Self-Healing Coatings

In the Netherlands, and especially at the TU Delft, the self-healing concept has been applied to most industrially-relevant materials. Coatings are very attractive from the perspective of implementing self-healing concepts, as coatings are present everywhere (e.g. all parts of an aircraft are coated), are very prone to damage, and offer broad functionalities such as mechanical, anti-corrosive and aesthetical, resulting in a range of functionalities potentially to be healed or restored. The group NovAM at the Faculty of Aerospace Engineering is performing relevant research in the field of selfhealing coatings for aircraft applications with the objective of extending the lifetime of aircraft structures.

^{TEXT Dr. Santiago J. Garcia, Assistant Professor, AMM, Novel Aerospace Materials (NovAM)}

LESSONS FROM NATURE

When designing new engineering materials and systems, humans can learn a lot from the observation of natural species. After millions of years of evolution, plants and animals have designed very interesting approaches to improve their survival skills which can be useful for us. Water striders, for instance, use a determined hairy pattern in their legs that allows them to walk on the water surface without sinking despite their weight; while the lotus leaf uses a concept based on two length scale topography to repel water and stay permanently clean. Geckos, on the other hand, use a similar strategy but for a diff erent purpose: to walk on vertical walls. These strategies are now being reproduced and copied by humans to design new materials with special functionalities such as: self-cleaning, ultra strength adhesion, reflectance, thermochromism (i.e. surfaces that change color with temperature), anti-icing, low drag… These developments have lead to products available in the market for many years as hook and loop fasteners, better known as Velcro, which was invented after the Second World War after observing and understanding how certain seeds were able to stick to clothes and fur of animals.

When observing nature (including ourselves), one of the most common and interesting survival mechanisms can be easily overlooked for being so common. This mechanism is the one that allows us, from early age, to play without too many worries in case we fall. We, and our parents, implicitly ‘know’ that if we fall very hard and break a bone, after some time of stress-free state, the bone will be repaired and we will be able to function normally again. We also know that if we suffer a superficial wound on our skin, it will only be a matter of time before the skin will form a scar and avoid continuous bleeding, being thus a completely necessary mechanism for our survival. This mechanism is known as bone or wound healing, or simply healing.

What we have learned from nature is that, in terms of material development, natural systems are not the strongest materials (e.g. human skin). To be even more honest, natural materials are very prone to suffering substantial damage during their life existence. Rather than striving for stronger materials, nature has invested more effort in a different survival concept: “damage management”.

So far, engineering materials and systems are traditionally designed to obtain the highest performance during the longest time possible, and once damage occurs the material loses its value and needs to be replaced. But this means high costs related to replacement or repair of materials together with permanent maintenance costs. Moreover, humans design materials and systems that work in very extreme conditions and that are very complicated to reach, for instance, spacecraft or wind turbines. In this respect, it would be fantastic to also emulate nature here as well and to design a completely new type of materials based on damage management: materials and systems that self-sustain themselves for long periods of time.

Triggered by examples from nature, but respecting the character of man-made materials, about ten years ago the concept of “self-healing engineering materials” was introduced. In the Netherlands, and especially at the Delft University of Technology, the concept has been applied to most industrially-relevant materials and industrial systems: metals, concrete, asphalt, fiber composites, ceramics, metaloceramics, polymers, and coatings.

SELF-HEALING COATINGS

Coatings are very attractive from the perspective of implementing self-healing concepts as coatings are present everywhere (e.g. all parts of an aircraft are coated), are very prone to suffer damage, and offer broad functionalities such as mechanical, anti-corrosive and aesthetical, resulting in a range of functionalities potentially to be healed or restored.

At the Novel Aerospace Materials group (NovAM) of the faculty of Aerospace Engineering, the research line of self-healing materials has been pursued from the very start and for almost two years we also have a strong research line on self healing anti-corrosive coatings. Particularly for protective coatings, the application of the self-healing concept can have a high positive impact due to the high probability of incurring damage (e.g. local delaminations, scratches, holes) and the crucial importance of maintaining the potential of protection of the underlying substrate. For example, when a coating(system) is scratched, the damage not only leads to the local direct exposure of the substrate to the corrosive environment (e.g. salty water) but also to much more extensive coating debonding and subsurface corrosion leading to severe corrosion mechanisms such as pitting, filiform and crevice corrosion.

In our search for ideal self healing coatings with applications for aircraft, we have identified three interesting and potentially successful concepts:

1. Corrosion inhibitors (healing species) released into scratches or underfilm zones where corrosive species have reached the metallic surface. Environmentally friendly anticorrosion species are embedded in the coating matrix directly or carried by nano charges to control their release. Upon damage, the anticorrosive species are released and react with anodes and/or cathodes of the underlying metallic surface (Figure 1) creating a protective layer.

2. Damage closure. The intention of this approach is that the damage is not visible anymore, while at the same time offering some barrier protection against corrosive species (Figure 2). While the concept has attractive features and has been shown to work, it does not off er much protection as long as the adhesion between coating and substrate is not restored and the corrosive species as chlorine and water are entrapped below the repaired coating.

3. Surface coverage. This concept aims to cover the metallic surface as a result of a reaction of the released healing agent with species released during the corrosion processes or species present in the environment (Figure 3). One of the approaches using this concept studied at NovAM is the use of water and surface reactive silyl esters; NovAM was the first group to show that embedded silyl esters can give excellent corrosion protection of damaged sites due to the formation of a hydrophobic layer on top of the exposed metallic surface that keeps the metal away from the corrosive environment (Garcia, 2011). One of the used techniques to monitor the halting of the local corrosion process was the Scanning Vibrating Electrode Technique (SVET). Figure 4, shows a map produced with this technique, where the colors are related to very weak currents in the aluminium base metal produced by the local corrosion processes. The figure indicates that after one day immersion in a corrosive environment, the scratched area with the healing agent presents no activity (i.e. surface presents same color and close to 0μA/cm2), while in a non self-healing system the activity would increase considerably (i.e. scratch visible by a diff erent color and higher currents).

^{Figure 1. Controlled release of corrosion inhibitors}

^{Figure 2. Damage closure}

^{Figure 3. Surface coverage with hydrophobic film}

^{Figure 4. Effect of silyl ester on the corrosion protection (SVET measurements)}

While much more work needs to be done to optimize our novel coating systems and to create new approaches, we  remain convinced that in a few years the aerospace coatings industry may start implementing these new concepts, leading to more reliable aircraft coatings for the future. With a lot of innovative research to be done, we look forward to even more AE undergraduates joining our research team and to make their mark on this emerging field in Aerospace Engineering relevant materials science.

_{References: S.J. Garcia, H.R. Fischer, P.A. White, J. Mardel, Y. González-García, J.M.C. Mol, A.E. Hughes, Progress in Organic Coatings 70 (2011) 142–149}