The Nano Hardness Tester (NHT) from CSM Instruments has already established itself for the characterisation of thin films and coatings with force and displacement resolutions of 10mN and <1nm respectively, using a differential capacitive measuring technique. Although quantitative hardness and modulus data are of great importance in judging the applicability of a given material to a specific function, the actual response of the material to an indentation at such low loads can give substantial additional information about the indentation process.
Scanning Force Microscope Integration
This article describes a new option for the NHT where a scanning force microscope (SFM) can be integrated into the optical microscope of the instrument, in place of a standard objective lens. This efficient and compact solution is shown in Figure. 1, and has a very user-friendly PC interface where all functions can be controlled directly from a powerful software package. The SFM uses an interferometric beam deflection system for detecting cantilever displacement and a piezoelectric scanner which is feedback controlled in the x, y and z directions. This latter feature greatly reduces the effects of piezo distortion and non-linearity and allows a standard lateral scan range of 20 µm, although larger scanners can also be supplied on special request.
The net advantages of combining surface topographic and indentation data in one instrument, with an accurate electromechanical positioning system, are that a particular sample site can be located and measured before repositioning either under an optical objective lens or the SFM.
Figure 1. The new SFM mounted as a standard objective on the optical microscope of the instrument
Figure 2. Indentation curves (max. load = 100mN) for ferrite and austenite phases, together with corresponding SFM images of the residual imprints.
An interesting example of the use of this instrument is the characterisation of multiphase materials where the local mechanical properties of separate phases need to be measured. Duplex stainless steels, having a microstructure consisting of ferrite and austenite, have been investigated in order to establish their susceptibility to long term ageing at intermediate temperatures (300-400°C).
Figure 2 shows load-displacement curves for each phase with a maximum applied load of 100mN, together with SFM images of the residual imprints. The difference in imprint shape between both phases is particularly evident. For the austenite, the edges are concave which confirms the high work-hardenability and elastoplasticity of this phase, due to radial relaxation of material around the indentation during unloading of the indenter. In contrast, the ferrite imprint exhibits convex edges together with bulging and pile-up of material around it which suggests weak work-hardenability and a rigid plastic response to indentation. The respective Vickers hardness of the austenite and ferrite phases was measured to be 280 and 740 after 8000 hours ageing at 350°C.