• PDF / 694,340 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 97 Downloads / 287 Views

DOWNLOAD

REPORT

---

T3.32.1

A Surface Coating for Silicon Carbide Whiskers Heng. Zhang1, T.D. Xiao1, D.E. Reisner1, and P. Santiago2 1. Inframat Corporation, 74 Batterson Park Road, Farmington, CT 06032, U.S.A. 2. Central Microscopy Research Facilities, Physics Institute, UNAM A.P. 20-364, C.P. 01000 04510, Mexico D.F., Mexico ABSTRACT A mullite coating process has been conducted to form an electrically insulating layer on the surface of the silicon carbide whiskers for whisker reinforcement application by a chemical deposition and hydrolysis. The phase, structure, morphology and electrically insulating property of the coated silicon carbide whiskers have been investigated by using x-ray diffraction, scanning electron microscopy, transmission electron microscopy and electrical measurement. The study indicates that a crystalline mullite coating layer is formed after heat-treating the precursor coating at elevated temperature. The formed mullite coating significantly increases the electrical resistance of the silicon carbide whiskers. INTRODUCTION Silicon carbide in the form of whisker (SiC-w) has excellent mechanical properties, and is one of the best components for whisker toughening reinforcement for ceramic matrix composites [1]. However, as a semiconductor, it will dramatically reduce the electrical resistivity of an insulating composite system, which has critical requirement for high electrical resistivity. Hence, silicon carbide whiskers have to be coated with an insulating layer to increase its electrical resistance to fulfill the combined functions of superior mechanical and electrical insulating properties. Mullite, with a formula of 3Al2O3.2SiO2, possessing good electrical resistance and interface characteristics [1], is a good candidate for such a purpose. Recently, it was reported that mullite coated Hi-Nicalon SiC fibers exhibit higher tensile strength than uncoated SiC fibers [2] at temperatures of 1200oC. A few methods have been used to elaborate thin films on fibers and whiskers, including chemical vapor deposition (CVD), sputtering and the sol-gel process. Among the numerous advantages of the sol-gel process compared to conventional methods [3], the low temperature and the possibility of controlling the porosity by adding various templates were of great interest. In this work, a sol-gel process combining a hydrolysis has been selected for producing mullite coated silicon carbide whisker. The process, morphology and electrical insulating properties will be reported in this presentation. EXPERIMENTAL DETAILS SiC whiskers with an average diameter. of 1-2 µm and a length of 15-20 µm, purchased from Alfa Aesar, were used as the substrate of mullite deposition. In order to provide the SiC whiskers with a suitable surface functionality for subsequent mullite coating treatment, the SiC whiskers were first heated at 600 oC for 0.5 h to form a thin SiO2 layer on the surface of SiC

T3.32.2

whiskers. A sol-gel dipping technique, combined hydrolysis, has been employed to fabricate mullite coating on the SiC-w surfaces. The start