The approaches to thin film preparation and TEM observations on slurry Si‐modified aluminide coatings

Abstract

Transmission electron microscopy (TEM) can be used as a precision characterization tool to identify very small precipitates in diffusion aluminide coatings. However, in order to successfully prepare the appropriate samples for TEM observation, often non-traditional thin film preparation techniques need to be employed. In this work, two sample preparation methods of twin jet electro-polishing and ion milling were experienced to characterize fine precipitates (< 1 lm), in Si-aluminide coatings applied on Ni-base superalloy In-738LC by slurry technique. These precipitates are concentrated throughout the topcoat zone. It was found that the preparation of thin film exactly from the outer zone of the coating is only possible using ion milling process. The ion-milled specimens were utilized to observe by JEOL high resolution TEM operating at an accelerating voltage of 300 kV. Electron diffraction patterns, bright field and EDS were used to identify the precipitate phases as well as the coating matrix. The results showed that the fine precipitates are typically chromium silicides in nature, mostly as Cr 3 Si and CrSi, distributed in the b-NiAl matrix phase.

Microstructure and Oxidation Resistance of a Si Doped Platinum Modified Aluminide Coating Deposited on a Single Crystal Superalloy

“…It can be seen that the Si content in the outer layer is lower than that in the inner layer. As mentioned above, the average Si content in the outer layer is about 0.7 at.%, which is much lower than its solubility in NiAl phase (2-4 at.%) [23], revealing that Si is dissolved in the (Ni, Pt)Al phase, which is consistent with the XRD pattern ( Figure 1) where no silicide is detected. The dissolution of Si is beneficial for retarding the formation of the metastable phase PtAl 2 , which might be attributed to the fact that Si has a very low diffusion rate in the PtAl 2 phase [17] and the deposition of Si might hinder the nucleation of the PtAl 2 phase.…”

Internal surface protection of gas turbine blade by Si‐aluminide coating

“…From the viewpoint of the microstructure, the silicon-enriched nickel aluminide coating formed on GTD-111 superalloy was found to be consistent with previous work, which dealt with the same coating on In-738 superalloy [6]. Accordingly, it can be predicated that the fine particles precipitated in the top coat of the Si-AL coating are silicides (mostly as chromium silicides), as has been indicated in previous publication [8].…”

“…Among potential coating modifying elements, Si has been demonstrated by several researchers, including the senior author, to be effective in improving the oxidation and Type II hot corrosion performance of the aluminide coatings [3][4][5][6][7][8][9]. Our previous work [6] demonstrated that Si may be added easily into the aluminide structure through a slurry silicoaluminizing technique.…”

The effect of aluminide coating on the steam oxidation behavior of SS321 steel at 700 °C

“…Among the coating techniques, aluminide diffusion coatings are widely used in the aerospace industry to coat turbine blades, which are mainly made of nickel-based superalloys, as well as superheater tubes, especially the inner part [9]. One of the most important reasons for using the aluminide coating method is its good resistance against oxidation, hot corrosion, and, in some cases, wear; its strong bond with the substrate; its sufficient resistance against thermal and mechanical stresses; its diffusion stability; its good flexibility; its small and uniform thickness; its lack of local destruction; and its favorable surface quality in terms of aerodynamics, as well as the practicality of the process and its economy [10]. In recent years, many studies have been published on the potential of porous coatings for power plants and their services to enhance corrosion resistance.…”