Comparative Isothermal Oxidation Behaviour of New Aluminide Coatings from Slurries Containing Al Particles and Conventional Out-of-Pack Aluminide Coatings

This paper investigates the microstructural differences of an out-of-pack (SVPA) aluminide coating with a TBC/aluminide coating deposited by slurry (PARTICOAT) onto Ren, N5 superalloy material. Their isothermal oxidation behaviour in air at 1,100 A degrees C up to 1,000 h is compared with the support of Thermocalc(A (R)) modelling. Whereas the kinetics appears lower in the SVPA than in the slurry for short oxidation term (< 100 h), the mass gains are similar after long exposure time (1,000 h). Both coatings develop a NiAl2O4/Al2O3 duplex scale, the slurry showing a thicker spinel layer than the SVPA. Interdiffusion brings about the transformation of beta-NiAl into gamma'-Ni3Al in both systems which extends more in the slurry coatings, thereby inducing coarser precipitation of refractory elements. However, in contrast to the out-of-pack coatings, both the precipitates and the top coat of alumina hollow spheres developed in the slurry coatings significantly limit rumpling of the oxide scale.