MANUFACTURING AND CHARACTERIZATION OF NANO-COMPOSITE ABLATIVE MATERIALS

Introduction: An efficient Thermal Protection System (TPS) is mandatory to protect re-entry space vehicles from the severe heating encountered during hypersonic flight through a planet or the Earth atmosphere. Ablative materials represent a traditional approach to thermal protection, used for over 50 years in a broad range of applications. The Laboratory of Materials and Surface Engineering (LIMS) of Sapienza University of Rome has developed carbon-phenolic ablative materials with several densities and compositions. This work has the aim of improving the ablative materials performances by means of nano-fillers addition. The addition of ZrO2 nanoparticles can modify bulk properties, such as mechanical strength and thermal stability, with negligible consequence on density. At the same time, nanoparticles are prone to agglomerate and this phenomenon can drastically reduce the desired beneficial effect: for this reason the ZrO2 nanofillers were modified by several surface functionalization treatments to optimize the particles dispersion in the polymeric matrix.Materials and methods: A resole phenolic resin was selected as matrix material because of the high oxidation resistance, high heat of ablation, high char yield (55-60%) at temperatures above 650°C in inert atmosphere and low viscosity (about 250 cP at T = 25°C). Two rigid graphitic felts (SGL Carbon SE), consisting of long carbon fibers connected together by a carbon binder, were selected as reinforcement materials. ZrO2 nanoparticles (30-60 nm by IoLiTec GmbH) were selected as nano-fillers due to the high melting point (2680°C), low thermal conductivity (2.5 W/mK) and the possibility to be dispersible in many solvents after surface modification. The ablative materials were characterized through SEM micrographs, fourpoint bending tests, compression tests on virgin and charred materials. Composite materials with different concentrations of nano-ZrO2 were exposed in an oxyacetylene ablation testing facility in order to compare the ablation resistance and thermal protection capability of manufactured materials. Results: FE-SEM micrographs highlight the good dispersion of ZrO2 nanoparticles in the polymeric matrix; mechanical tests show that a nano-ZrO2 content of 2 wt.% does not provide significant variation in the rupture stress values while a nanofiller content of 5wt.% drastically improves the mechanical performances, both of the virgin and charred material. The tests performed in the oxyacetylene ablation facility also confirm the enhanced properties of the manufactured nanocomposite materials in terms of weight loss and thermal stability. Discussion: The addition of ceramic nanoparticles has the aim of improving the mechanical properties (rupture stress and elastic modulus) and the ablation performance (mechanical stabilization of char and reduction of recession rate) of the manufactured materials. The ZrO2 nanoparticles provide an effective strengthening and stiffening effect only with a minimum content of 5 wt.%: in this case the charred material exhibits an evident enhancement of rupture stress, leading to a higher mechanical resistance of the surface exposed during the re-entry manoeuvre and, finally, to a lower recession rate of the thermal shield.