Rheology of aqueous boehmite-coated silicon nitride suspensions and gels
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The rheological properties of boehmite-coated silicon nitride aqueous suspensions and gels are reported. In unidirectional rheological tests, it was found that the boehmite coating reduces the viscosity of the suspensions over a wide range of shear rates and volume fractions of particles. The suspension shear stress as a function of shear rate can be described by the Bingham model, and the Bingham yield stresses of boehmite-coated silicon nitride suspensions are lower than those of the uncoated suspensions. The reduction in the viscosity and the Bingham yield stress is attributed to a shallower secondary minimum in the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential between coated particles than that for uncoated silicon nitride particles. Moreover, at low values of pH, the coated silicon nitride suspensions gelled over time, and the viscoelastic behavior of the gels was studied by dynamic oscillatory tests. It was found that the shear modulus (G') and loss modulus (G") remain constant up to a certain strain amplitude, y°, beyond which G' and G" begin to vary. The value of G' in the linear region increases exponentially, whereas y° decreases exponentially with the volume fraction of coated silicon nitride particles. The exponential behavior of the shear modulus G' of the gels is similar to the exponential pressure-density relationship found in the previous pressure filtration study, indicating that particulate rearrangement occurs as volume fraction of particles is increased.
I. INTRODUCTION Silicon nitride-based ceramics have a wide variety of applications such as cutting tools, bearing balls, turbocharger rotor, gas turbine rotors, etc., depending on the application temperature.1 The superior properties of silicon nitride result from the strong covalent bonding between Si and N atoms. However, the strong covalent bonding also makes the sintering and forming of silicon nitride difficult. Diffusion of atoms is limited and the deformation under external influence is restricted. In addition, due to the complex surface chemistry of silicon nitride particles, the solids loading of silicon nitride in water is relatively low compared to that of oxide ceramics such as alumina. To overcome these processing difficulties, several research groups have used a coating approach to introduce processing aids more uniformly and at the same time modify the surface chemistry of silicon nitride. Mah et al.,2 Shaw and Pethica,3 and Garg and De Jonghe,4 coated sintering aids on silicon nitride particles using sol-gel precursors. Malghan et al.5 and Liden et al.6 coated sintering aids on silicon nitride particles using colloidal particles. Although the distribution of sintering aids has been studied as indicated above, the improvement on the consolidation and forming has not been addressed. In a recent study,7 we coated boehmite (A1OOH) on silicon 2808
J. Mater. Res., Vol. 10, No. 11, Nov 1995
http://journals.cambridge.org
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nitride using a sol-gel coating approach and showed that the consolidation and formin
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