**3.3 Self-healing ceramics**

Ceramics are very important engineering material and are widely applied in electrical, magnetic, chemical, nuclear and biomedical fields [35]. However, ceramics have major shortcomings of being inherently porous and brittle in nature. As a result, ceramics have low strengths and fracture toughness as the components are prone to catastrophic failure by crack damage even at subcritical loading [36, 37]. Thus, self-healing concept in ceramics targets induced healing of structural defects (cracks and pores) in order to prolong lifetime [38].

**219**

*Exploits, Advances and Challenges in Characterizing Self-Healing Materials*

Self-healing in brittle ceramics has also witnessed extensive studies like polymers, but healing in ceramics is difficult to achieve at temperatures below 1000°C [35]. This is because self-repairing in ceramics takes place readily via solid-state diffusion, which requires high activation energy. This thermally activated solidstate reaction has the disadvantage of inhibiting long-range transport of material required to heal macro-cracks. More so, the healing of nano-cracks is inhibited by crack surface relaxation phenomena triggered by the ionic and covalent bonding character in ceramics [39]. Although, processing of self-healing ceramic materials is regarded as a high-temperature healing process, processing at lower temperatures is

Some healing routes have been adopted to repair crack damages in ceramics at elevated temperatures. The important ones include crack closure enabled by diffusion-controlled sintering; crack opening rebonding promoted by viscous flow of glass phase; filling the crack opening space with products of oxidation reaction as obtainable in silicon carbide composites [35, 40] and healing of multicomponent and multiphase ceramic materials through local particle rearrangement-induced eutectic melt or phase transition [41]. Damage mechanisms and crack healing have been widely studied in various ceramic materials including single crystalline,

Concrete is the most popular cement-based material and most widely used constructional material [42, 43]. Concrete is regarded as a composite material made up of water (H2O), cement, fine and coarse aggregates. It has many good qualities such as availability and affordability of its constituent materials, versatility, durability and low maintenance [42–45]. Concrete exhibits superior compressive strength, but low tensile strength. In order to improve its tensile strength, it is reinforced with steel bars [43]. However, the major limitation of concrete is its high susceptibility to cracking [43, 44]. The causes of cracks at both micro- and macro levels include preparation processes, temperature differences, shrinkage, fatigue loads and settlement of structures [42, 44]. The cracks serve as channels for water, dissolved particles in fluids and unwanted acidic gasses to penetrate the concrete [42, 45], attacking the concrete and corroding the steel reinforcement [44, 45]. Thus, the susceptibility of concrete to crack leading to structural failures is a major concern

However, concrete has been known to exhibit natural or autonomous selfhealing to a certain extent under a long-term hydration [45]. It has been proved that some initial cracks in concrete can be suddenly closed when un-hydrated cement reacts with carbon dioxide dissolved in water, producing calcium carbonate, CaCO3 [46, 47]. Thus, self-healing in cementitious materials can be obtained naturally or artificially [42]. The blocking of cracks through natural routes occurs owing to the following: expansion of hydrated cementitious matrix; precipitation of CaCO3; presence of impurities in H2O and further hydration of unreacted cement [42]. The artificial approach toward the development of self-healing cementitious materials targets enhancing the natural abilities of cement-based materials by engineering artificial healing abilities [42]. The artificial route focuses on filling of cracks by use of microorganism, polymers and addition of supplementary cementing materials to the concrete mix or steel fibers [45]. Microorganisms are biological agents and they are added in cement directly or in encapsulated forms to promote the precipitation of sealing compound such as CaCO3 in a crack opening [48, 49]. Crack closure can also be achieved by addition of extra cement or other additives like fly ash to initial mix design to promote continuing hydration or stimulate a

*DOI: http://dx.doi.org/10.5772/intechopen.93031*

polycrystalline and amorphous glasses.

and has remained unsolved in industry [43].

**3.4 Self-healing concrete materials**

being pursued.

