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CONSOLIDATION OF Si3 N4/SiC WHISKER COMPOSITES BY COLLOIDAL FILTRATION

J.P. ERAUW, J. LUYTEN and J. SLEURS Materials Research Unit - Flemish Institute for Technological Research SCK/V1TO, Boeretang 200, B-2400 Mol (Belgium)

ABSTRACT Stable Si3N4/SiCw co-dispersions containing up to 20 wt.% of whiskers have been prepared. Homogeneous green bodies with relative green density above 55% and pore size in the range 40-50 nm have been obtained by pressure assisted colloidal filtration. By optimizing the temperature and/or the amount of sintering additives, densities above 90-95 % have been achieved by pressureless sintering of these consolidated compacts. INTRODUCTION The incorporation of SiC whiskers in a Si 3N4 matrix material can result in a general improvement of the mechanical properties [1]. However, obtaining dense, flaw-free composites with a homogeneous distribution of the reinforcing phase is not straightforward. Balling and clumping of the whiskers, on the one hand, leads to reduced packing efficiency and excessive porosity in the samples [2]; network forming at high whisker loading, on the other hand, drastically inhibits densification [3]. It has however been recognized that both problems can, to some extent, be overcome by a suspension processing route which favours the formation of green compacts with homogeneous microstructure, high relative density and small pore size [4,5]. The present research aims thus primarily at assessing the potential benefits of such a "colloidal approach". Preliminary results concerning the suspension behaviour of Si3 N4/SiCw co-dispersions with up to 20 wt.% of whiskers, and the characteristics of the related green and sintered products are reported.

EXPERIMENTAL PROCEDURE The silicon nitride powder (SN-E-10, UBE Industries) used for this study, has a narrow size distribution with a mean spherical equivalent diameter of about 0.4 pm. The BET specific surface area and the oxygen content are 12 m2 /g and 1.8 wt.% respectively. The 13silicon carbide whiskers (Silar SC-9, ACMC) have an average diameter of 0.6 p.m and lengths in the range 10 - 80 g~m. The oxygen content amounts 0.5 wt.% and the specific surface area, 3 m2 /g. The whiskers were used as received. A 71.4 wt.% Y20 3 - 28.6 wt.% A12 0 3 powder mixture was used as sintering aid. The electrokinetic mobility of both Si3N 4 and SiC raw materials was measured by microelectrophoresis (System 3000, Penkem) in a constant background electrolyte of-5 10-2 M NaCl, the pH being adjusted with HCl or NaOH addition. The dilute suspensions were aged before measurement. The zeta potential was calculated from the average mobility using the Helmholtz-Smoluchowski equation.

Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society

312

Aqueous slips with a solid content of 60 wt .% were prepared at pH 10-10.5 by ball milling the matrix powder and sintering additives for several hours in order to achieve a highly deagglomerated state. SiC whiskers were subsequently added and the suspension was further homogenized by an addit