Si-modified NiAl coating has been studied by researchers due to its good performance in high temperature corrosive environment, but the effect of Si content on the microstructure and high temperature oxidation resistance of the aluminide coatings has been seldom studied systematically. To figure out this issue, three modified NiAl coatings with different Si contents were prepared by varying the Si/(Si+Al) ratios eg. 8 wt.%, 17 wt.% and 30 wt.% in the slurry. Phase structures and microstructures of the simple NiAl coatings and three Si-modified NiAl coatings before and after oxidation were analyzed using XRD, SEM and TEM etc. Results show that the δ-Ni2Al3 and β-NiAl are the primary phases of the four aluminide coatings. Si mainly locates in the upper zone of the Si-modified NiAl coatings in the form of slilicides as CrSi2, Cr5Si3 and Ni2Si. With increasing Si/(Si+Al) ratio, the Si content increases and the Al content decreases. After oxidation at 1000 °C for 500 h, the simple NiAl coating has the maximum weight gain of 1.93 mg/cm2, while the Si-modified NiAl coating with Si/(Si+Al) ratio of 8 wt.% has the smallest weight gain. The dopant of Si into NiAl coatings can promote the formation of protective α-Al2O3 film, retard the outward diffusion of refractory metal elements and improve the surface quality of the oxide film, resulting in reduced oxidation rate. However, the addition of Si reduces the Al content on the surface layer of the coating and may make the Al reservoir β phase insufficient to maintain the selective oxidation of Al in the later period of long-time oxidation. Therefore, when the Si/(Si+Al) ratio is 8 wt.% and the Si content is 9.0 at.%, the Si modified NiAl coating possesses the best oxidation resistance.