## **Figure 1.**

*Self-healing concepts in metals and metal matrices adapted from Ref. [22].*

*Exploits, Advances and Challenges in Characterizing Self-Healing Materials DOI: http://dx.doi.org/10.5772/intechopen.93031*

Self-healing in brittle ceramics has also witnessed extensive studies like polymers, but healing in ceramics is difficult to achieve at temperatures below 1000°C [35]. This is because self-repairing in ceramics takes place readily via solid-state diffusion, which requires high activation energy. This thermally activated solidstate reaction has the disadvantage of inhibiting long-range transport of material required to heal macro-cracks. More so, the healing of nano-cracks is inhibited by crack surface relaxation phenomena triggered by the ionic and covalent bonding character in ceramics [39]. Although, processing of self-healing ceramic materials is regarded as a high-temperature healing process, processing at lower temperatures is being pursued.

Some healing routes have been adopted to repair crack damages in ceramics at elevated temperatures. The important ones include crack closure enabled by diffusion-controlled sintering; crack opening rebonding promoted by viscous flow of glass phase; filling the crack opening space with products of oxidation reaction as obtainable in silicon carbide composites [35, 40] and healing of multicomponent and multiphase ceramic materials through local particle rearrangement-induced eutectic melt or phase transition [41]. Damage mechanisms and crack healing have been widely studied in various ceramic materials including single crystalline, polycrystalline and amorphous glasses.

#### **3.4 Self-healing concrete materials**

*Advanced Functional Materials*

able in Ref. [22].

**3.3 Self-healing ceramics**

(cracks and pores) in order to prolong lifetime [38].

*Self-healing concepts in metals and metal matrices adapted from Ref. [22].*

The approaches that have been proposed and attempted in developing selfhealing in metallic systems according to [22] include: (a) precipitation-induced self-healing approach at low and high temperatures [26]; (b) dispersion of nanoshaped memory alloy (SMA) in off-eutectic metal matrix [27–29]; (c) SMA-clamp and melt [30]; (d) solder tubes/capsules; (e) coating agent [31] and (f) electrohealing [32, 33]. Blazej Grabowski and C. Cem Tasan [22] classified these concepts into two based on the healing length scale as (i) healing of nanoscale voids (which includes approaches a and b) and (ii) healing of macroscale cracks (which includes approaches c, d, e and f) [22]. One (I) and two (II) above were earlier classified into damage prevention and damage management by Van der Zwag et al. [34], respectively. This implies that healing at nanoscale targets prevention of macroscale damage while healing at macroscale focuses on management of macroscale damage to prevent total failure. Manuel [30] further classified approaches a and b as solid-state healing; approaches c, d and e as liquid-state healing and approach f as electrolyte-assisted healing. The self-healing concepts in metals and metal matrices are summarized in **Figure 1** and details of the features of these concepts are avail-

Ceramics are very important engineering material and are widely applied in electrical, magnetic, chemical, nuclear and biomedical fields [35]. However, ceramics have major shortcomings of being inherently porous and brittle in nature. As a result, ceramics have low strengths and fracture toughness as the components are prone to catastrophic failure by crack damage even at subcritical loading [36, 37]. Thus, self-healing concept in ceramics targets induced healing of structural defects

**218**

**Figure 1.**

Concrete is the most popular cement-based material and most widely used constructional material [42, 43]. Concrete is regarded as a composite material made up of water (H2O), cement, fine and coarse aggregates. It has many good qualities such as availability and affordability of its constituent materials, versatility, durability and low maintenance [42–45]. Concrete exhibits superior compressive strength, but low tensile strength. In order to improve its tensile strength, it is reinforced with steel bars [43]. However, the major limitation of concrete is its high susceptibility to cracking [43, 44]. The causes of cracks at both micro- and macro levels include preparation processes, temperature differences, shrinkage, fatigue loads and settlement of structures [42, 44]. The cracks serve as channels for water, dissolved particles in fluids and unwanted acidic gasses to penetrate the concrete [42, 45], attacking the concrete and corroding the steel reinforcement [44, 45]. Thus, the susceptibility of concrete to crack leading to structural failures is a major concern and has remained unsolved in industry [43].

