Morphology, microstructure evolution and optical properties of Al-Si-N nanocomposite coatings

The microstructure evolution of Al-Si-N coatings with increasing Si content is described based on data from a multi-technique approach with data from X-ray diffraction, SEM, XPS and UV-vis. At Si contents below about 6 at.% Si the coatings form solid solutions of Si in wurtzite (w-) AlN, while at Si concentrations between 6 and 15 at.% Si nanocomposites consisting of a Si-saturated nanocrystalline w-Al-Si-N phase exist with Si3N4 as a tissue phase. The tissue phase formation already starts at 4 at.% Si and is completed at 7 at.%. Values of the refractive index and extinction coefficient as well as data from X-ray diffraction for concentrations up to 12 atomic % (at.%) of Si are consistent with the assumption of an epitaxial Si3N4 tissue phase layer of one monolayer thickness, which separates the w-Al-Si-N nanocrystals from each other in nanocomposites. Silicon contents above ca. 12 at.% lead to the formation of a Si3N4 tissue phase with properties similar to amorphous Si3N4. A quantitative model is presented that unifies the experimental evidences into a consistent image of the microstructure at different silicon contents.