Thermal shock behavior of a three-dimensional SiC/SiC composite
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INTRODUCTION
SILICON carbide fiber-reinforced SiC-matrix composites fabricated by the chemical vapor infiltration (CVI) process (SiC/SiC (CVI)) have been intensively researched as the most promising thermal structural materials.[1,2] Besides fundamental thermal and mechanical properties, thermal shock and thermal cycle effects are important factors limiting the performance of the composites for hightemperature application. Thermal shock resistance of fiber-reinforced silicon carbide matrix composites has been the subject of increasing interest recently.[3–9] These studies have shown that the characteristics of thermal shock damage in fiber-reinforced composites were directly affected by the woven structure of the fiber preforms. Yin et al. studied the thermal shock behavior of a three-dimensional (3-D) C/SiC composite quenched in air.[4] The results indicated that there was a critical thermal shock cycle number of 50 cycles for 3-D woven C/SiC composite. When the cycles of thermal shocks were less than 50, the residual flexural strength of C/SiC composites decreased with the increase of the number. When the number of thermal shocks of C/SiC was greater than 50, the strength of C/SiC did not further decrease because the crack density was saturated. Kagawa documented the microdamage processes of a SiC/SiC composite consisting of a SiC matrix reinforced with a twodimensional (2-D) SiC fiber fabric (Nicalon) fabricated by the CVI process.[8] Further, the observed microdamage processes of the composite were as follows: (1) microcracks initiated from the corners of pores; (2) crack growth between bundles and inside the bundles at the transverse plane; and (3) saturation of the crack density and formation SHOUJUN WU, Doctor, LAIFEI CHENG, LITONG ZHANG, and YONGDONG XU, Professors, are with the National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted January 19, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
of long cracks after three repeated thermal shock events. The surface of 2-D HI-NICALON* SiC/SiC composites *HI-NICALON is a trademark of Nippon Carbon Co., Tokyo, Japan.
under thermal shock indicates sublimation and trace melting.[9] However, the thermal shock behavior of HI-NICALON/SiC composites, especially those that are 3-D and quenched in water, has not been reported up to now. In the present article, a 3-D HI-NICALON/SiC composite was prepared. Much of the analysis and discussion will then focus on the effects of thermal shock quenched in water on the evolution of microstructural damage, the flexural strength and the corresponding Young’s modulus of the composite. II.
EXPERIMENTAL PROCEDURE
A. Fabrication of the Specimens HI-NICALON silicon carbide fiber from Japan Nippon Carbon (Tokyo, Japan) was employed. The fiber preform was prepared using the four-step three-dimensional (4-step 3-D) braiding method and was supplied by the Nanjing Institute of Glass Fiber (Nanjin
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