Main properties of silicides of the Mo - Si system

. Using magnetron-ion sputtering, a layer of metallic molybdenum 1 –2 μm thick was deposited on the surface of a silicon single crystal grown by the Czochralski method in an atmosphere of an inert argon gas at ambient temperature. According to the results of the experiment, pure Mo layers 2 μm thick deposited by magnetron reactive sputtering from a highly pure metal molybdenum target onto a cold silicon wafer substrate with a thickness of 1.5 mm. They have simultaneously high conductivity and transparency only in the case when a strictly defined deposition rate of molybdenum metal corresponds to a given partial pressure of argon in the volume. The sputtered targets are disks 40 mm in diameter with a thickness of 3-4 mm. The technological cycle of product processing includes the stage of target cleaning. Sputtering of a metal Mo target in pure argon Ar without the addition of oxygen promotes the formation of opaque metal films with very good conductivity. X-ray diffraction analysis of silicon monocrystals with a Mo metal-coated surface revealed compounds Mo3Si and MoSi0.65 in the molybdenum-silicon system. Silicon silicide MoSi2 was found to undergo an allotropic transformation in the temperature range 1850 ÷ 1900 °C, and the low-temperature variety  -MoSi2 has a tetragonal structure. The high-temperature form of  -MoSi2 has a hexagonal structure. The results of the study with an atomic scanning microscope showed that the chains of silicon atoms, connecting with Mo atoms, form zigzags passing through the prisms of the Mo structure along parallel X and Y axes.


Introduction
Molybdenum, like other refractory metals, is characterized by a unique set of physical, mechanical and chemical properties, which ensured its wide application in various fields.The heat resistance and heat resistance of a silicide coating on molybdenum increase significantly if the MoSi2 layer is doped with metals, for example, chromium and iron [1][2][3][4][5].The high heat resistance of coatings in the temperature range of 1300÷1450°C is due to the formation of a continuous vitreous silicon dioxide film, which, in the presence of a small number of alloying elements in it, becomes more fusible compared to unalloyed, has greater fluidity, is able to heal the resulting defects and facilitates the relaxation of thermal stresses during the time of a sharp change in the temperature of the product.With an increase in the operating temperature, the molybdenum silicide compound in the diffusion volume is the dominant factor in the diffuse dissolution of molybdenum disilicide into the metal [6].
In the study of the structure of the disilicide MoSi2, X-ray was detected in the molybdenum-silicon system, and later compounds Mo3 Si and MoSi0.65 were found.
According to the equilibrium phase diagram of the state of Mo -Si, molybdenum forms a refractory silicide with silicon (Fig. 1) in this system there are three compounds: molybdenum disilicide MoSi2, lower molybdenum silicides Mo5Si3 and Mo3Si [7,8].The solubility of silicon in solid molybdenum is 3.35 at.% at 1820 o C and 9 at.% at 2025 o C. The region of solid solutions based on the Mo3Si compound is practically absent.

Experiment
As a substrate, a single crystal of p-and n -type silicon with a purity of at least 99.9%, 5×12 mm in size and 1.5 mm thick, was used.Before coating, the surface of the silicon substrate was cleaned with a cambric cloth with a mixture of polyrite and rectified alcohol diluted in distilled water.At present, both in our country and abroad, new polishing compositions, polyrites from cerium and zirconium dioxides, as well as ready-made suspensions based on silicon and aluminum oxides are widely used [9,10].After the substrates were loaded into the working chamber, the surface of the silicon single crystal substrates was subjected to processing using a high-voltage ion source in an argon atmosphere with a 5% oxygen content.
Pure molybdenum metal Mo is used as a target.The physical thickness of the resulting metal layer was calculated based on the optical thickness nh=λ0/4, measured by interference extrema, was 1÷2 μm.The film deposition rate (nm/min) was determined based on the physical thickness obtained and the coating time.
Next, silicon single crystals with a Mo metal-coated surface were subjected to thermal hardening at various temperatures in quartz ampoules in a vacuum.X-ray analysis revealed the formation of compounds of various types of molybdenum silicides at the Mo-Si interface and in the bulk of the diffuse silicon layer.
Silicides of transition metals, including molybdenum, do not belong to interstitial phases, since large silicon atoms cannot be introduced into the pores of metal lattices.Silicon atoms replace metal atoms and form complex crystalline structures in the form of graphite-like networks.Silicides are characterized by layered structures with rather sharp separation of layers of metal atoms and silicon atoms, which facilitates shear deformation and reduces creep resistance at elevated temperatures.

