Interactions Between Crystalline Si 3 N 4 and Preceramic Polymers at High Temperature
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ABSTRACT Polymeric precursors to Si3 N4 were mixed with Si3 N4 powder, in various combinations, to form coatings in Si 3N 4 and to control microstructure of Si 3N4 . The microstructure of polymerderived ceramic and preceramic materials was examined by transmission electron microscopy (TEM) after heat treatment at 900'C and 1200'C and sintering at 1650'C. The observed microstructure suggests that the polymer-derived-material properties will approach those of conventionally formed Si 3 N4. With proper viscosity control, gaps between a-Si 3 N 4 powder particles as narrow as 5 to 10 nm are filled by the polymer-derived-material, ensuring full wetting of the a-Si3N4. Voids as large as 0.3 JLrm between a-Si3N4 particles are filled by the ceramic precursor, reducing the size of pores formed during subsequent sintering processes. Intimate bonding between the amorphous, polymer-derived-material and the crystalline Si 3 N4 grains is observed after heat treatments or sintering, a necessary condition for achieving high-quality properties similar to conventional material. Acicular grains of f3-Si3N4 formed from the equiaxed a-Si 3N4 powder/PCMS mixture upon sintering are also observed. INTRODUCTION Preceramic polymers consist of inorganic skeletons substituted (in most cases) with organic pendant groups. They can be fabricated similarly to organic polymers and converted to a ceramic on heat treatment. Heat treatment initially crosslinks the polymer to prevent melting during the second heating stage, pyrolysis, and to increase ceramic yields. The polymer's organic substituents are next removed as volatile compounds by pyrolysis at 900'C, and the polymer
skeleton is converted into an amorphous, three-dimensional ceramic network. The pyrolysis time, temperature, and atmosphere affect the final conversion yield, composition, and integrity of the product and can differ for various applications. The amorphous ceramic material can be crystallized by further heat treatment. The crystallization temperature is a function of the ceramic type, composition, impurities, and incorporated additives. The feasibility of using preceramic polymers as binders and additives for controlling microstructure is illustrated by the variation of grain structure with polymer concentration and temperature as shown in Figure 1.1 Other applications under investigation include joining and strengthening ceramics by a coating approach. Understanding the independent variables that control microstructural development is necessary if commercially successful preceramic polymers are to be developed. The effort reported in this paper centered on characterizing the microstructure and crystallinity of mixtures of polycyclomethylsilazane (PCMS, [CH 3SiHNH]x) and Ube SNE-10 a-Si3N4 powder. Special attention was paid to how well PCMS wets the ax-Si3N4 particles. Secondary efforts centered on the interface between Si 3 N4 substrates and polymer/powder mixture and Si 3N4 grain growth and glassy phase formation upon sintering. These efforts will be extended in future studies
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