UCLA

Ultra-incompressible and superhard materials are good candidates for applications in cutting, grinding, polishing tools and coatings. It has been found that transition metal borides, combining highly incompressible transition metals with strong covalently bonded boron network, are potential candidates for machining tools due to good thermal stability and great mechanical properties. In the course of our studies involving a family of metal borides, such as tungsten diboride (WB2), rhenium diboride (ReB2), tungsten tetraborides (WB4) and dodecaborides (MB12, M = metal), and its solid solutions, we demonstrated that hardness could be enhanced through both intrinsic (chemical bonding) and extrinsic (grain boundary) approaches. In this thesis, a complete discussion was made for each diboride-, tetraboride- and dodecaboride- system, aiming to study the deformation mechanism in a lattice specific manner and the intrinsic hardening effects. In addition to intrinsically modify the crystal structure to improve the hardness, extrinsic hardening effect, specifically modifying grain boundaries, was investigated in ReB2 system to understand the size dependence of strength and texture in the nano-system.