Influence of process conditions on the surface oxidation of silicon nitride green compacts

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The surface chemistry of silicon nitride is strongly influenced by the processing environment. Mixing and forming steps can modify the oxidation state at the surface of the particle. The aim of the present investigation is to determine the possible changes and interactions on the surface of the particle induced during processing by different techniques of formation, such as isostatic pressing and slip casting. The effect of mixing conditions on surface oxidation have also been analysed on the green cast and pressed bodies prepared by dry and wet mixing using both water and alcohol as the mixing vehicles. X-ray photoelectron spectroscopy (XPS) has been used to characterize surface oxidation of the silicon nitride taking into account the Si2p, N1s and O1s binding energies, as well as the corresponding photoelectron line intensities. All processing treatments result in some surface oxidation. The Si2p peaks have two components in all studied conditions (due to nitride and oxynitride species) whereas N1s peaks are doublets for slip-cast samples and singlets in the pressed samples, thus suggesting a displacement of oxidized species by silica. In the case of slip-cast samples, the dispersant tetramethylammonium hydroxide (TMAH) seems to form a protective screen on the particles preventing further oxidation.

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