Indentation response and cracking of sub-micron silica films on a polymeric substrate

Indentations at low forces with small spherical tipped indenters have been made on physical vapour deposited (PVD) films of silica on an allyl diglycol carbonate (CR-39) polymer. Film thicknesses investigated were 320 and 80 nm with indenters ranging from 0.25 to 10 mu m nominal radius. Contact pressure and effective elastic modulus of the films as a function of penetration depth of the contact diameter of the various indenters were calculated. In all instances the substrate appeared to yield prior to cracking of the film. Cracking of the films, in the form of circular cracks just outside the contact area, was evident as a discontinuity in the force-displacement response, particularly for the smaller indenter radii. Estimates of the maximum contact stress for the onset of cracking indicated that it decreased with increasing indenter radius. Finite element analysis was used to calculate the stress fields in the films and substrate during loading and to estimate the extent of the plastic zone within the substrate and the fracture strength of the films. (C) 1998 Elsevier Science Ltd. All rights reserved.