Effect of ion bombardment on properties of hard reactively sputtered Ti(Fe)Nx films

This article reports on results of a systematic investigation of properties of hard Ti(Fe)Nx films reactively sputtered using a d.c. unbalanced magnetron. The Ti(Fe)Nx films with a low (≤15 at.%) Fe content were selected as a typical single-phase material to investigate an effect of the energy Epi, delivered to them during their growth by bombarding ions, on their physical and mechanical properties. In this investigation, the energy Epi per deposited volume was varied by the magnitude of a deposition rate aD because Epi[J/cm3]=Usis/aD, where Us is the substrate bias, is is the substrate ion current density and aD is the film deposition rate. It was found that: (i) properties of sputtered films are a result of a combined action of physical and chemical processes controlled by the energy Epi and the film stoichiometry x=N/(Ti+Fe), respectively; (ii) Ti(Fe)Nx films can form a superhard material with hardness H≥40 GPa; and (iii) superhard films with the highest hardness are: (a) formed in a transition region; (b) nearly stoichiometric with x≈1; and (c) composed of a mixture of grains of different crystallographic orientations. The last finding makes it possible to explain the origin of the superhardness of single-phase materials. A special attention is devoted to mechanical properties of Ti(Fe)Nx films, particularly to relationships between hardness H, Youngʹs modulus E, elastic recovery We and the ratio H3/E∗2, which is proportional to a resistance of the material to plastic deformation, but also to dependences of these mechanical properties on energy Epi, deposition rate aD, average size Lc of grains and microstrain Eg generated in the film during its growth; here E*=E/(1−ν2) is the effective Youngʹs modulus and ν is the Poissonʹs ratio. Correlations between mechanical properties and modes of their sputtering are discussed in detail.