Silicon carbide fiber-reinforced alumina extrusion
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(Received 10 November 1994; accepted 20 June 1995)
Silicon carbide fiber-reinforced alumina bodies have been produced by ram extrusion. The A1203 powder and S i c fiber were milled together to give a dry dispersion of up to 30 vol % fiber which was subsequently mixed to a paste by high shear kneading using hydroxypropylmethylcellulose solutions as the binder phase. Extruded bodies with green densities ranging between 56 and 63% full theoretical density were achieved. The paste flow behavior was characterized using physically based equations which show that for any given moisture content the pressure drop and the constituent paste parameters are all systematically reduced as the fiber loading is increased. This observation can be explained almost completely by combining packing theory with the paste rheology data. Fiber interactions within the paste and the die system appear not to greatly influence the rheological character of the material. It is shown statistically that the fibers are homogeneously dispersed throughout the paste mass after extrusion. Image analysis has been used to aid in macrodefect analysis, and it is shown that the optimum concentration of fiber was 20 vol90 with a quantity of binder sufficient to give an initial yield stress of 2 MPa.
1. INTRODUCTION
Recent work has shown that the strength and fracture toughness of alumina can be enhanced by the addition of silicon carbide in various forms. These materials have been considered candidates for high temperature applications and include whiskers, platelets, fibers, and nanoparticulate varieties.’,2 Good results have been achieved with the addition of whisker^,^ but chopped fibers have also been successful. Components fabricated in these composite materials are normally pressed or cast and then densified using either hot pressing or hot isostatic pressing methods. Alternative fabrication routes include plasticforming methods, such as injection molding and extrusion. Extrusion has been used for many years in the manufacture of simple monolithic shapes, and in recent years has found favor with some workers as a method of producing green bodies of superior quality.435Fiber loaded plastics are commonly extruded and molded. The possibilities of extruding ceramic matrix composite materials may be considered in this light. The rheological character of “plastically” deformable pastes can now be more fully evaluated using physically based equations6; it has more recently been shown that these equations are applicable to composite ceramic materials containing short chopped In this paper the extrusion of A1203/SiC chopped fiber composites is evaluated in terms of flow characa)Also a member of the School of Chemical Engineering J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
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teristics, fiber and defect distribution, the green body density, and surface characteristics. II. EXPERIMENTAL PROCEDURE
A calcined low soda (0.05% Na20) alumina of median particle size of 0.5 p m (Baco RA107LS, BA Chemicals Ltd., U.K.
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