Silicon Carbide and Silicon Nitride Ceramics Based on Colloidal Processing and Sol-Gel Techniques
The work in this dissertation deals with the development of silicon carbide and silicon nitride-based materials using colloidal processing and sol-gel techniques. Special attention has been given to the optimization of the homogeneity in ceramic multicomponent systems. Most ceramic materials are multicomponent systems, and important questions are how to disperse powders with different surface characteristics and how to add the secondary component/components in the most optimal way.
Powders with different surface properties have been mixed and stabilized by anionic polyelectrolytes (lignosulphonates or polyacrylic acids) or by the addition of highly charged nanometre-sized particles. It was found that the highly charged lignosulphonate molecules and inorganic sol particles gave a stabilizing effect without being adsorbed to the powder surfaces. This type of stabilizing mechanism, which at present is not well- characterized, is particularly useful when dispersing multicomponent systems with different surface properties. In non-aqueous solvent, a steric stabilizer with both acidic and basic head groups was used to stabilize the different powders. Ceramic materials were processed from the stabilized slips by either slip casting, pressure slip casting or freeze granulation/pressing. In pressure slip casting of Si3N4, partially flocculated slips were also used.
A surface-modification technique was developed to obtain controlled surface properties of the ceramic powders. By reacting the particle surfaces of Si3N4 or SiC with aluminium alkoxide, the particles obtained alumina-like surfaces and could be treated as if they were Al2O3 powders.
An ordered mixing of multicomponent systems was obtained by various particle coating techniques. Sintering additives were homogeneously distributed by either coating SiC powders with polyphenylene (carbon source) or by adding nanometre-sized sol particles (Y2O3 and Al2O3), which electrostatically adsorbed on Si3N4 and SiC particle surfaces. Stabilized SiC slips with Y2O3 and Al2O3 sol particles (50 and 10 nm, respectively) were used for freeze granulation and pressing. A technique for quantifying the homogeneity of ceramic materials was developed, and it was clearly shown that the sol additions enhanced the homogeneity of the microstructure as compared with the use of conventional powder mixing techniques.