Effect of Pack Composition on the Characteristic and Wear Behaviors of Fe-Al Intermetallics Grown on AISI 304 Stainless Steel

The diffusional powder-pack aluminizing method stands as one of the well-established techniques to enhance the high-temperature oxidation resistance of stainless steels through the formation of Fe-Al coatings. However, it has been observed that the detailed impact of the costly pure Al powder on the characteristics of intermetallic coatings (coating thickness, hardness, fracture toughness, phase structure, grain size, and microstrain) has not been thoroughly investigated, particularly regarding the optimal proportions of Al powder in coating formation. This study is dedicated to the characterization of aluminide coatings formed on AISI 304 stainless steel surfaces using wt.%5-20 Al powders. The obtained aluminide coating layers were examined through scanning electron microscopy, energy-dispersive spectroscopy, x-ray diffraction spectroscopy, and Vickers microhardness tests. The microstructure, phase composition, microhardness, fracture toughness, grain size, and microstrain conditions of the coatings were evaluated based on the experimental findings. Characterization studies revealed a significant influence of increasing Al ratios on coating thickness and fracture toughness values, while the effect on the phase structure and microhardness of Fe-Al coatings was found to be less pronounced. In aluminide coatings created using 5% Al to 20% Al, Fe₂Al₅ was identified as the dominant phase across all coatings. Despite the similarity in hardness values (1168 ± 142- 316 ± 174 HV_0.1) for these coatings, fracture toughness values increased from 0.65 to 2.45 MPa.m^1/2 with the increase in Al ratios and coating thicknesses. This is attributed to the increase in average crystallite size in Fe-Al coatings with the increase in Al ratios and the consequent reduction in microstrain values of the Fe-Al coatings. Fe-Al coatings exhibited superior wear resistance compared to untreated AISI steel at both room temperature and 500 °C. The increase in the Al content used in the formation of Fe Al coatings allowed for thicker and harder FeAl coatings, which positively impacted mechanical properties.