Processing-Microstructure-Mechanical Properties of Freeze-cast Porous Alumina Scaffolds with 3-Levels of Structural Hierarchy

Abstract

Unidirectional freeze casting (ice-templating) evolved as a promising processing route in the past decade to fabricate unique porous materials where pores are oriented in a single preferential direction. This simple and straightforward technique offer better tailorability of the pore structures, and hence the material processed by this route has potential applications in various fields like filtration, catalysis, biological scaffolds, energy generation and storage, electrodes for SOFC, impact protection etc. However, this wide diversity in applications demand large-scale ordered porosity with controlled pore morphology. In the quest of developing such materials, control of several processing/freezing conditions which are directly associated with the underlying principles of the technique, is utmost necessary. To this end, a holistic study on the processing microstructure-property is imperative to produce novel directional porous materials with desired pore structures and porosity fractions. The primary work of the dissertation involves directional freezing of anisotropic alumina platelets and unfold the effect of shape anisotropy on the growing freezing front, vis-à-vis, the final microstructure. Unidirectional freeze casting was carried out under large vertical temperature gradient in a custom-built setup, that resulted in large-scale ordered lamellar structure; mainly due to the fast and straightforward self-assembly of platelets during directional solidification. A microstructural montage was presented to delve into the effect of both smaller (4 μm) and bigger (8 μm) size platelets on the freezing induced microstructural evolution along the ice-growth direction of the scaffold. This observation was explained on the basis of ice physics and the interaction of ceramic platelets with the advancing freezing front. An array of microstructures was produced by freeze casting over a wide range of freezing velocities (1.4 – 2600 μm s-1), platelet sizes (4 and 8 μm) and solid loading (10 to 40 vol%) to study their influence on various structural parameters, viz. wavelength (λ), lamella thickness (δ), and bridge density ρb). The ensemble of micrographs presented indicate the influence of processing variables on the transition between platelet rejection and engulfment by the ice lamellae. This corresponds to the microstructural transition of either (a) lamellar to dendritic to isotropic (more accurately dendritic overlayer structure) or (b) directly from lamellar to isotropic (precisely lamellar overlayer structure). The microstructures were further quantified with a specific dimensionless parameter m. For smaller platelet (4 μm) scaffolds, the microstructure to be lamellar with low bridge density and m > 4. The wavelengths and bridge spacing were comparable for 2 < m < 4 and led to dendritic structure. For the morphologies characterized by m < 2, the spacing among the numerous interlamellar bridges was smaller than the structural wavelength and hence, the scaffolds revealed usually isotropic structure. For bigger platelet (BP) scaffolds, the microstructures are classified as lamellar for m > 2 and isotropic for m < 2. Finally, the specific processing conditions that yielded different morphologies and the parameter m were utilized to construct a unique ‘3D morphology map’ to establish the processing-microstructure relationship for the freeze-cast porous alumina scaffolds. In the second part of the present research, the microstructure-mechanical property relationship for the freeze-cast porous alumina scaffolds was established. 3D morphology map guided the suitable processing conditions correspond to three different pore regimes such as lamellar, dendritic and isotropic. The influence of various pore structures on the uniaxial compressive response of platy-alumina scaffolds (level-1 hierarchy) was examined at quasistatic strain rate (~ 10-4 s-1) regime. Despite significant microstructural differences, all the scaffolds exhibited progressive, cellular like failure behavior which is attributed to the high porosity fraction (>85%). The microstructural evidences along with the Gibson-Ashby model prediction for out-of-plane deformation of honeycomb structure clearly suggest that buckling induced elastic instability of the lamella walls sets the limit for maximum compressive strength of the scaffolds (level-1 structure). The highly porous alumina platelet scaffolds (level-1) resulted in lower compressive strength mainly due to the presence of large amount of macropores as well as micropores present within the lamella walls. To overcome this and to strengthen and stiffen the freeze-cast scaffolds, a unique strategy was adopted by introducing a silica-calcia liquid phase sintering aid (LP), and creating another level of structural hierarchy (i.e. level-2). The idea of using SiO2-CaO liquid phase with molar ratio of 75:25 is due to its better wettability that facilitates the platelet rearrangement during liquid phase sintering, and thereby reducing the inter-platelet gap (i.e. micropores) which results in the enhanced densification of the ceramic walls. The response of the scaffolds (with level-2 hierarchy) under uniaxial compressive stress was strongly influenced by both pore morphologies and LP concentration. At fixed LP content, the compressive load bearing capability gradually increased from lamellar to dendritic to isotropic structures. Again, the higher amount of liquid phase (20 vol%) caused remarkable increase in the compressive strength of the freeze-cast scaffolds with level-2 architecture. More importantly, the failure mode of the scaffolds was changed from damageable, cellular- like (localized damage) to brittle-like failure (global fracture) at higher LP content. The transition of failure behaviour from cellular to brittle was observed to occur at ‘porosity fraction at transition’ of ~70%, and the corresponding critical buckling load (Pcrit) for the transition was calculated to be about 2.3. The robust walls produced by the addition of liquid phase sintering aid resulted in extraordinarily improvement of both strength and stiffness of level-2 hierarchy structure, i.e. almost two orders of magnitude higher than level-1. Further, 3rd level of structural hierarchy was created by adding submicron size equiaxed alumina particles in the level-2 structure containing alumina platelets and liquid phase precursors. The notion for this inclusion is to modify the lamella wall architecture by creating inter-platelets bridges, thus promoting extrinsic toughening of the lamella walls by large-scale crack deflection by these bridges. The particle induced interlocking of the platelets also offers additional resistance to the lateral bending (buckling) of the lamella walls by restricting the platelets movement during uniaxial compression. As a result, the compressive properties of level-3 architecture was remarkably increased (than level-2) due to the synergistic effect of alumina platelets, liquid phase sintering aid and alumina particles. The increase in the mechanical properties due to the reduction in porosity from level-1 to 3 was quantitatively assessed to be insignificant as compared to the combined effect of all 3 structural components. Overall, the present thesis elucidated an in-depth investigation on the processing-microstructure-property (mechanical) correlations for a multi-level hierarchical freeze-cast porous alumina scaffolds which would offer valuable insights to produce novel porous ceramics. The approach of improving the strength and stiffness of the directional porous ceramics as delineated in the present work can be utilized for the design and development of several bioinspired materials.