22.08.2015   Comprehensive nuclear materials

Creep Behavior

CVI SiC/SiC and CVI SiC/Si-B-C composites exhibit primary creep only, even during long tests (Figures 11 and 12).54

Creep of CMCs involves local stress transfers depending on the respective creep rates of the fiber and the matrix. Such stress transfers may lead to fiber failures or matrix cracking and debonding, and sliding at the interfaces. When the matrix is elastic and creep-resistant, fiber creep induces stress trans­fers from the fibers onto the matrix, which may cause matrix cracking. This creep-induced matrix damage has been observed on CVI SiC/SiC composites.55-57

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Figure 11 Creep rate curves for a damage strain e0 = 0.8% and for various applied constant stresses for the SiC/Si-B-C composite, and under 450 MPa at 1200°C in argon for a Nicalon NL 202 fiber.

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Figure 12 Creep rate curves for the SiC/SiC composite under a constant stress of 150 MPa (e0 = 0.14% and e0 = 0.22%) at 1200°C.

In CVI SiC/SiC composites, the SiC matrix is far more creep-resistant than the SiC fibers, which creep at 1100 ° C.55,58,59

The creep behavior of CVI SiC/SiC composites with a multilayered matrix (SiC/Si-B-C) is caused by the creep of the Nicalon SiC fibers, whatever the extent of initial damage created upon loading (Figure 11). The Si-B-C matrix is less creep — resistant and stiffer than the SiC matrix.