How far are self-healing vehicles from becoming a reality?
30 April 2021
Here, Ben Smye, Head of Growth at materials search engine Matmatch, explores the potential for self-healing materials in the transport industry.
Imagine a car that never experiences a puncture, a plane that remains structurally intact over thousands of flights, or a ship that’s insusceptible to corrosion. These may sound far-fetched, but that hasn’t stopped materials scientists from developing advanced self-healing materials that could one day achieve this.
It’s inevitable that transport vehicles will experience scratches, micro-cracks or dents during their lifecycles – whether due to collisions or environmental conditions. The maintenance cost of vehicles will always be relatively high due to routine tasks such as cleaning, inspection and component replacement. Vehicle design engineers play a crucial role in minimising future maintenance costs, and material selection is key.
One class of materials that holds a lot of promise in the transport industries is self-healing materials. As the name suggests, self-healing materials have the capability to repair themselves – without manual intervention. The goal is to extend the lifespan of materials and their applications and, in some cases, the healed material boasts greater properties than it did in its pre-damaged state.
Design engineers might think this too good to be true, and they’d be right, to an extent. Numerous projects in the study of self-healing materials are underway for the transport industry. They include Lamborghini’s partnership with the Massachusetts Institute of Technology (MIT) that announced a self-healing concept car, though many of the developments remain conceptual.
However, this should not stop engineers from considering the potential of these materials or how they will affect vehicle designs in the future.
Before examining how self-healing materials might benefit transport applications, we must first define what a self-healing material is. As outlined in Matmatch’s A-Z of smart materials whitepaper, self-healing materials include mainly polymers and elastomers. There is a growing focus on other materials, such as ceramics and metals, though it’s more difficult to imbue these with self-healing properties – particularly within metals with less atomic mobility.
Although many types of material technology fall under the ‘self-healing’ label, there are many ways for materials to heal themselves. Instead, materials scientists develop the materials with specific mechanisms and techniques in place that determine how the healing takes place.
There are two types of self-healing process: autonomic, where the process is fully self-contained and requires no external input; and non-autonomic, which requires external stimuli to trigger the healing process. Additionally, there are three core mechanisms used to promote self-healing in materials: encapsulated healing agents, microvascular healing networks, and intrinsic healing.
Encapsulated healing is arguably the most common technique for incorporating self-healing properties into polymers and polymer composites. This mechanism incorporates micro- or nano-capsules of healing agent into the material’s structure.
Materials using this mechanism feature catalysts dispersed throughout the material matrix, alongside the microcapsules of healing agent. When a crack occurs, the microcapsule ruptures and releases the healing agent. The released agent reacts with the catalyst and hardens, filling the crack.
This mechanism is, however, limited by design. Each capsule of healing agent is microscopic to prevent compromising the material’s overall integrity. The size constraints also limit the amount of healing agent each capsule contains, restricting the amount of damage they can repair.
There is also only a finite number of capsules, each of which can only heal the material once. If an application is likely to experience repeated localised damage – such as aircraft components – then this mechanism may not be the best long-term option.
Microvascular healing systems
To overcome the single-use limitations of the embedded capsules approach, materials scientists took a leaf out of Mother Nature’s book. In 2007, Dr Kathleen Toohey led a study that incorporated a vascular network similar to those in biological systems into the substrate of an epoxy resin.
This material contains a network of microtubes that allow the healing agent to flow to the location of damage. When a crack occurs, the change in surface tension causes the healing agent to pump to the point of damage, where it reacts to embedded catalyst particles and then hardens to seal the crack.
This design boasts great potential for several materials that are commonly used in transport. For example, carbon-fibre reinforced plastic (CFRP) is a composite material regularly used in transport. CFRPs with self-healing properties have been developed by leveraging capillary networks to transport healing agents, with the healed material often not showing a significant reduction in bend strength.
For some researchers, the goal is to develop intrinsic self-healing polymers that regenerate using dynamic chemical bonds. Whereas the lifespan of extrinsic healing polymers is limited by the quantity of healing agent present in the material, intrinsic healing materials could offer a near-endless capacity for reparation.
Achieving intrinsic healing is no easy feat, especially for materials that would offer practical benefit to the transport industry. The typical challenges are that healable materials are often soft, deformable, and usually need to be heated to extremely high temperatures to repair. Progress has been made, and one study achieved low elastic modulus polymers that can heal in response to temperatures of 70ºC, but research continues.
A notable example of another material type that exhibits intrinsic self-healing properties is self-healing ceramics. The reaction between oxygen molecules and silicon carbide in the ceramic forms silicon dioxide, which reacts with alumina to fill the gap and crystallise into a hardened form. Research conducted in 2018 found adding manganese oxide to alumina grains allowed the ceramic to heal in under 60 seconds at 1,000ºC – the operating temperature of the aircraft engines where this ceramic might be used.
These materials might promise an exciting prospect for design engineers, but most have not yet been scaled-up to a commercial stage. While some brands have teased self-healing cars and tyres, these applications remain largely conceptual.
However, that doesn’t stop us from imagining where self-healing materials might lead. It’s possible that futuristic cars constructed of a lightweight, self-healing CFRT chassis with hybrid rubber tyres will travel on concrete roads that autonomically repair potholes using embedded limestone-producing bacteria.
Until this becomes reality, however, the best step that design engineers can take is to select materials that meet the strenuous demands of transport applications. To this end, design engineers should use material databases such as Matmatch to research materials and compare those which are best suited for the task.
For example, molybdenum copper alloy (MoCu30) is ideal for demanding automotive and aerospace applications. This material is lightweight with high thermal conductivity – 205 Watts per meter-Kelvin at 20ºC – and good performance across a wide range of temperatures, with a low thermal expansion.
Likewise, an aluminium-copper metal matrix composite, such as AL427915 as offered by Goodfellow, makes an excellent choice for low-stress applications that still require stiffness, good strength and fatigue levels, and a high tensile modulus. It’s suitable for automotive pistons, chassis components and even aircraft structural components and brakes.
The developments in self-healing materials are picking up pace. However, as with most things, it’s important to establish the reality of the current situation. Until these materials begin scaling up, it’s best that transport engineers design for longevity by choosing materials that can go the distance.
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