The current and potential future methods of developing self-healing materials will be discussed, and their specific applications within the automotive industry, will be explored in the following article.
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Self-healing materials are those with some capacity for automated self-maintenance without external intervention. Similarly to the way in which biological tissue heals, by bringing in replacement material from other sources in the body and utilizing a series of chemical reactions in order to re-create the damaged structure, synthetic self-healing materials typically operate by incorporating chemicals able to interact only with damaged regions of the material, causing growth only in these locations.
How Do Self-Healing Materials Work?
Current self-healing materials are typically constructed from polymers with relatively non-complex structures, meaning that most or all of the bonds within the polymer are identical. Owing to this, material damage that causes bond breakage can be repaired universally within the polymer by the same chemical mechanism.
For example, the Diels-Alder reaction between a conjugated diene and substituted alkene can be facilitated by lewis acid activation, catalyzing the reformation of a bond or introducing new material if provided. Therefore, by placing a suitable catalyst and potentially also replacement material within close range of the broken bond, it can be autonomously repaired.
Since self-healing materials rely on additional chemicals to facilitate bond formation and provide replacement material, these chemicals must be able to reach the damaged site autonomously. This has been achieved using porous polymers in a liquid or gel medium or otherwise by vascular approaches, wherein fine networks of channels containing the necessary chemicals interlace the material with enough density to facilitate easy access to any damaged site.
Capsule-based approaches have also been employed, wherein the encapsulated healing agent is attached to or embedded within the self-healing material and releases the chemicals upon damage. As micro-fractures within the material progress between interior capsules containing the healing agent, a vascular structure is generated, allowing flow throughout the material. This type of system is intended to be one-use only, initially repairing the damaged material and then requiring replacement once the healing agent is released.
Can Ceramics and Metals Become Self-Healing?
Besides polymers, ceramics and cements also show potential as self-healing materials and have even been employed historically for this purpose. For example, Roman-era cement has shown a capacity for self-healing when used for building in contract with fresh water, as dissolved calcium hydroxide is deposited within exposed cracks, sealing up those that are small enough.
Interestingly, calcium carbonate-secreting microorganisms have shown some capacity to facilitate the self-healing of cementitious materials by the same mechanism, and cement mixtures containing these microorganisms have been produced in research. Typically, vegetable or other type of oil is included as nourishment, though in any case, the self-healing properties of the material are thereby limited temporally at this stage.
Ceramics known as MAX phases are named from their construction, entailing a layer of early transition Metal, a group A element (typically Al or Si), and either carbon or nitrogen (X). The layered carbides, nitrides, early transition metals, and some p-block metals demonstrate remarkable self-healing properties. When oxidized at high temperatures in air, metal and A group elements migrate to the damaged site, oxidizing and producing new bulk material.
Self-healing metals have also been explored, though given the general immobility of metal atoms within the bulk structure, it has proven difficult to achieve. Healing agents containing metals capable of precipitating at the damaged site have shown some capacity to repair microfractures. For example, Zhang et al. (2015) produced an iron matrix containing mobile gold atoms able to migrate to the exposed damaged site and deposit there, which significantly extended the lifetime of the metal at 550⁰C compared to iron only.
How Would Self-Healing Materials Be Useful in the Automotive Industry?
One of the most immediately apparent applications of self-healing materials, particularly within the automotive industry, is surface coatings. Paint and other coating materials protect metal from exposure to oxygen in the air and water, which leads to oxidation and rust, and small chips or scratches in paint can provide an ingress point, leading to extensive corrosion. Self-healing surface coatings would therefore protect structural materials from oxidation and significantly improve operational lifespan.
Other high-wear areas of vehicles would benefit from some self-healing capacity, such as tires. Tires are typically damaged by small cracks and fissures that develop, and thus are the perfect application for many types of existing self-healing polymers. Similarly, wear and tear associated with high temperatures and pressures within the engine and other mechanical components of motor vehicles could be the ideal application for many of the forms of self-healing ceramics and metals already discussed, where these conditions act to promote self-healing functionality.
Further, looking beyond the vehicles themselves, self-healing abilities could be incorporated into road surfaces directly. As discussed, cementitious materials are capable of self-healing autonomously by re-forming bonds in aqueous conditions, given that replacement material is provided. Partially autonomous self-healing materials are those that require some external input in order to heal, and in this case, the required materials could be delivered to roads in high infrastructure areas by piping, significantly reducing the occurrence of potholes developed from small cracks and saving money in the long term.
Self-Healing Materials of the Future
At this stage, self-healing materials require close contact with the damaged material, i.e., only small cracks and fissures are autonomously repairable, as the chemical forces driving regrowth require a constant supply of new material and other reagents. Repairing wide cracks or other extensive damage will likely require some level of external input, but by automating the delivery of this material, human maintenance is significantly lessened.
Additional components within vehicles dedicated to the distribution and delivery of healing agents to the appropriate location could therefore become a reality, besides any healing agents incorporated directly into surface coatings and other materials. Highly sensitive electronic components could also potentially be imbued with self-healing functionality, where chemical bonds between electrically conductive molecules or metals are reformed by similar mechanisms as other materials.
Scrap and waste originating from the automotive industry pose a massive burden, with only a relatively minor fraction of the materials involved recoverable, besides losses associated with manufacture and distribution during the lifetime of the vehicle. Self-healing vehicles would produce a paradigm shift in the automotive industry, wherein parts are expected to last much longer and only require replacement owing to serious damage. Even then, with some external input, partially self-healing materials could potentially re-grow to their original form, significantly lessening waste generation.
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References and Further Reading
Sloof, W. G., Pei, R., Mcdonald, S. A., Fife, J. L., Shen, L., Boatemaa, L., Farle, A.-S., Yan, K., Zhang, X., Van Der Zwaag, S., Lee, P. D., & Withers, P. J. (2016). Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy. Scientific Reports, 6(1), 23040. https://doi.org/10.1038/srep23040
Zhang, S., Kwakernaak, C., Sloof, W., Brück, E., Van Der Zwaag, S., & Van Dijk, N. (2015). Self Healing of Creep Damage by Gold Precipitation in Iron Alloys. Advanced Engineering Materials, 17(5), 598–603. https://doi.org/10.1002/adem.201400511
Song, H., Wang, Z.-J., He, X.-D., & Duan, J. (2017). Self-healing of damage inside metals triggered by electropulsing stimuli. Scientific Reports, 7(1). https://doi.org/10.1038/s41598-017-06635-9
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