Scripta materialia ELSEVIER ota Materialia 54(2006)163-168 Damage mechanisms of C/Sic composites subjected to constant load and thermal cycling in oxidizing atmosphere Hui Mei*, Aifei Cheng, Litong Zhang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical Unirersity, 547 Mailbox. Xian Shaanxi 710072, People's Republic of china Received 4 April 2005: received in revised form 9 September 2005: accepted 26 September 2005 Available online 20 October 2005 Abstract Properties of a carbon fiber reinforced silicon carbide matrix composite were investigated in controlled environments including con- stant load, thermal cycling and wet oxygen atmosphere. Damage was assessed by residual mechanical properties and scanning electron microscopy characterization. Thermal strain was shown to change with cyclic temperatures over the same period (120 s) Strain varies approximately from the initial linear elastic strain of 0.63% to the final nonreversible damage strain of 1. 6% during the short time of the test. The experimental strain difference between two selected temperatures is about 0. 16% and the theoretical calculation value is 0. 1566%. After 50 thermal cycles, the Youngs modulus of the composites is reduced by a factor of 0.5 while the residual strength still retains 82% of the initial strength. It is observed that matrix cracks transversely and wave-shaped cracks are arranged on the coating surface at relatively regular spacing. A typical superficial oxidation can be found along the opening and propagating cracks beneath e coating o 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved Keywords: Fiber; Ceramic matrix composites; Thermal cycling: Creep; Residual properties 1. Introduction ation, the composites will be also subjected to both thermal cycling and some rigid constraint conditions in an oxidiz Carbon fiber reinforced SiC-matrix composites(C/Sic) ing atmosphere during its service. Consequently, thermal fabricated by the chemical vapor infiltration process cycling damage to the composites under such conditions (CVI) have been proposed as advanced materials suitable must be well understood before actual use in these for aerospace and gas turbine engine parts [1, 2]. In partic- environments ular, in recent years many efforts have been devoted to the The effects of temperature cycling on the structural high-temperature applications of C/SiC composites. These integrity of polyester matrix composites have been investi composites show some attractive properties and advanta- gated using a large scale model composite [3]and a compu ges over traditional ceramics: higher tensile and flexural tational model of delamination of a two-layer composite strength, enhanced fracture toughness and impact resis- laminate subjected to the cyclic loads, both mechanical tance, lower density and no cooling requirement. In partic- and thermal, was obtained [4]. On the other hand, experi- ular, the mechanical properties of C/SiC composites can be mental thermal shock studies have also been conducted retained at high-temperatures and under severe service on unidirectional, two-dimensional and three-dimensional environments. In many of the instances under consider-(3D) woven-fiber composites [5-7] and newly-developed ascending thermal shock test equipment has also been Corresponding author. Tel: +86 29 88494616: fax: +86 29 88494620 applied to study thermal shock and thermal fatigue of cera- E-mailaddresseshuimeiofchina(@yahoo.com.cn,phdhuimeiayahoo.micmaterials[8].However,the mechanical response and damage features of C/SiC composites subjected to thermal 1359-6462/S- see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved doi: 10.1016/j. scriptamat. 2005.09.044Damage mechanisms of C/SiC composites subjected to constant load and thermal cycling in oxidizing atmosphere Hui Mei *, Laifei Cheng, Litong Zhang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, 547 Mailbox, Xi
an Shaanxi 710072, People
s Republic of China Received 4 April 2005; received in revised form 9 September 2005; accepted 26 September 2005 Available online 20 October 2005 Abstract Properties of a carbon fiber reinforced silicon carbide matrix composite were investigated in controlled environments including con￾stant load, thermal cycling and wet oxygen atmosphere. Damage was assessed by residual mechanical properties and scanning electron microscopy characterization. Thermal strain was shown to change with cyclic temperatures over the same period (120 s). Strain varies approximately from the initial linear elastic strain of 0.63% to the final nonreversible damage strain of 1.6% during the short time of the test. The experimental strain difference between two selected temperatures is about 0.16% and the theoretical calculation value is 0.1566%. After 50 thermal cycles, the Young
s modulus of the composites is reduced by a factor of 0.5 while the residual strength still retains 82% of the initial strength. It is observed that matrix cracks transversely and wave-shaped cracks are arranged on the coating surface at relatively regular spacing. A typical superficial oxidation can be found along the opening and propagating cracks beneath the coating.
2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Fiber; Ceramic matrix composites; Thermal cycling; Creep; Residual properties 1. Introduction Carbon fiber reinforced SiC-matrix composites (C/SiC) fabricated by the chemical vapor infiltration process (CVI) have been proposed as advanced materials suitable for aerospace and gas turbine engine parts [1,2]. In partic￾ular, in recent years many efforts have been devoted to the high-temperature applications of C/SiC composites. These composites show some attractive properties and advanta￾ges over traditional ceramics: higher tensile and flexural strength, enhanced fracture toughness and impact resis￾tance, lower density and no cooling requirement. In partic￾ular, the mechanical properties of C/SiC composites can be retained at high-temperatures and under severe service environments. In many of the instances under consider￾ation, the composites will be also subjected to both thermal cycling and some rigid constraint conditions in an oxidiz￾ing atmosphere during its service. Consequently, thermal cycling damage to the composites under such conditions must be well understood before actual use in these environments. The effects of temperature cycling on the structural integrity of polyester matrix composites have been investi￾gated using a large scale model composite [3] and a compu￾tational model of delamination of a two-layer composite laminate subjected to the cyclic loads, both mechanical and thermal, was obtained [4]. On the other hand, experi￾mental thermal shock studies have also been conducted on unidirectional, two-dimensional and three-dimensional (3D) woven-fiber composites [5–7] and newly-developed ascending thermal shock test equipment has also been applied to study thermal shock and thermal fatigue of cera￾mic materials [8]. However, the mechanical response and damage features of C/SiC composites subjected to thermal 1359-6462/$ - see front matter
2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.09.044 * Corresponding author. Tel.: +86 29 88494616; fax: +86 29 88494620. E-mail addresses: huimeiofchina@yahoo.com.cn, phdhuimei@yahoo. com (H. Mei). www.actamat-journals.com Scripta Materialia 54 (2006) 163–168