Bi-directional freezing to fabricate freeze-cast ceramics with orientation gradient and uniaxial compressive deformation behavior of infiltrated composites

Abstract

Structural orientation gradient in living organisms is attributed to their enhanced surface protection to penetration and deformation, providing bioinspiration to fabricate engineering materials with gradient in mechanical properties. Freeze-casting is a promising technique to fabricate bioinspired materials. However, in the conventional freeze-casting process, due to the applied unidirectional temperature gradient, all the material layers are also unidirectional, oriented in the direction of applied gradient. While bi-directional freezing conditions have been used but still preserving the characteristic unidirectional alignment. The main objective of this work was to demonstrate that bi-directional freezing conditions could be employed to create and control orientation gradient in freeze-cast materials and thereby develop multilayered composites with orientation gradient. Bi-directional freezing conditions were used to generate both horizontal freezing front (HFF) and vertical freezing front (VFF), where the ice crystals associated with VFF tilted the ice crystals associated with HFF, resulting in orientation gradient. 4 different bi-directional freezing conditions were achieved to change gradient microstructure. In situ studies were performed to investigate different growth front movements and calculate the velocities. Porous ceramics were infiltrated to develop bi-directionally freeze-cast composites. While the current literature sheds invaluable insights into the mechanical behavior of freeze-cast composites with unidirectional orientation, this investigation characterized uniaxial compressive mechanical response and damage characteristics of bi-directionally freeze-cast composites, which is another objective of this work. It was revealed that bi-directional freezing conditions can tune material layer orientation throughout freeze-cast microstructure, strongly influencing compressive mechanical response and damage characteristics in the resulting composite materials.