Cracking during nanoindentation and its relation to fracture toughness

Abstract

Cracking during indentation has long been used to measure the fracture toughness of ceramics, most often with a microhardness tester equipped with a Vickers square-based pyramidal indenter and at loads in the range 10-10,000 g. The same techniques can be used in nanoindentation at much smaller loads and size scales, but two significant problems are encountered. First, the square-based Vickers pyramid suffers from a chisel-edge tip defect that results in the loss of geometrical similarity for indentations with characteristic dimensions on the order of a micron or so. Geometrical similarity is crucial in the interpretation of results. Second, there are threshold loads, typically in the range 10-100 g, below which no cracking occurs in many ceramic materials indented by a Vickers indenter. These problems can be circumvented by testing with sharper indenters with the triangular pyramidal geometry, but the relevant mechanics of elastic-plastic contact with them is not well understood. In this study, the effect of indenter sharpness on the indentation cracking behavior of two brittle materials - silicon and germanium - is investigated for a series of triangular pyramidal indenters. The sharpness of the indenters is characterized by the centerline-to-face angle, which was varied from 35 to 85 degrees. The influences of indenter angle on cracking thresholds and the relationship between the crack length and indentation load are documented and discussed in terms of prevailing theories and models.