Effect of Si3N4 starting powders on alfa-SiAlON ceramics microstructure

Master of Science Thesis EFFECT OF Si3N4 STARTING POWDERS ON cc-SiAlON CERAMICS GRAIN MORPHOLOGY ŞENİZ REYHAN KUŞHAN Anadolu University Graduate School of Natural and Applied Sciences Ceramics Engineering Program Supervisor: Assoc.Prof.Dr. Hasan MANDAL 2000 Silicon nitride and SiAlON has advantages of good mechanical, thermal and chemical properties for engineering applications. Although (3 is the first developed group among a and P-SİA10N ceramics, cc-SiA10N ceramics have attracted more attention because they show excellent hardness and offer the possibility of reducing the residual glassy grain boundary phase by the incorporation of sintering aids into the crystal structure. But according to their equiaxed microstructure they have relatively low toughness. There has been successful experimentation to develop elongated form of cc-SiA10N to improve the fracture toughness. But in these studies either an oxygen and aluminium rich composition which lowers the high temperature mechanical properties according to the residual glassy grain boundary phase or an expensive and difficult sintering technique, HIP, is required. The difference of this study from the previous ones is the nitrogen rich starting composition and providing desification by gas pressure sintering which is a cheap and easy way of sintering. Microstructural development according to both Sİ3N4 starting powder composition and sintering conditions are observed. It was revealed that P-Sİ3N4 grains act a seeding role for elongated microstructure. Beside this, two-stage-sintering instead of one stage also provided the formation of elongated grains by means of allowing nucleation time. Thus, a hard and tough a-SiA10N is produced which has a potential application as ball bearing. Keywords: ct-SiA10N, Sİ3N4 Starting Powder, Sintering Conditions, Grain Morphology