Oxidation Behavior of Si-Modified Aluminide Coatings on K438 Superalloy Prepared Using a Hybrid Slurry/Pack Cementation Process

A Si-modified aluminide coating was prepared on nickel-based K438 superalloy using a hybrid slurry/pack cementation process. The microstructures indicated that the coatings exhibited a two-layer structure: the outer Al-rich Ni0.9Al1.1 layer with the dispersion of minor Ni2Al3 and Cr-rich phases of Al13Cr2 and Cr5Si3 and the inner interdiffusion layer. For comparison, simple aluminide coatings were also produced using normal pack cementation. Compared with the K438 superalloy and simple aluminide counterpart, the Si-modified aluminide coatings exhibited a lower oxidation rate and improved alumina scale adhesion in air at 1,100°C. Furthermore, the Si-modified aluminide coatings showed longer service life compared to the simple aluminide coatings because the addition of Si mitigated the degradation of coatings. Therefore, the simple aluminide lost protection while the Si-modified aluminide coatings were still protective after 300 h.