Phase stability tuning in the NbxZr1-xN thin-film system for large stacking fault density and enhanced mechanical strength

Joelsson, T.; Hultman, L.; Hugosson, H. W.; Molina-Aldareguia, J. M.
March 2005
Applied Physics Letters;3/28/2005, Vol. 86 Issue 13, p131922
Academic Journal
The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is pred-icted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verifica-tion, epitaxial NbxZr-1xN(0 ≤ x ≤1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyz-ed with transmission electron microscopy sTEMd and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x=0.5 composition. The nanoindentation hardness of the films varied between 21 GPa for the binary nitrides, and 26 GPa for Nb0.5Zr0.5.5N. Unlike the cubic binary nitrides, no slip on the preferred <110>{110} slip system was observed. The increase in hardness is attributed to the increase in defect density at x=0.5, as the defects act as obstacles for dislocation glide during deformation. The findings present routes for the design of wear-resistant nitride coatings by phase stability tuning.


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