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Gastvortrag am 08.04.2014

Microstructure and Mechanical Properties of Alumina and Silicon Carbide Armor Ceramics

ABSTRACT: The ever increasing sophistication of rapid-fire arms has in the past years forced a switch from metal armors to ceramic and composite armors which offer better protection, lightweight and mobility. Current high-performance protective armors feature projectile blunting, eroding or breaking ceramic plates backed up or enclosed by ductile metals, fabrics or composites. However, ceramic plates have the disadvantage of lacking multi-hit capability due to brittleness that can lead to fracture at first impact which substantially compromises protection at subsequent impacts. Therefore, improving the fracture toughness while maintaining appropriate levels of hardness, strength and lightweight is currently the key focus in ceramic armor research. To meet this challenge, manufacturing processes, microstructure design and appropriate reinforcements need to be developed. Such development requires the definition of appropriate, accurate and efficient testing and analysis methodologies and standards. Our current research focuses on two of the most commonly used armor ceramics: SiC and Al2O3. Chemical composition, grain structure, porosity and second phases, including ZrO2 particles, are studied using optical and scanning electron microscopy as well as XRD. Studied mechanical properties include hardness, fracture toughness, strength, density, Young modulus and fracture mode. The appropriateness of indentation fracture toughness measurements as well as impact, four-point bend and ring-on-ring tests is investigated among others. Microstructure-property relationships are studied. Results so far show a strong indentation size effect on Young modulus, microhardness and fracture toughness measurements. Second phases, porosity and grain structure are highlighted as key factors determining the fracture toughness and ultimately armor performance. While second phases are shown to increase the fracture toughness by transformation toughening and crack deviation, porosity seems to promote energy absorption by local deformation and densification. Future work will include carbon and boron nitride nanotube reinforcements that have proven very promising for further toughness improvement.