Investigation of the stresses and stress intensity factors responsible for fracture of thin protective films during ultra-micro indentation tests with spherical indenters

This paper reports on the mechanical characterization of thin hard films deposited on softer substrates using the micro-indentation technique together with spherically tipped indenters. After a short review of the deformation and fracture behavior the indentation process is simulated using a finite element analysis in order to determine the stresses responsible for the first fracture event of the film. A fracture mechanics analysis is presented that uses the weight function method to calculate the stress intensity factors K-I and K-II within the near surface region of the film, A fracture criterion for multiaxial loading conditions is used to calculate the stress intensity factor K-I appl. It will be shown that for films which are thin compared to the radius of the indenter first fracture should occur under a Mode-I loading condition. For the cases where the film was thick compared to the indenter radius mixed mode loading conditions prevail and the resultant fracture path is dependent on the nature of the film microstructure.