Self-healing ceramic coatings: a game-changer for transportation

At a glance

  • Light alloys are ideal in aircraft and car manufacturing - providing lightweight materials that improve the vehicle’s efficiency, while reducing emissions
  • However, they are quick to corrode. Previously, corrosion performance was managed using chemical surface treatments but these treatments are now banned due to carcinogenic qualities, and industry is facing huge, unexpected costs in maintain materials - until an efficient alternative is found.
  • Researchers at Manchester are developing high-performance smart coatings, that not only protect the allow from corrosion but also - once compromised, for example by a scratch - have the capability to ’self-heal’
  • This environmentally friendly coating technology could be the key to improving the long-term performance of lightweighting components, which is critical to ensuring the future energy-efficient vehicles and supporting a sustainable consumption of resources.

Light alloys, such as aluminium, magnesium and titanium, are materials with a very low density, ideal for use in manufacturing aircraft and cars. Lightweight, they can improve a vehicle’s energy efficiency, while reducing emissions, and are especially important in the manufacture of electric vehicles, where batteries extremely heavy.

When alloying elements such as copper are added, mechanical performance is improved - but corrosion resistance is decreased. This trade-off means it’s difficult to have a single strategy that improves all elements at the same time. Traditionally, the corrosion performance was optimised using chemical surface treatments. But due to their highly toxic, carcinogenic properties, they are now banned.

In their place, traditional passive coatings are the only alternatives. But because they only work as a physical barrier to the environment, once they fail, they no longer protect the material underneath. Which means the industry is bearing the unexpected cost of replacing corroding light alloys more frequently.

An environmental and economically sustainable solution

To address this challenge, researchers at Manchester are pioneering a new technology: a high-performance, environmentally-friendly smart coatings, that not only protect the allow from corrosion but once compromised - for example by a scratch - have the capability to ’self-heal’, correcting this fail and regenerating a new, protective layer.

Smart coatings can interact with the environment, and respond selectively to specific triggers such as mechanical fractures, or changes in temperature and humidity, for example.

The technology isn’t new - it’s been achieved in organic coatings such as paint, which can have, for example, hydrophobic properties and provide anti-corrosion. But they have low thermal stability and wear-resistance; for example, you can easily scratch a car painted with conventional paint. This means they can’t be used in something as vital as the engine of a car, the landing gear of an aeroplane or the rotor system of a helicopter - these materials will fail in extreme environments.

The team of materials experts, led by Dr Beatiz Mingo, Senior Lecturer and Royal Academy of Engineering Fellow, based at the Henry Royce Institute at The University of Manchester, are exploring two key considerations: Can we transfer these active functionalities, which have so far been restricted to organic materials, to ceramics, which are much more resistant and robust? and is it possible to manufacture ceramic coatings with anti-corrosion and self-healing properties?

Establishing a breakthrough in self-healing ceramic

Using a technique called Plasma Electrolytic Oxidation (PEO), Dr Mingo’s team is looking to achieve active multi-functionalisation of ceramic coatings.

By developing a method to encapsulate corrosion inhibitors into nano-containers, which fit into the holes caused by porosity in the ceramic - normally a bad thing in coatings - her team has proven that ceramic coatings can provide corrosion protection and self-healing on demand.

Through their research, they’ve discovered a thermally stable nano-container, able to interact with the inhibitor, and sensitive to a specific trigger. Various materials fit this criteria, for example halloysite nanotubes, which have a pH release trigger (pH changes can occur at the onset of corrosion).

By inserting this nano-container into the pores in a ceramic coating, they’ve pioneered a process that means, when corrosion starts, the pH change triggers the release of inhibitors from the nano-containers. These then precipitate on top of the metallic substrate, ’healing’ the fault by generating new corrosion protection.

A technology to revolutionise transport - and health

With this fundamental breakthrough established, Dr Mingo’s team is now refining the process to create a single-step approach, where it’s possible to functionalise coating at the same time as synthesising it, to make this process far more effective.

But this pioneering advance is not just a game changer for transport - ensuring providing a coating technology capable of improving the long-term performance of lightweighting components - but has potential in health care.

For example, magnesium can be used as a biomaterial to manufacture bio-absorbable pins and screws for broken bones, which can be allowed to degrade and be absorbed into the body. However, magnesium corrodes rapidly. Using the same principles, Dr Mingo’s technology could devise a way of applying a coating on top of the magnesium implant, to delay this process and match the time that it takes for the bone to heal.