The discussion of the results
The present diagram (Figure 1) has been modeled using some thermodynamic data.The (Mo) phase was assumed to obey Henry's law, and the solubility of Mo in the solid state in (Si) was assumed to be zero.Liquidus is partly speculative due to a lack of experimental evidence.However, modern thermodynamic calculations show that the enthalpy of mixing in a liquid is strongly negative.The authors of [11] determined that the eutectic between Mo3Si and Mo5Si3 occurs at 2020 °C and 26.4 at .%Si, and also determined that the intermediate phase MoSi2 undergoes a polymorphic transformation at 1900 °C.He also pointed to the presence of a reaction involving MoSi2, Mo5Si3 and MoSi2 at 1850 °C, which appraisers consider speculative, requiring further study.Mo3Si has a structure of the closest cubic packing type or a type close to it.The large compactness of the lattice emphasizes the metallic nature of the Mo-Si bond, but there are also covalent bonds between metal atoms in the phases.
The melting point of Mo5Si3 silicide is 2180 20 o C, the width of the homogeneity region at 1700 o C is from 37 to 40.35 at .% silicon.
The MoSi2 compound melts at 2020 ± 20 °C, the homogeneity range is from 65.8 to 66.7 at.% silicon, has a tetragonal structure.
According to [13], the homogeneity region of MoSi2 obtained by diffusion saturation in vacuum can be several percent and tends to increase with increasing siliconization temperature.The differences in the concentration of elements are: for Si = 2.52 0.5%; for Mo = 2 0.5%.
The low-temperature form of -MoSi2 is a tetragonal cell with 2 Mo atoms and 4 Si atoms.Si atoms form a frame, in the voids of which is Mo.The structure can also be considered as consisting of layers parallel to the (010) plane with the closest hexagonal packing.The layers alternate in the order ABAB..., layer.B is displaced in the direction of the X axis by a/2.The shortest Mo-Si distance is c/3.Chains of silicon atoms form zigzags passing through Mo prisms parallel to the X and Y axes.Comparison of the obtained results with a set of standard X-ray diffraction patterns (ASTM) [14] also made it possible to determine the composition of the Mo-Si structure.Figure 3 shows the X-ray diffraction pattern of the Mo-Si structure.The lattice parameters of the structure are calculated by the formula where,  -is the Bragg angle determined from the x-ray - = ℏ 2 +  2 +  2 Miller indices.The x-ray shows characteristic peaks of the initial components of molybdenum MoSi2 and silicon SiO2, which are shown in Fig. 3.During the diffusion process, silicon is partially oxidized with residual oxygen in the volume, as a result, the single crystal undergoes structure and phase formation -crystallization and recrystallization.Contiguous grains of crystalline matter form a common boundary.The intergranular contact expands, and the process of coalescence of grains develops.The X-ray diffraction analysis of the obtained samples shows a uniform distribution of molybdenum disilicide grains over the entire volume of the doped layer.
The protrusions of the chains are directed towards each other, the distances between the Si atoms in the protrusions and in the chains are equal, i.e., a three-dimensional silicon framework is created.High-temperature form -MoSi2 has a hexagonal structure (table 1).

Conclusions
The use of thicker Mo coatings on the silicon surface does not give desirable results, because with an increase in the thickness of the metal, cracks appear in the transition layer of the structure, which leads to a decrease in the service life of the structures.It is possible to significantly slow down undesirable diffuse processes at the interface between the coating and the base by creating barrier layers.Diffusion inhibition is observed when the diffusing element forms multicomponent compounds.A promising direction in the protection of molybdenum products from oxidation is the creation of composite multilayer coatings, in which each layer performs a specific function -provides heat resistance, prevents parasitic interaction of the coating with the base; assists in the relaxation of stresses in the coating, smoothes out the drops in the system.Studies of the properties of protective silicide molybdenum components made it possible to formulate the main factors initiating the degradation of the protective properties of single crystals, ensuring their thermal stability and coatings during their hightemperature operation in an oxidizing atmosphere.

E3SFig. 1 .Fig. 2 .
Fig. 1.Silicon-molybdenum state diagram [4] Singular points of the Mo-Si system.There are three eutectics in the system:  Mo3Si -Mo5Si3 at 26.4 at .% silicon and a temperature of 2020 o C ;  Mo5Si3 -MoSi2 at 54 at .% silicon and a temperature of 1900 o C;  MoSi2 -Si at 98.5 at.% silicon and a temperature of 1400 o C.In addition, according to[10], at a temperature of 1850 o C, there is a eutectoid MoSi2  Mo5Si3 + MoSi2 and at 1900 o C peritectic MoSi2 + P MoSi2.Mo 3 Si silicide is formed by the peritectic reaction Mo + Si = Mo3Si at 2025 ± 20°C and has a cubic structure with a period a = 0.4890 ± 0.0002 (Fig.2).

Table 1 .
Crystal chemical characteristics of molybdenum silicides Some physical properties metal silicides Mo is given in table.2.