Numerical and experimental study of internal flow field for a carbon fiber tow pneumatic spreader

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INTRODUCTION

OVER the years, carbon fibers have been considered as one of the most important reinforcements for aluminum and its alloy to fabricate advanced composite materials. The carbon fiber (CF) reinforcement/aluminum (Al) matrix composites are of great interest because of their high specific strength and stiffness, low coefficient of thermal expansion, and high thermal/electric conductivity. Therefore, CF/Al composites have the most potential application as structural and functional materials in future. The primary concern in achieving the potential has been the difficulties experienced in combining Al with continuous fiber tows and the chemical reaction at the interface between CF and Al. The interface plays a most vital role in the overall performance of the composite materials. Improper wetting and chemical reaction occuring at the interface during synthesis or under service conditions can degrade the mechanical properties of the composites.[1–11] Certain coatings can promote the wettability between CF filaments and Al as well as prevent the molten aluminum coming into direct contact with carbon; thus, the chemical reaction can be eliminated. Many methods have been proposed for the preparation of metallic or nonmetallic coated carbon fiber tow.[9,12–15] However, the observed variation in the coating is mainly attributed to nonuniform activation on the surface of the fibers prior to deposition. The nonuniform activation is caused by the compact carbon filaments. Abraham et.al.,[12] Bobka and Lowell,[16] and Clark et al.[17] reported that the oxidation treatment led to a nonuniform etching of the fibers, while carbon fiber tows were treated by oxygen to modify the J.C. CHEN, Postdoctoral Candidate, and C.G. CHAO, Professor, are with the Institute of Materials Science and Engineering, National ChiaoTung University, His-Chu, Taiwan 300, Republic of China. Manuscript submitted April 3, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B

surface of the filaments. Therefore, the outer filaments are strongly attacked, while those in the interior of the bundle are hardly attacked. Some variations in the coating techniques of carbon fiber tows have been developed. Ceramic coatings (SiC, TiC, TiB2) or functional gradient coatings (C/SiC/Si) were deposited by chemical vapor deposition (CVD) on carbon filaments for CF/Al composites; however, similar results—that fiber and composite strength decrease with coating thickness and the variations of coating thickness in carbon fiber tow are obvious.—were found[2,7,9] The main problem still exists, which comes from the different treatment between the outer and the interior filaments due to the carbon fiber tows containing thousands of filaments, in spite of the deposition technologies and the deposited materials developed. Therefore, if the carbon fibers are separated uniformly, it is advantageous for the improvement of mechanical properties.[18] Processes and an apparatus were developed for pneumatically spreading graphite or other carbon filaments from a tow bundle to form a