**5. Prospects for standardization of characterization techniques**

Most of the current characterization methods used to quantify the healing performance focused on the macroscopic evaluation of recovery from macroscopically applied damage. However, early stage of damage and recovery occur at sub-macro level. Therefore, macroscopic evaluation cannot by itself be sufficient enough for self-healing quantification [4]. As the materials' failure normally starts at nano- and microscale levels, a sufficient and necessary quantification approach should take

**229**

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

ization of quantification in hard self-healing materials.

irrespective of testing method and material class [71].

understanding of damage mechanism and healing process.

into consideration the damaging and healing events at these small length scales [71], more so, when they can easily be prevented or healed faster at the sublevels. However, inflicting of macroscopic damage on hard self-healing materials, though far from the actual utilization conditions, can be easily replicated and holds the promise of easier standardization and comparison across similar materials [71]. Since damage initiation inherently starts at the microscopic/nanoscale level, inflicting of damage protocol can be done at microscopic level using nano- or/and microindentation techniques. Mechanical damage can be induced using micromachining, accompanied by imaging of the mending process using high-resolution imaging equipment such as environmental scanning electron microscope (ESEM) [4]. Currently, micro-indentation testing protocol is widely accepted and standardized tool for mechanical testing of materials [113]. *In situ* measurement techniques can also be very helpful in such environment, where dynamic and on process conditions can be captured or easily replicated. At the heart of this standardization prospect is

Microscopic evaluation has limitations. One challenge is the requirement of very small volume of samples, which makes sampling difficult due to the nonhomogeneous composition of some materials [4]. Another is that it is not very suitable for testing soft materials. However, most of the self-healing material classes are hard materials. Some polymers, metals, ceramics, concrete and some coatings, especially inorganic coatings are all hard materials. Testing using micro-indentation technique complemented with macroscopic methods could be a useful step toward standard-

In practical situations, a single applied load affects more than one specific material's property and self-healing efficiency is regarded as the ability of a given material to recover a specific property relative to the virgin specimen. In this situation, efficiency calculation should take the combined properties of the material affected by the load into consideration and should be reported as an average or overall efficiency. The use of overall efficiency as a method of assessing performance of distinct materials using different test methods opens a path toward standardization

The adopted routine of assessing the performance of self-healing materials has been to characterize the modified self-healing material and compare its properties to the unmodified, virgin material. An extensive literature survey indicated that it was difficult to find researchers who evaluated self-healing performance at macro-, micro- and nanostructural levels simultaneously. This is probably because the field is relatively new. Today, it is becoming obvious that an appropriate performance assessment method should take into account the damaging and healing at macroscopic as well as microscale/nanoscale levels. In order to achieve this, a combined suitable and reproducible evaluation procedure must be exploited for a better

It is equally hard to find researchers who investigated several properties at the same time in one material. In real-life situations, the damage initiation and eventual failure can be caused by combined factors/loads-tensile, compressive, cyclic, bending, creep, thermal loads and others. The self-healing efficiency is defined as the ability of a given material to recover a specific property relative to the virgin or undamaged specimen [75]. For instance, a single applied load affects more than one specific material's property and efficiency calculation should take into account other properties of the materials affected by the load [71]. Therefore, the most

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

the operator's skill.

**6. Future trends**

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

into consideration the damaging and healing events at these small length scales [71], more so, when they can easily be prevented or healed faster at the sublevels.

However, inflicting of macroscopic damage on hard self-healing materials, though far from the actual utilization conditions, can be easily replicated and holds the promise of easier standardization and comparison across similar materials [71]. Since damage initiation inherently starts at the microscopic/nanoscale level, inflicting of damage protocol can be done at microscopic level using nano- or/and microindentation techniques. Mechanical damage can be induced using micromachining, accompanied by imaging of the mending process using high-resolution imaging equipment such as environmental scanning electron microscope (ESEM) [4]. Currently, micro-indentation testing protocol is widely accepted and standardized tool for mechanical testing of materials [113]. *In situ* measurement techniques can also be very helpful in such environment, where dynamic and on process conditions can be captured or easily replicated. At the heart of this standardization prospect is the operator's skill.

Microscopic evaluation has limitations. One challenge is the requirement of very small volume of samples, which makes sampling difficult due to the nonhomogeneous composition of some materials [4]. Another is that it is not very suitable for testing soft materials. However, most of the self-healing material classes are hard materials. Some polymers, metals, ceramics, concrete and some coatings, especially inorganic coatings are all hard materials. Testing using micro-indentation technique complemented with macroscopic methods could be a useful step toward standardization of quantification in hard self-healing materials.

