Metallothermic Reduction of MoO₃ on Combustion Synthesis of Molybdenum Silicides/MgAl₂O₄ Composites

Abstract

Combustion synthesis involving metallothermic reduction of MoO₃ by dual reductants, Mg and Al, to enhance the reaction exothermicity was applied for the in situ production of Mo₃Si–, Mo₅Si₃− and MoSi₂–MgAl₂O₄ composites with a broad compositional range. Reduction of MoO₃ by Mg and Al is highly exothermic and produces MgO and Al₂O₃ as precursors of MgAl₂O₄. Molybdenum silicides are synthesized from the reactions of Si with both reduced and elemental Mo. Experimental evidence indicated that the reaction proceeded as self-propagating high-temperature synthesis (SHS) and the increase in silicide content weakened the exothermicity of the overall reaction, and therefore, lowered combustion front temperature and velocity. The XRD analysis indicated that Mo₃Si–, Mo₅Si₃– and MoSi₂–MgAl₂O₄ composites were well produced with only trivial amounts of secondary silicides. Based on SEM and EDS examinations, the morphology of synthesized composites exhibited dense and connecting MgAl₂O₄ crystals and micro-sized silicide particles, which were distributed over or embedded in the large MgAl₂O₄ crystals.

1. Introduction

Molybdenum silicides, Mo₃Si, Mo₅Si₃ and MoSi₂, are promising intermetallic materials for ultrahigh-temperature structural applications. Besides a high melting point over 2020 °C, they possess high strength, excellent oxidation resistance, corrosion resistance, creep resistance and good compatibility with ceramic reinforcements [1,2,3,4,5,6,7,8]. To improve the refractory property of transition metal silicides, magnesium aluminate spinel (MgAl₂O₄) has been one of the potential additives, because of its unique combination of properties, including a high melting point (2135 °C), relatively low density, chemical inertness, high hardness, high mechanical strength and good thermal shock resistance [9,10,11,12]. However, preparation of MgAl₂O₄ via either wet chemical methods or solid-state reactions required several complicated steps under the long processing time [9,10,11,12].

As an alternative, metallothermic reduction reactions (MRRs) of metal oxides with Mg and Al as reducing agents produce MgO and Al₂O₃ as precursors for the formation of MgAl₂O₄ and such oxidation reactions are highly exothermic [13,14]. When combining Mg/Al-based MRRs with combustion synthesis, such a fabrication route is effective in producing MgAl₂O₄-containing composites. Moreover, the highly-exothermic MRRs render reduction-based combustion synthesis fit for self-propagating high-temperature synthesis (SHS). Many merits such as high energy efficiency, short reaction time, simplicity of operation and high-purity products have been recognized for the SHS process [15,16,17]. According to Horvitz and Gotman [18], reduction-based combustion synthesis using 2TiO₂–Mg–4Al samples was performed to produce TiAl–Ti₃Al–MgAl₂O₄ composites. Omran et al. [19] conducted co-reduction of WO₃ and B₂O₃ by Mg in the presence of Al₂O₃ to fabricate the composites of MgAl₂O₄–W–W₂B. By means of adopting pre-added MgO, Zaki et al. [20] obtained MgAl₂O₄ composites with MoSi₂ and Mo₅Si₃ from co-reduction of SiO₂ and MoO₃ by Al in argon at a pressure of 5 MPa. The high Ar pressure was to suppress the volatilization of MoO₃. Recently, Radishevskaya et al. [21] synthesized MgAl₂O₄ by the SHS method using the reactant mixtures consisting of MgO and Al₂O₃, along with Al as the fuel, Mg(NO₃)₂⋅H₂O as the oxidizer, and NaCl as the mineralizer. Results indicated that NaCl of 1 wt.% contributed to the completion of the formation of MgAl₂O₄ and mechanical activation of the green mixture for 60 s facilitated the production of MgAl₂O₄ without oxide impurities.

