Self Healing Materials

Materials science is one of the most fascinating and challenging areas of science. It is both an old and a new science. Materials science not only determined our oldest history and even named it (i.e. Bronze Age, Iron Age), but still determines the pace of breakthrough developments in our daily life. Our mobile telephone/camera systems only became reality because of new battery materials, novel light-sensitive detector materials and new developments in silicon IC technology. On the other hand, transport of electrical currents in the national grid system still leads to large energy losses because materials scientists have not been able to create materials showing superconductivity at room temperature. Quantum computing would lead to a quantum jump in computing power, but is still some time away due to the fact that materials with the required properties and stability have not been created yet.

Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Damage in polymeric coatings, adhesives, microelectronic components, and structural composites can span many length scales. Structural composites subject to impact loading can sustain significant damage on centimeter length scales, which in turn can lead to subsurface millimeter-scale delaminations and micron-scale matrix cracking (Figure 1).

Polymers have become an indispensable material resource, representing billions of dollars worth of material consumption every year. The rising prices and exhaust of natural resources such as petroleum, combined with rising environmental concerns, have prompted the development of recyclable and degradable polymers. Polymers that can be reverted back to their monomers or to shorter repolymerizable oligomers, hence, reversible polymers are particularly enticing in this respect because they essentially prevent any material loss with multiple recycling. While reversible polymers have been known for a long time, there has been recent renewed interest in such polymers, since their reversibility can be exploited for repair at the molecular level.

Advanced composite materials combine high-performance-reinforcing fibres with matrix resins. Most high-performance applications currently utilise thermosetting resins because they provide stable high-modulus matrix systems. There is some interest in thermoplastic polymer matrices. These are often found in short-fibre-reinforced thermoplastic materials and occasionally in high-performance composites. The latter are likely to receive more interest in future because the weldability of thermoplastics can be advantageous in tape placement fabrication. In recent times there has been an increased interest in the development of durable systems in which the polymer can self heal. There are many opportunities for self healing because the micromechanical mechanism of first ply failure of an advanced composite is transverse or matrix cracking. Transverse cracking of a laminated composite arises because the ply failure strain at 90? to the fibre is normally less than that of the polymer alone. Furthermore, these cracks propagate within the interphase material adjacent to the fibres. In addition, the fibres are configured at angles to each other so that these transverse cracks are pinned at the higher modulus ply interface and do not have a large-crack displacement. There is, therefore, ample scope for the repair of first ply failure cracks. The micromechanics of laminates often involves the initiation of delamination at transverse cracks. Thus, the damage accumulation mechanisms involve fracture of the polymeric matrix which provides the opportunities for self healing of fibre composites.

Ionomers are a class of polymers which have up to 20 mol% of ionic species incorporated into the structure of the organic polymer. These ionic species create interactions or aggregates [17, 28], not present in comparable non-ionic polymers that have a profound effect upon the mechanical and physical properties of the polymer. As a result there has been much research in both academia and industry over the last 30 years [4, 22] aimed at increasing the understanding of structure–property relationships in these polymers while also exploring new commercial applications. The self healing phenomenon exhibited by ionomers is a particularly interesting property arising from their unique chemical structure and is the subject of this chapter.

Lightweight, high-strength, high-stiffness fibre-reinforced polymer composite materials are leading contenders as component materials to improve the efficiency and sustainability of many forms of transport. For example, their widespread use is critical to the success of advanced engineering applications, such as the Boeing 787 and Airbus A380. Such materials typically comprise complex architectures of fine fibrous reinforcement e.g. carbon or glass, dispersed within a bulk polymer matrix, e.g. epoxy. This can provide exceptionally strong, stiff, and lightweight materials which are inherently anisotropic, as the fibres are usually arranged at a multitude of predetermined angles within discrete stacked 2D layers. The direction orthogonal to the 2D layers is usually without reinforcement to avoid compromising in-plane performance, which results in a vulnerability to damage in the polymer matrix caused by out-of-plane loading, i.e. impact. Their inability to plastically deform leaves only energy absorption via damage creation. This damage often manifests itself internally within the material as intra-ply matrix cracks and inter-ply delaminations, and can thus be difficult to detect visually. Since relatively minor damage can lead to a significant reduction in strength, stiffness and stability, there has been some reticence by designers for their use in safety critical applications, and the adoption of a ‘no growth’ approach (i.e. damage propagation from a defect constitutes failure) is now the mindset of the composites industry. This has led to excessively heavy components, shackling of innovative design, and a need for frequent inspection during service (Richardson 1996; Abrate 1998).

This chapter deals with aspects to be considered with respect to the self healing of polymer coatings. After discussing the scope and limitations of self healing concepts in polymer coatings, an overview of present approaches and technologies is given. The differences between reversible and irreversible networks are discussed. Surface self-replenishing as well as active species self-replenishing are dealt with. A currently applied industrial example is elaborated: scratch-healing automotive coatings. Finally, possible future scenarios are discussed.

The phenomenon of self healing in concrete has been known for many years. It has been observed that some cracks in old concrete structures are lined with white crystalline material suggesting the ability of concrete to self-seal the cracks with chemical products by itself, perhaps with the aid of rainwater and carbon dioxide in air. Later, a number of researchers [1, 2] in the study of water flow through cracked concrete under a hydraulic gradient, noted a gradual reduction of permeability over time, again suggesting the ability of the cracked concrete to self-seal itself and slow the rate of water flow. The main cause of self-sealing was attributed to the formation of calcium carbonate, a result of reaction between unhydrated cement and carbon dioxide dissolved in water [1]. Thus, under limited conditions, the phenomenon of self-sealing in concrete is well established. Self-sealing is important to watertight structures and to prolonging service life of infrastructure.

