Abstract
Ceramic texturing can be achieved during colloidal processing of ceramics by aligning the suspended particles in a strong magnetic field and retaining this alignment in the green body. Use needs to be made of anisotropy in the magnetic dia-or paramagnetic susceptibility in the compound of interest. The alignment is most effective for the larger particles. As the microstructure coarsens during sintering the aligned larger particles will gradually consume the smaller ones and thus the texture in the material is enhanced by this grain growth process. We have made use of slip casting and electrophoretic deposition as colloidal shaping methods. We will discuss a number of important process parameters in these methods and illustrate the principles by examples taken from our work in this area on alumina, lanthanum titanate and yttria stabilized zirconia.
According to the theoretical model, the energy of the crystal anisotropy increases with increasing particle size, which was confirmed by studies on slip casting of alumina suspensions which revealed that the orientation in the green deposit increased with increasing starting powder size. After sintering however, anisotropic grain growth accompanied with excellent texture formation was observed for the ceramic fabricated from the finest starting powder grade, whereas the modest orientation factor of the largest starting powder based ceramic hardly changed after sintering.
Lanthanum titanate (La2Ti2O7) ceramics were crystallographically aligned in a 9.4 Tesla strong magnetic field. The magnetic alignment mechanism was analyzed according to monoclinic crystal symmetry. The ( ) plane of La2Ti2O7 was aligned perpendicularly to the imposed magnetic field direction. The three principal magnetic axes were determined relative to the crystallographic axes: one of the magnetic axes has an angle of 74.94° to the a axis, the other has an angle of 66.30º relative to the c axis and the third magnetic axis is along the b axis. Moreover, a uniform and isotropic microstructure is observed in the sintered La2Ti2O7 with a substantial crystallographic alignment.
A reactive synthesis technique was used to obtain (001)-oriented 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP). Undoped pure monoclinic zirconia having magnetic anisotropy was first aligned in a strong magnetic field and co-sintered in a randomly distributed cubic 8Y-ZrO2 fine matrix powder. The reactive sintering resulted in a 3Y-TZP ceramic with a (001) orientation. The (001)-oriented 3Y-TZP showed a substantial toughness anisotropy, i.e. the toughness along the [001] direction is 54% higher than that of its perpendicular direction. Moreover, the toughness along the [001] direction is 49% higher than that of a non-textured isotropic reactively synthesized 3Y-TZP and 110% higher than that of an isotropic co-precipitated powder based 3Y-TZP. The substantially enhanced toughness was interpreted in terms of the tetragonal to monoclinic martensitic phase transformability.
CV:
Omer Van der Biest is full Professor at the Department of Metallurgy and Materials Engineering of the Katholieke Universiteit in Leuven (Belgium). He holds a degree of Master in Metallurgical Engineering of the K.U. Leuven (1970) and a Ph. D. in Materials Science and Engineering of the University of California, Berkeley (1974). Prior to his appointment in Leuven in 1987 he was with the Lawrence Berkeley Laboratory, Lecturer at the University of California and Scientific Officer with the Energy Research Centre in Petten (Nl). Keywords describing his research interests include: high temperature materials and properties, ceramic composites, ceramic and powder metallurgical processes, mechanical properties, analytical electron microscopy. He is an author on more than 350 publications in refereed journals and on numerous contributions to Conferences. |
