【作者】 孙玮；

【导师】 周延春； 刘家臣；

【作者基本信息】 天津大学 ， 材料学， 2017， 博士

【摘要】 硼化物超高温陶瓷材料具有高熔点、高热导、高强度等优良特性,在高超声速飞行器领域具有巨大的应用前景,但是其较差的高温抗氧化能力和抗热震性成为了限制其应用的主要障碍。因此深入理解其本征性能与行为机制对指导材料设计与性能改进具有非常重要的意义。鉴于材料的表面性能对其宏观表现的重要影响,本文从以下几个方面对硼化物和硼碳化物的表面性质展开了研究:（1）ZrB₂的表面稳定性与氧吸附行为的各向异性以及过渡金属元素掺杂对ZrB₂表面稳定性的影响。通过一系列双区域模型,各表面的表面能和氧吸附能得以精确计算。预测了在近平衡态生长条件下获得的晶粒形貌应是沿[0001]方向的柱状晶粒。此外,晶粒的（0001）表面会比其它表面表现出更高的吸附惰性,预示着具有大尺度（0001）表面的片状晶粒可能具有更好的抗氧化性。当Sc、Y、Nb、Ta、Mo和W等元素的掺杂浓度较高时,将使（0001）面成为最稳定表面,这将促进片状晶粒的形成,并有可能提升ZrB₂晶粒的抗氧化能力。（2）TiB₂的表面的稳定性和表面成键特点。TiB₂的（112ˉ0）表面比（0001）和（101ˉ0）表面具有更好的稳定性。推测未经处理的TiB₂粉体的晶粒表面可能是由（0001）-B和（112ˉ0）表面组成的,并呈现出六方片状的晶粒形貌。TiB₂各表面均比ZrB₂中的同类型表面具有更高的表面能,这可能是导致TiB₂容易发生晶粒粗化的原因。（3）过渡金属二硼化物的表面稳定性和表面氧吸附中存在的一般性规律。随着金属元素价电子数的增加,二硼化物各表面的表面能会先增大后减小,而各表面上的氧吸附作用均有减弱的趋势。综合考虑表面稳定性以及表面氧吸附作用的差异,推测YB₂、HfB₂和TaB₂可能具有比ZrB₂更好的抗氧化能力。（4）一种新型超高温陶瓷Y₂B₃C₂的单晶和表面性能。Y₂B₃C₂单晶具有较高的轴向强度以及非常弱的切变强度,在剪切形变过程中能表现出应变强化行为。C-B-C原子链在剪切形变过程中极易发生转动是导致应变强化的主要原因。Y₂B₃C₂的表面能普遍较低,而且具有显著的各向异性,容易形成沿[010]方向的长柱状晶粒。这种长柱状晶粒形貌使得晶粒在材料受力变形过程中容易受到垂直于其长轴方向的切变应力而发生剪切变形并表现出应变强化效应。（5）YB₂C₂的表面稳定性以及相应二维单层材料的可能结构。在两种YB₂C₂结构中,（001）-BC表面均是最稳定表面。表面稳定性的各向异性预示着YB₂C₂容易形成具有较大比例（001）表面的片状晶粒形貌。YB₂C₂的（001）表面的低表面能同时也反映出了较弱的层间相互作用,预示着从其晶体中剥离出二维层状材料的可能性。由两种晶体直接获得的游离二维BC单层结构并不稳定,通过加氢修饰可以大大提高单侧结构的稳定性。更多还原

【Abstract】 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.（112ˉ0）surface were found to be more stable than others.It was predicted that the as-synthesized TiB₂ grain is covered by（0001）-B and（112ˉ0）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.更多还原