Molybdenum disilicide (MoSi2) is a silicon compound of molybdenum, because the radius of the two atoms is not different, the electronegativity is relatively close, so it has similar properties to metals and ceramics. The melting point is as high as 2030 ° C, with electrical conductivity, at high temperatures the surface can form a silica passivating layer to prevent further oxidation, its appearance is gray metal color, due to its quadrangular α-type crystal structure, there is also a hexagonal but unstable β-modified crystal structure. Insoluble in most acids, but soluble in nitric acid and hydrofluoric acid.

First, nature

        MoSi2, a Dalton intermetallic compound with fixed composition, is an intermediate phase with the highest silicon content in Mo-Si binary alloy system. With the dual characteristics of metal and ceramic, it is a kind of high temperature material with excellent performance. Excellent high temperature oxidation resistance, antioxidant temperature up to 1600℃ or more, comparable to SiC; Has a moderate density (6.24g /cm3); Low coefficient of thermal expansion (8.1×10-6K1); Good electrothermal conductivity; The higher brittleness and toughness transition temperature (1000 ° C) has ceramic-like hard brittleness. It has metal-like soft plasticity above 1000℃. MoSi is mainly used as heating element, integrated circuit, high temperature antioxidant coating and high temperature structural material.
        Resistance at high temperature: In an oxidizing atmosphere, a protective film layer is formed on the surface of dense quartz glass (SiO2) burned at high temperature to prevent continuous oxidation of molybdenum disilicide. When the temperature of the heating element is higher than 1700 ° C, SiO2 protective film is formed, which thickens at the melting point of 1710 ° C and fuses with SiO2 into molten drops. Due to the action of its surface extension, it loses its protective ability. Under the action of an oxidizing agent, when the element is used continuously, the form of a protective film is formed again. It should be noted that due to the strong oxidation at low temperatures, the element cannot be used for a long time at 400-700 ° C.

Second,usage

        Molybdenum disilicide, as a structural material used in high temperature components, gas burners, nozzles, high temperature filters and spark plugs of aviation and automobile gas turbines, has become the latest hot spot in the research of intermetallic compound structural materials. The biggest obstacle to its application in this regard is its high brittleness at room temperature and low strength at high temperature. Therefore, low temperature toughening and high temperature reinforcing of molybdenum disilicide are the key technologies for its practical application as structural materials. The results show that alloying and compounding are effective means to improve the toughness and high temperature strength of molybdenum disilicide at room temperature. Generally used for molybdenum disilicide alloying components are only those and molybdenum disilicide with the same or similar crystal junction WSi2, NbSi2, CoSi2, Mo5Si3 and Ti5Si33 and a few other silicides, of which the most ideal is WSi2. However, the advantages of molybdenum disilicide in specific gravity will be obviously lost by WSi2 alloying, and the application is limited to a certain extent. Practice has proved that molybdenum disilicide has good chemical stability and capacitance with all ceramic reinforcing agents (such as SiC, TiC, ZrO2, Al2O3, T iB2, etc.). Therefore, the preparation of molybdenum disilicide matrix composite is the most effective way to improve the mechanical properties of molybdenum disilicide.

Third, preparation method

The main preparation method of MoSi2

        Due to the high melting point and room temperature brittleness of silicides, its preparation and performance testing are difficult, so far there is no complete production specification of structural silicides and their composites. However, since the discovery of molybdenum silicide in 1906, people have developed a variety of preparation methods, summarized as follows:

1. Mechanical alloying (MA)

        Mechanical alloying is a method of synthesizing new materials by high energy ball milling of pure element mixture through mechanical-chemical action, which is a process of continuous cracking and continuous welding of raw material powder particles.

        In the MA process, some factors such as the size of the cold welded body, the degree of pollution of the fracture interface between the lamella and the lamella have a great influence on the formation of the compound. This technology has the following advantages: 1 ball milling can also produce atomic alloying at room temperature; ○2 can produce alloys with very low impurity content; ○3 can flexibly control the solid solution or second phase addition and the grain/particle size of the product, and has a good effect on the final processing and performance. MA technology can be used in the production of MoSi2 powder, and the preparation of MoSi2 by MA technology has been extensively reported. In the MA process, MoSi2 formation mechanism is divided into two types: the system of mixing Mo powder and Si powder according to stoichiometric ratio, MoSi2 is formed by the mechanism of high temperature self-diffusing combustion (SHS), the reaction rate is fast, the reaction product is the low temperature C11b type body-centered square structure α-MoSi2 phase, and the MoSi system is mixed according to non-stoichiometric ratio. The reaction requires a longer incubation period and the reaction process is slower, and it is generally believed that the mechanism of this mechanical alloying is mechanical alloy-induced diffusion control reaction (MDR). The reaction products include the α-MoSi2 phase of C11b body centered square structure at low temperature and the β-MoSi2 phase of C40 hexagonal structure at high temperature. In the MA process, MoSi2 grains can be refined to only 5-10nm. Compared with other preparation techniques, MoSi2 produced by MA technology has no significant difference in hardness and conductivity. However, the ultra-fine structure of the mechanically alloyed powder reduces the hot compression consolidation temperature (about 400 ° C lower than that of the ordinary powder sintering), and the final density exceeds 97%, and the oxygen content can be reduced, and the chemical uniformity is quite good. MA technology has the characteristics of simple process, low production cost and high production efficiency, and is suitable for industrial development and application. However, in the operation process, attention should be paid to avoid the pollution of the powder by the grinding medium and the grinding atmosphere.

