200C). The retained properties of all specimens that achieved run-out were characterized. D Fractography Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degra- dation appears to be the cause of reduced creep lifetimes in steam Published by Elsevier Ltd. 1 Introduction development of CMCs based on environmentally stable oxide con- stituents 6-11 Advances in power generation systems for aircraft engines, The main advantage of CMCs over monolithic ceramics is their land-based turbines, rockets, and, most recently, hypersonic mis- superior toughness, tolerance to the presence of cracks and de- siles and flight vehicles have raised the demand for structural fects, and non-catastrophic mode of failure. It is widely accepted materials that have superior long-term mechanical properties that in order to avoid brittle fracture behavior in CMCs and im- and retained properties under high temperature, high pressure, prove the damage tolerance, a weak fiber/matrix interface and varying environmental factors, such as moisture [1. Typical needed which serves to deflect matrix cracks and to allow subse- omponents include combustors, nozzles and thermal insulati quent fiber pullout [12-14]. It has been demonstrated that similar Ceramic-matrix composites(CMCs), capable of maintaining excel- crack-deflecting behavior can also be achieved by means of a fi lent strength and fracture toughness at high temperatures are nely distributed porosity in the matrix instead of a separate inter- prime candidate materials for such applications. Additionally, low- face between matrix and fibers [15. This microstructural design with a reduced need for cooling air, allow for improved high-ten The concept has been successfully demonstrated for oxide -oxide perature performance when compared to conventional nickel- composites [6,9, 11, 16, 17]. Resulting oxide/oxide CMCs exhibit based superalloys [2]. Concurrent efforts in optimization of the damage tolerance combined with inherent oxidation resistance. CMCs and in design of the combustion chamber are expected to An extensive review of the mechanisms and mechanical proper accelerate the insertion of the Cmcs into aerospace turbine engine ties of porous-matrix CMCs is given in [18, 19 tions require exposure to oxidizing environments, the thermody- with monolithic ceramics, two-dimensional laminated CMCpared pplications, such as combustor walls 3-5]. Because these applica While CMCs exhibit improved damage tolerance cor namic stability and oxidation resistance of CMCs are vital issues. more susceptible to failure in the matrix-rich interlaminar regions. The need for environmentally stable composites motivated the The interlaminar failure or delamination may ultimately lead to loss of stiffness and accelerate structural failure [20]. A number of studies assessed the behavior of CMCs in shear [20-23]. choi expressed are those of the authors and do not reflect the official tion of the United States Air Force, Department of Defense or the Us et al.[24-27]assessed the high-temperature life limiting behavior in interlaminar shear of several non-oxide CMCs, including three ding author.Tel:+19372553636x4641;fax:+19376567053 Sic fiber-reinforced CMCs and one carbon -fiber-reinforced cmc .ruggles-wrenn@afit.edu(MB. Ruggles-Wrenn) Choi and co-workers determined the interlaminar shear strength ront matter Published by Elsevier Ltd.Creep behavior in interlaminar shear of NextelTM720/alumina ceramic composite at elevated temperature in air and in steamq M.B. Ruggles-Wrenn *, P.D. Laffey Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765, United States article info Article history: Received 14 March 2008 Received in revised form 4 April 2008 Accepted 9 April 2008 Available online 15 April 2008 Keywords: A. Ceramic–matrix composites (CMCs) A. Oxides B. Creep B. High-temperature properties D. Fractography abstract The creep behavior in interlaminar shear of an oxide–oxide ceramic matrix composite (CMC) was evalu￾ated at 1200 C in laboratory air and in steam using double-notch shear test specimens. 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 interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 4–6.5 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air. In steam, the composite exhibited primary, secondary and tertiary creep. In air, creep run-out defined as 100 h at creep stress was achieved in all tests. In the presence of steam, creep perfor￾mance deteriorated rapidly and run-out was achieved only at 4 MPa (50% of the interlaminar shear strength at 1200 C). The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degra￾dation appears to be the cause of reduced creep lifetimes in steam. 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]. Typical components include combustors, nozzles and thermal insulation. Ceramic–matrix composites (CMCs), capable of maintaining excel￾lent strength and fracture toughness at high temperatures are prime candidate materials for such applications. Additionally, low￾er densities of CMCs and their higher use temperatures, together with a reduced need for cooling air, allow for improved high-tem￾perature performance when compared to conventional nickel￾based superalloys [2]. Concurrent efforts in optimization of the CMCs and in design of the combustion chamber are expected to accelerate the insertion of the CMCs into aerospace turbine engine applications, such as combustor walls [3–5]. Because these applica￾tions require exposure to oxidizing environments, the thermody￾namic 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 con￾stituents [6–11]. The main advantage of CMCs over monolithic ceramics is their superior toughness, tolerance to the presence of cracks and de￾fects, and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fracture behavior in CMCs and im￾prove the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subse￾quent fiber pullout [12–14]. It has been demonstrated that similar crack-deflecting behavior can also be achieved by means of a fi- nely distributed porosity in the matrix instead of a separate inter￾face between matrix and fibers [15]. This microstructural design philosophy implicitly 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 combined with inherent oxidation resistance. An extensive review of the mechanisms and mechanical proper￾ties of porous-matrix CMCs is given in [18,19]. While CMCs exhibit improved damage tolerance compared with monolithic ceramics, two-dimensional laminated CMCs are more susceptible to failure in the matrix-rich interlaminar regions. The interlaminar failure or delamination may ultimately lead to loss of stiffness and accelerate structural failure [20]. A number of studies assessed the behavior of CMCs in shear [20–23]. Choi et al. [24–27] assessed the high-temperature life limiting behavior in interlaminar shear of several non-oxide CMCs, including three SiC fiber-reinforced CMCs and one carbon-fiber-reinforced CMC. Choi and co-workers determined the interlaminar shear strength 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2008.04.009 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 Science and Technology 68 (2008) 2260–2266 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech