Low-stress superhard TiB films prepared by magnetron sputtering

The article reports on structure and mechanical properties of TiB alloy films sputter deposited from a sintered TiB2 target using an unbalanced dc magnetron. We present results of a systematic investigation of the effect of negative substrate bias, Us, substrate ion current density is, and substrate temperature, Ts, on properties of TiB films. The X-ray diffraction (XRD) analysis shows that the TiB films consist of the hexagonal TiB2 phase with the typical (0001) texture only. The TiBx films are over-stoichiometric with the ratio x=B/Ti≈2.4. All TiB films sputter ion plated in argon magnetron discharge are superhard films with hardness H>40 GPa and exhibit high values of (i) effective Youngʹs modulus E*=E/(1−ν2) up to approximately 600 GPa and (ii) elastic recovery, We, up to approximately 82%; here E and ν are the Youngʹs modulus and the Poissonʹs ratio, respectively. Besides, it was found that the value of the Braggʹs angle 2θ of the (0001) reflection line can be easily controlled by the energy delivered to the film during its growth by (1) the substrate heating Ts and (2) ion bombardment (Us, is). The angle 2θ of the (0001) reflection increases with increasing Ts from 300 to 550 °C and decreasing Us from −150 to −50 V. In this range of process parameters, the energy Ep delivered to the growing film per condensing atom by ion bombardment can be adjusted to a value, at which the (0001) reflection from sputtered films is close to that of the TiB2(0001) powder standard. These films exhibit a low macrostress, which approaches to zero. It enables to sputter thick (up to 8 μm) superhard (H>40 GPa) TiB films. The optimum value of Ep is achieved when the TiB film is sputtered at Us=−50 V, is=1 mA/cm2, Ts=550 °C with a deposition rate aD=52 nm/min. The TiB film prepared under these conditions exhibits a maximum hardness of H≈77 GPa, measured using a computer controlled microhardness tester Fischerscope H100 at the Vickers diamond indenter load L=50 mN.