Molybdenum disilicide (MoSi2) is a silicon compound of molybdenum. Because the radii of the two atoms are similar and the Electronegativity is similar, it has properties similar to those of metal and ceramics. The melting point is as high as 2030 ℃, with conductivity. At high temperatures, a silicon dioxide passivation layer can be formed on the surface to prevent further oxidation. Its appearance is gray metal color, originating from its four directions α- Type B crystal structure, also present in hexagonal but unstable β- Modified crystal structure [3]. Not soluble in most acids, but soluble in nitric acid and hydrofluoric acid.

1、 Nature

MoSi2 is the intermediate phase with the highest silicon content in the Mo Si binary alloy system, and is a Dalton type intermetallic compound with fixed composition. Having the dual characteristics of metal and ceramic, it is an excellent high-temperature material with excellent performance. Excellent high-temperature oxidation resistance, with an oxidation resistance temperature of over 1600 ℃, equivalent to SiC; Moderate density (6.24g/cm3); Lower coefficient of thermal expansion (8.1 × 10-6K-1); Good thermal conductivity; Below a higher brittle to ductile transition temperature (1000 ℃), there is a ceramic like hard brittleness. It exhibits a metal like soft plasticity above 1000 ℃. MoSi is mainly used as heating elements, integrated circuits, high-temperature oxidation resistant coatings, and high-temperature structural materials.

Resistance at high temperature: in the oxidation atmosphere, a protective film is formed on the surface of the dense silica glass (SiO2) that is burned at high temperature to prevent continuous oxidation of Molybdenum disilicide. When the temperature of the heating element is above 1700 ℃, a SiO2 protective film is formed, which thickens at a melting point of 1710 ℃ and merges with SiO2 to form molten droplets. Due to the extended movement on its surface, it loses its protective ability. Under the action of oxidants, when elements are continuously used, they form a protective film again. It should be noted that due to its strong oxidation at low temperatures, this element cannot be used for a long time in temperatures ranging from 400 to 700 ℃.

2、 Purpose

Molybdenum disilicide, as a structural material, has become the latest hot spot in the research of intermetallic compound structural materials because of its application in high-temperature components of aviation and automobile gas turbines, gas burners, nozzles, high-temperature filters and spark plugs. The biggest obstacle to its application in this area is its high room temperature brittleness and low high-temperature strength. Therefore, low temperature toughening and high temperature reinforcement of Molybdenum disilicide are the key technologies for its practical use as structural materials. This research shows that alloying and compounding are effective means to improve the room temperature toughness and high temperature strength of Molybdenum disilicide. The components commonly used for Molybdenum disilicide alloying are only a few silicides that have the same or similar crystal junction with Molybdenum disilicide, such as WSi2, NbSi2, CoSi2, Mo5Si3 and Ti5Si33, among which WSi2 is the most ideal. However, WSi2 alloying will obviously lose the advantages of Molybdenum disilicide in specific gravity, and its application will be limited. Practice has proved that Molybdenum disilicide has good chemical stability and compatibility with all ceramic reinforcements (such as SiC, TiC, ZrO2, Al2O3, TiB2, etc.). Therefore, the most effective way to improve the mechanical properties of Molybdenum disilicide is to prepare Molybdenum disilicide matrix composites through compounding.

3、 Preparation method

The main preparation method of MoSi2 is difficult due to the high melting point and room temperature brittleness of silicides, making its preparation and performance testing difficult. So far, there is no complete production standard for structural silicides and their composite materials. Since the discovery of molybdenum silicide in 1906, various preparation methods have been developed, summarized as follows:

1 Mechanical alloying (MA)

Mechanical alloying is a method of synthesizing new materials by high-energy ball milling a mixture of pure elements through mechanochemical interaction. It is a process of continuous fracture and welding of raw material powder particles.

In the MA process, some factors such as the size of the cold welded body, the interlayer spacing between thin layers, and the degree of contamination at the fracture interface have a significant impact on the formation of compounds. This technology has the following advantages: ① Ball milling can also produce atomic level alloying at room temperature; ○ 2 Able to produce alloys with very low impurity content; ○ 3 Can flexibly control the addition of solid solution or second phase, as well as the grain/particle size of the product, and has a good effect on the best processing and performance.

MA technology can be used in the production of MoSi2 powder, and there have been numerous reports on the use of MA technology to prepare MoSi2. During the MA process, there are two mechanisms for the formation of MoSi2. The system of stoichiometric mixing of Mo powder and Si powder forms MoSi2 through the mechanism of high-temperature spontaneous combustion (SHS), with a fast reaction rate and a low-temperature C11b type body centered square structure as the reaction product α- MoSi2 phase, in a non stoichiometric Mo-Si system, requires a longer incubation period and a slower reaction process. It is generally believed that the mechanism of this mechanical alloying is the mechanical alloying induced diffusion controlled reaction (MDR). The reaction product has a low-temperature C11b body centered square structure α- MoSi2 phase with high-temperature C40 hexagonal structure β- MoSi2 phase.

During the MA process, MoSi2 grains can be refined to only 5-10nm, and compared to other preparation techniques, MoSi2 produced using MA technology has no significant differences in hardness and conductivity. However, the ultra-fine structure of mechanically alloyed powder reduces the hot pressing consolidation temperature (about 400 ℃ lower than that of ordinary powder sintering), resulting in a final density of over 97%, and can reduce the oxygen content, with considerable chemical uniformity.

MA technology has the characteristics of simple process, low production cost, and high production efficiency, making it suitable for industrial development and application. However, during the operation process, attention should be paid to avoiding contamination of the powder by the ball milling medium and atmosphere.

2. Dip coating sintering method coating (coating thickness issue)

Grind the graphite into a 15mm * 15mm * 15mm * test block, use a CSF-1A ultrasonic cleaner to clean its surface, dry it, and reserve it.

Mix Si powder, water, and polyvinyl alcohol in a certain proportion to form a slurry, and grind it in a ball mill for 1 hour. Apply approximately 500 coats on graphite blocks using the dip coating method μ M thick slurry. After drying at 110 ℃ for 12 hours, and then treating in a 1450 ℃ vacuum furnace for 2 hours, a gradient SiC inner coating can be prepared on the graphite substrate.

Weigh Mo powder and Si powder in a certain proportion and prepare Si Mo slurry using the same process as mentioned above. Si Mo slurry is coated on the inner layer of SiC using the dip coating method, and different thicknesses of Si Mo slurry precoating are coated on the inner layer of SiC by controlling the dip coating frequency of Si Mo slurry. Dry at 110 ℃ for 12 hours, then calcine at 1420 ℃ for 2 hours in a vacuum resistance furnace. Performance: (1) The outer layer thickness of Si MoSi2 has a significant impact on the oxidation resistance of the prepared SiC/Si MoSi2 coating. Si MoSi2 outer layer thickness is 80 μ The coating exhibits good oxidation resistance at around 1400 ℃. Too thin or too thick is not conducive to oxidation resistance.

(2) The formation of a complete and dense SiO2 glass layer on the surface of the coating after oxidation is the fundamental reason for the improvement of the material's antioxidant performance

The 2-liquid silicon infiltration and slurry sintering method (plasma spraying) uses Mo and Si powder as raw materials, and mixes them in a mixer at a ratio of Mo: Si=1:2 (atomic ratio) for 24 hours. The mixed powder is then self propagating at high temperature in an argon atmosphere to synthesize MoSi2 powder, which is one of the spraying powders (self propagating synthesis powder). MoSi2 powder synthesized by self-propagating high-temperature synthesis was subjected to granulation and vacuum heat treatment to obtain agglomerated powder for spraying.

Preparation and microstructure analysis of MoSi2 coating: Using K403 nickel based alloy as the matrix material, its size is 10mm × 15mm, first remove oil, rust, and sandblast the surface of the substrate material. APS-2000 atmospheric Thermal spraying equipment is used. Self propagating synthetic powder and agglomerate powder are used as spraying materials respectively. The spraying process parameters are: power 50kW, relative distance between spray gun nozzle and sample 150 mm, argon flow 40L/min, and powder feeding rate 18 g/min. The phase composition of MoSi2 powder and coating was detected by D8 Advance Diffractometer; The microstructure morphology of the coating was observed using JSM 6380LV scanning electron microscopy.

The MoSi2 coating prepared by self-propagating synthesis powder contains more Mo5Si3 and Mo phases, which is not conducive to the oxidation resistance of the coating; MoSi2 coatings with small amounts of Mo5Si3 and Mo phases and good compactness can be prepared using agglomerated powder as the spraying material. (Self propagating synthesis of MoSi2 powder has poor performance, so it will not be discussed here)

Performance: (1) For agglomerated powders, the MoSi2 coating mainly consists of MoSi2 and only contains a small amount of Mo5Si3 phase and a very small amount of Mo phase, resulting in a good phase composition. It can be seen that the production of Mo5Si3 and Mo phases in the MoSi2 coating prepared by agglomerate powder Thermal spraying can be significantly reduced, thus the oxidation resistance can be greatly improved. (The average particle size of the agglomerated powder is relatively large, and the smaller surface area reduces its oxidation during the spraying process, resulting in the coating containing only a small amount of Mo5Si3 and Mo phases.)

(2) Under the same spraying process parameters, if the powder particles are too large, it will lead to poor particle melting and make the coating loose and porous; If the powder particles are too small, although they can fully melt, they will undergo severe oxidation during the flight from the nozzle to the sample, resulting in more light white areas on the coating cross-section, which generates more molybdenum rich phases, which is not conducive to preparing a coating with better phase composition; In addition, if the powder particles are too small, it can also cause "powder blockage" during the powder feeding process, causing discontinuous powder feeding and hindering the preparation of the coating. Both self-propagating synthesis and agglomerated powder particles can fully melt during the spraying process, resulting in better cross-sectional and surface morphology of the coating. (1) Due to its small particle size, the self-propagating synthesized powder in MoSi2 coating contains more Mo5Si3 and Mo phases, which is not conducive to the oxidation resistance of the coating.

