Method of Estimated Unloading Curve for Evaluation of Interfacial Strength by Nanoindentation Test

Abstract

A new evaluation method was proposed for the interfacial strength between a thin film and its substrate by nanoindenation test. In this method, the released energy due to a coating delamination is obtained by estimating the shapes of the unloading curve at the points just after and before delamination on the load-displacement curve. Shape of unloading curves can be assumed by obtaining the contact stiffness of thin films. To confirm the validity of the method, nanoindentation tests were conducted for polyimide (PI) coating / Si substrate specimens and the interfacial strength was evaluated. Three specimens that have different thickness were prepared. Contact stiffness of PI film bears a linear relationship with indentation load. Irrespective of the coating thickness, the values of the interfacial strengths converge. In addition, the values of strengths agree with the results obtained by the authors in the previous paper for the case that small scale yielding conditions are satisfied. For the case that indentation loading rate is high and the delamination occurs before the relaxation of film, it is necessary to measure the contact stiffness with each indentation loading rate.