Cracking During Nanoindentation and its Use in the Measurement of Fracture Toughness
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ABSTRACT Results of an investigation aimed at developing a technique by which the fracture toughness of a thin film or small volume can be determined in nanoindentation experiments are reported. The method is based on the radial cracking which occurs when brittle materials are deformed by a sharp indenter such as a Vickers or Berkovich 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 the toughness from the crack lengths. However, a problem is encountered in extending this method into the nanoindentation regime with the standard Berkovich indenter in that there are well defined loads, called cracking thresholds, below which indentation cracking does not occur in most brittle materials. We have recently found that the problems imposed by the cracking threshold can be largely overcome by using an indenter with the geometry of the corner of a cube. For the cube-corner indenter, cracking thresholds in most brittle materials are as small as I mN ( - 0.1 grams). In addition, the simple, well-developed relationship between toughness and crack length used for the Vickers indenter in the microindentation regime can be used for the cube-corner indenter in the nanoindentation regime provided a different empirical constant is used. INTRODUCTION Nanoindentation is a widely accepted tool for measuring the mechanical properties of thin films and small volumes of material [1,2]. One of the great advantages of the technique is its ability to probe a surface and map its properties on a spatially resolved basis, sometimes with a resolution of better than 1 gtm. Nanoindentation has been used to characterize elastic properties such as the modulus, E [3,4], plastic properties such as the hardness, H [3,4], and time dependent properties, such as the stress exponent for creep, n [5]. To date, however, little attention has been given to how the technique may be useful in the measurement of properties important in fracture, such as the fracture toughness, Kc. Here, the results of a study aimed at establishing a method by which nanoindentation can be used in the measurement of fracture toughness are reported. The method we are pursuing is based on the radial cracking which occurs when brittle materials are indented by a sharp indenter such as a Vickers or Berkovich diamond (see Fig. 1). A theoretical description of the mechanics of this indentation cracking has been developed by Lawn, Evans, and Marshall [6] which leads to a simple relation between the fracture toughness, Kc, and the lengths of the radial cracks, c, of the form: Kc= a(E
(1)
Here, P is the peak indentation load and ox is an empirical constant which depends on the geometry of the indenter. A particularly attractive feature of using this method in nanoindentation 663 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
Vickers indentation Fig. 1. Schematic illustration of radial cracking at a Vickers inde
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