Reversing Structural Damage: Self-healing Materials
No matter how robust an automated control system might be, physical damage is a reality that plagues all machines. Thanks to new breakthrough materials, some failures may be prevented—or even reversed.
A newly emerging field of ‘self-healing materials’ offers a lot of promise in terms of increasing the longevity and reliability of manufactured goods. Consider a material that, when damaged, begins to repair itself either automatically or through changing the environment to promote repair. This has many potential applications, as it is a way to repair a part instead of removing it and replacing it.
What are Self-healing Materials?
Self-healing materials are those which can repair themselves after experiencing some form of damage. Small cracks, such as those that grow during continuous or repeated stresses, thermal shock or cycling, or other mechanisms can mean the premature failure of a component.
Consider the repeated stress on a gear. Every time the gear cycles, the teeth experience a repeated, equal stress. Even though this stress is (hopefully) contained to the elastic region of deformation, meaning the teeth should not fail, the repetitions will lead to fatigue, where small cracks begin to develop, eventually causing the gear’s teeth to fail. This may point to a new transmission or another major repair in short-term order.
Figure 1. This microscope view of a material shows many regions of mixed material solidified as crystals. The edge of each crystal structure is a small crack which could grow under repeated stress until it spreads across the entire component. Image used courtesy of Canva
Suppose that instead of gradually getting larger, those small cracks slowly heal themselves and that the gear’s material was able to patch these small cracks over time, in a similar manner to the way skin repairs itself after a small cut.
Self-healing materials offer this promise. In the immediate future, they will not be able to repair large cracks or catastrophic damage but are instead targeted towards microcracks from fatigue and other failure mechanisms.
How do Self-healing Materials Work?
There are several common methods of healing small amounts of damage, such as microcracks. The most common method is by applying heat to the material to increase the rate of diffusion, polymerization, and crosslinking, depending on the specific material.
By applying controlled heat to a polymer, bonds begin to form across tiny cracks, returning the material to its original state.
Consider chocolate pudding, a polymer, in a bowl. After a few scoops of it have been removed, the pudding’s shape is different. However, if a little heat is applied (or a lot of time has passed), the pudding will eventually return to the shape of the bowl, with very few remaining marks from the spoon. The same property can be seen in many high-viscosity common liquids like peanut butter or honey. These same mechanisms are in play to build continued polymerization across the cracks of more structural materials.
Diffusion healing may be possible for materials that can withstand higher temperatures. In diffusion, the faster atoms are moving (the higher their temperature), the more likely they are to break bonds and then reform them somewhere else. This breaking and reforming of bonds means a crack will begin to have atoms jump across it and form new bonds, slowly healing the crack. Because diffusion is a function of temperature, it only becomes a significant part of self-healing materials at higher temperatures, but it may lead to self-healing ceramics and metals in the future.
Figure 2. Solder is a great example of heat-treatable material in which the melting/reflow temperature of the solder is lower than the components adjacent to it, allowing repairs to be performed with special tools. Image used courtesy of Canva
Self-healing Material Challenges
The overall concept of ‘self-healing’ sounds great, but the goal is certainly not without challenges.
The problem with most self-healing materials is that parts must be removed and heat-treated, otherwise the extreme heat would transfer to nearby components, accelerating their own failure. This means the labor cost for removal and replacement is virtually the same, or even more expensive than simply replacing the failing part.
Another big problem is that research on self-healing properties indicates a limit of just a few cycles in most materials. The most likely scenario would be that a part heals a few times, but only at a significant expense in labor and capital.
Composite Material and Future Possibilities
Composite materials are combinations of different polymers, ceramics, and metals, constructed in a way to try and maximize the advantages of each material while minimizing the disadvantages. This often yields mixed results. However, in the case of self-healing materials, a self-healing layer inside of the composite can help strengthen and improve the lifetime of another material, meaning the second material need not be a self-healing material. This means a ceramic fiber and thermoplastic composite is possible, where the ceramic fibers provide the strength and the thermoplastic might be able to provide self-healing properties.
In composites, one of the major problems is delamination between layers. Many composites are made in a sandwich structure, where different materials are layered one over top of the other. This layered structure can be created with oriented fibers laid in one direction in one layer, and then another direction in a different layer, all bound together with an adhesive. Unfortunately, the weak spot often is the bonds between the layers, and these types of composites can delaminate under repeated stresses.
How Could this Self-healing Process be Controlled?
New composite materials are possible, thanks to 3D printing and other additive manufacturing processes. Recently, a team of researchers at North Carolina State University combined the concept of self-healing materials, layered composites, and 3D printing into one system.
In this system, the 3D printer can lay down the different layers, and a special self-healing epoxy is used. During the printing process, a small heater coil layer is embedded into the composite material. This coil will generate enough heat to begin the healing process when a voltage is applied to its terminals.
Figure 3. Heating coils can be placed inside polymer layers. When a proper control voltage is applied, the material can be returned to its original state. Note that not all materials can achieve this property, which is why much research is still required. Image used courtesy of Canva
This has a tremendous advantage over existing self-healing composites as parts made with this system do not need to be removed to heal. Instead, a voltage can simply be applied to the exposed terminals to begin the healing process, saving both time and labor.
Applications of Self-healing Materials
Virtually all applications can benefit from materials that repair their own damage. In particular, these self-healing materials can lower the costs of replacement parts, reduce the time spent in maintenance and installation, as well as increase the reliability of parts in service. A few industries are already recognizing the possibilities of these materials based on these advantages.
The aerospace industries stand to realize massive gains from these self-healing materials. The leading edges of wings, elevators, and flaps are placed under repeated stresses during flight and need frequent maintenance. In fact, cracks in these structures used to be remedied by drilling a small hole at the end of the crack. This dulled the crack tip and theoretically slowed the growth of the crack. With self-healing components, these cracks could be repaired automatically.
On the space end, satellites and spacecraft are difficult and extremely expensive to repair. However, they are subject to damage, thermal cycling, and other fatigue-inducing stresses. If a component can be repaired by applying a signal to the heater coil instead of being captured and repaired, this is a tremendous savings.
Future Work to Increase Material Reliability
The versatility of 3D printing means many complex geometries can be laid up in a self-healing composite structure. As this process reaches the commercialization, it will make headway into other markets, such as automotive panels, sporting goods, and virtually anywhere scratches and cracks diminish the strength or the aesthetics of a component.
Figure 4. Thanks to the constant advance of 3D printing, more opportunities for embedded components, strategic materials, and even on-the-fly construction, material science has been making serious advancements in recent times. Image used courtesy of Canva
Eventually, these composites may be paired with a sensor and processor system, such that small strain gauges detect when cracks may form and then send the control signal to the heater to start healing the composite. Ultimately, this will catch cracks earlier than visual inspection and save energy by only applying the heat when it is needed.