By using Mg and Al simultaneously as dual reductants, this work aims at investigating the in situ production of MgAl₂O₄-containing molybdenum silicide (Mo₃Si, Mo₅Si₃, and MoSi₂) composites by the SHS process with reducing stages. That is, a solid-state combustion reaction involves the synthesis of MgAl₂O₄ from the metallothermic reduction of MoO₃ and the formation of molybdenum silicides from elemental interactions between Mo and Si. Three different silicide phases were produced and their influence on reaction exothermicity and combustion wave kinetics was explored. Compositional and microstructural analyses were performed on the final composites. Moreover, some products were selected for Vickers hardness and fracture toughness measurements.

2. Materials and Methods

The raw materials utilized by this study include MoO₃ (Acros Organics, 99.5%), Mg (Alfa Aesar, <45 μm, 99.8%), Al (Showa Chemical Co., <45 μm, 99.9%), Mo (Strem Chemicals, <45 μm, 99.9%), Si (Strem Chemicals, <45 μm, 99.5%), and Al₂O₃ (Alfa Aesar, 99%). According to different molybdenum silicides, three reaction systems, R(1), R(2), and R(3), are formulated for the synthesis of Mo₃Si–, Mo₅Si₃–, and MoSi₂–MgAl₂O₄ composites, respectively.

$$\frac{4}{3}Mo{O}_3+Mg+2Al+\left(3x-\frac{4}{3}\right)Mo+xSi\to xM{o}_{3}Si+MgA{l}_2{O}_4$$

(1)

$$\frac{4}{3}Mo{O}_3+Mg+2Al+\left(5y-\frac{4}{3}\right)Mo+3ySi\to yM{o}_{5}S{i}_3+MgA{l}_2{O}_4$$

(2)

$$\frac{13}{12}Mo{O}_3+Mg+\frac{3}{2}Al+\frac{1}{4}A{l}_2{O}_3+\left(z-\frac{13}{12}\right)Mo+2zSi\to zMoS{i}_2+MgA{l}_2{O}_4$$

(3)

where stoichiometric coefficients x, y, and z are associated with the quantities of Mo and Si powders in the green mixtures, and also represent the molar proportion of silicide phase to MgAl₂O₄. The same composition of metallothermic reagents of 4/3MoO₃ + Mg + 2Al is adopted in R(1) and R(2), but R(3) has a different metallothermic mixture of 13/12MoO₃ + Mg + 3/2Al because R(3) comprises pre-added Al₂O₃. Because of metallothermic reduction of MoO₃, the source of Mo for the formation of molybdenum silicides (Mo₃Si, Mo₅Si₃, and MoSi₂) from R(1), R(2), and R(3) included both reduced and elemental Mo.

It has been realized that magnesiothermic and aluminothermic reductions of MoO₃ are highly exothermic and have an adiabatic temperature (T_ad) exceeding 4200 K [22], which plays an important role in facilitating self-sustaining combustion for R(1), R(2) and R(3). When compared with the reduction of MoO₃ by Mg and Al, the formation reactions of Mo₃Si, Mo₅Si₃ and MoSi₂ are much less energetic. Among three molybdenum silicides, MoSi₂ is the most exothermic phase to form [23], and therefore, Al₂O₃ at one-quarter of the required amount was added in the starting mixture to regulate the degree of violence of combustion.

Experimental ranges of x, y, and z conducted in this study were determined based on the reaction exothermicity of R(1), R(2) and R(3), which was assessed by computing T_ad as a function of stoichiometric coefficients according to the following energy balance equation [24,25] with thermochemical data taken from [23].

$$\Delta H_r+{{\displaystyle \int }}_{298}^{T_{ad}}{{\displaystyle \sum }}^{ }{n}_j{C}_p\left(P_j\right)dT+{{\displaystyle \sum }}_{298-T_{ad}}{n}_{j}L\left(P_j\right)=0$$

(4)

where ΔH_r is the enthalpy of reaction at 298 K, n_j is the stoichiometric coefficient, C_p and L are the specific heat and latent heat, respectively, and P_j refers to the product.

