Electron Microscopy Characterization of the High Temperature Degradation of the Aluminide Layer on Turbine Blades Made of a Nickel Superalloy

Abstract

The effects of exposure to overheating (temperature above 1000 °C) on the degradation (modification) of layers of coatings (coatings based on aluminum) of uncooled polycrystalline rotor blades of aircraft turbine jet engines were investigated under laboratory conditions. In order to determine the nature of the changes as well as the structural changes in the various zones, a multi-factor analysis of the layers of the coating, including the observation of the surface of the blades, using, among others, electron microscopy, structural tests, surface morphology, and chemical composition testing, was carried out. As a result of the possibility of strengthening the physical foundations of the non-destructive testing of blades, the undertaken research mainly focused on the characteristics of the changes occurring in the outermost layers of the coatings. The obtained results indicate the structural degradation of the coatings, particularly the unfavorable changes, become visible after heating to 1050 °C. The main, strongly interacting, negative phenomena include pore formation, external diffusion of Fe and Cr to the surface, and the formation and subsequent thickening of Fe-Cr particles on the surface of the alumina layer.

Investigation of the Relationship between Degradation of the Coating of Gas Turbine Blades and Its Surface Color

“…They are distinguished by their efficiency, which depends on the temperature of exhaust gases at the turbine inlet. It should be noted that over time, excessive temperature concerning operating temperature can lead to the degradation of the blade's structure and thus to turbine failure [1][2][3][4][5][6]. One external symptom of blade degradation is a change of its surface color [7][8][9] (Figure 1a,b), which is a diagnostic signal indicating alteration of its technical condition.…”