The addition of zirconium to aluminide coatings: The effect of the aluminide growth mode

Alumina-former coatings such as nickel aluminides have been the essential approach to combating the aggressive service environment of gas turbine hot section components. The positive effects of zirconium on the cyclic oxidation lifetime of aluminide coatings have been reported. The current paper has reported the effect of the growth mode of the aluminide coating on the microstructure of the Zr-modified aluminide coating. The aluminization process was carried out using the halide activated pack cementation process in the powder mixtures of aluminum, ammonium chloride, zirconium oxide, and alumina at 760 and 1040 degrees C for the inward and outward growth, respectively. The microstructures of the coatings were studied using FESEM, EDS and XRD. The results showed that the outward coatings contain a high level of Zr mainly incorporating it in the second phases such as AlNi2Zr in a matrix of NiAl. In contrast, the Zr concentration in the inwardly grown aluminide coatings did not exceed the 1 atomic percentage and the typical microstructure of the high activity aluminide coatings was observed. The chemical reactions involved in the pack cementation process and the mechanism of the coating formation were discussed.