**4. Characterization of self-healing materials**

It has been obvious from past reviews on different materials systems that the major source of failures in materials is the presence of faults/damage such as voids,

**221**

operation.

strength etc.

**Figure 2a**.

efficiency is shown in Eq. (1).

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

nanoscale events, responsible for initiation of material failure [71].

η

cracks or other defects introduced in the materials during processing or developed during utilization. Damage initiation usually starts with one or more cracks at microscopic level [70], which gradually propagate and grow in size and eventually lead to unexpected failure of the material [70, 71]. Hence, damage can be regarded as an accumulation of micro-damages that ultimately lead to material rupture if it is not repaired in due time [71]. Great efforts are spent in designing, processing and characterization to prevent the development of the damage or manage it using different self-healing strategies enabling longer service lives, greater safety and

For the above, self-repairing strategies target preventing, curing, by closing or filling or sealing of these cracks or voids on the surface or in the bulk of the materials by exploiting different self-healing approaches, whereas characterization focuses on proving that self-healing has taken place by determining the extent of recovery or restoration of initial properties after encountering damage(s). Much earlier than now, the characterization of self-healing capabilities in material systems concentrated mainly on the well-known macroscale evaluation, neglecting micro- and

On the other hand, macroscale evaluation is not always sufficient, because it cannot provide comprehensive information about self-healing at all length scales or levels as it is focused mainly on the restoration of observable properties after the damage has occurred. However, it has the advantage of easier standardization when compared to microscale methods [72] as it is observable and the procedures most times are well known. To obtain a quantitative assessment and better understanding of the materials' self-healing abilities, there is need to complement macroscopic investigation with microscopic and nanoscale measurements [4, 6, 42] as self-healing processes take place on various length scales that might require sophisticated experimental procedures and equipment to reveal the mechanism of self-healing

The quantification formula for estimating the capability of a given material to self-repair is the self-healing efficiency ɳ, defined as the ability of a given material to recover a particular property relative to the virgin or undamaged material [73]. This formula, which was initially applied to polymeric materials [74], is now commonly adopted for comparing healing efficiencies in many material classes subjected to macroscopic quasi-static and dynamic tests. The expression for healing

> = X 100% *healed virgin*

where f is a certain property of a particular material such as tensile strength, fracture toughness, tear strength, fatigue strength, flexural strength, creep rupture

However, unlike other testing methods, monitoring or testing of self-healing materials in most cases entails inducing controlled damage such as a crack in the material and allowing it to heal using a particular healing treatment. This is followed by testing both the healed and virgin materials to failure. The extent of recovery of properties of the healed material is compared to original or virgin material properties using Eq. (1). The sequence of characterization is shown in

Numerous quantification methods have been used for the assessment of selfhealing capabilities for the different material systems. The characterization method adopted to quantify self-healing effectiveness should take into consideration the

*<sup>f</sup>* (1)

*f*

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

reduced maintenance costs.

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

*Advanced Functional Materials*

**3.5 Self-healing coatings**

storing and release abilities [64–67].

**4. Characterization of self-healing materials**

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].

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,

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

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].

It has been obvious from past reviews on different materials systems that the major source of failures in materials is the presence of faults/damage such as voids,

abrasions, changes in pH, surface tension and temperature [54, 55].

**220**

cracks or other defects introduced in the materials during processing or developed during utilization. Damage initiation usually starts with one or more cracks at microscopic level [70], which gradually propagate and grow in size and eventually lead to unexpected failure of the material [70, 71]. Hence, damage can be regarded as an accumulation of micro-damages that ultimately lead to material rupture if it is not repaired in due time [71]. Great efforts are spent in designing, processing and characterization to prevent the development of the damage or manage it using different self-healing strategies enabling longer service lives, greater safety and reduced maintenance costs.

For the above, self-repairing strategies target preventing, curing, by closing or filling or sealing of these cracks or voids on the surface or in the bulk of the materials by exploiting different self-healing approaches, whereas characterization focuses on proving that self-healing has taken place by determining the extent of recovery or restoration of initial properties after encountering damage(s). Much earlier than now, the characterization of self-healing capabilities in material systems concentrated mainly on the well-known macroscale evaluation, neglecting micro- and nanoscale events, responsible for initiation of material failure [71].

On the other hand, macroscale evaluation is not always sufficient, because it cannot provide comprehensive information about self-healing at all length scales or levels as it is focused mainly on the restoration of observable properties after the damage has occurred. However, it has the advantage of easier standardization when compared to microscale methods [72] as it is observable and the procedures most times are well known. To obtain a quantitative assessment and better understanding of the materials' self-healing abilities, there is need to complement macroscopic investigation with microscopic and nanoscale measurements [4, 6, 42] as self-healing processes take place on various length scales that might require sophisticated experimental procedures and equipment to reveal the mechanism of self-healing operation.

The quantification formula for estimating the capability of a given material to self-repair is the self-healing efficiency ɳ, defined as the ability of a given material to recover a particular property relative to the virgin or undamaged material [73]. This formula, which was initially applied to polymeric materials [74], is now commonly adopted for comparing healing efficiencies in many material classes subjected to macroscopic quasi-static and dynamic tests. The expression for healing efficiency is shown in Eq. (1).

$$\eta = \frac{f\_{\text{heated}}}{f\_{\text{võrg}}} \mathbf{X} \mathbf{1} \mathbf{o} \mathbf{o} \mathbf{y} \mathbf{6} \tag{1}$$

where f is a certain property of a particular material such as tensile strength, fracture toughness, tear strength, fatigue strength, flexural strength, creep rupture strength etc.

However, unlike other testing methods, monitoring or testing of self-healing materials in most cases entails inducing controlled damage such as a crack in the material and allowing it to heal using a particular healing treatment. This is followed by testing both the healed and virgin materials to failure. The extent of recovery of properties of the healed material is compared to original or virgin material properties using Eq. (1). The sequence of characterization is shown in **Figure 2a**.

Numerous quantification methods have been used for the assessment of selfhealing capabilities for the different material systems. The characterization method adopted to quantify self-healing effectiveness should take into consideration the

**Figure 2.**

*(a) Steps in characterization of self-healing materials, and (b) interrelationship among material properties, self-healing mechanism, characterization method and application adapted from Ref. [6].*

material properties developed, repairing mechanism and its intended application as shown in **Figure 2b** adapted from Ref. [6]. This subchapter considers quantification of healing efficiencies at different length scales in the above material classes.
