Raman study of Hi-Nicalon-fiber-reinforced celsian composites: II, residual stress in fibers

Band shifts in Raman spectra were used to assess, on a microscopic scale, the residual strain existing in Hi-Nicalon silicon carbide fibers reinforcing celsian-matrix composites, Uncoated as well as p-BN/SiC-coated and p-B(Si)N/SiC-coated Hi-Nicalon fibers were used as the reinforcements. We unambiguously conclude that the fibers were in a state of compressive residual stress. Quantitative determination of the residual stress was made possible by taking into account the heating induced by laser probing and by using a reference line, of fixed wavenumber, We found fiber compressive residual stress values between 110 and 960 MPa, depending on the fiber/ matrix coating in the composite, A stress relaxation-like phenomenon was observed at the surface of p-BN/SiC-coated Hi-Nicalon fibers whereas the uncoated or p-B(Si)N/SiC-coated Hi-Nicalon fibers did not show any stress relaxation in the celsian-matrix composites.