Availableonlineatwww.sciencedirect.com COMPOSITES °" ScienceDirect CIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 68(2008)1588-1595 w.elsevier. com/locate/compscitech Creep behavior of Nextel TMz 720/alumina ceramic composite with ±45° fiber orientation at1200°C☆ M B. Ruggles-Wrenn , G.T. Siegert,SS.Baek Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433-7765, US.A Received 18 January 2007; received in revised form 2 May 2007: accepted 19 July 2007 Available online 1 august 2007 Abstract The tensile creep behavior of an oxide-oxide continuous fiber ceramic composite(CFCC)with +45 fiber orientation was investigated at 1200. in labor) fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance.The ten- in steam and in argon. The composite consists of a porous alumina matrix reinforced with laminated, woven mull- sile stress-strain behavior was investigated and the tensile properties measured at 1200C. The elastic modulus was 46 GPa and the ultimate tensile strength (UTS)was 55 MPa. Tensile creep behavior was examined for creep stresses in the 15-45 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run- out(set to 100 h) was achieved in all test environments for creep stress levels <35 MPa. At creep stresses >35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated Keywords: A Ceramic-matrix composites(CMCs); A Oxides: B Creep; B. High-temperature properties; D. Fractography 1. Introduction Concurrent efforts in optimization of the CMCs and in Advances in aerospace propulsion technologies have design of the combustion chamber are expected to accelerate raised the demand for structural materials that have supe- the insertion of the CMCs into aerospace turbine rior long-term mechanical properties and retained proper- applications, such as combustor walls [3-5]. Because these ties under high-temperature, high pressure, and varying applications require exposure to oxidizing environments, environmental factors, such as moisture [l]. Ceramic-matrix the thermodynamic stability and oxidation resistance of omposites (CMCs), capable of maintaining excellent CMCs are vital issues. The need for environmentally stable strength and fracture toughness at high-temperatures are composites motivated the development of CMCs based on prime candidate materials for such aerospace applications. environmentally stable oxide constituents[6-11] Additionally, the lower densities of CMCs and their higher The main advantage of CMCs over monolithic ceramics use temperatures, together with a reduced need for cooling is their superior toughness, tolerance to the presence of air, allow for improved high-temperature performance when cracks and defects, and non-catastrophic mode of failure It is widely accepted that in order to avoid brittle fracture * The views expressed are those of the authors and do not reflect the behavior in CMCs and improve the damage tolerance, a official pol ion of the United States Air Force, Department of weak fiber /matrix interface is needed, which serves to Defense or the US government Corresponding author. Tel: +l 937 255 3636x4641: fax: +1 937 656 deflect matrix cracks and to allow subsequent fiber pullout [12-14]. It has been demonstrated that similar crack-deflect arina-ruggles-wrenn(@afit. edu(MB. Ruggles- ing behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate 0266-3538S.see front matter Published by Elsevier Ltd. doi: 10.1016/j. compscitech. 2007.07.012Creep behavior of NextelTM720/alumina ceramic composite with ±45 fiber orientation at 1200 C q M.B. Ruggles-Wrenn a,*, G.T. Siegert a , S.S. Baek b a Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433-7765, USA b Agency for Defense Development, Daejeon, Republic of Korea Received 18 January 2007; received in revised form 2 May 2007; accepted 19 July 2007 Available online 1 August 2007 Abstract The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) with ±45 fiber orientation was investigated at 1200 C in laboratory air, in steam and in argon. The composite consists of a porous alumina matrix reinforced with laminated, woven mull￾ite/alumina (NextelTM720) fibers, has no interface between the fiber and matrix, and relies on the porous-matrix for flaw tolerance. The ten￾sile stress–strain behavior was investigated and the tensile properties measured at 1200 C. The elastic modulus was 46 GPa and the ultimate tensile strength (UTS) was 55 MPa. Tensile creep behavior was examined for creep stresses in the 15–45 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in all test environments for creep stress levels 635 MPa. At creep stresses >35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated. Published by Elsevier Ltd. Keywords: A. Ceramic–matrix composites (CMCs); A. Oxides; B. Creep; B. High-temperature properties; D. Fractography 1. Introduction Advances in aerospace propulsion technologies have raised the demand for structural materials that have supe￾rior long-term mechanical properties and retained proper￾ties under high-temperature, high pressure, and varying environmental factors, such as moisture [1]. Ceramic–matrix composites (CMCs), capable of maintaining excellent strength and fracture toughness at high-temperatures are prime candidate materials for such aerospace applications. Additionally, the lower densities of CMCs and their higher use temperatures, together with a reduced need for cooling air, allow for improved high-temperature 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 applications require exposure to oxidizing 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 pullout [12–14]. It has been demonstrated that similar crack-deflect￾ing behavior can also be achieved by means of a finely distributed porosity in the matrix instead of a separate 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2007.07.012 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). www.elsevier.com/locate/compscitech Available online at www.sciencedirect.com Composites Science and Technology 68 (2008) 1588–1595 COMPOSITES SCIENCE AND TECHNOLOGY