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

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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.

Introduction

Hot dipping is a simple and easy-echo method to synthesize various coatings on the surface of different substrates. In this process, usually Ti-, Ni-, Mo-, or Fe-based alloys are immersed into a molten alloy bath for a specific time. The melt reacts with, or diffuses into the surface of the alloy and leads to the formation of coatings [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Zinc is a common molten bath for galvanizing steels [1], [2], [3], [4]. Moreover, molten Al can be used to form different intermetallic compounds depending on the substrate type and alloy composition [7, [11], [12], [13]]. There are plenty of studies on the aluminizing of different steels like AISI 321 [12, 14, 15] and low carbon steels [1, 13] by hot dipping method in the molten aluminum. Titanium alloys are other interesting materials that can be immersed into the molten Al to form titanium aluminide coatings [7, 16]. Providing that silicon is used in the molten bath, different silicide and aluminosilicide compounds like TiSi2, WSi2, MoSi2, and Mo(Si, Al)2 can be obtained on the surface of alloys [6, 8,17].

Molybdenum and its alloys can be used in many applications like high-temperature crucibles and electrodes due to their high melting point (2617 °C), high thermal and electrical conductivity and low coefficient of thermal expansion (5.8 × 10−6) [18]. However, the oxidation resistance of these alloys is poor and consequently, they cannot be used at temperatures above 600 °C for a prolonged time in an oxidation environment [18]. Synthesis of protective coatings like molybdenum disilicide (MoSi2) is a facile way to solve this problem. MoSi2 is an attractive compound because of its interesting properties such as a high melting point of 2030 °C, the moderate density of 6.24 g/mm3, and excellent oxidation resistance at high temperatures [19, 20]. The addition of Al to this compound and the formation of Mo(Si, Al)2 phase improves the adherence of the scales during oxidation and improves its oxidation resistance at intermediate temperatures around 523 °C [21].

Up to now, there are some studies related to the formation of MoSi2 and Mo(Si, Al)2 compounds on the surface of Mo by hot-dip siliconizing process [18, 21]. Zhang et al. [18] synthesized MoSi2 coating on the surface of pure molybdenum through hot dipping in Si bath at 1490 °C for 5 to 20 min. Their results showed that molybdenum silicide layers with columnar grains and a thickness between 20 and 50 µm were developed. In another study, Nanko et al. [21] synthesized Mo(Si, Al)2 layer on the Mo surface via dipping in a molten Si-rich aluminum bath. The concentration of silicon was 28 at.% in the melt. Moreover, the coating temperature was kept constant at 700 °C and the processing time varied from 86 ks up to 580 ks. Their results showed Mo(Si, Al)2 layer with columnar grains grew on the surface and its thickness changed between 25 and 140 µm. Then, the mechanism of the layer formation was proposed [21].

Meanwhile, it is obvious that the coating time considered by Nanko et al. [21] was too long due to the low coating temperature which increases the production cost, significantly. The problem can be overcome by increasing the dipping temperature. It may have a great influence on the nature of the synthesized layer and its formation kinetic, and also can decrease the processing time and subsequently its cost. Furthermore, from the traditional point of view, it is important to explore if Mo(Si, Al)2 coatings can be synthesized on the surface of Mo using the commercial Al-12 wt.%Si alloy with lower Si content. Therefore, the present study aims (a) to synthesize Mo(Si, Al)2 coatings on the surface of Mo by hot-dipping into a commercial Al-12 wt.%Si alloy melt, (b) to investigate the effect of dipping time and temperature on the characteristics of the synthesized layers, and (c) to study the growth kinetic of the coatings.

Section snippets

Experimental procedure

A pure molybdenum rod (purity higher than 99.5 wt.%) as substrate and commercial Al-Si piston alloy with the chemical composition of 11.24% Si, 1.48% Fe, 1.00% Cu, 0.79% Mg, 1.30% Ni, 0.11% Zn, and 0.08% Ti (all in wt.%) as Al and Si source were used as starting materials. The disks of molybdenum with dimensions of 10 mm in diameter and 10 mm thickness were prepared. The surfaces of the samples were ground with a 1500# mesh SiC emery paper, and then ultrasonically cleaned using acetone. The

Structure and microstructure characterization

Fig. 1(a) to (c) separately show the optical micrographs of the samples dipped at 750 °C for 120 min, 850 °C for 60 min, and 950 °C for 60 min. For the samples treated at 750 °C for dipping times less than 60 min, no clear coating was distinguished in the optical micrograph. So, the dipping time increased to 150 min. It is shown in Fig. 1(a) that a layer with 38 to 42 µm thickness was developed on the surface of sample dipped at 750 °C for 120 min. A similar coating was formed on the surface of

Conclusions

The present study deal with the formation of Mo(Si, Al)2 coatings on the surface of pure Mo substrate by hot-dip aluminosiliconizing into a commercial Al-12 wt.%Si alloy melt. The main results can be summarized as follows:

  • 1

    A uniform Mo(Si, Al)2 coatings with a thickness of 40 to 60 were obtained after dipping at 850 °C for 60 min, and an increase in the processing time to 120 min led to higher coating thickness up to 150 µm.

  • 2

    A two-layer coating was obtained after treatment at 950 °C. The first

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (25)

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