Perperation And Ablation Properties Of Vitreous Silica Fabric/phenolic Resins Modified With Preceranic PPOSS

Vitreous silica fabric/phenolic resin(VSF/PR)as a kind of resin-matrix thermal protective materials is widely used in the ablation and thermal protection of longtime flight vehicles.However,under high-temperature environment aerodynamic heating,long-time ablation and shear of high-temperature flow can cause thermal deformation of the thermal protective VSF/PR material and its structure,which makes VSF/PR heat-insulating property decrease greatly.Strength and loading capacity of the VSF/PR material significantly degrade under aerodynamic force and thermal loading.Therefore,it is necessary to modify the VSF/PR material and increase in anti-ablation and high-temperature mechanical properties.In this thesis,three kinds of pre-ceramic polyhedral oligomeric silsesquioxanes(PPOSS)were employed as modifiers to design and create a kind of modified anti-ablation composites,POSSs/VSF/PR.The PPOSS modifiers in PPOSS/VSF/PR can be conversed to crystal/partial crystal,Si O2 and Si C under high-temperature environment,and then VSF/PR can be modified by Si O2 and Si C in time,anticipating increases in two characters of ablation resistant and mechanical strength for VSF/PR at a higher temperature.Three POSSs were selected as modifiers for the modification of the VS/PR composite:(1)tetramethylammonium-POSS(TPP),containing the porous Si O2 crystalline structure;(2)(γ-glycidyloxy-propyl)-POSS(GPP),cured to form a crosslinked network;(3)Dodecaphenyl-POSS(DPP),containing 12 phenyls,making be rich-carbon and stiffness states.The POSSs can be prepared through sol-gel method and then characterized by:FTIR,NMR(1H,13 C and 29Si),MS and molecular dynamic simulation and quantum calculation.Three POSS modifiers were employed as preceramic POSSs(PPOSS).The structural evolution and transformations of PPOSS under three pyrolysis steps(25–500、500–1000 和 1000–1500°C)were studied.The transformations of PPOSS were characterized by XRD patterns,SEM images,FTIR spectra,TG-DTA curves and SEM/EDS images and data for calculations of the transformation trends.A law of the transformation nature related to PPOSS structure directly was given;method and technology for transformation and modification of the composites as preceramics on time were established.By comparing and assessing,the TPP modifier is the best one among three modifiers for the transformation.Ablation properties of the composite modified with TPP were enhanced greatly,and mass and linear retios of the ablations were decreased in 4.0 and 3.3 times by 20 s for the ablation under the temperature of 2000°C.The specimen of the materials before and after the modified,VSF/PR matrix and PPOSS/VSF/PR,can be prepared using chemical reactions under conditions of temperature,pressure and pressing mould,and then the properties of the specimen were tested,including,mechanics,thermal and ablation resistance.The results shown: since PR is macromolecular resin,all other properties are poorer than those of PPOSS/VSF/PR,because of PPOSS special structures.As the results of above,TPP and DPP are the better modifiers.DPP transformation was completed with richcarbon and more phenol rings and its high-temperation tensil was 3.2 MPa,increasing in 1.9 times under the temprature of 1000°C.The ablation and protecting from heat mechanism of the organic and inorganic parts was studied based on a 3-layer ablation structure model and TG-DSC curves,respectively.SEM images of layer ablation structures from the surface melting layer and the carbonization layer were studied and indicated that the protection mechanisms of the ablating and protecting from heat by absorbing heat,self-closing porousness to block heat and melting with crystal particle resistant to tracks from strong treating flows were given by contributions of PPOSS/VSF/PR.TG-DSC curve of the PPOSS/VSF/PR organic parts from the decomposition layer were studied and indicated that the protection mechanisms of self-absorbing heat and decomposition absorbing heat were given by contributions of PPOSS/VSF/PR.Meanwhile,mechanism of protection for the two parts with shield each other and dynamic three-layer mould for the ablation structure,were given.