Imagine a car that would never experience a tyre puncture, a plane that can remain structurally intact over thousands of flights, or a ship that’s insusceptible to corrosion. These may sound like far-fetched ideas, but that hasn’t stopped materials scientists from developing advanced self-healing materials that could one day achieve this.
It’s inevitable that vehicles frequently used as transport will experience scratches, micro-cracks or dents during their lifecycles – whether due to collisions or environmental conditions. For most private car owners, the consequences range from aesthetic imperfections to personal expense for repairs. However, the effects of damage can be even more impactful for owners of fleets or airlines, where damage to vehicles can significantly increase operational costs. Even without damage, the cost of maintaining larger vehicles quickly mounts up. In 2018, the International Air Transport Association carried out its maintenance cost task force survey. The survey found that, in 2017, airlines spent an average of US$828 per flight hour on maintaining narrowbody aircraft and an average of US$1,410 per flight hour for widebody aircraft.
The total maintenance cost of vehicles such as aircrafts will always be relatively high, due to the expense of routine tasks such as cleaning, inspection and component replacement. Furthermore, in most cases, if cracks occur in key components while a vehicle is in operation, it would be difficult – if not impossible – to tend to immediately. However, vehicle design engineers play a crucial role in helping to minimise future costs of maintenance, and it all comes down to smart material selection.
Self-healing materials promising for the transport industry
One class of materials that has held a lot of promise for design engineers in recent years, especially those in the transport industries, is self-healing materials. As the name suggests, self-healing materials are designed with the capability to repair themselves if they are damaged – without needing manual repair. Here, 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.Too good to be true?
Design engineers might think this too good to be true, and they’d be right, to an extent. In fact, many developments in the study of self-healing materials, and numerous projects, are underway for the transport industry. They include Lamborghini’s partnership with the Massachusetts Institute of Technology (MIT) that, in 2017, announced a self-healing concept car, the Terzo Millennio, for which many of the developments remain conceptual in nature.
However, this should not stop engineers from considering the potential of these materials or how they will come to affect vehicle designs in the years ahead.
What are self-healing materials?
Before examining how self-healing materials might benefit transport applications, we must first define what a self-healing material is. As outlined in the Matmatch 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. But it has, understandably, proven more difficult to imbue these materials with self-healing properties – particularly within metals with less atomic mobility.
Although many types of material technology fall under the ‘self-healing’ label, there is no single way for a material to heal itself. 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. In addition, there are three core mechanisms used to promote self-healing in materials, and they come in the form of encapsulated healing agents, microvascular healing networks, and intrinsic healing. Let’s look at each in turn.
Encapsulated healing is arguably the most common technique for incorporating self-healing properties into materials such as polymers and polymer composites. This mechanism incorporates micro- or nano-capsules of healing agent into the material’s structure, during the manufacturing stage.
Materials that use this mechanism will feature catalysts dispersed throughout the material matrix, alongside the microcapsules of healing agent. When a crack occurs, it will lead the microcapsule to rupture and release the healing agent into the crack. The released agent reacts with the catalyst and undergoes a process that causes it to harden and fill the crack.
One of the earliest practical examples of this was demonstrated in 2001, by Professor Scott R. White and his colleagues, in a polymer epoxy material. In the study, White et al. developed an epoxy composite that mixed 100 parts Epon Resin 828, a blend of bisphenol-A and epichlorohydrin, with 12 parts diethylenetriamine (DETA).
As White reported, “the reaction polymerises dicyclopentadiene at room temperature in several minutes to yield a tough and highly crosslinked polymer network.”
This mechanism is, however, limited by design. Each capsule of healing agent is microscopic to prevent it from compromising the overall integrity of the material. It would be counter-intuitive to employ self-healing mechanisms that repair damage to a material with lower fracture toughness than its non-healing counterparts. The size constraints of the capsules limit the amount of healing agent that each contains. This therefore limits the amount of damage it can adequately 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 that are frequently exposed to extreme changes in pressure and temperature – 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. As Dr Kathleen Toohey noted in a seminal 2007 study in the field of self-healing materials, “Healing in biological systems is accomplished by a pervasive vascular network that supplies the necessary biochemical components.” Toohey led a study that incorporated a similar vascular network into the substrate of an epoxy resin.
Such a material contains a network of microtubes that allow the healing agent to flow to the location of damage by leveraging capillarity, or the ability of liquids in capillary tubes to flow independently of external forces. When a crack occurs, the change in surface tension in the vascular network causes the healing agent to pump to the point of damage, at which point it reacts to embedded catalyst particles and then hardens and seals the crack.
