High-density twin boundaries in transition metal nitride coating with boron doping

The formation mechanism of twin boundaries in transition metal nitrides has remained largely unexplored owing to their high stacking fault energy that hinders twin boundary formation. Herein, we fabricated TiB0.11N1.16 coatings with high-density twin boundaries, including coherent twin boundaries and incoherent twin boundaries. The experimental results revealed that boron doping facilitates the coating orientation transition from [200] to [111], which provides an orientation advantage and indirectly promoting the formation of twin boundaries. Theoretical analyses indicated that boron segregation leads to the formation and stabilization of incoherent twin boundaries via two aspects: i) it reduces incoherent twin boundary energy and ii) forms large internal stress, which stabilizes the reduced incoherent twin boundary energy and prevents detwinning. The coupling of the two effects not only forms and stabilizes the incoherent twin boundaries, but also provides a possible precursor structure for the formation of coherent twin boundaries. Owing to the stress field change caused by the nonuniform distribution of boron, some twinned columns transform into the matrix orientation driven by system energy minimization, thus forming coherent twin boundaries. Moreover, the hardness and toughness of the TiB0.11N1.16 coatings were superior to those of twin-free ceramic coatings. The proposed mechanisms will help to introduce twins in materials with high stacking fault energy.