Chemical and Morphological Analysis of Sol-derived Kca 2 Nb 3 O 10
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Chemical and morphological analysis of sol-derived KCa2 Nb3 O10 Steven T. Kim and Vinayak P. Dravid Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208
Sankar Sambasivan ACTG/Northwestern University, Evanston, Illinois 60201 (Received 22 May 1998; accepted 18 December 1998)
The chemical and morphological properties of a sol-derived layered perovskite compound, KCa2 Nb3 O10 (KCN), are presented. Development of this compound is motivated by its use as an interphase fiber-coating material for ceramic matrix composites (CMC’s). In such systems, this material is to be placed between the fiber and matrix to control crack propagation in the vicinity of the fiber, thereby enhancing toughness. Comparative analyses are performed between known bulk specimens of KCN and the sol-derived product using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The suitability of the sol-derived KCN for CMC applications is demonstrated through microstructure and chemical composition similar to that of the known bulk KCN samples. I. INTRODUCTION
Fiber reinforced ceramic matrix composites (CMC’s) rely on designed weakness in the immediate vicinity of the fiber/matrix interface to enhance toughness. This designed weakness can play a crucial role in delaying the onset of catastrophic failure by dissipating the energy of a propagating crack through a variety of mechanisms. These mechanisms primarily include extensive microcracking within the interface region and frictional energy dissipation through fiber pullout. In the case of nonoxide composite systems, the presence of weak and compliant pyrocarbon and/or boron nitride phases around the circumference of the reinforcement phase provides the requisite toughening mechanisms.1,2 However, such composites are not suitable for sustained service in high or intermediate temperature (.1200 ±C) oxidative environments where there is much interest in employing CMC’s. For this reason, there is significant investigation into the development of oxide-based CMC’s. Within the area of oxide-based CMC’s, one common design approach involves placing a thin oxide layer between the fiber and matrix. The role of this coating phase is to govern the fracture behavior of the composite in the immediate vicinity of the fiber. While the specific strategy involved with such coating interphases is varied,3–19 the overall design goals among approaches are similar. The primary function of the coating phase is to enhance toughness through the mechanisms outlined above. One coating approach involves the use of easy cleaving oxides.9–19 This idea is very similar to the use of pyrocarbon layers in SiC reinforced composites, albeit with an oxide-based system. In this design, a coating material with easy-cleaving crystallographic planes is employed. The functionality of this approach lies in producing coplanar orientation between the easily cleaved planes of the coating and the plane of the fiberJ. Mater. Re
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