How does transformation toughening work in zirconia ceramics?

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advanced ceramics Steps in doctor blading, a tape-casting process employed in the production of ceramic films. Ceramic powder and solvent are mixed to form a slurry, which is treated with various additives and binders, homogenized, and then pumped directly to a tape-casting machine. There the slurry is continuously cast onto the surface of a moving carrier film. The edge of a smooth knife, generally called a doctor blade, spreads the slurry onto the carrier film at a specified thickness, thereby generating a flexible tape. Heat lamps gently evaporate the solvent, and the dry tape is peeled away from the carrier film and rolled onto a take-up reel for additional processing.

Transformation toughening is a mechanism used to improve the fracture toughness of zirconia ceramics by utilizing a phase transformation.1

In this process, extremely fine particles of tetragonal zirconia are dispersed within a matrix of cubic zirconia or alumina. Because these particles are very small and in intimate contact with the matrix, they remain in the tetragonal phase at room temperature.1

When a crack begins to propagate through the material, the stress field at the crack tip triggers the embedded tetragonal particles to transform into a monoclinic form. This transformation is accompanied by a 3 percent volume expansion. As the crack advances, a process zone forms behind the crack front where all the particles have undergone this transformation. The cumulative increase in volume exerts a closing force on the crack, effectively resisting further propagation.1

Because this strain response to stress resembles that of steel, transformation-toughened zirconia (TTZ) is often referred to as "ceramic steel."1