Optimization and Characterization of Chemical Vapor Infiltrated SiC/SiC Composites

Abstract

SiC/SiC composites were fabricated by the forced-flow, thermal gradient chemical vapor infiltration (FCVI) method at the Oak Ridge National Laboratory (ORNL) and by the iso-thermal chemical vapor infiltration (ICVI) method at the Japanese National Institute for Materials Science (NIMS). The FCVI approach can fabricate relatively large composites in relatively short time, while the ICVI has significant controllability of fiber/matrix interphase formation. Fiber types included the near stoichiometric Tyranno SA and Hi-Nicalon Type-S. SiC/SiC composites 12.5 mm in thickness with either75 or 300 mm in diameter were fabricated at ORNL. SiC/SiC composites with 40 mm in diameter and 1.5∼3.0 mm in thickness were fabricated at NIMS. The microstructure of these materials was studied using SEM with EDS and TEM while their mechanical properties were evaluated by tensile, flexural and single fiber push-out testing. Density, the uniformity of fiber/matrix interphase and mechanical properties improved by increasing fiber volume fraction, optimizing processing conditions for both the FCVI and the ICVI processes. Porosity was decreased to approximately 15%. The effect of the interphase on mechanical properties and fracture behavior were studied. Tensile strength of 2D composites reinforced with Tyranno SA fibers and with optimized multilayer SiC/C interphase was approximately 300 MPa.