Measurement of Fracture Toughness in Thin Films and Small Volumes Using Nanoindentation Methods

Abstract

Nanoindentation methods are now used routinely in the characterization of the elastic modulus and hardness of thin films and small volumes of material. Here, we report preliminary results of an investigation aimed at developing a technique by which the fracture toughness of a thin film or small volume can be determined. The method is based on the radial cracking which occurs when brittle materials are indented by a sharp indenter such as a Vickers diamond. In microindentation experiments, the lengths of radial cracks have been found to correlate reasonably well with fracture toughness, and a simple, semi-empirical method has been developed to compute toughness from measured crack lengths. However, a problem is encountered in extending the method into the nanoindentation regime in that there are well defined loads, called cracking thresholds, below which indentation cracking does not occur in most brittle materials. For a Vickers indenter, cracking thresholds in most ceramics are about 25 grams or more, i.e., loads well above those which would normally be used in nanoindentation. We have recently found, however, that the problems imposed by the cracking threshold can largely be overcome by changing the indenter geometry. Preliminary studies using a three sided indenter with the geometry of a corner of a cube have revealed that cracking thresholds can be reduced to loads as small as 0.5 grams, for which indentations and crack lengths in most materials are sub-micron in dimension. The studies also indicate that the simple relationship between toughness and crack length used for Vickers indenters still applies provided a different empirical constant is used. Results of these studies are presented along with a brief review of the theory on which the method is based.