Investigation Of Preparation And Performence Of Fiber Reinforced Ceramifiable Silicone Rubber Composites

With the development of science and technology,the flight speed of spacecraft is obviously accelerated,and the flight distance and time are longer.For aerospace vehicles with high Mach number and long flight time,thermal protection materials are particularly important in improving the performance of the whole aircraft.In recent years,the ceramifiable silicone rubber composite has been widely used in the field of thermal protection of aerospace vehicle engines because of its excellent properties.In this paper,low density fiber reinforced ceramifiable silicone rubber composite filled with ZrSi2 was prepared.The physical properties,vulcanization properties and ablative resistance of the composite were analyzed and discussed.The thermal decomposition and the ceramifying reaction mechanism of the ceramifiable silicone rubber composite at high temperature were studied.The effects of fiber type and content on mechanical properties,structural integrity and microstructure evolution in the process of ceramization were studied.In addition,the low density component was introduced into the ceramifiable silicone rubber composite by foaming technology to reduce the thermal conductivity of the composite.First of all,using graphite and ZrSi2 as ceramifiable fillers,the ceramifiable silicone rubber composite with different ZrSi2 content was prepared.The density of the ceramifiable silicone rubber composite increased from 1.28 g/cm~3 to 1.51 g/cm~3 with the addition of ZrSi2,and the increase of density was basically consistent with the theoretical prediction value.The tensile strength increased first and then decreased with the increase of ZrSi2 content.The elongation at break decreased with the addition of ZrSi2,while the permanent deformation increased.With the 30 phr of ZrSi2 content,the maximum tensile strength was 5.08 MPa,the elongation at break was 364.3%,and the permanent deformation was 5.24%.The addition of ZrSi2 increased the thermal conductivity of the ceramifiable silicone rubber composite from 0.553 W/(m·K)to 0.694 W/(m·K).In addition,the thermal conductivity of the composite decreased with the increase of temperature.The addition of ZrSi2 improved the glass transition temperature of the ceramifiable silicone rubber composite,reducing the low temperature flexibility of the composite.The results of thermal analysis showed that the addition of ZrSi2 increased the initial decomposition temperature of the composite from 473.5?to 490.2?.It has little effect on the peak temperature of decomposition.The addition of ZrSi2 significantly reduced the linear and mass ablation rate of the composite.When the addition amount was 30 phr,the linear and mass ablation rate of the ceramifiable silicone rubber composite were 0.055 mm/s and 0.054 g/s,respectively,which were 56.7%and 37.2%lower than that of the composite without ZrSi2.Secondly,the analysis of the thermal decomposition process and the mechanism of the ceramifying reaction of the ceramifiable silicone rubber composite showed that the thermal decomposition temperature of the composite increased with the increase of the heating rate,but the residual rate of the final thermal decomposition products did not change.According to the different peak decomposition temperature at different heating rate,the kinetic equation of thermal decomposition reaction of composite was obtained.In the process of thermal decomposition,the thermal cracking of silicone rubber occurred,the molecular chain broken and rearranged,and a large number of alkanes and oligosiloxanes were produced.A series of thermochemical reactions took place in the process of ceramifying transformation.In the presence of oxygen,all components of the composite underwent oxidation reaction,and the ceramifying products formed in the oxygen poor area were finally oxidized,forming the ceramic products containing Zr-Si-O-C finally.In the microstructure of ablation products,the protective layer produced by ceramization was relatively compact,which had a good barrier effect on the external heat flow and oxidizing atmosphere,and improved the erosion resistance of the composite.In addition,during the ablation process,the ceramic layer,pyrolytic layer and original layer were formed from the ablation surface inward.There was no obvious interface between the layers.Thirdly,the ceramifiable silicone rubber composite with fiber was prepared using aramid pulp,chopped carbon fiber,chopped zirconia fiber and chopped high silica fiber as reinforcement materials.Aramid pulp had a great influence on the hardness of the ceramifiable silicone rubber composite.After adding 10 phr aramid pulp,the hardness of the composite increased from 70.5 to 85.The addition of