| Reaction Bonded silicon carbide ceramics have high performances such as high thermo conductivity, high resistance and antioxidant, so it has acquired lots of applications in industrial fields. Previous studies in recent years just focused on fabrication methods of RBSC as well as modeling technics. Attentions paid on the influences of the particles size are also just focused on the difference of SiC powder, but not the graphite powder or carbon fiber. The air pores occurred at the composite materials cannot be avoided because of the fabrication methods at present. The influences of the air in composite materials cannot be neglected as the reason that the distribution, shapes and amount of pores will have great influence on microstructure of composite materials and it cannot be explained just using a empirical formula. As the matter of this, in order to improve the applications of RBSC, studies on influence of microstructures and mechanical performances were carried out on the thesis. A kind of Finite Element Methods software "Palmyra" were used to help to carry out the simulation of influences of the pores in RBSC, suing a digital way to study the influences while some parameters were changed. The main tasks on the thesis are listed as follow:1. Four different kind of graphite powder or carbon fiber will be added to see the different with another kind of sample that just include SiC powder. The volume fraction of graphite powder, SiC powder and phenolic resin powder are 19%,70% and 11% respectively while two batches of experiments were carried out. After that, density testing, three-points bending test and microstructures observation will also be carry out.2. Compare the 5 different samples with different added materials. the results indicate that the samples with larger particle size added have worse mechanical performances and density compared with finer particle size. The samples in first batch contain large volume of pores and acquire worse mechanical properties. The results show that residual silicon and carbon are the main reasons lead to bad performances. Studies on microstructures. XRD experiments indicate that the samples from second batch only contain a-SiC,β-SiC and residual silicon, with out any residual carbon. This demonstrates that the reaction between C and Si are fully. Microphotographs indicate that certain amount of pores remain in RBSC.3. Monte Carol simulation methods are used in the thesis to find out the relationship between the shapes, distribution and volume fraction with elastic modulus. Three different kind of aspect ratio were used, they are 0.2,1 and 5, and respectively which represent the oblate spheres, spheroid and prolate spheres. The outcomes show that under certain scale, the larger the aspect ratio the shape has, the better performances the materials will have. On the same shape model, the finer pores size is, the better performance it will acquire. But the worst performance and reached 99.5%of the best so it cannot be told that finer size have great influence on the performances. If there is a same shape of pores with different size, which means the factor K (size ratio) is not equal to 1, in most of the case, it can improve the ability. But in some cases, worse circumstance also appears. The large volume fraction the prolate shapes the model has, the better performance the model will acquire.4. Comparison between experiments and simulation are also included and purpose to find out the reasons that cause the differences. The reason cause the differences are although prolate pores can acquire better properties, however carbon fiber has larger particle size which could relate to stopping the silicon to fill in. volume fraction of pores and residual silicon did not be considered while carrying out the simulation work. |
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