First-principles quantum molecular dynamics study of Ti _<i>x</i> Zr _{1− <i>x</i>} N(111)/SiN _<i>y</i> heterostructures and comparison with experimental results

Abstract

The heterostructures of five monolayers B1-Ti _<i>x</i> Zr_1-<i>x</i> N(111), <i>x</i> = 1.0, 0.6, 0.4 and 0.0 (where B1 is a NaCl-type structure) with one monolayer of a Si₃N₄-like Si₂N₃ interfacial layer were investigated by means of first-principles quantum molecular dynamics and a structure optimization procedure using the Quantum ESPRESSO code. Slabs consisting of stoichiometric TiN and ZrN and random, as well as segregated, B1-Ti _<i>x</i> Zr_1-<i>x</i> N(111) solutions were considered. The calculations of the B1-Ti _<i>x</i> Zr_1-<i>x</i> N solid solutions, as well as of the heterostructures, showed that the pseudo-binary TiN-ZrN system exhibits a miscibility gap. The segregated heterostructures in which Zr atoms surround the Si _<i>y</i> N _<i>z</i> interface were found to be the most stable. For the Zr-rich heterostructures, the total energy of the random solid solution was lower compared to that of the segregated one, whereas for the Ti-rich heterostructures the opposite tendency was observed. Hard and super hard Zr-Ti-Si-N coatings with thicknesses from 2.8 to 3.5 <i>μ</i>m were obtained using a vacuum arc source with high frequency stimulation. The samples were annealed in a vacuum and in air at 1200 °C. Experimental investigations of Zr-Ti-N, Zr-Ti-Si-N and Ti-Si-N coatings with different Zr, Ti and Si concentrations were carried out for comparison with results obtained from Ti _<i>x</i> Zr _{<i>1</i>-<i>x</i>} N(111)/SiN _<i>y</i> systems. During annealing, the hardness of the best series samples was increased from (39.6 ± 1.4) to 53.6 GPa, which seemed to indicate that a spinodal segregation along grain interfaces was finished. A maximum hardness of 40.8 GPa before and 55 GPa after annealing in air at 500 °C was observed for coatings with a concentration of elements of Si≽ (7-8) at.%, Ti ≽ 22 at.% and Zr ⩽ 70 at.%.