aramid pulp reduced the tensile strength,elongation at break and permanent deformation of the composite.However,with the increase of the content,the tensile strength of the composite increased first and then decreased,and the elongation at break and permanent deformation decreased.When the amount of aramid pulp was 8 phr,the tensile strength reached the maximum value of 4.68MPa,the elongation at break and permanent deformation were 90.2%and 3.7%respectively.Among the four kinds of fibers,composite filled with aramid pulp had higher tensile strength and the lower elongation at break,while composite filled with chopped carbon fiber had the lowest permanent deformation.The addition of short fiber increased the thermal conductivity of the composite.The composite with 8 phr chopped carbon fiber had the largest thermal conductivity,which was 0.81 W/(m.K),and the composite with aramid pulp had the smallest thermal conductivity,which was 0.68 W/(m.K).The addition of aramid pulp reduced the linear and mass ablation rate of the ceramifiable silicone rubber composite,improving the ablation resistance of the composite.After adding 8 phr aramid pulp,the linear and mass ablation rate were 0.035 mm/s and 0.036g/s,respectively,which decreased by 36.3%and 33.3%respectively.The microstructure showed that the aramid pulp was entangled in the ablation products,which greatly improved the erosion resistance of the composite.Fourthly,the mechanical properties,ablative resistance and microstructure evolution of the ceramifiable silicone rubber composite with quartz fiber fabric,carbon fiber fabric and high silica fiber fabric were studied.It was found that the tensile strength of the composite with quartz fiber fabric was 5.63 MPa,increased by about 20.3%,elongation at break was 27.8%,and permanent deformation was 2.12%.Carbon fiber fabric had a great influence on the thermal conductivity of the composite.Quartz fiber fabric could effectively reduce the ablative rate of the composite.The linear and mass ablation rate ware 0.012 mm/s and 0.022 g/s,respectively.When carbon fiber fabric and high silica fabric cloth were added,the linear ablation rates of composite were 0.015 mm/s and 0.027mm/s,respectively,and the mass ablation rates was 0.022 g/s and 0.031 g/s,respectively.The microstructure showed that the ablation structure with quartz fiber fabric was more compact,which had better erosion shielding effect on heat and oxygen.The fiber fabric and aramid pulp played the role of network reinforcement and dispersion reinforcement respectively in the ablation process of composites.The combination of the fiber greatly improved the ablation resistance and airflow erosion resistance of composites.The bending strength of the decomposition products with quartz fiber fabric increased to 14.38MPa,while the bending strength of the decomposition products of the composite with carbon fiber fabric and high silica fiber fabric decreased to 9.01 MPa and 12.62 MPa,respectively.Finally,the low density fiber reinforced ceramifiable silicone rubber composite was prepared by the method of expandable microsphere foaming.The addition of frits with low melting point reduced the thermal stability of silicone rubber composite,increased the quality and the bending strength of thermal decomposition residue at different temperatures,and formed a self-supporting ceramic structure with certain mechanical strength.The addition of foaming agent greatly reduced the density and hardness of fiber-reinforced ceramifiable silicone rubber composite.The optimum amount of foaming agent was 6 phr.The density of the free foaming composite is 0.521 g/cm~3,the thermal conductivity of the limited foaming composite was 0.328 W/(m.K),and the compression modulus was 1.26 MPa.From the microstructure,the size distribution of the foamed microspheres in the composite was uneven,the diameter was about 50-80?m.There was an obvious interface between the spherical shells and the silicone rubber matrix.The expansion of the microspheres in the foaming process was greatly limited by the composite system.The addition of foaming agent improved the linear and mass ablation rate of the fiber-reinforced ceramifiable silicone rubber composite.When the optimal addition of foaming agent was 6 phr,the linear and mass ablation rate of the composite are 0.031 mm/s and 0.043 g/s,respectively.It can be seen from the microstructure of ablation products that there were many ablation pits on the ablation surface,and a relatively dense ceramic protection structure was formed around the pits.The melting of frits in the ablation process provided a better liquid environment for the ceramifying reaction,improving the mechanical strength and compactness of the ablation products.The erosion resistance of the composite material was also improved.