Characterization of magnetron sputtered sub-stoichiometric CrAlSiNx and CrAlSiOyNx coatings

The influence of varying nitrogen and oxygen partial pressures on microstructure, mechanical and optical properties of magnetron sputtered CrAlSiNx and CrAlSiOyNx coatings has been studied. The partial pressure of nitrogen reactive gas was varied from 0.037 Pa to 0.15 Pa for CrAlSiNx films, and the N2/O2 (85%:15%) partial pressure was varied from 0.046 Pa to 0.21 Pa for CrAlSiOyNx layers. Transmittance and reflectance of samples were measured and were modeled to obtain the spectral optical constants, n and k. Chemical state, composition, morphology and microstructure of films were analyzed by XPS, RBS, XRD, Raman Spectroscopy and SEM. Films' hardness was evaluated using nanoindentation method. XRD results revealed that the two samples CrAlSiNx with PN = 0.15 Pa and CrAlSiOyNx with PNO = 0.21 Pa are polycrystalline with cubic (fcc-B1) structure. On contrary, all other films prepared with lower reactive gases partial pressures are amorphous. The chemical composition changed with the variation of reactive gases partial pressure, although the Cr: Al: Si composition ratio remained approximately constant, 1.25:1.5:1. All samples showed low hardness, mainly due to lower content of reactive gases and higher content of Si. However, the sample CrAlSiNx with PN = 0.15 Pa has the highest value of 11.1 GPa. Optical constants are seen to be very sensitive to reactive gases partial pressure. The refractive index and extinction coefficient were lower for coatings with higher reactive gases partial pressure. These coatings are good candidates for designing selective solar absorber stacks for different applications.