Anisotropic Shrinkage in Tape-Cast Alumina: Role of Processing Parameters and Particle Shape

Aqueous suspensions of alumina and other powders containing no visible morphological anisotropy were tape cast under different conditions to study the role of processing parameters and powder characteristics in the origin of in-plane sintering shrinkage anisotropy. High anisotropic shrinkage, with the typical shrinkage in the transverse direction exceeding that in the casting direction by more than 10%, was observed for tape-cast alumina cast at high solids loading and high shear rates. Tape casting without any binder also resulted in similar anisotropy. Lower solids loading with increased agglomeration resulted in a drastic reduction of shrinkage anisotropy. Under similar casting conditions, BaTiO₃ did not show any shrinkage anisotropy unlike alumina. Cubic zinc ferrite and titania powders also showed some consistent shrinkage anisotropy. A correlation was found between in-plane particle orientation and anisotropic shrinkage proving that powder characteristics, particularly particle shape, control anisotropic shrinkage even in apparently equiaxed powders.