Fabrication of Silicon Nitride / Oxynitride by Reaction Bonding and Post Sintering
Reaction bonding and post sintering of silicon nitride /oxynitride was investigated as a route to fabricate a material with good oxidation resistance and good high temperature strength. Silicon powders, mixed with 0,2,4,8 and 16 wt% silica, were CIPed at 150 MPa and nitrided using the nitrogen demand principle. Four different nitriding gas compositions were used, consisting of different amounts of hydrogen, argon and helium mixed with nitrogen. Post sintering at atmospheric pressure and by HIP at 160 MPa were investigated to densify the materials. Samples were characterized by X-ray diffraction, SEM and Hg-porosimetry in both the nitrided and sintered state.
The α/β silicon nitride ratio after nitriding varied with gas composition and decreased with increasing silica content. During nitriding, some of the silica was transformed to silicon oxynitride, and the porosity of the nitrided samples was 25–35 %. Nitridation in the presence of hydrogen and helium, respectively, resulted in a larger amount of residual silicon. Fracture surfaces show a submicron grain size with grains in clusters. The porosity becomes coarser with increasing silica percentage, and the clusters become more obvious. This has been confmned by Hg-porosimetry measurements. Post sintering at atmospheric pressure at 1750 °C for two hours does not increase the bulk density, but increases the density locally in the clusters, increases the pore size and decreases the α/β silicon nitride ratio. HIPing at 1750 °C and 160 MPa for two hours resulted in complete transformation to β and a considerable increase in density.
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