Molten siliconizing process is a common densification method for graphite, carbon/carbon, carbon fiber reinforced silicon carbide and silicon carbide fiber reinforced silicon carbide composites. This method densifies by introducing molten silicon into porous bodies at high temperatures to react with carbon. The advantages of melt siliconizing process are low cost and short cycle; The disadvantage is that there is free silicon residue in the matrix after melting, which affects the high temperature performance of the material.

The usual methods to remove free silicon from the matrix include vacuum evaporation and in situ reaction elimination of dissimilar elements, in which vacuum evaporation needs to be processed at a higher temperature, and the volatilization of silicon produces pores, which on the one hand is easy to cause damage to the material, on the other hand, the long-term oxidation resistance of the material is unfavorable. The method of in situ reaction elimination can reduce the content of free silicon in the process of melting without secondary treatment, and the prepared material can still maintain low porosity. Often added reaction elements are molybdenum, zirconium, titanium, chromium, hafnium and so on. The research results of Esfehanian et al. show that Si-Ti-MoSi2 can effectively reduce the content of free silicon in the matrix after fusion, and if the process temperature and parameters are suitable, the matrix without free silicon can be obtained. Tong et al. used Si-10Zr alloy to infiltrate C/C porous bodies, and the results showed that only SiC, ZrSi2, ZrC and carbon phases were detected in the materials after infiltration, but no free silicon was detected. In addition, SiC, ZrC, C and a small amount of ZrSi2 phase were detected when Si-Zr25 alloy was used for infiltration, but no free silicon was detected. In order to reduce the content of free silicon in the carbon fiber reinforced silicon carbide composite matrix, Kim et al., Si-Cr alloy was used for melting and infiltration, and the results showed that the added Cr element generated CrSi2 in the matrix, which reduced the content of free silicon in the matrix.

Because of its high temperature resistance, graphite is often used as a high temperature sealing material. A significant reduction in porosity can be achieved by melt siliconizing, but the existence of free silicon restricts its application in high temperature sealing scenarios. Because molybdenum can react with silicon to produce molybdenum disilicide, thereby reducing the content of free silicon, and molybdenum disilicide itself has high melting point and oxidation resistance, so it is suitable as an added component in the fusion reaction. In this paper, molybdenum disilicide powder was added to silicon powder to reduce the content of free silicon in the sample after fusion. The influence of molybdenum on the micromorphology, composition and mechanical properties of the sample after fusion was studied, and the influence on the mechanism of fusion was proposed.

1 Experiment

1.1 Experimental process

The graphite used is high strength graphite (Jinglonte Carbon Technology Co., LTD.) prepared by molding process, the grade is JT2, the density is 1.78g/cm3, the porosity is 15%. First, the graphite is processed into a 50 mm×50 mm×4 mm flat plate, and the dust on the surface after processing is cleaned with deionized water, and then the graphite plate is ultrasonic cleaned in alcohol, and finally dried in an explosion-proof oven at 100 ℃ for 2 hours. The graphite was impregnated with silica powder (325 mesh, purity > 99, Jinan Yinfeng New Material Co., LTD.), silica powder and molybdenum disilicide (5 μm, purity > 99%, Guohua reagent). When silicon powder is used, the quality of silicon powder is 1.1 times that of graphite plate; When the mixed powder of silicon powder and molybdenum disilicide is used (the mass ratio of silicon powder and molybdenum disilicide is 2 ∶ 1), the mass of the mixed powder is 1.7 times that of the graphite plate. The temperature of molten siliconizing is 1500 ℃, the heating rate is 5 ℃ / min, the maximum temperature holding time is 60 min, and the environment is vacuum. The sample after silicon powder infiltration was named GS, and the sample after mixed powder infiltration was named GM.

1.2 Test Characterization

The upper and lower surfaces of the silicated samples were polished by a grinder and processed to a thickness of (3±0.1) mm. The analyte phases were composed by X-ray diffractometer (XRD:D8Advance, Bruker, Germany). The bending strength and modulus of graphite and the samples were tested by universal material testing machine. The morphology and composition were analyzed by field emission scanning electron microscopy (SEM: FEI NANOSEM 450, FEI Corporation, USA) and its accompanying EDAX spectrometer.

