Availableonlineatwww.sciencedirect.com e Science Direct COMPOSITES CIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 67(2007)1425-1438 www.elsevier.com/locate/compscitech Influence of hold times on the elevated-temperature fatigue behavior of an oxide-oxide ceramic composite in air and in steam environment JM.Ehrman , M.B. Ruggles-Wrenn , S.S.Baek b Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765, USA Agency for Defense Development, Daejeon, Republic of Korea Received 2 June 2006: received in revised form 25 August 2006: accepted 7 September 2006 Available online 15 November 2006 Abstract The effect of hold times at maximum stress on fatigue behavior of an oxide-oxide ceramic composite was investigated at 1200Cin laboratory air and in steam environments. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (NextelM720)fibers, has no interface between the fiber and matrix, and relies on the porous matrix for faw tolerance Tension-tension fatigue tests with a ratio R (minimum to maximum stress)of 0.05, and hold times of 10 and 100 s were performed for fatigue stresses of 125 and 154 MPa in laboratory air, and for fatigue stresses of 100 and 125 MPa in steam environment. Block loading tests incorporating periods of cyclic and sustained loading were carried out to assess the effects of loading history on material behavior and environmental durability. In laboratory air, lives produced in fatigue tests with hold times exceeded those produced in creep but were shorter than those obtained in fatigue. Prior fatigue resulted in an order of magnitude improvement in creep life. Prior creep had no effect on subsequent fatigue life. Presence of steam significantly degraded the material performance In steam, lives produced in fatigue tests with hold times were close to those obtained in creep. Prior fatigue reduced the creep resistance, and prior creep degraded the subsequent fatigue life Composite microstructure, as well as damage and failure mechanisms were investigated. a qualitative spectral analysis showed evidence of silicon species migration from fiber to matrix, especially in steam. o 2006 Elsevier Ltd. All rights reserved Keywords: A Ceramic-matrix composites: A Oxides; B. Fatigue: B Creep: B Mechanical properties 1. Introduction tions. Additionally, the lower densities of CMCs and their ratures, together with a reduced need for Advances in aerospace propulsion technologies have cooling air, allow for improved high-temperature perfor raised the demand for structural materials that have supe- mance when compared to conventional nickel-based super rior long-term mechanical properties and retained proper- alloys [l]. Ceramic-matrix composites are being evaluated les under high temperature, high pressure and various for use in aerospace turbine engines and are likely to be environmental conditions such as moisture. Ceramic- incorporated in combustion chambers and nozzle exten matrix composites(CMCs ), capable of maintaining excel- sions of the advanced rocket propulsion systems [2, 3]. lent strength and fracture toughness at high temperatures Because these applications require exposure to oxidizing are prime candidate materials for such aerospace applica- environments, the thermodynamic stability and oxidation resistance of CMCs are vital issues The views expressed are those of the authors and do not reflect the Non-oxide fiber/non-oxide matrix composites generally ion of the United States Air Force, Department of show poor oxidation resistance [4, 5]. The degradation Defense or the us government Corresponding author. Tel. +1 937 255 3636x4641; fax: +1 937 656 involves oxidation of fibers and fiber coatings, and is typi 4032 cally accelerated by the presence of moisture [6-8]. Using a E-mail address a.ruggles-wrenn(@afit. edu (M.B. Ruggles. non-oxide fiber/oxide matrix or oxide fiber/non-oxide natrix composites generally does not substantially improve 02663538/S. see front matter 2006 Elsevier Ltd. All rights reserved doi:10.1016j.compscitech.2006.09.005Influence of hold times on the elevated-temperature fatigue behavior of an oxide–oxide ceramic composite in air and in steam environment q J.M. Mehrman a , M.B. Ruggles-Wrenn a,*, S.S. Baek b a Department of Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson Air Force Base, OH 45433-7765, USA b Agency for Defense Development, Daejeon, Republic of Korea Received 2 June 2006; received in revised form 25 August 2006; accepted 7 September 2006 Available online 15 November 2006 Abstract The effect of hold times at maximum stress on fatigue behavior of an oxide–oxide ceramic composite was investigated at 1200 C in laboratory air and in steam environments. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (NextelTM720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension–tension fatigue tests with a ratio R (minimum to maximum stress) of 0.05, and hold times of 10 and 100 s were performed for fatigue stresses of 125 and 154 MPa in laboratory air, and for fatigue stresses of 100 and 125 MPa in steam environment. Block loading tests incorporating periods of cyclic and sustained loading were carried out to assess the effects of loading history on material behavior and environmental durability. In laboratory air, lives produced in fatigue tests with hold times exceeded those produced in creep but were shorter than those obtained in fatigue. Prior fatigue resulted in an order of magnitude improvement in creep life. Prior creep had no effect on subsequent fatigue life. Presence of steam significantly degraded the material performance. In steam, lives produced in fatigue tests with hold times were close to those obtained in creep. Prior fatigue reduced the creep resistance, and prior creep degraded the subsequent fatigue life. Composite microstructure, as well as damage and failure mechanisms were investigated. A qualitative spectral analysis showed evidence of silicon species migration from fiber to matrix, especially in steam. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic–matrix composites; A. Oxides; B. Fatigue; B. Creep; B. Mechanical properties 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 various environmental conditions such as moisture. Ceramic– matrix composites (CMCs), capable of maintaining excel￾lent strength and fracture toughness at high temperatures are prime candidate materials for such aerospace applica￾tions. Additionally, the 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 super￾alloys [1]. Ceramic–matrix composites are being evaluated for use in aerospace turbine engines and are likely to be incorporated in combustion chambers and nozzle exten￾sions of the advanced rocket propulsion systems [2,3]. Because these applications require exposure to oxidizing environments, the thermodynamic stability and oxidation resistance of CMCs are vital issues. Non-oxide fiber/non-oxide matrix composites generally show poor oxidation resistance [4,5]. The degradation involves oxidation of fibers and fiber coatings, and is typi￾cally accelerated by the presence of moisture [6–8]. Using a non-oxide fiber/oxide matrix or oxide fiber/non-oxide matrix composites generally does not substantially improve 0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.09.005 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 4032. E-mail address: marina.ruggles-wrenn@afit.edu (M.B. Ruggles￾Wrenn). www.elsevier.com/locate/compscitech Composites Science and Technology 67 (2007) 1425–1438 COMPOSITES SCIENCE AND TECHNOLOGY