Effect of Al and Cr alloying by arc cladding on the high-temperature oxidation resistance of MoSi2 materials

Different proportions of Al and Cr alloying MoSi2 were prepared by arc cladding with different mass fractions of MoSi2, Al, and Cr as raw materials. The work studied the effects about of Al and Cr on the phase, high-temperature oxidation morphology, products, and high-temperature oxidation property of MoSi2. The results confirmed that the volume expansion coefficient of Al2O3 generated by material oxidation was low which could reduce the degree of mismatch and the cracks in the oxide film. Cr element had a poor effect on enhancing the continuity and compactness of the oxide film, however, it could improve the stability of t-MoSi2 formation after arc cladding. MoO3 was not formed in the MoSi2 material added with Al and Cr show the surface oxidation. A dense and continuous Al2O3–SiO2 oxide film was formed, and no cracks holes were found in the oxide film. After oxidation at 800 °C–1200 °C for 120 h, the sample still maintained good oxidation protection. According to the calculation of oxidation kinetics, the MoSi2 material added with Al and Cr had good oxidation resistance. 3% Al + 9% Cr + 88% MoSi2 had the optimal high-temperature oxidation resistance about four times than pure MoSi2.


Introduction
In the past few decades, experts have developed new material systems, providing many material systems with better performance than nickel-base super-alloy for the aerospace industry with increased energy efficiency [1][2][3][4][5][6][7][8]. Perepezko J H et al [1,2] have made great achievements in the study of Mo-Si alloys and compounds. They found that Mo-Si alloys and compounds can work at higher temperature than nickel-based alloys, and keep high temperature properties. Zhai R X [4] et al found that the Mo substrate shows good high temperature oxidation resistance at 1300°C after adding Mo-Si coating. Ouyang G Y et al [5] prepared Mo-Si composite coatings on Mo-W-Si-B alloy and studied their high temperature oxidation resistance. They found that the material shows good oxidation resistance and almost zero mass change.
MoSi 2 with a C11b structure is considered as one of the ideal high-temperature service materials in Mo-Si system because of its good electric conductivity, excellent thermal conductivity, and low thermal expansion. However, its engineering application is limited due to the limited mechanical properties at room temperatures and poor oxidation resistance at intermediate temperatures [9][10][11][12][13][14][15][16]. In high-temperature oxidation, MoSi 2 forms dense and continuous SiO 2 film on the surface, which prevents oxygen from diffusing into MoSi 2 and has good oxidation resistance [17][18][19][20][21][22][23][24][25]. Whereas, Mo prefers O to Si form MoO 3 at the medium temperature, hindering the protection of MoSi 2 by SiO 2 films and leading to 'pesting' phenomenon.
The 'pesting' of MoSi 2 have been studied in [26][27][28][29][30]. The results show that the oxidation rate of MoSi 2 changes with the increased temperature, and the 'pesting' just occurs in the air at about 500°C S H Wen et al [31] found that MoSi 2 with micro-crack is broken after oxidation, but the single-crystal MoSi 2 without cracks is opposite. Westbrook et al [32] also suggested that the catastrophic oxidation of MoSi 2 is attributed to preferential diffusion between elements and grain boundary embrittlement. Many researchers have also pointed out that oxidation occurs preferentially in cracks and holes. The internal stress generated by the oxide growth in these defects leads to the pulverizes the samples. Therefore, the service temperature and internal defects are the key factors affecting the pulverization of MoSi 2 .
Alloying is one of the main methods to improve the poor oxidation resistance of MoSi 2 at medium temperatures. The properties of MoSi 2 can be improved by adding alloying elements. L IngeMarsson et al [33] added Al element in MoSi 2 , and Mo (Si, Al) 2 composite forms a continuous alumina protective layer at 1500°C to improve the high-temperature oxidation of MoSi 2 . Hou et al [34] studied the high-temperature oxidation behaviors of MoSi 2 materials with different Al contents. When oxidized at high temperatures, Al and Si form a mixed oxide film to protect MoSi 2 materials. The content of Al is inversely proportional to the oxidation resistance. Jiang et al [35] studied the influence of Al and Cr elements on the oxidation property of MoSi 2 materials at the atomic scale by density functional theory. The calculation results show that the Si-O bond is the main factor to prohibit diffusion. Al and Cr elements can improve the oxidation resistance of MoSi 2 . Dou Hu et al [36] prepared MoSi 2 /Cr coatings on C/C materials. Cr element changed the crack propagation path under the thermal cycle environment at 1500°C, thus releasing the thermal stress in coatings and further extending the service life of coatings. Adding Al and Cr elements can significantly improve the service performance of MoSi 2 materials.
In recent years, TIG arc cladding has become a high-efficiency, low-cost surface treatment technology, widely used in surface repair and modification of various metals [37]. However, the modification of refractory metal silicide coatings by TIG arc cladding is rarely reported. In this work, MoSi 2 materials were prepared by TIG arc cladding. Different contents of Al and Cr elements were added to study the synergistic alloying of Al and Cr elements on the high-temperature oxidation resistance of MoSi 2 , expecting to improve the service performance of MoSi 2 -coating materials. It provides the interface structure and performance of the refractory-metal silicide coating.
Pure Mo alloy (Mo >99.99%) were selected as substrate and cut into 20×20×8 mm by a wire-cutting machine. The wire cutting traced on the substrate surface was removed by 240, 400, 600, 800 sandpapers. The substrates were cleaned in an ultrasonic cleaner for 1 h to remove surface stains.

