Influence of Spray-Dry Slurry Flocculation on the Structure of Sintered Silicon Nitride

The influence of spray-dry slurry flocculation on the structure of sintered silicon nitride, especially on the pore structure and size distribution, was investigated as a function of slurry pH. The flocculation of spray-dry slurry considerably affected the character of spray-dried granules and also the microstructure of sintered bodies. Decreasing the slurry pH to a certain value (e.g., 7.9) caused slurry flocculation and reduced granule density as well as the diametral compression strength of granule. The reduction of granule strength contributed to the decrease in the large pore population in sintered bodies and also to the increase in its modulus of rupture (MOR). However, the population of pores with size less than 3μm increased an order of magnitude by decreasing the slurry pH from 10.8 to 7.9. The microscopic observation of spray-dried granule showed flocculated clusters consisting of several particles in a low pH slurry system. The void space in/between the clusters was not removed during forming and sintering and remained in the sintered body possibly because of the hard agglomeration of particles. Numerous remaining micropores in sintered body appeared to be the origin of relatively low strength of sintered bodies (i.e., 1000MPa) which contained no clear fracture origin in the flocculated slurry system.