I. Tsao et al., RHEOLOGICAL BEHAVIOR OF INJECTION-MOLDABLE SILICON POWDER-SILICON CARBIDE WHISKER FORMULATIONS, Journal of the American Ceramic Society, 76(12), 1993, pp. 2977-2984
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.