| Carbon/carbon composites have a series of excellent properties,such as low density,high specific strength,high specific modulus,high temperature resistance,corrosion resistance,thermal shock resistance and so on.They are widely used in aviation,aerospace and brake.During the preparation of carbon/carbon composites by liquid impregnation-carbonization method,the physical and chemical properties of precursors and processing parameters will have a significant impact on the properties of final products.Systematic study of these factors has an important guiding significance for the process optimization and product performance improvement.In the paper,carbon/carbon composites are prepared by liquid impregnation-carbonization method using 2.5D needle-punched carbon felt as reinforcement and phenolic resin solution as precursor.The physical and chemical properties of phenolic resin precursor are analyzed comprehensively,and the effect of different concentrations of phenolic resin on the densification process of carbon/carbon composites is studied.The preparation process of carbon/carbon composites by liquid impregnation-carbonization is optimized.The mechanical properties of carbon/carbon composites with different densities prepared by the optimum process are analyzed.At the same time,the effect of density on the mechanical properties and fracture mechanism of carbon/carbon composites are studied by micro-morphology analysis.Specific research contents and results are as follows:1.The curing behavior,weight loss,gel time,and rheological properties of phenolic resin precursor are studied.The results show that:(1)When the heating rate was 5℃/min,the curing rate of phenolic resin at 150℃is the fastest,and the carbon yield at 800℃was 59%.(2)The gel time of phenolic resin solution is more than 4 hours below 80℃.When the temperature exceeds 160℃,the gel time is only a few minutes.(3)The viscosity of phenolic resin solution increases with the increase of concentration at room temperature.The viscosity of phenolic resin solution decreases with the increase of temperature from 40℃to 90℃.The activation energy of viscous flow of phenolic resin is 63.07 kJ/mol.The viscosity of phenolic resin is sensitive to temperature.In four hours,60℃is the appropriate impregnation temperature,and the viscosity of phenolic resin is sensitive to time at higher temperature.2.The effect of the concentration of phenolic resin on the densification process of carbon/carbon composites is studied.The results show that:the density of preform increases and the density increment decreases with the increase of impregnation,curing,and carbonization cycles.With the increase of the concentration of phenolic resin,the density of preform first decreases and then increases.The preform impregnated with 75%phenolic resin solution has the highest density,and its surface is more compact under optical microscope.Under scanning electron microscope,the preform has the better fiber-matrix bonding at the section and the smaller and less pore.3.Carbon/carbon composites with the densities of 1.1,1.2,1.3,and 1.4 g/cm~3 are prepared using phenolic resin solution with a concentration of 75%as the precursor.Tensile,flexural and compressive properties of the composites are analyzed.The results show that:(1)The tensile and flexural strength first increase and then decrease with the increase of the density of composites,and the sample with a density of 1.3g/cm~3 reaches the maximum;(2)The compressive strength increases with the increase of the density of composites,and the sample with a density of 1.4g/cm~3 reaches the maximum;(3)The tensile fracture of the composites shows brittle fracture characteristics,and the flexural fracture of the composites with a lower density shows plastic fracture characteristics.Compression fracture of X-Y direction shows brittle fracture characteristics,and the compression fracture of Z direction of the composites with a lower density shows plastic fracture characteristics. |
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