Nanoscale Mechanics of Ductile Interphases in Solid Solution Directionally Solidified Eutectic Composites

Public Deposited

Ceramic matrix composites are known for their low density, high strength and high stiffness, but lower fracture toughness compared to metal matrix composites. The addition of a reinforcing agent within the matrix can increase the toughness of the composite via many strain energy absorption mechanisms such as plastic deformation. This dissertation attempts to shed light on the competing deformation and fracture mechanisms in ductile/brittle nanoscale lamellar systems where the conventional deformation mechanisms may not apply. Ni<sub>x</sub>Co<sub>1-x</sub>O/ZrO<sub>2</sub> Directionally Solidified Eutectic (DSE) composite series has been chosen as a model system for this study. In the first part of this dissertation, it is demonstrated that formation of a novel metal-ceramic multi-layered structure is feasible via reduction of Ni<sub>x</sub>Co<sub>1-x</sub>O/ZrO<sub>2</sub> composite as a result of the interfaces forming an electrochemical cell in a reducing atmosphere at high temperatures. The second part of the thesis is dedicated in understanding the correlative deformation and fracture mechanisms in the reduced Ni<sub>x</sub>Co<sub>1-x</sub>O/ZrO<sub>2</sub> model system with a nanoscale Ni(Co) confined interphase. These investigations were inspired by a novel observation that there is striking dissimilarity in the interfacial fracture behavior of the reduced Ni<sub>x</sub>Co<sub>1-x</sub>O/ZrO<sub>2</sub> composite compared to that of the fully oxidized Ni<sub>x</sub>Co<sub>1-x</sub>O/ZrO<sub>2</sub> system. A multitude of conventional and analytical electron microscopy techniques are utilized to investigate the role of the size scale, chemistry of this model system on the strain energy absorption upon deformation. FIB TEM lift-off technique is further employed to investigate the crack tip interactions with the nanoscale confined interphases in this model system. To study the role of size scale, the nanoscale deformation mechanism within the metallic interphase is investigated across 50-300 nm thickness range for the confined Ni(Co) interphase. The role of chemistry on the small scale deformation mechanisms in this model system is investigated by choosing two different compositions in the Ni<sub>x</sub>Co<sub>1-x</sub>O/ZrO<sub>2</sub> composite series: CoO/ZrO<sub>2</sub> with x=0 and Ni<sub>0.5</sub>Co<sub>0.5</sub>O/ZrO<sub>2</sub> with x=0.5. These investigations confirm that the metallic interphase, with the thickness of above 100 nm, contributes to strain energy absorption through plastic deformation. With decreasing the interphase size scale to values below 100 nm, the extent of plasticity is reduced within the metallic interphase

Last modified
  • 05/22/2018
Date created
Resource type
Rights statement