SLURRY ALUMINIZATION MODIFIED WITH REACTIVE ELEMENTS

Diffusion aluminide coatings are widely employed to increase oxidation resistance of superalloy components. However, at temperatures above 1050°C, especially under cyclic conditions, or under severe hot corrosion, these simple aluminide coatings offer limited protection. Thus, from the 1970s onwards, developments in diffusion coating technologies included modification of the aluminide coatings. Modified aluminides have been fabricated by additions of chromium, silicon, platinum, and most recently, various reactive element additions, including Hf, Zr, Ce, and Y. This work presents diffusion aluminide coatings modified with the addition of reactive elements such as Si and Zr and a mixture of Si-Zr. Material and Methods Diffusion aluminide coatings were obtained by slurry approach. The composition of the slurry includes a binder-gel, activating salts (NH4Cl and AlF3), and Cr-Al (55-45w%) particles with an average size of 45um. The slurry obtained was modified by a dual approach: by directly adding metal particles of the modifying element to the mixture or a fluoride salt of that element. Diffusion heat treatments were performed in a tubular furnace in an inert atmosphere while isothermal oxidation heat treatments were conducted in an air atmosphere. The coatings obtained and tested in isothermal oxidation were analysed by XRD diffractometry and microstructure characterization by SEM-EDS microscopy. Results The modified aluminide protective coatings by the slurry method were successfully formed on GTD-111 superalloy. Three families of coatings were obtained: aluminides modified by addition of Zr, by addition of Si and by addition of a Si-Zr mixture. Parabolic isothermal oxidation curves of standard aluminides were obtained and compared with the same results for the oxidation of Si- and Zr-modified aluminides. Discussion Zr is known as an element which is helpful to enhance alumina scale adhesion, a phenomenon which was attributed to a change of the oxide microstructure due to incorporation of Zr into the oxide scale. This effect of Zr was shown to occur at the interface between alumina scale and several alloys: NiAl and NiCrAl. The present investigation attempts to highlight the effect of Zr addition into aluminide coatings on isothermal oxidation behaviour to increase lifetime of the coating through adhesion enhancement between the alumina scale and substrate. Additions of silicon are also known to improve hot corrosion resistance, but may also benefit the high temperature oxidation performance of aluminide coatings. The addition of Zr and Si can be alternatives to platinum-modified aluminides as low-cost solution for providing environmental protection.