A ceramic that can repair itself

Myrthe Bosch thinks that we have passed the point where self-healing materials are science fiction. She proved the self-healing capacity of a composite material made of ceramic.

Myrthe Bosch: "The MAX-phase particles are heat-resistant and hard for a metal, but quite soft for a ceramic." (Photos: Marcel Krijger)
Myrthe Bosch: "The MAX-phase particles are heat-resistant and hard for a metal, but quite soft for a ceramic." (Photos: Marcel Krijger)

As its name suggests, a self-healing material can repair tiny fractures by itself as a result of chemical reactions. This will extend the lifespan of the material and prevent sudden fracture. The idea has been used previously on other materials, but has only just started to be used in ceramics. Bosch (26) used so-called MAX-phase particles, a type of material with properties in between metals and ceramics. "MAX-phase is not special due to its elements, but because of its crystal structure," said Bosch. She researched the influence of these particles in the ceramic alumina. The MAX-phase particles are heat-resistant and hard for a metal, but quite soft for a ceramic. The idea is that the composite has the best of both worlds. It keeps the qualities of alumina, like hardness and resistance to wear and heat, but has extra toughness and crack-healing ability due to the MAX-phase particles. "The toughness will prevent swift micro fractures. And if fractures occur, the MAX-phase particles will use their self-healing capacity to repair the composite," Bosch said.

Oxidation

At the Department of Materials, Science and Engineering (at the Faculty of Mechanical, Maritime and Materials Engineering), Bosch made her own MAX-phase particles in a pure form. If a facture arises in the alumina composite, some MAX-phase particles are exposed to air. Under the influence of heat, the particles will oxidise. The resulting product is aluminium oxide, the same material out of which alumina is made. The result of this chemical process is that the crack fills with the main material. It only takes 15 minutes to fill the majority of the crack. After this short time the MAX-phase particles have not fully oxidised yet. "If the particles oxidise just long enough to fill the gap, then still some material is left to oxidise again at a new fracture," said Bosch.

Turbine blades

The aerospace industry cannot wait for such a composite to be available on the market. Turbine blades, for example, are now made of steel superalloys. Ceramic materials are much better suited for such applications due to their high heat-resistance, but they are very fragile. A ceramic composite could be the solution. Furthermore, the material itself repairs the fractures. Despite this promising application, Bosch is not sure if MAX-phase particles will be used any time soon. "I'm not sure if this specific composite will be ever used, but I'm convinced that self-healing ceramics will be applied in the future," she said.

Title of thesis: Self-healing alumina composite with MAX-phase particles

Grade: 7.5