Self-crack-healing by oxidation of a pre-incorporated healing agent is an essential property of high-temperature structural ceramics for components with stringent safety requirements, such as turbine blades in aircraft engines. Here, we report a new approach for a self-healing design containing a 3D network of a healing activator, based on insight gained by clarifying the healing mechanism. We demonstrate that addition of a small amount of an activator, typically doped MnO localised on the fracture path, selected by appropriate thermodynamic calculation significantly accelerates healing by >6,000 times and significantly lowers the required reaction temperature. The activator on the fracture path exhibits rapid fracture-gap filling by generation of mobile supercooled melts, thus enabling efficient oxygen delivery to the healing agent. Furthermore, the activator promotes crystallisation of the melts and forms a mechanically strong healing oxide. We also clarified that the healing mechanism could be divided to the initial oxidation and additional two stages. Based on bone healing, we here named these stages as inflammation, repair, and remodelling stages, respectively. Our design strategy can be applied to develop new lightweight, self-healing ceramics suitable for use in high- or low-pressure turbine blades in aircraft engines.

Self-crack-healing behavior under a combustion gas atmosphere with a low oxygen partial pressure, P O2 , is important for actualizing ceramic gas turbines, but to date only self-crack-healing behavior in air has been investigated. In this study, we investigated crack-healing behaviors at 1273-1773 K under several levels of P O2 . Crack-healing in atmospheres with P O2 ! 50:0 Pa gave rise to the complete strength recovery of cracked specimens, resulting from passive oxidation. Based on the obtained results, the kinetics for strength recovery by self-crack-healing was expressed as a function of healing temperature, T H (K), and P O2 (Pa). The strength recovery rate for complete crack-healing, v H (s À1 ), could be expressed as v H ¼ 6:95  10 5 expðÀ4:65  10 4 =T H ÞP O 2 0:835 Using this rate equation, one can evaluate the healing time for complete strength recovery under combustion gas in a gas turbine. B. Derby-contributing editor

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The available temperature range of the self-healing induced by high temperature oxidation of SiC can be controlled by the particle size of the contained SiC particles. In this study, three types of alumina–SiC composites were prepared. The SiC particle sizes of the composites were 270, ∼30 nm, and less than 10 nm. The self-healing abilities were estimated by the strength recovery behavior at several temperatures. The use of nanometer-sized dispersed SiC particles as healing agent decreases the activation energy of the SiC oxidation obtained from the differential thermal analysis with several heating rates. This implies that smaller SiC particles can give rise to the oxidation at lower temperature. Moreover, the lowest temperature at which the cracked strength was completely recovered for 10 h was strongly affected by the SiC particle size. As the SiC particle size varied from 270 to ∼30 nm, the lowest temperature varied from 1300 to 950 °C. However, alumina composite containing SiC particles whose particle size is less than 10 nm cannot recover completely the cracked strength under every condition, because the space between crack walls cannot be filled with the formed oxide due to the small volume of SiC on the crack walls. Therefore, it was found that there is an optimal SiC particle size for endowing self-healing ability.