UCLA

This dissertation presents the investigation of the effects of size-scale and crystallographic orientation on room-temperature plastic deformation of ceramics. Using in situ electron microscopy based nanomechanical testing, I show that sub-micron-scale single-crystalline refractory carbides exhibit size- and orientation-dependent room-temperature plasticity under uniaxial compression. Refractory carbides such as ZrC, TaC and SiC - chosen as candidate materials - owing to their high hardness (≥ 20 Gpa), high elastic modulus (≥ 350 GPa) and melting point (≥ 3000 K), are widely used in various high temperature/high hardness structural applications. Most of the previous studies conducted on these materials are on bulk samples at temperatures ≥ 0.3 Tm. While these studies have helped understand the thermomechanical behavior, relatively little is known concerning their room-temperature mechanical properties. Here, I investigated simultaneous morphological and microstructural changes during mechanical deformation of sub-micron-size cylindrical pillars, identified slip systems and measured yield strength as a function of crystal size. I show that for ZrC pillars, loading along [100] and [111] directions activate {11-0}<110> and {001}<110> slip systems, respectively. For both the orientations, yield strengths increase with decreasing crystal size. Unexpectedly, ZrC(111) is found to be up to 10� softer than ZrC(100). For TaC pillars, loading along [100] and [011] directions activate {110}<110> and {11-1}<110> slip systems. In contrast to ZrC, I did not observe any size effects on yield strength. In the case of 6H-SiC, loading along the basal direction <0001> results in brittle fracture, while loading at 45 °C with respect to <0001> leads to dislocation glide-controlled plasticity at room-temperature. Molecular dynamics simulations and density functional theory calculations helped in identification of the most energetically favorable slip systems and the mechanisms governing the plastic deformation. My results provide important insights into room-temperature plastic deformation modes operating in refractory carbides.