On the anisotropic shear resistance of hard transition metal nitrides TMN (TM=Ti, Zr, Hf)

The anisotropic shear moduli and strengths of hard fcc-TMN (TM=Ti, Zr, Hf) have been calculated by ab initio density functional theory in order to better understand their shear resistance. TiN shows the largest shear strength among the three nitrides, being consistent with the known facts that TiN is harder than HfN which is harder than ZrN. The electronic origin is further addressed. Based on the smooth shapes of the stress-strain curves and on the variation of electronic structure during shear deformations, the abrupt lattice instability mode reported previously for TiN (110)⟨11¯0⟩ shear deformation is not supported by our data.