Study On Toughening Method Of Fine-grained Isotropic Graphite

For a long time,the inherent brittleness of graphite materials limited their engineering applications.Especially for fine-grained graphite,the brittleness problem is particularly prominent.Once the crack in the body expands,the component will fail rapidly.At present,the industry and academia mainly focus on process optimization and mechanism analysis for the brittleness of graphite materials,but there is almost no precedent for the method of improving toughness.Therefore,this paper explores two toughening methods for fine-grained isotropic graphite,filling the gap in this research field.The first method is to use spherical graphite（SFG）to control the pore structure of artificial graphite,and explore the possibility of preparing isotropic graphite from spherical graphite.Another method is to add chopped pitch-based carbon fibers to fine-grained isotropic graphite,and use the crack bridging effect of fibers to enhance the resistance of graphite to cracks.Based on the excellent pressing properties of SFG,it was found that adding a certain proportion of SFG can effectively improve the bulk density of coke-based isotropic graphite.Through optical texture analysis,it is found that SFG can effectively improve the pore structure of the samples.The porosity and pore size decrease with the increase of SFG content,and the pore shape also changes from elongated to elliptical.This change in microstructure has a positive effect on the fracture toughness of the samples,increasing the fracture toughness value from 0.77 MPa m^1/2 without SFG to0.90 MPa m^1/2 with 10%addition.Through R curve analysis,this toughening phenomenon may be related to the change of pore structure hindering the crack initiation process.The strength and thermal conductivity of the graphite samples were also found be increased by SFG through flexural strength testing and thermal diffusivity testing.At the same time,the optical texture,fracture toughness and R curve of IG-11 and NBG-18 nuclear graphite were also measured in this test.The effect of filler particle size on toughness was verified,and based on this,judged the toughness test method of this experiment is effective.In this paper,while studying the use of SFG to toughen isotropic graphite,it also explores the method of preparing isotropic graphite with this new filler.SFG is randomly arranged during molding,so that the product avoids the preferred orientation in the macroscopic view,and has an isotropy degree of 1.10.There are a large number of pores formed by the bending of graphite sheets inside the SFG particles.These pores are parallel to the basal plane of graphite and is therefore able to accommodate the c-axis thermal expansion.Therefore,SFG-based isotropic graphite has a coefficient of thermal expansion（CTE）as low as about 2.7·10^-6 K^-1,which is 40%lower than that of nuclear grade graphite IG-110.Compared with coke and microcrystalline graphite,the bulk density of SFG products is higher,but the baking performance is relatively poor.Experiments confirmed that the addition of coke or microcrystalline graphite is an effective measure to avoid cracking behavior,where coke increases the CTE of the sample,while microcrystalline graphite continues to maintain the low CTE of the sample.In terms of fracture toughness,the K_IC value of SFG-based graphite is lower,but it still has a large improvement effect compared with unspheroidized flake graphite.Whisker toughening is a commonly used toughening method for ceramic materials,and it is also applicable to graphite materials.The addition of chopped pitch-based carbon fibers to coke fillers has a very significant effect on improving the fracture toughness of fine-grained isotropic graphite.The fracture toughness value of 3.5mm coke samples after adding chopped fibers can reach 1.01 MPa m^1/2,which is 36.49%higher than that before adding chopped fibers.Through comparative experiments with different fiber contents and different particle sizes coke,it was found that with the increase of fiber content,the fracture toughness of the sample first increased and then decreased,and the smaller particle size in coke particle,the more obvious the fiber toughening effect on graphite.The fracture morphology of the samples shows that the chopped fibers have three different fracture modes in the product,among which the fiber pulling method is the most important toughening method.The R curve analysis proves that the addition of fibers can not only enhance the resistance of the graphite matrix to crack initiation,but also increase the effect of external toughening mechanisms.In addition,chopped fibers can also improve the strength and thermal conductivity of graphite samples.