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

        The graphite is ground into a test block of 15mm*15mm*15mm, and its surface is cleaned by CSF1A ultrasonic cleaner, and dried for reserve use. Si powder, water and polyvinyl alcohol were prepared into slurry according to a certain proportion and ground in a ball mill for 1h. The graphite block was coated with slurry about 500μm thick by dipping coating method. After drying at 110℃ for 12h, and then processing in a vacuum furnace at 1450℃ for 2h, the gradient SiC inner coating can be prepared on the graphite matrix. Mo powder and Si powder are weighed in a certain proportion, and Si-Mo slurry is prepared by the same process as above. Si-Mo slurry was coated on SiC inner layer by dipping coating method, and SiMo slurry pre-coating with different thickness was coated on SiC inner layer by controlling the dipping coating times of SiMo slurry. It is dried at 110℃ for 12h, and then calcined at 1420℃ in a vacuum resistance furnace for 2h.

Properties: (1) The thickness of Si-MoSi2 outer layer has a great influence on the oxidation resistance of the prepared SiC/SiMoSi2 coating. The outer layer thickness of SiMoSi2 is about 80μm, and the coating shows good oxidation resistance at high temperature of 1400℃. Too thin or too thick has poor resistance to oxidation.

(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 oxidation resistance of the material 2 Liquid silicon penetration and slurry sintering method (plasma spraying) with Mo and Si powder as raw materials, according to Mo∶Si=1∶2(atomic ratio) mixed in the mixer for 24h. The mixed powder was self-propagated into MoSi2 powder under argon atmosphere at high temperature, which was used as one of the spraying powders (self-propagating synthetic powder). The self-propagating MoSi2 powder was synthesized at high temperature through granulation and vacuum heat treatment to obtain the aggregate powder for spraying. Preparation and microstructure analysis of MoSi2 coating: K403 nickel-based alloy is used as the base material, its size should be 10 mm×15 mm, and the surface of the base material is first treated with oil removal, rust removal and sand blasting. APS2000 atmospheric plasma spraying equipment was used, and self-propagating synthetic powder and aggregate powder were used as spraying materials, respectively. The spraying process parameters were as follows: power 50kW, relative distance between spray gun nozzle and sample 150mm, argon flow rate of 40 L/min, powder feeding rate of 18 g/min. The phase composition of MoSi2 powder and coating was detected by D8-Advance X-ray diffractometer. The microstructure of the coating was observed by JSM6380LV scanning electron microscope. The MoSi2 coating prepared by self-propagating synthetic powder contains more Mo5Si3 and Mo phases, which is not conducive to the oxidation resistance of the coating. The MoSi2 coating containing a small amount of Mo5Si3 phase and Mo phase with good density can be prepared by using the aggregate powder as the spraying material.

Properties: (1) For the aggregate powder, the MoSi2 coating is mainly composed of MoSi2 phase and contains only a small amount of Mo5Si3 phase and a very small amount of Mo phase, and its phase composition is better. It can be seen that MoSi2 coating prepared by plasma spraying of aggregate powder can significantly reduce the generation of Mo5Si3 and Mo phases in the coating, and thus greatly improve the oxidation resistance. The average particle size of the aggregate powder is larger, and the smaller surface area reduces its oxidation during the spraying process, resulting in the coating containing only small amounts of Mo5Si3 and Mo phases.

