Composites Science and Technology 69(2009)663-669 Contents lists available at ScienceDirect Soong Composites Science and Technology ELSEVIER journalhomepagewww.elsevier.com/locate/compscitech Creep of Nextelmm720/alumina-mullite ceramic composite at 1200C in air, argon, and steam M B. Ruggles-Wrenn*, C L. Genelin Air Force institute of Technology, wright-Patterson Air Force Base, AFT/ENY, 2950 Hobson Way, WPAFB, OH 45433-7765, USA ARTICLE IN F O ABSTRACT Article history: The tensile creep behavior of an Received 28 October 2008 Received in revised form 11 December 2008 1200C in laboratory air, in stean matrix reinforced with laminated, woven mullitealumina(Nextel 720)fibers, has no interface between Available online 14 January 2009 he fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress-strain behavior was investigated and the tensile properties measured at 1200C. The elastic modulus was 74.5 GPa and e ultimate tensile strength was 153 MPa Tensile creep behavior was examined for creep stresses in the A Ceramic-matrix composites(CMCs) 70-140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out(set to 100 h)was achieved in laboratory air for creep stress levels <91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage B High-temperature properties and failure mechanisms were investigated D Fractography Published by Elsevier Ltd. 1 Introduction and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the advances in power generation systems for aircraft engines, damage tolerance, a weak fiber/matrix interface is needed, which land-based turbines, rockets, and, most recently, hypersonic mis- serves to deflect matrix cracks and to allow subsequent fiber pull siles and flight vehicles have raised the demand for structural out [10-12]. It has been demonstrated that similar crack-deflecting materials that have superior long-term mechanical properties behavior can also be achieved by means of a finely distributed Ind retained properties under high temperature, high pressure rosity in the matrix instead of a separate interface between ma nd varying environmental factors, such as moisture [1]. Typic x and fibers [13]. This microstructural design philosophy implic components include combustors, nozzles and thermal insulation. itly accepts the strong fiber/matrix interface. The concept has been Ceramic-matrix composites(CMCs), capable of maintaining excel- successfully demonstrated for oxide-oxide composites lent strength and fracture toughness at high temperatures a [4,7, 9, 14, 15]. Resulting oxide/ oxide Cmcs exhibit damage toler prime candidate materials for such applications. Additionally, low- ance combined with inherent oxidation resistance. An extensive er densities of CMCs and their higher use temperatures, together review of the mechanisms and mechanical properties of porous with a reduced need for cooling air, allow for improved high-tem- matrix CMCs is given in [1, 16]. perature performance when compared to conventional nickel Porous-matrix oxide/oxide CMCs exhibit several behavior based superalloys 2]. Advanced reusable space launch vehicles trends that are distinctly different from those exhibited by tradi- will likely incorporate fiber-reinforced CMCs in critical propulsion tional CMCs with a fiber-matrix interface. Most Sic-fiber-contain- components [3. Because these applications require exposure to ing CMCs exhibit longer life under static loading and shorter life oxidizing environments, the thermodynamic stability and oxida- under cyclic loading [17. For these materials, fatigue is signifi tion resistance of CMCs are vital issues. The need for environmen- cantly more damaging than creep. Zawada et al. [18 examined tally stable composites motivated the development of CMCs based the high-temperature mechanical behavior of a porous matrix on environmentally stable oxide constituents [4-9). Nextel 610/aluminosilicate composite Results revealed excellent The main advantage of CMCs over monolithic ceramics is their fatigue performance at 1000C, the material exhibited high fatigue superior toughness, tolerance to the presence of cracks and defects, limit, long fatigue life and near 100% strength retention. Con ersely, creep lives were short, indicating low creep resistance Because creep was shown to be much more damaging than cy author.Tel:+19372553636:fax:+19376567053 lic loading to oxide-oxide CMCs with porous matrix[18-20]. hisx temperature creep resistance remains among the key issues the E-mail address: marina. ruggles-wrenneafit edu(MB. Ruggles-Wrenn) must be addressed before using these materials in advanced aero-Creep of NextelTM720/alumina–mullite ceramic composite at 1200 C in air, argon, and steam q M.B. Ruggles-Wrenn *, C.L. Genelin Air Force Institute of Technology, Wright-Patterson Air Force Base, AFIT/ENY, 2950 Hobson Way, WPAFB, OH 45433-7765, USA article info Article history: Received 28 October 2008 Received in revised form 11 December 2008 Accepted 8 January 2009 Available online 14 January 2009 Keywords: A. Ceramic–matrix composites (CMCs) A. Oxides B. Creep B. High-temperature properties D. Fractography abstract The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite 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 tensile stress–strain behavior was investigated and the tensile properties measured at 1200 C. The elastic modulus was 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels 691 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, 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]. 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]. Advanced reusable space launch vehicles will likely incorporate fiber-reinforced CMCs in critical propulsion components [3]. Because these applications require exposure to oxidizing environments, the thermodynamic stability and oxida￾tion resistance of CMCs are vital issues. The need for environmen￾tally stable composites motivated the development of CMCs based on environmentally stable oxide constituents [4–9]. 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 [10–12]. 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 [13]. This microstructural design philosophy implic￾itly accepts the strong fiber/matrix interface. The concept has been successfully demonstrated for oxide–oxide composites [4,7,9,14,15]. Resulting oxide/oxide CMCs exhibit damage toler￾ance combined with inherent oxidation resistance. An extensive review of the mechanisms and mechanical properties of porous￾matrix CMCs is given in [1,16]. Porous-matrix oxide/oxide CMCs exhibit several behavior trends that are distinctly different from those exhibited by tradi￾tional CMCs with a fiber–matrix interface. Most SiC-fiber-contain￾ing CMCs exhibit longer life under static loading and shorter life under cyclic loading [17]. For these materials, fatigue is signifi- cantly more damaging than creep. Zawada et al. [18] examined the high-temperature mechanical behavior of a porous matrix NextelTM610/aluminosilicate composite. Results revealed excellent fatigue performance at 1000 C, the material exhibited high fatigue limit, long fatigue life and near 100% strength retention. Con￾versely, creep lives were short, indicating low creep resistance and limiting the use of that CMC to temperatures below 1000 C. Because creep was shown to be much more damaging than cyc￾lic loading to oxide–oxide CMCs with porous matrix [18–20], high￾temperature creep resistance remains among the key issues that must be addressed before using these materials in advanced aero- 0266-3538/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.compscitech.2009.01.002 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 3636; fax: +1 937 656 7053. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. Ruggles-Wrenn). Composites Science and Technology 69 (2009) 663–669 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech