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

This article reports on results of a systematic investigation of properties of hard Ti(Fe)N_x films reactively sputtered using a d.c. unbalanced magnetron. The Ti(Fe)N_x films with a low (≤15 at.%) Fe content were selected as a typical single-phase material to investigate an effect of the energy E_pi, delivered to them during their growth by bombarding ions, on their physical and mechanical properties. In this investigation, the energy E_pi per deposited volume was varied by the magnitude of a deposition rate a_D because E_pi[J/cm³]= U_si_s/ a_D, where U_s is the substrate bias, i_s is the substrate ion current density and a_D 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 E_pi and the film stoichiometry x =N/(Ti+Fe), respectively; (ii) Ti(Fe)N_x 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)N_x films, particularly to relationships between hardness H , Young's modulus E , elastic recovery W_e 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 E_pi, deposition rate a_D, average size L_c of grains and microstrain E_g generated in the film during its growth; here E^*= E /(1- ν ²) is the effective Young's modulus and ν is the Poisson's ratio. Correlations between mechanical properties and modes of their sputtering are discussed in detail.