(2) Under the same spraying process parameters, the powder size is too large, which will lead to poor melting of the particles, making the coating loose and porous; If the powder particle size is too small, although the particles can be fully melted, serious oxidation will occur during the flight from the nozzle to the sample, resulting in more light white areas in the coating section, that is, more molybdenum rich phase is produced, which is not conducive to the preparation of a coating with better phase composition. In addition, the powder particle size is too small, it will also cause the "powder blocking" phenomenon in the powder feeding process, resulting in discontinuous powder feeding and not conducive to the preparation of the coating. Both the self-propagating synthesis and the aggregate powder particles can be fully melted during the spraying process, which makes the cross section and surface morphology of the coating better. 【(1) Due to the small particle size of the self-spreading synthetic powder, MoSi2 coating contains more Mo5Si3 and Mo phases, which is not conducive to the oxidation resistance of the coating. (2) Using the aggregate powder as the raw material of plasma spraying, MoSi2 coating containing a small amount of Mo5Si3 and Mo phases can be prepared with good density. 】

3. Preparation method of spark plasma sintering

(1)Kuchino et al. mixed the MoSi powder according to the atomic ratio of 1:2, put it into the graphite mold, put the graphite mold into the vacuum chamber of 6 Pa, applied 40 MPa pressure to the mold, and then injected pulse current into the powder to heat up at 0.17℃/s. The highest sintering temperature was 1 400℃ and kept 600 s in situ synthesized MoSi2 material with a density of 99%, dense MoSi2 contains a small amount of SiO2. Using MoSi2 powder as raw material, the same process is used to synthesize the density of 99% material, and the synthesized material has good oxidation resistance in the accelerated oxidation region (400~700℃).

(2) Shimizu et al. prepared MoSi2 powder by SHS process, and then sintered it in SPS equipment for 10 min at 1 254℃ and 30MPa pressure, and the density reached 97.3%, the grain size was 7.5μm, the Vickers hardness was 10.6 GPa, and the fracture toughness KIC was 4.5 MPa•m1/2, material with a bending strength of 560 MPa. At 1 000℃, the strength of MoSi2 can be maintained at about 325 MPa.

(3)Krakhmalev et al first conducted high-energy ball milling of raw material powder, and then sintered it with SPS, and obtained Mo(Si0.7al0.25)2, Mo(Si0.7al0.25)2/SiC, Mo(Si0.7al0.25)2/0,10,20,30 of C40 structure, respectively %(volume fraction) Al2O3 and Mo (Si0.75Al0.25)2/ZrO2 composites. The hardness of Mo(Si0.75Al0.25)2 matrix material is 14 GPa, the indentation fracture toughness is about 1.84Mpa.m1/2, and the fracture of the material is mainly cleavage. The addition of 20% SiC does not improve the hardness of the material, but the fracture toughness of the material can be increased to 2•48 MPa•m1/2, and the fracture surface has the characteristics of intergranular. When the Al2O3 content is less than 20%, the hardness and indentation fracture toughness of the material are not significantly improved, and the fracture of the material is mainly cleavage, while the hardness of the composite material containing 30% Al2O3 is reduced to 10.2 GPa, the fracture toughness is increased to 3.67 MPa•m1/2, and the fracture surface shows obvious intergranular characteristics. Mo, Mo5Si3, Al2O3 and Mo0.34Zr0.20Si0.46 were found in Mo(Si0.7al0.25)2/ZrO2 composites, that is, Mo(Si,Zr)2 phase, the hardness of the material is about 14 GPa, and the indentation fracture toughness is 2.69 ~ 2.94 MPa•m1/2, compared with Mo(Si0.7al0.25)2, the fracture toughness is increased by 50%, and the reaction of Zr replacing Al in Mo(Si,Al)2 May occur in the synthesis process: AlMo(Si,Al)2+ZrO2=ZrMo(Si,Zr)2+Al2O3 spark plasma sintering (SPS) is still a relatively new process, from the above research can be seen that this process can obtain relatively dense matrix materials and can prepare composite materials, this process in the preparation of MoSi2 The surface has not been widely used, so the performance improvement of the material is not very obvious. However, compared with other preparation methods, the performance of pure MoSi2 has been greatly improved. Because of its high density, at least to prevent the low temperature "Pesting" phenomenon of MoSi2, so this process should be greatly developed in the future.

4. Low Vacuum Plasma Deposition (LVPD)

        Plasma spraying process combines the advantages of fine particle technology and in situ reaction process, in some cases, the net size processing can be carried out. Low vacuum plasma deposition (LVPD) is a low vacuum environment, so that the inert gas (argon or neon) in the formation of high-speed plasma, the spray material powder is melted and deposited with the plasma flow impact on the substrate, forming a very small grain size, good chemical uniformity, unbalanced solubility and close to the final shape of the product material. The relative density of MoSi2 synthesized by LVPD method is 95 ~ 98%, and the microstructure is highly refined, and the hardness and fracture toughness are greatly improved.

5. ReactionSynthesis

        Reaction synthesis is a technique in which solid (liquid) reaction occurs in raw material mixture or solid - gas (liquid) reaction occurs in raw material mixture with external gas (liquid) body to synthesize material. It can be specifically divided into the following methods.

