MICROSTRUCTURE, CONNECTIVITY, AND MECHANICAL BEHAVIOR IN ALUMINA AND SIALON MICROSTRUCTURE COMPOSITES
Open Access
- Author:
- Soublet, Brandy A
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- May 04, 2011
- Committee Members:
- Gary Lynn Messing, Thesis Advisor/Co-Advisor
Gary Lynn Messing, Thesis Advisor/Co-Advisor - Keywords:
- microstructure property relations
alumina
connectivity
ceramic processing
mechanical properties - Abstract:
- Composite designs are useful in developing new materials with unique property sets tailored for any application. Mitigating the processing difficulties associated with processing dissimilar materials, however, can be expensive and limit their application. In this work, a new class of materials recently proposed called microstructure composites were fabricated and mechanically tested to gain insight into the mechanical response of these composites. Microstructure composites are composites in which the constituent components are not dissimilar materials, but different microstructures of a material with the same composition. The material used in this work is α-Al2O3 and the constituent microstructures are a fine-grained, equiaxed Al2O3 microstructure and a textured Al2O3 microstructure. Textured alumina grains exhibit crack deflective properties due to the low fracture energy of the basal plane, and can therefore be used to toughen alumina. Crack deflection increases the amount of energy absorbed by a ceramic during fracture and can be quantified by work of fracture. The textured microstructure is achieved by templated grain growth (TGG); template α-Al2O3 particles added to a colloidal slurry are aligned during forming. The addition of 0.14 wt% CaO + SiO2 liquid-forming dopant induces anisotropic growth of the template particles during densification. Microstructure composites can be fabricated with a variety of complex architectures according to the concept of connectivity, as used for piezoelectric applications. The connectivity of a composite describes the dimensions in which each component is connected to itself. A novel processing method, slurry co-casting, which involves the simultaneous tape casting of two ceramic slurries, was utilized to cast composite tapes. These were subsequently cut and stacked in specific sequences to create microstructure composites of 1-1, 2-2, and 3-3 connectivities. Microstructural control between the equiaxed and textured layers was achieved by sintering at 1550°C. A sharp interface between the constituent components ensured that distinct connectivities were achieved. Combining constituent components in such specific configurations increases the control a researcher has over the properties of the material. The alumina microstructure composites were mechanically tested in equibiaxial flexure using the ring-on-ring method. Load/deflection curves, fracture surfaces, and failure behavior were analyzed. Crack deflection and step-wise fracture were observed for all connectivities tested. Extended crack deflection (≥ 1 mm) was observed in only the 1-1 and 3-3 connectivities. So-called “graceful failure” was observed in all connectivities and work of fracture reached values in excess of 7 kJ/m2. The mechanical response of the 2-2 connectivity was variable; it was the only connectivity to exhibit catastrophic, instead of graceful, failure in some cases. The 2-2 connectivity, however, is unique from the 1-1 and 3-3, in that due to the nature of the architecture and the chosen loading geometry, it is possible that a crack traveling through the specimen would never encounter a reinforcing, textured layer. For this reason, the 2-2 connectivity studied in this work is not recommended for structural applications. The 1-1 and 3-3 connectivities, however, demonstrated crack deflective and energy absorbing properties, which increase the toughness of the material. The fabrication and mechanical testing of equiaxed/textured microstructure composites in this work offered new insights into the effect of connectivity (namely the 1-1, 2-2, and 3-3) on the mechanical response of alumina. Microstructure composites of SiAlON were also fabricated in which the constituent microstructures were the α- and β-SiAlON phases. α-SiAlON was used for its high hardness and β-SiAlON was used for its high toughness, due to the elongated nature of its grain structure. Microstructure composites of these phases could exploit the advantageous mechanical properties of both. Many processing issues, however, including failure in both the green state and during densification (by hot-pressing), were encountered. Initial mechanical testing of the 3-3 connectivity in SiAlON microstructure composites reveals catastrophic failure and low biaxial strength. This behavior is most likely the result of one of the processing issues encountered in the fabrication of these composites, that is, the “pre-cracking” of the green sample in the hot-press prior to densification.