This research work is part of the GRAPHYPRE thesis project, which aims to elaborate multi-layer alumina materials by alternating pure alumina and mullite-alumina layers shaped by uni-axial pressing. The principle is to combine layers of materials with different thermal expansion coefficient. Thus, residual stresses appear during cooling from the sintering temperature. These residual stresses and the presence of mullite crystals in the alumina matrix and at the interfaces are expected to have a significant effect on the fracture strength and toughness of the developed multilayer materials.
Kaolinite is a promising raw material to be considered in the formulation to supply silicon and produce mullite Natural kaolinite generally crystallizes in the form of pseudo-hexagonal platelets exhibiting a slight thickness (from 1 µm to 100 nm). They are composed of both stacked layers of SiO2 tetrahedral sheets and AlO2(OH)4 octahedral sheets. The key point of the project is to control the transformation of kaolinite during the heat treatment to lead to mullite crystallization during the increase of temperature and residual stresses during the descent.
In this context, different compositions of a mixed system based on alumina and kaolinite were prepared by cryogenic granulation by varying the kaolinite content between 0 and 25%vol. The coefficients of thermal expansion of the mixed compositions were calculated through a dilatometric study in order to select a suitable composition for multilayer architecture configurations. Several multilayer material architectures were developed by co-pressing and also analysed by dilatometry.