The value of ΔH_r was calculated from the difference in enthalpy of formation (ΔH_f) between the reactants (ΔH_f of MoO₃: −745 kJ/mol, Al₂O₃: –1675.7 kJ/mol, and Mg, Al, Mo, and Si: 0 kJ/mol) and products (ΔH_f of Mo₃Si: −118.4 kJ/mol, Mo₅Si₃: −310.6 kJ/mol, MoSi₂: −131.4 kJ/mol, and MgAl₂O₄: −2299.1 kJ/mol) [23]. The values of C_p of the products as a function of temperature are expressed as follows [23].

$$C_p\left(M{o}_{3}Si\right)=85.23+22.68\times {10}^{-3}\times T+0.03\times {10}^6\times T^{-2}(\mathrm{J}·{\mathrm{mol}}^{-1}·{\mathrm{K}}^{-1})$$

(5)

$$C_p\left(M{o}_{5}S{i}_3\right)=183.36+35.01\times {10}^{-3}\times T-1.2\times {10}^6\times T^{-2}(\mathrm{J}·{\mathrm{mol}}^{-1}·{\mathrm{K}}^{-1})$$

(6)

$$C_p\left(MoS{i}_2\right)=67.84+11.95\times {10}^{-3}\times T-0.66\times {10}^6\times T^{-2}(\mathrm{J}·{\mathrm{mol}}^{-1}·{\mathrm{K}}^{-1})$$

(7)

$$C_p\left(MgA{l}_2{O}_4\right)=146.78+35.56\times {10}^{-3}\times T-3.68\times {10}^6\times T^{-2}(\mathrm{J}·{\mathrm{mol}}^{-1}·{\mathrm{K}}^{-1})$$

(8)

The SHS experiment was performed in a windowed combustion chamber filled with high-purity argon (99.99%) at 0.2 MPa. Reactant powders were dry mixed and then were uniaxially pressed to form cylindrical test specimens with 12 mm in height, 7 mm in diameter, and 55% in the relative density. In this work, a cylindrical bottle partially filled with the raw materials and alumina (Al₂O₃) grinding balls rotated about the longitudinal axis of a tumbler ball mill machine for 8 h to fully blend the reactant powders. The size of the alumina ball is 5 mm in diameter. The ball mill operated at 90 rpm. Because Al₂O₃ is one of the precursors to form MgAl₂O₄, no contamination from grinding balls was detected.

The combustion wave propagation velocity (V_f) was determined from the time series of recorded combustion videos. The combustion temperature was measured by a 125 μm bead-sized thermocouple with an alloy composition of Pt/Pt–13%Rh. Details of the experimental setup were previously reported [25,26]. Phase components of the synthesized products were identified by an X-ray diffractometer with CuK_α radiation (Bruker D2 Phaser, Billerica, MA, USA). Analyses of scanning electron microscopy (SEM) (Hitachi S3000H, Tokyo, Japan) and energy dispersive spectroscopy (EDS) were performed to examine the fracture surface microstructure and composition ratio of elements of the final products.

Measurement of Vickers hardness and fracture toughness of the products was performed [27]. For such measurements, only selected experiments under stoichiometric coefficients of x = y = z = 2 were carried out by placing the sample compact in a stainless-steel mold. Densification of the product was conducted by a hydraulic compressor. Upon the completion of the SHS reaction, the burned sample was rapidly pressed when the product was still hot and plastic, which was held for about 15 s. The product density after compression reached about 93–95% of theoretical density and then the product surface was polished for the measurement. Microhardness was measured with a Buehler Micromet microhardness tester at a load of 1000 g and a dwelling time of 10 s. Five indentations were made to obtain the average values of the indentation imprint and crack length measurements.