6. Self-propagating high-temperature synthesis (SHS) 
        Self-propagating high-temperature synthesis is a method of synthesizing new materials using the heat energy released by the chemical reaction of reactants. The raw material powder mixture is isostatic pressed, heated at one end so that it ignites a reaction within a small volume range, and then gradually spreads throughout the compact specimen. In the synthesis of MoSi2 by SHS method, the heat of formation released by the high exothermic function between Mo-Si (HfMoSi2= -131.8 kJ/mol) causes the temperature of the adjacent material to rise abruptly and trigger a new chemical reaction
And spread through the entire reactant in the form of a combustion wave, allowing the synthesis reaction to complete spontaneously in the system.
SHS technology has the following advantages: simple production, low investment, full use of energy, short reaction time (a few seconds to a few minutes), fast heating speed, but it is also because of this, so that the reaction is difficult to control, the resulting material
The density is low.

7. insituReactionSintering

        In situ reactive sintering is a technology that heats the elemental raw material powder with a certain chemical mixing ratio to a certain temperature, generates thermodynamically stable compounds in situ chemical reaction and completes the sintering process. The reaction may be fast or slow, exothermic or endothermic, depending on the chemical composition of the starting and ending reactants, the microstructure of the reactants, the presence of pressure, and the thermal boundary conditions. This is a very effective synthesis method for silicides with high melting point and other intermetallic compounds that are not easy to sinter. Compared with the self-spreading high temperature synthesis method, the in situ reactive sintering process can be artificially controlled by changing the process parameters, and the densification can be promoted by pressure and liquid phase sintering. MoSi2 can use the reaction of MoSi and the liquid phase sintering method that melts Si, so that the two process technologies can cooperate, not only reduce the production cost of materials, but also improve the mechanical properties of materials. It is an effective technique for the preparation of so-called "designable materials".

8. Solid displacement reaction (Solid - statedisplacementreaction)

        A solid-state displacement reaction can also be simply described as a diffusion phase transition reaction, a process in which two or three elements and compounds react in situ to form a thermodynamically stable new compound. The morphology of the product is lamellar and agglomerate. The morphology of the product is controlled by thermodynamic and kinetic parameters such as the reaction system and the solubility between the reactant and the product. Ceramic and intermetallic matrix composites can be successfully machined with this technique. However, the slow reaction speed and high cost of solid-state replacement limit its application.

9. XDTM technology (ExothermicDispersiontechnique)

        The XD process was originally developed by MartinMarietta Laboratory as a way to prepare composite materials with ceramic or intermetallic compound particles subtly dispersed in a metal or intermetallic compound matrix. In this process, the component elements of the high-temperature ceramic reinforcement phase are mixed in the matrix phase (usually a metal matrix), and the matrix phase is heated at a temperature much lower than the formation of the ceramic phase to become a solvent, and the component elements produce an exothermic reaction to form micron-sized ceramic particles in the dissolved matrix. Because the dispersion is formed in the in-situ reaction, it is possible to produce a clean matrix contaminated by foreign substances and the interface of the reinforced phase. The MoSI2-matrix composite prepared by XDTM technology is exposed to the air at 450℃ ~ 550℃ for 48 hours without PEST phenomenon. XDTM technology can be reduced to a temperature-controlled casting technology, so there are also disadvantages of macro and micro porosity and macro and micro element segregation.

        In addition to the above, there are several more traditional methods, such as arc melting, casting or powder pressing sintering. However, the traditional melting method is hindered by the high melting point of MoSi2, and oxygen reacts with Si to form a second phase of SiO2 in the matrix and grain boundaries during the material synthesis process, which reduces the mechanical properties of the material. The densification process of Mo-Si compound powder pressing sintering is difficult because of high temperature and long period.

Fourth, concluding remarks

        Although MoSi2 materials have been around for a century and have been studied as high-temperature structural materials for more than a decade, the research on structural materials is still in the experimental stage, except for heating elements and coatings, and Ni3Al and other gold There is still a big gap between the properties of intergeneric compounds and SiC and Si3N4 ceramic materials that have been applied in practice, and many problems need to be solved and explored. MoSi2 material itself also has poor toughness at room temperature, low strength at high temperature and low temperature PEST phenomenon. As a coating, MoSi2 must pay attention to the matching performance of the coating and the substrate CET, and as a heating element, it is necessary to further expand the application field and improve the use temperature. In the future, MoSi2 research should still pay attention to the preparation process and properties of basic materials, prepare dense MoSi2 and MoSi2 matrix composites, and introduce the second phase by compounding and alloying to achieve room temperature toughening and high temperature reinforcement of the material Phase can not reduce the high temperature oxidation resistance of materials, functional gradient materials and laminated composite materials are the future development trend of structural materials.