Analyses of the work required to delaminate a coating film from a substrate under oscillating load conditions and the film-substrate contact behavior after delamination

Abstract

Nanoindentation tests in which oscillating loads were applied to specimens were carried out to study the work (Wf) required for a coating film to delaminate from its substrate. A sharp increase in the indentation depth occurred during coating film delamination. A theoretical model was developed for the present study to evaluate the difference between the work in the load-depth profiles obtained with and without delamination. Arranging that the maximum load was reached at the end of the loading process allowed us to obtain an approximately constant value for the indentation depth propagation rate during in one cycle. This allowed a determination of the number of oscillating cycles and, thus, the work required by assuming that there was no delamination during this delamination period. The depth propagation rates and the work required for coating film delamination were investigated by varying Pmax, Pmean, P0, and frequency (f). The depth propagation rates were slightly increased by increasing the Pmax value but were significantly lowered by increasing the frequency (f). The effects due to the changes in Pmean and P0 on the depth propagation rate were insignificant. The Wf value was slightly lowered by increasing the Pmax value but was significantly increased by increasing the oscillating load frequency (f). The effects of varying P0 and Pmean on the Wf value were found to be small. The Wf values showed an exponential drop when the depth propagation rate was increased; they were asymptotic to a constant value when the depth propagation rate was sufficiently high. Coating film delamination produced a sharp increase in the indentation depth even in the case of a very small load increase. The oscillating load-depth cycles appearing after delamination allowed us to establish the film-substrate contact behavior occurring during the loading/unloading process of one oscillating cycle.