Study on the Technology of Introducing SiC Micropowder into Carbon Fiber Woven Fabric by Vacuum Impregnation

The precursor impregnation and cracking process is a more advanced and effective method for manufacturing ceramic matrix composites and components, and has obvious advantages compared with other ceramic matrix composites manufacturing processes. However, there are still some problems in the process of the preparation of Cf/SC composites from precursors of polycarbosilane (pcs) by high temperature pyrolysis to silicon carbide as the matrix: (1) The porosity of the material is large. In the organic pyrolysis process of organic precursors, due to the release of a large number of small molecules, the yield of ceramics is not high (50%~60%), and the cleavage products are porous and low in strength. In addition, due to the large change in density before and after matrix cracking (precursors PCS density is 10 ~ 1.2g /, therefore, how to shorten the densification cycle, reduce the internal pores and cracks in the material is the key to improving the performance of ceramic matrix composites. Currently, the use of heat The preparation of Cf composites by impregnation and cracking with precursor assisted by molding can significantly increase the density of the materials, shorten the preparation cycle of the materials, and reduce the production cost of 1451. In addition, the precursor 1h31 with high ceramic yield can reduce volume shrinkage and porosity, or By adding reactive fillers such as AiCrMoW and TaT10 to the carbon fiber braid, chemical reaction between the active filler and the PCS cracking product or chemical reaction can effectively reduce or even eliminate volume shrinkage and pore generation of the precursor pyrolysis process. The addition of inert fillers such as SCSiN4BN, AN, and A2O3 improves the initial density of the braid preform, and also shortens the densification cycle of the material. At the same time, the inertial micropowder does not undergo mass and volume change during the cracking of the precursor, and to a certain extent Inhibits the shrinkage of the fired product and reduces the porosity of the material. Study on the preparation of an inert filler powder SC ceramic matrix composite material impregnated precursor cleavage.

1 Experiment 1 Raw materials and their properties Carbon fiber weaving was used as the skeleton of the material and was woven by Tianjin Textile Engineering Institute in a two-step process. The volumetric content of XY and Z in the three-dimensional direction was 8:1. The X direction was M40JB carbon fiber, Y and Z directions. The basic properties of carbon fiber (JiCi) produced for the Jilin Carbon Plant are listed in Table 1. The silicon carbide micropowder is used as an inert filler. The Zhengzhou Dongfang Mechanical Polycarbosilane (PCS) precursor PCS is synthesized by this key laboratory. The molecular weight is about 1300, and the softening point is 166~176*C. The cross-linking agent and solvent of divinylbenzene (DVB) PCS, Table 1 The basic performance of carbon fiber 2 The process of introducing SC micropowder into the braid The vacuum impregnation method will first The paste prepared from the SC fine powder and ethanol is strongly stirred and mixed well. The slurry is placed in an ultrasonic bath and dispersed by ultrasonic vibration for 3 cmin. The SC fine powder does not sink. The knitted fabric is then placed in a closed container and vacuumed. Slurry, when completely immersed, continue to vacuum for 2cmin and finally stand for 6h to remove and dry.

The vacuum suction filtration method adopts a self-made vacuum suction filtration impregnation device (). First, the braid is placed in the middle of the suction filtration device, and the surrounding is sealed with paraffin. Then the vessel is sealed, the vacuum is sucked for 2Cmin, the slurry is suctioned, and then vacuum is applied. 2Qnin is left standing for 6 hours. drying.

13SC micro powder distribution characterization using Japan-made OLYMPUS universal microscope to observe the distribution of SC micropowder.

143DBCf/SC composites were prepared by vacuum impregnation mass ratio of 1Q flooding solution, 120 °C, 6h cross-linked curing, followed by N2 protection under 1000*C atmospheric pressure pyrolysis to prepare 3DBCf/SC composites, final multiple impregnation Cleavage to increase the degree of densification of the material.

