2 Fracture toughness
Fracture toughness is a performance index that reflects the material's resistance to unstable crack expansion. At present, there is no unified standard for the quantitative characterization of the fracture toughness of ceramic coatings. There are mainly three characterization methods: critical stress intensity factor, critical crack expansion energy release rate, and crack density. Figure 2 compares the results of the cupping test of the two coatings. Conventional ceramic coatings show obvious cracking and peeling, while no macroscopic cracks are observed in nanostructured coatings.