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

It is generally accepted that the most significant source of uncertainty in nanoindentation measurement is the geometry of the indenter tip. In this p aper results are presented on the determination of indenter area functions by two methods: (1) direct determination from co-ordinate measurements obta ined using a scanning force microscope (SFM); (2) indentation into referenc e materials with known Young's modulus and Poisson ratio. The direct measur ement of geometry of a Berkovich indenter was determined using a traceably calibrated Thermomicroscopes 'Autoprobe CP' SFM that of other mentioned ind enters (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 i n the co-ordinate axes x, y and z, thus realizing directly traceable measur ements. Recently, the original arrangement of the laser-interferometers was improved such that an Abbe error is minimized. The paper reports the resul ts achieved with this arrangement and analyses the uncertainty of the area functions obtained. Good knowledge of the machine compliance is a second pr erequisite for the accurate determination of the mechanical properties of t hin films, in particular elastic modulus. Using the indenter area calculate d from SFM measurements, an iterative procedure of high load indentation in to a stiff reference material is assessed. In the alternative method, impro vements in the determination of the area function by means of indentation i nto reference materials, based on the Oliver and Pharr method (J. Mater. Re s. 7 (1992) 1564-1583), is reported. Due to the uncertainty of the thus det ermined area functions essentially depending on the uncertainty of the mach ine compliance, a combined iterative approach using two reference materials is described. The machine compliance again was determined by high load ind entation into a stiff reference material (as in the first method). The inde nter area function, however, is derived from another reference material all owing accurate stiffness measurements at shallow depths. This allows measur ement of the area function near the indenter tip. Both methods are compared to assess the accuracy of the indentation method for determining the inden ter area function and the machine compliance. (C) 2000 Elsevier Science B.V . All rights reserved.