2. Results and analysis

The microstructure of the graphite sample after melting. It can be seen from the figure that molten silicon can penetrate into the pore structure of the graphite. The micromorphology of GS samples is similar to that of GM samples. A homogeneous SiC diffusion reaction layer can be observed around the graphite pores in the fusion zone. The silicon carbide coated by the diffusion reaction layer has a regular shape and larger grain size. In the process of melting siliconizing, the region near the graphite is the silicon carbide grain with small grain size, that is, the SiC diffusion reaction layer, while the region far away from the graphite is the grain growth region. It can be observed from the microstructure that there are pores in the region covered by the diffusion reaction layer. On the one hand, the silicon carbide with large grain size falls off during the sample preparation process. On the other hand, 1mol of carbon reacts with 1mol of silicon to produce 1mol of silicon carbide in the process of melting. The molar volume of silicon carbide is 12.4cm3, the molar volume of silicon is 11.1 cm3, and the molar volume of carbon is 6.53 cm3. Since the molar volume of silicon carbide (12.4 cm3) is less than the sum of the molar volume of silicon and carbon (17.63 cm3), the volume after the reaction is reduced by 5.23 cm3. If the space reduced after the reaction is not filled with silicon, pores will be generated.

FIG. 3 Line scan map of GM sample

Figure 4 Morphologies of electrons (a) and backscattered electrons (b) of GM samples after grinding (1000 times)

The electron images and backscattered electron images of GS and GM samples after grinding. By comparison, it can be found that only 2 elements are detected in the GS sample, the lighter color area is silicon carbide or silicon, the darker color area is graphite, while the GM sample can observe 3 different shades of areas, the darker color area is graphite, the gray color area is silicon carbide or silicon, and the bright color is the area containing molybdenum element. The molybdenum element is mainly distributed in the region wrapped by the reaction layer, which may be due to the stronger binding force of carbon and silicon.

FIG. 3 shows the line scanning results of GM samples. It can be seen from the figure that the region containing molybdenum also contains silicon, and the region where molybdenum exists is surrounded by silicon, which is consistent with the results observed in FIG. 2.

FIG. 4 shows the electron and backscattered electron morphologies of the polished GM sample amplified by 1 000 times. It can be seen from Figure 4(a) that the sic/Si phase is dispersed in the graphite matrix. The arrow region in Figure 4(b) is the region containing Mo element, indicating that molybdenum element can be distributed relatively evenly in the matrix and concentrated in the SIC/Si phase.

Figure 5 XRD patterns of GS and GM samples

FIG. 6 Schematic diagram of the melting process of GS(a) and GM(b) samples

Figure 5 shows the XRD pattern of the sample. It can be seen from the XRD results that graphite, silicon carbide and a small amount of silicon phase exist in the GS sample, while only graphite, silicon carbide and molybdenum disilicide phase are detected in the GM sample, and no silicon is detected. At the same time, XRD results show that molybdenum exists in the form of molybdenum disilicide in GM samples, which is consistent with the line scanning results in FIG. 3, indicating that adding molybdenum disilicide to the fusion agent can eliminate the free silicon in the sample after fusion.

FIG. 6 shows the mechanism diagram of the fusion process between GS and GM samples. When only silicon is used as the melting agent, after the melting temperature reaches the melting point of silicon, silicon melts and enters the pore structure of graphite matrix under the action of capillary force, and then reacts with silicon to form silicon carbide. Due to the high temperature of the melting process (1 500 ℃), the carbon in the graphite near the reaction region reacts with silicon through the diffusion reaction layer to form silicon carbide. When the mixed powder of silicon and molybdenum disilicide is used as the melting agent, some molybdenum elements are dissolved in silicon during the melting process, and after entering the pore structure of graphite matrix, carbon reacts with silicon preferently to form silicon carbide. With the consumption of silicon, molybdenum is enriched in the melt; When the temperature drops, the excess silicon combines with the molybdenum element to form molybdenum disilicide.

In terms of material properties, the bending strength and bending modulus of the initial graphite samples were (47±2)MPa and (7.5±0.2)GPa, and the properties of the samples were significantly improved after fusion. The bending strength and bending modulus of GS samples were (115±10)MPa and (24±0.3)GPa. The bending strength of GM samples is (119±6)MPa and the bending modulus is (24±1.3)GPa. The results of mechanical properties show that the addition of molybdenum disilicide has no obvious effect on the bending strength of GM samples at room temperature.

3 Conclusion

Silica powder and silica powder + molybdenum disilicide mixed powder were respectively used to melt and densify graphite. The results showed that:

(1) When mixed powder infiltration is used, molybdenum can enter the graphite matrix, and the molybdenum element is mainly distributed in the area surrounded by the silicon carbide diffusion reaction layer.

(2) The use of mixed powder fusion can eliminate the free silicon content in the sample after fusion, and molybdenum exists in the matrix in the form of molybdenum disilicide.

(3) The performance of the material after penetration has been significantly improved, the addition of molybdenum disilicide has little effect on the performance of the material after penetration, and the bending strength and modulus are not different.