Texture development in Si3N4/BN fibrous monolithic ceramics

Preferred orientation was measured in Si3N4/BN fibrous monolithic ceramics using x-ray diffraction. The materials were manufactured by co-extrusion of polymer binder/ceramic blends which were subsequently pyrolized and then hot-pressed to produced a fully dense ceramic composite. A very strong modified wire texture was present in the BN with the basal planes aligned parallel to the axis of extrusion due to shear-induced reorientation of the platelet-shaped BN particles during co-extrusion. Texture was also observed in the Si3N4 and was attributed to a combination of co-extrusion and hot-pressing. After hot pressing, the basal planes of the rod-shaped β-Si3N4 were observed to be preferentially aligned perpendicular to the extrusion direction. Measurements prior to hot-pressing revealed that a small amount (≈5%) of β-Si3N4 was present in the α-Si3N4 starting powder. Although texturing of the predominant α-Si3N4 did not occur during co-extrusion, significant texturing of the β-Si3N4 was observed. During subsequent hot-pressing, the pre-existing textured β-Si3N4 particles appeared to act as seeds for transformation and preferred growth of rod-shaped β grains parallel to the axis of extrusion.