Ablation behavior and mechanical characterization of 2.5d needle-punched high silica–phenolic composites compared with laminated structures

Abstract

The simultaneous physicochemical processes occur both internally and on the surface in materials exposed to high temperature flux and oxidizing/reactive environmental conditions. While temperature dependent decomposition (pyrolysis), phase transformations, and volatile product formation occur within the material, mechanisms such as oxidation, sublimation/thermal decomposition, and flow-induced erosion are activated on the surface. The combined effect of these processes leads to mass and volume loss in both the internal structure and surface of the material. This phenomenon, called ablation, is characterized by material recession and mass loss, especially in thermal protection systems operating under high heat flux. Materials that dissipate heat load by consuming material in a controlled manner through these mechanisms are defined as ablative materials. In the past, thermal protection materials used in high-temperature environments largely relied on solutions such as monolithic ceramics, refractory metals, and single-phase heat-resistant coatings; however, today, ablative composite materials are widely preferred to simultaneously manage heat load and meet structural requirements. Composite materials offer several advantages thanks to the compatibility of the reinforcement phase (fiber/woven structures) and the matrix phase. These advantages include high specific strength at low density, heat absorption through controlled decomposition (pyrolysis), reduced heat conduction due to the formation of a charred layer on the surface, and the ability to manage damage through specific mechanisms. Therefore, in modern thermal protection systems, ablative composites, especially phenolic-based and textile-reinforced composites, stand out as an approach that provides a higher performance-to-weight ratio and more effective thermal protection compared to traditional monolithic solutions. However, in laminated composite structures commonly used in ablative composites, volatile products and gas phases formed because of matrix pyrolysis under high temperatures are transported from the material to the surface along the thickness direction (z-direction). The internal pressure created during the escape of gases from the surface, the growth of microvoids, and stress accumulation at the interfaces can lead to weakening of the interlayer bond. In addition, local material loss and mechanical ablation/erosion on the surface caused by gas escape accelerated damage propagation at layer boundaries, increasing the tendency for delamination. Therefore, it is critical to increase not only the in-plane properties but also the out-of-plane (z-direction) strength and toughness under ablation loads. This need is one of the main reasons why architecture capable of providing reinforcement in the z-direction (e.g., two and a half dimensional (2.5D) needle-punched/three-dimensional (3D) textile-reinforced structures) are preferred over laminated structures. In line with these requirements, reducing delamination and increasing the load-carrying/bonding capacity in the z-direction (thickness direction) of laminated ablative composites has become a critical design objective. High-silica textile reinforcements support insulation performance by limiting heat transfer due to their thermal stability at high temperatures and especially their low thermal conductivity; conversely, phenolic resin consumes endothermic energy through pyrolysis under high heat flux and strengthens the thermal barrier effect thanks to the char layer formed on the surface. However, in classical laminated architecture, internal pressures and interfacial stress generated during the transport of pyrolysis gases in the thickness direction can trigger ply-by-ply separation and delamination due to weak interlayer bonding; this is one of the main damage mechanisms limiting performance, especially in thermal protection system applications where extreme heat loads such as re-entry are encountered in aerospace. At this point, needle-punch technique stands out as an architectural improvement tool based on textile engineering. Needle punching creates through-thickness fiber bridging by mechanically "locking" high silica fabric layers with silica felt structures along the thickness line; thus, interlayer toughness and load transfer are increased while delamination progression is suppressed. In addition, felt-like porous textile layers can improve the predictability of ablation behavior by providing a more stable surface morphology and more homogeneous damage development during gas release and char formation. Therefore, while maintaining the thermal protection advantages provided by the combination of high silica textile reinforcement and phenolic matrix, mechanical integrity and delamination resistance can be simultaneously improved thanks to the z-directional coupling achieved by needle punching. This study aims to prevent delamination in high-silica phenolic ablative composites using needle-punch technique and silica felt structure. In high-silica fiber-reinforced phenolic composites, conventional laminated and needle-punched configurations were experimentally investigated in terms of both mechanical response and torch-ablation behavior. High-silica fabric-felt preforms with different fabric/felt layer ratios and approximately the same fiber volume ratios were produced using a laboratory-type needle-punch setup; impregnated with phenolic resin by vacuum-assisted resin transfer molding and consolidated by hot pressing. Tensile, short beam shear, and three-point bending tests were applied for each configuration; in addition, linear and bulk ablation rates were determined using oxy-propane torch tests. Needle-punched configurations demonstrated a 25–60% reduction in tensile strength and roughly a 25% reduction in flexural strength when compared to the reference laminated structure; however, short beam tests revealed a 40–50% increase in interlaminar shear strength because of better fiber bridging in the thickness direction. While retaining a comparatively high interlayer shear capacity, the silica fabric–silica felt arrangement without needle-punch application only slightly reduced tensile and flexural strength when compared to the reference laminate. This suggests that in situations where process durability and cost are more important than maximum delamination resistance, this more straightforward and easier to manufacture structure might be a viable substitute. Ablation experiments showed that the layered construction had the highest linear ablation rate and the most noticeable layer-by-layer separation. In comparison to the laminated reference, an optimized needle-punched configuration with a balanced fabric/felt stack arrangement and roughly 60–65% fiber volume ratio decreased the bulk ablation rate by about 40–50% and the linear ablation rate by about 90% while maintaining comparable levels of tensile and bending performance. According to post-test analysis, needle-punched composites considerably decreased delamination and produced a more stable char layer under intense heat loading. In conclusion, it was determined that needle-punched high-silica/phenolic composites, with optimized architecture and process parameters, significantly reduced delamination tendency and improved ablation behavior compared to reference laminated structures; however, they maintained acceptable levels in key mechanical performance metrics such as tensile/flexural strength. These findings indicate that these structures are viable and strong candidates for aerospace applications, particularly for thermal protection system components operating under high heat flux.