Anisotropic kinetic of the kaolinite to mullite reaction sequence in multilayer ceramics

Multilayer ceramics with a composite and organized microstructure were realized from kaolin and alumina fibers to improve strength and fracture toughness. Dilatometry experiments along 3 directions reveal anisotropic shrinkages, which are in correlation with different activation energy for sintering. Mullite growth is strongly anisotropic, inducing the formation of an organized microstructure, where larger mullite crystals are mainly oriented in plane of layer and perpendicular to alumina fibers. Kinetic data from thermal transformations show that the starting reaction mechanism is mullite nucleation, and it is continued by a strongly anisotropic grain growth. It is explained by topotactic transformations at phyllosilicate faces and along alumina arrangements. Mullite growth kinetics is also favored perpendicularly to fiber main dimension by the anisotropy of alumina diffusion coefficient. It shows the limited importance of mullite crystallization in microstructural transformation, but it also shows that controlled mullite growth is central in microstructural arrangement.