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.
Keywords: NT-cBN; cBN; dislocation; plastic deformation; size-dependent hardness.