**4.3 Characterization of self-healing ceramics**

*Advanced Functional Materials*

η

scattering, rheological and thermal techniques [6].

based on the tensile strength criterion given by Eq. (7):

η

**4.2 Characterization of self-healing metals**

 = *healed* 100% *virgin <sup>E</sup> <sup>X</sup> E*

Besides the macroscale methods used for mechanical performance evaluation of self-healing materials, evaluation at smaller length scale (micro- and nanoscale levels) is necessary to reveal the underlying healing mechanisms in polymers and deepen understanding of self-healing [6]. Characterization techniques at this scale enable the monitoring of the whole process of self-healing from stage of inflicting damage to identifying interactions and confirmation of healing functionality at molecular/nanolevels [16, 80]. The techniques include imaging, spectrometric,

Self-healing efficiency quantification of metals based on the bulk material properties can also be carried out by subjecting pre-cracked samples to various mechanical tests and self-healing efficiency determined using Eq. (1). For instance, the self-healing efficiency of metallic system based on precipitation-induced approach can be evaluated by subjecting the age-hardened alloy (virgin alloy) and its pre-cracked counterpart but filled low-melting alloy to tensile loading to fracture. Alaneme and Omosule [81] used this method to determine the self-healing efficiency of underaged Al-Mg-Si alloys and 60Sn-40Pb alloy-reinforced aluminum metal–metal composites. The self-healing efficiency, ɳ, was estimated using relation

σ

σ *tensile* <sup>=</sup> *healed* 100% *virgin*

Other experimental techniques used to characterize solid materials subjected cyclic or creep loads can also be adapted to evaluate bulk metallic material systems. Mechanical evaluation of materials using micro-indentation techniques is a widely accepted tool to reveal information on the surfaces of bulk hard materials [4] and it can be readily applied to study healing at micron scale. Self-mending at this length scale can be studied by inducing mechanical damage through micromachining accompanied by imaging of the repairing process. The imaging can be done using low and high imaging equipment such as optical microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS) and environmental scanning electron microscope (ESEM) or X-ray micro-tomography instrument to provide details concerning crack propagation arrest [31] and evolution of self-healing reactions and to reveal evolved microstructures and morphology [4]. The results obtained at micro- and nanolevels

Most research conducted on self-healing metals focused on either solid-state diffusion healing of micro-cracks, or shape memory alloy (SMA)-reinforced "off-eutectic" matrices. It is worthwhile to conduct tests at nanolevels to elucidate the bonding at the interface between the diffusing species and the metal matrices. Laha et al. [82] applied nanoscale investigation using Auger spectroscopy to show that boron (B) atom acts as a solute healing agent in 347-austenitic stainless steel by diffusing to the nano voids and precipitating at the void surfaces. He et al. [83] equally used positron annihilation spectroscopy to confirm that the addition of B

*virgin* is the tensile strength of the virgin specimen and

*X* (7)

σ

*healed* is the tensile

(6)

**224**

where σ

strength of the healed specimen.

are used to buttress results at macrolevels.

The current fracture mechanic tests used to assess self-healing capabilities in hard materials are also applicable to advanced ceramics, but the difficulty in creating controlled cracks in ceramic materials limits this application. Unlike other hard materials that exhibit some level of plasticity, ceramic materials are very brittle in nature and are prone to unwanted fast fracture under mechanical and thermal stresses [71]. Another limitation is that it is difficult achieve crack closure in ceramics at low temperatures. The most common procedure for creating controlled micro-cracks and quantifying healing efficiency is indentation method [84]. The healing efficiency is evaluated in terms of crack closure by comparing the control samples with the healed specimens.

For instance Nam and Hwang [40] investigated crack healing behavior of ZrO2/SiC composite ceramics with TiO2 additive. Cracks of about 100 μm were made on the sample surfaces using Vickers indenter. Since self-healing in ceramics is a high-temperature process, the indented samples were heat-treated to stimulate healing, but the test was conducted at room temperature. This was followed by observing the pre- and post-healed indents with X-ray diffractometer (XRD). The strength of crack closure was determined using three-point bending test. Li et al. [85] also used flexural test to evaluate the multiple healing of titanium aluminum carbide (Ti2AlC) ceramic damaged by indentation in terms of crack propagation.

Besides using bending tests, tensile and the biaxial ball-on-three balls (B3B) tests have been used to study healing efficiencies in ceramics at room temperatures. Gao and Suo [86] assessed adhesion healing efficiency in a ceramic coating by performing tensile tests and correlating the healing time and residual stress while Harrer et al. [87] studied the healing of surface defects induced by different machining conditions on silicon nitride ceramic using biaxial ball-on-three balls.

All these tests are usually performed at room temperature while the healing process takes place at elevated temperatures >1000°C with the attendant oxidative atmosphere and thermal stresses. At high temperatures, the internal structure and mechanical performance of the ceramics could change due to the accompanying local melting and phase transformations [88]. Since mechanical performance under the above atmosphere will be different from that at low temperatures, there is the need to develop a more suitable method for the quantification of self-healing efficiency in ceramics [71] that takes into consideration the real service conditions. Attempts in this direction have been made by Ando et al. [89, 90], who determined *in situ* crack healing ability by conducting mechanical tests on some ceramics at elevated temperatures [89, 90].
