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 diglyco l 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 pres sure and effective elastic modulus of the films as a function of penetratio n depth of the contact diameter of the various indenters were calculated. I n all instances the substrate appeared to yield prior to cracking of the fi lm. Cracking of the films, in the form of circular cracks just outside the contact area, was evident as a discontinuity in the force-displacement resp onse, 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 t he stress fields in the films and substrate during loading and to estimate the extent of the plastic zone within the substrate and the fracture streng th of the films. (C) 1998 Elsevier Science Ltd. All rights reserved.