Synthesis and kinetic study of Mo(Si,Al)2 coatings on the surface of molybdenum through hot dipping into a commercial Al-12 wt.%Si alloy melt

Abstract

Mo(Si, Al)2 coatings were synthesized through hot dipping of Mo rods into a commercial Al-12 wt.% Si alloy melt in the temperature range of 750 to 950 °C for 15 to 150 min. The structure of the coatings was characterized by X-ray diffraction analysis. In addition, optical and scanning electron microscopes along with energy dispersive spectroscopy were employed for microstructural and chemical analyses, respectively. For the sample treated at 750 °C, Mo(Si, Al)2 layers were only formed at high dipping times. An increase in the temperature up to 850 °C facilitated the formation of the coatings and a uniform layer of Mo(Si, Al)2 with 40 to 60 µm thickness was obtained after 60 min. However, a two-layer coating was observed after processing at 950 °C for 60 min. The first layer attached to the substrate mainly consisted of nano-sized Mo5Si3 particles dispersed in an Al8Mo3 matrix. The second layer had a microstructure containing Al8Mo3 and Mo(Si, Al)2 phases. Furthermore, some transverse cracks were detected across the coatings obtained at both 850 °C and 950 °C. The rate of the formation of the coating obeyed the linear law and its activation energy was estimated by the Arrhenius equation as 110 kJ/mol.