ELECTRON LOCALIZATION INDUCES LARGE HARDNESS ENHANCEMENT IN CrN

电子局域引起的 CrN 硬度增强

闫明旗

11849151

硕士

物理学

王善民

2020

2020-07-14

哈尔滨工业大学

深圳

Early 3d transition-metal mononitrides are a class of extensively exploited hard mate-rials for fabricating a wide variety of cutting and machining tools. Origin of the hardness in such nitrides has previously been well explored to link to the electron filling of a partic-ular σ bonding state formed between metal d: eg and nitrogen p orbitals. Over filling of the σ bonds with valence-electron concentrations (VEC) above 8.4 would lead to a sub- stantial reduction in hardness. However, chromium nitride (CrN), one of most frequently studied nitrides, possesses a large VEC value of 11, its hardness has an anomalously high values, which cannot be explained in terms of the scenario of bond filling. In this work, in conjunction of experimental observations, we present ab initio calculations for CrN to investigate the unexpectedly enhanced mechanical properties at a fundamental level. We find that the 3d electrons in half-filled Cr: t2g orbitals have a higher density than non- magnetic calculations. Therefore they are tightly localized for producing strong t2g − t2g magnetic exchanges in the metal sublattice, resulting in a suitably occupied σ bonding state with a VEC of 8. Based on the electron localization model, the determined shear modulus for magnetic CrN is around 160 GPa, nearly triple that of non-magnetic calcu- lation with completely delocalized 3d electrons (i.e., 55 GPa). This suggests the electron localization can profoundly strengthen the shear modulus in CrN to strongly resist the shearing deformation that prevails under indentation, as manifested with high hardness. These findings provide a new avenue for design novel refractory transition-metal com- pounds with extraordinarily enhanced mechanical properties by the contro of valence- electron concentration.

元素周期表中前几个 3d 过渡金属氮化物是一类被广泛利用的硬质材料，可用 于制造各种切削和加工工具。之前已经很好的理解了第 4 族碳氮化物的硬度起源， 这与金属 d 轨道中的 eg 电子和氮的 p 轨道之间形成的 pdσ 键以及金属 t2g 电子之 间形成的 ddσ 键的填充有关。当价电子浓度(VEC)低于 8.4 时，电子倾向于填充 对硬度有益的 pdσ 键，而当价电子浓度(VEC)高于 8.4 时，电子则开始填充对硬 度有害的 ddσ 键，此键的过度填充将导致硬度大幅降低。然而，实验发现，具有 11 个价电子的氮化铬(CrN)具有极高的硬度(16GPa)，高于价电子浓度为 10 的 氮化钒(10GPa)，因而无法仅用上述电子填充理论来解释。在这项工作中，我们利 用密度泛函理论来研究这种意外的硬度增强。我们发现，在不考虑磁性的情况下， 过渡金属氮化物的弹性性质的变化均具有相似的趋势，但是磁性使得弹性性质发 生了很大的变化。在磁性 CrN 中，半填充状态的 Cr 原子的 t2g 电子密度远高于非 磁情况，因而是空间局域的。这种局域不仅在金属亚晶格中产生了强的 t2g − t2g 磁 交换，而且也等效地减少了价电子数目，形成了价电子数目接近于为 8 的状态，减 少了硬度有害的 ddσ 键的填充，从而增强了硬度。计算的磁性 CrN 的剪切模量为 160 GPa，远大于 3d 电子完全巡游下的非磁计算的结果(60 GPa)。这表明电子局 域可以显着增强 CrN 中的剪切模量，以强烈抵抗压痕下普遍存在的剪切变形，结 果表现为高硬度。这些发现启示我们可以通过控制化合物价电子的浓度来调控机 械性能，并为设计新型难熔过渡金属化合物提供了途径。

chromium Nitride (CrN) transition metal nitrides hardness elastic properties first principles calculations

氮化铬(CrN) 过渡金属氮化物 硬度 弹性性质 第一性原理计算

英语

联合培养

南方科技大学

Yan MQ. ELECTRON LOCALIZATION INDUCES LARGE HARDNESS ENHANCEMENT IN CrN[D]. 深圳. 哈尔滨工业大学,2020.