Microstructure and mechanical properties of Al–Si–N transparent hard coatings deposited by magnetron sputtering

Motivated by the success of transition metal nanocomposite hard coatings, a new optically transparent coating based on the Al–Si–N ternary system was developed. Al–Si–N thin films were deposited by reactive DC magnetron co-sputtering of Al and Si targets in an Ar/N2 atmosphere at 200 °C and 500 °C sample temperature. The structure and mechanical properties of the coatings were investigated by XPS, XRD, TEM, nanoindentation and mechanical profilometry. The chemical composition was varied from pure AlN to Al–Si–N with 23 at.% of Si. The films are crystalline with the hexagonal AlN structure up to 12–16 at.% of Si as found from XRD and TEM analyses. A shift of X-ray diffraction peaks indicates a substitutional incorporation of silicon in the h-AlN lattice up to a solubility limit identified at 6 at.% of Si. By further increasing the silicon content, a nanocomposite nc-Al0.44Si0.06N0.5/a-SiNx is formed. From TEM analysis it follows that the crystalline material consists of columnar grains composed of crystallites with a (002) texture. The mean crystallite size decreases from 60 nm to about 5 nm upon addition of silicon, as revealed by XRD. Hardness measurements show a diffuse hardness maximum exceeding 30 GPa around 10 at.% of Si. At this value a negligible residual stress in the coating is measured.