Investigation Of Preparation And Performance Of Ablation Resistant Silicone Rubber Composites

With the development of solid rocket technology,the thermal protection requirements of the combustion chamber and afterburning chamber are becoming more and more severe.Especially,the ablation rate of the carbon-carbon skeleton insulations tends to increase in the afterburning chamber due to the oxygen enriched environment.During the ablation process,silicone rubber?SR?composites could change into a ceramic-like char with good anti-oxidation and mechanical erosion resistance,so it has attracted significant interest in recent years.In this paper,silicone rubber ablative composites were prepared by adding single-phase or multi-phase fillers to a basic formula using high-temperature vulcanized silicone rubber as matrix,and the mechanical properties,thermal stability and ablation properties of the resulting composites were investigated.The main work and detailed results are summarized as follows:The thermal decomposition behaviors of methyl vinyl silicone rubber?MVS?and methyl phenyl vinyl silicone rubber?MPVS?under nitrogen and air atmosphere were investigated by thermogravimetric analysis?TGA?.Moreover,the effects of different silicone rubber ratio,the content of fumed silica and phenolic fiber on the tensile properties and ablation rates of the rubber composites were studied to determine an optimal basic formula.An organically modified montmorillonite?MMT?masterbatch was prepared by solution intercalation under high mechanical stirring,and then it was applied to fabricate the MMT/SR composite.The MMT content did not take apparent effect on the tensile properties of the composites.After incorporation of MMT,the TGA results showed that the resulting composites exhibited lower initial decomposition temperature but higher maximum decomposition temperature in comparison to the basic formula.The linear and mass ablation rates presented earlier decrease and later increase trend with increasing MMT content.Incorporation of 10 phr MMT,the linear ablation rate reached the minimum value,a decrease of 22.5%compared to the basic formula.There was a sintering phenomenon on the ablated surface,meanwhile,smaller cracks and more pores were observed in the fracture char surface than that of the basic formula.The addition of two-dimensional MMT and the formation of a viscous binder were considered to provide a barrier effect,slowing the diffusion of pyrolysis gases,which could increase the accumulation of gas in the charring layer.According to X-ray diffraction,the peak associated with basal reflection of stacked layers did not appear after ablation,suggesting that most of MMT platelets randomly distributed in the char.The surface of graphite nanoplatelet?GnP?was treated with different methods such as aminopropyltriethoxysilane?APTES?,vinyltrimethoxysilane?VTMS?and Triton X-100,the results showed that MVS composite had a better overall performance after VTMS treatment.The addition of VTMS modified GnP as a second filler to the 10 phr MMT/SR system resulted in a sharp decrease in elongation at break of the composites,however,the values were still above 300%.The TGA results obtained in a nitrogen atmosphere showed that GnP improved the initial decomposition temperature of the composites by nearly 40°C,and promoted the formation of char residue.In an air environment,the presence of GnP filler did not lead to significantly improved resistance to oxidation.Compared to 10 phr MMT/SR sample,the linear and mass ablation rates of the composite after further incorporating4 phr GnP decreased by 38.7%and 26.7%,respectively.The char surfaces of the ablated composites were dense and compact in the presence of GnP filler,however,some microcracks appeared on the surface with increasing GnP content.The fracture char surfaces of the ablated composites were continuous with no evident cracks,and a large number of nanowires were observed.The formed nanowires exhibited reinforcing effects that served as local reinforcements to maintain the char strength,which was expected to improve the anti-oxidation and mechanical erosion resistance of the char.It was found that most of the resultant nanowires exhibited a heterostructure composed of a?-SiC core and an amorphous SiO₂ shell layer.The nanowires were formed in-situ through the vapor-solid mechanism,and GnP acted as the carbon source as well as the active substrate for growth of nanowires.SiC_w/MMT/SR composites were fabricated by selecting VTMS modified SiC whisker?SiC_w?as the second filler,and the resulting composites exhibited significantly higher tensile strength,but slightly lower elongation at break in comparison to 10 phr MMT/SR sample.The ablation properties of the composites were greatly increased after the addition of SiC_w.When the SiC_w content was 6 phr,the linear ablation rate and mass ablation rates were reduced by 47.3%and 45.2%compared to 10 phr MMT/SR sample,respectively.In the ablation process,the ablated SiC_w with molten silica coating could easily adhere and bond the other components,in this case,the char was far denser and harder than the 10 phr MMT/SR sample.With low SiC_w content in the composite,some large-scale microcracks were formed in the fracture char surface,but not throughout the entire section.Afterwards,the large-scale microcracks disappeared gradually with increasing SiC_w content,and a lot of newly formed nanowires distributed in the char.The newly formed nanowires combined with oxidized SiC_w acted as micro-skeleton and bridge to improve the tenacity of the char.In our present work,no additional metal droplet was detected on the tips of the nanowires,suggesting that the growth mechanism changed from vapor-liquid-solid to vapor-solid at high temperature in the presence of oxide impurities in SiC_w acting in the role of catalyst.