Based on the success of metallic nanocomposite hard coatings, we have developed new nanostructured thin films based on the Al-Si-N system combining similar wear resistance with excellent transparency in the visible range of light. The films were deposited by reactive DC magnetron co-sputtering of Al and Si targets in Ar/N₂ at 200°C. They were characterized by XRD, XPS, TEM, UV-Vis spectrometry and nanoindentation. Chemical compositions from pure AlN to Al-Si-N with 30 at.% of Si were investigated. Under conditions of sufficient nitridation, an average optical transparency approaching 100% was obtained. The hardness of these films shows a weak maximum around 25 GPa and a resilience increased by about 60% at silicon concentrations of 12-15 at.%. However, in contrast to known silicon-containing ternary nitrides, this material shows no phase segregation during deposition into nc-AlN/a-SiN_x, as would be expected from thermodynamical data. XRD analysis proves the presence of h-AlN (wurtzite), with a decreasing lattice parameter and a texture change as silicon is introduced into pure AlN. This suggests the formation of a metastable Al_1-xSi_xN solid solution, formed by the substitution of Si atoms on Al sites in the AlN crystalline lattice. TEM images reveal a columnar growth throughout the entire film thickness of about 1 μm. Each column is composed of elongated nanocrystallites whose length decreases down to 25 nm with addition of silicon into AlN. The films are stable upon annealing at 800°C under N₂ for 2h. Above 13 at.% of Si, the coatings are X-ray amorphous, which may be interpreted as the solubility limit of Si in AlN. The observed properties make this material promising for wear protection applications on a variety of surfaces which require optical transparency.

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