In this study, Vickers hardness (H_v) was calculated from the applied load (P) and the average diagonal impression length (d) in the equation below [28,29]. The fracture toughness (K_IC) was determined by the indentation method using the following equation proposed by Evans and Charles [29].

$$H_v=1.8544\frac{P}{d^2}$$

(9)

$$K_{IC}=0.16H_v{a}^{1/2}{\left(\frac{c}{a}\right)}^{-3/2}$$

(10)

where a is the half of the average length of two diagonals of the indentation and c the radial crack length measured from the center of the indentation.

3. Results and Discussion

3.1. Combustion Exothermicity of Reactions

Calculated values of T_ad of R(1), R(2) and R(3) as a function of their respective stoichiometric coefficients are presented in Figure 1 in order to evaluate combustion exothermicity. A significant decrease in T_ad with increasing silicide content is observed for all three synthesis reactions, mainly because the formation of molybdenum silicides is much less exothermic than the metallothermic reduction of MoO₃. As revealed in Figure 1, the value of T_ad associated with the formation of Mo₃Si–MgAl₂O₄ composites from R(1) decreases considerably from 3964 °C to 2415 °C as the coefficient x increases from 1 to 5. On account of a large heat capacity for Mo₅Si₃, R(2) is the weakest exothermic reaction and shows a decrease in T_ad from 3475 °C at y = 1 to 2162 °C at y = 5. In spite of the dilution effect of pre-added Al₂O₃ on combustion, R(3) intended for the synthesis of MoSi₂–MgAl₂O₄ composites is still very energetic with T_ad ranging from 3840 °C to 2745 °C. Figure 1 indicates that R(3) has the highest T_ad except for the case of z = 1. According to the analysis of combustion exothermicity, R(1) and R(3) were conducted in this study with the experimental variables of x = 2–5 and z = 2–5, respectively, and R(2) with y = 1–4 was carried out. Reactions with x = 1 and z = 1 were avoided, since the resulting combustion was often violent enough to melt down the powder compact and led to incomplete phase conversion.

3.2. Combustion Wave Velocity and Temperature

A typical sequence of recorded combustion images from R(1) with x = 3 is illustrated in Figure 2, showing a stable and self-sustaining combustion process. A distinct combustion front allowed the propagation velocity to be determined. Variations of combustion wave velocities of R(1), R(2) and R(3) with the molar ratio of silicide to MgAl₂O₄ are presented in Figure 3. A declining trend consistent with the adiabatic combustion temperature was observed. This can be explained by the fact that the combustion wave propagation rate is essentially governed by layer-by-layer heat transfer from the thin combustion zone to the unreacted region, and therefore, is subject to the reaction front temperature. Specifically, Figure 3 points out a decrease in V_f from 5.9 to 2.9 mm/s for R(1) with x from 2 to 5. For the similar range of stoichiometry of z = 2–5, R(3) has a faster combustion wave with V_f ranging from 6.7 to 4.3 mm/s. On the other hand, the combustion front of R(2) has a slower speed and its V_f decreases from 5.9 mm/s at y = 1 to 2.7 mm/s at y = 4.

Figure 4a,b depict combustion temperature profiles measured from R(1), R(2) and R(3) under equal stoichiometric coefficients of 2 and 4, respectively. A steep rising gradient followed by a rapid cooling rate is characteristic of the temperature profile of the SHS reaction. The highest value is considered as the combustion front temperature (T_c). A comparison of T_c among three synthesis reactions in Figure 4a indicates that R(3) has the highest T_c of 1637 °C (z = 2), R(2) has the lowest 1442 °C (y = 2), and R(1) is in-between at 1574 °C (x = 2). A similar ranking of T_c can be seen in Figure 4b, which is associated with the synthesis of composites with a molar ratio silicide/MgAl₂O₄ equal to 4. When compared with T_c shown in Figure 4a, lower values of T_c = 1330 °C, 1103 °C and 1470 °C are observed in Figure 4b for R(1), R(2) and R(3), respectively. This confirms the decrease in reaction exothermicity with an increasing fraction of silicide formed in the composite.

