Silicon molybdenum rod coating design magnesium aluminum spinel (MgAl2O) has a wide single-phase region in a large temperature range and a high melting point (2105 ℃), which makes it have great potential as a high-temperature antioxidant coating. A stable spinel coating with good crystallization, fine grain and orderly cation distribution can be prepared on SiC substrate by plasma spraying. Some researchers believe that it can be used as a coating to avoid the changes of microstructure and thermal stability at high temperature. However, the impurities in magnesia alumina spinel and the free a12o and MgO phase transition will cause serious damage to the coating. Moreover, in the process of use, SiO2 in the surface glass sealing layer and SiO2 generated by SiC oxidation will form new minerals with mgal2o3. The volume effect during the transformation between minerals has a great destructive effect on the coating. Therefore, as a high temperature oxidation resistant coating, mgal2o3 24 still needs a lot of modification

Research work.

(3) Molybdenum disilicide and tungsten disilicide coatings

Molybdenum disilicide MoSi2 (melting point 2030 ℃) and tungsten disilicide (WSi2, melting point 2180 ℃) are silicon-based intermetallic compounds. Due to their high melting point, SiO2 film can be formed on the surface during use, which plays a role in sealing and preventing oxygen diffusion, making the system a common coating material in the production of CC and SiC Based Composites, especially electric heating elements. MoSi2 and WSi2 high temperature oxidation resistant coatings were prepared on the surface of niobium substrate by slurry sintering. Metallurgical bonding can be achieved between the coating and substrate, and a transition layer can be formed by diffusion. Mosi / SiC and MoSi / Si prepared by embedding method or infiltration method

MoSi2 / WSi2 and WSI / SiC anti-oxidation coatings form gradient distribution through the diffusion of Mo and W, which can greatly increase the anti-oxidation temperature of the matrix material.

In addition, the addition of MoSi2 and WSi2 to ZrO2 aii gradient thermal barrier coating can make the cracks in ZrO2 AIF coating appear and slow down the crack growth rate; Moreover, SiO2 produced by the oxidation of MoSi2 and WSi2 can repair the thermal barrier coating. However, the thermal expansion coefficient of silicon-based intermetallic compound is much larger than that of SiC, and the sintering temperature is very high, which limits the application of the coating system. In addition, MoO3, wo and other volatile substances are generated when moos2 and WSi2 are at low temperature (600C), resulting in catastrophic damage to the coating and sharp deterioration of oxidation resistance of the coating.

(4) MULLITE COATING

Among these oxide systems, mullite, as an oxide with high melting point, has a coefficient of thermal expansion close to that of SiC based materials, good environmental durability and chemical compatibility, and the melting point is as high as 1800 ℃, and there is no crystal transformation when the temperature changes. Therefore, the coating as silicon carbide material will not fall 14 이 due to temperature changes. Therefore, the researchers believe that mullite coating is the most promising silicon carbide protective coating, and the deep oxidation coating is prepared by sol-gel method on the surface of Si3N4. It can reduce the oxidation of matrix at 1300 degree. Similarly, mullite coating is prepared by sol-gel process on the surface of recrystallized silicon carbide (R-SiC). When oxidized at 1500 degree, it can effectively prevent oxygen from contacting with silicon carbide material. During cyclic oxidation, no oxidation product spalling occurs, which greatly enhances the high temperature oxidation resistance of recrystallized silicon carbide. Moreover, with the increase of mullite coating thickness, the oxidation resistance of the coating is further improved by 4 이. However, the research shows that the mullite coating on the surface of SiC matrix is the same as the thin layer of mullite without matrix, and cracks occur during thermal cycle at 1000 ℃. According to the measured thermal expansion coefficient of plasma sprayed mullite coating (see Figure 1.2), During the first thermal cycle (25 ~ 1000), the volume shrinkage of the coating starts from 600 ℃, which may be caused by the precipitation of mullite crystals from the glassy state. The thermal expansion coefficient of the coating after Mullitization is very close to that of SiC. The researchers believe that the crystallization of the glassy mullite coating during plasma spraying is the key to the crack of the coating