Cracking in brittle materials during low-load indentation and its relation to fracture toughness
Harding, David Scott
Doctor of Philosophy
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 ($\sim$250 mN) or more for most ceramic and glass materials. It is shown that the problems imposed by the cracking threshold can largely be overcome by using an indenter with the geometry of a cube corner. With a cube corner indenter, the cracking threshold of most brittle materials can be reduced to loads as small as 0.1 gram ($\sim$1 mN). In addition, the well-developed relationships between indentation crack length and fracture toughness used for the Vickers indenter at high loads, generally above 1000 grams ($\sim$10 N), can be used with the cube corner indenter at very small loads, 0.1 gram ($\sim$1 mN) and above, provided different empirical constants are used. The Berkovich indenter, which is the three-sided analog of a Vickers indenter, is also found to obey the fracture toughness relations of the Vickers indenter. A model is developed which explains the differences in cracking thresholds for the Vickers, Berkovich, and cube corner indenters. The model is based on the hardness impression itself serving as the precursor flaw from which cracks extend.
Engineering; Materials science