In practical situations, a single applied load affects more than one specific material's property and self-healing efficiency is regarded as the ability of a given material to recover a specific property relative to the virgin specimen. In this situation, efficiency calculation should take the combined properties of the material affected by the load into consideration and should be reported as an average or overall efficiency. The use of overall efficiency as a method of assessing performance of distinct materials using different test methods opens a path toward standardization irrespective of testing method and material class [71].

## **6. Future trends**

*Advanced Functional Materials*

able unless improvised.

these electrochemical techniques should be complemented by conducting non-electrochemical or physicochemical analysis. Physicochemical analysis is majorly aimed at studying mechanism of self-healing coating protection—elemental analysis, interaction mechanisms between the metal substrate and the coating, morphology or phase transformation of the coating before and after healing. Surface analysis can be carried out with OM, SEM and CLSM, while energy dispersive X-ray (EDX), EPMA and XPS are useful for elemental analysis. The detailed description of the

Developing an artificial material to self-heal like a biological system comes as a huge task to the scientist or engineer. But more challenging is proving the material's self-healing capability and suitability for a particular application via characterization. Different approaches are used to achieve self-healing in different material classes based on their inherent properties. A self-healing material is an advanced material, a new product with new properties different from its original properties. It is important that the characteristics of the new product should be determined qualitatively or quantitatively. It is also important to monitor the changes during processing that led to the new properties. As a result, specific characterization methods are required to quantify self-healing efficiencies in each of the material classes. Most times, the equipment might not be common and sometimes unavail-

Unlike other characterization methods, evaluation of self-healing materials in most cases entails inducing a minimal expected mode of damage or failure during utilization on the material and using different methods to determine the extent of recovery/restoration of properties compared to original or virgin materials properties and to understand the mechanism of recovery. The methods of inducing the damage are different for the various material classes and even in the same material group. Inducing the appropriate damage simulating the real-life scenario is challenging. Also tasking is the selection of an appropriate testing procedure from an extensive range of known materials testing procedures that is adaptable and suitable for a particular self-healing concept. The field of self-healing materials is relatively new but a richly rewarding venture. The understanding of self-healing mechanisms in a variety of self-repairing material classes is still evolving. So also are the characterization methods needed to elucidate the dynamics of self-healing process. The challenges of harmonization of these methods in various research groups are yet to be resolved. More so, the mode of damage is different and unique to the damaged material and its intended applications. Even within the same material class, there are various self-healing approaches and evaluation strategies. This makes the adopted routine of assessing the performance of the modified material and comparing its properties to the unmodified, virgin material complex. This makes it equally arduous to establish a common testing procedure for similar or for different materials classes.

**5. Prospects for standardization of characterization techniques**

Most of the current characterization methods used to quantify the healing performance focused on the macroscopic evaluation of recovery from macroscopically applied damage. However, early stage of damage and recovery occur at sub-macro level. Therefore, macroscopic evaluation cannot by itself be sufficient enough for self-healing quantification [4]. As the materials' failure normally starts at nano- and microscale levels, a sufficient and necessary quantification approach should take

above test methods can be found in Refs. [55, 109, 112].

**4.6 Challenges in the characterization of self-healing materials**

**228**

The adopted routine of assessing the performance of self-healing materials has been to characterize the modified self-healing material and compare its properties to the unmodified, virgin material. An extensive literature survey indicated that it was difficult to find researchers who evaluated self-healing performance at macro-, micro- and nanostructural levels simultaneously. This is probably because the field is relatively new. Today, it is becoming obvious that an appropriate performance assessment method should take into account the damaging and healing at macroscopic as well as microscale/nanoscale levels. In order to achieve this, a combined suitable and reproducible evaluation procedure must be exploited for a better understanding of damage mechanism and healing process.

It is equally hard to find researchers who investigated several properties at the same time in one material. In real-life situations, the damage initiation and eventual failure can be caused by combined factors/loads-tensile, compressive, cyclic, bending, creep, thermal loads and others. The self-healing efficiency is defined as the ability of a given material to recover a specific property relative to the virgin or undamaged specimen [75]. For instance, a single applied load affects more than one specific material's property and efficiency calculation should take into account other properties of the materials affected by the load [71]. Therefore, the most

#### *Advanced Functional Materials*

appropriate evaluation approach should be the one that takes into account more than one property and reports combined efficiencies or overall efficiency, equal to prime average of the efficiency obtained for each material property [71].

A case could also be made to suggest or define effective predictive approaches or methods that would lead to faster evaluation and design of self-healing quantifications in materials at various length scales.
