Stabilisation, rheology and application of aqueous suspensions of oxidised vs. non-oxidised SiC powder

The study aimed at evaluating the stabilisation and rheological properties of aqueous suspensions (30 vol%) of pristine (non-oxidised) and oxidised fine SiC powders which were stabilised either with NaOH or TMAH (tet-ramethylammonium hydroxide). The extensive oxidation of the SiC powder surface was confirmed by the results of XPS and FT-IR analyses. The optimal amount of a given dispersant depended, according to the results of zeta potential measurements, on a pH range guaranteeing formation of a stable suspension. This effect allowed the authors to propose a mechanism of stabilisation of the SiC suspensions. Herein, the maximum amount of silanol groups is formed on the SiC particles surface in the pH range of 9.5-11 which then adsorb Na+ or TMA+ cations, thus creating a favourable system of electrostatic interactions. In turn, it increases the electrostatic repulsion between SiC particles and improves the stabilisation of the suspensions and their rheological characteristics. An increase in the surface oxidation of the SiC particles leads to reducing the repulsive forces between the powder particles. Apparent viscosity of the suspensions made of SiC UF10 powder, regardless of the type and amount of the fluidizer, is very low (5-10 mPa s) and shows a slight increase as the shear rate increases. The oxidation of the SiC powder changes the rheological characteristics of the suspensions, which may be caused by their rapid sedimentation. Stability of the prepared SiC suspensions was also assessed by casting them into a plaster mould and then by sintering and characterisation of the resulting samples.