Preparation And Properties Of Hybrid Fiber Reinforced Boron Phenolic Ablative Composites

Large-area cabins such as missiles and rockets in aerodynamic thermal-force coupling environment are facing with short-time high heat flux and strong airflow during service,while the existing ablative resin-based composites for cabin thermal protection show a poor performance to high-speed airflow.In order to improve the anti-scouring performance of ablative resin-based composites,the hot-melt boron phenolic resin（BPR）is used as the matrix,organic-inorganic hybrid fiber is served as reinforcement,and laminated paving,stitching,three-dimensional weaving are employed to prepare fabric preforms.Then,the hybrid fiber reinforced plastic（HFRP）finishes are prepared by VIMP-molding process to investigate mechanical properties,ablation resistance and erosion resistance.The main contents include:（1）The viscosity,curing characteristics and thermal stability of the hot-melt BPR were analyzed to determine the optimized processing temperature and curing conditions.The results indicate that the injection temperature in VIMP process is 90℃.The preferably curing system was 120°C/3h+150°C/2h+180°C/2h+220°C/2h,and thethermal decomposition temperature of the cured resin is 120～170℃higher than thetraditional phenolic resin counterparts.（2）The mechanical properties and ablation resistance of laminate prefabricated reinforced BPR composites were investigated.The results show that the mechanicalproperties decrease slightly after 300℃heat treatment,owing to fiber pyrolysiscarbonization plays a dominant role in phenolic fiber/boron phenolic（P/BPR）composites.While,the mechanical property of quartz fiber/boron phenolic（Q/BPR）composites experience an ascent because of the further improvement for joint structurepredominates during the heat treatment.After heat treatment at 550℃,the BPR matrixwas pyrolyzed and the organic components in the sample were completely carbonized,which results in a sharply decrement of mechanical properties for both P/BPR and Q/BPR.For P-Q interlaminar hybrid composites with 3:1,1:1 and 1:3 mixing radio,the 1:1one exhibits the best interlaminar shear due to synergy after carbonization.The interlaminar shear strength of the inter-layer hybrid P-Q/BPR composites is 23%higher than that of the inter-layer hybrid P-Q/BPR composites after completely carbonization.The oxygen-acetylene flame mass ablation rate is reduced by 8%,and the mass ablation rate after engine tail flame test is reduced by 4%,indicating that the interlaminar hybrid P-Q/BPR composite has better interlaminar shear strength,ablation resistance and erosion resistance.The reason for the improvement is that lattice structure of the quartz molten product after high temperature treatment can effectively support the pyrolytic carbon for the interlaminar hybrid composites,while the fiber segment structure of the intralayer hybrid composites after the treatment is easily broken due to its poor adhesion.The bulk carbon product of the composites makes the carbon layer structure denser and more stable than the intralayer hybrid composites carbon layer structure.（3）The effect of stitching on the properties of plain fabric reinforced BPR composites was investigated.The results show that quartz fiber is more suitable to serve as stitching yam than PBO fiber which has a weak interface between BPR matrix.The interlaminar shear strength of the quarts stitched intermixed P-Q/BPR composites was13%higher than that of the unstitched composites after complete carbonization.The flame ablation rate of the intermixed P-Q/BPR composites with quartz fibers after engine tail test is 3.46 g·s^-1,which is 8%and 11%lower than the interstitial P-Q/BPR and intralayer hybrid PQ/BPR composites,respectively.The results indicate that the suture technique can effectively enhance the erosion resistance of ablation resistant HFRP.（4）The effect of 3D weaving on the properties of fiber reinforced composites was investigated.The results show that the shear strength of the original sample and the carbonized sample of the three-dimensional braided P/BPR composite is higher than that of the laminated P/BPR due to a better combination between the phenolic fiber and BPR.However,the 3D braided Q/BPR and hybrid samples are prone to be resin-lacking after prepared by VIMP-molding method,which can greatly weaken the mechanical properties.The mass ablation rate after the engine tail flame test is 3.94 g·s^-1 owing to the higher porosity.It indicates that the ablation resistance,erosion resistance and thermal insulation performance of the 3D braided HFRP are lower than those of the laminated HFRP and the stitched HFRP composite.