X-ray absorption spectroscopy study of the effects of Si addition on the properties of hard nitride coatings

Nanostructured ceramic coatings are being intensively investigated because they open a wide field of new materials with improved mechanical and tribological properties. The final behaviour of such coatings depends on their composition and microstructure. Si addition in nitride compounds has shown to improve significantly their hardness. For instance, in the TiSi-N coating system, coating hardness ranging from 40 to 80 GPa has been reported in the composition range of 2-20 at.% Si. To explain this improvement, a detailed investigation on composition, structure, and atomic short-range order surrounding Ti and Si atoms is needed. X-ray absortion spectroscopy (XAS) is a very appropriate tool for this kind of studies since it is very sensitive to the local environment around a specific atomic element, giving information on the composition, structure, and short range symmetry In this work, several TiSiN and AlCrSiN coating compounds with different Si content have been studied by XAS in order to investigate the effects of Si addition on the composition, structure and phase development of these coatings. AlCrSiN thin films have been obtained by cathodic arc evaporation, whereas TiSiN coatings have been grown by magnetron sputtering. Changes in the structure and chemistry of the Si-containing coatings as compared with those without Si can influence the mechanical properties, grain size and morphology, defect content, film porosity or phase content of this films. To get some insight on these changes, X-ray absorption spectra have been measured at the Si, Cr, and Ti K-edges in fluorescence yield mode. XAS measurements were carried out at the BESSY KMC1 line, using a standard chamber equiped with a solid state fluorescence detector. Figure 1(a) shows XAS spectra at the Ti K-edge of TiSiN samples with different Si content. In this case, all spectra keep a similar lineshape to that of titanium nitride, independently of Si content. The pre-edge region, which is associated to transitions from Ti1s states to Ti-3d states hybridized with Si-2p states, does not show significant changes with Si-addition. This suggests a weak interaction between Ti and Si. Fig. 1(b) shows XAS spectra at the Si K-edge of the same TiSiN samples, as well as a SiO2 single crystal, as a reference sample for comparison. In all cases the spectral lineshape is typical of amorphous Si3N4 [1], with a main peak at 1845 eV, and a weaker structure at 1861 eV. The main peak of SiO2 is located at 1847, so we can not exclude the presence of SiO2 for very low Si content. Except for this small oxygen presence no significant changes in the lineshape is observed with silicon content. Hence, both Ti-K and Si-K XAS results suggest that the interaction between Ti and Si is weak and that amorphous Si3N4 aggregates are formed inside a TiN matrix. Fig. 2(a) shows XAS spectra at the Cr K-edge of AlCrSiN with different Si content. Similarly to the TiSiN samples, the pre-edge region, at 5960 eV, is associated with transitions from Cr-1s states to Cr-3d states hybridized with 2p empty states of the ligand atoms. This hybridization allows 1s-3d transitions, which are otherwise dipole forbidden. The spectrum for the AlCrN sample shows only a tiny pre-edge peak at this position, very similar to that reported in previous works for Cr2O3 [2-4], suggesting that Cr in this compound is in the form of Cr. The intensity of this peak slightly increases for sample with a Si content of 5%. For samples with even higher Si content, the intensity increase is still more predominant. The increase of this feature could be explained in the case of oxide compounds in terms of a valence increase from +3 to +4, +5 or +6, or in terms of a reduction of crystalline domains down to the nanoscale [12]. The presence of high valence Cr would involve a shift to higher energies of the main absorption edge, which is not observed in our samples, so the presence of