**Author details**

*Advanced Functional Materials*

**7. Concluding remarks**

tions in materials at various length scales.

microscale aspects of damaging and healing.

self-healing systems decreases over time.

appropriate evaluation approach should be the one that takes into account more than one property and reports combined efficiencies or overall efficiency, equal to

The development of self-healing materials comes as a huge challenge to the scientist or engineer—the challenge of synthesizing smart materials that can self-repair, elucidating the healing mechanism and proving their self-healing capabilities through characterization. There are many material classes and different approaches have been attempted to achieve self-healing capabilities in these materials. For close to two decades, several self-healing materials have been developed and many methods have been employed to assess self-healing behavior and determine healing efficiency of these materials. As the materials are different, so are the evaluation techniques utilized to characterize the healing behaviors. Besides taking into consideration the type of material, the healing-enabling preparatory route and repairing mechanism, the most suitable test method should fit the intended application. For completeness, effective characterization should be the one encompassing all length scales—macro-, micro- and nanoscale levels. Thus, an ideal quantification approach needs to take into account the macroscopic as well as

Among the several characterization methods utilized to investigate self-healing

In ceramics and concrete, initiation of controlled damage is somewhat tasking due to their inherent brittleness and low diffusion rate of healing agents. This is also likened to metals, whose self-healing mostly occurs faster at high temperature, but damage initiates at low temperature. Therefore, simulating the real condition of damage and healing simultaneously is a herculean task. On the other hand, coating is used in various conditions, but the nanoscale evaluation of indentation is carried out at controlled environments, which are different from its real application

The authors declare no conflicts of interest with respect to the authorship and

behavior and determine healing efficiency in metals, polymers and polymer composites, ceramics and concrete at macroscale evaluation focusing on restoration of mechano-physical properties is popular. Typically, most characterization methods in metals are carried out at the macroscale level, but healing takes place at nanoscale level. This can be a fundamental limitation in the characterization process for metals. However, evaluation techniques at micro- and nanoscale levels have been employed to link and correlate mechanical healing with underlying molecular processes in particular polymeric materials. However, testing of polymers and other material systems do come with their own challenges, including getting reliable information from testing of modified materials only with the available small-scale samples and at laboratory conditions. Long-time instability of polymers is also a problem as it has been demonstrated that the healing efficiency of extrinsic

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 quantifica-

prime average of the efficiency obtained for each material property [71].

**230**

condition.

**Conflict of interest**

publication of this chapter.

Camillus Sunday Obayi\* and Paul Sunday Nnamchi Department of Metallurgical and Materials Engineering, University of Nigeria, Nsukka, Nigeria

\*Address all correspondence to: camillus.obayi@unn.edu.ng

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
