RHEOLOGICAL BEHAVIOR OF INJECTION-MOLDABLE SILICON POWDER-SILICON CARBIDE WHISKER FORMULATIONS

The rheological behavior of injection-moldable formulations for reacti on-bonded Si3N4 toughened with silicon carbide whiskers was studied us ing capillary rheometry. The effects on rheology of the following para meters were examined: solids loading, powder/whisker volume ratio, par ticle size and type, and binder composition. Two important aspects of the flow behavior were delineated. First, corrections for end effects and slippage along the wall were made in order to interpret the experi mental data properly. At high shearing rates (i.e., 10(3) s-1) slip ma y account for more than 50% of the total flow. Such slippage promotes flow into the smallest channels or corners of the mold and may appreci ably facilitate molding. Consequently the careful study of slippage is an inherent requirement of the rheological characterization of these concentrated suspensions. Second, the suspension viscosities were deli neated. An empirical equation for predicting relative viscosity was de veloped for formulations containing up to approximately 30 vol% of sil icon carbide whiskers (with more than 50 vol% total solids). Suspensio n viscosities generally increased with decreasing particle size and in creasing whisker contents. Particle surface roughness appears to affec t the shearing behavior. Binders of low molecular weight resulted in h igher relative viscosities than higher molecular weight binders, indic ating possibly better dispersion of solids when more viscous binders a re employed.