Magnetron sputtered TiAlON composite thin films. I. Structure and morphology

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Abstract
Composite TiAlON thin films were deposited by rf magnetron sputtering from TiN–Al2O3 ceramic targets. Samples deposited under conditions that minimize the adatom mobility are attractive as decorative coatings due to their lustrous, black color and their wear resistance. The critical relationships between the processing parameters and the resulting microstructural features were investigated. Coatings with thicknesses ranging from 0.05 to 5 μm were characterized by x-ray and electron diffraction, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Thus, both the small scale ‘‘nanostructure’’ and the larger scale ‘‘microstructure’’ are described. It is shown that the observed features fit in the evolutionary model of microstructures. For ‘‘standard’’ conditions, the nanostructure consisted of crystallized grains (up to 10 nm in size) of a mixed nitride (Ti,Al)NX embedded in an amorphous mixed oxide (Ti,Al)O2. These two phases formed nanocolumns with a base diameter of 10 nm, separated by voided regions for very thin films (50 nm). These small scale features merged into microcolumns with a lateral size of 0.1 μm in films thicker than 1 μm. The low mobility regime produced important intercolumnar porosity. Increased adatom mobility induced by low energy particle bombardment affected both structural levels. The lattice parameter of the (Ti,Al)NX grains relaxed to 0.417 nm, value in the reported range for single phase metastable (Ti0.5Al0.5)N coatings. The bombardment interrupted the columnar growth and yielded a denser material. If in addition, the deposition conditions increased the energy of ejected species (for low argon pressures), segregation of metallic aluminium was observed together with the (Ti,Al)N grains. The microstructure became totally featureless up to a thickness of 1.8 μm.

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