3.3. Composition and Microstructure Analyses of SHS-Derived Products

The XRD spectrum graphs of final products synthesized from R(1) with x = 2 and 4 are plotted in Figure 5a,b, respectively. Besides MgAl₂O₄, two silicide compounds were detected with Mo₃Si the dominant and Mo₅Si₃ the minor. Because of the presence of Mo₅Si₃, there was a small amount of elemental Mo left in the end product. It should be noted that the production of MgAl₂O₄ justifies a combination reaction between in situ formed Al₂O₃ and MgO from the metallothermic reduction of MoO₃ by dual reductants. Phase constituents associated with the products of R(2) are identified in Figure 6a,b, indicative of the Mo₅Si₃–MgAl₂O₄ composites with a trivial amount of Mo₃Si. Because Mo₅Si₃ has a homogeneity range from 37.5 to 40 at% Si [20], no remnant Si was found in the Mo₅Si₃–MgAl₂O₄ products even containing some Mo₃Si.

Figure 7a,b shows the XRD spectra of the MoSi₂–MgAl₂O₄ composites produced from R(3) with z = 2 and 4, respectively. It should be pointed out that MoSi₂ formed from R(3) is α-MoSi₂ (the low-temperature phase). This was due to the fact that the reaction temperature of R(3) was below 1900 °C [22], the phase transition temperature from α-MoSi₂ to the high-temperature phase of β-MoSi₂. As revealed in Figure 7a,b, there are small amounts of Mo₅Si₃ and Si in the as-synthesized MoSi₂–MgAl₂O₄ composites.

When compared with the work of Zaki et al. [20], MgAl₂O₄ composites with MoSi₂ and Mo₅Si₃ were produced from MoO₃, SiO₂, Al and MgO powder mixtures by self-sustaining combustion. They indicated the presence of small amounts of Mo₅Si₃, Al₂SiO₅ and free Si in the synthesized MgAl₂O₄–MoSi₂ composites. The impurity Al₂SiO₅ was formed via a combination reaction of Al₂O₃ with SiO₂. Moreover, the increase in MgO led to the formation of the other impurity Mg₂SiO₄ which was produced from the reaction between MgO and SiO₂. Therefore, it is believed that the formation of Al₂SiO₅ and Mg₂SiO₄ could be due to incomplete reduction of SiO₂, since these two phases were not found in the products of the present study. On the other hand, Zaki et al. [20] obtained MgAl₂O₄–Mo₅Si₃ composites without impurities and secondary silicides, on account of a larger heat release from combustion and a lesser amount of SiO₂ contained in the sample.

In the work of Radishevskaya et al. [21], MgO and Al₂O₃ were added into a combustible mixture composed of Al, Mg(NO₃)₂⋅H₂O and NaCl to produce MgAl₂O₄ through the SHS scheme. Results showed that the pre-added MgO and Al₂O₃ failed to be fully combined into MgAl₂O₄ unless mechanical activation of initial components in a planetary mill was conducted. In contrast, MgO and Al₂O₃ were not detected in the final composites of R(1), R(2) and R(3). This could be because these two precursors of MgAl₂O₄ were in situ produced from metallothermic reduction reactions in the present study.

For the Mo₃Si–MgAl₂O₄ composite of R(1) with x = 3 illustrated in Figure 8, the SEM image shows the fracture surface microstructure and EDS spectra provide the atomic ratios of constitution elements. The micrograph exhibits that MgAl₂O₄ crystals are dense and continuous and small Mo₃Si grains tend to agglomerate into clusters. Moreover, the atomic ratios of Mo:Si = 76.16:23.84 and Mg:Al:O = 14.83:26.49:58.68 are close to those of Mo₃Si and MgAl₂O₄.

