Polyborosilazane-Derived Ceramic Fibers in the Si-B-C-N Quaternary System for High-Temperature Applications

Abstract

Amorphous Si-B-C-N ceramic fibers prepared at 1000°C from a melt-processable boronmodified polysilazane [B(C2H4SiCH3NCH3)3]n were annealed in the temperature range 1000-1800°C in a nitrogen atmosphere to identify the changes in the thermal and structural stability as well as in the related fiber strength. Fibers were shown to be extremely stable up to 1600°C without decomposition and measurable changes in their amorphous structure. At higher temperatures, X-ray diffraction and thermogravimetry analyses indicated that the structural and thermal properties of fibers were probably controlled by the carbothermal decomposition of a minor part of the silicon nitride phase providing SiaNVSiC nanograins in the material at high temperature. The excellent strength retention after heat-treatment at 1600°C (1.3 GPa) is clearly related to the high structural and thermal stability of fibers. Between 1600°C and 1700°C, the fiber strength decreased to 0.9 GPa then dropped to about one-quarter the original value at 1800°C while structural changes were evident. With an excellent stability in air at 1300°C, these Si-B-C-N fibers are potential candidates for Continuous Fiber-reinforced Ceramic-matrix Composites (CFCCs). INTRODUCTION There is long interest and demand of aerospace industry for ceramic matrix composites because of their superior properties compared to monolithic ceramic materials. In a general way, such lightweight structural materials are made from a variety of continuous fibers and matrix combinations in which fibers, embedded in the ceramic matrix, act as the reinforcing phase in Continuous-reinforced Ceramic matrix Composites (CFCCs). Performances of CFCCs for hightemperature aerospace and energy-related areas depend upon judicious selection of fibers with the proper chemical and physical properties of the ceramic which is employed. Non-oxide ceramic fibers are particularly attractive for their light weight, thermal shock resistance, creep resistance and relatively high tensile strength and modulus values making them useful for aerospace applications. Among them, those fibers prepared from organometallic/inorganic polymers are of particular interest because of their small diameter and flexibility to allow weaving To the extent authorized under the laws of the United States of America, all copyright interests in this publication are the property of The American Ceramic Society. Any duplication, reproduction, or republication of this publication or any part thereof, without the express written consent of The American Ceramic Society or fee paid to the Copyright Clearance Center, is prohibited.