Superhard nanocomposite Ti–Al–Si–N films deposited by reactive unbalanced magnetron sputtering

Abstract

Nanocomposite Ti–Al–Si–N films (Al content ranging from 0 to 16.7 at.% and Si from 0 to 11.8 at.%, respectively) were prepared on Si(1 0 0) substrates at 500 °C by reactive close-field unbalanced magnetron sputtering in an Ar–N 2 mixture. The chemical composition, bonding structure, surface morphology, microstructure, stress and mechanical properties of these films were systematically investigated by means of energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD), optical interference system and nanoindentation measurements. The XPS measurements reveal evolution of chemical composition and bonding structure of the deposited Ti–Al–Si–N films. The observed nonlinear peak shift of Ti 2p and N 1s indicates the bonding evolution of crystalline TiN. The spectra from Al 2p at 73.7 eV and Si 2p at 101.1 eV indicates the evolution of film composition from TiSiN to TiAlN with the incorporation of Al, and also implies that their chemical states are mainly in the form of SiN x and AlN, respectively. The (1 1 1) diffraction peak from the XRD θ–2 θ patterns shows a variation either on peak width or on peak position, indicating the variation of grain size and lattice constant. The calculated lattice constant reveals a mixture of different phases whose structures are similar to TiN. The nonlinear variation of grain size and lattice constant is considered due to the competition between two processes, i.e. Al addition and the underlying Si reduction. In order to further understand the microstructure evolution, the XRD spectra and corresponding calculations of the Ti–Al–Si–N films without heating during deposition are also shown for comparison. The effect of substrate heating is discussed. The present results show that the influence of Si and Al addition on the grain growth is different. The surface roughness of Ti–Al–Si–N films also exhibits a nonlinear variation, which is due to the variation of grain size and the competitive growth of different phases. The influence of stress on the hardness can be ignored due to its relatively low value (1–2 GPa) compared with the super-hardness of the films. All the quaternary Ti–Al–Si–N films show enhanced mechanical properties when compared with the nanocomposite Ti–Si–N and Ti–Al–N films deposited under the same condition. The best hardness, reaching superhard, is obtained for the Ti–Al–Si–N film with 7.5 at.% Al and 6 at.% Si. The combined effect of vacancy hardening and some of extrinsic hardening are considered responsible for the enhanced mechanical properties of Ti–Al–Si–N films.