| The characteristics and oxidation behavior of TiAlCr(N) coatings were investigated as a function of nitrogen content in the coatings. Eight coating samples were sputter deposited on silica wafers with different discharge gas mixtures of N2 and Ar where the fraction of N2 ranged from 0 to 0.25. Characterization of the coatings employed x-ray diffraction (LRD), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy/selected area diffraction (TEM/SAD), and coating stress analysis via wafer curvature techniques. Coating nitrogen content was found to increase linearly with N 2 discharge gas fraction up to a limit of saturation, above which coating nitrogen content remained constant. The intrinsic compressive stresses of the coatings increased with N2 discharge gas fraction due to increased surface bombardment during deposition. Coatings below the nitrogen saturation limit were found to be amorphous, while coatings above the saturation limit were polycrystalline. Nitrogen in the amorphous coatings formed bonds exclusively with aluminum, while nitrogen bonding in the polycrystalline coatings included all three metals (Ti, Cr, and Al). The polycrystalline coatings consisted of primarily Ti1−xAlxN and Cr1−yAl yN phases. Stress relaxation was more pronounced in the amorphous coatings than in the polycrystalline coatings.; Four coatings compositions were selected for the oxidation studies: one without nitrogen, two amorphous coatings, and one polycrystalline coating. These samples were oxidized in air at 800 and 1000°C for one hour. Digital photographs and optical micrographs of the oxidized coatings showed very little physical coating degradation in the amorphous coatings at either temperature. The coatings without nitrogen degraded significantly at both temperatures, while the polycrystalline coating flaked and spalled at 1000°C. The amorphous coatings, however, showed very little resistance to oxidation at either temperature. Beneath the oxidized surface layers formed at both temperatures, the nitrogen-less coatings contained γ-TiAl, and the polycrystalline coatings indicated the presence of TiN, both of which imply a degree of oxidation resistance. |
