Measurement of Residual Stress in Silicon Carbide Fibers of Tubular Composites Using Raman Spectroscopy

Silicon carbide (SiC) fiber-reinforced ceramic matrix (SiCf/SiCm) composites have been identified as potential materials for nuclear fuel cladding. These composites are fabricated by braiding the SiC fibers around a mandrel into a cylindrical tube and then densifying this preform with SiC matrix using chemical vapor infiltration (CVI). However, during this fabrication process residual stresses develop in the composite, either due to the mechanical braiding process or due to high temperature CVI deposition of the SiC matrix phase. Micro-Raman spectroscopy has been employed to measure the residual stress in the SiC fibers at various stages of the fabrication process. Samples of the composite were analyzed after mechanical braiding and at 33%, 66% and 100% CVI densification of the composites. Raman spectra obtained at the above stages revealed significant band shifts of the SiC peaks and these peak-shift corresponded to -716 MPa of residual stress following the braiding process and -1075 MPa after the densification process. Interestingly, the Raman spectra also revealed carbon bands whose origin was traced to the presence of nanoscale turbostratic graphite throughout the nanograined SiC microstructure. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.