Water-based Si 3 N 4 suspensions: Part I. Effect of processing routes on the surface chemistry and particle interactions
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Water-based Si3N4 suspensions: Part I. Effect of processing routes on the surface chemistry and particle interactions C. Galassi and F. Bertoni IRTEC-CNR, Via Granarolo 64, I-48018 Faenza, Italy
S. Ardizzone and C.L. Bianchi Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, I-20133 Milan, Italy (Received 28 November 1997; accepted 3 May 1999)
Si3N4 powders manufactured by two different preparative routes were characterized for the solid–liquid interfacial reactivity and surface composition. Three mixing processes were tried to investigate the modifications of silicon nitride particle surface in aqueous suspensions. The surfaces of the starting powders and the dried mixed powders were investigated by x-ray photoelectron spectroscopy to determine the nature and ratios of surface groups. Electroacoustic measurements show that no change occurs in the isoelectric point for the mixed Si3N4 powders while the milling/mixing process has a great influence on the zeta potential magnitude and particle size distribution.
I. INTRODUCTION
Silicon nitride is one of the most investigated advanced ceramic materials as a result of its great potential for high-temperature, wear and structural applications such as in heat engines and turbine devices or in cutting tools.1–3 Sintered silicon nitride exhibits very advantageous properties: low density, high hardness and strength, good thermal shock resistance, and good corrosion and oxidation resistance. However the difficulties linked to the reliability and the cost of processing have prevented up to now the industrial widespread use of Si3N4-based ceramics. Colloidal processing is a way to partially overcome these problems4,5 by mixing the additives more intimately in the dispersed state and hence to improve sintering aid distribution that can reduce the additives concentration and allow for pressureless sintering. Furthermore the control of suspension dispersion and stability prevents the formation of aggregates and inhomogeneities in green bodies which influence the dense Si3N4 microstructure and therefore all the final properties.6 The mixing/milling process of the additives is an important step in colloidal processing7,8 as it modifies the surface structure of the powders, reduces the particle size, drives the homogeneous distribution of the sintering aids and influence the interparticle interactions. A previous study9 has investigated various mixing/milling processes to introduce La2O3 and Y2O3 as sintering aids in Si3N4 and their impact on the mechanical properties and sintering behavior. The results show that, with J. Mater. Res., Vol. 15, No. 1, Jan 2000
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constant additive amounts, the process history has a strong effect. From this study, ultrasonication, attrition and ball milling appeared the most promising processing steps. Based on these results, in this paper, two commercial Si3N4 powders, obtained from two different manufacturing m
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