Progress in determination of the area function of indenters used for nanoindentation

Abstract

It is generally accepted that the most significant source of uncertainty in nanoindentation measurement is the geometry of the indenter tip. In this paper results are presented on the determination of indenter area functions by two methods: (1) direct determination from co-ordinate measurements obtained using a scanning force microscope (SFM); (2) indentation into reference materials with known Young's modulus and Poisson ratio. The direct measurement of geometry of a Berkovich indenter was determined using a traceably calibrated Thermomicroscopes ‘Autoprobe CP’ SFM that of other mentioned indenters (Berkovich and ball shaped indenters) was determined with a modified SFM Veritekt-3. The modification of the latter is that the movement of the SFM scanner is directly calibrated by three laser interferometers fitted in the co-ordinate axes x, y and z, thus realizing directly traceable measurements. Recently, the original arrangement of the laser-interferometers was improved such that an Abbe error is minimized. The paper reports the results achieved with this arrangement and analyses the uncertainty of the area functions obtained. Good knowledge of the machine compliance is a second prerequisite for the accurate determination of the mechanical properties of thin films, in particular elastic modulus. Using the indenter area calculated from SFM measurements, an iterative procedure of high load indentation into a stiff reference material is assessed. In the alternative method, improvements in the determination of the area function by means of indentation into reference materials, based on the Oliver and Pharr method (J. Mater. Res. 7 (1992) 1564–1583), is reported. Due to the uncertainty of the thus determined area functions essentially depending on the uncertainty of the machine compliance, a combined iterative approach using two reference materials is described. The machine compliance again was determined by high load indentation into a stiff reference material (as in the first method). The indenter area function, however, is derived from another reference material allowing accurate stiffness measurements at shallow depths. This allows measurement of the area function near the indenter tip. Both methods are compared with assess the accuracy of the indentation method for determining the indenter area function and the machine compliance.