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The University of Freiburg is developing a waterproof material that can repair itself by shedding its skin like a snake.

Waterproof materials have the potential to self-clean every last drop of syrup on anything from windshields to glass bottles, but these materials tend to have a fragile side. One possible solution is to create waterproof coatings that can heal themselves, as a research team led by Jürgen Rühe at the University of Freiburg in Germany has already done. Their superhydrophobic material repairs itself after damage by shedding its skin like a snake.

Inspired by the discovery of natural water-repellent surfaces, such as the discovery of lotus leaves and water striders, many different superhydrophobic materials have been developed. Some of the successful ones are those that use nanostructure-coated substrates supported by larger microstructures. Although such materials are useful, they are easily damaged and a single scratch is enough to destroy their hydrophobic properties.

To overcome this, Ruhe’s team developed a new self-healing material composed of three layers stacked on top of each other. The top layer is a hydrophobic membrane composed of poly 1H, 1H, 2H, 2H-perfluorodecyl acrylate (PFA), also known as “nanograss,” which has sharp, cone-shaped stems extending from its surface. Nanostructures acquire hydrophobic properties. Underneath this layer is polyvinylpyrrolidone (PVP), a water-soluble polymer used in a variety of medical applications. The bottom layer is a superhydrophobic thin film of silicon nanograss.

Cross-section of self-healing hydrophobic material (Source: American Chemical Society)

The idea is that if the top layer is scratched, the material can be flooded with water that penetrates the scratch and dissolves the middle polymer layer. This allows the damaged top layer to loosen and fall off, exposing the new hydrophobic layer underneath.

According to the team, the molting material is still in the laboratory stage and more work needs to be done to strengthen the top layer to prevent scratches from damaging the lower layers. But if they succeed, their multi-layer approach could lead to more resilient self-cleaning and waterproof materials.

A self-healing pure SiC coating was prepared on carbon/carbon (C/C) composites through a two-step process to protect the C/C composites from oxidation. First, SiC nanowires are obtained on the substrate surface through thermal decomposition of hydrogen silicone oil (H-PSO). Second, a coating with a thickness of 100-200 μm is produced by high-temperature infiltration of silicon in an argon atmosphere. The morphology and phase composition of the coating were observed by X-ray diffraction (XRD) and scanning electron microscope (SEM). Oxidation experiments were conducted on the samples, and the results showed that the obtained coating was composed of pure SiC and had certain self-healing ability. The SiC self-healing coating significantly increases the oxidation resistance of C/C composites, and it allows the C/C composites to survive in air conditions of 1400°C for more than 4 hours without oxidation, with a corresponding mass loss of only 0.71 %.

Due to their high specific strength, high specific modulus, low linear expansion coefficient, and good wear resistance, composite materials are widely used in many industries, including but not limited to aerospace, metallurgy, pharmaceuticals, biotechnology, chemical industry and other industries. However, C/C composite materials are prone to oxidation above 370°C and the oxidation rate increases rapidly when the temperature exceeds 500°C. Oxidation mass loss significantly reduces its performance, which limits its wide application at high temperatures.

The key to solving this problem lies in improving the oxidation resistance of C/C composites at high temperatures. SiC coatings can isolate carbon materials from oxygen, significantly improving their oxidation resistance.

In addition, it also protects against corrosion and significantly increases impact strength. In recent years, considerable efforts have been devoted to the preparation of SiC coatings. In order to reduce the coefficient of thermal expansion (CTE) mismatch between the coating and the C/C matrix, the researchers added heterogeneous elements such as Al2O3, CrSi2 and MoSi2.14-16 during the preparation process. However, foreign elements introduced during the preparation of SiC coatings may contaminate the use environment at high temperatures, making this coating unusable in some applications. Due to their excellent properties, such as thermal stability, high strength, and high thermal conductivity, SiC nanowires have great potential as ceramic reinforcement materials. They are expected to provide effective protection for SiC-coated C/C composites and ensure the purity of the SiC coating.

Source: New Materials Online