Huang Cheng, Yang Bo, Peng Xianghe, Chen Shaohua
Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China.
Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures, Beijing Institute of Technology, Beijing 100081, China.
ACS Appl Mater Interfaces. 2020 Nov 4;12(44):50161-50175. doi: 10.1021/acsami.0c15275. Epub 2020 Oct 23.
A very interesting experimental finding shows that a nano-twinned cubic boron nitride (NT-cBN) ceramic has size-dependent hardness. In order to reveal the hardening mechanism of NT-cBN, the plastic deformation mechanism of single-crystalline cubic boron nitride (SC-cBN) under nano-indentation is first studied, and then that of NT-cBN is further investigated using atomistic simulations with a parameter-modified Tersoff potential. It is found that the plastic deformation of SC-cBN under nano-indentation is mainly attributed to serial dislocation behaviors, such as the formation of dislocation embryos, shear loops, and prismatic loops. In comparison, for NT-cBN, the plastic behavior is much more complex, which is influenced by a dislocation blockage, absorption, dissociation, and re-nucleation due to the interaction between dislocations and twin boundaries (TBs). From the plastic deformation mechanism of NT-cBN, it is found that the size-dependent hardening behavior of NT-cBN is a competitive result between the hardening sources, including slip transfer, dislocation accumulation, and suppression of dislocation nucleation, and the softening sources, including TBs being destroyed, parallel slips of dislocations, and the formation of new sites for dislocation nucleation. The smaller the distance between the adjacent TBs, the more dominant the role of hardening sources is, resulting in the high size-dependent hardness of NT-cBN. The results in this paper should be helpful for the optimized design of high strength and toughness of nano-structured cBN ceramics.
一项非常有趣的实验发现表明，纳米孪晶立方氮化硼（NT-cBN）陶瓷具有尺寸依赖性硬度。为了揭示NT-cBN的强化机制，首先研究了纳米压痕下单晶立方氮化硼（SC-cBN）的塑性变形机制，然后使用参数修正的Tersoff势通过原子模拟进一步研究了NT-cBN的塑性变形机制。研究发现，纳米压痕下SC-cBN的塑性变形主要归因于一系列位错行为，如位错胚胎、剪切环和棱柱环的形成。相比之下，对于NT-cBN，塑性行为要复杂得多，它受到位错与孪晶界（TBs）相互作用导致的位错阻塞、吸收、解离和重新形核的影响。从NT-cBN的塑性变形机制可以看出，NT-cBN的尺寸依赖性硬化行为是硬化源（包括滑移转移、位错积累和位错形核抑制）和软化源（包括TBs被破坏、位错的平行滑移以及位错形核新位点的形成）之间竞争的结果。相邻TBs之间的距离越小，硬化源的作用越占主导，导致NT-cBN具有高尺寸依赖性硬度。本文的结果有助于纳米结构cBN陶瓷高强度和韧性的优化设计。
