CONTROLLED CRYSTALLIZATION IN SELF-REINFORCED SILICON-NITRIDE WITH Y2O3, SRO, AND CAO - CRYSTALLIZATION BEHAVIOR

The controlled crystallization of the amorphous grain boundary phase h as been examined in a series of self-reinforced Si3N4 materials with a dded Y2O3, SrO, and CaO. The effects of time, temperature, atmosphere, glass content, glass chemistry, and matrix Si3N4 on the crystallizati on have been investigated, The stability of the crystallized product, the crystallization kinetics (T-T-T curve), and crystallization mechan isms have also been examined, Crystallization produced an oxynitroapat ite containing Y, Sr, and Ca over a broad range of heat-treatment cond itions and glass compositions, The oxynitroapatite was compatible with Si3N4 and remained stable up to 1600 degrees C, At low temperatures ( <1350 degrees C), the rate-limiting crystallization mechanism was oxyg en diffusion in the glass, and at higher temperatures (>1350 degrees C ) the rate-limiting crystallization step changed to either the formati on of new Si3N4 grains or solute diffusion in the glass.