Preparation of MoSi 2 material
A powder layer was prepared on the substrate by presetting powders. The thickness of the powder layer was 5 mm, and alcohol was used as the adhesives. The preset powder was dried in the oven (DHG101-3A, 4.5 kW, ShangHai) at 120°C for 10 min. TIG arc cladding (EASB 4300iw, Poland) was used cladding experiments on the preset powder.
The parameters of TIG arc cladding were as follows: a cladding current of 160 A, a cladding voltage of 220 V, a welding torch height of 5 mm, an argon content 8 l min −1 , and a welding torch scanning speed of 120 mm min −1 .

High-temperature oxidation test
The oxidation experiment was carried out in the KBF1400 box-type heat-treatment furnace. The oxidation samples were cut into 6×6×8 mm cubes by a wire-cutting machine. The wire cutting traced on the sample surface was removed by 240, 400, 600, 800, 1000, 1200, 1500, 2000 sandpapers. The polished samples were cleaned in an ultrasonic cleaner for 1 h, and the oil pollution left by wire cutting was removed and dried. The oxidizing temperature was 800, 1000, 1200°C with an oxidizing time of 120 h. The samples were removed and weighed from the furnace every 12 h in the corundum crucible.

Morphology and phase analysis
Bruker D8 Advance x-ray diffractometer (Cu Ka radition; θ=0.03°) was used to detect the phase composition of the material, and Jade software was used to characterize phases in XRD. The morphology of the material was observed by Sigma500 field emission scanning electron microscopy (SEM, ZEISS, Germany). An energy dispersive spectrometer (EDS, Oxford) was used to analyze the element distribution in the material. Figure 1 shows the x-ray diffraction patterns of Al and Cr alloying MoSi 2 . MoSi 2 was an intermetallic compound with a metastable C40 structure (h-MoSi 2 ) above 1900°C and a stable C11b (t-MoSi 2 ) structure at room temperatures [38]. After arc cladding, there was macro residual stress in the sample causing the diffraction peak to shift slightly [39]. The unstable h-MoSi 2 phase could not be completely transformed into the t-MoSi 2 phase at high temperatures due to the high cooling rate of the sample. Therefore, there were a large number of h-MoSi 2 phases in the material at room temperatures.

