Energy dissipation, fracture toughness and the indentation load-displacement curve of coated materials

The energy dissipated during normal indentation of coated materials is analyzed and related to the coating and interfacial fracture toughness. Hybrid organic-inorganic coatings, which were prepared using a sol-gel process, were used as model materials. The loading and unloading curves are integrated and the differences in irreversibly dissipated energy after delamination and chipping are used to calculate the energy release rates. The calculated energy release rates are inversely proportional to the coating thickness and a factor of up two tens larger than previous results. Furthermore, it is shown that the energy dissipated during indentation is a measure of the system's response and that the system's response is altered by the fracture events. Extrapolation to infinite coating thickness leads to values that are in agreement with previously published results. The relationship between irreversibly dissipated energy and the ratio of the hardness to elastic modulus is analyzed in detail and it is shown that this relationship is similar as for monolithic materials, although various fracture events occurred, thus suggesting that the underlying relationship is independent of the history of the coating-substrate system. (C) 2000 Elsevier Science B.V. All rights reserved.