Thermal stability of Al-O-N PVD diffusion barriers

The use of Al-O-N films as a diffusion barrier for components with a high thermal load, e.g. in gas turbines, was investigated. Therefore, films with compositions along the quasibinary section Al2O3-AlN were deposited onto Ni base superalloy (CMSX4) substrates by means of magnetron sputtering ion plating (MSIP) at 373 K substrate temperature and characterized with regard to their composition and structure using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (XRD). The phase stability of the films was examined by annealing under inert atmosphere at temperatures up to 1473 K for 4 h and subsequent XRD analyses. To investigate a possible application of these films as a diffusion barrier between Ni base superalloys and MCrAlY, the latter in technical applications serving as corrosion protection for superalloys, a MCrAlY coating was deposited onto selected samples. Without a diffusion barrier and operating temperatures of and above 1373 K, noticeable interdiffusion could be observed. After annealing the CMSX4-Al-O-N-MCrAlY composites for 4 h the interfaces CMSX4-Al-O-N and Al-O-N-MCrAlY were investigated by XRD and energy-dispersive X-ray spectroscopy (EDX). The analyses showed that ternary Al-O-N films were grown in X-ray amorphous structure and remained in that state after annealing the CMSX4-Al-O-N-MCrAlY composites for 4 h at 1373 K. Al2O3 was also deposited in X-ray amorphous structure, but converted into alpha-Al2O3 after annealing. In the case of the Al2O3 interdiffusion between CMSX4 and MCrAlY was observed during annealing, especially evident regarding titanium. In contrast to this, ternary Al-O-N films showed a better performance as a diffusion barrier, e.g. no titanium was detectible in the Al-O-N or MCrAlY film. A possible explanation is that the ternary films remained in their amorphous structure, whereas Al2O3, which at the low substrate temperature supposedly was deposited nanocrystalline in the cubic gamma-Al2O3 structure, converted into the hexagonal or-modification during annealing. This transition is assumed to be responsible for the permeability for interdiffusing elements. (C) 1998 Elsevier Science S.A. All rights reserved.