Microstructure of Al and Cr alloying
The Mo-Al and Cr-Si bonds were slightly enhanced compared with the Mo-Si bond [35], and the MoSi 2 material added with Al consisted of Mo (Si, Al) 2 , h-MoSi 2 , t-MoSi 2 and a small amount of Mo 5 Si 3 . The MoSi 2 material with added Cr consisted of t-MoSi 2 , h-MoSi 2 , CrSi 2 and a small amount of Mo 5 Si 3 . The diffraction peaks of t-MoSi 2 after alloying with Al and Cr elements occupied a dominant position in the pattern. Figures 2, 3 and table 1 are show the changes in element distribution before and after oxidation at 1200°C. After arc cladding, Mo (Al, Si) 2 was formed by Al, Mo, and Si. Cr uniformly distributed in the material (see figure 2).
After oxidizing at 1200°C (see figure 3), the content of Al decreased (see table 1). Al2O3-SiO2 mixed oxide film form at low temperature because of the low formation energy of Al. Cr element changes from the dispersion state to the segregation state. A part of Cr exists in the oxide film in the form of agglomeration (see figure 7), and the other part of Cr exists in the material in the form of segregation (see figure 3). It indicated that adding Al and Cr elements can improve the stability of forming the t-MoSi 2 phase at room temperatures. Figure 4 shows the surface x-ray diffraction patterns after oxidation at 800 1000 and 1200°C for 120 h, respectively. Mo preferentially forms volatile substance MoO 3 with O at 800°C, which hinders the continuity of SiO 2 on the oxidizing surface. According to thermodynamic analysis [34,39], Al 2 O 3 has low free energy and critical partial-pressure value; therefore, Al is preferentially oxidized to Si. The driving force of Al 2 O 3 formation weakens with the increased Al 2 O 3 phase, and Si gradually oxidizes to form SiO 2 with the increased activation energy. Figure 4 shows that the oxidation product of Cr belongs to the Cr 2 O 3 phase with few diffraction peaks. Meanwhile, the t-MoSi 2 and trace MoO 3 phases are detected in the surface layer. Although Cr cannot form a stable and effective oxide layer in the oxidizing environment, it inhibits the formation of volatile substance MoO 3 and improves the high-temperature stability of MoSi 2 .  In summary, the synergistic addition of Al and Cr improves the continuity and compactness of the oxide film. Figure 6 shows the oxidized morphology of Al and Cr alloying MoSi 2 at 1200°C. After oxidation for 120 h, cracks and holes appear in the oxide film formed by pure MoSi 2 (see figure 6(a)). The formation and volatilization of MoO 3 destroy the continuity of the oxide film, leading to cracks and holes in the oxide film eventually. The low volume expansion coefficient of Al 2 O 3 formed by adding Al element can reduce the degree of mismatch and cracks. Therefore, figure 6(b) shows the holes without crack defects.

High-temperature oxidation behaviors
After adding Cr, cracks and oxidized holes of the oxide film decrease (see figures 6(b) and (d)). In table 3, the Mo contents in areas 3 and 4 decrease significantly, and Cr inhibits the volatilization of MoO 3 and improves the stability of MoSi 2 . Table 3 Figure 8 shows the mass change relationship of Al-and Cr-Alloying MoSi 2 at oxidizing temperatures. Mo preferentially reacts with O to form volatile MoO 3 at medium temperatures. Al 2 O 3 has low free energy, so preferential Si is oxidized to form an oxide film to stop the loss of Mo. Therefore, the sample with Al shows increased oxidation, with an average oxidation increase of 4.42 mg cm −2 . The above results show that Cr element has a poor effect on improving the oxidation resistance of MoSi 2 . Therefore, the samples with added Cr show oxidation loss. The average oxidation losses are 0.22 (100% MoSi 2 ), 1.33 (3% Cr+97% MoSi 2 ) and 0.17 mg cm −2 (9% Cr+91% MoSi 2 ), respectively. After adding Al and Cr, the sample shows increased oxidation, with the average increase of 0.18 mg cm −2 (3% Al+3% Cr+94% MoSi 2 ) and 0.06 mg cm −2 (3% Al+9% Cr+88% MoSi 2 ). The synergistic addition of Al and Cr has high oxidized protection for MoSi 2 materials.

