The fine structure of Ni-La_2O_3 electrodeposited composite + aluminum diffusional coating on M38 superalloy was studied by high resolution electron micrography (HREM) observation firstly after oxidation at 1273K for 25hrs. The tiny La_2O_3 particles distributed in the grain boundary of β-NiAl coating prior to oxidation. After oxidation, the dispersived or agglomerated La_2O_3 particles (10～50nm in diameter) was incorporated into Al_2O_3 scale/β-NiAl coating matrix interface. Moreover, compared to the Ni-plating and aluminizing coating, β-NiAl coating with La_2O_3 dispersoids has a slower scale-thinken rate. SEM images showed that there was an obvious difference in surficial morphology of oxide scale between the coatings with and without La_2O_3 particles. The former surface distributed dense and small oxide grains, and the latter had oxide blocks with winskers. It is belived that the growth mechanism of the scale has a strong relationship with its microstructure. During the oxidation process of β-NiAl-La_2O_3 composite coating. La_2O_3 particles (<50nm in diameter) in NiAl phases can migrate to the interface between the scale and the coating and resulted in a rapid increase in quantity of these tiny particles there. An amount of the above-mentioned La_2O_3 dispersoids inserted into Al_2O_3 scales may inhibit the diffusion of major ionic species in the scales, which is benifical to reduce the growth rate of the scale and modify the scale adhension. The intermediate may help to increase the interfacial stability. The detailed discussion was given about these explanations above.
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