A discussion of the chemical mixing process for in situ preparation of silicon carbide whiskers in silicon nitride powde
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Shiushichi Kimura Department ofInorganic Materials, Faculty of Engineering, Tokyo Institute of Technology, Yokohama 22 7, Japan
Eiichi Yasuda, Yasuhiro Tanabe, and Yuuichi Asami Research Laboratory ofEngineering Materials, Tokyo Institute of Technology, Yokohama227, Japan (Received 14 April 1987; accepted 13 January 1988) To produce a homogeneous composite of silicon carbide whiskers in a silicon nitride matrix, the whiskers were formed in situ. The resulting bulk density was higher than that obtained by a physical mixing of the components using a dispersion procedure. Bending strength at room temperature of the in situ formed composite was about the same as that for the physically mixed composite when the small amount of carbon remaining in the in situ formed composite was removed. I. INTRODUCTION During the last few years, several papers on whisker-reinforced ceramic composites1"" were published. Whiskers are used to obtain stronger and tougher ceramics for structural uses. Unlike plastics and metals, ceramic matrices do not show any liquid state at the mixing stage with reinforcing fibrous materials. Homogeneous mixing of solids having different sizes and shapes is very difficult. It is well known that homogeneous mixing is essential for preparing reliable composite materials. The preparation of silicon carbide whiskers [SiC(w) ] in situ in silicon nitride powder [Si3N4 (p)], followed by homogenizing and sintering, was studied and the results were compared with those of composites obtained by physical mixing. The former process can be called chemical mixing because of the in situ preparation of the whiskers. Flow sheets of both chemical and physical mixing processes are illustrated in Fig. 1.
space-forming agent for the growth of the whiskers because its boiling point is at the whiskerizing temperature. This provides whisker growth porosity in the Si3N4 at the time of reaction. In the physical mixing process, TOKAWHISKER (Tokai Carbon Company, Ltd.) was used. These SiC(w) are 0.2-0.5 /jm in diameter, 50-200 /um in length. The Si 3 N 4 (p) was the same as described above. The hydroxide sintering aids were Y(OH) 3 (Mitsubishi Chemical Industries Company, Ltd.), 5.87% aqueous solution, and La(OH) 3 (Nissan Rare Elements Chemical Company, Ltd.). The oxide sintering
Chemical
Si3N4(p),
Si3N,(pl,
SiO 2 , C e t c .
Sintering
Sintering aids
II. EXPERIMENTAL PROCEDURE A. Materials The a-Si3N4 (p) used was A grade, prepared by Toshiba Ceramics Company, Ltd. The Si3N4 content was 99%. The average diameter of the particles was 0.67 /nm and the silica content was 1.57%. The carbon black used to prepare the SiC(uj) was "Diablack-II," with a DBP value of 124 ml/g and an ash content of 0.28% (Mitsubishi Chemical Industries Company, Ltd.). Silica, the silicon source of SiC(iti), was a 30% solution dispersed in methyl alcohol (Nissan Chemical Industries Company, Ltd.). A surfactant, Nissan Polystar DSM-0545, was used for a smooth mixing. An aqueous solution (1 g/100 ml) of CoCl2 (analytical grade reagent) was added as a c
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