Effects of N doping on mechanical properties of Fe and interlayer adhesion of Fe with Cr, Ti, and Si: A spin-polarized density functional theory study

Interlayers between a substrate and a functional film are used extensively to tailor film adhesion and avoid failure induced by delamination. With nitrogen doping increasingly used to improve the surface properties of steel, how to select appropriate interlayers for high film adhesion on substrate remains elusive in theory. Using spin-polarized density functional theory computations, the authors investigated the effects of N doping on the mechanical properties of Fe and its adhesion with widely used Cr, Ti, and Si interlayers. The results showed that N atoms doped at the Fe octahedron center increased hardness by 30% under 4 wt. % N. N atom at Cr/Fe, Ti/Fe, Si/Fe interfaces interestingly increased the interlayer adhesion of Ti/Fe and Si/Fe, while reducing that of Cr/Fe. This behavior is attributed to competition between magnetic coupling and atomic bonding. For Cr/Fe with strong magnetic coupling, the reduction in adhesion was resulted from doped N that inhibited magnetic coupling at the interface. For Ti/Fe and Si/Fe with weak magnetic coupling, the formation of covalent bonds at the interface by the N atom increased interfacial adhesion. This study provides insights into the nitrogen doping of steel and the selection of proper interlayers for strong film-substrate adhesion.