Time-dependent deformation in an enhanced SiC/SiC composite
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Time-Dependent Deformation in an Enhanced SiC/SiC Composite SHIJIE ZHU, JIAN-WU CAO, MINEO MIZUNO, and YUTAKA KAGAWA Time-dependent deformation in an enhanced SiC/SiC composite has been studied under constant load at high temperatures of 1200 °C, 1300 °C, and 1400 °C. Creep damage evolution was evaluated by a Young’s-modulus change of partial unloading and microscopic observation. The addition of the glassy phase in the matrix is very effective for protecting the composite from oxidation. The transient creep is dominant in creep life at all the temperatures. An empirical equation is proposed to describe creep behavior of the composite. It is found that creep activation energy increases with creep time at stresses lower than matrix cracking stress, but the activation energy remains constant at stresses higher than the matrix cracking stress. The creep strain rate of the composite is considered to be controlled by creep of fibers based on examining the time, strain, stress, and temperature dependencies of creep strain rates.
I. INTRODUCTION
THE advantages of continuous-fiber-reinforced ceramicmatrix composites (CMCs) are high fracture toughness, high thermal-shock resistance, notch insensitivity, and no scaling effect compared to monolithic ceramics.[1,2,3] Furthermore, it is feasible to manufacture large components using CMCs, which are difficult to make out of monolithic ceramics. However, the oxidation resistance of CMCs at high temperatures is a fatal problem, since the interphase in CMCs is sensitive to environment, e.g., carbon interphase in SiC/SiC.[4,5] To overcome this problem, three improving ways have been explored, that is, modification of the interphase (e.g., use boron nitride to replace the carbon coating),[6,7] surface glass sealing,[8,9] and the addition of glassy phases in the matrix.[10,11] In SiC/SiC, glassforming, boron-based particulates can be added to the matrix, which react with oxygen to produce a sealant glass that inhibits oxidation of the carbon layer.[10,11] The glass-forming particulates in the matrix of the enhanced SiC/SiC composite will become glass and flow to seal the cracks at high temperatures. As a result, the glass phase prevents further oxidation and heals cracks. The SiC/SiC modified in this way was called the enhanced SiC/SiC composite.[10] In SiC-fiber-reinforced pure SiC matrix composites, since the creep resistance of SiC fibers (NICALON*) is lower than *NICALON is a trademark of Nippon Carbon Co. Ltd., Tokyo, Japan.
that of the pure SiC matrix, the matrix cracking is the dominant damage mechanism.[12–17] Creep of fibers transfers the stress onto the matrix and causes matrix cracks. The matrix cracking reloads the fibers. This is undesirable for creep and environmental resistance of the composites, since the increase in matrix stress by stress redistribution causes the development of periodic matrix cracking.[14–17] Flexural creep experi-
SHIJIE ZHU, Associate Professor, and YUTAKA KAGAWA, Professor, are with the Institute of Industrial S
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