Properties of textured silicon nitride ceramics produced by seeded tape-casting

Previous studies have shown preferred orientation in silicon nitride (Si3N4) imparts anisotropic properties to the material as a result of both anisotropy in the Si3N4 single crystal and the morphology of the Si3N4 grains. In this study, oriented Si3N4 materials were produced by the seeded tape-casting method wherein elongated beta-Si3N4 "seeds" were added to predominately alpha-Si3N4 ceramic slurries. The tapes were subsequently cut, stacked, laminated and hot-pressed with various stacking sequences. Through x-ray pole figure measurements and the calculation of the orientation distribution function (ODF), planar textures were measured when individual seeded layers were stacked randomly and an orthotropic texture was observed when the layers were aligned in the tape-casting direction. Maximum (002) multiples of a random distribution (MRD) for these two sample types were measured as 2.1 and 5.8, respectively. Comparing these results with those of unseeded samples indicates the beta-Si3N4 seeds are entirely responsible for the orthotropic texture recorded in the seeded tape-cast materials. Property anisotropy in the textured samples was found to be subtle. Indentation toughness measurements showed slightly higher values normal to the tape-casting plane (9.5 MPa·m1/2) compared to values in the tape-casting direction (8.0 MPa·m1/2) and the elastic constants normal to the tape-casting plane are generally lower than those in the tape-cast and cross-cast sample directions. The R-curve behavior of these samples showed only a slight increase in toughness with increasing crack length and no difference in toughness between sample types, all falling within the range of 4.5 to 5.8 MPa·m 1/2.