However, concrete has been known to exhibit natural or autonomous selfhealing to a certain extent under a long-term hydration [45]. It has been proved that some initial cracks in concrete can be suddenly closed when un-hydrated cement reacts with carbon dioxide dissolved in water, producing calcium carbonate, CaCO3 [46, 47]. Thus, self-healing in cementitious materials can be obtained naturally or artificially [42]. The blocking of cracks through natural routes occurs owing to the following: expansion of hydrated cementitious matrix; precipitation of CaCO3; presence of impurities in H2O and further hydration of unreacted cement [42].

The artificial approach toward the development of self-healing cementitious materials targets enhancing the natural abilities of cement-based materials by engineering artificial healing abilities [42]. The artificial route focuses on filling of cracks by use of microorganism, polymers and addition of supplementary cementing materials to the concrete mix or steel fibers [45]. Microorganisms are biological agents and they are added in cement directly or in encapsulated forms to promote the precipitation of sealing compound such as CaCO3 in a crack opening [48, 49]. Crack closure can also be achieved by addition of extra cement or other additives like fly ash to initial mix design to promote continuing hydration or stimulate a

reaction process releasing self-sealing products [50, 51]. Self-healing in cementitious materials has also been attempted by incorporating polymers containing healing agents and shape memory materials into cement matrices [52, 53].

#### **3.5 Self-healing coatings**

Coatings can be defined as any thin layer of covering applied to the surface of a material. The basic objective of traditional coating is to separate material surfaces, especially metal surfaces from environmental corrosive attack. Most metallic materials have the intrinsic weakness of being corrodible in aqueous service environments. Corrosion normally starts at the surface and is adjudged one of the major causes of material failures. Coating acts as a barrier, limiting the diffusion of oxidation species such as oxygen and moisture to the metal surface [54]. For effective protection, the coating must maintain its adherence, structural integrity and not break down in the presence of operating factors such as mechanical stresses, abrasions, changes in pH, surface tension and temperature [54, 55].

However, over time, these operating factors lead to formation of scratches, surface and internal micro-cracks or even delamination, requiring human intervention to prevent or stop the interaction of the coated material surface and the unfriendly environment [55, 56]. The development of self-healing/smart coatings is driven by the need for damaged protective coatings to automatically sense or respond to damages and repair without human intervention when in service. Besides corrosion sensing, smart coatings have been applied to achieve self-cleaning and antifouling functions [54]. Intrinsic and extrinsic strategies have been adopted to impart self-healing capabilities in coatings using different materials of both organic and inorganic origins [54, 56], such as polymeric compounds [57], metals [58], ceramics [59] and composites [60].

Intrinsic self-healing coating can be obtained by using organic materials that undergo reversible chemistry [56] or self-reactions [61]. Extrinsic self-repair in organic coatings can be achieved by embedding self-healing agents or corrosion inhibitors in the structure of a polymer coating. The two popular methods of doing this are encapsulating healing agent in microcapsules (microencapsulation) or storing healing agents in capillary tubes (vascular networks) [55, 62]. Healing takes place when microcapsules or capillary tubes containing the healing/anticorrosion agents are ruptured by damage and release their contents, which flow into and heal the damaged areas [55, 62, 63]. The process of healing can come in form of blocking the active sites on the exposed metal surface after encountering damage. Besides storing healing agents in open polymer structures, nano-sized containers based on inorganic systems such silica, ceramic and TiO2 have been reported to have high storing and release abilities [64–67].

In order to overcome the limitation of low storage capacity in microencapsulation-based polymeric and inorganic self-healing systems, nano-sized core-shell and microfiber containers are being exploited [68]. The major limitation of polymeric and inorganic containers is low storage capacity, depletion and noncontinuous replenishment of the healing agent contained within it on rupturing of a microcapsule. Currently, these concepts are being extended to layer-by-layer deposition, multi-shell-core microcapsules that can contain artificial or green anticorrosive agents that enable two-in-one action of self-healing and anticorrosion [69].