With this design, microvascular healing materials can overcome the issue of repeated localised damage that limits the application of microencapsulated healing. It also boasts great potential for several materials that are commonly used in transport, manufacturing and design.
For example, carbon-fibre reinforced plastic (CFRP) is a composite material that is regularly used in transport due to its comparatively high stiffness and strength at low density and weight. For this reason, it’s increasingly used in everything from chassis and roof frames in cars to train frames and the fuselage of Boeing’s 787 Dreamliner aircraft. CFRPs have been developed with self-healing properties by leveraging capillary networks to transport healing agents, with the healed material often not showing a significant reduction in bend strength. This is noted by G Williams in the 2007 paper, a self-healing carbon fibre reinforced polymer for aerospace applications.
Both encapsulation mechanisms and microvascular structures offer promise for the field of self-healing materials. However, each of these techniques involves the development of extrinsic healing polymers, which require the use of a separate healing agent.
For some materials scientists and researchers, the goal is to develop intrinsic self-healing polymers that regenerate using dynamic chemical bonds within the material itself. Whereas the lifespan of extrinsic healing polymers is limited by the quantity of dormant healing agent present in the material, intrinsic healing materials could theoretically 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, as identified by Yu Yanagisawa in a 2018 research paper, are that “These healable materials are usually soft and deformable. Some healable materials with high mechanical robustness have also been developed by cross-linking with dynamic covalent bonds. However, in most cases, heating to high temperatures (of 120°C or more) to reorganise their cross-linked networks is necessary for the fractured portions to repair.”
Research is still ongoing in this area, particularly in developing robust materials that heal autonomically. Progress has been made, and one study achieved low elastic modulus polymers that can heal in response to temperatures of 70°C.
There have also been a handful of other materials that exhibit intrinsic self-healing properties. A notable example is self-healing ceramics, where the ceramic can repair due to the reaction between oxygen molecules, which enter a crack, and silicon carbide in the ceramic. This reaction forms silicon dioxide, which reacts with another ceramic component, alumina, to form a material that fills the gap and crystallises into a hardened form.
Research conducted in 2018 found that adding trace amounts of manganese oxide to alumina grains allowed the ceramic to heal in under 60 seconds at 1,000°C. This was noted as the operating temperature of the aircraft engines where this ceramic might be used as turbine material.
However, most of these materials are not at a stage where they are suitable for the rigorous conditions of transport. Similarly, these non-autonomic healing processes make them ineffective for transport applications, where the biggest value comes from materials that heal in immediate response to damage during operation.
Applications of self-healing materials in the automotive industry
These materials might promise an exciting prospect for design engineers, but most have not yet been scaled-up to a commercial stage. Although brands such as Lamborghini and Goodyear have teased self-healing cars and tyres, respectively, in recent years, these applications remain largely conceptual.
However, that doesn’t stop us from imagining where self-healing materials might lead or conceiving of how they will fit into future designs. Decades from now, it’s possible that cars constructed of lightweight, self-healing CFRP chassis will travel on concrete roads that autonomically repair potholes using embedded limestone-producing bacteria. Any minor punctures or cracks in the tyres could also be healed by hybrid rubber tyres.
Until this becomes reality, however, the best step that design engineers can take is to select materials that adequately meet the strenuous demands of transport applications. To this end, design engineers should use material databases such as Matmatch to research materials with ideal characteristics, and compare those that are best suited for the task ahead.
For example, molybdenum copper alloy (MoCu30) is ideally suited to demanding automotive and aerospace applications. This material is lightweight with high thermal conductivity – 205 Watts per metre-Kelvin at 20°C – 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 ideal choice for low-stress applications that still require stiffness, good strength and fatigue levels, and a high tensile modulus. Its characteristics make it suitable for automotive pistons, chassis components and even aircraft structural components and brakes.
For protecting materials during demanding applications, one of the current best courses of action is to support materials with specialist coatings. This is another area where developments are being made to achieve self-healing coatings that can reinforce existing structures. Yet it is another area in which there are limited commercial offerings.
Until that changes, design engineers should consider coatings that can protect surfaces against wear and corrosion. As with material selection, choosing a coating with the right properties is invaluable. On Matmatch’s online materials database, there is a selection of alloy powders for coatings that help protect applications against corrosion, while offering a desirable strength-to-weight ratio.
The developments in self-healing materials are exciting and are undoubtedly picking up pace. However, as with most things, it’s important to establish the facts and reality of the current situation. Specifically, that until these materials begin scaling up, it might be best that engineers design for longevity by choosing materials that can go the distance.
Ben Smye is with Matmatch