Binder System Containing Polycarbosilane and Polysiiazane for Injection-Molded Si 3 N 4 Parts
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BINDER SYSTEM CONTAINING POLYCARBOSILANE AND POLYSIIAZANE FOR INJECTION-MOLDED Si3 N 4 PARTS Sophia R. Su GTE Laboratories Incorporated 40 Sylvan Road Waltham, MA 02254, U.S.A. ABSTRACT Polycarbosilane and polysilazane, precursors of SiC and Si 3N 4 , have been incorporated into the binder system for injection-molded Si 3N 4 -based parts. Property requirements for these precursors as the injection-molding binder are briefly described. The binder removal mechanism is proposed and discussed based on the experimental results. Instead, the gas diffusion effect, rather than fluid wicking, is a key factor. Controlling the thermal decomposition rate of the binder is the key for achieving crack-free dense sintered parts. INTRODUCTION In recent years, injection molding has been a major technique for fabricating complex net-shape ceramic parts. This process provides economical, rapid production of high-quality precision parts from a wide selection of ceramic materials. Injection molding consists of softening the material in a heated cylinder and injecting it under pressure into a relatively cold mold, where it resolidifies to produce a part in the desired shape. 1 Therefore, an organic vehicle is needed to provide the ceramic powders with the fluid mobility necessary for molding. Traditionally the binder system for injection molding is composed of thermoplastic resin, thermosetting resin, lubricant, and plasticizer. However, the major problem in the injection-molding process for ceramic parts is the removal of the organic constituents prior to sintering. This binder removal procedure demands extremely slow heating rate, and requires long time cycles to avoid internal cracks. The mechanism involves fluid wicking, diffusion control, and permeation control as proposed by German. 2 Because of the complexity of the binder system, dewaxing, in practice, doesn't usually provide crack-free ceramic parts, especially when the cross-section of the samples is large. As a result, the production yield is low and the manufacturing cost ishigh. The ceramic precursors - polycarbosilanes, polysilanes, and polysilazane containing Si-C and Si-N bonds - shown a wide range of applications. For example, hexaphenylcyclotrisilazane and3 methyphenylpolysilane were used to infiltrate porous reaction-bonded sintered silicon nitride. Si 3 N4 and SiC fibers have been produced by melt-spinning from polycarbosilanes and polysilazanes. 4' 5 The use of polycarbosilanes as binding material in ceramic fabrication as well as in powder production have also been reported. 6 ,7 These polymeric precursors act as binder during shape forming, and then decompose to ceramic residues in a subsequent thermal treatment. The highly reactive ceramic residue surrounding the original ceramic particles can enhance densification and improve microstructure of the ceramic compact. In this paper, we present results of the study on the applicability of these precursors as the major binder component in injection-molded Si 3N 4 parts. Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materi
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