www.afm-journal.de FULL PAPER © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4371 www.MaterialsViews.com wileyonlinelibrary.com Adv. Funct. Mater. 2010, 20, 4371–4374 By Daniel Coillot, François O. Méar,* Renaud Podor, and Lionel Montagne 1. Introduction Many efforts in the research and development of smart mate- rials are currently driven towards higher performance. In recent years, it has been realized that a strategy based on damage management can make materials stronger and more reliable. These materials have a built-in capability to repair the damage that may occur during use. When damage through thermal, mechanical, chemical, or another origin is formed, the mate- rial has the ability to heal and restore itself to its original set of properties. Thus, self-healing polymers, metals, ceramics, and their composites have attracted broad attention. The self-healing concept has been developed in many applica- tion fields such as polymers for coatings, microelectronic pack- aging, [1] medical uses, [2] concrete or cementitious structures, [2] and composites materials for aerospace applications. [3,4] A robust self-healing effect must have the following character- istics: [2] efficiency (recovers both mechanical and chemical prop- erties); repeatable (ability to self-repair from multiple damage events; long shelf life (remains in operation during the service life, which may span decades); reliable (consistent self-healing in a broad range of environments); pervasiveness (ready for activation when and where needed); and economical (economically feasible, for instance for the highly cost-sensitive con- struction industry). Most of the work in the development of self-healing materials has concentrated on the field of polymers, for which several self- healing mechanisms have been reported. One of the earliest one is by Dry, [5] who used glass capillaries to carry a liquid resin to the dam- aged region. This process was then extended to hollow glass fibres. [6–8] Another self-repair process is achieved by incorporating a micro- encapsulated healing agent and a catalytic chemical trigger within a polymer matrix. [9–11] Chen et al. [12] have developed a polymer composition that remains in a dynamic polymerisation–depolymerisation equilibrium, which ensures the reformation of broken bonds on heating. Finally, Hayes et al. [13] have developed a smart composite system which com- bines structural health monitoring with a self-healing resin. Passive self-healing has also been developed for cementi- tious composite by Li et al., [2] based on the process developed by Dry [7] in polymers. Superglue contained in hollow, brittle glass fibres serves as the sealing/healing agent. Another active application field of self-healing is that of ceramic matrix composites (CMCs). For instance, carbon- fiber-reinforced carbon-matrix composites (C/C composites) are widely used as aeronautic and spatial structural materials because of their outstanding mechanical properties at high temperature, in association with their low weight. These C/C composites are protected with antioxidation coatings, but micro- cracks often occur owing to the thermal expansion coefficient (TEC) difference between the C/C substrates and their coatings. Therefore, self-healing of the coating was developed to enable long-term antioxidation protection. For this purpose, Cho et al. [10] have extended the polymer-self-healing concept by incorpo- rating into the coatings a healing agent such as SiC, B 4 C, ZrB 2 , or HfB 2 . [14–19] Self-healing is activated when the healing agents come into contact with air and form oxides that heal the cracks. Self healing has also been claimed to enable an increase of the operating duration of glass seals for solid-oxide fuel cells (SOFCs). It was indeed shown that the SOFC lifetime was limited by cracks that form within the glass seal used to join elec- troactive ceramic parts to the metallic structure. [20] However, con- trary to the conventional self-healing processes described above, here the self-healing effect was not obtained by the addition of a healing agent, but simply by heating the sealing glass above its softening temperature. [21–25] This effect was shown to Autonomic Self-Repairing Glassy Materials A new process that enables glassy materials to self-repair from mechanical damage is presented in this paper. Contrary to intrinsic self-healing, which involves overheating to enable crack healing by glass softening, this process is based on an extrinsic effect produced by vanadium boride (VB) particles dispersed within the glass matrix. Self-repair is obtained through the oxida- tion of VB particles, and thus without the need to increase the operating temperature. The VB healing agent is selected for its capacity to oxidize at a lower temperature than the softening point of the glass. Thermogravimetric analyses indeed show that VB oxidation is rapid and occurs below the glass transition temperature. Solid-state nuclear magnetic resonance spectroscopy indicates that VB is oxidized into V 2 O 5 and B 2 O 3 , which enable the local for- mation of glass. The autonomic self-healing effect is demonstrated by an in situ experiment visualized using an environmental scanning electron micro- scope. It is shown that a crack could be healed by the VB oxidation products. DOI: 10.1002/adfm.201000147 [*] D. Coillot, Dr. F. O. Méar, Prof. L. Montagne Unité de Catalyse et Chimie du Solide UMR-CNRS 8181 Université Lille Nord de France F-59652 Villeneuve d’Ascq, France E-mail: francois.mear@univ-lille1.fr Dr. R. Podor Institut de Chimie Séparative de Marcoule UMR 5257 CEA-CNRS-UM2-ENSCM Site de Marcoule BP 17171, F-30207 Bagnols sur Cèze cedex, France