Abstract
The use of Al–O–N films as diffusion barriers for components with a high thermal load, e.g. in gas turbines, was investigated. Films with compositions along the quasibinary section Al2O3–AlN were deposited onto Ni based superalloy (CMSX–4) substrates by means of magnetron sputtering ion plating at 373 K substrate temperature and characterised with regard to their composition and structure using X-ray photoelectron spectroscopy and grazing incidence XRD. The phase stability of the films was examined by annealing under inert atmosphere at temperatures up to 1473 K for 4 h and by subsequent XRD analyses. To investigate the possible application of these films as a diffusion barrier between Ni based superalloys and MCrAlY, with the latter serving in technical applications as corrosion protection for superalloys, an NiCoCrAlY coating was deposited onto selected specimens. Without a diffusion barrier and operating temperatures of and above 1373 K, noticeable interdiffusion was observed. After annealing the CMSX–4/Al–O–N/NiCoCrAlY composites for 4 h, the CMSX–4/Al–O–N and Al–O–N/NiCoCrAlY interfaces were investigated by XRD and energy dispersive X-ray spectroscopy. The analyses showed that interdiffusion between CMSX–4 and NiCoCrAlY was observed in the case of the Al2O3 during annealing, and was especially evident in titanium. In contrast to this, ternary Al–O–N films showed better performance as diffusion barriers, i.e. no titanium was detectable in the Al–O–N or NiCoCrAlY film. A possible explanation is that the ternary films were grown in an amorphous structure and remained in that state after annealing the CMSX–4/Al–O–N/NiCoCrAlY composites, whereas Al2O3, which at the low substrate temperature was supposedly deposited nanocrystalline in the cubic γ-Al2O3 structure, converted into the hexagonal α modification. This transition is assumed to be responsible for the permeability of interdiffusing elements.
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