Cracking in brittle materials during low-load indentation and its relation to fracture toughness.

摘要

Nanoindentation is a widely recognized method for characterizing the mechanical properties of thin films and small volumes. This thesis reports the results of an investigation aimed at developing a technique by which the fracture toughness of a thin film or small volume can be determined from low-load indentation experiments. The method relies on the fact that most brittle materials form radial cracks when indented with a sharp indenter such as a Vickers indenter. The lengths of the radial cracks produced during microindentation experiments have been shown to correlate reasonably well with fracture toughness. As a result, simple semi-empirical relations have been developed to calculate fracture toughness based on the measurement of indentation crack length. The one problem encountered in applying the indentation cracking method for measuring fracture toughness to low loads is that there are threshold loads below which most materials do not form radial cracks. For Vickers and Berkovich indenters, the cracking threshold is 25 grams (;It is shown that the problems imposed by the cracking threshold can largely be overcome by using an indenter with the geometry of a cube corner. With a cube corner indenter, the cracking threshold of most brittle materials can be reduced to loads as small as 0.1 gram (