(2) Using agglomerate powder as raw material for Thermal spraying, MoSi2 coating with small amount of Mo5Si3 and Mo phase and good compactness can be prepared.]

3 Spark Plasma Sintering Preparation Method (1) Kuchino et al. mixed Mo-Si powder in an atomic ratio of 1:2 and loaded it into a graphite mold. The graphite mold was placed in a vacuum chamber at 6 Pa, and a pressure of 40 MPa was applied to the mold. Then, a pulse current was applied to the powder, which was heated at a temperature of 0.17 ℃/s. The highest sintering temperature was 1400 ℃, and the MoSi2 material with a density of 99% was synthesized in situ for 600 seconds. The dense MoSi2 contained a small amount of SiO2. Using MoSi2 powder as the raw material and using the same process to synthesize a material with a density of 99%, the synthesized material exhibits excellent oxidation resistance in the accelerated oxidation region (400-700 ℃).

(2) Shimizu et al. prepared MoSi2 powder using the SHS process, and then at 1254 ℃, 30

Sintering in SPS equipment at a pressure of MPa for 10 minutes resulted in a density of 97.3% and a grain size of 7.5 μ m. The Vickers hardness is 10.6 GPa, the Fracture toughness KIC is 4.5 MPa • m1/2, and the bending strength is 560 MPa. At 1000 ℃, the strength of MoSi2 can be maintained at around 325 MPa.

(3) Krakhmalev et al. first performed high-energy ball milling on the raw powder and then sintered it with SPS to obtain C40 structured Mo (Si0.75Al0.25) 2Mo (Si0.75Al0.25) 2/SiC, Mo (Si0.75Al0.25) 2/0,10,20,30% (volume fraction) Al2O3 and Mo (Si0.75Al0.25) 2/ZrO2 composite materials, respectively. The hardness of Mo (Si0.75Al0.25) 2 matrix material is 14 GPa, and the indentation Fracture toughness is about 1.84 Mpa. m1/2. The fracture of the material is mainly cleavage. The addition of 20% SiC does not improve the hardness of the material, but can increase the Fracture toughness of the material to 2.48MPa • m1/2, and the fracture surface appears intergranular characteristics; When the content of Al2O3 is less than 20%, the hardness and indentation Fracture toughness of the material have no obvious improvement. The fracture of the material is mainly cleavage, while the hardness of the composite containing 30% Al2O3 decreases to 10.2 GPa, and the Fracture toughness increases to 3.67 MPa • m1/2. The fracture surface shows obvious intergranular characteristics. Mo, Mo5Si3, Al2O3 and Mo0.34Zr0.20Si0.46 are found in the Mo (Si0.75Al0.25) 2/ZrO2 composite, that is, Mo (Si, Zr) 2 phase. The hardness of the material is about 14 GPa, and the indentation Fracture toughness is 2.69~2.94MPa • m1/2. Compared with Mo (Si0.75Al0.25) 2, the Fracture toughness is increased by 50%. During the synthesis process, Zr may replace Al in Mo (Si, Al) 2:

AlMo (Si, Al) 2+ZrO2=ZrMo (Si, Zr) 2+Al2O3

Spark plasma sintering (SPS) is still a relatively new process. From the above research, it can be seen that this process can obtain relatively dense matrix materials and can prepare composite materials. This process has not been widely used in the preparation of MoSi2, so the performance improvement of the material is not very significant. However, compared to other preparation methods, the performance of pure MoSi2 has been significantly improved. Due to its high density, it can at least prevent the low-temperature "Pesting" phenomenon of MoSi2, so this process should be greatly developed in the future.

4 Low vacuum plasma deposition (LVPD) Thermal spraying technology combines the advantages of particle refining technology and in-situ reaction technology, and in some cases, it can carry out net size processing. Low Vacuum Plasma Deposition (LVPD) is a process in which an inert gas (argon or neon) is formed into a high-speed plasma in a low vacuum environment. The sprayed material powder is melted and deposited with the plasma stream hitting the substrate, resulting in a material with very small grain size, good chemical uniformity, strong unbalanced solubility, and close to the final shape of the product. The relative density of MoSi2 synthesized by LVPD method is 95~98%, and it shows highly refined microstructure, and its hardness and Fracture toughness are greatly improved.

ReactionSynthesis is a technology that involves the solid (liquid) phase reaction of a raw material mixture or the solid-gas (liquid) reaction of the raw material mixture with an external gas (liquid) body to synthesize materials. It can be further divided into the following methods.

Self propagating high-temperature synthesis (SHS) is the synthesis of new materials using the heat energy released by the chemical reactions of reactants