Characterization of the Tyranno SA4 third generation SiC fiber surface and comparison with Tyranno SA3 and HNS fibers

While presenting similar properties, the Hi-Nicalon Type S (HNS) and Tyranno SA3 (TSA3) SiC fibers exhibit different mechanical behaviors when used as reinforcement in SiC/SiC composites. Indeed, the HNS-reinforced composites exhibit a pseudoductile mechanical behavior whereas the TSA3-based composites show low ductility. Even though the differences in their grain size and surface roughness could explain a part of this phenomenon, the chemical composition and microstructure of the fibers outermost surface play a key role. The recent availability of the new Tyranno SA4 (TSA4) SiC fiber allowed the processing of composites showing the expected pseudoductile mechanical behavior in ceramic matrix composites, even without an interphase. Therefore, this result shows that the TSA4 surface should be different from its predecessors. In order to characterize the surface, X-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), and transmission electron microscopy (TEM) were performed on the HNS, TSA3, and TSA4 fibers. The presence of an organized boron nitride layer of dozens of nanometers in thickness on the TSA4 fiber surface was evidenced. This layer already acts as an interphase material, guaranteeing cracks deflection, and is responsible for the pseudoductile behavior of composites made of this new fiber, reducing the interfacial shear stress at the fiber/matrix interface.