Why can large transformation-toughened zirconia ball bearings bounce on concrete without visible damage?

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advanced structural ceramics Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.

Large transformation-toughened zirconia (TTZ) ball bearings can bounce on concrete without visible damage due to a mechanism known as transformation toughening.1

In this process, extremely fine tetragonal zirconia particles are dispersed within a matrix. While these particles are stable at room temperature, the stress field at the tip of an advancing crack triggers a phase transformation from the tetragonal form to a monoclinic form. This transformation is accompanied by a 3 percent volume expansion.1

As the crack propagates, the cumulative increase in volume from these transforming particles exerts a closing force on the crack, effectively resisting its growth. Because of this mechanism, transformation-toughened zirconia exhibits a fracture toughness significantly higher than conventional ceramics—often referred to as "ceramic steel" because its strain response to stress resembles that of steel. This enhanced toughness allows the material to withstand impacts that would otherwise cause brittle failure in standard ceramics.1