5 Material performance test The bending strength adopts the three-point bending loading method, and the fracture toughness adopts the single-side notch beam method. It is tested on the CSS1101 electronic universal testing machine. Loading speed: 0* 2 Results and discussion 1 The vacuum impregnation method uses carbon fiber braid with vacuum The impregnation of different particle size materials was conducted to examine the volume fraction of SC micropowder introduced into the woven fabric. The numbering rule of the sample is: 01 represents a sample with a particle size of 0.4*SC; a micropowder slurry concentration of 1:1 impregnated; 11 represents a particle size of 1.0 mSC. 1:1 impregnated sample; Vi, V2V3 represent the volume fraction of SC micropowder introduced in the knit after the 1st and 23rd impregnation, respectively. The specific experimental results are shown in Table 2. Table 2 Experimental results of the introduction of SC micropowder by vacuum impregnation The use of vacuum impregnation in the introduction of sc fine powder in the knit, SC fine powders of different particle sizes have a greater impact on the impregnation effect, in the same process The particle size is 0.4% under the conditions! The SC fine powder introduced into the SC micro-powder with the volume fraction of carbon fiber braid higher than 1.0m (the volume fraction of the micro-powder of the A0 group sample is obviously higher than that of the A1 group sample) may be due to the small particle size of the micropowder and the uniform slurry. Less aggregation, less resistance in the impregnation process, easy to fill the pores in the braid; and when the particle size of the fine powder is large, it will inevitably agglomerate on the surface of the braid and block the pores in the braid. It is difficult to carry out subsequent immersion. Therefore, the small particle size of the fine powder facilitates the impregnation.

The effect of different slurry concentrations on the impregnation of SC fines was investigated. The experimental results show that when the mass ratio of SCEOH is 1:1 and 1:2, the impregnation effect is better. The reason may be that when the concentration is low, the amount of micropowder contained in the slurry is small, and the volume fraction of immersed fine powder in the braid is relatively small. At the same time, it was found that when the mass ratio of SCEOH reached 1.5:1, the fine powder could hardly enter the interior of the braid because of the large slurry concentration and poor dispersion. It was easy to create multiple vacuum impregnations on the surface of the braid and introduced the braid. The impact of SC micro-powder volume fraction is also greater. The relationship between the SC micro-powder volume fraction and the number of impregnation times introduced in the braid is found. If the sample is found to be vacuum impregnated with SC micro-powder again, the introduced volume fraction increases more, but the third repeat When impregnated, the increase in the volume fraction of the SC fines introduced is small. Therefore, when the vacuum impregnation method is used to introduce SJC fine powder into the braid, the number of times of immersion is preferably 2 times.

22 Vacuum suction filtration The experimental results of the introduction of SC micropowder in carbon fiber braids by vacuum suction filtration are shown in Table 3. During the experimental process, it was found that the dissolved paraffin wax penetrated into the pores of the braid, affecting the quality of the braid, and the slurry was found during impregnation. The solvent in the extraction speed is too fast, a large number of SC micro-powder deposited on the surface of the braid, resulting in obstruction, making it difficult to further impregnation of micro-powder, and the introduction of micro-powder within the distribution of non-uniform. At the same time, the volume fraction of SC micropowder introduced into the woven fabric by vacuum filtration was only 5.4%, which was not as good as that of the vacuum method A01 at the same concentration (6.9%). As a result, only one sample was prepared for the process and no further research was conducted.

Table 3 Experimental results of the introduction of SC micropowder by vacuum suction filtration Characterization of the 23SC micropowder distribution Metallography of micropowder distribution in a fabric after vacuum impregnation of the SC micropowder As can be seen from the figure, the distribution of the SC micropowder introduced in the braid is uniform and can Effectively fills the pores between the fiber bundles.

24 Influence of the introduction of SC micropowders on the properties of the material Experimentally preliminarily introducing SC micropowder into the woven fabric and impregnating the prep vacuum into the SC micropowder Distribution Metallography Preparation of 3DBCf/SC composites, the mechanical properties of the materials are shown in Table 4 Pre-impregnated SC micropowder The density of the 3DBCf/SiC composites prepared after 5 cycles of impregnation and cracking of the carbon fiber braid reaches about 1.7 g/cm3, which is higher than the blank-like density, which significantly shortens the densification cycle of the material, and the bending strength of the composite reaches 300~ 31 (MPa fracture toughness of 16 ~ 17MPam1/2, strength increased by 20% ~ 25%, toughness increased by 30% ~ 40% show that under the same immersion cycle, can improve the mechanical properties of 3DBCf/SC composite density.

Table 4 Mechanical properties of 3DBCf6C composite materials 3 knots using vacuum method The introduction of SC fines with a particle size of 4 m in carbon fiber braids is superior to SC fines with a particle size of 1.0 m.

When SiC micropowder was introduced by vacuum method, the mass ratio of SiC and SiO2 was better than that of 1:2. The vacuum impregnation could introduce SC powder with the volume fraction of about 10%.

The effect of introducing SC micropowder into carbon fiber braid in vacuum impregnation method is obviously better than vacuum suction filtration method.

The density of 3DBCf/SC composites prepared by impregnating carbon fiber braids pre-impregnated with SC fines for 5 times is about 7g/cm3, which shortens the densification cycle of the materials. The bending strength of the material reaches 300~310MPa and the fracture toughness reaches 16~.