The microstructure of the Mo₅Si₃–MgAl₂O₄ composite of R(2) with y = 3 in Figure 9 also reveals agglomeration of small Mo₅Si₃ grains with a particle size of about 2–4 μm. Most of the large MgAl₂O₄ crystals are covered with Mo₅Si₃ grains. The atomic ratios of Mo:Si = 61.64:38.36 and Mg:Al:O = 13.61:28.36:58.03 confirm the formation of Mo₅Si₃ and MgAl₂O₄. A similar morphology can be seen in Figure 10, unveiling the MoSi₂–MgAl₂O₄ composite of R(3) with z = 3. It is evident that MgAl₂O₄ crystals are dense and relatively large. Small MoSi₂ grains are distributed over or embedded in MgAl₂O₄ crystals. Atomic ratios of Mo:Si = 32.98:67.02 and Mg:Al:O = 13.85:28.86:57.29 were obtained from the EDS analysis.

Selected test conditions (x = y = z = 2) were conducted to prepare product samples for the measurement of hardness and fracture toughness. For the composite of 2Mo₃Si–MgAl₂O₄ produced from R(1), Vickers hardness of H_v = 1.41 × 10⁴ MPa and fracture toughness of K_IC = 3.3 MPa m^1/2 were determined. Values of H_v = 1.42 × 10⁴ MPa and K_IC = 3.1 MPa m^1/2 were obtained for 2Mo₅Si₃–MgAl₂O₄ synthesized from R(2). For the product of 2MoSi₂–MgAl₂O₄ from R(3), H_v = 1.48 × 10⁴ MPa and K_IC = 2.8 MPa m^1/2 were determined. The error of hardness values was estimated as about ±10% and the error of fracture toughness values was within ±20%. The uncertainty of K_IC determination using the indentation fracture method could result from the residual stresses induced by specimen densification, the existence of pores or cracks, surface finish, and possible inhomogeneous microstructure. Compared with monolithic Mo₃Si, Mo₅Si₃, and MoSi₂ (H_v ≈ 1.3 × 10⁴ MPa and K_IC = 2~3 MPa m^1/2) [7,30,31], MgAl₂O₄ as an additive improved the hardness and toughness of molybdenum silicides.

4. Conclusions

The in situ fabrication of Mo₃Si–, Mo₅Si₃– and MoSi₂–MgAl₂O₄ composites was investigated by the SHS process integrating metallothermic reduction of MoO₃ with combustion synthesis. Mg and Al were simultaneously used as dual reductants to produce MgO and Al₂O₃ as precursors of MgAl₂O₄. Molybdenum silicides were synthesized from the elemental reactions between Mo and Si. Experimental results showed that the formation of MoSi₂–MgAl₂O₄ composites was the most exothermic and characterized by the highest combustion front temperature and fastest combustion velocity, while that of Mo₅Si₃–MgAl₂O₄ composites was the least. Composites with molar ratios of Mo₃Si/MgAl₂O₄ from 2 to 5, Mo₅Si₃/MgAl₂O₄ from 1 to 4, and MoSi₂/MgAl₂O₄ from 2 to 5 were synthesized. An increase in silicide content brought about a decrease in reaction exothermicity because the formation of molybdenum silicides was much less exothermic than the metallothermic reduction of MoO₃. Based on the XRD patterns, phase conversion from the reactants to products was essentially completed except for trivial amounts of secondary silicide and Mo or Si present in the end products. SEM and EDS analyses revealed that MgAl₂O₄ formed large connecting grains with a dense morphology. Granular Mo₃Si, Mo₅Si₃ and MoSi₂ were relatively small and were distributed over or embedded in MgAl₂O₄ crystals. Hardness and fracture toughness of molybdenum silicides were improved by adding MgAl₂O₄. This study demonstrated an effective fabrication route adopting dual reductants to increase combustion exothermicity for the in situ production of Mo₃Si–, Mo₅Si₃– and MoSi₂–MgAl₂O₄ composites with a broad compositional range.