Synthesis of mullite–MoSi2 nano-composite through a mechanochemical processing
Graphical abstract
MoO3, Al and Si powders were ball milled for different durations. XRD diffraction was used to detect the product phases. The mullite-α-MoSi2 composite was formed and its particles were analyzed by scanning electron microscopy.
Introduction
Molybdenum disilicide (MoSi2) has attracted many researchers over the last three decades due to its unique properties such as high melting point (2030 °C), relatively low density (6.28 g/cm3), metallic-like thermal and electrical conductivities, very good oxidation and corrosion resistance at high temperatures [1], [2], [3], [4]. These properties make it ideally suited for high temperature structural applications. However, it has some intrinsic limitations like low ductility at low temperatures (roughly below 1000 °C) and poor high-temperature creep strength [4], [5], [6]. Thus, it is essential to increase room-temperature toughness and high-temperature strength for practical applications. Molybdenum disilicide is also widely used in the production of heating elements. A continuously adhering silica film that forms on a heater surface protects it from oxidation reliably up to 1700 °C. Above 1700 °C, the oxygen diffusivity in SiO2 is high because of the high equilibrium vacancy concentration that would result in an enhanced rate of oxidation. Below 700 °C, the SiO2 layer is not continuous and that leads to penetration of oxygen and pest disintegration in the porous MoSi2 product in the temperature range of 400–600 °C [2], [7], [8], [9], [10]. Therefore, another challenge lying in MoSi2 is to control the pest oxidation.
Fortunately, MoSi2 is thermodynamically stable with a wide variety of potential ceramic reinforcements, including SiC, Si3N4, ZrO2, Al2O3, mullite, TiB2, and TiC. Thus many material researchers focused on the field of synthesis and characterization of MoSi2-based composites and reached satisfactory results [4], [5], [6], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].
Mullite ((Al4SiO8)1.2) possesses many attractive properties as follows; high melting point (1830 °C), excellent high temperature strength and creep resistance, low density (3.2 g/cm3), good chemical stability, good thermal shock behavior, inherent oxidation stability, low thermal expansion coefficient, low thermal conductivity and dielectric properties.
Due to the aforementioned properties, it is a strong candidate material for structural applications under thermal and mechanical loads [20], [21], [22], [23], [24], [25], [26], [27].
A composite of mullite–MoSi2 can compensate for the oxidation issues and low creep resistance of MoSi2. As a heating element, MoSi2 could play the role of a passing current through the mullite matrix [28]. In the work of Kisly and Kodash[9], the mullite-based coating formed on the Al2O3–MoSi2 alloy surface during annealing reliably protected the alloy up to 1800 °C in the air. Fei et al.'s study [11] indicated that mullite formation in the Al2O3–MoSi2 coating improved its oxidation resistance behavior. Composites of molybdenum disilicide, reinforced with mullite whiskers and particles, were made with the aid of a powder processing method by McFayden et al. [29]. Zaki et al. [28] reached a mullite–MoSi2 composite using combustion synthesis, which showed a good oxidation resistance in the range of 1000–1300 °C. Furthermore, the heat produced by the MoSi2 formation reaction [30] can promote the endothermic reaction of mullite formation (ΔHf ≅ 120 kJ·mol− 1 at 298 K for (Al4SiO8)1.2) [31].
In the present investigation, an attempt has been made to make a nano-structure mullite–MoSi2 composite from the raw materials of MoO3, Si and Al through mechanochemical reactions. Different Al amount and milling times were tested to obtain the composite. XRD, EDX and SEM were used to study the product composition and microstructure. Additionally, TEM was implemented to examine the particle size and morphology of the products. It is interesting to note that few researches have tried to make mullite–MoSi2 composite and so far ball milling has not been used as a method of synthesis.
Section snippets
Materials and methods
The raw materials used in this study were comprised of powders of MoSi2, Si and Al with purity of 95%, 99% and 99.5% and particle sizes of 250, 300 and 100 μm, respectively. Experiments of ball milling were carried out in a planetary ball mill with a steel vial of 39 cm3 volume and steel balls with diameters of 8, 10 and 12 mm. The ambient was under dry condition with air pressure of 1 atm. The rotation speed of 250 rpm and a ball-to-powder ratio (BPR) of 40:1 were used with the primary powder
Thermodynamic calculations
ΔH298, Tad and the amount of vaporized Al2O3 were calculated for reactions (1), (2), (3)by writing a thermodynamic model with Excel software. Table 1 shows that ΔH298 increases with the increment of Al amount. Tad is also higher for the systems with more Al amount, although it remains constant in the vaporizing temperature of Al2O3 (3253 K) for reactions (2), (3).
Results and discussion
Fig. 1 shows the comparison of XRD results of samples containing 1 mol Al with different ball milling durations. It can be seen that after 30 min, no reaction was detected. However, the reaction took place in sample X1-1 h so that the primary mixture was converted to Mo, α-MoSi2 and MoO2. This means that the MSR reaction occurs between 30 min and 1 h after starting the ball milling; therefore, activation time of at least 30 min is required. The peaks of the Al2O3 phase were not revealed by XRD. There
Conclusions
A mullite-α-MoSi2 nano-composite was successfully prepared by mechanochemical synthesis. The composite is mullite-based, containing around 60 wt.% mullite and its MoSi2 is a stable kind of α with tetragonal crystals. The crystallite size of around 18 nm and average particle size of about 155 nm were obtained for the composite after 5 h of ball milling. The temperature of the ball milling vial wall confirmed the XRD results, showing that the MSR reaction in X1.5 samples took place in a duration
References (38)
- et al.
Oxidation behaviour of a Mo(Si, Al)2 based composite at 1500 °C
Intermetallics
(2011) - et al.
Effect of small aluminum additions on microstructure and mechanical properties of molybdenum di-silicide
Intermetallics
(1999) - et al.
MoSi2/Al2O3 FGM: elaboration by tape casting and SHS
J. Eur. Ceram. Soc.
(2001) - et al.
In situ pressureless sintering of SiC/MoSi2 composites
Ceram. Int.
(2012) - et al.
Fabrication and characterization of TiC-particle-reinforced MoSi2 composites
J. Eur. Ceram. Soc.
(2002) - et al.
Oxidation of MoSi2-based composites
Mater. Sci. Eng.
(1992) - et al.
Effect of minor alloying with Al on oxidation behavior of MoSi2 at 1200 °C
Mater. Sci. Eng.
(1999) - et al.
Engineering limitations of MoSi2 coatings
Mater. Sci. Eng.
(1992) - et al.
The mullite coatings on heaters made of molybdenum disilicide
Ceram. Int.
(1989) - et al.
Influence of long term oxidation on the microstructure, mechanical and electrical properties of pressureless sintered AlN–SiC–MoSi2 ceramic composites
J. Eur. Ceram. Soc.
(2003)