Concrete can be considered as a kind of artificial rock with properties more or less similar to certain natural rocks. As it is strong, durable, and relatively cheap, concrete is, since almost two centuries, the most used construction material worldwide, which can easily be recognized as it has changed the physiognomy of rural areas. However, due to the heterogeneity of the composition of its principle components, cement, water, and a variety of aggregates, the properties of the final product can widely vary. The structural designer therefore must previously establish which properties are important for a specific application and must choose the correct composition of the concrete ingredients in order to ensure that the final product applies to the previously set standards. Concrete is typically characterized by a high-compressive strength, but unfortunately also by a rather low-tensile strength. However, through the application of steel or other material reinforcements, the latter can be compensated for as such reinforcements can take over tensile forces.

Any treatise on healing in asphalt pavements must begin with an answer to the question: “How important is healing in asphalt pavements?” Experience clearly tells us that it is substantially important. Shift factors from laboratory-predicted to fieldobserved fatigue cracking demonstrate that laboratory data underpredict field observations. A variety of reasons are responsible for the use of shift factors including traffic wander, time of crack propagation, difference in stress states between the laboratory and the field, and crack healing. The shift factor developed by the Asphalt Institute from AASHTO Road Test (laboratory-to-field) data is 18.3.

Metals are known to degrade, or even worse, lose functionality during service [1, 2]. The major factors shortening the service life of these materials are internal defects, such as cracks and voids [3, 4]. These internal defects are very difficult to detect, and even more difficult to repair [5–7]. During service, they might coalesce into a major crack, leading to failure [8]. When damage defects are generated in materials, the internal energy of the material increases together with some entropy increment, that is, the system is in a metastable state of the thermodynamic equilibrium. If some energy is imported from the environment, the system can overcome the energy barrier and automatically evolve along the way of minimizing the total Gibbs free energy of the system, so that the defects could be self healed [9–11] and the material performance can be partially restored [12–14]. It is therefore essential to understand the evolution of defects so that the healing mechanisms can be understood and employed to achieve the desired specific engineering requirement.

In this chapter, we will first discuss the details of the healing processes of cracks by finite element (FE) modeling. Next, void healing processes will be analyzed and some analytical solutions are presented. From the numerical and analytical analyses, the parameters controlling the rate of the healing processes are derived.

In the field of transmission electron microscopy fundamental and practical reasons still remain that hamper a straightforward correlation between microscopic structural information and self healing mechanisms in materials. We argue that one should focus in particular on in situ rather than on postmortem observations of the microstructure. In this contribution this viewpoint has been exemplified with in situ TEM nanoindentation and in situ straining studies at elevated temperatures of metallic systems that are strengthened by either solid solution or antiphase boundaries in intermetallic compounds. It is concluded that recent advances in in situ transmission electron microscopy can provide new insights in the interaction between dislocations and interfaces that are relevant for self healing mechanisms in metallic systems.

Alloys for high temperature applications in an oxidizing environment depend on the formation of a protective and slow growing oxide scale. The composition of these alloys is such that a continuous layer of a thermodynamically stable oxide is formed through selective oxidation of one of the constituting elements. Then, the oxide layer forms a barrier between the environment and the underlying alloy. The alloys for high temperature applications can be divided into alumina (Al₂O₃), silica (SiO₂), or chromia (Cr₂O₃) formers, such as stainless steels, superalloys (Reed 2006), and intermetallics (MX, where M is Ti, Fe, Co or Ni, and X denotes Al, Si, or Cr). These materials are successfully applied in for example gas turbine engines (aero, marine, and industrial), heating equipment and automotive converters etc. In this chapter, the focus will be on alumina forming alloys encountered as coating material for blades and vanes in gas turbine engines. However, the principles addressed also apply to the other mentioned classes of high temperature alloys.

The design of natural materials follows a radically different paradigm as compared to engineering materials: organs are growing rather than being fabricated. As a main consequence, adaptation to changing conditions remains possible during the whole lifetime of a biological material. As a typical example of such a biological material, bone is constantly laid down by bone forming cells, osteoblasts, and removed by bone resorbing cells, osteoclasts. With this remodelling cycle of bone resorption and formation, the skeleton is able to adapt to changing needs at all levels of structural hierarchy. The hierarchical structure of bone is summarized in the second part of this chapter.

A suite of mathematical models for epidermal wound healing is presented. The models deal with the sequential steps of angiogenesis (neovascularization) and wound contraction (the actual healing of a wound). An innovation is the combination of the two processes which do not take place in a complete sequential manner but overlap partially. The models consist of nonlinearly coupled diffusion–reaction equations, in which transport of oxygen, growth factors, and cells, and mitosis are taken into account. Further, Adam’s alternative model, which is based on the assumption of the presence of an active layer at the wound edge, is described and some implications are presented. An important feature of the model due to Adam is that the wound edge is tracked explicitly as a part of the solution. In this work several numerical methods to solve the moving boundary problem are described.

A number of self healing mechanisms for composite materials have been presented in the previous chapters of this book. These methods vary from the classical concept of micro-encapsulating of healing agents in polymer systems to the autonomous healing of concrete. The key feature of these self healing mechanisms is the transport of material to the damaged zone in order to establish the healing process. Generally, this material is a fluid and its motion is driven by capillary action which enables transportation over relatively large distances requiring little or no work. In the microencapsulated polymers as developed by White et al. [1], this liquid material is a healing agent, which is enclosed in the material by micro-encapsulation. When the capsule is ruptured by a crack, the healing agent will flow into the crack, driven by capillary action. Polymerisation of this healing agent is triggered by contact with catalysts which are inserted in the material and whose position is fixed. The new polymerised material will rebond the crack surfaces.