High-temperature oxidation kinetics
The driving force of oxidation increases gradually with the increased temperature. Al, Cr and Si prefer Mo and O to form an oxide film to isolate oxygen from the erosion of MoSi 2 materials. The synergistic addition of Al and Cr show increased oxidation. The average increases of oxidation at 1000°C (see figure 8(b)) are 1.10 mg cm −2 (3% Al+3% Cr+94% MoSi 2 ) and 0.56 mg cm −2 (3% Al+9% Cr+88% MoSi 2 ), respectively. The average weight gains of oxidation at 1200°C (see figure 8(c)) are 1.98 mg cm −2 (3% Al+ 3% Cr+94% MoSi 2 ) and 1.47 mg cm −2 (3% Al+9% Cr+88% MoSi 2 ), respectively.   The mass change of samples is approximately the same as parabolic law, expressed by: where, Δm is mass change per unit area, mg; T the oxidation time, h; C the constant; k p the parabolic rate followed the Arrhenius equation, as shown in equation (2):  where, k 0 is the former factor; R the common gas constant, 8.31 (J mol −1 ); T the oxidized temperature, K; Q the activation energy of oxidation reaction (see table 4). Equation (3) is obtained by taking the logarithm on both sides of equation (2). Figure 9 shows the relationship between the oxidation rate and temperature of Al-and Cr-alloying MoSi 2 . Table 4 shows the oxidized activation energy, which is the maximum after the synergistic addition of Al and Cr. It is lower when a single element is added. Figures 4 and 5 show Cr can improve the stability of the MoSi 2 , and the oxidation resistance of the MoSi 2 material is poor. Therefore, the activation energy of MoSi 2 materials with 3% and 9% Cr is lower than that of pure MoSi 2 .
The oxygen affinity of Al is higher than Si, and the free energy of Al 2 O 3 is lower than SiO 2 [33,34]. Thus, the activation energy of the MoSi 2 material added with Al is also lower than that of pure MoSi 2 . Al 2 O 3 -SiO 2 oxide film forms on the surface after adding Al and Cr, which improves the high-temperature oxidation resistance of MoSi 2 materials. 3% Al+9% Cr+88% MoSi 2 has the best high-temperature oxidation resistance, about four times that of pure MoSi 2 . It confirms the conclusions of the above-oxidized interfaces and oxidation products.

Oxidation mechanism of Al-and Cr-alloying MoSi 2 materials
MoSi 2 was selectively oxidized to form the SiO 2 film. The film prevented oxygen from diffusing inward and played an effective protective role in MoSi 2 materials. However, the volatilization of MoO 3 formed holes and cracks in the SiO 2 film (see figure 10(a)). At high temperatures, cracks formed stable oxidation channels. Oxygen diffused to MoSi 2 material through cracks and formed volatile substance MoO 3 with Mo. The mass loss of MoSi 2 materials was fast, with its service ability decreased.
After adding Al and Cr (see figure 10(b)), Al and Cr were uniformly mixed with MoSi 2 and formed the Mo (Si, Al) 2 -CrSi 2 -MoSi 2 materials by arc cladding. Al and Cr reduced the forming temperature of the oxide film that formed at a lower temperature to reduce the oxidation volatilization of Mo. The oxide film after alloying by Al and Cr mainly consisted of Al 2 O 3 , Cr 2 O 3 , and SiO 2 by equations (4)-(8) because the free energy of Al and Cr was lower than Si [34,36]. Cr 2 O 3 existed in the oxide film through agglomeration, and its main function was to hinder the oxidation of MoSi 2 through cracks and to improve the stability of the oxide film. The Al 2 O 3 wrapped cracks formed by preferential Si oxidation of Al to prevent oxygen from entering. Cracks generated by the oxide film were repaired through the flow of SiO 2 to form stable, dense mixed oxide film.

Conclusions
Al-and Cr-alloying MoSi 2 materials were prepared by arc cladding to study the effects on the high-temperature oxidation resistance of MoSi 2 materials. The main conclusions are as follows: (1) MoSi 2 prepared by arc cladding had a large amount of h-MoSi 2 at room temperature because of its high cooling rate. Al and Cr alloying could improve the stability of t-MoSi 2 phase formation at room temperature.
(2) Al 2 O 3 reduced the mismatching degree and the cracks of the oxide film because of its low volume expansion coefficient. Cr had the advantages such as inhibiting the formation of MoO 3 , reducing the formation of holes, and improving the stability of the SiO 2 film. The synergistic addition of Al and Cr could promote the continuity and densification of the oxide film.  (3) A dense and continuous oxide film could be formed at 800°C-1200°C by adding Al and Cr. The increased oxidation increment obeyed parabolic law. From the oxidized activation energy, the oxidized effect of a single element alloyed on MoSi 2 was poor. The synergistic addition of Al and Cr had a better oxidized effect and oxidation resistance. 3% Al+9% Cr+88% MoSi 2 had the best high-temperature oxidation resistance, about four times that of pure MoSi 2 .