PROCESSING OF SILICON NITRIDE POWDERS

Center for Ceramics Research, Rutgers, The State University Piscataway, NJ 08855-0909, USA 

Conventional powders (in this author s terminology) have the following general characteristics  

1) a large average size, 1-10 microns, with some particles in the 20-100 micron size range, 

2) a very wide distribution of particle sizes, with Rmay/Rave 5, 

3) a high degree of agglomeration, both weak and strong inter-particle bonds, 

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Properties of Silicon Nitride Powders In Aqueous Environment" Accepted for publication in Powder Processing III, Ed% G. L. Messing, ACerS, February 1990,... 

All the processes produce silicon nitride powder. Most of the commercially available powder is produced by direct nitridation. This reaction is carried out in the temperature range of 1250°C-1400°C. The reaction is shown in Equation 12.15. 

Considerable recent activity in the area of ceramic processing is aimed toward the formulation of materials with high strengths, comparable to the room temperature strength of metal alloys, at high temperatures (of the order of 2000 K). The impetus comes from the significant gains made in the last 20 years with materials formed from submicron powders of silicon nitride and silicon carbide and the promise of similar improvements in the near future. 

Nakamura M, Kuranari Y, Imamura Y (1987) Characterisation and synthetic process of Si3N4 material powders. In Somiya S, Mitomo M, Yoshimura M (eds) Silicon Nitride I. Elsevier, London, p 40... 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

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