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  • 1
    ISSN: 1432-1130
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The suitability of PVD films of γ-Al2O3 and of ternary Al-O-N as diffusion barriers between a nickel based superalloy CMSX-4 and NiCoCrAlY for a possible application in gas turbines was investigated. Therefore, an Al2O3 film and, alternatively, an Al-O-N film were deposited on CMSX-4 at 100 °C substrate temperature by means of reactive magnetron sputtering ion plating (MSIP). After characterization of composition and structure of the films by X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (XRD), a NiCoCrAlY coating was deposited onto the diffusion barriers and, for comparison, directly onto CMSX-4 by MSIP as well. The composites were annealed for 4 h at 1100 °C under inert atmosphere. Wavelength dispersive X-ray (WDX) element mappings and line-scans of the cross-sectional cut served to evaluate the suitability of the films as diffusion barriers. After detachment of the coatings from the substrate, the phase stabilities of the two metastable phases γ-Al2O3 and Al-O-N were determined by means of grazing incidence XRD. Without a diffusion barrier, enhanced interdiffusion was observed. Analyses of the composite with the γ-Al2O3 interlayer revealed diffusion of Ti and Ta from the substrate into the NiCoCrAlY coating. No interdiffusion of Ni, Ti, Ta, and Cr could be detected in case of the ternary Al-O-N film. Whereas the ternary Al-O-N film remained in the as-deposited X-ray amorphous structure after annealing, a phase change from the γ to the α modification could be observed in case of the Al2O3 film, presumably responsible for its lower efficiency as a diffusion barrier.
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  • 2
    ISSN: 1432-1130
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The oxidation behavior of cubic Ti1-xAlxN films was improved by decreasing the Ti/Al ratio from 50/50 in the direction of the phase transition between cubic and hexagonal structure. Metastable, polycrystalline, single-phase Ti1-xAlxN films were deposited on high speed steel (HSS) substrates by reactive magnetron sputtering ion plating (MSIP). The composition of the bulk was determined by electron probe microanalysis (EPMA), the crystallographic structure by thin film X-ray diffraction (XRD). A Ti1-xAlxN film with a Ti/Al atomic ratio of 38/62 was deposited in cubic NaCl structure, whereas a further decrease of the Ti/Al ratio down to 27/73 led to a two-phase film with both cubic and hexagonal constituents. The Ti0.38Al0.62N film was oxidized in synthetic air for 1 h at 800 °C. The oxidic overlayer was analyzed by X-ray photoelectron spectroscopy (XPS) sputter depth profiling, EPMA crater edge linescan analysis, and secondary neutrals mass spectroscopy (SNMS). Scanning electron microscopy (SEM) micrographs of the cross sectional fracture were taken for morphological examination. With higher Ti content, the Ti1-xAlxN formed a TiO2-x rich sublayer beneath an Al2O3 rich toplayer, whereas the oxide layer on the Ti0.38Al0.62N film consisted of pure Al2O3. The thickness of the oxide layer was determined to 60–80 nm, about a quarter of the oxide layer thickness detected on Ti0.5Al0.5N films. The absence of a TiO2-x sublayer was also confirmed by XRD. The results show a distinct improvement of the oxidation resistance of cubic Ti1-xAlxN films by increasing the Al content from x = 0.5 to 0.62, whereas a further increase leads to the hexagonal structure, which is less suitable for tribological applications due to its tendency to form cracks during oxidation.
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