Effect of Forming Techniques on the Orientation of Platelet Composites
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EFFECT OF FORMING TECHNIQUES ON THE ORIENTATION OF PLATELET COMPOSITES
YEONG-SHYUNG CHOU AND DAVID J. GREEN Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802
ABSTRACT Two forming techniques, i.e., slip casting and dry compaction, have been employed to make SiC(platelet)-A120 3 composites. The orientation of SiC platelets in the composites, before and after hot pressing, was studied using optical microscopy and a pole-figure X-ray device. It was found that the preferred orientation in the dry-pressed green bodies increased with increasing pressure, whereas the platelets in the slip cast bodies were close to a random orientation. The pressure during the hot pressing process did, however, align the platelets of slip-cast composites during densification. Optical micrographs confirmed the platelets tend to align with their faces perpendicular to the hot pressing direction. The effect of platelet size on the orientation was also investigated. X-ray analysis showed that small platelets (- 1.6 g±m) do not give rise to preferred orientation both in green and hot pressed bodies; whereas larger platelets (-24 gam) with larger aspect ratio do yield a stronger preferred orientation. The causes for the platelet alignment will be discussed. INTRODUCTION In the past few decades, ceramic matrix composites reinforced with whiskers have been extensively studied due to their promising improvement in mechanical properties, especially fracture toughness. For example, the fracture toughness, Ktc, of alumina can be doubled or tripled to about 9.0 MPa-m 1/2 when reinforced with 20 v% of SiC whiskers [1-2]. Mullite composites reinforced with Si 3 N4 whiskers have also shown a two fold increase in fracture toughness [3]. However, there are certain problems associated with these whiskers. For example, most of the whiskers possess sizes in the micron or sub-micron range. Consequently, they represent a health hazard when inhaled. In addition, ceramic materials reinforced with whiskers often experience difficulties during sintering as a result of constrained densification [45]. The long and needle shape of SiC whiskers can easily form a continuous network in the matrix due to their low percolation limit and they are difficult to disperse, as they tend to entangle. Hence, the densification of whisker reinforced ceramic composites requires not only applied pressure but also a much higher sintering temperature than the matrix alone. For the case of SiC/A120 3 composites, sintering temperature as high as 1850'C have been reported, compared to - 16001C for alumina matrix [1]. One possible solution to the above problems is the replacement of the whiskers with single crystal disc-shaped particles, such as SiC platelets, with the assumption that those toughening mechanisms operating in whisker composites, such as crack deflection and grain bridging, could also occur in platelet composites. Furthermore, it may be feasible that platelet composites with preferred orientation, i.e., with the face
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