Sic-Sic Composite Interphase Evaluation via Small Scale Mechanical Testing

Abstract

Silicon carbide (SiC) has emerged as a highly capable engineering material for extreme environment applications. It has proven particularly useful for high temperature applications where it exhibits exceptional strength, relative chemical inertness, and low coefficient of thermal expansion. Recent studies have shown that SiC also has excellent neutronic properties including low thermal neutron absorption, low tritium permeability, and high radiation tolerance. As a result, SiC-SiC composites are being considered for application in accident tolerant fuel cladding and other structural components for fission and fusion reactors. The standard crystalline form of SiC is inherently brittle and therefore subject to low damage tolerance. In order to increase the material toughness, SiC-SiC fiber-matrix composites have been developed. These composites are divided into three constituents; a highly crystalline and near stoichiometric SiC fiber (commonly Hi-Nicalon{sup TM} Type-S or Tyranno{sup TM}-SA), a (layered) interphase of graphitized pyrolytic carbon (PyC) around the fiber, and a chemical vapor infiltrated (CVI) SiC matrix that consolidates the material. Increased toughness is achieved through two primary mechanisms: increasing the crack length by introducing more barriers, and fiber pullout via interphase shear within the matrix. As a result, it has become critically important to understand the fundamental properties of SiC-SiC composites. However, comprehensive material properties are difficult to predict unless we have an in depth knowledge of the individual constituents (fiber, matrix, interphase) and how their properties interact with one another at the micro-scale. Of specific interest to this work are the interfacial properties of the fiber-matrix interface. This includes the internal friction coefficient and interfacial debond shear stress. (authors)