The Influence Of ZrB₂on The Ablation, Insulation And Mechanical Properties Of Lightweight Carbon/Phenolic Composites

A spacecraft, entering or traveling through an atmosphere, requires a thermal protection system (TPS) to insure it return safely. Ablative materials are the most reliable TPS. Charring ablative materialscurrently widely used are suitable for the high enthalpy and heat flux change flight environment, and carbon/phenolic composites are the typical example. Light weight is the eternal pursuit of aerospace, and traditional dense carbon/phenolic composites can’t meet the demand. Therefore, lightweight carbon/phenolic composites (C-Ph) are hot research topic in recent years.Introduction of ZrO2improves obviously the insulation performance of C-Ph composites. On the same time, the low melting point of B2O3might fill in cracks of charred phenolic resin and reduce oxygen diffusion into the inside. Addition of ZrB2not only can improve the insulation performance of C-Ph composites, but also oxidation resistancet, since the oxidation products of ZrB2are ZrO2and B2O3. This article employs impregnation method to introduce ZrB2into the porous low density C-Ph composites, and researches effects of zirconium diboride particles on the ablation performance、insulationperformance、ablation mechanism、ablation behavior of carbon-phenolic composites under the oxyacetylene flame. In addition, the mechanical property is also discussed.Infrared spectrum analysis results show that the existence of ZrB2doesn’t interference curing reaction of phenolic resin. However, ZrB2can react with the pyro lysis products CO and CO2of phenolic resulting in ZrO2、B2O3and glass carbon. With increasing content of ZrB2, a tendency of decrease in ablation rate of ZrB2-C-Ph composites is observed in oxygen acetylene torch, where the ration of oxygen to acetylene ratio is1.25, and the existence of ZrB2affects obviously linear ablation rate. The optimum content of ZrB2in C-Ph composites is9%, which shows best ablation resistance and insulation performance.Only carbon fibers are existed on ablated surface of C-Ph composites, and charred matrix is hardly existed on ablated surface.Therefore, the ablation rate of phenolic matrix is higher than carbon fiber in C-Ph composites.The main ablation mechanisms of C-Ph composites are oxidation and char spallation in oxidizing oxyacetylene flame, and the ablation occurs not in surface, but in volume. When the content of ZrB2is1%、3%、5%, ablation rate of phenolic matrix is still higher than carbon fiber, and composites display similar ablation mechanisms to that of C-Ph composites. Moreover, oxidation and char spallation are also the main ablation mechanisms. When the content of ZrB2in C-Ph composites is7%、9%、11%, the ablation rates of carbon fibers and matrix are close to each other, and a continuous ZrO2layer is formed on ablated surface. Because it is difficult for the fragments resulted from char spallation to travel through this ZrO2layer to escape off, the continuous ZrO2layer forms a "net" that can prevent the charred matrix from spallation. That means the protective ZrO2layer might keep the char undergo a longer ablation duration, which can reduce oxygen diffusion into the virgin material and provide an insulation layer. Therefore, oxidation is the main ablation mechanisms of these composites with a continuous ZrO2layer formed, and the char spallation is the minor.The influence of ablation time on performance of9%ZrB2-C-Ph composites is discussed. Results demonstrate mass loss rate and ZrO2layer vary during ablation process. The mass loss rate is the maximummass during the first40seconds, and mass ablation rate falls, wkile ablation process continues. When the ablation process exceeds160seconds, pyrolysis of phenolic is the minor reason for mass loss, and oxidation is the main reason. After200seconds, composites behavior steady-state ablation.The thickness ofZr02layer formed on ablation surface doesn’t always grow, and the cracks emerge in ZrO2layer when thickness grows to a certain value. Local ZrO2layer is stripped, and ZrO2layer regenerates in that place. There fore, the forming-stripping process of a protective ZrO2layer is repeated during ablation process.The fact that ZrB2can improve ablation performance of C-Ph material happens only in oxidizing environment. Introduction of9%ZrO2or9%B2O3also can enhance ablation performance of C-Ph material, but the effects are inferior to9%ZrB2.Compression experiment shows9%ZrB2-C-Ph composites have higher compression strength, compared with C-Ph composites. Phenolic shrinks during curing and pyrolysis course, which will produce a compressional stress to ZrB2particles. On the same time, ZrB2particles impose the opposite stress to the matrix of composites, thus the strength of matrix and the interface strength between carbon fiber and matrix are increased. Introduction of9%ZrB2influences hardly tensile stress of C-Ph composites. Tensile stress of9%ZrB2-C-Ph composites greater than the C-Ph composites when temperature is below1000℃, and tensile strength are all close to55MPa when temperature is above1000℃.