Paper TF-ThM2
Electronic Mechanism for Toughness Enhancement in V_xM_1-xN Thin Films

Thursday, October 21, 2010, 8:20 am, Room Ruidoso

We use Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA) to predict the properties of a number of novel V-M-N thin films in the B1 (NaCl) structure. The new compounds are obtained by alloying VN with Nb, Mo and W, in concentrations of 50 %. We evaluate the elastic moduli and constants of these ternaries and perform a detailed analysis of their electronic structure. These results are compared with the corresponding properties of typical binaries such as TiN and VN. Our calculations show that the new ternaries have hardness comparable with TiN and VN, and significantly, a resolute ductile behavior. This unique combination of hardness/ductility, which is in contrast to the hardness/brittleness relationship typically found in hard coatings, equates to significantly increased toughness, as confirmed by the stress-strain relationship we obtain for all these compounds. The electronic structure results presented herein reveal a layered charge density, consisting in alternating high and low electron density regions, similar to that recently reported for Ti-Mo/W-N thin films [1]. To fully understand the mechanism responsible for this interleaved arrangement of electrons, we carry out crystal orbital overlap population (COOP) and electron localization function (ELF) calculations, and succeed to energetically resolve the bonding/antibonding contributions, of the first and second neighbors, to the chemical bonds in these compounds. Based on the results of this analysis, we find that the electronic mechanism responsible for the observed toughness enhancement in these compounds is rooted in the increased metal-metal (second neighbors) interaction of d-t_2g orbitals.

[1] D.G. Sangiovanni, V. Chirita and L. Hultman, Phys. Rev. B 81, 104107 (2010)