Cracks Pattern Formation and Spalling in Functionalized Thin Films

Functionalized thin films of ceramic material are used in catalytic converters as a catalyst support. A degradation of the catalytic performances can be caused by thermal and chemical effects leading to the formation of cracks and the detachment of fragments. Understanding the formation of the lattice of cracks and the spalling is challenging in fracture mechanics. The need for two conditions for predicting crack nucleation, one involving energy and the other stresses, is shown. The stress condition defines a threshold below which the pattern formation is inhibited. As long as it is not reached, the energy accumulates. Then, at onset, depending on the strength and toughness of the material, the amount of energy can be sufficiently large to give rise to a more or less dense lattice of cracks. Following initiation, the newly created small fragments tend to separate from it by debonding.