MICROSTRUCTURAL EVOLUTION AND HARDNESS PROPERTIES OF Si-MODIFIED ALUMINIDE COATING ON 304 STAINLESS STEEL VIA SLURRY ALUMINIZING: EFFECT OF ALUMINIZING TEMPERATURES

Abstract

Surface modification of austenitic steel with a Si-modified aluminide coating enhances its lifespan by improving resistance to corrosion, oxidation, and high-temperature strength. To achieve this, a slurry composed of silicon, alumina, and aluminium was applied on the surface of 304 stainless steel (304SS) substrates. The samples were subjected to aluminizing at 750 °C, 800 °C, and 850 °C for 6 hours. Microstructural analysis was carried out using a field scanning electron microscope (FESEM) equipped with energy-dispersed X-ray spectroscopy (EDX), while X-ray diffraction (XRD) was employed for phase identification. The hardness of the coating was measured using Vicker microhardness. The study revealed the presence of various binary intermetallic compounds, including Fe₂Al₅, Fe₃Al, and FeAl, as well as ternary phases like Al₃Fe₂Si₃ and Fe_1.7Al₄Si, within the coatings. The addition of silicon reduced the intermetallic compound (IMC) layer thickness by occupying vacancy sites along the crystal structure c-axis of Fe₂Al₅, thereby restraining the growth of this brittle IMC in favor of more ductile phases. Notably, specimens heat treated at 850 °C exhibited the highest thickness of Fe-Al IMC layers. As temperature increases, the number of voids at the interface between the aluminide layer and the steel substrate also grew. Microhardness measurement revealed that Fe₂Al₅, FeAl, and Fe₃Al layers had a hardness value of about 850-990 HV, 570-630 HV, and 320-410 HV respectively for all the temperatures. Fe₂Al₅ has the lowest toughness and is confirmed to be the hardest zone in the aluminide coating. Si-modified aluminide coatings on 304 stainless steels could be considered as a material candidate for high temperature application.