Surface modification of SiC powders by hydrolysed aluminium coating

SiC powders are surface modified to behave like alumina in aqueous suspensions by coating the powders with in situ generated hydrolyzed aluminium from dilute aqueous aluminum nitrate solutions in a pH range of 3–4.5 using hexa-methylene-tetramine as the base generator at ambient temperature. By examining the zeta potential and rheological properties of aqueous suspensions of the powder with different Al-coverage, it was observed that the coated powder begins to show alumina-like surface properties at an Al-coverage of 0.1 mg/m2, in contrast to 0.5 mg/m2 that was the minimum value reported earlier for observing the effect. This is explained by proposing a coating mechanism which proceeds through adsorption of a layer of cationic hydrolysed aluminum molecules, such as [Al(OH)(H2O)5]2+ and [Al2(OH)2(H2O)8]4+, during which the particles attain alumina-like surface charge properties. The modified powder retained alumina-like surface characteristics when stored under moist conditions and in acidic suspensions. The low value of Al-coverage and the resulting low increase (<10%) of powder surface area are advantageous for preparation of concentrated SiC and SiC–Al2O3 composite suspensions in acidic aqueous medium with rheological properties similar to that of their counterparts containing alumina only.