Design And Reaction Mechanism Of HfB₂/phenolic Ceramcizable Phenolic Composites With High-temperature Resistance

Fiber-reinforced ceramicizable phenolic resin composites have many advantages,such as low density and high strength,simply molding process.Also,they can be made into a large-area products.Under high temperature environments,the composites can undergo phase transformation and rapidly form fiber-reinforced porous carbon-containing composite ceramic structures,which have good high temperature resistance and mechanical strength.Now,fiber-reinforced ceramicizable phenolic resin composites have been widely used in the field of thermal protection materials,even ceramicizable technology has become a key technology for future space exploration and space-to-earth round-trip plans.At present,the temperature resistance grade of the ceramicizable phenolic heat-resistant materials is limited,about 600℃-1400℃.However,at higher temperatures,the resin matrix and fiber of the porcelain phenolic heat-resistant material still have the problem of insufficient heat resistance.Based on this,in this paper,the ceramic system and fiber of the ceramizable phenolic composite material were designed through experimental research and theoretical calculation verification,to prepare ceramizable phenolic composites that can maintain the structural integrity at high temperatures.At first,Hf B₂and SiB₆were selected as ceramic systems,and different formulations of Hf B₂-SiB₆/boron phenolic resin composites were prepared by controlling the content of SiB₆in the system.The effect of SiB₆content on the properties of the composites was studied by testing the density,heat resistance and ablation performance of the composites.The results show that the addition of SiB₆in a certain range can effectively improve the thermal stability,high-temperature sintering density and ablation resistance of the composites.In order to explore the high-temperature reaction mechanism of composites,the reaction process of composites with optimal ceramic system at high temperature was analyzed by XRD,XPS,SEM-EDS and thermodynamic calculation.The results show that Hf B₂-SiB₆/boron phenolic resin composites gradually underwent many complex physical and chemical changes at different high temperatures,such as cracking reaction,oxidation reaction,carbothermal reaction,eutectic melting reaction,and nitriding reactions,and finally formed ceramic-glass-cracked carbon multiphase structure.Based on the optimal ceramic system,high silica fiber reinforced composites were prepared,and the influence of ceramic filler content on the basic properties of fiber reinforced composites was studied,and the optimal filler content was determined.The results show that the addition of ceramic filler can improve the thermal stability,dimensional stability at high temperature,the residual rate after static ablation,ablation resistance and mechanical properties of the composites after heat treatment,but at the same time,it can also improve the density of the composites.When the filler content was 50%,the density of the composite was 1.88g/cm³,the bending strength reached 116.96MPa,and the linear ablation rate and mass ablation rate were 0.083mm/s and 0.053g/s,respectively.Based on the optimum content of ceramics,four different fibers were selected as reinforcements to explore the influence of fibers on the properties of composites.It is found that different fibers have great influence on the density,heat resistance,mechanical properties,ablation properties and high temperature stability of the composites.Among them,the HSBPR/CF composite has lower density and better mechanical properties,of which density was 1.54 g/cm³,and its bending strength before and after high temperature treatment reached 288.37MPa and 22.08MPa,respectively.But its ablation performance and heat resistance are poor.Its linear ablation rate and mass ablation rate were 0.078mm/s and 0.056g/s,respectively.Relatively,the HSBPR/ZF composite has good thermal stability and ablation resistance,with linear ablation rate and mass ablation rate as low as 0.005mm/s and0.048g/s,respectively.But its mechanical properties are very poor,of which bending strength was 25.03MPa and 2.37MPa,respectively,before and after high temperature treatment.Based on different fiber reinforced characteristics,hybrid fiber reinforced composites were designed and these properties were studied.The results show that hybrid fiber reinforced composites have good mechanical properties at room temperature.The bending strength of HSBPR/ACF composite and HSBPR/ZCF composite reached 362.89MPa and 291.34MPa,respectively,and the impact strength reached 28.65k J/m²and 28.45k J/m²,respectively.After different high-temperature treatments,the mechanical properties of the composites decreased,but also remained at a high level.When the treatment temperature reaches 1600℃,the bending strength of HSBPR/ACF composite and HSBPR/ZCF composite reached 43.74MPa and39.69MPa,and the impact strength reached 4.99k J/m²and 4.56k J/m²,respectively.At the same time,the composite reinforced by carbon fiber mixed with zirconia fiber has good heat resistance and ablation resistance,with linear ablation rate as low as0.005mm/s and mass ablation rate as low as 0.035 g/s.