Text

theses

ETD doctoral

Boron carbide is a superhard ceramic that has attracted considerable research attention for decades. However, hard ceramics are generally considered brittle due to the lack of energy dissipation mechanisms, such as dislocation slip, deformation twinning, and micro-cracking, to accommodate irreversible deformation. The highly covalent nature of boron carbide and the low symmetry in its crystal structure imply that localized dislocation movement is difficult. In fact, despite numerous transmission electron microscopy (TEM) studies, localized dislocation movement has never been identified. The lack of plasticity hinders the widespread use of boron carbide in common engineering applications that demand a level of compliance in materials. Boron carbide also suffers from stress-induced amorphization that has a deleterious effect on its mechanical properties. Amorphization occurs when the consolidated boron carbide experiences pressure exceeding its strength, leading to the formation of nano-scale amorphous bands inside the crystalline matrix. The network of amorphous bands acts as a “path of least resistance” for crack growth leading to catastrophic failure. This thesis focuses on resolving the two prevailing issues of boron carbide (brittleness and susceptibility to stress-induced amorphization) and proposes a design strategy for fabricating the next-generation boron carbide with enhanced toughness and resistance to amorphization.

In this thesis, dislocation mediated plasticity was enabled in boron carbide by tuning its bond characteristics by adding Al to its crystal structure. The addition of Al-B and Al-C bonds increased the propensity of ionic bonds and allowed inter-icosahedra (planar) glide to occur. The toughness of boron carbide could be further increased by making eutectic composite with ZrB2 and particulate composite with TiB2. The tougher diborides provided a means to blunt crack tips and accumulated residual stress at the phase boundaries. The presence of residual stress deflected the crack path and preferentially guided the crack to travel between the phase boundaries.

Stress-induced amorphization was resolved through Si/B co-doping. Si/B co-doped boron carbide was synthesized in various ways (arc melting, diffusion couple, and reaction hot-pressing) and characterized to understand the role of Si in reducing amorphization. We found that as little as 1 at.% addition of Si substantially suppressed amorphization when compared to B-doping alone. Our TEM study comparing the single crystal undoped and Si/B co-doped boron carbide revealed a dramatic change in deformation behavior. Instead of forming long distinct parallel amorphous shear bands, Si/B co-doped boron carbide manifested short and diffusion micro cracks. The mechanism implies that the involvement of Si facilitates local fragmentation as opposed to large-scale amorphization and microcracks.

Lastly, a composite comprised of Si/B co-doping and 10 wt% TiB2 reinforcement was synthesized. Compared to Si/B co-doped boron carbide, the composite showed an improvement in toughness (10%), hardness (6%), strength (21%), and smaller grain size while minimizing a density increase (4%). Our composite has similar overall mechanical properties to a commercial boron carbide tiles while having 6% higher in elastic modulus and 44% higher in amorphization resistance.

ETD_11071

Ph.D.

Includes bibliographical references

doi:10.7282/t3-2wns-hg54

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