First-principles Study Of The Surface Propertiesof Ultra-high Temperature Ceramics

Boride based ultra-high temperature ceramics are promising candidates for applications in hyper sonic vehicles owing to a combination of properties including high melting point,high strength,thermal conductivity et al.But poor thermal shock and oxidation resistance are still the main barriers to their more extensive applications.Hence,understanding their intrinsic properties and behavior mechanism are significant in the design of materials and tailoring their oxidation and mechanical properties.Since the performance of materials can be usually affected by their surfaces,the surface properties of borides were investigated with respect to following:?1?The anisotropic surface stability and oxygen adsorption behavior of ZrB₂,and the effect of transition metal on the surface stability of ZrB₂.By employing two-region models,the surface energies and adsorption energies were calculated accurately.The hexagonal rod-like ZrB₂ grain morphology was predicted during the crystal growth under equilibrium conditions.The?0001?surfaces were found to be less favorable for oxygen adsorption which indicates the plate-like grains with large area of?0001?surfaces should have better oxidation resistance.With higher concentrations,Sc,Y,Nb,Ta,Mo and W resulted in the most stable?0001?surfaces and the plate-like grain morphology,which might improve the oxidation resistance of ZrB₂ grains.?2?The anisotropic stability and bonding features of TiB₂ surfaces.?112?0?surface were found to be more stable than others.It was predicted that the as-synthesized TiB₂ grain is covered by?0001?-B and?112?0?surfaces which results in a hexagonal plate-like grain morphology.It was also found the surface energies of TiB₂ are much higher than those of ZrB₂ with similar structure,which might be responsible for the easy coarsening of TiB₂.?3?The general trends in surface stability and oxygen adsorption of transition metal diborides.With the increasing number of valence electrons of transition metals,the surface energies first increased and then decreased,while the oxygen adsorptions were gradually weakened.Combing the anisotropic surface stabilities and oxygen adsorptions,YB₂,HfB₂ and TaB₂ were predicted to have better oxidation resistance than ZrB₂.?4?Single crystal performance and surface stability of Y₂B₃C₂.Y₂B₃C₂ presented strong axial stiffness and remarkable weak shear resistance.Strain-stiffening behaviors of Y₂B₃C₂ were predicted under shear deformations.The strain-stiffening effect was originated from the easy tilting of C-B-C chain.Y₂B₃C₂ kept low surface energies and preferred to crystalline in a rod-like shape along[010]direction.This morphology made it possible for the internal grains to suffer shear strain at both end of their rod-like shape and then display strain-stiffening behaviors.?5?The surface stability and possible two-dimensional monolayer structures of YB₂C₂.For both the two kinds of YB₂C₂ structures,?001?-BC surface was the most stable surface.The anisotropy in surface stability resulted in plate-like grain morphologies with large area of?001?surfaces.The low surface energies of?001?surfaces also indicated the possibility of exfoliation of two-dimensional BC monolayer from YB₂C₂.The as-derived BC monolayers were unstable,but can be stablized by hydrogen modification.