The role of analytical electron microscopy in investigating interfacial phenomena in continuous-fiber-reinforced ceramic composite interfaces

Stemming from the general concept that the fiber/matrix interface plays a critical role in the mechanical performance of continuous fiber reinforced ceramic matrix composites (CFCC's), the use of mechanically functional interphase coatings placed between the fiber and matrix phases has received considerable attention. Analytical electron microscopy (AEM) is particularly useful for investigating several interfacial phenomena that can govern the fracture behavior of these CFCC's. Because AEM techniques combine high spatial resolution of imaging and spectroscopy in a parallel manner, spatially resolved spectral data can be transparently linked to morphology. This study employs AEM techniques to form microstructure-property relationships for two different CFCC systems.; First, an “easy cleaving” oxide coating, potassium calcium niobate (KCN), is evaluated. KCN is a layered perovskite with very weak basal planes that can in principle be oriented to redirect cracks around fibers. Scanning electron microscopy (SEM) analysis of microcomposite samples indicates that the coating promotes debonding at the fiber/coating interface. However, transmission electron microscopy (TEM) analysis shows that the coating is chemically unstable with the sapphire fibers due to a sodium impurity. TEM analysis shows that the fiber has reacted with the coating to produce a highly textured beta-alumina around the circumference of the fiber that is responsible for the observed debonding behavior.; The second portion of this study covers calcium tungstate, a “Weakly bonded” oxide coating. The “weakly bonded” interface concept involves coating fibers with a material that produces an inherently weak fiber/coating interface, causing cracks to circumvent the fiber. Working on a similar coating material, other researchers have hypothesized a possible atomistic arrangement explaining how this weak interface may be formed. To assess the validity of this hypothesis, this study makes extensive use of highly resolved spectroscopy. TEM and scanning transmission electron microscopy (STEM) analyses show that the calcium tungstate is chemically and morphologically inert with alumina fibers. The use of electron energy loss spectroscopy (EELS) reveals aspects of local atomic coordination and electronic structure that is consistent with the formation of discrete and weakly bonded interfaces in the calcium tungstate CFCC material.